Water. desalination + reuse

PROJECTS | 22 | Desalination & Water Reuse | August-September 2014 DESign Using analytical results from the pilot and results from batch tests, a biological process design model was calibrated to BAPCO's wastewater and pilot plant configuration which was used for the final process design. This was a two-train system each composed of six equal cells in series, four of which could be operated in either an anoxic or aerobic mode depending on the feed composition. This configuration allows flexibility to operate the system with a two-stage (anoxic/aerobic) or four-stage (anoxic/ aerobic/anoxic/aerobic) configuration with the capacity to change volumes at each stage. Sizing of the biological system was based on a SRT of 35 days with a food-to- mass ratio of about 0.1 kgCOD/kgTSS/d. Using kinetic and stoichiometric parameters common to municipal systems made it possible to more than halve the size of the BAPCO wastewater treatment plant. Equalization system, sludge de-watering, spent caustic management and chemicals dosage systems were included in the design, which required the use of exotic materials due to high chloride content. The final design incorporates an innovative double-stage cooling system to decrease wastewater temperature from 48°C to 35°C. STaRT uP anD PERfORmanCE TESTing BAPCO's wastewater treatment plant was started up by seeding the biomass from Sitra's municipal wastewater treatment plant which comprised pretreatment, a conventional activated sludge biological system with pre-denitrification tank and aeration tank, and secondary clarifiers. Sludge used to seed the BAPCO plant was recycled activated sludge, which was transferred into one of two biological trains, already full of the Sitra-treated effluent used for previous hydraulic testing phases. After seeding the biomass, the plant was operated in recycle mode without any permeation, with the addition of acetic acid, phosphoric acid and urea. This phase, lasting approximately four weeks, promoted sludge growth. Figure 3 shows the increase in TSS. The type of wastewater treated meant that the salinity of the Sitra sludge was around 2,500 mg/l - an order of magnitude lower than the refinery wastewater. So in the subsequent phase, TDS was increased by 10% a day to avert the osmotic shock from a sudden increase of salinity which could have disrupted the cells and destroyed the biological population performing the organic substance removal and nitrification. TDS was increased by using recycled permeate in the biological system (with a closed loop) and increasing the amount of BAPCO wastewater daily while bleeding the same amount of permeate from the system. During this period of about one month, acetic acid, phosphoric acid and urea were added to promote the growth of heterotrophic and autotrophic organisms capable of sustaining high salinity levels. Removal of COD and nitrogen was monitored during this phase. Figure 4 compares the calculated theoretical increase in TDS value and the actual value obtained during the real operation. Moreover, it shows the trend of the removal efficiencies for COD and TKN observed during the TDS increase phase. These efficiencies were calculated day by day, considering the amount of COD and nitrogen added as acetic acid and urea and the amount recycled with the permeate. The system stabilized after TDS was higher than 10,000 – 12,000 mg/l, with COD removal at around 85 – 90% and nitrification efficiency - based on TKN values - of around 60%. In mid November 2013, spent caustic was injected into the first anoxic cells of the two biological trains over some Figure 3. TSS increase and COD addition during the sludge growth phase. Figure 4. Comparison between predicted and actual TDS Increase and trend of the removal efficiency of COD and TKN during the TDS increase step.

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