Comparative Studies on the Primary Treatment of Municipal Wastewater for Ocean Disposal

Abstract

The objective of this study is to evaluate the effectiveness of primary treatment on municipal wastewater before it is discharged into the ocean. A series of filter papers of specified pore size was applied to measure the removal of organics and solids in wastewater through various primary treatment processes. For plain sedimentation, the removal of suspended solids was always less than 50% while COD and BOD removals were 23-41% and 15-27%, respectively. For chemical coagulation with the addition of poly aluminium chloride (PAC), 70% SS removal was obtained with a PAC dose of 30 mg/l. If polyelectrolyte is added (about 1 mg/l), the dosage of PAC can be reduced to around 10 mg/l to maintain the SS removal efficiency at the same level. For air flocculation treatment by preaeration followed by sedimentation, the most achievable removal of solids from wastewater of more than 80% was obtained at an aeration rate of 0.5-1.0 Nl air/l. In particular it is more effective for smaller solids than for larger solids in wastewater. On organic removal, about 15-40% removal in terms of COD or BOD was obtained by treating wastewater with either plain sedimentation or air flocculation and sedimentation processes. The efficiency of organic removal from wastewater increased to about 60% by utilizing chemical coagulation and sedimentation treatment. Based upon the characteristics of tested municipal wastewater, in order to maintain the SS removal at 65% or above as regulated, the treatment processes of chemical coagulation or air flocculation followed by sedimentation should be employed.