Artificial wetlands and water quality improvementR.B.E. Shutes*Urban Pollution Research Centre, Middlesex University, Bounds Green Road, London N11 2NQ, UKAbstractThis paper illustrates the role of plants to assist the treatment of water pollution in man-made wetlands in tropical and temperate climates.It also considers the potential for environmental education of these wetland systems. The management and natural treatment of pollution isdescribed in the Mai Po Marshes, Hong Kong and a wetland in London which is also an important site for birds. The design of the PutrajayaLake and Wetland system in Malaysia is compared with a constructed wetland and lake for the treatment of urban surface runoff in a newresidential development in the United Kingdom. The benefits of these natural systems are discussed in the context of the global trend forintroducing sustainable methods of environmental management and low cost pollution treatment systems. D 2001 Elsevier Science Ltd. Allrights reserved.Keywords: Constructed wetlands; Sustainable management; Urban runoff; Wastewater treatment1. Introduction1.1. Wastewater treatment by aquatic plantsThe ability of large aquatic plants (macrophytes) to assistthe breakdown of human and animal derived wastewater,remove disease-causing microorganisms and pollutants hasonly recently been scientifically investigated (Kadlec andKnight, 1996).A range of plants have shown this property, but thecommon reed (Phragmites australis), and the reedmace(Typha latifolia) are particularly effective. They have a largebiomass both above (leaves) and below (underground rhi-zome system) the surface of the soil or substrate. Thesubsurface plant tissues grow horizontally and verticallyand create an extensive matrix which binds the soil particlesand creates a large surface area for the uptake of nutrientsand ions. Hollow vessels in the plant tissue enable air tomove from the leaves to the roots and to the surroundingsoil. Aerobic microorganisms flourish in a thin zone (rhizo-sphere) around the roots and anaerobic microorganisms arepresent in the underlying soil. Natural filtration in thesubstrate also assists the removal of many pollutants andpathogenic microorganisms.1.2. Artificial or constructed wetlandsConstructed wetlands were initially developed about 40years ago in Europe and North America to exploit andimprove the biodegradation ability of plants. The advan-tages of these systems include low construction and operat-ing costs and they are appropriate both for smallcommunities and as a final stage treatment in large munic-ipal systems (Cooper et al., 1996). A disadvantage of thesystems is their relatively slow rate of operation in compar-ison to conventional wastewater treatment technology.Constructed wetland designs include horizontal surfaceand subsurface flow, vertical flow and floating raft sys-tems. Surface flow wetlands are similar to natural marshesas they tend to occupy shallow channels and basinsthrough which water flows at low velocities above andwithin the substrate. The basins normally contain a combi-nation of gravel, clay- or peat-based soils and crushedrock, planted with macrophytes.In subsurface flow wetlands, wastewater flows horizon-tally or vertically through the substrate, which is composedof soil, sand, rock or artificial media. The purificationprocesses occur during contact with the surface of the mediaand plant rhizospheres. Subsurface flow systems are moreeffective than surface flow systems at removing pollutants athigh application rates. However, overloading, surface flood-ing and media clogging of the media of subsurface systemscan result in a reduced efficiency.0160-4120/01/$ ± see front matter D 2001 Elsevier Science Ltd. All rights reserved.PII: S 0160-4120(01)00025-3* Tel.: +44-20-8362-5000; fax: +44-20-8362-6580.E-mail address: [email protected] (R.B.E. Shutes).www.elsevier.com/locate/envintEnvironment International 26 (2001) 441 ± 447The lifespan of constructed wetlands has been demon-strated as being approximately 20 years for organic wastetreatment. They can be designed to form an aestheticallypleasing and functional landscape which can be incorpo-rated into residential developments. In addition, they pro-vide a valuable ecological habitat for wildlife.The performance of these systems is influenced bytheir area, length to width ratio, water depth, rate ofwastewater loading and the time for it to pass through thewetland. For the removal of disease-causing microorgan-isms, an efficiency above 90% is normally achieved, fororganic material and suspended solids 80% removal maybe expected but nutrient removal efficiency is normallybelow 60%.Constructed wetlands have also been designed to treaturban and highway runoff (Shutes et al., 1997, 1999). Thevariable quality and quantity of urban and highway runoffrequires a more complex design for a constructed wetlandtreatment system. Fig. 1 shows the use of pretreatmentstructures including an oil separator, silt trap, spillagecontainment basin and settlement pond in an idealisedhighway runoff treatment system incorporating a horizontalsubsurface flow constructed wetland. A final settlement tankprovides posttreatment followed by discharge to a receivingwater body. An overflow structure from the settlement basinprevents excessive flows passing through the wetland anddamaging the plants.The treatment of airport surface runoff, industrial wasteand mine drainage, landfill site and composting systemleachate and the drying of sludge are also applications ofconstructed wetlands.These natural systems are often referred to as exam-ples of green technology because of the use of plants.However, `green' is a widely used and often misleadinglabel attached to any product or system which claims toreduce the impact on the environment. The factorswhich need to be considered when assessing how`green' a waste water treatment system is have beenlisted by Brix (1998):treatment performance in relation to effluent standards;robustness of process;emissions of various pollutants to environment;waste production (e.g. sludge);recycling or reuse potential;energy consumption, including source of energy used;use of chemicals;area use;environmental nuisance;environmental benefits.Brix suggests that an environmental life-cycle assess-ment approach, which quantifies energy and resource inputsand outputs at all stages of the life cycle, could be applied toconstructed wetland systems. However, he