Onsite aerobic package treatment systems

Several studies were conducted to evaluate the performance of onsite aerobic package treatment systems (APTS), and to suggest operational and design modifications. In the first phase, five APTS in southwest Virginia were found to be operating poorly (average BOD5 = 69.4 mg/l, TSS = 99 mg/l, TKN = 18.6 mg/l), often violating Virginia effluent standards. Mechanical failures, improper maintenance and design were identified as causing the poor performance. In the second phase, excessive hydraulic retention time (HRT) in field systems (over 3 days) was identified as a significant design problem. Laboratory systems operating with aeration basin HRTs of 0.5 to 2 days produced better effluent quality (average BOD5 = 19 mg/l, TSS = 29 mg/l, TKN = 0.7 mg/l). In a separate experiment relatively high levels of laundry detergent did not significantly affect laboratory system performance. Finally, a field experiment was conducted to determine the effect of equalizing washing machine flow on the operation of an APTS. An improvement in system performance and stability was observed. Based on these studies, potential design and operational modifications are suggested, including improved maintenance, reduction of aeration basin HRT, and equalization of wastewater flow.