The Use of Constructed Wetlands - Literature review Example

Wetlands Name Institution Date Wetlands Introduction Environmental awareness has become a sensitive issue both to the individual nations and the global community over the years. This situation has been stimulated by the increasing decline in environmental conditions that has been witnessed since the onset of the industrial era. Subsequently, as a result of the elevated emphasis on the environmental management, the use of constructed wetlands has generally expended internationally. This provides mechanisms of managing the industrial wastewater and is a counteractive measure of ensuring improvement in handling pollution cases related to both abandoned and active mines. Essentially, constructed treatment wetlands are biologically active systems engineered by man. These systems, which include swamps, sloughs, bogs, and marshes, are characterized by water tables above the surface of the soil or near surface as well as saturated soil conditions and are strategically designed to treat the pollutants in the surface water, waste streams and ground water. This article provides a critical assessment of the importance of the constructed wetland with regard to the processes of mine pollution amelioration. Constructed Wetlands The design and construction of affects the manner in which water treatment occurs. According to Skousen (1998), there are two construction styles that are currently being predominantly employed globally. The first style is referred to as aerobic wetlands while the second is known as anaerobic. Ford (2003) observes that both aerobic and anaerobic wetlands are both types of passive treatment systems. The passive systems are cost effective approaches of enhancing wastewater treatment. In this case, wetlands ensure reduction of waste water treatment costs by removing metal contents either partially or completely. In addition, they do no require regular maintenance which is a further relief on the costs of treatment. Aerobic wetlands are generally shallow ponds which facilitate natural precipitation of iron, magnesium and manganese. Skousen (1998) explains that aerobic wetlands are used to collect water and provides time to allow aeration of which in turn allows the metals that are present in the waters to precipitate. Precipitation refers to the reaction between two different dissolved species. Aerobic wetlands are characterized by vegetations which have diverse functionalities. They filter water and introduce oxygen which results in precipitation of iron and other acidic minerals out of water. As a result, during construction wetland species are planted in the relatively impermeable sediments that are made up of clay, mine spoil and soil so as to add organic matter. In this away, the wetland plants improve the uniform flow thus elevating the effectiveness of wetlands. Ashraf et al (2011) observe that the vegetation in the wetlands such as Phragmites australis has general positive impacts on the treatment efficiency. In this case, they reduce he velocity of the flow of water in the wetlands, stabilize the surface of wetlands and facilitate sedimentation. Furthermore, they absorb nutrients from sediments and store them in their tubers, roots and other green parts, they are efficient fixation sites for microorganisms and they adsorb metals. Anaerobic styles on the other hand are constructed to neutralize the acidity of metals and reduce the metals to sulphides forms. In other words, anaerobic wetlands are constructed to reduce the acidity of AMD. In this case, anaerobic wetlands depend in organic rich substrates to enhance generation of reducing condition. This style encourages passage of water through substrates that are rich in organic components. This has a significant positive impact on treatment of polluted water. Anaerobic wetlands may be underlain with limestone gravel during construction so as to neutralize the acidity of AMD. The limestone dissolve in acid mine drainage and releases CO2. The end product is bicarbonate which increases alkalinity. Alkalinity buffers the acidity of minerals thus allowing the pH level to be maintained at degrees that can support precipitation. Effectiveness of Constricted Wetlands in Managing Acid Mine Drainage i) Acid Mine Drainage The mining sector has persistently been subjected to negative environmental criticism as well as strict governmental regulations (Smith 1997). Both abandoned and active mines are strongly associated with environmental degradation, pollution and interference with the ecosystem. Mining activities disorient large quantity of soil and rock materials by exposing them into the environment. As result of this exposure, mineral components which are highly toxic react with moisture and air. These processes are referred to as oxidation, where the mineral components react with oxygen and hydration where they react with water or atmospheric moisture. Particularly, some of the minerals such as coal and metal ores containing iron, magnesium and aluminum are deleterious in nature and have high potential of emitting acid into the atmosphere and water bodies. Other minerals that contain elements of sulfur such as metal sulfides like pyrites also bear the potential of discharging acids into the environment. Acid mine drainage is one of the major environmental challenges that faces mining industry (Michelutti and Wiseman 2012). Acid mine drainage (AMD) is the reaction between the