IN Pakistan lakes and wetlands cover an estimated 9.7 per cent of the total surface area. Around 225 listed significant wetland sites are located across the country from sea coast in the south to high mountains in the north.

Natural wetlands have been used for centuries as a sink for waste. They act as biological filters and are a viable option for helping to solve a wide range of environmental and water quality problems.

Pakistan is facing critical water shortage and pollution. Drinking water sources, both surface and groundwater are contaminated with coliforms, toxic metals and pesticides. Pakistan’s drinking water ranking is at 80 among 122 nations. Drinking water in densely populated cities like Karachi, Lahore, Rawalpindi, and other major cities/towns is polluted due to various anthropogenic activities and cannot be recommended for human consumption. An analysis of water samples from Islamabad and its twin city Rawalpindi revealed that 94 per cent and 34 per cent of water samples were seriously contaminated.

In recent years, wetlands have been effectively used to treat polluted waters across the globe. Wetlands have a relatively low cost and are an efficient choice, because these are easy to install and maintain and produce limited quantities of by-products. They are used not only to degrade organic substances and remove nutrients from municipal sewage, storm water, and agricultural runoff, but also to remove metals from mining effluent and industrial waste water.

Both constructed and natural treatment wetlands can be utilised for water quality purification. Natural treatment wetlands have been used for the disposal and treatment of secondary and tertiary wastewater effluents for many years. Constructed wetland systems follow the assimilative properties of natural wetlands in an environment that can be controlled and manipulated.

Constructed surface flow treatment wetlands are typically shallow, man-made impoundments planted with emergent, rooted vegetation. Water flows overland through the wetland and primarily above the sediment surface. These wetlands may be planted manually or naturally colonised by volunteer plant communities. Metals and phosphorus can be sequestered in plant materials and wetland sediments. Molecular diffusion and subsequent microbial activity transform or permanently sequester dissolved pollutants in the incoming wastewater and those released from sedimented solids.

Several classification systems have been developed to distinguish wetland types. For the purpose of evaluating water-quality benefits, wetlands can be separated into two broad categories: riparian wetlands and inter-stream divide wetlands. Even though riparian and inter-stream divide wetlands may be equally wet, they make different contributions to water quality. Wetland vegetation species are primary candidates for use in phytofiltration because they play a major role in the removal of toxic trace elements from waters passing through the wetland. Wetland plants directly take up the metals or indirectly affect the removal efficiencies, by increasing the residence time and reducing water velocities and ultimately enhancing sedimentation of heavy metals. Different categories of wetland plants i.e. emergent, surface floating, free floating rooted leaves, sub-merged macrophytes and trees have different ways of taking up the contaminants. For example emergent and surface floating plants absorb metals through roots, while submerged plants and free floating plants take up metals mostly through roots as well as leaves.

Treatment wetlands provide following major benefits compared to more conventional treatment alternatives:

•    Wetlands are less expensive to construct than traditional secondary and tertiary wastewater treatment systems.

•    They require less maintenance and are less expensive to operate than traditional treatment systems.

•    They introduce a reuse option (environmental enhancement) for the wastewater facility.

•    Discharge from treatment wetlands can augment surface water sources for existing water bodies year-round, thus ensuring flow and enhancing local natural aquatic resources.

•    With proper design, portions of the treatment wetland may provide important wetland wildlife habitat, as well as human recreational opportunities such as birdwatching, hiking, and picnicking.

•    Treatment wetlands are viewed as an asset by regulatory agencies in many regions and as a potentially effective method for replacing natural wetlands lost through agricultural practices, industrial and municipal development, and groundwater withdrawal. These systems may provide self-mitigation for any unavoidable impacts to on-site natural wetlands.

To sum up, conventional wastewater treatment plants involve large capital investments and operating costs, and are not economically acceptable solutions. Constructed wetlands are gaining in importance as an effective alternative for the treatment of industrial effluents in developing countries. Wetlands can be established in the same place where the wastewater is produced; can be maintained by relatively untrained personnel; have relatively low energy requirements; and are low cost systems.

The writer is Assistant Professor, Department of Plant Sciences. Quaid-i-Azam University, Islamabad