Boris Liberman

Bioflocculation: Chemical free, pre-treatment technology for the desalination industry

Rapid sand filtration (RSF), proceeded by chemical coagulation and flocculation, is a commonly used, effective pretreatment in the desalination industry. We designed and tested a novel, large pilot-scale, two-stage granular Rapid Bioflocculation Filter (RBF) based on a first-stage Bioflocculator (BF) unit followed by a mixed-media bed filter (MBF). The BF filter bed consisted of an extremely porous volcanic Tuff granular medium which provided an enlarged surface area for microbial development and biofilm proliferation. We compared the efficiency of the pilot RBF to that of a full-scale RSF, operating with upstream chemical coagulation, by measuring the removal from the same untreated seawater feed of key factors related to membrane clogging: SDI, turbidity, chlorophyll a (Chl a) and transparent exopolymer particles (TEP). After 2 weeks of operation, the Tuff grains were colonized extensively by coccoid bacteria that formed biofilm along the entire BF. With bacterial colonization and biofilm development, numerous aggregates of bacteria and some algal cells embedded in an amorphous organic matrix were formed on and within the Tuff grains. By 1-3 months, the biotic diversity within the Tuff filter bed had increased to include filamentous bacteria, cyanobacteria, fungi, protista and even crustaceans and marine worms. During and for ≈ 24 h after each cleaning cycle (carried out every 5 to 7 days by upward flushing with air and water), large numbers of floc-like particles, from ≈ 15 μm to ≈ 2 mm in size were observed in the filtrate of the BF unit. Microscopic examination of these flocs (stained with Alcian Blue and SYTO(R) 9) showed that they were aggregates of many smaller particles with associated bacteria and algae within a polysaccharide gel-like matrix. These biogenic flocs (bioflocs) were observed to form during normal operation of the RBF, accumulating as aggregates of inorganic and organic material on the Tuff surfaces. With each flush cleaning cycle, these bioflocs were released into the BF effluent but were retained by the second phase MBF unit. No flocs were seen in the MBF filtrate. Over a year-long study, both the pilot RBF and the full-scale RSF showed similar filtration efficiencies, measured as the percentage removal of Chl a, TEP, turbidity and SDI from the same seawater feed. These results indicate the potential of the bioflocculation approach with no chemical additives as an alternative to conventional RSF pretreatment for large SWRO facilities.

Analysis and monitoring: MSC – a biologically oriented approach

Dr Boris Liberman and microbiologist Tom Berman introduce us to Microbial Support Capacity (MSC) – a biologically-oriented approach to check the efficiency of processes, such as pretreatment, in front of RO and NF membranes.

Larnaca desalination plant

The Larnaca desalination plant has been designed for extended capacity of 54000 m 3 /d high-grade potable water. Deep-seated open intake, low velocity dual-media filters and low flux desalination process will give high plant availability. Lower power consumption is achieved by dividing the product in two streams, front and rear. The separated rear product is sent to the second RO stage. This, together with the gravity permeate movement through post-treatment to the product tank, and the gravity brine disposal, contributes to decreased power consumption. Effective limestone post-treatment process and absence of suck-back tanks above the RO (reverse osmosis) trains are the process singularity of this desalination plant.

Starting procedure of high-pressure pump with de-rated motor for large-scale SWRO trains☆

The target today to produce desalinated water at the lowest cost requires the design of a small number of large RO (reverse osmosis) trains in a desalination plant. The increased capacity of RO trains, operated by one high-pressure pump, has become more prevalent. The Eilat 8000 m 3 /d seawater RO plant incorporates one high-pressure pump and an energy recovery turbine driven by a de-rated motor on the same shaft. This pump is manufactured by Sulzer Pumpen GmbH. Low power consumption and capital cost are characteristic of this SWRO plant.