Water is one of the most important and indispensable resources required for various development activities. The exponential growth of population, increasing use of various agrochemicals in rural areas, urbanization, industrialization, and generation of large quantities of wastes have led to contamination of water.
Various chemical, physical and biological methods like; chemical oxidation, adsorption, chemical precipitation, coagulation/flocculation, sedimentation/flotation, air stripping, membrane filtration: microfiltration, ultrafiltration, nanofiltration, reverse osmosis, UASB, stabilization ponds, activated sludge, and trickling filters are used for treating polluted wastewater. But these conventional treatment methods are very costly and used some other chemicals for the treatment process so such types of technologies were required that could be environmentally friendly, easy to operate, less energy-intensive, and cost-effective. Constructed Wetlands (CWs) have been used to achieve wastewater treatment goals by using natural components and processes which significantly minimize the use of energy-intensive mechanical devices and technological complexity. Wetlands can be described as a zone between aquatic and terrestrial ecosystems, and are characterized by saturated substrates (soil), presence of water, and vegetation distinctly from other ecosystems and fauna. Wetlands perform other several functions like; water quality improvement, cycling of nutrients and other material, provide habitat for aquatic organisms, provide passive recreation, such as bird watching and photography, help in education and research along with aesthetics and landscape enhancement.
The use of constructed wetlands for water purification is becoming more and more accepted and effective in many parts of the world including developing as well as developed countries. Constructed wetlands are complex manmade ecosystems that include physical, chemical, and biological processes. It consists of a perfectly designed basin that contains water, a substrate, and most commonly, macrophytes (plants). These systems are prepared to imitate natural wetland systems using locally available plants, soil, and associated microorganisms to remove contaminants from wastewater effluents.
According to the United States, Environmental Protection Agency Constructed wetland is defined as “a wetland specifically constructed for the purpose of pollution control and waste management, at a location other than existing natural wetlands.”
There are several types of constructed wetlands: surface flow wetlands, subsurface flow wetlands, and hybrid systems that involve both SF and SSF systems. CW with the above-ground flow are called surface-flow CWs (SF) and the other with a belowground flow are referred to as subsurface-flow CWs (SSF). SSF system is also called the root zone treatment system (RZTS), microbial rock reed filter, and plant-rock filter systems. The microbiological activity in the root zone of aquatic plants is the key parameter for the performance of RZTS.
Subsurface flows further categories into horizontal and vertical flow CWs on the basis of water flow. Dr. Kathe Seidel was the first who conducted an experiment to find out the possibility of wastewater treatment by wetland plants in Germany at Max Plancks Institute in the late 1950s. She designed a constructed wetland system for treating sewage using vertical and horizontal flow beds planted with Phragmites australis plant species. After that CW systems became the most widely used concept for treating various kinds of wastewater like; Domestic and Municipal wastewater, dairy wastewater, coal mining, landfill leachate, and runoff waters all over the world.
Plants and bacteria in Constructed wetland systems play an important role in the treatment process. The main pollutant removal mechanisms involved in a constructed wetland system are sedimentation, filtration, Biochemical transformations, adsorption, precipitation, volatilization, nitrification/denitrification, microbial, and plant uptake of pollutants.
Increasing urbanization and industrialization have resulted in the production of large amounts of waste by both rural and urban activities, which may lead to serious environmental problems. Hence, this is a need of the time to promote such type of holistic approach like Constructed Wetland technology, which is environment-friendly, easy to operate, less energy-intensive, and cost-effective, especially in a developing nation like India.