Posts under this category displays latest news, useful articles, terms definitions, scientific explanations and more topics related to the water treatment industry.
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
What is Nitrate and Nitrite Element Definition
Nitrate is one of the major anions in natural waters, but concentrations can be greatly elevated due to leaching of nitrogen from farm fertilizer or from feed lots or from septic tanks. The mean concentration of nitrate nitrogen (NO3-N, nitrate measured as nitrogen in testing) in a typical surface water supply would be around 0.2 to 2 mg/L; however, individual wells can have significantly higher concentrations. Adult daily dietary nitrate intake is approximately 20 mg, mostly from vegetables, like lettuce, celery, beets, and spinach (National Academy of Sciences Committee on Nitrite and Alternative Curing Agents in Food, 1981).
Nitrite does not typically occur in natural waters at significant levels, except under reducing conditions. It can also occur if water with sufficient ammonia is treated with permanganate. Sodium nitrite is widely used for cured meats, pickling, and beer. Rarely, buildings have been contaminated by faulty cross connections or procedures during boiler cleaning with nitrous acid. Nitrite, or nitrate converted to nitrite in the body, causes two chemical reactions that can cause adverse health effects: induction of methemoglobinemia, especially in infants under one year of age, and the potential formation of carcinogenic nitrosamides and nitrosamines. Methemoglobin, normally present at 1 to 3 percent in the blood, is the oxidized form of hemoglobin and cannot act as an oxygen carrier in the blood. Certain substances, such as nitrite ion, act as oxidizers. Nitrite is formed by reaction of nitrate with saliva, but in infants under one year of age the relatively alkaline conditions in the stomach allow bacteria there to form nitrite. Up to 100 percent of nitrate is reduced to nitrite in infants, compared with 10 percent in adults and children over one year of age.
Furthermore, infants do not have the same capability as adults to reconvert methemoglobin back to hemoglobin. When the concentration of methemoglobin reaches 5 to 10 percent, the symptoms can include lethargy, shortness of breath, and a bluish skin color. Anoxia and death can occur at high concentrations of nitrites or nitrates. Carcinogenic nitrosamines and nitrosamides are formed when nitrate or nitrite are administered with nitrosatable amines, such as the amino acids in proteins. However, epidemiological studies, primarily on gastric cancer, have not yielded consistent results . The carcinogenicity of nitrate and nitrite is currently under review. The data on the role of nitrates in developmental effects, such as birth defects, are regarded as being inconclusive. Several epidemiologic case-control studies found an increased risk of developmental brain defects, but more studies are needed. If there is an association, maternal methemoglobinemia might be the critical risk factor. The MCLGs and MCLs are 10 mg/L for nitrate measured as nitrogen (or 45 mg/L nitrate) and 1 mg/L for nitrite measured as nitrogen. In addition, the MCL for total nitrate-N and nitrite-N is 10 mg/L.
Learn more about other elements and their effects in water supplies by exploring our water treatment blog.
- Published in Water Treatment
What is the Hydrologic Cycle Definition
The classic hydrologic cycle shows the relationship between surface and groundwater and the constant movement of water in the environment. Moisture on the earth’s surface evaporates to form clouds, which deposit precipitation on land in the form of rain, snow, or hail. That moisture is absorbed by the soil and percolates into the ground.When the precipitation rate exceeds infiltration, overland flow occurs and streams, rivers and reservoirs receive the runoff. Water from surface sources will reach its final destination in the ocean, where the hydrologic cycle begins again with the evaporation process. The hydrologic cycle is affected by land features and ocean currents that determine changing weather patterns and deliver precipitation unevenly, with some areas getting ample rainfall and others getting little.
Sources of water contamination occur throughout the hydrologic cycle. Contaminants can be diluted, concentrated, or transported through the cycle and affect drinking water.The objective of source water quality management is to minimize (or eliminate, if possible) contaminant input within a watershed basin, the geographic area that drains to the source water intake. Water sources can be affected by both chronic and acute water quality impacts. Chronic impacts may be subtle and persist over an extended period of time, resulting in a gradual deterioration of the source water. An example of a chronic impact is increased human activity within a watershed, producing increased nutrient levels in a lake or reservoir and accelerated eutrophication. An acute impact occurs as an incident, like an oil spill, which can be remedied quickly. The water quality manager needs to appreciate all source-water impacts in order to respond at the plant or on the watershed to minimize the impact on drinking water quality. The protection of water quality may require taking a treatment plant out of service for a time or installing additional treatments to meet the changing quality in the source water.
The geology within a watershed basin influences source quality, as do seasonal flow variations and climate. Microbiological activity in lakes and reservoirs can affect water quality. A variety of human activities can introduce pathogens and increase nutrient levels contributing to eutrophication of lakes and reservoirs.
BQUA – What is the Hydrologic cycle definition
- Published in Water Treatment
What is Cadmium – Cadmium Element Definition
Cadmium enters the environment from a variety of industrial applications, including mining and smelting operations, electroplating, and battery, pigment, and plasticizer production. Cadmium element occurs as an impurity in zinc and may also enter consumers’ tap water by galvanized pipe corrosion. Cadmium is also in food, with 27 ug/day in the average diet.
