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6. DEVELOPMENT AND EVALUATION OF ALTERNATIVES TABLE 6.2.5-3 Summary of Odor Control Alternatives Comparison MWMC Facilities Plan, Eugene-Springfield Capital Cost Non-Monetary Alternative (millions of dollars) Ratinga I - Conventional Open-Space Organic Media Biofilter $3.75 17 2 -Conventional Open-Space Soil Media Biofilter $4.14 21 3 - Bioscrubbers (Biotrickling Tower) $4.55 24 Notes: a Non-monetary score is out of a possible maximum score of 30 points. MWMC views odor control as an extremely important issue. One of MWMC's highest priorities is to maintain their status as environmental stewards in the community by providing facilities that are neighborhood friendly. Bioscrubber and biofilter technologies produced a nearly equivalent result in the evaluation. Considerations such as site space, more advanced technologies, and higher atmospheric dispersion make bioscrubber technology more favorable over biofilter technology. It is recommended that bioscrubbers be incorporated into the overall odor control design for future use and to replace the existing biofilter operations. The biofilters will be phased out over the next 20 years of facility operation so that they achieve their original intended design life. 6.2.6 Tertiary Filtration Unchanged dry weather mass limits will drive effluent TSS and CBOD concentrations down below the current effluent concentration limits of 10 mg/L. As effluent concentrations are driven down over time it will become exceedingly more difficult to achieve the required effluent limits unless tertiary filtration is added. Figure 6.2.6-1 show how the effluent TSS concentration limits are reduced over time as the maximum month flows increase over the planning period. Operations must provide an effluent quality that is sufficiently below the effluent TSS requirements so that any process upsets may be absorbed by the system without resulting in permit violations. Figure 6.2.6-1 shows the desired operating performance to meet this requirement. Process upsets can occur in the secondary biological treatment process during periods of operational transition and can result in poor settling suspended solids for periods sufficiently long to adversely impact the monthly effluent CBOD and TSS concentrations. It is well established that process upsets may occur during periods of significant wastewater temperature changes, or during periods when the process is transitioning from winter operations (where no nitrification is required), to summer operations (where nitrification is required). Effluent filtration can provide an effective defense against these types of situations. Filtration also provides the added benefit of operational flexibility, consistent performance reliability, and provides the opportunity to develop reuse programs. For those reasons previously outlined, it is recommended that 30 mgd of partial secondary effluent filtration be implemented over the planning period. The pre-filtered flow would be removed from the secondary effluent flow stream, pumped up to filtration for treatment, and then re-blended with secondary effluent prior to disinfection. Filtration will provide performance reliability MWMC_6.0_REV11 .DOC 6-19