The Use of Constructed Wetlands to Treat Effluent Mark Torczon ABSTRACT The use of constructed wetlands to treat various types of effluent is becoming an increasingly popular practice. Depending on the application, the use of constructed wetlands can be a cost effective and ecological alternative to the traditional treatment processes of today. Before one can understand the intricate reactions and processes of a constructed wetland, it is necessary to gain an understanding of the simpler components that make up these systems. A clear understanding of the elementary elements is the foundation to understanding the complexity of the constructed wetland system. The purpose of this paper is to provide the foundation to understanding constructed wetland systems by providing an overview of the components of a constructed wetland, as well as descriptions, advantages, and disadvantages of the different types of constructed wetlands. This paper will also discuss design considerations, applications, and limitations of the constructed wetland systems as well as recent research findings concerning the performance of these systems under varying conditions and circumstances. KEYWORDS Constructed wetland, surface flow (SF), subsurface flow (SSF), substrate,. INTRODUCTION The first constructed wetland was built in Australia in 1904. Even though constructed wetlands have been around for over a century, the advancement in technology within the field, and the widespread use of these systems was incredibly slow. Today there are thousands of these systems in place all over the world, and the use of constructed wetlands over the traditional treatment processes is becoming an increasingly popular idea. The increasing popularity can be attributed to the numerous benefits derived from the use of constructed wetlands. Similar to natural wetland systems, the benefits of constructed wetlands include: wildlife habitat, flood control, and improved water quality. Wetlands are considered one of the most fertile and productive ecosystems next to rainforests and coral reefs (Ramsar, 1971). Their implementation and integration is the next logical step in providing a world where human kind and nature can coexist symbiotically. HYDROLOGY, THE CRITICAL DESIGN ASPECT Hydrology is the most critical aspect one must keep in mind when designing a constructed wetland. It is the primary factor that determines whether a constructed wetland succeeds or fails. A balance between water entering and water leaving must be reached in order to maximize the efficiency of a wetland. Simple water balance equation: S=Q+R+I-O-ET (Davis) S= Net change in storage Q= Water contributed due to surface flow, which includes the inflow of storm/wastewater R= Water contributed due to rainfall I= Net infiltration O= Outflow ET = loss from evapotranspiration Above are the variables one must take into account when designing a constructed wetland. However the listed items above are the basic considerations. The climate of the geographicalarea also plays an integral role in the design of constructed wetlands. Seasonal fluctuations in precipitation and temperature need to be accounted for. Increased rainfall and rapid snowmelts can lead to a higher velocity and volume of flow. This, in turn, increases the hydraulic loading rate and turbidity of the water while reducing the hydraulic retention and contact time. This can result in a drastic decline in the treatment efficiency of a constructed wetland. Processes within a constructed wetland continue even in freezing temperatures. As long as the water within the basin does not freeze solid, sedimentation, decomposition, and microbial activity continue. The speed at which these processes occur however is substantially slower. It is important to take corrective action and implement countermeasures to maximize the efficiency during adverse weather conditions. A common practice is to raise water levels prior to freezing weather. Once a layer of ice forms on the top, the water level is dropped. This allows space for under-ice flow. Another common practice is to increase the size of the constructed wetland, or store the effluent in a pretreatment unit prior to, and during, winter months (Davis). This ensures that the effluent receives an adequate amount of retention time within the wetland. As stated above, snowmelts and heavy rains associated with spring time have a tendency to overload a constructed wetland and prevent proper treatment. Accounting for these natural events minimizes possible inconsistencies associated with constructed wetland treatment and maximizes the treatment capabilities. COMPONENTS OF A CONSTRUCTED WETLAND The design components of a constructed wetland are unique to its location. Once the hydrological design aspects are complete, the design of the actual structure can begin. A constructed wetland is made up of three components: 1. A basin containing water 2. A substrate 3. Plant life A basin can be constructed by using the topography of the land and various grading operations. During the construction of a wetland it is important to remember that the soil lining the wetland be relatively impermeable. This will keep the effluent collected within the basin from seeping into the ground. Lastly, the location and size of the basin should be relative to the topography of the land and drainage area, meaning the wetland should be in a location where it can effectively collect and accommodate the amount of effluent collected. (Davis) Substrates used in a constructed wetland are dependent on the site location and function of the constructed wetland. Various soil, gravel, sand, rock, and organic material are typically used as substrates. Selecting the proper substrate is a very important decision when designing a constructed wetland for a variety of reasons: (Davis) 1. Substrate permeability regulates the movement of water through the wetland. 2. Many chemical and biological transformations take place within the substrate 3. Substrates provide storage for many contaminants 4. In a saturated substrate, water replaces the atmospheric gases in the pore spaces and microbial metabolism consumes the available oxygen. Oxygen is then consumed faster than it is replaced by process of diffusion from the atmosphere thus making the substrates anoxic. This reducing environment is crucial for the removal of pollutants. There are