Cadmium element acts as an emetic and can cause kidney dysfunction, hypertension, anemia, and altered liver function. It can build up in the kidney with time. Chronic occupational exposure has resulted in renal dysfunction and neuropsychological impairments. The mechanisms remain uncertain, but cadmium element competes with calcium inside cells and across cell membranes. Cadmium has been shown to induce testicular and prostate tumors in laboratory animals injected subcutaneously. Lung tumor incidence is increased in people exposed to cadmium by inhalation.
As yet, USEPA considers cadmium element unclassifiable as a human carcinogen regarding oral exposure because the observed human carcinogenicity occurs via inhalation. It is regulated based on its renal toxicity in humans, and, using an uncertainty factor of 10, an MCLG and an MCL of 5 ug/L have been adopted (U.S. Environmental Protection Agency, 1991e).
- Published in Water Chemistry, Water Treatment
What is Asbestos – Asbestos Element Definition
Asbestos is the name for a group of naturally occurring, hydrated silicate minerals with fibrous appearance. Included in this group are the mineral chrysotile, crocidolite, anthophyllite, and some of the tremolite-actinolite and cummingtonite-grunerite series. All except chrysotile fibers are known as amphibole. Most commercially mined asbestos is chrysotile. Asbestos element occurs in water exposed to natural deposits of these minerals, asbestos mining discharges, and asbestos-cement pipe.
The physical dimensions of asbestos fibers rather than the type are more important in health effects, with the shorter, thinner fibers more highly associated with cancer by inhalation. Human occupational and laboratory animal inhalation exposures are associated with the cancer, mesothelioma, found in lung, pleura, and peritoneum. An NTP study also observed gastrointestinal cancers in rats dietarily exposed to intermediate range fiber (65 percent of the fiber larger than 10 um, 14 percent larger than 100 μm) for their lifetime. Epidemiologic studies of asbestos element in drinking water have had inconsistent results, but there are suggestions of elevated risk for gastric, kidney, and pancreatic cancers (Cantor, 1997). The USEPA based its 1/1,000,000 cancer risk estimate and the MCLG and MCL on 7 × 106 fibers/L > 10 micron observed in the NTP rat study (USEPA).
- Published in Water Chemistry, Water Treatment
What is Arsenic – Arsenic Element Definition
Arsenic concentrations in U.S. drinking waters are typically low. However, an estimated 5,000 community systems (out of 70,000) using groundwater and 370 systems (out of 6,000) using surface water were above 5 ug/L. These were primarily in the western states. Dissolution of arsenic-containing rocks and the smelting of nonferrous metal ores, especially copper, account for most of the arsenic in water supplies. Until the 1950s, arsenic element was also a major agricultural insecticide.
Arsenic element may be a trace dietary requirement and is present in many foods such as meat, fish, poultry, grain, and cereals. Market-basket surveys suggest that the daily adult intake of arsenic is about 50 ug, with about half coming from fish and shellfish. In fish, fruit, and vegetables, it is present in organic arsenical forms, which are less toxic than inorganic arsenic. However, arsenic is not currently considered essential (National Research Council, 1989). Extrapolating from animal studies, Uthus (1994) calculated a safe daily intake of between 12 and 40 ug.
In excessive amounts, arsenic element causes acute gastrointestinal damage and cardiac damage. Chronic doses can cause Blackfoot disease, a peripheral vascular disorder affecting the skin, resulting in the discoloration, cracking, and ulceration. Changes in peripheral nerve conduction have also been observed. Epidemiological studies in Chile, Argentina, Japan, and Taiwan have linked arsenic in drinking water with skin, bladder, and lung cancer (reviewed by Smith et al., 1992; Cantor, 1997). Some studies have also found increased kidney and liver cancer.
Ingestion of arsenical medicines and other arsenic exposures have also been associated with several internal cancers, but several small studies of communities in the United States with high arsenic levels have failed to demonstrate any health effects. Micronuclei in bladder cells are increased among those chronically ingesting arsenic in drinking water. Inorganic arsenate and arsenite forms have been shown to be mutagenic or genotoxic in several bacterial and mammalian cell test systems and have shown teratogenic potential in several mammalian species, but cancers have not been induced in laboratory animals.
USEPA has classified arsenic as a human carcinogen, based primarily on skin cancer (U.S. Environmental Protection Agency, 1985).The ability of arsenic to cause internal cancers is still controversial. Under the NIPDWR regulations, an MCL of 50 ug/L had been set, but it is under review. Currently, USEPA’s Risk Assessment Council estimates that an RfD (for non-carcinogenic skin problems) ranges from 0.1 to 0.8 ug/kg/day−1, which translates into an MCLG of 0 to 23 ug/L. Based on a 1-in-10,000 risk of skin cancer, USEPA estimated that 2 ug/L might be an acceptable limit for arsenic in drinking water.
- Published in Water Chemistry, Water Treatment
What is Aluminum – Definition
Aluminum element occurs naturally in nearly all foods, the average dietary intake being about 20 mg/day. Aluminum salts are widely used in antiperspirants, soaps, cosmetics, and food additives. Aluminum is common in both raw and treated drinking waters, especially those treated with alum. It is estimated that drinking water typically represents only a small fraction of total aluminum intake. Aluminum element shows low acute toxicity, but administered to certain laboratory animals is a neurotoxicant. Chronic high-level exposure data are limited, but indicate that aluminum affects phosphorus absorption, resulting in weakness, bone pain, and anorexia.
