6.5. SR 05-16-2005 - Laserfiche WebLink

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1.0 EXECUTIVE SUMMARY The City of Elk River is a quickly growing municipality located northwest of Minneapolis, Minnesota. Wastewater generated by the City is treated through processes that include preliminary, primary, secondary treatment, and disinfection. Secondary treatment is accomplished using a trickling filter with provisions for upgrading to activated sludge. Treated wastewater is discharged to the Mississippi River. The Minnesota Pollution Control Agency (MPCA) is the regulatory agency responsible for Elk River's National Pollution Discharge Elimination System (NPDES) permit. The MPCA has classified the Mississippi River as an Outstanding Resource Value Water (ORVW) at the reach to which the City of Elk River discharges. Stricter standards apply to ORVW's to protect the receiving water. The MPCA requires evaluation of alternative discharge and advanced treatment options to ensure protection of the Mississippi River at this reach. Mass loading to the river, with respect to organics and suspended solids, has been fixed since 1984. Effluent loading for five-day, carbonaceous biochemical oxygen demand (CBODs) is fixed at 216 pounds per day and total suspended solids (TSS) is fixed at 260 pounds per day. More advanced treatment is needed to meet the mass loading requirements. New permit conditions have been suggested as highly probable by the MPCA. Phosphorus concentrations in the effluent are to be reduced to less than 1 mgP/L, and mercury concentrations are to be reduced to less than 6.9 ng/L. A facility plan has been prepared to assess the wastewater treatment processes, analyze the sanitary sewer collection system, and provide recommendations for upgrades to meet future flows, loadings and permit requirements for operation through the year 2025. The existing plant was evaluated to determine the condition of the equipment and tanks. Wastewater flow was characterized and future flows and loadings were projected. Three areas of development have been identified for trunk sanitary sewer expansions, and new flows from those areas have been identified. Based on projections, the plant must be capable of treating an average wet weather flow of 3.7 million gallons per day (mgd) with a peak hourly flow of 7.0 mgd. Organic loading to the facility was projected based on typical residential, commercial and government contributions. Industries are major contributors of organics to municipal wastewaters, and a typical industrial contribution was incorporated into the wastewater projections. A number of upgrades to the facility are needed to ensure successful treatment of increasing wastewater flows in accordance with the anticipated permit requirements. Improvements are needed for preliminary treatment, primary treatment, secondary treatment, and disinfection. Additionally, a phosphorus removal system is necessary to meet the new 1 mgP/L, and filtration is required to polish the treated wastewater to meet strict requirements for CBODs, TSS, phosphorus, and mercury. The MPCA mandates a nondegradation review to be performed for treatment facilities that discharge to an ORVW. Alternatives have been evaluated as part of the nondegradation review for Elk River. Advanced treatment processes are a practical option that can be implemented at the treatment facility at a consequential expense. Specifically, sand or membrane filtration are options that would polish the wastewater to meet the requirements for an ORVW. An analysis shows that Elk River wastewater, treated to secondary standards, would have little effect on the Mississippi River with respect to CBODs and TSS due to the high dilution provided by the river. Phosphorus 1-1