How It Works: Constructed Wetland
Artifical wetlands are designed to act like natural ones in treating waste water. Courtesy.
The school had already been warned several times by the National Environmental Management Authority (NEMA) to treat its wastewater before letting it out to the surrounding environment. The septic tank was malfunctioning and the effluent partially treated, and the place where it was being discharged was smelly and filthy. Seeta High School in Mukono was on the verge of serious action being taken against it by NEMA.
This was the backdrop against which the school got involved in a collaboration with two universities and a research network to construct a wetland where its wastewater would be discharged for treatment.
The project was carried out by Makerere University, University of Dar es Salaam and BIO-EARN—a biotechnology research network—supported by the Swedish aid agency—SIDA.
The project
The scientists and engineers constructed the wetland and also monitored it afterwards to ensure it was functioning well, the school’s plumbing system was retrofitted to align it with the new set up and a new septic tank was also constructed.
The wastewater and sewerage flows into the septic tank and the effluent from the septic tank then flows to the constructed wetland, where it is treated.
The wastewater treatment system at Seeta High School has attracted interest from similar institutions, which have expressed willingness to install it on their campuses to deal with the same kind of issue that Seeta High School was facing.
Suitable disposal of wastewater depends on this prior treatment and there are a variety of methods to remove harmful micro-organisms and prevent contamination. Usually it will involve collecting the wastewater in a single location to undergo various processes. Organic matter is removed by physical filtration within the gravel and plant roots but is ultimately removed through biodegradation. Biological treatment may be anaerobic (as in a septic tank where there is little or no oxygen) or aerobic, with oxygen from the atmosphere and from the plant roots.
Since there are often large volumes are involved, these processes are conducted on continuous flow rather than at intervals. While most processes are continuous flow, certain operations are done at specific periods.
Wastewater treatment can be categorised by the nature of the process used, for example, physical, chemical or biological.
A complete treatment system may consist of the application of physical, chemical and biological processes to the wastewater just like the wetland constructed at Seeta High.
Constructed wetlands are shallow (usually less than one metre deep) ponds planted with aquatic plants such as papyrus, water lilies, algae and duckweed.
There are four parts: the liner, distribution media, plants and underdrain system. The liner keeps the wastewater in and groundwater out.
Polyvinyl chloride (PVC) is the most common and reliable material for liners. Clay liners are not recommended because they can crack if too thin, allowing untreated wastewater to move into the soil and contaminate groundwater.
The types
There are cells connected to a septic tank and a mechanism for returning the treated wastewater to the environment.
The wetland cell is a bed of graded stone where the plants are grown. The bed is usually gravel but can be any material that resists corrosion or being dissolved by effluent. For instance, limestone is not advised because it breaks down in the acidic conditions of the cell. The cell should have a length-to-width ratio to ensures proper water flow. If it must be enlarged, the ratio should be kept constant as well.
There are two main types: surface-flow and sub-surface constructed wetlands. In surface-flow types, wastewater flows on top of existing soil. They are more economical for treating large volumes. In the sub-surface, wastewater remains beneath the surface. This controls mosquitoes and offers little chance of human contact with the wastewater.
Size and time
If the site characteristics are suitable, effluent flows to a drainfield. If the soil is unsuitable for a drainfield, the effluent goes to a polishing lagoon or wildlife habitat pond, where further treatment occurs. Here, effluent either evaporates into the air or seeps into the soil.
The size of the system is based on temperature, which affects the pace of the processes, and the amount of effluent is determined by the size of community or facility it serves. This information is used to determine hydraulic retention time—the time effluent needs to stay in the wetland for proper treatment. The longer it stays, the more time that the micro-organisms and plants have to act on it.
This treatment system is appropriate for institutions in areas not connected to a central sewerage system. There should enough land available for construction. The advantage is it can be designed as an integrated system whereby the effluent can be used in agriculture, aquaculture and agro-forestry.