Carcinogenicity, mutagenicity, and teratogenicity tests have all been negative. Associations between aluminum and two neurological disorders, Alzheimer’s disease and dementia associated with kidney dialysis, have been studied. Current evidence suggests that Alzheimer disease is not related to aluminum intake from drinking water, but other sources of aluminum appeared to be associated with Alzheimer. Dialysis dementia has been reasonably documented to be caused by aluminum. Most kidney dialysis machines now use specially prepared water.
Aluminum element was included on the original list of 83 contaminants to be regulated under the 1986 SDWA amendments. USEPA removed aluminum from the list because it was concluded that no evidence existed at that time that aluminum ingested in drinking water poses a health threat (U.S. Environmental Protection Agency, 1988b). USEPA has a secondary maximum contaminant level (SMCL) of 50 to 200 μg/L to ensure removal of coagulated material before treated water enters the distribution system.
- Published in Water Chemistry, Water Treatment
What are the USA Drinking Water Standards
Drinking water standards have been set by a number of countries and international organizations. The number of standards published by these organizations and the frequency of their revision is increasing. Hence, only references for the current standards will be provided so that the most recent version can be obtained from the appropriate agency.
Summary Listing of USA Drinking Water Standards
Current and proposed drinking water standards are summarized in Tables 1.9 to 1.11.The background and current status of anticipated USEPA drinking water regulations are reviewed annually in the Journal of the American Water Works Association (Pontius, 1990a, 1992, 1993a, 1995, 1996a, 1997b, 1998; Pontius and Roberson, 1994).
USEPA Drinking Water Standards USA Table
USEPA Drinking Water Standards USA Table continued
USEPA Drinking Water Standards USA Table continued 2
USEPA Drinking Water Standards USA Table Continued 3
USEPA Drinking Water Standards USA Table Continued 4
Drinking water standards in Canada
In Canada, provision of drinking water is primarily the responsibility of the provinces and municipalities. The federal Department of Health conducts research, provides advice, and in collaboration with the health and environment ministries of the provinces and territories, established guidelines for drinking water standards quality under the auspices of the Federal-Provincial Subcommittee of Drinking Water.A publication entitled Guidelines for Canadian Drinking Water Quality (Health and Welfare Canada, 1996) identifies substances that have been found in drinking water and are known or suspected to be harmful. For each substance, the Guidelines establish the maximum acceptable concentration (MAC) that can be permitted in water used for drinking. The MAC is similar to the USEPA MCL.
Drinking water standards in Mexico
In Mexico, the federal Secretariat of Health has the authority for setting drinking water standards. A national law analogous to the U.S. SDWA does not exist, but Mexico has set standards for a number of microbiological and chemical contaminants that they refer to as norms (Secretariat of Health, 1993). Compliance with the DRINKING WATER STANDARDS, REGULATIONS, AND GOALS 1.39 norms established by the federal government is mandatory.The norms are similar to the USEPA MCLs in that they are set at the federal level, and then the water purveyors are required to conduct monitoring in accordance with the norms and to meet the values set. The standards include sampling and analytical requirements as well as reporting requirements. Implementation is carried out by the Secretariat of Health, other government entities, and the National Water Commission.
Drinking water standards by the World Health Organization (WHO)
The World Health Organization (WHO) is a specialized agency of the United Nations with primary responsibility for international health matters and public health. In carrying out that responsibility, it assembles from time to time international experts in the field of drinking water to establish Guidelines for Drinking water Quality (WHO, 1996).The primary aim of this publication is the protection of public health. These Guidelines are published primarily as a basis for the development of national drinking water standards, which, if properly implemented, will ensure the safety of drinking water supplies by eliminating known hazards. These Guidelines values are not mandatory limits. Each country must consider the Guidelines values in the context of local or national environmental, social, economic, and cultural conditions. They can then select which of the Guidelines are applicable to their situation and may choose to make adjustments to suit local conditions. The issues of monitoring, reporting, and enforcement are solely left to the discretion of the governmental entity using the WHO Guidelines. USEPA is an active participant in the development of the Guidelines, and the procedures used are in some ways similar to those used to develop the U.S. drinking water regulations.
Drinking water standards by the European Union (EU)
The European Union (EU) is a voluntary economic alliance of member states. Through the European Commission, directives are created that must then be adopted and implemented by member states. Therefore, EU members must have enforceable drinking water standards that cannot be less stringent than the limit values set out in the directive. Of course, member states can set more stringent standards if they wish. In July 1980, the Commission adopted directive 80/778/EEC relating to the quality of water intended for human consumption (EEC, 1980). On January 4, 1995, the European Commission adopted a proposal to simplify, consolidate, and update the directive. The proposal reduces the number of parameters from 66 to 48 (including 13 new contaminants), obliges member states to fix values for additional health parameters as needed, adds more flexibility to redress failures, allows efficient monitoring by including a number of indicator parameters, and finally, adds a requirement for annual reports to the consumer (EC, 1996).This proposal was adopted by the Council of the European Union on December 19, 1997 (EU, 1997).
