India generates 61,754 million litres per day (MLD) of sewage, but only 38% of it is treated, and the rest goes untreated into water bodies.

As per the estimates given by the Central Pollution Control Board, 38,791 MLD of wastewater finds its way into waterbodies, out of which 10,000 MLD alone comes from industrial sources. Because of untreated sewage discharge into waterbodies, the quality of the water is deteriorating while affecting aquatic ecosystems.

The existing conventional wastewater treatment facilities are unable to cope with the amount of wastewater generated, besides being expensive in terms of energy and chemical costs. There is a need for supplementary facilities which can reduce the burden on existing wastewater treatment systems.

Wetlands are one such solution. They are a natural system of water purification and treatment, which have been used for centuries as a sink for wastewater. Wetlands degrade organic substances, remove nutrients from municipal sewage, stormwater, agricultural runoff and are capable of removing metals from mining and industrial effluents. They support a rich diversity of wildlife and fisheries by being nesting areas for migratory birds and spawning grounds for fish and shellfish.

The wetlands along the coasts, riverbanks and lakeshores have a valuable role in stabilising shore lands and protecting them from erosion. One of the greatest benefits of inland wetlands is the natural flood control or buffering provided for downstream areas by slowing the flow of floodwater, desynchronising the contributions of tributary streams and reducing peak flows on main rivers.

There has been much research on wastewater depuration (the action or process of freeing water from pollution) capacity of wetlands that have led to a greater understanding of the potential of wetland ecosystems for pollutant assimilation and have encouraged the development of constructed wetlands systems for treatment of wastewaters from a variety of sources.

Constructed wetlands are engineered to imitate features of natural wetlands. Constructed wetlands have recently started receiving considerable attention as they are low cost, efficient in cleaning a variety of wastewater, including both point and non-point sources, which include acid mine drainage, agricultural effluents and runoff, landfill leachates, petrochemicals as well as industrial effluents.

Improved water quality

The potential for achieving improved water quality while creating a valuable wildlife habitat has led to a growing interest in the use of constructed wetlands. While land intensive, these systems offer an effective means of integrating wastewater treatment and resource enhancement.

However, use of wetlands at every water pollution source is not possible due to unavailability of space, but installation of constructed floating wetlands along discharge drains can be an option for curing polluted water by absorbing their pollutants and nutrients (nitrates and phosphates). Floating wetlands have proven as an effective solution in substantially reducing nutrient levels in several studies across world and India involving smaller-scale lagoon treatment plants.

Worldwide, various floating wetland models have been developed for treating municipal and industrial wastewaters. Wiconisco floating wetland in Pennsylvania and Rehberg Ranch floating wetland in Montana are designed for biological nitrate removal. Various Indian cities like New Delhi and Pune are testing floating wetlands for treating sewage in the drains, lakes and rivers (Yamuna).

Floating wetlands offer enhanced microbial growth by expanding available underwater surface area or microbial habitat.

Technologies developed in India like Phytorid Wastewater Treatment involve a constructed wetland exclusively designed for treatment of municipal, urban, agricultural and industrial wastewater. Phytorid technology has been proven successful in Siddheshwar Lake in Pachpakhadi, Dativali Lake in Diva and Dawala Lake on Ovala for reducing Biological Oxygen Demand and Chemical Oxygen Demand.
With increasing urbanisation, there is a need to promote such sustainable options that can act as a supplementary treatment system to the conventional setup.

Government programmes focusing on urban development like Smart Cities and Atal Mission for Rejuvenation and Urban Transformation should promote installation of constructed wetlands to treat local polluting sources.

(The writers work for TERI)