sulfides that are contained in waste materials, especially in abandoned mines, with water and oxygen to form sulfuric acid. According to White (2006), AMD is basically caused by oxidative dissolution of sulphides minerals such as pyrites t form hydrogen irons, sulfates and hydroxides which are acid in nature. Mining activities expose these acidic minerals to the water bodies, microbial processes and the weathering air. ii) How wetlands Remove Acid Mine Drainage Smith (1997) observes that wetland is multifunctional systems that assist in the removal of the metals in the drainages and ameliorate AMD. Sobolewski (1996) points out that there are fundamentally different types of wetlands. There are those that are designated to deal with the coal generated mine drainage and those that deal with AMD from metal mines. The manganese, iron, sulfates and heavy metal concentrations are the major contaminants caused by AMD. Wetland processes are significantly associated with remediation of these contaminants within AMD. According to Laberge Environmental Services report (2000), wetlands have been used to treat acid mine drainage that resulted from coals mining since 1980s. Wildman et al (1991) argues that wetlands contain organic-rich substrates that exchange the dissolved metals. The exchange happens between the abundant humic and fulvic acids that exist in the substrates and the dissolved metals. One of the methodologies utilized to control AMD includes metal retention. According to Skousen (1998), the chemical and biological processes that enhance treatment of AMD in the wetlands in order of their significance include formation and precipitation of metal oxide, formation of sulfides, and complexation of organic reaction, ion exchange and uptake by plants. Additionally, Smith (1997) observes that some other mechanisms include adsorption, sedimentation, bioaccumulation, neutralization, bacteria and abiotic oxidation, reduction and dissolution of carbonate minerals. Abiotic processes are mechanisms that immobilize pollutants. Due to the extensive content of surface water which flows slowly, the aerobic wetlands improve the processes of hydrolysis and metal oxidation. Hydrolysis refers to the processes of breaking bonds using water whereby ions react with the water molecules. This process alters the acidity or basicity of solutions. According to Skousen (1998), hydrolysis causes precipitation and physical retention of iron, magnesium and aluminum hydroxides. The rate at which the metal components are retained relies on the concentration of the dissolved metals, the content of dissolved oxygen, net alkalinity and the pH of the mine water, availability of the active microbial biomass and the duration of water detention in the wetland. Another significant removal mechanism involves the use of bacteria (Michelutti and Wiseman 2012). In this case the sulfate reducing bacteria (SRB) such as De-sulfovibrio are used in the wetlands. The bacteria catalyze the reaction between the sulfate minerals and the water molecules thereby reducing the sulfates. The swamps and wetlands have low pH values as well as reducing conditions that which favor the existence of SRB in addition the bacteria also require carbon as nutrients. These are also readily available in the constructed wetlands. Additionally, the SRB can enhance the precipitation of the arsenosulphide minerals. Arsenic minerals are commonly found in the earth’s core as well as in clay and sulphides-rich components of earth’s crust. They are highly toxic when exposed to the atmosphere, especially for along period of time like in areas of abandoned mines (Lizama, Fletcher and Sun 2011). Yang et al. (2006) observes that there are some successful cases which indicate that wetlands effectively purify drainage waters that have been contaminated by metal mines. In addition, the wetlands technologies offer economical and self-maintaining mechanisms for managing mine pollutions. As such, they are preferred alternatives to other conventional treatment mechanisms used in removing pollutants from water bodies. Similarly, According to Laberge Environmental Services report (2000), while examining the impact of wetland treatment system that was constructed at Keno Hill mining district in Canada, it was observed that the wetland operated only for a short period of time but there was significant reduction in the concentration of manganese, nickel, sulfates, cadmium and zinc. The treated water was then considered fit for animal and human consumption and was discharged into the Christal Lake. Wetland and Reclamation of Wasteland As mentioned earlier, mining activities disorient ecological systems. The topsoil which supports vegetation is literally destroyed during such activities as they are being dug in order to reach the mineral rich ores. Besides, mining activities introduce sulfates and other harmful mineral contents that increase the acidity of remnant soil content. Ghermandi et al (2010) explain that ecologists have recognized wide range of benefits of wetland to human including use of economic friendly methodologies to restore the ecosystem. This implies that wetland can effectively be employed to restore the value of a derelict land. Land reclamation refers top the alteration of inadequate land through various mechanisms such as leveling the drainage system. Such land can therefore be used for economic enhancing activities such as farming and commercial fishing. In essence, the