Contact us for more information regarding this article or water treatment options for your water.
- Published in 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 Electronegativity – Electronegativity Definition
The electronegativity determines whether a bond between two atoms is polar or nonpolar which is defined by Polarity. The electronegativity is a measurement of how attractive an atom is to electrons. The more electronegative an element is, the more the shared electrons will spend around that atom. When one atom in a bond gets to keep the electrons more than the other, it assumes a partial negative charge. The other bonded atom has the electrons less often, so assumes a slight positive charge. Like the negative and positive poles of a magnet. One end of the bond is positive and one end is negative, so the bond is said to be polar. Carbon and hydrogen bonds, as in methane, are nonpolar because the electronegativity of both are very close. Carbon and oxygen bonds are polar because the electronegativity of them are substantially different.
Electronegativity example in Methane molecule
Polar substances dissolve in polar substances because the partial positive & negative charges of one molecule are attracted to the partial +ve and -ve charges of other molecules. Nonpolar substances will dissolve in nonpolar substances. This is because of other forces of attraction, such as Van der Waals’ forces, can come into play. Nonpolar compounds generally do not dissolve in polar compounds, and vice versa, because there is no attraction between things that are charged and things that aren’t. Since water is polar, compounds which are polar will generally dissolve well.
Therefore, if we have an organic which is polar, such as methyl alcohol, it will dissolve well in water. In general, the more polar an organic is, the more it will be dissolved. More importantly for this chapter, the more nonpolar a compound is, the less it will be dissolved and the more it will tend to be a suspended particle.
SIZE: One other aspect of suspended solids is their size. Very large molecules, even if they are polar, will tend to be particles. For example, we have discussed silica (SiO2) at length. If we look at the bonding between Si and O, we see that silicon has an electronegativity of 1.90 and O has an electronegativity of 3.44. Is this a polar bond? Given that water is polar and hydrogen has an electronegativity of 2.10, silica is more polar than water. Yet we see that sand is a particle and not dissolved.
To summarize, then, the degree of polarity and the size of an organic compound will determine if it is suspended or dissolved.
- Published in Water Chemistry, Water Treatment
What is Total Suspended Solids – TSS Definition
Total suspended solids (TSS) concentration is a measure of the total weight of solid residuals contained in the source water. It is customarily presented in milligrams per liter or parts per million. Total Suspended Solids can also be defined as a laboratory analysis that measures the weight of suspended solids in a specified volume. It is used to accurately (within the limitations of the test) measure suspended solids concentration. The TSS measurement is the standard method for measuring the weight of suspended solids. The method utilizes a 0.45 micron membrane filter.
Total Suspended Solids TSS measurement test procedure
Total Suspended Solids is the most accurate standard test for monitoring the amount of suspended material in a sample. The problems with it are:
1. It doesn’t tell us anything about the number of particles.
2. It doesn’t tell us anything about the size of particles.
3. It is only a crude measurement of the fouling potential of the particles.
4. It doesn’t measure any particles less than 0.45 micron in diameter.
Even with its limitations, however, it can be a valuable tool for monitoring the solids loading into a Reverse Osmosis System.
TSS is measured by filtering a known volume of water (typically 1 L) through a preweighed glass-fiber filter. Drying the filter with the solids retained on it at 103 to 105°C, and then weighing the filter again after drying. The difference between the weight of the dried filter and of the clean filter, divided by the volume of the filtered sample. Reflects the total amount of particulate (suspended) solids in the source water.
It should be pointed out that because saline water contains dissolved solids which will crystallize and convert into particulate solids when the sample is heated at 103 to 105°C. Often Total Suspended Solids TSS analysis of saline water completed in accordance with the standard methods. For water and wastewater analysis yields an erroneously high Total Suspended Solids TSS content in the water. The higher the source water’s salinity and the lower its particulate content. The more inaccurate this measurement is. In order to address the challenge associated with the standard method of Total Suspended Solids TSS measurement. It is recommended to wash the solids retained on the filter by spraying the filter with deionized water before drying. Unless the source water solids sample is washed before drying, the results of this sample are meaningless.
The laboratory Total Suspended Solids TSS test is completed properly and the filtered sample is well washed to give us a precise measure. This parameter usually provides a much more accurate measure of the actual content of particulate solids in the source water than does turbidity. Because it accounts for the actual weight of the particulate material present in the sample. For comparison, turbidity measurement is dependent on particle size, shape, and color. And typically is not reflective of particles of very small size (i.e., particles of 0.5 µm or less). Such as fine silt and picoalgae. In fact, a change in the ratio of Total Suspended Solids TSS to turbidity is a good indicator of a shift in the size of particles contained in the source water. Which may be triggered by algal blooms, storms, strong winds, and other similar events. Which can result in resuspension of solids from the bottom sediments into the water column.
Typically, an increase in the Total Suspended Solids/turbidity ratio is indicative of a shift of particulate solids toward smaller-size particles. For example, during non-algal-bloom conditions, the TSS/turbidity ratio of an appropriately processed sample is typically in the range of 1.5 to 2.5. Water with a turbidity of 2 NTU would have a Total Suspended Solids concentration of 3 to 5 mg/L. During heavy algal blooms dominated by small-size (pico- and micro-) algae, the TSS of the source water could increase over 10 times (e.g., to 40 mg/L). While the source water turbidity could be multiplied by 2 to 3 times only. For this example it would be in range of 4 to 6 NTU. With a corresponding increase in the TSS/NTU ratio from 2/1 to between 6/1 and 10/1.
