RO Membrane – Reverse Osmosis Membrane Materials, Types and Structures
Reverse osmosis RO membrane differs by the material of the membrane polymer and by structure and configuration. Based on its structure, RO membrane can be divided into two groups: conventional thin-film composite and thin-film nanocomposite. Based on the thin-film material, conventional reverse osmosis RO membrane at present is classified into two main groups: polyamide and cellulose acetate. Depending on the configuration of the membrane within the actual membrane elements (modules), the reverse osmosis membrane materials is divided into three main groups: spiral-wound, hollow-fiber, and flat-sheet (plate-and-frame).
Conventional Thin-Film Composite Membrane Structure
The reverse osmosis RO membrane most widely used for desalination at present are composed of a semipermeable thin film (0.2 um), made of either aromatic polyamide (PA) or cellulose acetate (CA), which is supported by a 0.025- to 0.050-mm microporous layer that in turn is cast on a layer of reinforcing fabric (Fig. 1.1 for a membrane with an ultrathin PA film). The 0.2-um ultrathin polymeric film is the feature that gives the RO membrane its salt rejection abilities and characteristics. The main functions of the two support layers underneath the thin film are to reinforce the reverse osmosis membrane structure and to maintain membrane integrity and durability.
Fig 1.1: Structure of a typical reverse osmosis RO membrane
The dense semipermeable polymer film is of a random molecular structure (matrix) that does not have pores. Water molecules are transported through the membrane film by diffusion and travel on a multidimensional curvilinear path within the randomly structured molecular polymer film matrix. While the thin-film RO membrane with conventional random matrix-based structure shown in Fig. 1.1 is the type of membrane that dominates the desalination industry, new thin-film membrane of more permeable structure are currently under development in research centers worldwide.
Thin-Film Nanocomposite RO Membrane Structure
Thin-Film Nanocomposite TFC membrane either incorporate inorganic nanoparticles within the traditional membrane polymeric film structure (Fig. 1.2) or are made of highly structured porous film consisting of a densely packed array of nanotubes (Fig. 1.3). In Fig. 1.2, part A shows the thin film of a conventional PA membrane, supported by the polysulfone support layer. Part B shows the same type of membrane with embedded nanoparticles (labeled “NP”).
Fig 1.2: Polyamide reverse osmosis RO membrane with nanoparticles
Fig 1.3: Reverse osmosis RO membrane with carbon nanotubes
Nanocomposite reverse osmosis membrane material reportedly has higher specific permeability than conventional RO membrane at comparable salt rejection. Which is the ability to transport more water through the same surface area at the same applied pressure). In addition, thin-film nanocomposite membrane have comparable or lower fouling rates in comparison to conventional thin-film composite RO membrane operating at the same conditions. And they can be designed for enhanced rejection selectivity of specific ions. If membrane material science evolved to a point where the membrane structure could be made of tubes of completely uniform size, theoretically the membrane could produce up to 20 times more water per unit surface area than the RO membrane commercially available on the market today. As membrane material science evolves toward the development of membrane with more uniform structure, the further development of RO desalination membrane technology has the potential to yield measurable savings in terms of water production costs.
Cellulose Acetate CA Membrane
The thin semipermeable film of the first membrane element – developed in the late 1950s at the University of California, Los Angeles – was made of cellulose acetate (CA) polymer. While CA membrane has a three-layer structure similar to that of PA membrane, the main structural difference is that the top two layers (the ultrathin film and the microporous polymeric support) are made of different forms of the same CA polymer. In PA membrane these two layers are of completely different polymers – the thin semipermeable film’s made of polyamide, while the microporous support’s made of polysulfone (see Fig. 1.1). Similar to PA membrane, CA membrane has a film layer that is typically about 0.2 um thick; but the thickness of the entire membrane (about 100 um) is less than that of a PA membrane (about 160 um).
One important benefit of CA membrane is that the surface has very little charge and is considered practically uncharged, as compared to PA membrane, which have negative charge and can be more easily fouled with cationic polymers if such polymers are used for source water pretreatment. In addition, a CA membrane have a smoother surface than the PA membrane, which also renders them less susceptible to fouling.
CA membrane has a number of limitations, including the ability to perform only within a narrow pH range of 4 to 6 and at temperatures below 35°C (95°F). Operation outside of this pH range results in accelerated membrane hydrolysis, while exposure to temperatures above 40°C (104°F) causes membrane compaction and failure. Significant use of acid for normal plant operation requires reverse osmosis RO permeate adjustment by adding a base (typically sodium hydroxide) to achieve adequate boron rejection; in order to maintain the RO concentrate pH below 6, the pH of the feed water to the CA membrane has to be reduced to between 5 and 5.5.
CA membrane experiences accelerated deterioration in the presence of microorganisms capable of producing cellulose enzymes and bioassimilating the membrane material. However, they can tolerate exposure to free chlorine concentration of up to 1.0 mg/L. Which helps to decrease the rate of membrane integrity loss due to destruction by microbial activity. Since CA membrane has a higher density than PA membrane, it creates a higher headloss when the water flows through the membrane. Therefore they have to be operated at higher feed pressures, which results in elevated energy expenditures. CA membrane is used in municipal applications for saline waters with very high fouling potential (mainly in the Middle East and Japan) and for ultrapure water production in pharmaceutical and semiconductor industries. That is despite their disadvantages and mainly because of their high tolerance to oxidants (chlorine, peroxide, etc.) as compared to PA membrane.
Aromatic Polyamide Membrane
Aromatic polyamide (PA) membrane is the most widely used type of RO membrane at present. They have found numerous applications in both potable and industrial water production. The thin polyamide film of this type of semipermeable membrane is formed on the surface of the microporous polysulfone support layer (Fig. 1.1). It is formed by interfacial polymerization of monomers containing polyamine and immersed in solvent containing a reactant to form a highly cross-linked thin film. PA membrane operates at lower pressures and have higher productivity (specific flux) and lower salt passage than CA membrane. Which are the main reasons they have found a wider application at present.
While CA membrane has a neutral charge, PA membrane has a negative charge when the pH is greater than 5. Which amplifies co-ion repulsion and results in higher overall salt rejection. However, when pH < 4, the charge of PA membrane changes to positive and rejection reduces significantly to lower than that of a CA membrane. Another key advantage of PA membrane is that they can operate effectively in a much wider pH range (2-12). This allows easier maintenance and cleaning. In addition, PA membrane is not biodegradable and usually have a longer useful life – 5-7 years versus 3-5 years. Aromatic polyamide membrane is used to produce membrane elements for brackish water and seawater desalination, and nanofiltration.
Comparison between PA and CA Membrane
It should be noted that PA reverse osmosis membrane material is highly susceptible to degradation by oxidation of chlorine and other strong oxidants. For example, exposure to chlorine longer than 1000 mg/L-hour can cause permanent damage of the thin-film structure and can significantly and irreversibly reduce membrane performance in terms of salt rejection. Oxidants are widely used for biofouling control with RO and nanofiltration membranes. Therefore, the feed water to PA membrane has to be dechlorinated prior to separation. Table 1.4 below presents a comparison of key parameters of polyamide and cellulose acetate RO membrane in terms of their sensitivity to feed water quality.
| Parameter | Polyamide Membrane PA | Cellulose Acetate CA Membrane |
| Salt rejection | High (> 99.5%) | Lower (up to 95%) |
| Feed pressure | Lower (by 30 to 50%) | High |
| Surface charge | Negative (limits use of cationic pretreatment coagulants) |
Neutral (no limitations on pretreatment coagulants) |
| Chlorine tolerance | Poor (up to 1000 mg/L-hours); feed de-chlorination needed |
Good; continuous feed of 1 to 2 mg/L of chlorine is acceptable |
| Maximum temperature of source water | High (40 to 45°C; 104 to 113°F) | Relatively low (30 to 35°C; 86 to 95°F) |
| Cleaning frequency | High (weeks to months) | Lower (months to years) |
| Pretreatment requirements | High (SDI < 4) | Lower (SDI < 5) |
| Salt, silica, and organics removal | High | Relatively low |
| Biogrowth on membrane surface | May cause performance problems | Limited; not a cause of performance problems |
| pH tolerance | High (2 to 12) | Limited (4 to 6) |
Table 1.4: Comparison between Polyamide PA Membrane and Cellulose Acetate CA Membrane materials
Polyamide PA membrane is the choice for most RO membrane installations today. Mainly because of their higher membrane rejection and lower operating pressures. Exceptions are applications in the Middle East, where the source water is rich in organics. Thus cellulose acetate membrane offers benefits in terms of limited membrane biofouling and reduced cleaning and pretreatment needs. CA membrane provides an acceptable tradeoff between lower fouling rates and chemical cleaning costs. Also higher operating pressures and power demand on the other. Because of the relatively lower unit power costs in the Middle East. There are newer generations of lower-fouling PA membranes today on the market. The use of CA membrane elements is likely to diminish in the future.
- Published in Seawater Desalination, Technology, Water Treatment
Clean In Place – Membrane Cleaning CIP System
A Clean in place CIP system is a very effective method widely used by RO Systems manufacturers and operators to preserve and also clean fouled or scaled reverse osmosis system RO membranes. Using certain chemicals and following procedures guided by each RO Membrane manufacturers. With few exceptions, all reverse osmosis systems and other membrane systems are subject to fouling by one or more source water components and therefore require periodic cleaning. Clean In Place CIP Membrane cleaning is usually performed without removing membranes from the pressure vessels or the system. A Clean In Place CIP system is designed to prepare and recirculate chemical solutions through some of or all membrane modules at low pressure.
The CIP system can also be used to feed special membrane post treatment chemicals. Not to be confused with membrane system post treatment. Membrane performance require post-treatment equipment in some cases. The clean in place CIP system also serves to prepare and transfer membrane storage solutions, or preservatives. Membrane “pickling” solutions prevent microbial growth and in some cases prevent freezing when the membrane system is shut down for extended periods, typically more than a week. The clean in place system for a Reverse Osmosis System or Nano-Filtration system should be designed to accommodate all cleaning and membrane storage solutions expected to be used at the plant.
Inadequate clean in place CIP System procedures will result in ineffective cleaning results. We will discuss the major parameters of membrane cleaning followed by cleaning instructions from RO membrane manufacturers and step-by-step procedure. The major parameters are:
- Chemicals
- Temperature
- Flow rate
- Time
Clean In Place Chemicals
We won’t remove a carbonate scale with caustic or remove a biofilm with low pH. We must use the right chemical(s) to dissolve as much of the foulant / scalant as possible. Things that dissolve will leave the Reverse Osmosis System easily. It is the things that don’t dissolve which give us the problems. Please contact BQUA for more information about choosing the right chemical for your cleaning.