wetlands are constructed to replicate processes such as flood retention, water storage and to improve water quality. However, they are also constructed with the wider aim of mimicking the foregone functions of lost ecosystems and to compensate the destroyed natural habitats. Laberge Environmental Services report (2000) state that constructed wetlands on mine lands are significantly beneficial to reclamation since they are recognized as vital ecological systems. They enhance the provision of important habitats for wide range of wildlife species. Besides various plants that grow in the wetlands Water birds and various fish species are some of the animal life that are found in wetlands. Additionally, they also restore the aesthetic of mine areas. Wetlands are generally covered with wetland plants which are considerably appealing to the eye. In this case, they can be utilized as tourist destinations. Furthermore, they can provide efficient grounds for research and training programs for scientists associated with wastewater facilities or environmentalists. Wetland and Water purification Both the biotic and abiotic functions of the constructed wetland effectively restore water quality by eliminating pollutants associated with mining influenced water. According to Wetlands International report (2003), constructed wetlands can be used to treat both small and large volumes of contaminated water, and with varying levels of contaminants. On that note, wetlands can e used to treat polluted water in the rivers, lakes, surface run-offs, industrial effluents and agricultural wastewater. Moreover, Wetlands International report (2003) point out that constructed wetlands has multiple functions which include treating mining leachate and management of sludge. Leachate constituents refer to all the substances that occur naturally that do not have the unique characteristics of mine wastes. If not well managed, mining activities pose a high threat to the underground and surface water. Therefore, there is need to establish proper ground water monitoring strategy. According to Plumb (1999), the effort to develop effect ground water managing techniques are met by significant amount of challenges. For instance, the composition and behaviors of the mine waste leachate on the environment are not properly understood. Secondly, the perimeters that should be governed with regard to the ground water that is adjacent to the sites used to dispose off the mine wastes so as to allow detection of fugitive mine waste leachate have not been determined and evaluated. However, these factors do eliminate the existence of technique that can be employed to manage water pollutions from mining activities and other sources of pollutions such as industrial effluents. Ashraf et al. (2011) argue that there are approaches that can be utilized in designing the man made wetland system to enhance amelioration of tin contaminated water from mining catchment. According to the study conducted by Ashraf et al (2011), some of the quality parameter that can be used to determine the need for treatment include color, temperature, pH value, turbidity, dissolved solids, salinity, and dissolved oxygen. The study also indicated that the quality of water and soil in the mining areas are significantly degraded as they are characterized by metal concentration. Wetland remediation of wastewater from mines can be enhanced through three distinctive compartments which include biota, media and water (Ashraf et al 2011). While designing the treatment wetland to enable removal of heavy metals, it is vital to encompass every process of removing metals from other compartments within water compartment. Transportation of metal is influenced by the movement of water which allows transportation of water in the entire system. The water gears the saturation of the media thus allowing the reduction of the redox value and allows the treatment processes to .take place in the media and associated with biota. Therefore, it is easy to observe that the components are interrelated and each depends on one another and that water is the most crucial component. Additionally, the media component provides water and nutrients that support both plants and microorganisms thereby sustaining the biota. Media compartment are made up of any substrates that are used to contain both the biota and water. The chemical composition and the type of the arsenic that is present are the major determinant factors affecting the ability of the media to adsorb the heavy metals. Both plants and animals that consist the biota absorb the heavy metals hence purifies the water (Ashraf et al 2011). Advantages of constructed wetlands Both the ancillary benefits of the artificial wetlands as well as their water quality improvement service have extensively been researched. Some of the advantages of constructed wetland include provision of remedial options that can operate for along period of time after construction. They are also associated with minimal operations as low maintenance costs. According to Ford (2003), wetlands do not require power or energy to operate. Other advantages include they can easily be applied in remote areas that lack utility access, they do not need any chemicals that are hazardous or any mechanical equipment or building, they are efficient mechanisms for controlling soil erosion, infiltration, surface runoffs and managing fugitive dust emissions. In addition, constructed wetlands have high design