- Published in Water Chemistry, Water Treatment
What is Coagulation Definition
Coagulation is simply the process that destabilizes colloidal particles so that they can come together to form larger, conglomerate particles. Low-pressure membrane technology is becoming significantly more prevalent in the drinking water industry. Low-pressure membranes are purely size-exclusionary devices. As a result, anything smaller than membrane pore sizes (approximately 0.01 to 0.1 micron) will pass through the membrane, Therefore, membrane feed waters with dissolved materials, such as organics and metals, require some form of additional treatment.
Often, in these cases, the most economical pre-treatment process is simple coagulation. Potential coagulants for membrane pre-treatment include those also used for conventional water treatment. Additionally, organic adsorption media such as PAC and MIEX, or oxidants such as potassium permanganate, chlorine, or chlorine dioxide can be applied upstream of a low-pressure membrane (assuming appropriate membrane compatibility) for enhanced dissolved material removal.
Similar to a direct filtration mode of operation for conventional technology, the goal of coagulation for membrane pretreatment is to produce a pinpoint floc that is capable of adsorbing dissolved matter, but minimizes solids loading onto the membrane filtration process. As noted briefly above, it is important to quantify membrane compatibility and performance with the coagulant of choice.
Each commercially available RO membrane utilizes different membrane materials. As a result, the compatibility and performance of a coagulant for membrane filtration pre-treatment will likely vary between membrane system and raw water supplies. As such, there are no specific guidelines for membrane system pre coagulation except the general guidelines that are associated with conventional treatment.
Coagulant Types
The coagulant most frequently used for membrane plant source seawater conditioning prior to sedimentation or filtration is ferric salt (ferric sulfate and ferric chloride). Aluminum salts (such as alum or polyaluminum chloride) are not typically used because it is difficult to maintain aluminum concentrations at low levels in dissolved form because aluminum solubility is very pH dependent. Small amounts of aluminum may cause mineral fouling of the downstream Seawater Reverse Osmosis membrane elements. In coagulation, coagulant dosage for a given source water should be determined based on jar test and/or pilot testing. The optimum coagulant dosage in a coagulation process is pH dependent and should be established based on an on-site jar or pilot testing for the site-specific conditions of a given application.
Overdosing of a coagulant used for seawater pretreatment is one of the most frequent causes for SWRO membrane mineral fouling. When overdosed during coagulation, coagulant accumulates on the downstream facilities and can cause fast-rate fouling of downstream cartridge filters following the pretreatment step and in iron fouling of the Seawater RO membranes. The effect of overdosing of coagulant (iron salt) on the SDI level can be recognized by visually inspecting the SDI test filter paper (Figure 6). In such situation, a significant improvement of source water SDI can be attained by reducing coagulant feed dosage or in case of poor mixing, modifying the coagulant mixing system to eliminate the content of unreacted chemical in the filtered seawater fed to the SWRO membrane system.
In line static mixer used in coagulation
The main purpose of the coagulation system is to achieve uniform mixing of the added coagulant with the source seawater and efficient coagulation of the particles contained the seawater. The two types of mixing systems most widely used in seawater desalination plant is in line static mixer and mechanical flash mixer installed in coagulation tanks. Although in-line mixers are simple and less costly, they have two disadvantages: (1) their mixing efficiency is a function of the flow rate; (2) static mixers are proprietary equipment and the project designer would need to rely on the equipment manufacturer for performance projections.
Static mixers also create additional head losses of 0.5 to 1.0 meters, which need to be accounted for in the design of the intake pump station. Another important issue is to provide adequate length of pipeline (at least 20 times the pipe diameter) between the mixer and the entrance to the pretreatment filters in order to achieve adequate flocculation. Mechanical flash mixing systems consist of coagulation tank with one or more mechanical mixers and chambers. The coagulation tank is designed for a mixing time (t) of 1 to 3 seconds and mechanical mixers that create velocity gradient (G) of 300 s, (optimum G x t = 500 to 1,600). The power requirement for the mechanical mixer is 2.2 to 2.5 horsepower/10,000 m3/day. This type of mixing usually provides a more reliable and consistent coagulation, especially for desalination plants with significant daily flow variations (i.e., more than 30 % of average annual production flow).
- Published in Water Chemistry, Water Treatment
What is Total Organic Carbon – TOC Definition
Total organic carbon is one of the most widely used measures for organic content of seawater. Total Organic Carbon concentration measures the content of both Natural Organic Matter (NOM) and of easily biodegradable organics, such as polysaccharides released during algal blooms. This water quality parameter is widely used, because it is relatively easy to measure. And it is indicative of the tendency of the seawater to cause Seawater Reverse Osmosis System SWRO membrane biofouling. Total Organic Carbon TOC is measured by converting organic carbon to carbon dioxide in high-temperature furnace in the presence of a catalyst.
