Role of HRT in Biological Treatment of Combined Industrial and Municipal Wastewater

A pilot scale wastewater treatment plant was designed, constructed and operated on site to treat wastewater from a combined effluent drain. The activated sludge process (ASP) was used at three different hydraulic retention times (HRT) of 12, 10 and 8 hrs at an average MLSS concentration of 3500 mg/L. Under each HRT, the pilot plant was operated for a period of 10-12 weeks. Chemical oxygen demand (COD), biological oxygen demand (BOD), total organic carbon (TOC) and total suspended solids (TSS) were the major variables of interest. Average COD removal efficiency at HRT of 12, 10 and 8 hrs was 77, 80, and 87% respectively. Average BOD removal efficiency was 76, 79, and 88 % at HRT of 12, 10 and 8 hrs respectively. For the same HRTs, the average TOC removal efficiency was 64, 65 and 75% respectively. The average TSS removal efficiency was found to be 87, 90 and 96% at the given HRTs. 8 hr HRT was found to be the most appropriate in almost all cases. The average values of BOD reduced from 274.6 to 35.5 (mg/L), COD from 459.7 to 63.9 (mg/L), TSS from 528.2 to 23.4 (mg/L) and TOC from 134.3 to 33.2 (mg/L). This study proved that ASP at HRT of 8 hrs would treat the combined effluent to a level well within National Environmental Quality Standards (NEQS).