MEETING SSO TREATMENT REQUIREMENTS WITH A HIGH RATE PROCESS TRAIN: PILOT RESULTS LEAD THE WAY

To achieve regulatory compliance associated with eliminating sanitary sewer overflows the City of Salem must increase its peak wastewater treatment capacity to over 300 mgd. To meet this requirement in a cost-effective manner the City is considering installation of a remote process train in place of expanded conveyance and conventional secondary treatment. The treatment train under consideration consists of preliminary treatment and high-rate clarification coupled with UV disinfection. As a first step in the investigation of this alternative, Salem has completed two years of wet weather pilot testing at its treatment plant. The primary goal of the program was to demonstrate the process train's ability to achieve secondary equivalent treatment. Secondary equivalence will need to be demonstrated in order for the process to be permitted by the State of Oregon Division of Environmental Quality, and is expected to be tied (in part) to percent removal of BOD and TSS. Two high-rate clarification processes and two UV systems were piloted. Results of the pilot programs indicate that effluent produced by a peak excess flow treatment facility would be equivalent to effluent from conventional secondary processes treating similarly dilute flow. Greater than 85 percent removal of TSS and 60 percent removal of BOD was consistently achieved. In addition, when disinfecting effluent from a high-rate process, E.coli limits of less than 126 per 100 mL can be achieved at UV doses between 30 and 40 mJ/cm2. Based on these testing results the City is prepared to begin permitting and predesign efforts for a full scale facility that will treat up to 160 mgd of dilute wastewater at a point where SSOs are currently discharged.