Subsurface Flow Constructed Wetlands
By Kellie Potucek
As the population in Texas’ rural and suburban regions rapidly increases, there is an escalating need for permitted onsite wastewater treatment systems. In order to qualify for a permit, stringent standards must be met to guarantee the quality of water released from a system.
In loamy soil areas, conventional treatment of wastewater via a septic tank and soil absorption field, or drain field, is generally adequate. In this type of system, wastewater enters the watertight septic tank, where solids separate out and are partially digested by microorganisms. The wastewater then empties into a soil absorption field through perforated pipes for final treatment and distribution.
“This system relies heavily on the soil, where microorganisms help remove organic matter, solids and nutrients left in the water,” explained Bruce Lesikar, extension specialist of the Texas A&M University Department of Biological and Agricultural Engineering.
However, many areas of Texas have soils that do not sufficiently absorb and treat organic and microbial pollutants. In these regions, drain fields are required to be very large, making them uneconomical. Therefore, these areas require additional secondary or advanced treatment of effluent.
When properly built, subsurface flow constructed wetlands (SFCW) are a means of providing necessary additional processing. In systems with SFCWs, a septic tank is used for initial treatment. Then, wastewater travels through a header and into the wetland for further cleansing. Graded stones are typically used to construct the bed of the wetlands and aquatic plants are introduced. SFCW vegetation naturally filters suspended solids, organic matter, excess nutrients and harmful pathogens, leaving treated wastewater ready for dispersion into an average size soil absorption field.
While SFCWs have become increasingly popular for domestic wastewater treatment, there has yet to be a standard model from which they can be designed. The United States Environmental Protection Agency’s (EPA) most precise model suggestion is that the organic matter, or biochemical oxygen demand, should have an area loading rate of 6 g/m2 per day with a maximum effluent concentration of 30 mg/L. Beyond this guideline, the EPA merely makes suggestions for parameters such as depth, length to width ratio and inlet/outlet design.
Rebecca Hobbs, of the Texas A&M University’s Department of Soil and Crop Sciences, is researching individual aspects of SFCWs in order to improve the efficiency and consistency of their output. “If the effects of wetland parameters can be better understood, the design criteria for those parameters can be further developed and optimized,” explained Hobbs.
In one aspect of her research, Hobbs is investigating how the method of wastewater introduction into the wetland affects the efficacy of treatment. There are currently several header designs that transfer wastewater from the septic tank into the wetland. Hobbs is focussing on two header designs: the traditional perforated pipe and the more contemporary leaching chamber. In the perforated pipe design, a slotted pipe is used in an effort to evenly distribute effluent into the wetland. The leaching chamber design involves a plastic compartment that allows a place for organic material to collect before it releases water into the wetland.
Early results of Hobbs’ bromide tracer studies indicate that leaching chambers do not evenly distribute water through the wetland. In one case, her team removed a chamber, replacing it with a perforated pipe that spanned the entire width of the wetland. “We saw immediate improvements in the quality of water coming out of that wetland,” reported Hobbs.
Hobbs is also testing several length to width ratios, also referred to as aspect ratios. Specifically, Hobbs is testing 2:1 versus 1:1 versus 4:1 aspect ratios in an effort to determine whether wetlands work better if they are long and narrow or short and wide. She is currently in the data collection stage of this aspect of the study.
Hobbs supports the use of SFCWs as a low cost operation that helps guarantee water quality. Her research will help standardize optimal construction of these wetlands, providing new rural and suburban homeowners with an economical and efficient method to treat their domestic wastewater.