Clean In Place Temperature
Temperature affects chemical reactions. In general, the rate of most chemical reactions will double with every 10°C increase in temperature. In other words, we can get the job done quicker if the cleaning solution is warmer. If the cleaning solution temperature is less than 16°C (60°F), then cleaning will have no effect since the water is too cold. The cleaning solution temperature should be at least 21°C (70°F). Even better, get the temperature above 27°C (80°F).
Clean In Place Flow Rate
Flow rate is critical for removing fouling particles. It is unlikely that we will be able to dissolve particles completely. We, therefore, must physically remove them. We do this with turbulence. The higher the flow rate, the higher the turbulence. The higher the turbulence, the more particles removed.
Clean In Place Time
Frequently RO membrane manufacturers and chemical cleaning vendors recommend a one-hour cleaning. If the scalant/foulant is stubborn, some soaking time prior to, or after, we recommend circulating the solution. This is fine for lightly fouled/scaled elements. This may work if cleaning is initiated when the Normalized Permeate Flow NPF has dropped no more than 10-15%Â and/or the Differential Pressure DP across a stage has increased no more than 15-25%. And will not work if fouling/scaling has been allowed to progress. It is not unusual to have to clean severely fouled RO Membrane elements for 72 continuous hours. Use clean in place CIP monitoring sheets during a cleaning, and trending graphs following a cleaning, to determine when cleaning completes. Much more time than usually recommended may be required.
BQUA Clean In Place CIP System design
Clean In Place CIP System Procedure
RO membrane manufacturers provide the following clean in place CIP System procedures. This is followed by an illustrated, procedure which contains the most important points of a good clean in place membrane cleaning.
RO Membrane Element Cleaning and Flushing
The RO membrane elements in place in the pressure tubes are cleaned by recirculating the cleaning solution across the high-pressure side of the membrane at low pressure and relatively high flow. That’s when we use a CIP system. A general procedure for cleaning the Reverse Osmosis membrane elements is as follows:
- Flush the pressure tubes by pumping clean, chlorine-free product water from the cleaning tank (or equivalent source) through the pressure tubes to drain for several minutes.
- Mix a fresh batch of the selected cleaning solution in the cleaning tank, using clean product water. Circulate the cleaning solution through the pressure tubes for approximately one hour or the desired period of time. At a flow rate of 35 to 40 gpm (133 to 151 L/min.) per pressure tube for 8.0(20.3 cm) and 8.5(21.6 cm) inch pressure tubes, 15 to 20 gpm (57 to 76 L/min.) for 6.0(15.2 cm) pressure tubes, or 9 to 10 gpm (34 to 38 L/min.) for 4.0 inch pressure tubes.
- After completion of cleaning, drain and flush the cleaning tank; then fill the cleaning tank with clean product water for rinsing.
- Rinse the pressure tubes by pumping clean, chlorine-free product water from the cleaning tank (or equivalent source) through the pressure tubes to drain for several minutes.
- After rinsing the Reverse Osmosis system, operate it with the product dump. Open valves until the product water flows clean and is free of any foam or residues of cleaning agents (usually 15 – 30 minutes).
If the system shuts down for more than 24 hours, the best procedure for storage is soaking the element in an aqueous solution. With 20 percent, by weight, glycerine or propylene glycol and 1.0 percent, by weight, sodium bisulfite or SMBS Sodium Metabisulfite.
Example on Before and After CIP Chemical Membrane Cleaning in a Brackish Reverse Osmosis System
CIP System in Multi-Array Systems
For multi-array (tapered) systems the flushing and soaking operations can happen simultaneously in all arrays. You should carry out separately high flow re-circulation, however, for each array. So the flow rate is not too low in the first or too high in the last. This can be accomplished either by using one cleaning pump and operating one array at a time, or using a separate cleaning pump for each array.
RO membrane clean in place cip system overall procedure
- Published in Technology, Water Treatment
What is Dissolved Air Flotation Definition – DAF Unit
Dissolved air flotation (DAF) technology is very suitable for removal of floating particulate foulants such as algal cells, oil, grease or other contaminants that cannot be effectively removed by sedimentation or filtration. Dissolved Air Flotation DAF system can typically produce effluent turbidity of <0.5 NTU and can be combined in one structure with dual-media gravity filters for sequential pretreatment of seawater. Dissolved Air Flotation (DAF) process uses very small air bubbles to float light particles and organic substances (oil, grease) contained in the seawater. The floated solids are collected at the top of the DAF tank and skimmed off for disposal, while the low turbidity seawater is collected near the bottom of the tank. The time (and therefore, the size of flocculation tank) needed for the light fine particulates contained in the seawater to form large flocs is usually 2 to 3 times shorter than that in conventional flocculation tanks, because the flocculation process is accelerated by the air bubbles released in the flocculation chamber of the DAF tanks. In addition, the surface loading rate for removal of light particulates and floatable substances by DAF is approximately 10 times lower than that needed for conventional sedimentation. Another benefit of DAF as compared to conventional sedimentation is the higher density of the formed residuals (sludge). While residuals collected at the bottom of sedimentation basins typically have concentration of only 0.3 to 0.5 % solids, DAF residuals (which are skimmed off the surface of the DAF tank) contain solids concentration of 1 to 3 %. In some full-scale applications, the DAF process is combined with granular media filters to provide a compact and robust pretreatment of seawater with high algal and/or oil and grease content. Although this combined DAF/filter configuration is very compact and cost-competitive, it has three key disadvantages:
(1) complicates the design and operation of the pretreatment filters;
(2) DAF system loading is controlled by the filter loading rate and therefore, DAF tanks are typically oversized;
(3) Flocculation tanks must be coupled with individual filter cells. The feasibility of Dissolved Air Flotation DAF unit use for seawater pretreatment is determined by seawater quality and governed by source water turbidity and overall lifecycle pretreatment costs.
In flotation, the effects of gravity settling are offset by the buoyant forces of small air bubbles. These air bubbles are introduced to the flocculated water, where they attach to floc particles and then float to the surface. Flotation is typically sized at loading rates up to 10 times that for conventional treatment. Higher rates may be possible on high-quality warm water. Dissolved air flotation (DAF) is an effective alternative to sedimentation or other clarification processes. Modern DAF technology was first patented in 1924 by Peterson and Sveen for fiber separation in the pulp and paper industry (Kollajtis, 1991). The process was first used for drinking water treatment in Sweden in 1960 and has been widely used in Scandinavia and the United Kingdom for more than 30 years. Previous uses of the process in the United States have been to thicken waste-activated sludge in biological wastewater treatment, for fiber separation in the pulp and paper industry, and for mineral separation in the mining industry. Only recently has this process gained interest for drinking water treatment in North America. It is especially applicable when treating for algae, color, and low-turbidity water. The first use in the United States was at New Castle, New York, in a 7.5 mgd (28 ML per day) plant that began operation in 1993. A typical DAF unit is shown below:
Typical Dissolved Air Flotation DAF Unit / System
The DAF unit can handle source seawater with turbidity of up to 50 NTU. Therefore, if the source seawater is impacted by high turbidity spikes or heavy solids (usually related to seasonal river discharges or surface runoff), then DAF system may not be a suitable pretreatment option. In most algal bloom events however, seawater turbidity almost never exceeds 30 to 50 NTU, so the DAF technology can handle practically any red tide event. Although a DAF system have much smaller footprint than the conventional flocculation and sedimentation facilities, it includes a number of additional equipment associated with air saturation and diffusion, and with recirculation of portion of the treated flow, and therefore, their construction costs are typically comparable to these of conventional sedimentation basins. Usually, the O&M costs of DAF system is higher than these of gravity sedimentation tanks due to the higher power use for the flocculation chamber mixers, air saturators, recycling pumps, and sludge skimmers. The total power use for a Dissolved Air Flotation – DAF system is usually 2.5 to 3.0 kWh/1 0,000 m3/day of treated source seawater, which is Significantly higher than that for sedimentation systems (0.5 to 0.7 kWh/1 0000 m3/day of treated seawater).
Theory and Operation of a Dissolved Air Flotation DAF Unit
Effective gravity settling of particles requires that they be destabilized, coagulated, and flocculated by using metal salts, polymers, or both. The same is true for DAF. In gravity settling the flocculation process must be designed to create large, heavy floc that settles to the bottom of the basin. In Dissolved Air Flotation DAF unit, flocculation is designed to create a large number of smaller floc particles that can be floated to the surface. For efficient flotation, flocculated particles must be in contact with a large number of air bubbles.
Three mechanisms are at work in this air/floc attachment process:
• Adhesion of air bubbles on the floc surface
• Entrapment of bubbles under the floc
• Absorption of bubbles into the floc structure
The size of air bubbles is important. If bubbles are too large, the resulting rapid rise rate will exceed the laminar flow requirements, causing poor performance. If bubbles are too small, a low rise rate will result and tank size may need to be increased. In a typical Dissolved Air Flotation tank, flocculated water is introduced uniformly across the end of the tank, near the bottom, into the recycle dispersion zone. Recycle is continuously introduced through a distribution system of proprietary nozzles, valves, or orifices. When the recycle flow pressure is suddenly decreased from its operating pressure of 60 to 90 psi (414 to 620 kPa) to atmospheric pressure, saturated air within the recycle stream is released in the form of microbubbles with a size range of 10 to 100/xm, and averaging around 40 to 50/xm. These microbubbles attach to flocculated material by the mechanisms described previously, causing flocculated material to float to the surface. At the surface, the bubble-floc forms a stable and continuously thickening layer of float, or sludge. If left at the surface, the float can thicken to as much as 3% to 6% dry solids. This can be an advantage if solids are to be mechanically dewatered, because solids may be suitable for dewatering without further thickening, or the thickening process can be reduced. Sludge thickness depends on the time it is allowed to remain on the surface and the type of removal system employed.
Dissolved Air Flotation – DAF System – Key Design Criteria
Dissolved Air Flotation DAF system include three key components: flocculation chamber; flotation tank and recycling system. The design criteria for these three components are presented below:
| Flocculation System: | |
| Minimum Number of Tanks | 4 |
| Velocity Gradient | 30 to 120 s^-1 |
| Contact Time | 10 to 20 min |
| Flocculation Chambers in Series | 2 to 4 |
| Water Depth | 3.5 to 4.5 m |
| Type of Mixer | Vertical-shaft with hydrofoil blades |
| Blade Area/Tank Area | 0.1 to 0.2 % |
| Shaft Speed | 40 to 60 rpm |
| Flotation Chamber: | |
| Minimum Number of Tanks | 4 (same as filter cells if combined with filters) |
| Tank Width | 3 to 10 m |
| Tank Length | 8 to 12 m |
| Tank Depth | 2.5 t0 3m |
| Surface Loading Rate | 10 to 40 m3/m3/h |
| Hydraulic Detention Time | 10 to 15 min |
| Treated Water Recycle System: | |
| Recycling rate | 6 to 10 % of intake flow |
| Maximum Air Loading | 10 g/m3 |
| Saturator Loading Rate | 60 to 65 m3/m2/h |
| Operating Pressure | 4.0 to 6.5 bars |
Dissolved Air Flotation DAF process with built-in filtration (DAFF) is used at the 136,000 m3/day Tuas seawater desalination plant in Singapore (Kiang et aI., 2007). This pretreatment technology has been selected for this project to address the source water quality challenges associated with the location of the desalination plant’s open intake in a large industrial port (i.e., oil spills) and the frequent occurrence of red tides in the area of the intake. The source seawater has total suspended solids concentration that can reach up to 60 mg/L at times and oil and grease levels in the seawater could be up to 10 mg/L. The facility uses 20 build-in filter DAF unit, two of which are operated as standby. Plastic covers shield the surface of the tanks to prevent impact of rain and wind on DAF operation as well as to control algal growth. Each Dissolved Air Flotation DAF unit is equipped with two mechanical flocculation tanks located within the same DAF vessel. Up to 12 % of the filtered water is saturated with air and recirculated to the feed of the DAF unit.
