Water. desalination + reuse

PROJECTS August-September 2014 | Desalination & Water Reuse | 21 | PilOT PlanT: OPERaTiOnS and mOdElling BAPCO selected CH2M HILL to conduct a pilot test phase to characterize the wastewater influent streams and assess the performance of the MBR process. After the initial pilot phase, the treatment of spent caustic by its addition to the anoxic biological compartment was investigated. Based on the results, an optimized biological process design (cost-effective from a capital and an operation and maintenance standpoint) was developed to meet discharge limits. Tables 1 and 2 summarize the main characteristics of the induced air flotation (IAF) effluent and pilot plant operation. Oxidized forms of nitrogen were usually negligible in the influent, and all nitrogen was measured as TKN. Ammonia-nitrogen was approximately 57% of the TKN, indicating that the wastewater contained a significant amount of organic nitrogen. The TDS concentration was 30-31 g/l from June to August and 26-28 g/l during other months. Due to relatively slow growth rate of autotrophic (nitrifying) bacteria in this wastewater matrix, the pilot plant was operated at a long solids retention time (SRT) of 35-45 days. After the first two months of operation, acetic acid was added to the anoxic zone to maximize nitrate removal. After adding spent caustic, acetic acid dosage was decreased to let biomass better acclimatize to degrade the COD in the spent caustic. Simultaneously, the dosage of acetic acid was decreased to encourage heterotrophs to exploit the available COD. While the TKN concentration of spent caustic is high, the added load was negligible because of the low flow rate, and the pilot plant's performance in COD removal and nitrogen removal were unaffected by addition of spent caustic. Stable nitrification confirmed that spent caustic does not have any negative or toxic effect on the biomass. Average spent caustic characteristics were: 68,571 ppm COD; 21,767 ppm sulphides, and 894 ppm TKN. Spent caustic also provided alkalinity, decreasing the need for caustic addition. The IAF effluent has unique characteristics and commonly applied methods for the analysis of the COD can lead to biased results. A suitable methodology, the correction factor chlorides/COD reported in figure 1 was developed to define the amount of "extra reading" of the COD needed due to interference of chlorides. Table 1. Characteristics of the IAF effluent, sampled with the automatic sampler. Table 2. Average values and standard deviations for pilot plant inlet and outlet. Figure 1. Plot of chloride correction curve (samples not diluted and treated with mercuric sulphate). Figure 2. Schematic of biological unit with return activated sludge (RAS) and waste activated sludge (WAS) pathways.

• Cover