COMPREHENSIVE EVALUATION SOLVES WET WEATHER TREATMENT CHALLENGES

Toledo, Ohio signed a Consent Decree with USEPA/OEPA to settle an 11-year dispute over wet weather bypassing at the City's Bay View Wastewater Treatment Plant (WWTP) on December 15, 2002. Facilities required by the Consent Decree to be designed and constructed to mitigate wet weather bypassing were sized based principally on studies completed in 1997. During Consent Decree negotiations, Toledo continued to implement planned combined sewer overflow control and wastewater treatment plant upgrade projects. By the time the Consent Decree was signed, many of these projects were completed and had resulted in 1) reduced peak flows delivered to the wastewater treatment plant, and 2) an increase in the plant's capacity to process the peak flows.A systematic evaluation was conducted in 2003-4 to ensure that Consent Decree required wet weather treatment facilities were still appropriate to meet the primary objectives of the consent decree: 1) provide secondary treatment for 99.5 percent of all flow delivered to the Bay View WWTP in a typical year; 2) provide high rate chemical treatment of all secondary bypass flow up to a total flow of 400 mgd; 3) meet existing permit limits with the combined discharge of secondary and wet weather treated effluent. The evaluation covered the period from 1991-2003. A key component of the evaluation was a flow simulation model that was developed to route wet weather flow through secondary treatment, flow equalization, and wet weather treatment facilities to predict treatment loadings and expected performance. The simulation model also determined pumping rates, chemical requirements, and solids production for the high rate chemical treatment facilities.Based on the new information provided by the above evaluations to the regulatory agencies, the City of Toledo has obtained permission to proceed with the following:Construction of only 25 MG (instead of 60 MG) of flow equalization until and unless it is determined by the LTCP that more storage is needed.Deferral of construction of the additional wet weather Secondary Clarifier until and unless it is determined by the LTCP that it is needed.The flow simulation model has proven to be of great value for simulating combined discharge effluent quality; predicting influent pumping, chemical usage, solids production, and flow equalization dewatering pumping requirements; and determining probable frequency of utilization and operating protocols for wet weather treatment facilities.