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City of Biscay Wastewater Treatment Upgrades

IntroductionThe City of Biscay (City) is located approximately five miles southeast of Hutchinson along StateHighway 22 in McLeod County, Minnesota. The population is 113 (2010 US Census). Historically,wastewater infrastructure consisted of septic tanks at each connection that discharged settled waste-water to a common sewer within City right of way. Untreated wastewater would flow via gravitywithin the sewer and ultimately straight-pipe to the South Fork Crow River; a tributary of theMississippi River. A handful of properties within the community were served by individual subsurfacesewage treatment systems (ISTS).

The City was aware of the pollution created by their wastewater infrastructure and in 2010, Wenckwas retained to assist in finding a solution. The City worked for five years with the MPCA, McLeodCounty, Midwest Assistance Program (MAP), Minnesota Public Facilities Authority (PFA) andWenck to evaluate potential improvements and procure funding for the project. System designoccurred in the spring and summer of 2014 and construction soon followed.

Construction of a community midsized subsurface sewage treatment system (MSTS) occurred in fall2014. The MSTS consists of a combination conventional gravity and grinder pump pressure collec-tion system conveying raw sewage to a lift station. The lift station conveys wastewater to the treat-ment site approximately 2/3-mile northeast of Biscay. The treatment system consists of septic tanks, adose tank, and a multi-celled mound soil dispersal system. The MSTS is designed to treat 9,600 gal-lons per day and serves 29 properties within the City. The remaining properties, not connected to theMSTS, are served by conforming individual septic systems.

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City of Biscay Continued

Background InvestigationPrior to implementing a solution, an assessment of the preexisting wastewater infrastructure wasrequired for potential future funding. A community assessment report (CAR) was prepared by Wenckin 2010 to determine the condition and probable compliance status of preexisting individual subsur-face sewage treatment systems (ISTS). The CAR was made possible through the Small CommunityWastewater Technical Assistance Grant from the PFA. These grants are available to unsewered com-munities to examine possible solutions to wastewater infrastructure problems associated with noncom-plying ISTS. A field assessment was completed and ISTS were inspected on 51 parcels. Wenck found75% of the preexisting ISTS to be an imminent threat to public health or safety or failing to protectgroundwater.

Also within the CAR, Wenck analyzed three wastewater treatment solutions including ISTS replace-ment on each lot, a communal system to serve select service areas, and regionalization to the City ofHutchinson. The analysis was based off monetary and nonmonetary factors and helped aid the City’sdecision. The City selected to pursue a communal collection and treatment system to serve selectparcels within the service area. Parcels not connected to the community system were upgraded withcompliant ISTS.

Collection SystemThe collection system is a conventional gravity sewer in combination with low pressure grinder pumpstations. A 10-inch diameter mainline was installed and homes directly adjacent the mainline wereconnected. Homes not adjacent to the mainline, where gravity sewer was unfeasible, are served byindividual grinder pump stations.

Wastewater flows within the main sewer line to a lift station located at the northern edge of town.The new 6-foot diameter lift station is 18-foot deep and consists of duplex 3-inch solids handlingpumps, guiderails, and a valve vault basin. The structure interior is coated to prevent concrete deteri-oration. The lift station conveys wastewater to the treatment system north oftown. Continue - Page 18

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Wastewater Treatment SystemWenck and the City evaluated several options for sites of the new treatment system. Field investiga-tions were conducted to determine the adequacy for soil dispersal systems. The site that was ultimatelyselected showed favorable soil characteristics to accommodate the treatment system. Field investiga-tions were conducted by Wenck soil scientists and consisted of soil test pits, hand augered borings,and hydraulic conductivity testing. In addition, piezometers were installed to determine the physicaldepth to static groundwater and groundwater flow direction. The site was concluded to be favorableand was approved by the MPCA and McLeod County.

The treatment system serving the City is anMSTS. Following MN Rules, four days ofdetention time is required of septic tank vol-ume. To accomplish this, four 12,000 gal-lon precast concrete tanks were installed inseries. All tanks are insulated, contain aninlet baffle, outlet baffle, and access risersbrought to grade. The last tank outlet isequipped with an effluent screen that pre-vents suspended solids greater than 1/16-inch from exiting the tank. In addition, theexterior walls and base are coated with awaterproofing membrane.

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City of Biscay Continued

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Incoming wastewater flow fluctuation occurs throughout the day as people use water. To accommo-date for these peaks, the dose tank was sized as an equalization tank. The tank is large enough toattenuate or store the surge flows that occur throughout the day. The tank is insulated and equippedwith eight submersible dose pumps on guiderails and timed dosing float switches. Forcemain fromeach pump is installed with positive slope to allow complete drainback and pipe evacuation through aweep hole within the tank to prevent freezing.

Final effluent dispersal to the soil is accomplished by an 8-celled mound soil dispersal system totalingover 900 feet. Each cell contains a 10-foot wide bed containing rock, a manifold, and three distribu-tion laterals. Lateral ends are equipped with flushing boxes to allow the operator access for periodicflushing. Each mound cell is dosed an equal volume of effluent two to three times per day. During anormal dosing event, one pump is engaged and doses a cell. After the pump has timed out and ceaseddosing, the system alternates and the next mound cell in sequence is ready to dose. This cycle repeatsitself throughout the day as water is generated by the City and conveyed to the treatment site.

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City of Biscay Continued

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City of Biscay Continued

AcknowledgmentsMany thanks to past and current members of the Biscay City Council and staff, MN Public FacilitiesAuthority, Midwest Assistance Program, McLeod County, Board of Water and Soil Resources,MPCA, Hjerpe Contracting, and all the subcontractors and equipment suppliers that were involved inthe project. For questions about the project, contact Peter Daniels, Wenck, at 763-479-4284.