BQUA Dissolved Air Flotation System – DAF unit
A combination of DAF followed by two-stage dual-media pressure filtration has been successfully used at the 45,400 m3/day El Coloso seawater reverse osmosis SWRO plant is Chile, which at present is the largest desalination plant in South America. The plant is located in the City of Antogofasta, where seawater is exposed to year-round red-tide events, which have the capacity to create frequent particulate fouling and biofouling of the SWRO membranes (Petry et aI., 2007). The DAF system at this plant is combined in one facility with a coagulation and flocculation chamber. The average and maximum flow rising velocities of the DAF system are 22 and 33 m3/m 2/h, respectively. This Dissolved Air Flotation DAF system can be bypassed during normal operations and is typically used during red-tide events . The downstream pressure filters are designed for surface loading rate of 25 m3/m’/h. Ferric chloride at a dosage of 10 mg/L is added ahead of the DAF system for source water coagulation. The DAF system reduces source seawater turbidity to between 0.5 and 1.5 NTU and removes approximately 30 to 40 % of the source seawater organics.
BQUA is a proud manufacturer of Dissovled Air Flotation Systems – DAF Units. Please feel free to contact us anytime with your inquiry and our team of specialists will be ready and glad to help you.
- Published in Technology, Water Treatment
What is a Reverse Osmosis System – RO System Definition
A Reverse Osmosis System is basically the application of the reverse of the Osmosis process. Where pure water is produced out of brackish or seawater by applying a pressure to the concentrated salt solution above the applied and osmotic back pressures. An Industrial Reverse Osmosis System works the same way as illustrated. Net driving pressure (NDP) forces water through the membrane. In an operating Reverse Osmosis system, feed water is pressurized by a high pressure pump. Due to Net Driving Pressure (NDP), a portion of the feed water is forced through the Reverse Osmosis semipermeable membrane.
The membrane is completely impermeable (won’t allow passage) to particles and only slightly permeable to dissolved substances. The water that passes through the membrane is called permeate. Permeate usually has very few particles in it. Unless there is a membrane defect (hole) or other problem, any particles found in the permeate were produced there (either from bacteria or equipment). Permeate is also low in dissolved substances (a small amount of dissolved solids does pass through the membrane). Permeate, therefore, is a relatively high purity water. Figure below illustrates a Reverse Osmosis System in the format we have been using so far.
Reverse Osmosis System (RO System) Illustration
Reverse Osmosis System Operation and Flushing
We know that when we pressurize the feed water and water passes through the membrane, the feed water is concentrated. If the concentration in the feed water gets high enough, the osmotic back pressure will rise to eventually give us a Net Driving Pressure (NDP) of zero and the flow will stop. In a Reverse Osmosis System, then, we must flush away the dissolved substances from the membrane surface so that the osmotic back pressure won’t keep going up. This is different from the other filters that we usually work with. Most of the filters that we have dealt with in our lives have been “full flowâ€, “accumulative†types of filters. “Full flow†means that there is one stream in (feed water) and one stream out (filtrate). “Accumulative†means that the filtered “stuff†accumulate in or on the filter.
From coffee filters, to cartridge filters, to multimedia filters, this has been the case. The feed water goes in, the filter removes the “stuff†that we want to take. When the filter gets full, we backwash or replace the filter. Membrane systems can’t be full flow or accumulative. With an RO membrane, we are filtering out ions which have an osmotic pressure. What would happen if we continue to filter out dissolved substances which produce an osmotic back pressure? Answer: The process would stop.
A Reverse Osmosis System therefore, must have a flushing flow which carries away the dissolved and suspended substances. This waste stream is called concentrate. A Reverse Osmosis System (RO System), therefore, has one stream going into it (Feed Water), and two streams coming out (Permeate & Concentrate).
The following illustration also shows a Reverse Osmosis System with a 100 gpm (22.7 m3/hr) feed water flow. The Net Driving Pressure (NDP) supplied by a high pressure pump forces around 75% of the feed water through the membrane. The water, suspended particles, and dissolved substances which don’t go through the membrane are concentrated and exit the Reverse Osmosis System as concentrate.
Reverse Osmosis System Operation Example
Reverse Osmosis System Contaminants Removal
Most water constituents retains on the feed side of the Reverse Osmosis membrane depending on their size and electric charge. While the purified water (permeate) passes through the membrane. Figure below illustrates the sizes and types of solids removed by Reverse Osmosis membranes as compared to other commonly used filtration technologies. RO membranes can reject particulate and dissolved solids of practically any size. However, they do not reject well gases, because of their small molecular size. Usually Reverse Osmosis membranes remove over 90 percent of compounds of 200 daltons (Da) or more. One Da is equal to 1.666054 × 10−24 g. In terms of physical size, RO membranes can reject well solids larger than 1 (Angstrom) Å. This means that they can remove practically all suspended solids, protozoa (i.e., Giardia and Cryptosporidium), bacteria, viruses, and other human pathogens contained in the source water. Reverse Osmosis membranes are designed to primarily reject soluble compounds (mineral ions) while retaining both particulate and dissolved solids.
BQUA Reverse Osmosis RO Membrane Contaminant Removal
The structure and configuration of Reverse Osmosis membranes is such that they cannot store and remove from their surface large amounts of suspended solids. If left in the source water, the solid particulates would accumulate and quickly plug (foul) the surface of the Reverse Osmosis membranes. Not allowing the membranes to maintain a continuous steady-state desalination process. Therefore, the suspended solids (particulates) in desalination feed water must be removed before reaching the RO membranes.
Over the past 20 years, RO membrane separation has evolved more rapidly than any other desalination technology. Mainly because of its competitive energy consumption and water production costs. Brackish Water Reverse Osmosis System (BWRO) yields the lowest overall production costs of all the desalination technologies. It is also important to note that the latest Multi-Effect Distillation MED projects built over the last 5 years have been completed at costs comparable to those for similarly sized Seawater Reverse Osmosis plants. Seawater Reverse Osmosis (SWRO) desalination – for the majority of medium and large projects – is usually is more cost competitive than thermal desalination technologies.
Example of an Industrial Brackish Water Reverse Osmosis System BWRO
- Published in Technology, Water Treatment
What is a Cartridge Filter Definition
A Cartridge filter is a fine microfilter of nominal size from 1 to 25 um (micron) made of thin plastic fibers or other fine filtration media that is installed around a central tube to form a standard-size cartridge. Cartridge Filters are often used as the only screening device between the intake wells and the Reverse Osmosis system. This is in case of brackish and seawater desalination plants with well intakes producing high-quality source water. Cartridge filters are RO membrane protection facilities rather than screening devices; the main purpose they serve is to capture particulates in the pretreated source water that may have passed through the upstream pretreatment systems in order to prevent damage or premature fouling of the RO membranes.
Although wound (spun) polypropylene filter cartridges are most commonly used for seawater and brackish water applications. Other types, such as melt-blown or pleated cartridges of other materials have also found application. Standard cartridge filters for RO desalination plants are typically 101.6 to 1524 cm (40 to 60 in.) long. And are installed in either horizontal or vertical pressure vessels (filter housings). Cartridges are rated for removal of particles of 1, 2, 5, 10, or 25 um, with the most frequently used size being 5 um.
Fig. 1: Example on Cartridge Filter Housing Manufacturer: Hydrodex Filter Housings
Fig. 2: Example on Cartridge Filters: Hydrodex FRP Cartridge Filter
It is useful to mention that Hydrodex is one of the market leaders in the filtration industry. Hydrodex manufactures a premium cartridge filter made of Glass Reinforced Polymer (GRP).
Cartridge Filter Planning and Design Considerations
Cartridge filters are typically installed downstream of the granular media filtration system. That is in case such a system is used for pretreatment to capture fine sand, particles, and silt that may be contained in the pretreated water. When the source seawater is of very high quality – a silt density index (SDI) below 2 – and does not need particulate removal by filtration prior to desalination. In this case cartridge filters are used as the only pretreatment device. Serving as a barrier to capture fine silt and particulates that can occasionally enter the source water during the start-up of intake well pumps or due to failure of intake equipment or piping.
Cartridge Filter in Horizontal Orientation
A typical indication of whether the pretreatment system of a given desalination plant operates properly is the SDI reduction through the cartridge filters. If the pretreatment system performs well, then the SDI of the source water upstream and downstream of the cartridge filters is approximately the same. If the cartridge filters consistently reduce the SDI of the filtered source water by over 1 unit. This means that the upstream pretreatment system is not functioning properly. Sometimes the SDI of the source water increases when it passes through the cartridge filters. This almost always occurs because the cartridge filters have not been designed properly or are malfunctioning and providing conditions for growth of biofouling microorganisms on and within the filters.
A frequently debated question is whether cartridge filters are needed downstream of MF or UF membrane pretreatment systems. Taking into consideration that the cartridge filter pores are one to two orders of magnitude larger than those of the membrane filters. The answer to this question is highly dependent on the quality of the pretreatment membrane’s fiber material. And the type of flow pattern through the pretreatment system.
For UF or MF filtration systems that have a direct flow-through pattern. Where the desalination plant feed pumps convey water directly through the membrane pretreatment system without an interim pumping. The pretreatment membranes are more likely to be exposed to pressure surges. If the fiber material of the pretreatment membranes is weak and breaks easily under pressure surge conditions, the pretreatment system is more likely to experience fiber breaks. Broken membrane fibers will release small amounts of particles into the RO feed water. Which could cause accelerated membrane fouling unless it is captured by cartridge filtration.
In addition, if the broken membrane fibers release sharp particles contained in the source water, these particles could also damage the RO membranes. Sharp broken-shell particles may find their way into the UF or MF pretreated water if shellfish plankton contained in the source water passes through the microscreens. Grows to adult shellfish organisms (e.g., barnacles) on the walls of the pretreatment system feed pump station. And releases portions of shells that have been broken into small, sharp particles by the feed pumps.
