Various Types of Constructed Wetland for Wastewater Treatment-A Review

As per the World Health Organization 80% of wastewater is released to the environment without satisfactory treatment. Constructed Wetlands (CWs) are one of the natural wastewater (WW) treatment methods. CWs have been recommended as a low technology, low maintenance, low operation cost, and green technology wastewater treatment system. Many types of CWs are currently in use. This paper studies these different types based on the climate, area, base materials, temperature, contaminant removal efficiency, removal mechanism and physicochemical analysis of various wastewater parameters. It is found that wetlands are successful in removing organic matter–Biochemical Oxygen Demand (BOD5), Chemical Oxygen Demand (COD) and suspended solids and nutrients–total Nitrogen (TN) and total Phosphorus (TP). CWs need to be studied as a promising solution not only for effective treatment of wastewater but also as an economical method to improve the fertility of soil. Further, the paper discusses the scope of future research in CW to further improve the wastewater treatment technology.


Introduction
Natural wetland comprises of vegetation, soil and either water or wastewater.
There are different conventional methods for wastewater treatment such as sand traps (grit chamber), septic tanks, Imhoff tank, baffled reactor (Anaerobic Baffled Reactor, ABR), anaerobic filter, green filters, Soil BioTechnology (SBT) or Constructed Soil Biofilter (CSB) or Constructed Soil Filter (CSF), anaerobic stabilization ponds, aerobic stabilization pond (Maturation Ponds/Oxidation Pond), Rotating Biological Contactor (RBC), Active Sludge Process (ASP), Upflow Anaerobic Sludge Blanket (UASB), Trickling Filter (TF), and micro-algae techniques etc. CWs are the manmade system with combination of base material, vegetation, and organic matter to provide wastewater treatment [1]. CWs require less infrastructure, investments, raw materials, energy consumption, operation, staff during operation, maintenance, odors, insects, flow variations, toxic substances, and by-products [2]. Categorization of CWs depends on flow direction, macrophytic growth and hydrology [3]. CWs are typically used as a secondary treatment unit. The CWs are used for treatment of domestic wastewater, animal wastewater, mine water, leachate remediation, industrial wastewater, urban stormwater and field runoff [4]. The main part of the CWs are aggregate, soil and gravel used as filter media and macrophytes (vegetation) such as Typha, Canna indica, Flax Lily, Banksias, Bottlebrush, P. australis, common rush, tapered rosette grass, Phragmites australis, common reed, Club-rush, Cattail, Common water plantain, Reed canary grass, Meadowsweet, Yellow flag, Compact rush used as vegetation [5]. According to water flow in substrate UN-HABITAT has divided CWs in three categories -1. Vertical Flow Constructed Wetland (VFCW) 2. Horizontal Flow Constructed Wetland (HFCW) 3.

Removal Mechanisms in Constructed Wetland
The main mechanism of treatment through CWs comprises biogeochemical transformation with solid and liquid separations. Macrophytes plays a very important role in removing various types of contaminants. The removal mechanism by macrophytes is shown in fig. 2. Contaminants uptake by root zone involves rhizodegradation process by microbial activity. The heavy metals are reduced through phytostablization. Plant enzymes break down the contaminants through phytodegradation. Plants and algae remove the contaminants from soil and sediments through phytoextraction. Plants release the contaminates from soil and sediments in atmosphere through phytovolatalization process [6,7,8,9]. Removal of various contaminants involves various operations and process in CWs. Organic material (BOD) is removed by biological degradation, sedimentation, and microbial uptake -organic contamination including pesticides are removed by adsorption, volatilization, photolysis, and biotic degradation, suspended solids are removed by sedimentation and filtration, Nitrogen is removed by plant uptake, sedimentation, nitrification/ denitrification, microbial uptake, volatilization, Phosphorus is removed by sedimentation, filtration, adsorption, plant and microbial uptake, pathogens are removed by natural die-off, sedimentation, filtration, UV degradation, adsorption, heavy metals are removed by sedimentation, adsorption by vegetation and substrate and plant uptake process. Due to easy availability, Typha latifolia is a common plant for CWs and it also generates the higher biomass.

