Chapter 1 Introduction
1.1 The development and application of constructed wetland
Constructed wetland(CW)treatment is engineered system that has been designed based on the natural functions of vegetation,soil,and organisms for treating municipal or industrial wastewater,greywater or storm-water runoff.The first experiments on the using to CW for wastewater treatment were carried out in the early 1950s by Dr.K.S.,indicated that the reeds planted in the wetland showed the good power in the removal of organic matter and microorganisms such as Coliform bacteria,Salmonella and Enterococcus.A full-scale CW was constructed and operated for wastewater treatment in the late 1960s.Since then,the free water surface CW systems have been more popular in North America and Australia while the subsurface CW has been commonly applied into wastewater treatment in Europe(Vymazal,2011).However,the application of CW for treating wastewater was spread rather slowly during the 1970s to 1980s.Under the background of over fertilization in water body,the high effective removal of nitrogen,including organic nitrogen,ammonia,nitrite,and nitrate,has caught the interest of numbers of international scholars.Therefore,the vertical and horizontal flow CW were developed to achieve the higher effective treatment of contaminants and applied worldwide since the 1990s(Figure 1.1).Today,with the development of CW,it has been considered as the suitable technology for the treatment of many types of wastewater in the sparsely populated areas.
Figure 1.1 Constructed wetland
(a)free water surface constructed wetland;(b)horizontal subsurface flow constructed wetland;(c)vertical subsurface flow constructed wetland
Among 1952 to 1956,a multitude of studies was carried out for daily wastewater,phenol wastewater,and livestock wastewater treatment based on the removal function of wetland plants by Dr.K.S.However,in these years,the views of wastewater treatment among experts were mainly focused on the physical,chemical,and biological processes,the use of vegetation for wastewater treatment was rarely considered for the engineered application(Vymazal,2011).To enhance the performance of vegetation-based system,she also designed sealed basins filled with sandy soils and planted with various types of plants such as P.australis and Common reed(Vymazal,2011).After then,the first horizontal subsurface flow CW was designed and employed to treat municipal wastewater until 1974 in Liebenburg-Othfresen,Germany.
Although the horizontal subsurface flow CW showed a good ability in the contaminant removal efficiency,the free water surface CW was also widely applied into wastewater treatment in European and North America during 1967-1977.This was mainly resulted from that the organic matter and nitrogen concentrations in the wastewater was maintained in a lower level.Nevertheless,the application of CW for wastewater treatment in the cold climate area was still a difficult work because of the activities of microorganism and vegetation were limited to the low temperature(Vymazal,2007).The first depth-research on the engineered wetland for wastewater treatment at a lower temperature region was conducted by Kadlec and his co-teamers(Kadlec et al.,1979).At about the same time,some industrial wastewaters,including oil-byproducts and phenol wastewater,were employed to evaluate the removal performance of CW(Litchfield,1990).After that,the use of CW for treating wastewater sprang up over China,USA,Australia,India,and Japan.However,due to the high costs in biomass disposal and large footprint,these CW systems were not gained widespread acceptance in city’s municipal wastewater treatment.Nevertheless,CW was gradually accepted as an effective technology for wastewater treatment in rural areas and undeveloped countries in the later 20 years.
In the 1980s,various international conferences held in different countries to exchange the information about the engineered CW designs and researches,which led to the use of CW for wastewater treatment worldwide.At about the same time,many scholars focused on the higher effective removal performance system of horizontal subsurface flow CW,especially in Austria and Germany(Geller G.Lenz,1982).During this Period,there are about 80 horizontal subsurface flow CW systems built in Germany.In addition,the substrate of CW systems was constituted with soil media in Danish,which resulted in a lower hydraulic conductivity.Meanwhile,to improve the removal performance of contaminant,these CW systems were designed with a larger width and short passage length.In the end of 1987,a CW was designed with coarse filtration material(average gravel 5-10mm)in United Kingdom,which presented a higher hydraulic conductivity during the wastewater treatment.In addition,several hybrid systems(vertical-horizontal subsurface flow CW)were also developed in the early of 1980s according with the design of Dr.K.S.At about the same time,in the sparsely populated areas of North America,free water surface CW spread quickly and lots of engineered systems were built for the municipal wastewater treatment at cities of Florida,California,Lakeland and Orlando.In the end of this 1980s,the European and Germany guidelines were published about the design and operation for CW.
In the end of 20th century and early stage of 21th century,with the development and application of CW in various wastewater treatments,CW has been regarded as a relatively mature technology for the treatments of many types of wastewater.During this period,CW systems were designed and operated for the effluents treatment from pig farms,cheese daily,fish pond,chemical industry and airport runoff(Vymazal,2011).In addition,to removal the ammonia-N quickly from the effluents,the hybrid CW systems were developed(Mander et al.,2003).However,the use of CW for wastewater treatment usually presented a lower removal efficiency of phosphorus.To overcome these problems,several materials,including lightweight aggregates and industrial byproducts,were used as the substrate to support the absorption of phosphorus(Jenssen and Krogstad,2003).However,at present,the land requirement is relatively small in many cities,therefore,the researches on the CW systems focused on the enhancement of removal efficiency of pollutant in effluents to reduce the use of land.