Problem: Widespread failure of septic systems resulting in more-stringent permit requirements for a small town
Solution: Installing a gravity-fed sewer with an attached growth process system
With a population of 107, the small town of Mt. Union, Iowa, relied on individual onsite septic tanks for wastewater management, with no secondary treatment. But without regular maintenance, many of these outdated conventional systems failed, leading to widespread releases of untreated wastewater into a local creek.
The Iowa Department of Natural Resources (DNR) cited Mt. Union for inadequate wastewater treatment and mandated that the community make improvements. The department issued permit requirements that the town meet specific concentrations for several parameters, including average biochemical oxygen demand (BOD) less than or equal to 25 mg/L, total suspended solids (TSS) less than or equal to 30 mg/L, ammonia–nitrogen of 2.6 m/L, total nitrogen of 14 mg/L, pH readings of 6 to 9, dissolved oxygen at 5 mg/L or greater, total residual chlorine less than or equal to 0.02 mg/L, and
less than or equal to 126 organisms per 100 mL.
To meet these requirements for an anticipated 56,800 L/d (15,000 gal/d), the town sought an alternative wastewater treatment solution. When looking for new treatment options, minimizing capital and life-cycle costs was a priority because the small rural community has a median household income of just $27,500.
Also, because the town had gone without a wastewater treatment system for many years, residents viewed any wastewater bill as excessive. The new treatment system also had to use minimal land area, because the town’s land is highly fertile and used to grow crops.
Finding a system for a tight budget
In 2007, the town retained French–Reneker–Associates (Fairfield, Iowa) to evaluate its options. Using a discharge lagoon was discarded because it would have required up to 4 ha (10 ac) of land that could be farmed. Other options were eliminated because of their inability to comply with the DNR-required limits or their operation and maintenance (O&M) requirements and costs.
French–Reneker–Associates concluded that the best available technology would be a gravity sewer collecting flows from septic tanks followed by an Orenco (Sutherlin, Ore.) AdvanTex® treatment system. The technology would remove ammonia–nitrogen at an affordable up-front and long-term cost with low O&M and repair and replacement requirements, said Kent Rice, French–Reneker–Associates project engineer.
Necessary treatment with additional benefits
In February 2010, the town commissioned its solution, installing 34 septic tanks located throughout the town to feed into a gravity sewer system. The wastewater then is treated by the two-stage systemfor enhanced ammonia–nitrogen removal.
The systemis a nonsubmerged attached growth system that includes recirculation/dilution (R/D) tanks and treatment media to provide biological nitrification. A timer-controlled pump in the tanks periodically doses effluent to a distribution system on top of the media. Pumps run intermittently, resulting in energy consumption of less than 2.5 kWh per 3785 L (2.5 kWh per 1000 gal). Each time the media are dosed, effluent percolates through and is treated by naturally occurring microorganisms. Effluent collects below and is conveyed back to the R/D tank. Effluent recirculates multiple times before final disposal.
The system achieves unsaturated flow and sustained contact by distributing wastewater evenly over the surface of the medium and by keeping doses small and frequent. This also ensures that all of the media are used, preventing clogging. And because hydraulic, organic, and inorganic loads are distributed uniformly, biochemical interactions are more effective, according to Terry Bounds, Orenco’s executive vice president and the developer of the AdvanTex technology.
The system’s medium has hydraulic loading rates of 0.4 to 2 m3/m2
d (10 to 50 gal/ft2
d), depending on permit requirements and organic and inorganic concentrations, Bounds said. It also features a solids retention capability of months or years and minimizes sloughing, which reduces solids management costs.
The Mt. Union system provides reliable performance, and effluent quality exceeds the requirements. BOD is less than 10 mg/L, TSS is less than 10 mg/L, and ammonia–nitrogen averages between less than 1 and 2 mg/L (depending on seasonal temperature fluctuations), said Tyler Molatore, Orenco’s community systems program manager.
“Performance is sustained with part-time operation and relatively low operational skills and maintenance requirements. Even with highly variable flows of [18,900 to 113,600 L/d] 5000 to 30,000 gal/d and part-time operation, the system is operating well within its permit limits,” Molatore said.
The system requires no chemical additions, produces high-quality effluent that can be reused in various ways, such as for subsurface irrigation and groundwater recharge after disinfection, and for other nonpotable uses after disinfection.
Calculating costs and savings to come out on top
The system, funded in part by U.S. Department of Agriculture Rural Development grant and loan funds, included a collection system contract for $308,299 awarded to Jackson Creek Enterprises (Allerton, Iowa) and a treatment system contract for $419,275 awarded to Pilcher Construction (Ottumwa, Iowa).
The system and its components are designed to provide cost-effective operation over its lifetime while energy requirements are minimized by the intermittent use of small-horsepower pumps.
Mt. Union’s system is monitored remotely with a telemetry control panel and visited periodically, generally requiring only part-time servicing with regular inspection of the main components, cleaning of pump screens and distribution nozzles, and adjusting timer settings as needed, Molatore said. Total annual onsite maintenance amounts to about 40 hours per year for about $1500.
“Based on a 40-year present worth analysis, the total cost of ownership for the AdvanTex Treatment System was lower than the other technologies evaluated,” Molatore said. “Lower costs were largely attributed to low energy consumption and low O&M requirements.”
According to the Water Environment Federation (WEF; Alexandria, Va.) Awards Subcommittee, “Orenco’s design is cost-effective, innovative and useful for small and decentralized wastewater treatment systems. As WEF members in small communities face ever-increasing regulatory requirements, Orenco’s cost-effective, reliable AdvanTex design comes along at the right time to help produce cleaner water and meet NPDES [National Pollutant Discharge Elimination System] permit requirements.” The committee selected the system to receive WEF’s 2011 Innovative Technology Award.