The shell particles will be pressurized onto the UF/MF membrane filter fibers, causing punctures and ultimately entering the filtered flow. In such cases, the use of cartridge filters downstream of the membrane pretreatment system is a prudent engineering practice. Cartridge filters are operated under pressure, and the differential pressure across them is monitored to aid in determining when filter cartridges should be replaced. In addition, valved sample ports should be installed immediately upstream and downstream of the cartridge filter vessels for water quality sampling and monitoring (including SDI field testing).
Cartridge filtration systems are designed for hydraulic loading rates of 0.2 to 0.3 L/s per 250 mm (3 to 5 gal/min per 10 in.) of length. Additional filtration capacity is normally provided to allow replacement of cartridges without interruption of water production. Pressure vessels are typically constructed of duplex stainless steel for seawater RO installations.
The pressure drop across a clean cartridge filter is usually specified as less than 0.2 bar (2.8 lb/in2). Commonly, cartridges are replaced when the filter differential pressure reaches 0.7 to 1.0 bar (10.1 to 14.5 lb/in2). The operational time before replacement depends on the source water quality and the degree of pretreatment. Typically, a cartridge filter replacement is needed once every 6 to 8 weeks. However, if the source seawater is of very good quality cartridge filters may not need replacement for 6 months or more.
For RO systems where sand in the feed water might be anticipated. Rigid meltblown cartridges or cartridge filters with single open ends and dual O-rings on the insertion nipple. Rather than conventional cartridges with dual open ends, are commonly used. The single-open-end insertion filters have positive seating and an insertion plate. Which does not allow deformation of the filter cartridge under pressure caused by sand packing. Double-open-end cartridge filters are held in place by a spring-loaded pressure plate.
Design Example of a Cartridge Filter
This example presents the sizing and configuration of the cartridge filtration system for a 40,000 m3/day (10.6 mgd) seawater desalination plant. With a total plant seawater intake flow of 98,440 m3/day (26 mgd).
| Design feed flow, Qin | 98,440 m3/day = 1140 L/s |
| Cartridge filter material | Pleated polypropylene |
| Cartridge filter size | 5 um (micron) |
| Cartridge filter length, Lcf | 1016 mm (40 in.) |
| Selected design loading rate, DLR | 0.25 L/s per 250 mm |
| Number of cartridge filters needed | Qin/[DLR x (Lcf/250)] (8.1) = 1140/[0.25 x (1016/250)] = 1122 |
| Number of cartridge vessels | 6 (selected to match RO trains) |
| Cartridge vessel material | Glass-reinforced plastic |
| Number of filter cartridges per vessel | 1122/6 = 187 (selected 180) |
| Actual cartridge filter loading rate | 1140/[180 x 6 x (1016/250)] = 0.26 L/s per 250 mm (4.2 gal/min per 10 in.) |
In summary, the cartridge filtration system for the 40,000 m3/day (10.6 mgd) desalination plant will consist of six cartridge vessels. Each of which will contain 180 cartridge filters of size 5 μm and length 40 in.
- Published in Technology, Water Treatment
What is Reverse Osmosis (RO) Definition
Reverse osmosis (RO) is basially the reverse of the osmosis process. Scientists found that all that is required to reverse the process of osmosis is a suitable semipermeable membrane and applying a pressure to the concentrated salt solution above the applied and osmotic back-pressures, thereby forcing pure water through the semipermeable membrane. In other words, reverse osmosis is the process where water containing inorganic salts (minerals), suspended solids, soluble and insoluble organics, aquatic microorganisms, and dissolved gases (collectively called source water constituents or contaminants) is forced under pressure through a semipermeable membrane. Semipermeable refers to a membrane that selectively allows water to pass through it at much higher rate than the transfer rate of any constituents contained in the water. Learn more about pressure driven membranes here. If water of high salinity is separated from water of low salinity via a semipermeable membrane, a natural process of transfer of water will occur from the low-salinity side to the high-salinity side of the membrane until the salinity on both sides reaches the same concentration. This natural process of water transfer through a membrane driven by the salinity gradient occurs in every living cell; it is known as osmosis.
The hydraulic pressure applied on the membrane by the water during its transfer from the low-salinity side of the membrane to the high-salinity side is termed osmotic pressure. Osmotic pressure is a natural force similar to gravity and is proportional to the difference in concentration of total dissolved solids (TDS) on both sides of the membrane, the source water temperature, and the types of ions that form the TDS content of the source water. This pressure is independent of the type of membrane itself. In order to remove fresh (low-salinity) water from a high-salinity source water using membrane separation, the natural osmosis-driven movement of water must be reversed, i.e., the freshwater has to be transferred from the high-salinity side of the membrane to the low-salinity side. For this reversal of the natural direction of freshwater flow to occur, the high-salinity source water must be pressurized at a level higher than the naturally occurring osmotic pressure.
If the high-salinity source water is continuously pressurized at a level higher than the osmotic pressure and the pressure losses for water transfer through the membrane, a steady-state flow of freshwater from the high-salinity side of the membrane to the low-salinity side will occur, resulting in a process of salt rejection and accumulation on one side of the membrane and freshwater production on the other. This process of forced movement of water through a membrane in the opposite direction to the osmotic force driven by the salinity gradient is known as reverse osmosis (RO).
The rate of water transport through the membrane is several orders of magnitude higher than the rate of passage of salts. This significant difference between water and salt passage rates allows membrane systems to produce freshwater of very low mineral content. The applied feed water pressure counters the osmotic pressure and overcomes the pressure losses that occur when the water travels through the membrane, thereby keeping the freshwater on the low-salinity (permeate) side of the membrane until this water exits the membrane vessel.
Osmosis and Reverse Osmosis Process
The salts contained on the source water (influent) side of the membrane are retained and concentrated; they are ultimately evacuated from the membrane vessel for disposal. As a result, the RO process results in two streams—one of freshwater of low salinity (permeate) and one of feed source water of elevated salinity (concentrate, brine or retentate), as shown in the figure above. While semipermeable RO membranes reject all suspended solids, they are not an absolute barrier to dissolved solids (minerals and organics alike). Some passage of dissolved solids will accompany the passage of freshwater through the membrane. The rates of water and salt passage are the two key performance characteristics of Reverse Osmosis membranes.
Reverse Osmosis Process Drawing
- Published in Technology, Water Treatment
What is Sedimentation ?
After coagulation occurs followed by a flocculation process before entering a sedimentation basin / clarifier. Flocculated particles enter a sedimentation basin and begin to settle. Particles also referred to as sediments agglomerate to form larger flocs. This process is called sedimentation. When flocs / particles enter a basin and then start to settle, particles’ settling velocities change as particles agglomerate and form larger floc. Because the settling properties of flocculant suspensions cannot be formulated, a clarifier / sedimentation basin performance cannot be accurately predicted. However, for new plants, settling rates can be estimated from batch settling data developed with laboratory jar tests. For expanding existing plants, settling rates can be derived from evaluating the performance of existing basin during various influent water quality conditions. These evaluations often allow for increasing rates for existing basins and establishing higher rates, as compared to published guidelines, for new basins. In an ideal continuous flow basin, sedimentation would take place as it does in the laboratory jar. However, in a real clarifier, wind, temperature density currents, and other factors cause short-circuiting, disruption of flow patterns, breakup of floc, and scouring of the settled sludge. The designer must learn as much as possible about the settling properties of the flocculated solids and then design to match these characteristics. When the designers do not have access to source water data, it is best to select design criteria known to have worked in similar applications, either from personal experience or from regulatory guidelines.
Sedimentation Design Approach
The primary approach in designing conventional sedimentation basins is to select a design overflow rate for maximum expected plant flow. This rate may be chosen based on all units being in service or on one unit being out of service, to allow for redundancy. After selecting a rate, the designer should determine the number of units needed and select the type of sludge collection and removal equipment. The sludge equipment for removing may limit basin dimensions, which could establish the size and num. of units. With the number of basins selected, the designer should proceed to design inlet and outlet conditions and finalize dimensions to suit all design parameters and site conditions. The following are suggested guidelines for the various design parameters.
Sedimentation Overflow Rates
Hydraulic overflow rate is the primary design parameter for sizing sedimentation basins. This rate is defined as the rate of inflow Q divided by the tank surface area A. Units are typically rated in gallons per day per square foot, gallons per minute per square foot, or cubic meters per hour per square meter. Acceptable rates vary with the nature of the settling solids, water temperature, and hydraulic characteristics of the settling basin.
After evaluation of both cold & warm water loading rates, the design rate is based on whichever is more critical. Plant flow variations between warm and cold water periods often allow selection of higher rates for summer operation than the typical suggested loading rates. Overflow rates can also be selected based on pilot studies. Piloting of conventional settling basins is not especially reliable, but it is often done using tube settlers. Data from such studies, along with jar testing, are often useful in design. Pilot testing of other types of settling, especially proprietary processes, is useful and is recommended.
Detention Time
Which is the flow rate divided by tank volume, is usually not an important design parameter. Many regulatory agencies (e.g., Great Lakes, 2003), however, still have a requirement for detention periods of 4 h. It is likely that this detention requirement is a carryover from the days of manually cleaned basins designed to provide the sludge storage zone. These basins were often 15 to 16 ft (4.6 to 4.9 m) deep or greater and operated so that more than one-half the volume could be filled with sludge before being cleaned. Real time could vary from 4 h when clean to less than 2 h just before cleaning. Modern designs with mechanical sludge removal equipment need not provide a sludge zone, and deep basins with long detention times are no longer required. Conventional basins with detention times of 1.5 to 2.0 h provide excellent treatment.
Basin Depth and Velocities
In theory, basin depth should not be an important parameter either, because settling is based on overflow rates. However, in practice, it is important because it affects flow-through velocity. Flow-through velocities must be low enough to minimize scouring of settled floc blanket. Velocities of 2 to 4 ft/min (0.6 to 1.2 rn/min) usually are acceptable for basin depths of 7 to 14 ft (2.1 to 4.3 m), the hallower depths often used with multiple-tray basins. Single-pass basins are generally deeper, to offset the effects of short-circuiting from density and wind currents.
Circular Sedimentation Basin / Clarifier design
- Published in Technology
What is Clarification ?
Clarification has more than one application in water treatment. Its usual purpose in a conventional treatment process is to reduce the solids load after coagulation and flocculation. A second application, a process called plain sedimentation, is removal of heavy settleable solids from turbid water sources to lessen the solids on treatment plant processes. Material presented deals primarily with settling flocculated solids. One way of designing the clarification process is to maximize solids removal by clarification, which generally requires lower clarifier loadings and larger, more costly units. Alternatively, the clarifier may be designed to remove only sufficient solids to provide reasonable filter run times and to ensure filtered water quality. This latter approach optimizes the entire desalination plant and generally leads to smaller, less expensive facilities. Typical loading rates suggested in other articles or by regulatory guidelines are generally conservatively selected to provide a high-clarity settled water rather than optimization of the clarifier – filter combination.
