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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
Ultra-Permeable Membranes (UPM) – Desalination Technology
Recent studies introduce the promise of developing new membrane materials. These materials can desalinate water while showing far greater permeability than traditional reverse osmosis (RO) membranes. But the question remains whether higher permeability means significant reductions in the cost of desalinated water. A research evaluates the potential of ultra-permeable membranes (UPM) to improve the performance and cost of Reverse Osmosis.
By modeling the mass transport inside a Reverse Osmosis pressure vessel (PV), the study assesses how much tripling water permeability lowers energy consumption. And also lowers the number of required pressure vessels for a particular desalination plant. The findings were very interesting, it proved that a tripling (3x) in permeability permits 44% fewer pressure vessels and 15% less energy for a seawater Reverse Osmosis plant (SWRO). This is done at a both given capacity and recovery ratio. Moreover, tripling permeability results in 63% fewer pressure vessels or 46% less energy for brackish water Reverse Osmosis (BWRO). However, it also shows that the energy savings of Ultra-Permeable Membranes (UPM) exhibits a law of diminishing returns due to thermodynamics and concentration polarization at the membrane surface.
Ultra-Permeable Membranes and Desalination Energy Consumption
The research shows that the development of ultra-permeable membranes helps reducing the energy consumption. It also reduces the number of pressure vessels required for Reverse Osmosis desalination. However, in terms of reducing energy consumption, the benefits of Ultra-Permeable Membranes (UPM) are limited to approximately 15% in the case of SWRO. It also shows that membranes with 3x higher permeability reduces number of pressure vessels by 44% for seawater reverse osmosis RO plants SWRO. And 63% in brackish water RO plants BWRO. This does not affect the energy consumption or permeate recovery.
ultra permeable membranes UPM thin film composite TFC for BWRO and SWRO
In case of energy consumption, ultra-permeable membranes proved to lower energy consumption of Seawater Reverse Osmosis systems – SWRO – by %15. While on the other hand lowered energy consumption of Brackish Water Reverse Osmosis systems – BWRO – by 46%. The research was made at the same permeate flow per pressure vessel as what is typical nowadays.
For greater permeability, the incremental energy savings are negligible but capital requirements continue to decrease. The reason behind it is using fewer pressure vessels. Despite concerns expressed in the research, it shows that concentration polarization does not neglect the benefits of Ultra-Permeable Membranes (UPM). Although it mitigates the benefits relative to what is expected in the absence of concentration polarization. As membrane permeability increases, also typical cross-flow velocities and mass transfer coefficients decrease. Permeate water flux increases routinely with membrane permeability. In spite of the fact that more advanced system designs are required to fully take advantage of greatly increased feed flow rates. Results suggest that advances in membrane science will continue to make desalination highly competitive as a fresh water supply in the near and far future.
- Published in Seawater Desalination
