Resolution No. 4793 - Laserfiche WebLink

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6 DEVELOPMENT A~D EVAIUAllON OF ALTERNAtiVES 38 percent volatile solids reduction. With the comb/nation of anaerobic digestion and treatment in the FSLs, it is anticipated that VSS reduction in projected solids production through the design period will continue to exceed the 38 percent reduction requirement. 6.5.2 Anaerobic Digestion Capacity, both near- and long-term, is the primary issue to be addressed with regards to the anaerobic digesters. A lithium chloride analysis to evaluate digester capacity was conducted approximately 10 years ago. Results from the test indicated that the active volume of the digesters was 63 percent. Since that analysis, no major modifications have been made to the digesters to increase the active volume. As flows and loads continue to increase, the solids retention time in the digesters decreases. This may affect MWMC's potential to meet effluent biosolids pathogen and VSS reduction requirements. Modification of the existing digester gas rnJxing system with a pump mixing system would increase the active volume of the digesters to above 90 percent. Long-term capacity of the digesters was also evaluated. Projected upper limit flows and loads presented kt Chapter 4.0 were used to determine future capacity requirements. It was assumed that digester mixing improvements had been implemented and had improved the active volume of the digesters to 95 percent. Solids and hydraulic loading rates to the digesters were evaluated and the annual average hydraulic loading, with a 20-day solids retention time (SRT), was determined to be the limiting criteria. Figure 6.5.2-1 i~ustrates the capacity of the digesters based on processing the projected annual average flows. As seen in Figure 6.5.2-1, additional capadty is needed in approximately year 2010. The fourth digester shown in Figure 6.5.2-1 assumes a traditional mesophilic digester of the same size as the existing units. However, to provide MWMC with future biosolids flexibility, advanced digestion treatment processes were also evaluated. Alternatives were evaluated on monetary and non-monetary issues. Non-monetary issues include siting, cons~actability, performmnce, affect on downstream equipment, operat4onal flexibility, and maintenance. Processes selected for preliminary consideration were: Conventional, mesophilic digester ,, Pre-pasteurization Temperature-phased anaerobic digestion (TPAD) Table 6.5.2-1 provides a process comparison sttmmary of the three alternatives.