Constructed Wetlands

Constructed wetlands are a type of system especially constructed at a place away from natural wetland systems for the treatment of wastew'ater (Stottmeister et al., 2003). Constructed wetlands are artificial shallow' ponds planted w'ith aquatic plants (macrophytes, microphytes) w'hich utilize nutrients and degrade pollutants thereby helping in the treatment of the wastewater (Scholz and Lee, 2005) (Figure 4.3). Wetland systems involve naturally available microorganisms for the removal of the pollutants from the w'astew'ater (Upadhyay et al., 2017). Constructed wetlands involve a similar process to that of natural wetlands (physical, biological and chemical), and thus can easily be used for the treatment of w'astewater from different origins (Upadhyay et al., 2017). Artificial wetlands contain a synthetic liner at the bottom in order to avoid the percolation of the w'ater and also

A small constructed wetland showing the remediation mechanism involved in the treatment of wastewater

FIGURE 4.3 A small constructed wetland showing the remediation mechanism involved in the treatment of wastewater.

a specially designed system for controlling flow direction, detention time and water level in the wetland, as these factors affect the performance of the wetland (Kadlec and Wallace, 2008). The filter material must be permeable to avoid clogging and enable the smooth flow of the wastewater (Akratos and Tsihrintzis, 2007). The treatment occurs through physical, chemical and biological processes as the wastewater horizontally or vertically passes through the filter material (Yalcuk and Ugurlu, 2009). A horizontal or vertical flow system with a smaller size range (100 times) and retention time (3 times) than a surface flow system (Akratos and Tsihrintzis, 2007) is a suitable option for developing countries. The land area is already scarce, but the batch flow systems use less land area with a high pollutant removal efficiency (Kaur et al., 2012) and a better acceptability under Indian conditions.

Microalgae being the first colonizers of newly constructed wetlands can remove toxic pollutants from wastewater in a sustainable manner. The efficiency of a wetland in the remediation of waste- water is dependent upon several factors, like vegetation, soil type, microbial population and weather conditions of the local area (Almuktar et ah, 2018). Microalgae and cyanobacteria can grow in artificial wetlands and play a significant role in the remediation of wastewater (Rawat et ah, 2011). The treatment of wastewater in wetlands can help to recover nutrient resources, reduce BOD, increase oxygen content and enhance the removal of toxic and emerging pollutants (Mahmood et ah, 2013). Constructed wetlands are simply the mutual association between plants and microorganisms that enhance the elimination of toxic pollutants from wastewater by making them non-toxic (Almuktar et ah, 2018). Constructed wetlands can be efficiently used for secondary or tertiary treatment of wastewater because of their higher biological activity (Almuktar et ah, 2018).

Aquatic plant treatment systems (floating technology) have also been used for the purification of wastewater (Zhao et ah, 2012). Floating wetlands consist of ‘treatment plant beds’ and ‘free-floating plants’ (Rozkosny et al., 2014). Floating treatment wetlands involve the use of free-floating plants for improving the quality of the water (Headley and Tanner, 2008). Free-floating aquatic plants are efficient in removing organic matter as well as heavy metals from the wastewater (Kivaisi, 2001). Vegetation can efficiently increase the dissolved oxygen content of the water, thereby providing the conditions necessary for the degradation of the various pollutants by microorganisms in the waste- water. A decrease in the organic load of the wastewater is also helpful for maintaining the flow and mixing of the wastewater (Kivaisi, 2001). In rural and urban areas, wetlands are the most promising systems for wastewater treatment as low costs are involved in the construction and maintenance of the wetlands (Kadlec and Wallace, 2008.). Their simple construction and the lack of sludge recycling make them preferable for dairy effluent treatment in developing countries (Ibekwe et al., 2003; Haberl et al., 1995). The major drawback of this treatment system is the use of a land area which is already limited due to increasing urbanization and population (Massoud et al., 2009). The other major problem associated with a constructed wetland system is the generation of ions (Fe3+, Mn2+ and Ca2+) which decrease the bed permeability and result in the development of anaerobic conditions (Britz et al., 2006).

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