Selection of wetland
Selection of the wetland depends on type of pollutants, material of substrates, local available vegetations, temperature, Hydraulic Retention Time (HRT= Volume (Area x Water Depth x Porosity) / Discharge) and Hydraulic Loading Rate (HLR) [10]. As per the United State Environmental Protection Agency (USEPA) performance of constructed wetland depends on depth of water, slope of ground, and flow velocity.

Vertical Flow Constructed Wetland (VFCWs)
In VFCWs the wastewater in flow is above the substrate. The wastewater is treated through percolates in root zone. Higher concentration of polluted water can be treated by VFCWs. . It has been observed in pharmaceutical compounds in wastewater removal by pilot-scale VFCW by using scirpus grossus plants and sand, gravel as media. They practiced with different HRT (3, 4, and 5 days) and has been observed that 99% ibuprofen, 88% COD, 99% ammonia and 83% orthophosphate are effectively removed (HRT 5 days and 2 L/min aeration) [14]. Soil base constructed wetlands can also use for VFCWs.

Hybrid Flow Constructed Wetlands (HCWs)
HFCWs are the combination of VFCWs and HFCWs. HFWS is effective for reduction of pathogens, ammonia and TN [9]. The various types of CWs can be arranged together to create a combined system, which is called hybrid constructed wetlands. Such mixed CWs are utilized to accomplish higher effectiveness of wastewater treatment as opposed to single CW. Investigation of HCWs was initiated by Seidel in Germany in 1980 [27] -ammonia was oxidized to nitrate, and in the following anoxic HF beds nitrate was reduced via denitrification [3]. Using a similar setup for the municipal wastewater, using the gravel, sand and soil with typha plant, the removal efficiency for BOD578%, TKN 66%, TSS 74% and P 86% while pilstia stratiotes removed BOD5 84%, TKN 76%, TSS 82% and P 83% [28]. Fig. 5 shows the layout of HCWs. HFCWs has some disadvantages like space consuming, capital cost, pretreatment, high supervision and not extremely lenient to cold atmospheres.  [35], different aeration methods like tide flow (TF), effluent recirculation (ER) and artificial aeration for all type of CW (VFCW, HFCW and HCW) improving the removal efficiency 89% TSS, 84% COD, 63% total Nitrogen (TN) and 81% ammonia nitrogen [35]. Recently, steel slag size 10 to 20 mm used as filer media in multistage pond CWs for removal of contamination in river water resulting in increase in total phosphorus (TP) reduction for first level HFCW by 151% [36].
Researchers have also studied different types of wastewaters such as removal of petrochemical waste through HFCW by using gravel and sand media, plants typha and phragmites, removing the BOD5 (85%), COD (89%) and turbidity (66%). Similarly study on different plant species, like acacia nilotica, cotton, brassica napus carried out for decontaminating oilfield wastewater. BTEX (benzene, toluene, ethylbenzene and xylene), PAH (Polycyclic Aromatic Hydrocarbons) and aliphatic/ aromatic petroleum hydrocarbon can be removed by CW for refinery wastewater [37]. Heavy metals like Cu, Zn, Ni, Fe, Pb, Cd, B, Cr, Al, Co, As and Se from industrial wastewater, mine tailing waste, tannery wastewater, oil refinery, domestic wastewater can be removed from SFCW. Media used for removal of heavy metals from CW are sediments, compost, gravel, soil, ash with plants macrophyte, typha latifolia, macrophyte myriophyllum spicatum and algal species [38,39]. The removal efficiency of heavy metal from CW are reported as 69%-96% [38,39,40]. Pharmaceutical, hospital and personal care products (PPCB), medical waste and cosmetic waste has been seen effectively removed by CWs. Phytoremidiation in various types of CW is a well stablished treatment method now [39,40,41]. The removal of various PPCB was observed to be in the range of 70% to 90% by various CWs [42,43]. Agro based synthetic fertilizer, pesticides, fungicides, insecticides and herbicides leach with soil and contaminate the river, lakes, ponds, and ground water. CW removal efficiency for agricultural wastewater for COD (92%), BOD5 (98%), TN (91%), TP (96%), TSS (96%), 94-99% E.Coli, Fecal Coliform (FC), and Total Coliform (TC) from single stage CWs in various conditions has been reported [44]. The researchers attributed the contribution of CW for treating agro-base runoff and industrial wastewater [45]. Group of researchers [46] recently observed that wastewater from textile dyeing process can be effectively treated by CW resulting in removal efficiencies-COD (87%), BOD5 (86%), TSS (49%), TDS (46%), Orange3R (84%), alcohol oxidase (85%), lignin peroxidase (150%) and color (75%). Compost and landfill leachates have a high amount of organic matter and it can be treated by CW [47]. A pilot-scale study was done in Iran where HSSFCW planted with vetiver grass and fine gravel is used as filter media. It is observed that 75% of BOD5, 53% of COD and 74% of TN were removed successfully [47]. Microbial fuel cell (MFC)-based horizontal flow (HF) constructed wetlands are novel systems that use interconnected electrodes (cathode and anode), saturated media, plant, and electrochemically active microbial population for wastewater polishing [48,49]. One study reported that the Microbial Fuel Cell (MFC) HFCW with aluminum plates anode and cathode in HFCW with stone aggregate as filter media and vetiver grass planted in CW, has 93% phosphorus, 83% coliform, 55-92% nitrogen and 80-100% organic impurities removal [50]. Microplastics (MP) are the emerging as one of the major environmental pollutants on the earth. In MP Synthetic Organic Polymer Plastics (SOPP) have been used majorly in households, clothing, agriculture, personal care products [51]. The increased dependency on plastic material has skyrocketed the demand for plastic products which eventually end up as a waste in the environment. The research shows that MPs travel from municipal solid waste landfill to aquatic systems [52]. Natural wetlands are also proving to be effective in removing MPs, a pollutant of emerging concern. Mangrove wetlands are proving to be effective in removing MPs from aquatic streams [53]. CWs can be a suitable treatment strategy in removing MPs before they reach natural water sources.

