Operational Optimization and Mass Balances in a Two-Stage MBR Treating High Strength Pet Food Wastewater

A two-stage membrane bioreactor (MBR) system was evaluated for the treatment of high strength pet food wastewater characterized by oil and grease, chemical oxygen demand (COD), biochemical oxygen demand (BOD)5, total suspended solids (TSS), total Kjeldahl nitrogen (TKN), NH4-N, and TP concentrations of 2,800, 25,000, 10,000, 4,500, 1,650, 1,300, and 370 mg/L, respectively, to meet stringent surface discharge criteria of BOD5, TSS, and NH4-N of <10 mg/L, and TP of <1 mg/L. Pretreatment of the dissolved air flotation effluent with FeCl3 at a dose of 3.5 g/L, corresponding to a Fe:P molar ratio of 1.3:1 affected TP, TSS, volatile suspended solids (VSS), COD, BOD5, and TKN reductions of 88, 72, 75, 11, 11, 36, and 17%, respectively. The two-stage MBR operating at a total hydraulic retention time of 5.3 days comprising 2.5 days in the first stage and 2.8 days in the second stage, and solids retention time of 25 days in the first stage consistently met the criteria despite wide variations in influent characteristics. Very high COD and BOD5 removal efficiencies of 97.2 and 99.8% were observed in the first stage, with an observed yield of 0.14 gVSS/gCOD. A modular approach for the quantification of simultaneous nitrification denitrification (SND) in the first-stage MBR was developed and verified experimentally. The model indicated that on average, 21% of the influent nitrogen was removed by SND and predicted nitrogen loss with an accuracy of 72%. Complete nitrification of the residual organic nitrogen and ammonia was achieved in the second-stage MBR.