The clarifier falls into two basic categories: those used only to remove settleable solids, either by plain sedimentation or after flocculation. And those that combine flocculation and clarification processes into a single unit. The first category also includes conventional sedimentation basins and high-rate modifications such as tube or plate settlers and dissolved air flotation (DAF). The second category does include solids contact units such as the sludge blanket clarifier and slurry recirculation clarifier. Also included in this category is contact clarification in which flocculation and clarification take place in a coarse granular media bed.
Conventional Clarification Design
Most sedimentation basins also known as a clarifier used in water treatment are the horizontal-flow type in rectangular, square, or circular design. Both long, rectangular basins and circular basins are commonly used; the choice is based on local conditions, economics, and personal preference. Camp (1946) states that long, rectangular clarifier exhibit more stable flow characteristics and therefore better sedimentation performance than very large square basins or circular tanks. Basins were originally designed to store sludge for several months and were periodically taken out of service for manual cleaning by flushing. A clarifier is now designed to be cleaned with mechanical equipment on a continuous or frequent schedule.
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Sedimentation Basin a.k.a Clarifier Design
- Published in Technology
Semipermeable Membrane Definition
Semipermeable membrane is a thin, soft, pliable sheet of material that allows certain substances to freely pass through it while restricting the passage of other substances. In the water treatment field, a semipermeable membrane allows water to pass freely while restricts the passing of dissolved materials.
Water passes through a semipermeable membrane into a solution of higher salt concentration. This phenomenon is called Osmosis. The semi-permeable membrane is a material through which water may pass, but prohibits the traveling of nearly everything else. Because of pressurized feed water enters each pressure vessel, a portion of the incoming water goes through the semipermeable membrane. The semi-permeable membrane selectively restricts the passage of suspended and dissolved elements in the raw water.
Semipermeable membrane separating TDS and TSSÂ
The incoming supply water and the dissolved and suspended substances which don’t pass throughout the semipermeable membrane go to waste. One stream enters an RO system and two streams exit. The permeate is the water stream and small amount of TDS which goes through the Semipermeable Reverse Osmosis membrane. While the concentrate is the waste stream containing the water and elements which don’t go through the semipermeable membrane. The concentrate is simply concentrated incoming stream of water. The concentrate contains the TDS and the TSS that are not allowable to pass through the semipermeable membrane.
Read more about the RO Membrane.
- Published in Technology, Water Treatment
What is Bioflocculation
Bioflocculation is marked as an advanced, non-chemical, microbial based pre-treatment technology. The technology of bioflocculation uses a novel Rapid Sand Filters RSF configuration together with an extremely porous volcanic Tuff filtration medium. This provides an enlarged surface area for microbial development and biofilm reproduction and propagation.
In the majority of large scale seawater reverse osmosis SWRO plants, the main pre-treatment method is based on granular multimedia filters. This is also known as rapid sand filters (RSF). The vast use of RSF is mainly the result of their simplicity, low energy consumption, and low maintenance and operational costs. Regardless the need to use coagulants such as Alum or ferric sulfate to feed water. RSFs main application is to remove suspended solids greater than 0.35mm in diameter from the feed water stream. It also lowers the level of SDI to around 4. Studies made on RSF also shows that such filters are capable of reducing the levels of suspended particles. Also reducing dissolved TOC, chlorophyll a and transparent exopolymeric particles (TEP). In operation, when a RSF becomes overloaded with particles, a backwash procedure applies. Backwash flushes out the suspended living and non-living particles that has accumulated in the filter bed.
A pilot for Rapid Bioflocculation Filter (RBF) was constructed with two fiberglass columns (each of 6m in height and 1m diameter). It is an upward flowing Bioflocculator (BF) unit, packed with 3m natural porous volcanic Tuff medium (by Tuff Merom Golan). And downward flowing, mixed-media bed filter (MBF) consisting of 80 cm Filtralite (by Filtralite Co.) over 80cm sand. The pilot scale effectiveness is monitored by measuring the efficiency of the removal from the feed water. Key factors were related to membrane clogging; silt density index (SDI), turbidity, chlorophyll a (Chl a) and transparent exopolymer particles (TEP). It is mainly designed to optimize microbiological activity within the filter bed. The results from one year of operation of a large-scale pilot, dual-stage granular filter, indicate that this pretreatment technology with no addition of coagulants. Also no other chemicals gave results equivalent to a conventional RSF with prior chemical (Fe2[SO4]3) treatment.
Bioflocculation Volcanic Tuff Media
Volcanic Tuff grain sizes ranged between 3 and 5 mm in diameter, with a bulk density of 2110 kg/m3 and porosity relative to volume of 26.7%. The total pore area was 20 m2, with extremely wide pore size ranges (0.05 to >10 mm). median pore diameter was 0.75 mm with a characteristic pore length of 62 mm. The large range of pore sizes enabled a wide diversity of microorganisms to colonize the medium pores as a result of reduced shear forces.
Conventional pre-treatment procedures in seawater reverse osmosis SWRO rely mainly on RSF. The process mechanically removes suspended solids greater than 0.35 mm in diameter formed upstream. This occurs after the addition of chemicals in a coagulation and flocculation step. When RSF is overloaded with particles as indicated by high differential pressure across the filter, backwash procedure is carried out. Backwash is mainly flushing and cleaning the filter bed. The same procedure should be taken with RBF filters.
The study made on this new discovery has demonstrated the potential of a biologically-based pre-treatment for SWRO desalination. The results are based on a year-long study. It shows a comparable performance by a large pilot, two-stage, granular Rapid Bioflocculation Filter (RBF) consisting of a Bioflocculator unit with volcanic Tuff medium. This is followed by a Mixed Bed Filter with no prior chemical additives and a standard RSF operating with addition of chemical flocculant [Fe2(SO4)3] upstream. This was at the Hadera SWRO facility in Israel. Much of the effective performance of the RBF is due to the bioflocculation process which occurs within the Tuff medium.
Some biodegradation may also take place. But because of rapid flow rates through RBF, this is unlikely to be a major factor in the filtration process. The study shows that during normal operation there is continuous microbial growth and development of a biofilm layer of organisms. The growth is within an organic matrix that effectively retains different types of colloidal and particulate matter. When shear forces increase during flush cleaning cycles, some of this bio-aggregated material are released into the BF filtrate as bioflocs. These bioflocs are large enough and are mechanically retained with high efficiency by the MBF.
The bioflocculation process that occurs in the first stage of the RBF depends on the metabolism of an extensive, biological food web. It involves different populations of bacteria, archaea, cyanobacteria, protozoa, and even crustaceans and marine worms. It is noteworthy that this kind of microbial environment only develops on the highly porous Tuff grains of the BF and not on the MB filter media.
Conclusion on bioflocculation study
In conclusion, the study demonstrates that with suitable filter bed media and some design modifications, it is possible to construct a rapid granular filter. The RGF achieves effective largescale pre-treatment filtration equivalent to that of currently operating RSF. But without the need for prior chemical coagulation. The research suggest that this approach of using bioflocculation without chemical additives could have considerable potential. It could act as an alternative to conventional RSF pre-treatment for large seawater reverse osmosis SWRO facilities.
- Published in Technology, Water Treatment
What is Continuous Electrodeionization (CEDI)
The continuous electrodeionization is a recent invention that was brought by a water treatment company which patented the CEDI technology. CEDI which is the abbreviation for the continuous electrodeionzation is considered as a smart evolution to conventional Electrodialysis Reversal (EDR) technology. It is introduced as a blend of ion exchange membranes, ion exchange resins and electricity. The difference between the resins used in this technology and the conventional Ion Exchange resins made of divinyl benzene (DVB) is that these resins are continuously regenerated in the continous electrodeionzation without the need for regeneration chemicals or salt.
The way Continuous Electrodeionization CEDI works is so simple. It is actually very similar to Electrocoagulation in respect to the use of plates/electrodes (anode and cathode). Applying a DC current to plates, it turns them to anode and cathode. This is done in order to attract dissolved solids consisted of mainly anions (negatively charges ions) and cations (positively charged ions). The anode electrode will attract negatively charged ions while the cathode will attract positively charged ions. When ion exchange membranes made of cation selective resins are inserted right close to the cathode, it will block the passage of anions and water molecules. On the other hand, when we insert an ion exchange membrane made of anion selective resin close to the anode, it will block the passage of cations and water molecules and only allow anions to pass.
Continuous Electrodeionization Operation and Resin Regeneration
This configuration of membranes and electrodes form the framework of a Continuous Electrodeionization CEDI module. This process is however slowed down by the slow speed of which ions move in water, in fact the low conductivity of water molecules impedes ions removal. Meaning, as ions move outward, the water in the dilute chamber become purified. As ion levels decreases, electrical resistance increases and eventually the whole process slows down. That was solved by adding anions and cations selective resin beads between the two Ion Exchange membranes which reduces the electrical resistance.
The surface of the beads in the continuous electrodeionization CEDI module acts as an ion transport bridge. So that the ions can move quicker through the membranes at to the electrodes. Continuously adding resin beads – Ion Exchange selective membranes sandwiches, creates a series of water purification compartments where product and brine exit the system. As feed water is pumped into the system, it is diverted into separate compartments: concentrate and purification compartments. These two streams remain separated throughout the process because only ions can pass through the membranes. Ions migrate and accumulate in the concentrating compartment where they are washed away into the reject stream; exit the system as concentrate. The water leaving this compartment contain a concentration of ions of approximately 10 – 20x higher than the original feed water. This water can be either drained, recycled or reclaimed for further treatment.
Continuous Electrodeionization CEDI module
At the top of the purification compartment the ion concentration is at its highest. The surface of the resin beads act as a conductive path effectively moving the ions to the membranes. At the lower end of the purification, the ion concentration is reduced to the parts per trillion (ppt) level. The electric field becomes concentrated between the resin beads and the surrounding water resulting an electrochemical reaction. Where water splitting occurring into Hydrogen and Hydroxide ions which is essentially acid and caustic. The acid and caustic generated is what regenerated the resin beads by replacing other trace ions remaining. This exactly what happens in conventional IX deionization systems. A result is a chemical free operation where the electrical potential does all the work and extends the life of the resin.
- Published in Technology, Water Treatment
What is a Pressure Membrane / Pressure-Driven Membrane
The pressure membrane processes are:
– Reverse osmosis (RO)
– Nanofiltration (NF)
– Ultrafiltration (UF)
– Microfiltration (MF)
A pressure membrane is a membrane that functions by applying a pressure. A pressure membrane is permeable to water but not to substances which are rejected and removed. All membranes including any pressure-driven membrane separate feedwater into two streams: permeate and concentrate streams. The permeate (for RO, NF, or UF) or filtrate (for MF) stream passes through the membrane barrier. The concentrate (or retentate) stream contains the substances removed from the feedwater after the pressure membrane barrier rejects it.
