Resolution No. 4793 - Laserfiche WebLink

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hlW~IC ~ACILFfIES PLAN detection and reduction projects, executing these projects according to the schedule recommended in Figure .6.1.2-2. 6.2 Wastewater Treatment Plant Liquid Stream Treatment Alternatives 6.2.1 Secondary Treatment Enhancement Secondary treatment facilities at the WPCF consist of biological treatment and clarification. Two 8.9-million gallon aeration bash'ts each with 4 cells (2.2-mill~on-gallons each), eight 130-foot-diameter secondary clarLfiers, an aeration system, and a PAS pump station provide biological treatment. Table 3.2.1-3 summarizes the existing secondary treatment unit processes and equipment. The capacity of the secondary treatment facilities is dependent on both the aeration basins and secondary darifiers working together. Either facility can limit capacity depending on the mode of operation and seasonal effluent limits. The original facilities could operate ~n four modes: plug-flow, step-feed, complete mix, and contact stabilLzation. The contact stabilization mode provided the highest capacity rating for the facility. The ex~sting original maximum month dry weather capacity was estimated at 49 mgd and the maximttm month wet weather capacity was estimated at 75 mgd. The existing secondary treatment capacity noted above was estimated prior to the dry weather nitrification reqmrement imposed by the 2002 NPDES permit. The new effluent ammonia limit requires a monthly average of 12 mg/L and a maximum day of 22 mg/L from May I through October 31. Current maximum month dry weatLher capacity assessments accounting for nitrification estimate the secondary treatment capacity with one aeration basin out of service and operating in a plug-flow mode at 35 to 40 mgd. High flows occurring in May, June, and late October combine with lower wastewater temperatures, making it difficult to achieve the monthly average ammonia limit. In addition, the higher solids loading to the secondary clarifiers as a result of nitrification make it difficult to achieve the low 10 mg/L monthly TSS concentration limit. Wet season (November 1 through April 30) operations do not require nitrification. The maximum month wet weather secondary treatment capacity is estimated at 75 mgd. The peak diurnal wet weather secondary treatment capacity has been assessed at 103- 111 mgd, limited by the secondary clarifiers' inability to provide sufficient effluent quality at high surface overflow rates. Historically, wet weather flows in excess of 103 mgd are diverted around secondary treatment and blended with primary effluent. Current and historical peak wet weather flows exceed 200 mgd. Exist~g secondary treatment capacity will need to be expanded to accommodate the future dry weather and wet weather peak day, peak week, and maximum month flows. In addition to providing a base capacity increase, this will also increase the facility's ability to handle peak wet weather flows. Alternatives have been developed to evaluate the most effective expansion approach to meet the future needs of the facility. These alternatives are presented below.