SSO ELIMINATION THROUGH EXPANDED PRIMARY TREATMENT CAPACITY AND BLENDED EFFLUENT

In April 1995, the City of Rocky River, Ohio, received orders from Ohio EPA to eliminate all bypassing at the headworks of the wastewater treatment plant by 1 April 2000. The treatment plant serves a separate sanitary sewer system with an average flow of approximately 11 mgd; however, wet weather flow peaks can be greater than 100 mgd due to I/I. The currently operating plant was designed some 15 years earlier with Ohio EPA approval to treat peak flows of 45 mgd and bypass the remainder after screening. To comply with the orders, six years of bypass data was examined and adjusted to correct for inaccurate measurements to develop the required facility sizing. Alternatives were analyzed, and costs were compared. The selected alternative was to expand the primary treatment capacity to 128 mgd and then blend the primary effluent with the secondary effluent to meet all NPDES permit requirements. New grit removal and primary clarifiers were constructed with provisions to chlorinate and dechlorinate the primary treated wastewater that is blended with the secondary effluent. The new facilities were designed to minimize the impact on dry weather operation and to require minimum operator attention during high flow events. The new primary clarifiers are equipped with Tipping Bucket Sediment Flushers instead of continuous sludge collectors to simplify operation and to improve tank cleaning after the flow subsides. Since the system has been in service, there have been no headworks bypasses and no effluent limit exceedances.