Typically, open ocean seawater which is not influenced by surface fresh water influx (nearby river confluence). By man-made activities (i.e., wastewater or storm water discharges, or ship traffic). Or by algal bloom event (i.e., red tide), has a very low Total Organic Carbon content ($0.2 mg/L). When an algal bloom occurs however, Total Organic Carbon TOC concentration of the ocean water could increase by an order of magnitude (2 to 8 mg/L). Similar magnitude of Total Organic Carbon (TOC) increase could be triggered by a storm water or river discharge during high-intensity rain event. Such as the rainy seasons in tropical and equatorial parts of the world.
Usually, an increase of Total Organic Carbon above a certain threshold (2.0 to 2.5 mg/L) is observed to trigger accelerated biofouling of Seawater RO membranes. The Carlsbad seawater desalination demonstration plant in California is supplied by seawater collected using near-shore open ocean intake. Observations indicate that Total Organic Carbon concentration in the source water at that location exceeds 2.0 mg/L during algal bloom events. Within a week to two week period, the Seawater Reverse Osmosis system experiences measurable biofouling and associated increase in operating pressure. Similar TOC level observations at the Tampa seawater desalination plant, in Florida, USA (where the typical background TOC level of the seawater is less than 4 mg/L) indicate that accelerated biofouling occurs when Total Organic Carbon concentration exceeds 6 to 8 mg/L.
Usually, accelerated biofouling at the Tampa facility is triggered by one of two key events – rain events, which increase the content of alluvial organics in the source seawater, or algal blooms which cause elevated organic levels due to massive die off of algae. The increase in alluvial organics during rain events is caused by the elevated flow and alluvial content of Alafia River. Which discharges into Tampa Bay several kilometers upstream of the desalination plant intake. During high-intensity rains during summer months, TOC level in the river water discharging in the bay may exceed 20 mg/L.
| Seawater Source | TOC (mg/l) | Polysaccharides (% of Total TOC) |
Humic Substances and Building Blocks (% of Total TOC) |
Low Molecular Weight Acids and Neutrals (% of Total TOC) |
Other Low Molecular Weight Compounds (% of Total TOC) |
| Surface Raw Seawater – Perth, Australia | 0.9 | 3 | 31 | 25 | 41 |
| Surface Raw Seawater -Ashkelon, Israel May 2005) |
1.2 | 14 | 39 | 25 | 22 |
| Surface Raw Seawater -Ashkelon, Israel May 2006) |
1 | 7 | 52 | 22 | 19 |
| Surface Raw Seawater -Carboneras, Spain | 0.9 | 8 | 38 | 18 | 42 |
| Well Seawater – Gibraltar, Spain | 0.6 | 1 | 26 | 22 | 51 |
| Surface Raw Seawater –Gibraltar, Spain | 0.8 | 5 | 26 | 25 | 42 |
Analysis of various sources of seawater indicates that TOC concentration of seawater may contain various fractions of organics. This depends on the origin of this water and the type of seawater intake. See Table above. These fractions may also change depending on the season as well. Analysis of the Table indicates that low-molecular weight organic compounds are typically the greatest fraction of the TOC in seawater (40 to 50%). Comparison of the data from the Ashkelon seawater desalination plant in Israel indicates that the most easily biodegradable organics (polysaccharides) change seasonally. And increase during the summer season along with the content of algal biomass in the ocean. This data also shows that TOC concentration of seawater may not always correlate with the content of polysaccharides in the water.
- Published in Water Chemistry, Water Treatment
Que es un ion
Un ion es simplemente un atomo con una carga. Para explicar que es un ion, revisaremos brevemente la estructura del atomo. La configuracion electronica nos ensena que un atomo es mas estable cuando su orbital mas externo tiene un conjunto completo de electrones. En realidad, la tendencia de un atomo a lograr un orbitario externo mas completo es muy poderosa y superara la fuerza electrostatica que mantiene unido al atomo.
Al igual que en el caso del atomo de fluor, aceptara un electron adicional para lograr un orbital mas externo, a pesar de que terminara teniendo uno mas electrones que protones. Basicamente, un atomo neutro tiene el mismo numero de protones y electrones.
Ademas, un elemento como el sodio abandonara un electron para alcanzar un orbital externo completo. Esto resulta en una diferencia y desequilibrio entre electrones y protones en el mismo atomo.
Cuando en un atomo, los protones (+ve) son mas que los electrones (-ve) = el resultado es un ion positivo (cation).
Algunos atomos abandonan mas de un electron para tener un orbital mas externo. Mientras que otros atomos adquiriran mas de un electron para llenar el orbital externo. Dado que los electrones tienen una carga negativa (ion negativo) y los protones tienen una carga positiva (ion positivo), una diferencia en el numero de electrones y protones da como resultado una carga neta. Esto es cuando un átomo se llama por lo tanto un ion.
Dependiendo de si gana o pierde un electron, un ion tendra una carga positiva o negativa. El catión es un ion que tiene una carga positiva neta y el anión es un ion que tiene una carga negativa neta. Sabemos que los cargos opuestos se atraen, mientras que los mismos cargos son rechazados. Eso significa que los cationes y los aniones se atraen entre si. La fuerza de la atraccion puede hacer que los iones formen un solido llamado retÃculo de cristal.
- Published in Water Chemistry
Que es la geometria molecular?
