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On-site sewage treatment developed

By Stewart MacInnis

Spectrum Volume 19 Issue 10 - October 31, 1996

Much land currently unsuitable for home building could be opened to development in an environmentally sound manner with a supplemental wastewater-treatment system being tested at Virginia Tech.

"State-wide, approximately 60 percent of the soil is unsuitable for conventional systems because of some soil limitations," said Ray Reneau Jr., professor of crop and soil environmental sciences. "Nationally, that figure is similar. We are trying to develop technologies that can be used in soil that is too shallow, too permeable, or has other limitations."

Reneau and Professor Chuck Hagedorn are developing criteria for using "constructed wetlands" to supplement conventional on-site wastewater treatment systems, such as septic tanks. Those criteria, he anticipates, will be considered for implementation by the Virginia Department of Health next year. That information will also be available to the appropriate regulatory agencies in other states.

"This is a relatively inexpensive, low-maintenance system for additional treatment," he said. "In the future, rapidly growing areas will be served mostly by on-site systems. Central (sewage-disposal) systems in Virginia are at or over capacity. That brings to the forefront the need for a system such as this."

The two researchers hope the system will ease pressure to develop prime farm land into homesites.

"The same characteristics that make land ideal for agricultural use also make it prized for homesites that need on-site systems," Hagedorn says. "This will offer an on-site option for areas with margin soils not suitable for agriculture that will work and will not degrade the environment."

In a traditional septic system, wastewater from a home flows into the septic tank, where most of the solids settle out. The effluent then passes to a drainfield, where it soaks into the soil and is consumed and digested by soil physical, chemical, and biochemical processes.

The constructed wetlands system takes advantage of biological processes that naturally occur in wetlands. The wetland bed is planted with aquatic plants-the Virginia Tech study is evaluating several species-that produce "a very hostile environment for fecal organisms, including those that can cause disease in humans," Hagedorn said.

At a test site at Virginia Tech's Kentland Research Farm in western Montgomery County, the effluent from a septic tank was routed into one of 16 small test wetlands. Each of the test wetlands was used to evaluate different combinations of plant species and detention times.

"We've studied a variety of detention times in the wetlands from 2.6 to 5.9 days," says Hagedorn. "Regardless of the season, all detention times in this range resulted in nearly 100-percent removal of fecal coliform. The fecal organisms passing through the wetland zone are simply gobbled up."

After passing through the wetland, the effluent then went to a facility to measure the effect soil depth had on the quality of the effluent. The study found that a soil depth of 12 inches could be safely used for the type of soil studied, compared to a soil depth of 18 inches for effluent treated only by a traditional septic system. That depth would be applicable to all but the very sandy soils.

In addition to eliminating fecal coliform, an indicator of the biological quality of wastewater, the combination of septic tank and wetland brought the levels of nutrients, minerals and heavy metals within limits.

Reneau said a typical household would use a series of two 7.5-foot by 50-foot beds of wetlands. From the wetlands, the effluent would be sent to a drainfield. He said there is no odor associated with the wetlands, and that with appropriate plant selection they can appear to be an attractive part of the landscaping around the home.

Reneau notes that homeowners are permitted to use constructed wetlands as a part of their on-site wastewater treatment system, but that current health-department rules still require traditional soil depths. The results of this study will provide a scientific basis for state health officials to revise those rules.