Conclusion and further directions
Due to a small pollution load in domestic and municipal wastewater, CWs can be an effectively part of treatment. CW provides high removal efficiency for organic contaminants, nutrients-nitrogen and phosphorus, and pathogenic microorganisms. Highly toxic contaminants and heavy metals from industrial wastewaters can also be effectively treated by CWs. Toxic heavy metals removal like lead, cadmium, iron, mercury, arsenic, copper, chromium, zinc, nickels, silver and manganese through CWs have been reported by many researchers. CWs provide many advantages over conventional wastewater treatment systems. CWs provide an economical, low technology, less expensive and high energy-saving treatment technology. CWs also show promise in providing effective treatment not only for the domestic, municipal wastewater but also for difficult to treat industrial wastewaters. Waterborne pathogens can also be reduced by wetland systems. CWs are environmentally friendly and natural resource treatment systems that contribute to the public health. CWs also show potential in removal of emerging contaminants of concerns that are difficult to remove by conventional treatment methods. Recent research shows that some endocrine disrupting chemicals were reduced by 48 to 99% by CWs [54]. More research is also required for removal of MPs by CWs. A long term monitoring of geo-physicochemical properties of CW under various operating conditions will give further research for better understanding of the key factors responsible for the optimization of CW [55]. CW has great potential efficiency for removal of all types of water pollution and wetland technology has made great strides in the last few years. [13] Saeed, T., Sun G., (2012). A review on nitrogen and organic removal mechanism in subsurface flow constructed wetland: depend on environmental parameters, operating condition and supporting media. J. Environ.Manag. 112, 429-448. [16] Wu, S., Austin, D., Liu, L., Dong, R., (2011). Performance of integrated household constructed wetland for domestic wastewater treatment in rural areas. Ecol. Eng. 948-954.