April 2007, Vol. 19, No.4

Problem Solvers

Designed With Expansion in Mind

Problem: Onsite septic systems are no longer viable for a rapidly growing city.
Solution: Subsurface groundwater disposal proves both practical and cost-effective.

Four Corners, Mont., located just west of Bozeman in popular and rapidly growing Gallatin County, overcame several hurdles in an effort to improve its wastewater management and allow for future development. Since Four Corners is not located within an incorporated area, there were no significant public water and wastewater systems in place. Most residents used onsite wells and septic systems. 

In 2002, a group of area property owners formed the Four Corners County Water and Sewer District to provide water and wastewater services to their subdivision projects and adjoining properties. Shortly after its forming, the district was sued by a few residents who claimed they were treated unfairly. As a result, the district lost the ability to finance the public water and wastewater systems through bonds. At that point, Utility Solutions LLC (Bozeman, Mont.), a privately owned public utility, was formed to plan, design, construct, and operate the new public water and wastewater infrastructure. 

A Plan Is Devised
Providing a new wastewater system involved collecting, transporting, treating, and disposing of the wastewater in the area. The district developed a facility plan to predict wastewater flows for the next 20 years. The plan provided for flows up to 2270 m3/d (0.6 mgd) that would be treated by an oxidation ditch primary treatment plant with secondary clarification, aerobic digestion, ultraviolet disinfection, and subsurface disposal in the form of rapid infiltration basins. 

By using industry standards and innovative ideas, the district developed infrastructure designs that ultimately were approved by the reviewing agencies, which included the Montana Department of Environmental Quality and the Gallatin County Health Department.

Many homeowners were deeply concerned about potential wastewater contamination that might affect their wells and the health of the nearby Gallatin River. In addition, many had their own opinions about how wastewater should be handled in Gallatin County, with some advocating a central wastewater treatment facility many miles to the northwest at the headwaters of the Missouri River. It was clear that public opinion varied and had to be taken into consideration.

The district knew that without action, development in the area would go beyond the responsible use of onsite wells and septic systems very quickly. Any other alternatives were, at best, many years away. While the debate is expected to continue for years, the first phases of the infrastructure construction moved ahead and now are complete.

The district considered a few different options for wastewater effluent disposal, including surface water discharge, spray irrigation, and groundwater discharge. Surface water discharge to any nearby waterway could be practical and probably would have been permitted, but the process would be lengthy and there would be political opposition. Spray irrigation also is viable but requires a large amount of land and can be used only during the summer growing season. Gravel trenches were considered for the subsurface discharge area, but they were not viable due to the cost of gravel and the storage dose volume requirements of the design.

The Solution
In this case, subsurface groundwater disposal made the most sense in terms of practicality, economics, and timing. Infiltrator Systems Inc. (Old Saybrook, Conn.) chambers ultimately were chosen.

In the new system, wastewater is collected via manholes and sewer piping. Then it is pumped by a series of lift stations to an upstream wastewater treatment plant for disinfection. Following treatment, the effluent is transported to the disposal site, where it is stored in two 42-m3 (11,000-gal) fiberglass dose tanks. A submersible duplex lift station also is located on the disposal site to finish moving the water from the dose tanks into the rapid infiltration basins that are composed of a subsurface chamber system. The complete wastewater system is designed for modular construction with future expansions in mind.

The basins were designed for use in gravelly soils 0.9 m (3 ft) below the ground and utilize 2592 chambers for the first 12 of the 36 potential zones. When all 36 zones are complete, there will be 6912 chambers beneath the ground surface with a capacity of 1890 m3/d (0.5 mgd). Capacity can be added in 189-m3/d (50,000-gal/d) increments according to the facility plan. A standard zone comprises four laterals with a length of 66 m (218 ft) each and a discharge rate of 2.3 m3/min (617 gal/min). 
The rapid infiltration basins work in conjunction with other important components to satisfy permitting requirements and provide efficient, reliable wastewater effluent disposal. They include a pressure-dosed design with automated butterfly valves operating each of the zones individually. High-water alarm floats connect to the chambers, flow metering, and a supervisory control and data acquisition system. This allows operators to monitor the system remotely, keeping them up to date on everything that happens on the site.

The disposal system currently is equipped for flows of 757 m3/d (200,000 gal/d). Ultimate buildout predictions indicate that by 2026 the system could be serving 6000 homes and handling flows of 5678 m3/d (1.5 mgd). Growth in the Four Corners area already is approaching the original facility plan flow predictions of 2270 m3/d (0.6 mgd).  The utility is in the process of expanding its service area to reach more properties that are beyond the boundaries of local municipalities and water and sewer districts. Future construction phases include the subsurface chamber rapid infiltration basins. 

The Four Corners area now has a viable, safe, modern, and dependable means to handle wastewater for those who wish to participate.