La geometria molecular es basicamente la disposicion tridimensional / forma / estructura de los atomos que forman una molecula. Cuando las moleculas estan formadas por un enlace quimico, lo que significa que los atomos se unen entre si, los suborbitales involucrados en el enlace o enlaces crean diferentes formas moleculares que dependen de muchos factores.
Por ejemplo, las moléculas de agua no son lineales, una molecula de agua en realidad tiene forma de “V” y el angulo formado entre los dos atomos de hidrogeno y el atomo de oxÃgeno es aproximadamente de 105 grados.
Cuando dibujamos moleculas en dos dimensiones, la mayoria de las veces pensamos que estas moleculas son planas. Pero en realidad existen en diferentes formas y formas.
La Composicion Quimica y la Geometria Molecular de una molecula es lo que determina principalmente las propiedades de la molecula. Como el sabor, el punto de ebullicion, el magnetismo, la dinamica, la polaridad, el color y todas las demas propiedades.
Geometria molecular y diferentes tipos de estructuras moleculares:
Geometria Molecular Lineal
En una estructura de Geometria Molecular Lineal, los atomos estan unidos entre si para formar una lÃnea recta. Los angulos de union son de 180 grados y, por ejemplo, el dioxido de carbono (CO2) y el oxido nitrico (NO).
Planar Triangular – Geometria Molecular Planar Trigonal
Trigonal Planar Molecular La geometria se forma cuando un compuesto tiene un atomo en el centro unido a otros tres atomos en una disposicion que parece un triangulo alrededor del atomo central. Los cuatro atomos estan en la misma linea y planos en un plano.
Geometria Molecular Piramidal Trigonal
La Geometria Molecular Piramidal Trigonal obviamente tiene la forma de una piramide con una base que parece un triangulo. La estructura piramidal trigonal se parece a la geometria molecular tetraedrica, las estructuras piramidales necesitan tres dimensiones para que puedan separar completamente los electrones. Un ejemplo de la orientacion piramidal trigonal es el amoniaco (NH3).
Geometria Molecular Tetraedrica
Tetra significa cuatro y tetraedrico significa basicamente un solido o piramide que tiene cuatro lados. La geometria molecular tetraedrica se forma cuando un atomo central tiene cuatro enlaces con cuatro atomos a la vez formando una forma de piramide con cuatro lados. Segun la VSEPR, que es la teoria de la repulsion del par de electrones de la carcasa de valencia, los ángulos de enlace entre los atomos en la orientacion tetraedrica son aproximadamente 109.47 grados. Un ejemplo de la Geometria Molecular Tetraedrica es el metano (CH4).
Geometria Molecular Plana Cuadrada
La geometria molecular plana y cuadrada se forma cuando un atomo central tiene cuatro enlaces y dos pares solitarios. El tetrafluoruro de xenon (XeF4) es un ejemplo de la estructura plana cuadrada; está formado por seis orbitales equidistantes dispuestos en ángulos de 90 grados. Que forma una forma octaedrica. Dos orbitales contienen pares de electrones solitarios en lados opuestos del atomo central. Y los otros cuatro atomos estan unidos al atomo central, lo que hace que la molecula tenga una estructura plana y cuadrada.
Geometria Molecular Bipiramidal Trigonal
La Geometria Molecular Bipiramidal Trigonal ocurre cuando el atomo central esta conectado a cinco atomos formando cinco enlaces y sin pares solitarios. Tres de los cinco enlaces se crean a lo largo del ecuador del atomo formando angulos de 120 grados mientras que los dos restantes se forman en el eje del atomo. Un ejemplo de la Geometria Molecular Bipiramidal Trigonal es el Pentacloruro de Fósforo (PCl5).
Geometria Molecular Octaedrica
Octa significa ocho y Geometria Molecular Octaedrica significa una piramide o solido que tiene ocho lados o caras. La estructura octaedrica tiene seis atomos unidos que forman angulos de 90 entre ellos. Un ejemplo de la estructura molecular octaedrica es el hexafluoruro de azufre (SF6).
Formas moleculares y diferentes tipos de estructuras moleculares.
- Published in Water Chemistry
What is Silt Density Index SDI Measurement
Silt Density Index SDI roughly measures the fouling potential of a Reverse Osmosis System feed water. In other words, it is supposed to tell us if a feed water is likely to foul a Reverse Osmosis System or not. For the SDI measurement, the filter used is a cellulose acetate membrane filter with pores of around 0.45 micron. The filters are generally 47mm in diameter.
Silt Density Index SDI Clean Dirty/Plugged Filter Example
These SDI membrane filters will plug with particulate matter when a sample is filtered through them. If the pressure is kept constant, the amount of flow through a filter will decrease as the pores become plugged.
Silt Density Index SDI Filter Measurement
SDI measurements may be taken on any stream. The one that the membrane manufacturers are concerned about, however, is the SDI of the feed water as it enters the elements. This is generally taken at the suction of the high pressure pump, after all of the feed water pretreatment has been accomplished.
Silt Density Index SDI Measurement Method in Reverse Osmosis
Generally, three-eighth inch (9.5 mm) tubing is used to pipe the feed water stream to the filter housing. A shutoff valve is required to isolate the filter when not in use. A pressure regulator is required to maintain the pressure at 30 (2.1) psi(bar) throughout the test. A pressure gauge is required to monitor the pressure. A filter housing is required to hold the 0.45 micron filter.
