Constructed wetlands for wastewater treatment in Tanzania

Adapted from full article by Masudi, A.S, Mashauri, D.A, Mayo, A.W, and Mbwette, T.S.A. University of Dar es Salaam, WSP & CW Research Project, P.O Box 31531, Tanzania

Many developing countries are presently experiencing rapid population and economic growth especially in the urban centres. The provision of services, including wastewater collection, treatment and disposal has however not kept pace with these developments. This makes it more likely that cholera and other water-borne diseases may persist. The principle constraints are the lack of financial, technical and institutional resources. The trends over the past years in construction of wastewater pollution control facilities have been toward "concrete and steel" alternatives. With the advent of higher energy prices and higher labour costs, these systems have become costly for the communities that operate them. For small communities in particular, this cost represents a higher percentage of the budget beyond their affordability.

The high costs of some conventional treatment processes has produced economic pressures and has caused engineers to search for creative, cost-effective and environmentally sound ways to control water pollution. Processes that use relatively more land and are lower in energy and labour costs are therefore, becoming attractive alternatives for many wastewater treatment applications. Natural wetlands have been used for wastewater treatment for many years. Waste stabilisation ponds (WSP) have been the treatment system favoured for a majority of the applications in developing countries especially those located in tropical climates. They include a wide range of shallow ponds, with treatment mechanisms that vary from anaerobic fermentation to aerobic mineralisation relying on sunlight to promote a symbiotic relationship of algae and bacteria. In many cases, however, the reasoning behind this use was disposal, rather than treatment and the wetlands simply served as a convenient recipient that was closer than the nearest river or other waterways. Uncontrolled discharge of wastewater led in many cases to an irreversible degradation of many wetland areas.

Wetlands have been considered for a long time as "wastelands", were scientifically neglected and, therefore, the impact of wastewater on different wetlands was not properly assessed. However, there has been an explosive growth of knowledge and radical change of attitude toward wetlands during the last few decades.  During the last three decades the multiple functions and values of wetlands have been recognised by not only scientists, but also the public. They are among the most important ecosystem on earth because of their unique hydrologic conditions and their role as ecotones between terrestrial and aquatic systems. The potential applications of wetland range from secondary treatment of municipal and various types of industrial wastewater, to polishing of tertiary treated water and diffuse pollution. Successful case studies indicate that wetlands significantly reduce suspended solids, biological oxygen demand, pathogens, heavy metals and excessive nutrients from wastewater.  In Tanzania, natural wetlands occupy over 7% of the country's surface area. Waste Stabilisation ponds are the wastewater treatment option recommended by the Government of Tanzania for treatment of industrial, municipal, institutional and domestic wastewater in water serviced areas. Usage of waste stabilisation ponds in Tanzania for wastewater treatment started in late sixties. There were twenty pond systems in Tanzania, five pond systems are used to treat textile, paper mill, tannery and other industrial wastewater' s. Unfortunately most of these Waste stabilisation ponds systems are ineffective due to over-design, poor operation and maintenance, use of wrong design criteria and configuration, mixed municipal and industrial wastes and vandalism. Four percent of the pond systems are malfunctioning and are used as animal grazing fields and other agricultural activities. Furthermore, the Waste Stabilisation Ponds are sometimes constrained by land availability, odour development when overloaded, and topography. The occasional high concentration of suspended solids is the major disadvantages of pond systems. Plagued with these problems, national research institutes become duty bound to research and redress any shortfalls.  One of the most important developments has been the introduction of constructed wetlands. World wide natural wetlands are still used for wastewater treatment but at present, the use of constructed wetlands is becoming more popular and effective around the world.

Constructed wetland treatment systems are engineered systems that have been designed and constructed to utilise the natural process involving wetland vegetation, soil and their associated microbial assemblages to assist in treating wastewater. They are designed to imitate the same processes that occur in natural wetlands.

Constructed wetlands can be built with a much greater degree of control, thus allowing the establishment of experimental treatment facilities with a well-defined composition of substrate, type of vegetation and flow pattern. In addition, constructed wetlands offer several additional advantages compared to natural wetlands, including site selection, flexibility in sizing and most importantly, control over the hydraulic pathways and retention time. The pollution in such systems is removed through a combination of physical, chemical and biological processes including sedimentation, precipitation, adsorption by soil particles, assimilation by plant tissue, and microbial transformation. Constructed wetlands can be classified according to the life form of the dominating macrophytes into: Free floating macrophytes-based systems, submerged macrophytes-based systems and rooted emergent macrophytes-based systems. Different types of macrophytes-based wastewater treatment systems may be combined with each other or with conventional treatment systems in order to exploit the specific advantages of the different systems. The quality of the final effluent from the systems improves with complexity of the facility. The most common floating aquatic plant used for treatment of wastewater is the hyacinth. They are widely used to provide nutrient removal and upgrade the performance of Waste stabilisation ponds. The use of submerged macrophytes is to provide a final polishing step after primary and secondary treatment.  The rooted emergent macrophytes are the most commonly found species in constructed wetlands for wastewater treatment. The most frequently emergent macrophytes used are reeds, bulrushes, cattails, rushes and sedges. A research team at the University of Dar es Salaam in Tanzania is pioneering research into the introduction of constructed wetlands as an alternative wastewater treatment technique. Their current research aims meant to shed light on the following areas:  · Suitable microphates for sewage treatment,  · Suitable combinations of Waste stabilisation ponds and Constructed Wetlands (CW),  · Suitable combinations of pre-treatments and CW,  · Suitable soil characteristics for CW

Here the construction, design and research of Constructed Wetlands, is expected to develop models that can predict more accurately the complex biological activities in anaerobic, primary facultative and secondary facultative Waste stabilisation ponds. These models, which take into account the interaction between algae and bacteria and important environmental conditions, are expected to be used for advancement of the process of the design of Waste stabilisation ponds. 

It is foreseen that the urban population in Tanzania will increase at a rate of more than 10% per year and thus without appropriate means of wastewater collection and treatment, more cases of cholera and other water borne diseases are likely to remain persistent in our country. Combating of frequent outbreaks of communicable diseases in tropical countries is more expensive. This could be avoided by planning for wastewater collection and treatment with Waste Stabilisation Ponds alone or in combination with Constructed Wetlands especially when re-use is given priority.

Adapted from full article by Masudi, A.S, Mashauri, D.A, Mayo, A.W, and Mbwette, T.S.A. University of Dar es Salaam, WSP & CW Research Project, P.O Box 31531, Tanzania

September 2001