Actually, the driving force for these pressure membrane processes may come from (1) a pressurized feedwater source with the membranes installed in pressure vessels, called modules. Or (2) a partial vacuum in the filtrate/permeate flow stream caused by use of a filtrate/permeate pump or gravity siphon. The vacuum-driven processes typically apply to MF and UF only and have membranes submerged or immersed in nonpressurized feedwater tanks.
pressure membrane process using feed or permeate pumps
Pressure membrane processes are designed for cross-flow or dead-end operating modes. In the cross-flow mode, the feed stream flows across the pressure membrane surface. And permeate (or filtrate) passes through the pressure-driven membrane tangential to the membrane surface. Moreover, cross-flow operation results in a continuously flowing waste stream. A cross-flow system design sometimes contain a concentrate recycle. Also with a reject stream (feed-and-bleed mode). Many MF and UF systems treating relatively low turbidity waters are also designed to operate in a dead-end flow pattern where the waste concentrate stream is produced by an intermittent backwash. The figure below shows the relative removal capabilities for pressure-driven membrane processes and compares these processes with media filtration.
pressure membrane processes RO NF UF MF difference pore size rejection
In fact, MF and UF separate substances from feedwater through a sieving action. Separation depends on the pressure membrane pore size and interaction with previously rejected material on the membrane surface. Furthermore, NF and RO separate solutes by diffusion through a thin, dense, permselective (or semi-permeable) membrane barrier layer, as well as by sieving action. The required pressure membrane feed pressure generally increases as removal capability increases.
- Published in Technology, Water Treatment
What is Forward Osmosis
Fundamentally, Forward Osmosis exploits a naturally occurring phenomenon (Osmosis). Which is simply the water transport over a semi-permeable membrane from a low concentration to a high concentration. In a perfect world, the semi-permeable membrane permits water to pass through it however rejects all salts and undesirable components. The high salinity solution acts as the draw solution, which has a higher concentration than the feed water. To initiate the passage and attract water to pass through the membrane from the feed side to itself. In this manner, Forward Osmosis requires less energy (applied pressure) to transport a water stream through the membrane. In comparison to the pressure driven membrane procedures, for example, reverse osmosis (RO). However, as opposed to Reverse Osmosis, the result of Forward Osmosis is not fresh water but rather a diluted draw solution. It is a blend of the feed water and draw solution. In this way, a following filtration or separation process must be performed to extract clean water and to recover the draw solution. The draw solution consists of either a single or multiple simple salts or can be a substance specifically tailored for forward osmosis applications.
The following step of filtration might be energy intensive relying upon the draw solution and the recycling procedure. Hence, for potable water production, one must consider the energy utilization of both the Forward Osmosis procedure and the DS recovery. So that we can eventually make a reasonable examination and run comps between Forward Osmosis and other water treatment advancements. Or else, the conclusion could be one-sided and deceiving. In any case, Forward Osmosis might be more practical than pressure driven membrane technologies for water reuse. Only if the recovery of the Draw Solution is not required. Consequently, Researches and developments made on Forward Osmosis will be used to organize those procedures and applications without reusing the Draw Solution.
One of the most significant applications for Forward Osmosis is done by NASA. Six forward osmosis kits will fly aboard space shuttle Atlantis on the STS-135 mission by NASA. Scientists from NASA’s Kennedy Space Center in Florida plan to test a space adapted version of a Forward Osmosis bag. This will happen aboard space shuttle Atlantis during the STS-135 mission summer 2017. The group at Kennedy, led by NASA Project Manager Spencer Woodward, will include in the shuttle’s cargo six forward osmosis bag kits for the astronauts to test. The bags’ manufacturer, made a few adaptations to their commercial product for spaceflight. The idea is to make a fortified drink that provides hydration and nutrients from all sources available aboard a spacecraft, such as wastewater and even urine.
Forward Osmosis bag kit NASA experiment
Up until today, Forward Osmosis still experiences issues as a cost-effective innovation for direct seawater desalination. As a result of its high energy utilization and absence of efficient DS with -close to – negligible reverse flux. Regardless of many innovative advances in DS recently made, challenges still exist to: First, limit the reverse flux of DS. Secondly, mitigate internal concentration polarization (ICP) and Lastly, find better and easier recovery strategies. In conclusion, Forward Osmosis represents a forthcoming technology with a very low impact on the environment.
- Published in Technology, Water Treatment
What is Electrodialysis ED
In electrodialysis (ED)–based desalination systems, the separation of minerals and product water is achieved through the application of direct electric current to the source water. This current drives the mineral ions and other ions with strong electric charge that are contained in the source water through ion-selective membranes to a pair of electrodes
of opposite charges. As ions accumulate on the surface of the electrodes, they cause fouling over time and have to be cleaned frequently in order to maintain a steady-state Electrodialysis ED process. A practical solution to this  challenge is to reverse the polarity of the oppositely charged electrodes periodically (typically two to four times per hour) in order to avoid frequent electrode cleaning.
An Electrodialysis ED process that includes periodic change of the polarity of the system’s electrodes is referred to as an electrodialysis reversal EDR process. At present, practically all commercially available ED systems are of the EDR type. Electrodialysis ED systems consist of a large number (300 to 600 pairs) of cation and anion exchange membranes separated by dilute flow dividers (spacers) to keep them from sticking together and to convey the desalinated flow through and out of the membranes. Each pair of membranes is separated from the adjacent pairs above and below it by concentrate spacers which collect, convey, and evacuate the salt ions retained between the adjacent membranes. The membranes used for ED are different from those applied for Reverse Osmosis RO desalination—they have a porous structure similar to that of microfiltration and ultrafiltration membranes. Reverse Osmosis membranes do not have physical pores. Electrodialysis ED membranes are more resistant to chlorine and fouling and are significantly thicker than RO membranes.
electrodialysis ED membrane desalination technology schematic process system
It is important to note that a single set of EDR stacks can only remove approximately 50 percent of salts. As a result, multiple EDR stacks connected in series are often used to meet more stringent product water Total Dissolved Solids TDS targets. It should be pointed out that compared to brackish water RO membranes, which typically yield only up to 85 to 90 percent recovery, Electrodialysis Reversal EDR systems can reach freshwater recovery of 95 percent or more. The energy needed for ED desalination is proportional to the amount of salt removed from the source water. TDS concentration and source water quality determine to a great extent which of the two membrane separation technologies (RO or ED) is more suitable and cost effective for a given application. Typically, ED membrane separation is found to be cost competitive for source waters with TDS concentrations lower than 3000 mg/L. This applicability threshold, however, is a function of the unit cost of electricity and may vary from project to project.
The TDS removal efficiency of Electrodialysis ED desalination systems is not affected by non-ionized compounds or objects with a weak ion charge (i.e., solids particles, organics, and microorganisms). Therefore, ED membrane desalination processes can treat source waters of higher turbidity and biofouling and scaling potential than can RO systems. However, the TDS removal efficiency of ED systems is typically lower than that of Reverse Osmosis systems (15.0 to 90.0 percent versus 99.0 to 99.8 percent), which is one of the key reasons why they have found practical use mainly for brackish water desalination. In general, Electrodialysis Reversal EDR systems can only effectively remove particles that have a strong electric charge, such as mono- and bivalent salt ions, silica, nitrates, and radium. EDR systems have a very low removal efficiency with regard to low-charged compounds and particles—i.e., organics and pathogens. Table below provides a comparison of the removal efficiencies of distillation, ED, and RO systems for key source water quality compounds.
| Contaminant | Distillation (%) | ED/EDR (%) | RO (%) |
| TDS | >99.9 | 50-90 | 90-99.5 |
| Pesticides, Organics/VOCs | 50-90 | <5 | 5-50 |
| Pathogens | >99 | <5 | >99.99 |
| TOC | >95 | <20 | 95-98 |
| Radiological | >99 | 50-90 | 90-99 |
| Nitrate | >99 | 60-69 | 90-94 |
| Calcium | >99 | 45-50 | 95-97 |
| Magnesium | >99 | 55-62 | 95-97 |
| Bicarbonate | >99 | 45-57 | 95-97 |
| Potassium | >99 | 55-58 | 90-92 |
One important observation from this table is that, as compared to distillation and RO separation, ED desalination only partially removes nutrients from the source water. This fact explains why EDR is often considered more attractive than RO or thermal desalination (which remove practically all minerals from the source water) if the planned use of the desalinated water is for agricultural purposes—i.e., generating fresh or reclaimed water for irrigation of agricultural crops.
Construction and equipment costs for brackish water reverse osmosis (BWRO) and EDR systems of the same freshwater production capacity are usually comparable, or EDR is less costly, depending on the Reverse Osmosis membrane fouling capacity of the source water. However, since the amount of electricity consumed by EDR systems is directly proportional to the source water’s salinity, at salinities of 2000 to 3000 mg/L the energy use of EDR systems usually exceeds that of BWRO or nanofiltration systems for source waters. Therefore, EDR systems are not as commonly used as RO systems for BWRO desalination and are never applied for seawater reverse osmosis (SWRO) desalination.
It should be pointed out, however, that salinity is not the only criterion for evaluating the cost competitiveness of EDR and BWRO systems. Often, other compounds such as silica play a key role in the decision making process. For example, at the largest operational EDR plant worldwide at present—the 200,000 m3/day Barcelona desalination facility in Spain—this technology was preferred to BWRO desalination because the brackish surface water source for this plant—the Llobregat River—contains very high level of silica, which would limit recovery from a BWRO plant to only 65 percent; the EDR system can achieve 90 percent recovery. In addition, the Llobregat River was found to have very high organic content, which was projected to cause heavy fouling and operational constraints on a BWRO plant of similar size.
Reference: “Desalination Engineering” by Nikolay Voutchkov
- Published in Technology, Water Treatment
What is Diatomaceous Earth DE Filter
Diatomaceous Earth DE filtration – DE Filter –  has been used effectively for drinking water treatment since 1942. Diatomaceous Earth DE Filter was adopted as a standard method for the U.S. Army. The DE filter was selected because of its portability and effectiveness in removing Entamoeba histolytica cysts (Black and Spaulding, 1944). These cysts are pervasive in some parts of the world and are difficult to control with disinfectants alone. The capability of DE filter to effectively remove particulates applies equally well to the latter concerns of Cryptosporidium and Giardia. Where cyst removals of approximately 6 logs have been achieved (Ongerth and Hutton, 1997). DE filter is commonly called a pre-coat filter because of the pre-coat of filter leaves that initiates every operating cycle.