flexibility. This enables remediation of sites that have diverse or mixed pollutants. Besides provision of habitats for microorganism, wetland birds and various fish species, the constructed wetlands also restore the land that has been rendered valueless after mining activities thus enabling its reclamation. Disadvantages There are a number of limitations that may pose negative impacts on the choice of the remedial plan. In this case the constructed wetland may not be effective if the land for wetland is not properly constructed. The initial construction costs may also be high. It requires constant monitoring of the concentration of the pollutants so as to sustain the ecological health .of the system. The burden of disposing off the materials and metals that have been accumulated could be overwhelming. Although wetlands do not require regular maintenance, they require major periodic maintenance. There is a general over dependence on the local climatic conditions. This aspect renders the constructed wetlands ineffective during extremely cold seasons. Ford (2003) argues that constructed wetlands may fail due to either poor construction or winter conditions. Compared to other mechanisms of treatment technologies, constructed wetlands have essentially slow performance rate. If not properly considered during construction, the artificial wetlands can be the potential breeding grounds of mosquitoes. Similarly, lack of proper planning strategies during construction may result in the generation of detestable odors which result from the natural biological functions and may be elevated during anaerobic processes. There is also high risk of adding the contaminants such as the phosphorus and nitrogen to the water that flow over them. This condition impedes the efficiency of removing some minerals from the acid mine drainage and polluted water (Ford 2003). Conclusion In conclusion, the treatment wetlands are extensively employed in the mining industry to manage waste streams. Firstly, treatment wetlands are applied to manage acid mine drainage, which increases the levels of the toxic elements in the environment. Secondly, constructed wetlands are also used to control the degrees of acidity and alkalinity. Wetlands help in accomplishment of theses two missions through various processes such as oxidation of manganese and iron and use of reactive medium, typically limestone which adds alkalinity and buffer the pH levels in the waste stream. Additionally, the wetlands are also largely being utilized in the mining industry to remove heavy metals. In this case, the wetlands are designed to enhance the reduction of sulfates so as to foster sulphides precipitation of heavy metal such nickel, iron and copper. Both the aerobic and anaerobic wetlands are passive treatment mechanisms and as such have been considered the most cost effective mechanism of managing waste stream. Basically, the paper has demonstrated that these models utilize the vegetations and communities of microbial sediments to reduce the acidity and enhance precipitation of metals. List of Reference Ashraf, MA, Maah, MJ, Yusoff, I & Gharibreza, M 2011, Proposed design of anaerobic wetland system for treatment of mining waste water at former tin mining catchment Scientific Research and Essays Vol. 6(28), pp. 6001-6022. Ford, KL, 2003, Passive Treatment Systems for Acid Mine Drainage Available from [February 27, 2015]. Ghermandi, A, Jeroen CJ, Brander, LM, Groot, HF & Nunes PA, 2010, ‘Values of natural and human‐made wetlands: A meta‐analysis’, Water resources research, vol. 46, pp 1-12. Laberge Environmental Services, 2000, Investigations into Passive Wetlands Treatment Of Mine Drainage to Remove Heavy Metals at Various Sites at UKHM, Available from [February 27, 2015]. Lizama, KA. Fletcher, TD & Sun, G 2011, ‘Removal processes for arsenic in constructed wetlands’, Chemosphere vol. 84, pp 1032-1043. Micehelluti, B & Wiseman, M 2012, Engineered Wetlands as Tailing Rehabilitation Strategy Available from [February 27, 2015]. Plumb, R H, 1999, Characterization of Mine Leachates and the Development of a Ground-Water Monitoring Strategy for Mine Sites, Available from [February 27, 2015]. Skousen, J 1998, Overview of Passive Systems for Treating Acid Mine Drainage, Available from [February 27, 2015]. Smith, K 1997, ‘Constructed Wetlands for Treating Acid Mine Drainage’, Student ON-Line Journal Vol. 2 (7), pp 1-7. Sobolewski, A 1996, Wetlands for Treatment of Mine Drainage, Available from [February 27, 2015]. White, S 2006, Wetland Use in Acid Mine Drainage Remediation, Available from < http://home.eng.iastate.edu/~tge/ce421-521/Steven%20White.pdf > [February 27, 2015]. Wetlands International, 2003, The use of constructed wetlands for wastewater treatment Available from [February 27, 2015]. Yang, B, Lan CY, Yang, CS, Liao, WB, Chang, H & Shu, WS, 2005, Long-term efficiency and stability of wetlands for treating wastewater of a lead/zinc mine and the concurrent ecosystem development, Available from < http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0CDMQFjAD&url=http%3A%2F%2Fwww.researchgate.net%2Fprofile%2FBing_Yang13%2Fpublication%2F7305884_Long-term_efficiency_and_stability_of_wetlands_for_treating_wastewater_of_a_leadzinc_mine_and_the_concurrent_ecosystem_development%2Flinks%2F53eccc2d0cf23733e804c69e.pdf&ei=9KHuVPO6Koz9ULzggZAD&usg=AFQjCNHKrrymu0JMZsCj5TnK3fw1v1CcKQ&sig2=Ll6tN1AKkmdxchgiAUtsNA&bvm=bv.86956481,d.d2s> [February 27, 2015]. Read More