Analysis of Residential Subsurface Flow Constructed Wetlands Performance in Northern Alabama

Constructed wetlands are becoming increasingly popular as an alternative onsite wastewater treatment technology in many areas of the U.S. This paper documents a sample collection study and statistical analysis of five such residential constructed wetlands over seasonal variations. Results of the data analysis show that constructed wetlands can reduce organic content and coliform appreciably, an average of 85 percent and 90 percent respectively. The operational parameters obtained from this study are being used by local and state health departments for developing future design guidelines. Kathleen M. Leonard Ph.D., P.E. One of the major causes of nonpoint source water pollution may be poorly operating wastewater treatment procedures, including conventional residential onsite systems. This problem is immense since over 27 million housing units in the U.S. are served by conventional onsite septic systems, and it has been documented that 2.5 million of these systems were malfunctioning (Eddy, 1999). in response to the need for practical treatment alternatives, many communities are investigating constructed wetlands (CWs) for these areas. However, there seems to be a reluctance to design and permit these innovative natural systems due to lack of knowledge of their performance and long-term viability (Cole, 1998). Constructed wetlands are commonly described as either free-water-surface (FWS) or subsurface flow (SF) systems. The FWS systems have visible standing water comparable to shallow lagoons and are usually found in larger, municipal systems. On the other hand, the SF type do not have visible flow, are similar to flow through a porous media (sand or gravel), and are quite similar to other types of attached growth processes (Reed, et al., 1995). Two major design approaches are being promulgated in the U.S. for designing CWs. The first approach was developed by the U.S. Environmental Protection Agency (EPA, 1988) for large municipal wastewater treatment systems and is based on plug-flow kinetics. The Tennessee Valley Authority (TVA) developed the second model for small onsite systems (e.g., less than 76 m3 /day) with design criteria applicable for temperate climates. A recent article (Sauter and Leonard, 1997) documented differences in these design methods in residential applications. Constructed Wetland Systems in Northern Alabama It is often difficult to site conventional septic tank/leach field systems in Northern Alabama due to the heavy clay soils, limestone geology, and high water tables. This is a significant problem especially for small communities and rural areas that do not have access to sewer lines. Therefore, innovative decentralized and onsite systems are of major interest to health departments and land developers in the region. As a result, several experimental SF constructed wetlands were permitted and installed three to five years ago in suburban neighborhoods near Decatur in North Alabama. To verify performance efficiency, these constructed wetland systems were sampled for a variety of water quality parameters including biochemical oxygen demand (BOD), nitrates, ammonia, total coliform bacterial, turbidity, and solids over a range of seasons (e.g., temperatures). Water samples were taken at the influent to the cells (at hydraulic control structures), at the midpoint of two-cell systems, and from treated effluent at the end of the cell. All of the systems studied had a 1,250-gallon septic tank upstream of the wetland for initial solids and BOD removal. They were all designed for four bedroom homes using CONTRIBUTING WRITER