Although DE filter has DE as the most common pre-coat material used, other pre-coat material such as ground perlite performs as well in many applications. For many years, the type of equipment available limited the use of a Diatomaceous Earth DE filter for municipal drinking water treatment. The use of stainless steel and plastics in the fabrication of equipment has significantly changed the performance capability of the filters by improving their ease of operations and maintenance. DE Filter named upon Diatomceous Earth which is mined from the fossilized remains of microscopic plants called diatoms, deposited in what were the beds of ancient oceans. A powdered medium is manufactured from the diatomite deposits that is almost pure silica. One of the more common diatomite media used for drinking water treatment has a mean particle size of 22.3/xm with 80% of the particles ranging in size from 5 to 64 p~m. This medium, when deposited on the filter septum, has an average pore size of about 7.0/xm.
Diatomaceous Earth Filter Operation
As illustrated below, Diatomaceous Earth DE filter operations occur in three steps:
1. A precoat of about % in. (3 mm) is deposited on the filter.
2. After the precoat has been deposited, filtering begins. And at the same time a small amount of Diatomaceous Eearth material (called body feed) is added to the source water. This is to maintain the porosity of the media.
3. Particulates in the source water are trapped in the pre-coat layer until it reaches maximum head loss. At which time the filter run is terminated and media material is cleaned from the septa.
diatomaceous earth DE filter operation design
Porosity Control of DE Filter
The principal requirement for maintaining effective Diatomaceous Earth DE filter runs is to maintain the porosity of the filter cake. Source water solids generally vary in size and are mixture of relatively inert matter and solids that are predominantly organic. If source water is filtered through the pre-coat alone, the buildup of solids and compression of the accumulated cake quickly reduces DE filter cake porosity. And head loss increases at an exponential rate. This may be avoided by adding body feed to the source water in sufficient amounts to produce a constant flow versus head loss relationship. Although the rate of flow does not affect effluent quality or turbidity breakthrough, the flow rate for pre-coat filters should generally be limited to about 2 gpm/ft 2 (7.8 m/h). The shape of the pre-coat filter head loss curve that reflects both feed and flow conditions is. Therefore, an important feature to control effective filter run performance.
Supplementary Treatment for Diatomaceous Earth DE Filter
Supplementary measures are added to the basic DE filter process to enhance the filtration process and to expand the process to remove some non-particulate constituents. Natural color in source water supplies is caused by either organic or mineral matter. Color results from the decay of plant matter or from the solubilization of iron in the soil. And in many instances the mineral and organic matter are bound together. Therefore color is present either in particulate form or in solution. Particulate color consists mostly of negatively charged colloids, and even though the pre-coat medium has low pore size, charged colloids pass through unless the charge is neutralized.
The use of a strong oxidant such as ozone has been demonstrated to be effective in conditioning color for removal. When color is particulate rather than dissolved, DE filters reduce source water color of about 25 color units (CU) and less to below 5.0 CU. With source color between 25 and 60 CU, filter effluent is generally no higher than 10 CU. Supplemental treatment such as pre-ozonation or alum-coated media may be required. They can improve removal of particulate color and to reduce dissolved color. Dissolved iron precipitates by aeration or by adding a strong oxidant so that the iron is removed as a particulate in DE filtration. The use of magnesite (magnesium oxide) is found to facilitate removal of some forms of iron. Magnesite mixed along with body feed is held for about 10 rain to form negatively charged suspension of magnesium oxide. MgO gradually undergoes hydration and solution.
Manganese may be removed with DE filer with potassium permanganate (KMnO4) added to the body feed. Followed by flow detention, usually from 10 to 20 min. Detention time is important and should be determined in bench and pilot tests. The rate of KMnO4 addition and body feed rate depend on the amount of manganese in the source water. And on also other water quality characteristics. Where iron and manganese are both present in source water, supplementary conditioning must usually be accomplished in separate steps. With iron treatment preceding the KMnO4 addition. When there is a large amount of iron to be removed, it may be necessary to have two filters in series. With iron conditioning preceding the first filter and the addition of KMnO4 and detention between the first and second filters.
Practically the only carryover of solids in DE filter effluents would be very fine Diatomaceous Eearth particles used in pre-coat and body feed. Although the presence of these particles is innocuous to health, effluent water turbidity must meet the established requirements. Use of a finer Diatomaceous Eearth for pre-coat can sometimes achieve lower turbidity levels. A slight reduction in the applied flow rate can also help to improve effluent turbidity. Where only slight additional turbidity reduction is needed. The use of a simple cartridge filter following the DE filter accomplishes the desired DE removal. Use of cartridge filters to polish the effluent will be more effective for the lower-capacity DE installations when solids carryover is minimal. Note that without the addition of separate, additional treatment processes, DE filtration will not reduce the organic content of source water.
You can learn more about the Diatomaceous Earth DE Filter Design
Reference: Water Treatment Plant Design
- Published in Technology, Water Treatment
What is Diatomaceous Earth DE Filter Design
Several options are available in designing each of the flat-leaf DE filter elements, as well as in the integrated assembly DE filter design.
diatomaceous earth DE filter design flat leaf element
Types of DE Filter
Two basic groups of DE filter are available. If source water is to be forced through the filter under pressure, the containment vessel must be closed. DE filter operates under a vacuum, on the other hand, may be open vessels. Although there is theoretically no limitation to what pressure may be applied to a pressure-type filter. Practical considerations of pumping costs have limited head loss to a maximum of 35 psi (241 kPa). Most systems used for drinking water filtration are designed for a maximum head loss of 25 psi (172 kPa).
DE Filter Design Construction
Pressure filters are always constructed as cylindrical pressure vessels mounted either vertically or horizontally. Most units fabricated today are made of stainless steel for the shell and most internal parts. The type of stainless steel used depends on the corrosivity of the water being treated. Vacuum filters are built as rectangular tanks. Because of the low differential heads they are subject to, vacuum filter containments and internal parts, except certain structural supports, are most often fabricated of plastics for their chemical resistance and reduced maintenance. For larger units, the containment vessel may be constructed of concrete.
DE Filter Elements
Inlet water flow is introduced to a DE filter through the containment wall, fitted with an internal baffling device to prevent disturbance of the filter cake. Filter cake may be cleaned from the filter by scraping, vibration, hydraulic bumping (surging), or manually hosing down the septa from the top of an open vessel. Many arrangements of filter elements are available, constructed in both a tubular and flat form, with the flat (or “leaf’) design being by far the most common.
DE Filter elements may be mounted either horizontally or vertically, and they may be either fixed in position or able to rotate. Vertical mounting of the leaves is used almost exclusively for water treatment applications. Most pressure and vacuum filters constructed today have fixed leaves mounted by means of spigot-type “push-on” outlets installed in sockets on a manifold and sealed by O rings or flat gaskets. The outlet manifold is usually located below the leaves to provide them with support while allowing gravity to assist in seating the push-on connections.
Variations for both leaf connections and manifold location are selected depending on operating conditions and requirements for inspection and maintenance. Fixed-leaf pressure filters may also be divided into retracting shell and retracting bundle types for internal access. Both options may be used for any size filter, but the retracting bundle type is is generally preferred for larger units. The retracting shell or tank design has the shell mounted on wheels on rails. An electric motor or hydraulic piston opens and closes the unit.
In the retracting bundle design, the shell head is suspended from an overhead monorail. The bundle, attached by the manifold and frame to the head, is retracted by means of the monorail, which is usually motor-driven. Internal rails attached to the shell support the end of the leaf bundles when the shell is opened. Thorough cleaning of the septa at the end of a filter run is important in maintaining peak efficiency. Most units used for water treatment sluice the cake with water sprays, which creates a slurry that can be easily handled and treated and does not require opening the filter vessel.
Fixed-leaf filters are usually cleaned with high-pressure spray jets mounted on oscillating spray heads, with single or multiple jets directed between the filter leaves. Rotating filter leaves usually have a stationary spray header, and coverage is obtained as the leaves rotate past the sprays. Open filters may be cleaned manually using high pressure sprays and may require covers over the units to contain the spray. Additional devices that may assist in the complete removal of the cake slurry from the filter containment include spray jets in the invert of the vessel or an air scour to suspend the material before the vessel is drained.
diatomaceous earth DE filter design rectangular circular tubular element
Leaf DE Filter Design
The flat DE filter leaf with a broad surface and limited thickness should be designed with the following goals:
• Leaf and outer frame must be stiff enough to resist warping under the force exerted at maximum differential pressure.
• The unit must have a backing screen to prevent the cloth septum from flexing under gradually increasing pressure.
• The path provided for filtrate flow must not restrict flow and must create minimal head loss through the leaf. It is essential that the filter cake remain undisturbed.
An adequate filter leaf design prevents the possibility of cake movement due to warping of the frame or flexing of the septum.
Central Drainage Chamber
There are three basic types of filter leaf drainage chambers. Heavy wire mesh with wire spacing up to 1 in. (2.5 cm); expanded metal sheets that provide a deeper chamber with increased rigidity. And the Trislot, a proprietary design having thin metal bars with welded transverse round or wedge-shaped wires.
DE Filter Backing Screen
The backing screen is an intermediate screen used when the irregular surface of the central drainage chamber may permit flexing of the cloth septa under conditions of varying pressure.
DE Filter Septum
Filter septum materials are cloth weaves made with either stainless steel wires or plastic monofilaments. The principal purpose of the septum is to retain the pre-coat, which must bridge the openings in the weave. Because openings in the weave are larger than the major portion of particulates in pre-coat material, the pre-coat is retained by bridging. The cloth septum must be uniformly woven to produce an even pre-coat that reduces the extent of the recirculation required to deposit the material. The weave must also be designed so that it sluices cleanly, drops the cake freely, and resists plugging and damage. One of the more common wire cloths for water filtration is a standard 24 × 110 Dutch weave. Another type of weave is the multibraid, composed of bundles of wire in both directions. This weave is less vulnerable to the entrapment of particles and blinding than the standard weave. Woven wire cloth may also be “calendered,” which involves passing the cloth through compression rollers to flatten the rounded wire at the surface of the weave. Calendering improves pre-coat retention characteristics and generally strengthens the cloth against rough treatment. Plastic cloth is used predominantly for vacuum-type DE filters and is available in a variety of weaves using either polyester or polypropylene monofilament. Plastic cloth may be supplied as a bag to envelop a filter leaf or as a cloth caulked into a leaf frame.
Binding Frame Closures
The binding frame surrounds the filter leaf to prevent leakage around the septum. The outside frame is also the principal structural element to provide rigidity and prevent warping. Depending on the shape, the outside binder may also collect flow from the central chamber and supplement flow routing to the outlet nozzle.
Vacuum Filter Leaves
Vacuum filter leaves used for drinking water treatment are often made of plastic. The central drainage chamber and outlet spigot are molded in a single piece, usually of high-impact styrene. Ridges or other raised patterns provide the required flow path. The raised pattern is spaced so that intermediate screens are not required. The septum, in the form of an envelope with zipperlike closures, is sealed at the bottom outlet by a gasket that also provides tight closure for the manifold connection.
