2.3 Wastewater treatment using wetlands

Technical description

Untreated wastewater is usually discharged into nearby streams or water courses. It is generally assumed that the waste assimilative capacities of these natural water sources are high and can be sustained in the long term. However, as the negative effects of this waste disposal philosophy are increasing, low cost and low energy alternative systems, such as utilization of nearby wetlands, is usually indicated. Wetlands which lie in the buffer zone between the municipal areas, agricultural fields and the water courses provide a sound means for filtering wastewater before it is discharged into a river or other surface water feature. In the past, natural wetlands have been used as natural nutrient sinks for the treatment of wastewater.

Wetlands act as natural purification systems. Their hydrological regimes, sediments, and biotic components enhance the ability of wetlands to process wastewaters. Hydrological regimes are influenced by precipitation, surface water inflows, groundwater inflows, evapotranspiration, surface water outflows, groundwater outflows and changes in the water storage capacity of the system. Wetland sediments accrete carbon through decomposition of organic matter. This may result in very low oxygen concentrations within sediments. The systems exhibit very high primary production rates with the resulting organic soils having low bulk densities, high water holding capacities, low hydraulic conductivities, high organic matter contents, and extremely high caution exchange capacities (Eassan et al., 1988), retaining most of particulate organic matter produced in the wetland. Biotic components include plants, phytoplankton, invertebrates and vertebrates.

Operation and Maintenance

Operation and maintenance requirements depend on the type of the reclamation system. Pumps require monitoring and a preventive maintenance system, which requires skilled personnel, especially if there are several pumps within the system. Periodic inspections and ecological monitoring are required to ensure the quality of the output water, and to maintain the wetland vegetation.

Level of Involvement

This technology may be implemented by government agencies and communities.

Costs

The capital costs of constructing and managing a wetland treatment system vary widely according to specific local conditions. In augmented natural wetland systems, the capital costs consist solely of the cost of pipes and pumps. In constructed wetlands, land acquisition and development costs are also incurred. Easson et al. (1988), citing Tuchobanoglous and Culp (1980), provide a general guideline to the capital costs of wetland wastewater treatment, in 1980 dollars, as shown in Table 7. Costs of wetland treatment could be lower in Asia. The per unit cost of wetland treatment of wastewater, as provided by Easson et al. (1988) citing Fritz and Helle (1979), was one-half of that of a conventional treatment system. The operation and maintenance costs are also comparatively low, as wetland treatment systems require only periodic inspection and ecological monitoring. Nevertheless, the environmental investigations needed to identify the linkages between ecosystem components in the case of augmented natural wetland systems may increase the cost of implementing this technology significantly.

TABLE 7. Cost of Wetland Treatment Systems

Effectiveness of the Technology

Biological treatment of wastewater by wetlands has been found to reduce the levels of virtually all contaminants, including those present in wastewater from mines (Fenessy and Mitsch, 1991). Wetlands are effective in reducing, by up to 90%, the concentrations of nitrogen, pathogenic bacteria and heavy metals in wastewater (Easson et al., 1988, citing Rogers et al., 1985). System performance, however, is determined by various factors, including water depth, temperature, pH, and dissolved oxygen concentrations, and by the type of wetland constructed or considered for use in wastewater treatment. Natural wetlands include shallow and deep water marshes, mangrove swamps, cypress domes, tidal marshes, bogs, and peatlands. Constructed wetlands may be artificially to reflect this diversity of wetland types.

Suitability

The suitability of wetland treatment systems for wastewater management depends on a wide range of conditions. Generally, large wetlands are more suitable for use as a treatment system because of their larger surface area, greater number and variety of aquatic plants and reduced susceptibility to flooding when wastewater is applied at a rate likely to be generated by a small municipality. Larger wetlands are also more likely to be able to treat wastewater on a year round basis. Smaller wetlands may become costly in the absence of mechanisms to control the rate and volume of wastewater applications.

Advantages

Wetland systems have several distinct advantages. Natural wetlands are immediately available without further need significant for the construction of facilities. In the case of constructed wetlands, wetland treatment systems also help to create additional wetland habitat. Wetlands may also provide an opportunity for partial cost recovery through the harvest of peat or vegetation for the use in the manufacture of pulp, compost, food for livestock, or vegetative material for biogas production.

Disadvantages

Disadvantages of the systems include climatic limitations on the active growth phase of the wetland vegetation and the land area required. A cold climate can become a limiting factor for the adoption of such a technology. The technology also requires relatively large areas which may not be readily available near cities or towns. Further, wetlands can produce nuisance insects. In cases where little is known of the relationships between various biotic and abiotic components of a wetland, the effects of using the wetland for water quality management purposes on the overall ecosystem may not be readily apparent.

Cultural Aspects

Health risks, along with other cultural barriers, make it difficult for the widespread adoption of wetland treatment technologies for wastewater treatment and reuse; people feel uneasy using wetlands where wastewater is treated for other economic purposes such as harvesting of vegetation or peat.

Further Development of the Technology

There is an high potential for the further development of wastewater treatment systems based upon wetlands in many parts of Asia. This potential can contribute to the reuse of wastewater in those areas where there is a growing demand for water. However, a cost effective means of pretreating wastewater to reduce pollution levels prior to discharging it to wetlands should be found.

References

Asian Institute of Technology (AIT) 1992. Sewage Purification Through Aquatic Plants, Final report. Division of Environmental Engineering, AIT, Bangkok.

Easson, M.E. et al. 1988. Sanitation Technologies for Cold/Temperate Climate. Environmental Sanitation Review, No. 25, AIT, Bangkok.

Fennessey, M.S. and W.J. Mitsch 1989. Design and Use of Wetlands for Renovation of Drainage from Coal Mines. In: Ecological Engineering: An Introduction to Ecotechnology, W.J. Mitsch and S.E.Jorgensen (eds), John Willey and Sons, New York.