Application of Constructed Wetland to wastewater treatment

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University of New South Wales *

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3303

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Geography

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Oct 30, 2023

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docx

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Uploaded by DeanEchidna5905

Application of Constructed Wetland to wastewater treatment While the development of city brings enormous benefits and convenience to our modern society, it is not uncommon to see different kinds of environmental problem arise from disposal of domestic and industrial wastewater. The increasing global population and urbanization have exacerbated the issue of wastewater treatment. In an era where water scarcity and pollution pose significant challenges, the incorporation of constructed wetlands(CW) in wastewater treatment as a sustainable wastewater treatment method in urban environment is worthy to discuss. CW are regarded as a green and sustainable technology for the treatment of domestic and industrial WW 2 . It replicates natural wetland processes, utilizes include wetland vegetation, soil and microbial assemblages, to conduct controlled physical, chemical and biological treatment. 1 The idea of constructed wetland is firstly introduced in 1901. 2 The system was a vertical flow CW. CWs are later classified according to their hydraulic level as free surface flow(FSF), subsurface flow(SSF), tidal flow and baffled flow. Vertical flow CW belongs to one of the two ways of SSF CW. The other way is horizontal flow CW. 3 WW will first enter preliminary treatment tank where floating substances, grit particles, oil are removed from WW. It is then transported to sedimentation tank. This process enables settling of solids and reduction of biochemical oxygen demand(BOD) to take plcae. 2 The WW eventually arrives CW via inlet to pass through substrate which is treated as a filter media. 4 These filters, in addition to being a physical support for plant, are capable to remove total suspended solid(TSS) and chemical oxygen demand(COD) for up to 84% and 75% respectively 5 . The high efficiency of removal can also be seen in biochemical oxygen demand(BOD), having around 85-90% of removal rate. 2 This attributed to the attachment area of substrate for the growth of microbial population. In addition to attachment of microbial population on substrate, wetland’s vegetation stem, which also provides large surface area for attaching microbial growth, is capable of transporting atmospheric gas(oxygen) into their root so that they can survive in anaerobic environment. 6 Such properties helps in removing nutrients like nitrogen and phosphorus although the rate of removal is dependence on the type of wetland. Some may argue that such purification procedure can also be done in centralized wastewater treatment plants(WWTP) undergoing coagulation and flocculation. However, such process might lead to secondary contamination of water. A high amount of aluminum or iron-based coagulant poses a risk of human exposure to residential aluminum or iron in drinking water 7 . Oppositely, CWs provide connection between living species and non-living species. Vegetation make use of nutrients in WW like nitrogen and phosphorus and the biomass produced may be collected and utilized in food and energy industry 8 . Studies have also 1 [ CITATION Mar22 \l 1033 ] 2 [ CITATION Jan222 \l 1033 ] 3 [CITATION ASh21 \l 1033 ] 4 [ CITATION ZEl16 \l 1033 ] 5 [ CITATION Bas22 \l 1033 ] 6 [ CITATION Ham20 \l 1033 ] 7 [ CITATION Min22 \l 1033 ] 8 [ CITATION Cas23 \l 1033 ]

reported that CW effluent met the local water quality standard for recreational/agricultural reuse 5 . Apart from the purification factor, decentralized sewage treatment plant, like CWs, has become more popular option rather than centralized sewage treatment plant. This can be explained in 2 aspects: Environmental and economical. Environmentally speaking, the biggest difference between CW and WWTP is the elimination of using chemical products. This can prevent secondary contamination of water and lower the operation cost from using coagulant. Moreover, CW enhances biodiversity, increased environmental aesthetic values, improved air quality, carbon sequestration, reduced urban heat island effects etc. 5 Therefore, CW systems are said to be environmental-friendly and sustainable technology for treatment of various type of WW. Economically, CW systems offers several economic advantages over traditional WWTPs. Firstly, CW generally have lower initial investment cost due to its simplicity process which require less complex infrastructure and machinery. A comparison between the investment and operation cost of activated sludge system (ASS) and CW is carried out in Mexico. The result shows that the average investment costs of CWs are 110$/p.e(population equivalent) while it costs 190$/p.e for the municipal ASS project. As for the operation costs, the costs on CW system is also substantially lower than that of ASS. (ASS:6.5$/p.e, CW:1.3$/p.e), about one-fourth of WWTP. Studies also show that CW systems posses greater durability owing to their limited number of moving parts. 4 As a conclusion, while concerns about land usage may arise, the aesthetic and recreational value brought by CW systems is non-negligible. The Hong Kong Wetland Park is an example which provide opportunities for recreational activities like birdwatching and nature appreciation, fostering a sense of environmental stewardship among communities. Studies also shows the high potential of CW systems to resist changing climates, for instance CW system have contributed to the reduction and mitigation of flood risks through storing water and regulating and managing the land. 9 9 [ CITATION Ahm22 \l 1033 ]

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