Research Indicates Wetland Used With Batch Dosing Produces Cleanest Water From Septic Tank

Dr. Joe Yelderman, professor of geology at Baylor, and Dr. Margaret Forbes, research associate of biology at Baylor, constructed five different submerged gravel wetlands and tested the contaminant-removal ability of each wetland against different dosing systems, ranging from a continuous dose to a more rapid batch dose coming out of a septic tank. The submerged wetlands rely on the gravel and plants to remove contaminants by mirroring the pollutant removal ability of nature.

“There are a lot of places where it would be nice to build a home, but if you can't put in a septic tank because the soil can't handle a drain field, you can't build a home there unless you have some sort of alternative treatment system,” Yelderman said. “Our goal was to improve the water quality coming out of the septic tank so residents could dispose of the treated wastewater into thinner soil or places where the water table is higher. It would just provide more options to them.”

In Texas, state law requires treated wastewater from a septic tank must be disposed of in the soil, however traditional septic tanks need a certain depth of soil and a certain type of soil to meet environmental standards. Once treated wastewater – known as effluent – leaves a residential septic tank, it flows into what's called a drain field, which is an arrangement of perforated pipes that carry the effluent into the soil. In theory, the soil will further decompose the effluent, making it safer for the environment. However in many areas, the water table is either too high, which means the effluent does not have a chance to fully decompose, or the type of soil can not adequately absorb the effluent, which is the case around much of north and central Texas. The end result produces contaminants like phosphorous and nitrate entering the groundwater.

After several tests on the wetlands to see what dosing system works the best with a specific wetland, the Baylor researchers found that the wetland with gravel and plants performed better, or discharged water that was cleaner, during batch dosing when compared against more continuous dosing. Yelderman said he believes the batch system performed better because of the interaction with the air in between the dosing. When the wetland dried out and was then re-wetted, the gravel and plants oxidized the wastewater better and allowed the aerobic bacteria to better decompose the organic matter. Yelderman said this process actually stressed the plants and they did not grow as large, but they adjusted to the fluctuations and sent their roots deeper.

The results also showed that the wetlands with a certain type of gravel – an expanded shale aggregate – did not perform as well as expected, however it performed as well if not better that just using “regular” gravel. Yelderman said the results also show that the majority of the wetlands significantly reduced Biological Oxygen Demand (BOD) and successfully reduced nutrients like phosphorus and ammonia.

The research was funded by the Texas Onsite Wastewater Treatment Research Council and was completed at the Baylor Wastewater Research Program research site located at the Waco Metropolitan Area Regional Sewerage System.