Silt Density Index SDI Measurement Method Reverse Osmosis System
A 500 ml graduated cylinder is placed under the filter to collect the filtered water (Figure 14.12). The
time required to filter 500 ml is measured at the beginning and at the end of a 15 minute period
during which the feed water is being filtered by the 0.45 micron filter.
Silt Density Index SDI Measurement Reverse Osmosis
If there were no particles in the feed water, the filter would not plug at all, so the amount of time required to pass 500 ml through the filter should be the same at the end of 15 minutes as it was in the beginning. If particulate matter plugs the pores of the filter during the 15 minute test, however it will take a longer period of time to pass 500 ml through the filter at the end of the test than it did in the beginning.
SDI is a relative comparison of the amount of time required to pass 500 ml through a 0.45 micron filter before and after a given time period. The time period for RO membranes, required by the membrane manufacturers, is 15 minutes. The actual calculation of SDI is the following:
SDI =Â 100x(1-t1/t2) / T
Where:
t1 = Time required to pass 500 ml in the beginning, in seconds.
t2 = Time required to pass 500 ml at the end, in seconds.
T = Time between the beginning of t1 and beginning of t2, in minutes.
SDI Measurement Procedure Summary
A quick summary of the procedure is given.
STEP 1:Â Â Flush the SDI apparatus without the filter disc installed in the filter housing. This is required in order to flush out any particles which have become lodged in the lines since the last measurement.
STEP 2:Â Â Install a 0.45 micron filter in the housing. Bleed out any air then tighten the housing nuts. Place the discharge line of the filter into a 500 mL graduated cylinder.
STEP 3:Â Â Open the isolation valve and simultaneously start a stopwatch to begin timing.
STEP 4:Â Â Measure the time required (t1) to pass 500 mL. The pressure to the filter MUST remain at 30 (2.1) psi(bar) the entire time.
STEP 5:Â Â Allow feed water to continue to flow at 30 (2.1) psi(bar) of pressure through the filter for a total (from the beginning of t1) of 15 minutes.
STEP 6:Â Â Right at the end of 15 minutes, again measure the amount of time required to pass 500 ml.
STEP 7:Â Â Perform the SDI calculation.
- Published in Water Treatment
What is Silt Density Index SDI Definition
The Silt Density Index (SDI) is an analytical procedure which measures the fouling potential of a sample stream, it is basically the parameter most used to determine how much pretreatment is required in designing Reverse Osmosis Systems. In reality, it is a very rough indicator of fouling potential. SDI is based upon the principal that when a filter gets plugged, less water will pass through at the same pressure. Less water passes through because of the resistance to flow caused by particles plugging the spaces in the filtering medium.
Silt Density Index SDI for Clean vs Dirty/Fouled Filter
A sample stream of the raw water is filtered through a new 0.45 micron membrane filter for fifteen minutes. The amount of time required to pass 500 ml through the filter is measured initially, (when the membrane filter is clean), and again after fifteen minutes worth of suspended material have been filtered. The reduction in flow rate (longer time to pass 500 ml at the end of the 15 minute period) is a crude measurement of the fouling potential of the stream.
SDI filter pads after testing turbidity in seawater
Most spiral wound RO membrane element manufacturers require that an RO System feed water has a fifteen minute SDI value (SDI15) of less than 5 (one manufacturer requires less than 4). An SDI15 of 5 is obtained when it takes four times longer to pass 500 ml after fifteen minutes worth of suspended solids have been deposited on the membrane than it did during the initial 500 ml. Most hollow fiber element manufacturers require that the SDI15 be less than 3.
When a membrane element supplier requires that an RO System feed water SDI15 be less than 5, this means that the feed water SDI15 must be less than 5 as it enters the Reverse Osmosis System The problem is that the particles may grow. In the case of living particles, they grow and reproduce like any organism. They add mass by accumulating nutrients from the environment. They add numbers by reproduction. Nonliving particles may grow in an Reverse Osmosis System as well. They grow by physical/chemical means called coagulation or agglomeration.
The purpose of our pretreatment equipment is to remove the relatively large particulate matter from the feed stream so it does not plug the Reverse Osmosis Systen and to limit the number of living particles entering a unit. The purpose of dispersant addition is to then keep the smaller, especially nonliving particles from agglomerating (growing larger) and remaining in the RO System.
Silt Density Index SDI and Reverse Osmosis System
Since SDI is a measurement of both living, dead, and nonliving particulate matter greater than 0.45 micron, it is a useful tool to determine the fouling potential of a feed water and how much pretreatment equipment will be required. Some general guidelines on what it will take to maintain an RO unit’s feed water SDI at less than 4 – 5 are:
| RAW WATER SDI | PRETREATMENT REQUIRED TO ACHIEVE SDI < 5 |
| Greater than 10 – 20 | Clarification, MMF, CF, +/- Dispersant (plugs in less than 5 – 10 mns) |
| 7 – 8 TO 10 – 20 | MMF (with polymer addition), CF, +/- Dispersant |
| 5 TO 7 – 8 | MMF, CF, +/- Dispersant |
| Less than 5 | CF, +/- Dispersant |
- Published in 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
