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 $(\mathrm{BOD}{)}_5$ , total suspended solids (TSS), total Kjeldahl nitrogen (TKN), ${\mathrm{NH}}_4–\mathrm{N}$ , and TP concentrations of 2,800, 25,000, 10,000, 4,500, 1,650, 1,300, and $370\mathrm{mg}∕\mathrm{L}$ , respectively, to meet stringent surface discharge criteria of ${\mathrm{BOD}}_5$ , TSS, and ${\mathrm{NH}}_4–\mathrm{N}$ of $<10\mathrm{mg}∕\mathrm{L}$ , and TP of $<1\mathrm{mg}∕\mathrm{L}$ . Pretreatment of the dissolved air flotation effluent with ${\mathrm{FeCl}}_3$ at a dose of $3.5\mathrm{g}∕\mathrm{L}$ , corresponding to a $\mathrm{Fe}:\mathrm{P}$ molar ratio of 1.3:1 affected TP, TSS, volatile suspended solids (VSS), COD, ${\mathrm{BOD}}_5$ , 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 $5.3\mathrm{days}$ comprising $2.5\mathrm{days}$ in the first stage and $2.8\mathrm{days}$ in the second stage, and solids retention time of $25\mathrm{days}$ in the first stage consistently met the criteria despite wide variations in influent characteristics. Very high COD and ${\mathrm{BOD}}_5$ removal efficiencies of 97.2 and 99.8% were observed in the first stage, with an observed yield of $0.14\mathrm{gVSS}∕\mathrm{gCOD}$ . 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.