Outlet Connections
Fixed-leaf filter leaves usually have spigot-type outlet connections made of castings machined to fit into the sockets of the outlet manifold. The central drain hub is used for rotating leaves and is of two-piece construction, clamped to the center of the circular leaf by bolts. The leaf outlet connection must be of sufficient size to allow full flow of the filtrate collected by the leaf at a minimum head loss. As leaf size and loading increase, the distribution of flow within the leaf and the transition to the outlet connection increase in importance.
Reference: Water Treatment Plant Design
- Published in Technology, Water Treatment
What is Osmotic Pressure
Osmotic pressure is a function of dissolved substances. As in Osmosis, water will pass through a semi-permeable membrane from the lower concentration compartment into the higher concentration compartment. Osmotic pressure is simply what makes osmosis process occurs. The force of this flow is measured by measuring how much pressure must be applied to the higher salt concentration side in order to stop osmosis. This pressure then must be the force of osmosis. This pressure is called osmotic pressure. Osmotic pressure is a function of the number of collisions of water molecules with either side of the membrane. The more dissolved substances, of any kind, there are, the fewer the water collisions.
A very rough rule of thumb is that for every 100 mg/L or ppm of TDS (Total Dissolved Solids) the osmotic pressure is roughly 1 psi (0.07 bar). If pure water is separated from a 1000 mg/L TDS solution by a semi-permeable membrane. It will take around 10 psi of pressure on the 1000 mg/L side in order to stop osmosis. Therefore, the osmotic pressure forcing water from the pure water side into the 1000 mg/L side is 10 psi. While the greater water collisions are from the pure water side of the membrane, the reason for water movement left to right is due to the higher salt concentration on the right hand side.Â
osmotic pressure is what makes osmosis process occurs
Amount of osmotic pressure generated, therefore, is directly proportional to the amount of total dissolved solids (TDS) in the solution. Since every 100 mg/L creates around 1 psi (0.07 bar) of osmotic pressure, we simply have to divide the TDS by 100 (move the decimal to the left two places) in order to calculate the approximate pressure.
Now what happens when we have salt solutions on both sides of a semi-permeable membrane? Net osmotic pressure becomes important. The net pressure is the difference between the pressure of each of the solutions separated by a semi-permeable membrane. If two 1000 mg/L salt solutions separated by semi-permeable membrane, roughly 10 psi of osmotic pressure is being exerted by each solution. In this case the pressures are equal and opposite in direction. Substracting one from the other, the net osmotic pressure is zero. This is when the osmosis process reaches equilibrium.
- Published in Technology, Water Treatment
What is a Cellulose Acetate Membrane
The thin semi-permeable film of the first Reverse Osmosis membranes was developed in the late 1950s at UCLA (University of California Los Angeles). It was made of cellulose acetate (CA) polymer. A Cellulose Acetate membrane have a three-layer structure similar to that of a Polyamide Thin Film Composite TFC membrane. The main structural difference is that the top two layers (the ultrathin film and the microporous polymeric support) are made of different forms of the same Cellulose Actetate polymer.
Cellulose is a polymer that is made up of repeating units (monomers) of C6H10O5. (Note: A monomer is a molecule which comes together with other identical monomers to form a chain of monomers, called a polymer). The number of acetates on the cellulose molecules affects the semi-permeability and other characteristics of the membrane. In general, the following are some of the most important differences between diacetate and triacetate membranes.
cellulose polymer cellulose acetate membrane
In a TFC membrane these two layers are made of completely different polymers. The thin semi-permeable film is polyamide, while the microporous support is polysulfone. Similar to a TFC membrane, a Cellulose Acetate membrane have a film layer that is typically about 0.2 um thick. But the thickness of the entire membrane (about 100 um) is less than that of a TFC membrane (about 160 um). One important benefit of a Cellulose Acetate membrane is that the surface has very little charge and is practically uncharged. As compared to a TFC membrane, which have a negative charge and can be more easily fouled with cationic polymers. If such polymers are used for source water pre-treatment.
cellulose acetate membrane pores under microscope
In addition, a Cellulose Acetate membrane have a smoother surface than a TFC membrane. Which also renders them less susceptible to fouling. Cellulose Acetate membrane have a number of limitations. Including the ability to perform only within a narrow pH range of 4 to 6 and at temperatures below 35 C (95 F). Operation outside of this pH range results in accelerated membrane hydrolysis, while exposure to temperatures above 40 C (104 F) causes membrane compaction and failure. In order to maintain the Reverse Osmosis concentrate pH below 6, the pH of the feed water to the cellulose acetate membrane are reduced to between 5 and 5.5. This results in significant use of acid for normal plant operation and requires Reverse Osmosis permeate adjustment by addition of a base (typically sodium hydroxide) to achieve adequate boron rejection.
Cellulose Acetate membrane experience accelerated deterioration in the presence of microorganisms. Since they’re capable of producing cellulose enzymes and bioassimilating the membrane material. However, they can tolerate exposure to free chlorine concentration of up to 1.0 mg/L. Which helps to decrease the rate of membrane integrity loss due to destruction by microbial activity.
Since Cellulose Acetate membrane have a higher density than a Polyamide TFC membrane. They create a higher headloss when the water flows through the membranes. Therefore a cellulose acetate membrane operates at higher feed pressures, which results in elevated energy expenditures. Despite their disadvantages, cellulose acetate membrane have high tolerance to oxidants (chlorine, peroxide, etc.). As compared to a PA TFC membrane. Cellulose Acetate membrane are used in municipal applications for saline waters with very high fouling potential. Mainly used in the Middle East and Japan in seawater reverse osmosis plants, and for ultrapure water production in pharmaceutical and semiconductor industries.
- Published in Technology, Water Treatment
What is Multi Stage Flash Distillation MSF
The Multi Stage Flash distillation MSF evaporator vessels is also referred to as flash stages or effects. What happens is that the high-salinity source water is heated to a temperature of 90 to 115°C (194 to 239°F) in a vessel (the heating section in the figure below) to create water vapor. The pressure in the first stage is maintained slightly below the saturation vapor pressure of the water. So when the high-pressure vapor created in the heating section enters into the first stage, its pressure is reduced to a level at which the vapor “flashes†into steam. Steam (waste heat) for the heating section is provided by the power plant co-located with the desalination plant. Each flash stage (effect) has a condenser to turn the steam into distillate. The condensers are equipped with heat exchange tubes, which are cooled by the source water that is fed to the condensers.
schematic multi stage flash distillation process
Entrainment separators (mist eliminators or demister pads) remove the high-salinity mist from the low-salinity rising steam. This steam condenses into pure water (distillate) on the heat exchange tubes and is collected in distillate trays. This is from where it is conveyed to a product water tank. Distillate flows from stage to stage and is collected at the last stage. The concentrate (brine) is generated in each stage and after collection at the last stage some of it typically is recycled to the source water stream in order to reduce the total volume of source water that must be collected by the intake for desalination. The recirculated brine flowing through the interior of the condenser tubes also removes the latent heat of condensation.
As a result, the recirculated brine is also preheated close to maximum operating temperature. Thereby recovering the energy of the condensing vapor and reducing the overall heating needs of the source water. This “brine recycle†feature has been adopted in practically all of the most recent MSF facility designs and allows significant improvement of the overall cost competitiveness of MSF installations. Each flash stage typically produces approximately 1 percent of the total volume of the desalination plant’s condensate. Since a typical MSF unit has 19 to 28 effects, the total MSF plant recovery (i.e., the volume of distillate expressed as a percentage of the total volume of processed source water) is typically 19 to 28 percent. For comparison, Reverse Osmosis seawater desalination SWRO plants have a recovery of 40% to 45%. The latest Multi Stage Flash Distillation technology has 45-stage units—i.e., can operate at 45% recovery. This feature allows it to compete with Reverse Osmosis systems in terms of recovery.
Historically, MSF was the first commercially available thermal desalination technology. It was applied to produce potable water on a large-scale, which explains its popularity. MSF plants represent over 80% of thermally desalinated water today. The gained output ratio GOR for MSF systems is typically between 2 and 8; the latest MSF technology has a GOR of 7 to 9. The pumping power required for the operation of the MSF systems is 2.0 to 3.5 kWh/m3 (7.6 to 13.3 kWh/1000 gal) of product water.
Reference: “Desalination Engineering” by Nikolay Voutchkov
- Published in Technology, Water Treatment
What is Multi Effect Distillation MED
In Multi Effect Distillation MED systems, saline source water is typically not heated; cold source water is sprayed via nozzles or perforated plates over bundles of heat exchange tubes. This feed water sprayed on the tube bundles boils, and the generated vapor passes through mist eliminators. Which collect brine droplets from the vapor. The feed water that turned into vapor in the first stage (effect) is introduced into the heat exchange tubes of the next effect. Because the next effect is maintained at slightly lower pressure, although the vapor is slightly cooler, it still condenses into freshwater at this lower temperature.
This process of reducing the ambient pressure in each successive stage allows the feed water to undergo multiple successive boilings without the introduction of new heat. Steam flowing through the exchange tubes is condensed into pure water and collected from each effect. Heating steam (or vapor) introduced in the heat exchange tubes of the first effect is provided from an outside source by a steam ejector. The Multi Effect Distillation MED system shown in the figure below is also equipped with a brine recycle system. It allows the introduction of warmer-than-ambient water in the first effects of the system. Thereby reducing both the volume of feed water that must be collected by the plant intake system and the overall energy needs of the system.
The main difference between the Multi Effect Distillation MED and Multi Stage Flash Distillation MSF processes is that while vapor is created in an MSF system through flashing, evaporation of feed water in MED is achieved through heat transfer from the steam in the condenser tubes into the source water sprayed onto these tubes.
schematic multiple effect distillation process system
This heat transfer at the same time results in condensation of the vapor to freshwater. Multi Effect Distillation MED desalination systems typically operate at lower temperatures than MSF plants. Maximum brine concentrate temperature of 62 to 75°C versus 115°C) and yield higher GORs.
The newest Multi Effect Distillation MED technologies include vertically positioned effects (vertical tube evaporators). This may yield a GOR of up to 24 kg of potable water per kilogram of steam. Pumping power required for operation of Multi Effect Distillation MED systems is lower than that typically needed for MSF plants. It is equal to 0.8 to 1.4 kWh/m3/3.0 to 5.3 kWh/1000 gallons of product water. Therefore, Multi Effect Distillation MED is now increasingly gaining ground over MSF desalination. Especially in the Middle East, where thermal desalination is still the predominant method for producing potable water from seawater.
Reference: “Desalination Engineering” by Nikolay Voutchkov
- Published in Technology, Water Treatment
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