COMBINED BIOLOGICAL AND MEMBRANE TREATMENT OF FOOD-PROCESSING WASTEWATER TO ACHIEVE DRY-DITCH CRITERIA

This study tested the applicability of a submerged vacuum ultrafiltration membrane technology in combination with the biological treatment system to achieve the dry-ditch criteria stipulated as follows: BOD5, TSS, NH3-N, andtotal phosphorous (TP)concentration not exceeding 10, 10, 1, and 0.5 mg/l respectively for the treatment of food-processing wastewater characterized by average BOD5, COD, TKN, NH3-N, and TP concentrations of 1450, 3120, 1250, 3, and 20 mg/l respectively. During the study, the biological system operated at average hydraulic retention time of 5–6 days achieved 95–96.5% BOD removal and 96–99% COD removal. The external membrane system ensured the achievability of the BOD and TSS criteria, with BOD and TSS concentrations in the permeate of 1–2 and 1–8 mg/l respectively. Ammonia, nitrate, and nitrite concentrations increased during membrane filtration. The most salient finding of this study is that, contrary to common belief, for industrial wastewaters, the filterability of the mixed liquor is influenced by the soluble organics, and may be low, thus necessitating operation of bioreactors at low mixed liquor solids. This study demonstrated that bioreactors operated at low SRTs and in combination with ultrafiltration can still achieve superior effluent quality that may meet reuse criteria at reasonable cost.