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
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
