Water Journal : Water Journal May 2011
membranes & desalination refereed paper technical features 106 MAY 2011 water membrane system, with four primary RO skids for first and second stages, and two secondary skids for third and fourth stages; • CIP system for RO membranes; • Product water storage tank and distribution. Figure 2 illustrates the water treatment component of the GTP at time of commissioning. The RO skids were fitted with 8" x 40" Dow® FilmtecTM membrane elements with six elements per vessel; BW30-400 in the primary skids and BW30-400/34 in the secondary skids. Each primary RO skid was designed to operate at 70% recovery and a permeate flux of 24.9L/m2h, with 120 elements in a 14:6 array. Each secondary skid was also designed to operate at 70% recovery, yielding an overall plant recovery of 91%. The design permeate flux was 19.9L/m2h, with 90 elements in a 10:5 array. One of the primary skids was fitted with Toray TML720-430 elements for commissioning. These elements were recovered from a small scale interim treatment process that was operated while the main plant was under construction. Early Operation The plant was started at 25% capacity, with only one primary RO skid and one secondary RO skid in operation. Treated water (permeate) was recycled to supplement primary RO reject feed to the secondary RO. Within days of start-up, the plant was plagued with severe biological fouling. The RO cartridge filters blocked within 48 hours of operation, and the final (fourth) stage of RO required cleaning within a few days. After a number of cycles of cleaning the RO skids and replacing filter cartridges, the RO process was bypassed, and filtered water diverted to the sewer. The cartridge filters were examined with a scanning electron microscope, and were found to be covered with a layer of slime (see Figure 3). EDAX (Energy Dispersive X-ray analysis) revealed that the layer largely consisted of carbon and oxygen, with only traces of inorganic crystals of aluminium, calcium and silica on top of the organic material. Cartridge filters are designed for particulate removal, which was being achieved, but they were being fouled by an amorphous material. After exhausting the stock of 20 micron filters, 40 micron filters were sourced and installed. This provided increased run times, but potentially allowed increased particulate loading on the membranes. Similar slimes were found elsewhere through the RO skids. At the same time, the GAC contactors required backwashing every one to two days, as opposed to the expected 14-day interval. Further examination of the GAC contactor units found: • High bacterial loads in GAC backwash, over 200,000 cfu/mL; • pH drop of 0.5--0.8 units; • TOC reduction of up to 13%; • Dissolved oxygen reduction of 3mg/L--6mg/L, with GAC outlet levels often below 1mg/L. This abundance of biological activity was attributed to the presence of readily biodegradable organic carbon (RBOC), predominantly as acetate. A series of sampling and analysis showed organic acid levels in the feed water ranging from 44mg/L to 88mg/L, compared to the 64mg/L feed specification. Chloramine Dosing Chloramine dosing of RO feed was proposed to mitigate biofouling of the membranes and cartridge filters. However, it was unclear whether the cartridge filter fouling was due to growth of biofilm on the cartridge surface, or from biological material sloughing off the GAC. To address the latter concern, the dual media filters were reconfigured so that five of the 10 filters would provide filtration of the GAC contactor product prior to cartridge filtration (effectively halving plant capacity). Chloramine formation was achieved by injection of aqueous ammonia and sodium hypochlorite into a carrier water stream. This stream was dosed into the GAC contactor product stream to also limit biofouling of the dual media filters. Dosing chloramine upstream of the GAC contactors was rejected, as the GAC would dechlorinate the water, and may adversely affect biological activity in the contactors which were removing non-volatile CHCs without exhaustion of the GAC. Table 1: GTP Water Specifications Parameter Feed Water (mg/L) Treated Water (mg/L) pH 4.5--6 (std units) 6.5--8.5 (std units) 1,2 DCE 169 0.003 Total volatile CHCs 226 0.4* Total non-volatile CHCs 0.25 0.05 Sodium 396 20 Calcium 50 15 Magnesium 17 7 Iron 14 0.3 Aluminium 1.2 0.0005 Silica 10 6 Chloride 617 30 Sulphate 184 20 Sulphide 5.5 0.015 Ammonia -- N 11 0.5 Nitrate + Nitrite -- N 0.08 1.0 Phosphorous as P 0.95 0.01 Acetate 46 To meet BOD Total Organic Acids 64 BOD 72 10 Alkalinity as CaCO3 57 40 Suspended Solids 0.5 10 * Various limits for individual CHCs Figure 2: Botany GTP -- initial flow diagram. Figure 3: SEM image of a fouled cartridge.
Water Journal April 2011
Water Journal July 2011