Scientists investigate microbial levels near on-site wastewater systems.

Researchers at Texas A&M University (TAMU) are studying how wetlands plants and the micro-organisms associated with them may enhance the performance of constructed wetlands used for on-site wastewater treatment. The goal is to understand how plants affect the survival of human-based intestinal micro-organisms, biochemical oxygen demand (BOD), and dissolved organic matter. The researchers hope to identify the best plants for treatment and to learn how best to manage those plants to reduce the organic-matter load, nutrient content, and populations of disease-causing organisms. They also want to determine the amount of organic matter that plant-associated microbes can consume. For drip irrigation systems, the researchers also are investigating the impact of factors such as soil type, flow rate, temperature, and moisture.

The projects are being led by researcher Richard Weaver and graduate student Srinivasan Neralla of the TAMU Soil and Crop Sciences Department. Funding is being provided through the Section 319 Program of the Texas Natural Resource Conservation Commission and by the Texas Agricultural Experiment Station.

Methods

The study involves monitoring constructed wetlands and other systems used for residential on-site wastewater treatment in Bryan-College Station, Houston, and Stephenville. For example, a subdivision in rural Brazos County contains a constructed wetland, a sand filter, a drip irrigation system, and a conventional septic tank and drainfield.

Samples are being taken where wastewater enters and exits the treatment system. At some sites, soil, water, and microbe samples are collected daily for a week or so. At other locations, samples are gathered monthly. Most of the samples are taken from near the surface. The soils are brought back to the laboratory, where they are analyzed for nitrogen, as well as for microbiological types and populations. In addition, tensiometers are being used to gather information on subsurface moisture levels near the drip irrigation emitters.

Preliminary Results

Preliminary results suggest that constructed wetlands with healthy growing plants may reduce BOD levels by roughly 70 percent, in part because plant-associated microbes aid the treatment process. Already, Weaver and colleagues have evaluated the suitability of canna lilies and other plants for wastewater treatment systems.

With drip irrigation systems, higher levels of human-based intestinal micro-organisms tend to survive when soils are moist and temperatures are relatively low. The populations decline significantly when soils dry out and temperatures increase. Preliminary data indicate that populations of fecal-coliform bacteria next to underground drip irrigation emitters may be as much as 100 times higher than populations near the soil surface. As the study continues, researchers will evaluate the potential of various management strategies to lower the survival rates of these micro-organisms. Examples of such strategies include alternate wetting and drying of drip irrigation fields, adjustments to flow rates, and changes in the amount of time flows are applied.

Related Studies

Also being investigated are public health issues associated with on-site systems. This study will inventory pathogens and micro-organisms present near sites used for the treatment and disposal of wastewater. According to Weaver, the nature and extent of microorganisms near many types of on-site systems have not been studied. Gathering baseline data on microbe populations located close to on-site systems will help in managing those populations to minimize public health risks.

Bruce Lesikar and George Sabbagh of the TAMU Agricultural Engineering Department are working with Weaver to develop management strategies for drip irrigation systems, as well as innovative designs that will improve performance and reduce microbiological risks.

Additional Information

For more information, contact Richard Weaver by telephone at [phone] or by e-mail at <[e-mail]>, or contact George Sabbagh by telephone at [phone] or by e-mail at <[e-mail]>.

(Source: Texas Water Resources Institute, Texas A&M University)