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9. ENVIRONMENTAL REPOFrr To provide a basis for comparison to System Alternative I and System Alternative 4, the features that would be added to the WPCF and the wastewater treatment process are described below. An analysis of the types of potential environmental impacts .under MWMC's preferred alternative are presented in sections 9.4.2 through 9.4.10. Water Pollution Control Facility Under the parallel primary/secondary treatment alternative (PP/ST), the peak wet weather flow capacity of the WPCF would increase from approximately 160 mgd to 300 mgd. All wastewater entering the plant would receive preliminary treatment (grit removal, etc.). Additionally, a new tertiary treatment system would be added with capacity to treat 30 mgd of secondary-treated wastewater. Under this alternative, wastewater treatment would vary depending on plant influent flows, as follows. 0-137 mgd: Total influent flows to the plant of 0-137 mgd would undergo the same series of treatment processes currently used to treat plant flows: preliminary screening followed by primary treatment, secondary treatment, and disinfection prior to discharge. · 137-160 mgd: With plant influent flows of 137-160 mgd, preliminary-treated wastewater (0-23 mgd) would bypass primary treatment and go directly to secondary treatment. · 160-300 mgd: When plant influent flows totaled 160-300 mgd, 23 mgd would bypass primary treatment and go directly to secondary treatment, as described above. The rest of the flow above 160 mgd (0-140 mgd) would undergo primary treatment and bypass secondary treatment, heading to high-rate disinfection. Table 9.4.1-1 describes how the influent would be routed under this project alternative. TABLE 9.4.1-1 Parallel Primary and Secondary Treatment FI0w Strategy MWMC Facilities Plan, Eugene-Springfield Plant Total Flow Through Preliminary Treated Primary Treated Primary Treated Effluent Influent Secondary Effluent to Secondary Effluent to Secondary Diverted Around Flow (mgd) Treatment Treatment Treatment Secondary Treatment 0-137 0 increasing to 137 0 0 increasing to 137 0 137-160 137 increasing to 160 0 increasing lo 23 137 0 160-297 160 23 increasing to 160 137 decreasing to 0 0 increasing to 137 297-300 160 160 0 137 increasing to 140 The diverted primary effluent and secondary treated effluent would be disinfected in separate flow streams and blended together prior to being discharged into the Willamette River. Implementing PP/ST at the WPCF would involve constructing the following features (Figure 9.4.1-1): · Pretreatment expansion MWMC_9.0_REV14.DOC 9-9