By Claude Goguen, P.E., LEED AP
City of Afton installs precast concrete large subsurface sewage treatment system.
Afton, Minn., is a picturesque little town located on the banks of the St. Croix River. Its Old Village area is a popular destination for tourists who enjoy the scenic beauty of the river valley bluffs, but the migration of visitors during tourism season increases demand on its infrastructure systems, including water and wastewater.
Afton did not have a centralized sewer system, so the town relied on individual septic systems to treat wastewater. Many septic systems were failing, releasing a sewage odor into the air. In addition, the flood levee in Afton contained many of the septic systems, so when the river would rise and flow over this levee, it would carry septic system waste into the streets. This spurred city officials to implement improvements that included an improved flood levee, a reconstruction of the St. Croix Trail and a new decentralized community sewer system.
The new decentralized community sewer system for Afton, a large subsurface sewage treatment system (LSTS), was designed by WENCK, a consulting engineering firm, to treat 50,550 gallons per day of wastewater. This made it the largest septic system in Minnesota. The Minnesota Pollution Control Agency (MPCA) imposed strict requirements on discharge limits prior to soil dispersal, which included a limit of 10 milligrams per liter total nitrogen. To achieve this level of treatment, the LSTS was designed to incorporate complex treatment components and processes. Wieser Concrete Products in Maiden Rock, Wis., supplied the precast concrete tanks used on this very effective LSTS.
The use of a decentralized, soil-based dispersal system is often overlooked when identifying solutions for wastewater treatment. Engineers will often default to a larger treatment facility, or in more rural areas, build a stabilization pond system. While outdated septic systems can cause problems, current systems are one of the best, sustainable and most cost-effective solutions for wastewater treatment when designed and sited correctly.
The LSTS includes primary, secondary, and tertiary wastewater treatment processes.
- Primary treatment: Primary wastewater treatment is accomplished by a series of large precast concrete septic tanks with a combined capacity exceeding 100,000 gallons. These tanks provide some treatment of raw sewage through floatation, settling, and biological and chemical reactions. Solids, organics, heavy metals, nutrients and pathogens are reduced within the tanks. Pretreated effluent from the septic tanks flows via gravity to an equalization chamber sized for peak wastewater flow. This equalization tank provides attenuation where surge flows are stored and metered out throughout a 24-hour period. Equalization is very important as downstream treatment components are nourished on a regular, timed basis instead of in large surges, which could overwhelm the bacteria.
- Secondary treatment: Secondary wastewater treatment encompasses suspended and attached-growth biological aerobic processes. One 40,000-gallon precast concrete tank contains microbubble diffusion aerators and serves as a continuous-flow, suspended-growth aerobic basin. The second stage of biological treatment occurs within a recirculating gravel filter. The RGF is an attached-growth, fixed-film aerobic treatment process. Gravel media supports the growth of biomass for purposes of both carbon oxidation and nitrification. As wastewater passes over the media, these bacteria further reduce biological oxygen demand and nitrify ammonia.
- Tertiary treatment: Denitrification is a key treatment process in the Afton wastewater treatment facility. Specifically, denitrification is the removal of nitrogen from the wastewater stream involving the conversion of nitrate to nitrogen gas under anoxic conditions. A carbon source is used to fulfill the denitrification process as wastewater will have gone through a secondary treatment, removing most organics. Also included within the facility is a suspended-growth denitrification process. To optimize nitrogen removal, nitrified effluent is recirculated back to the head of the system. This system works well to achieve the 10 milligrams per liter nitrogen limit.
- Soil dispersal: Pretreated effluent is dispersed below grade by a system of pressurized soil absorption beds. At this point, total nitrogen is treated to drinking water standards. Pretreated effluent is further refined within the soil where it infiltrates downward and assimilates within the local aquifer.
The MPCA identified over 100 small communities in Minnesota that are still unsewered. These communities discharge partially settled and untreated sewage effluent into surface waters, on the ground surface or directly to groundwater. The Old Village in Afton was one of those communities and did not receive the necessary funding from grant programs for centralized sewer infrastructure. Building this needed infrastructure was long overdue and several agencies (U.S. Army Corps of Engineers, Minnesota Department of Natural Resources, MPCA and the Minnesota Public Facilities Authority) all recognized the need and played a vital role.