ReviewDevelopment of constructed wetlands in performance intensifications for wastewater treatment: A nitrogen and organic matter targeted review
Graphical abstract
Introduction
The constructed wetlands (CWs) for wastewater treatment, also known as treatment wetlands, are engineered systems designed and constructed to utilize natural processes and remove pollutants from contaminated water within a more controlled environment (Faulwetter et al., 2009, Vymazal, 2011a). These systems have developed rapidly over the last three decades, and CWs have been established worldwide as an alternative to conventional more technically equipped treatment systems for the sanitation of small communities (Garcia et al., 2010). These systems are robust, have low external energy requirements, and are easy to operate and maintain, which makes them suitable for decentralized wastewater treatment in the areas that do not have public sewage systems or that are economically underdeveloped (Brix, 1999, Vymazal, 2009).
The technology of wastewater treatment by CWs was especially spurred on by Käthe Seidel in the 1960s (Seidel, 1961) and by Reinhold Kickuth in the 1970s (Kickuth, 1978, Brix, 1987). At the early stage of CW development, the application of CWs was mainly used for the treatment of traditional tertiary and secondary domestic/municipal wastewater (Kivaisi, 2001) and was often dominated by free-water-surface CWs in North America and horizontal subsurface-flow (HSSF) CWs in Europe and Australia (Brix, 1994b, Vymazal, 2011a). Aiming at inexpensive and effective ecological wastewater purification, CW development has received great attention from both scientists and engineers in the last decades. The application of CWs has also been significantly expanded to purify agricultural effluents (Zhao et al., 2004b, Wood et al., 2007), tile drainage waters (Borin and Toccheto, 2007, Kynkäänniemi et al., 2013), acid mine drainage (Wieder, 1989), industrial effluents (Mbuligwe, 2005; Calheiros et al., 2012), landfill leachates (Justin and Zupancic, 2009), aquaculture waters (Trang and Brix, 2014), and urban and highway runoff (Scholes et al., 1999, Istenič et al., 2012).
The removal of contaminants in CWs is complex and depends on a variety of removal mechanisms, including sedimentation, filtration, precipitation, volatilization, adsorption, plant uptake, and various microbial processes (Vymazal, 2007, Kadlec and Wallace, 2009, Faulwetter et al., 2009). These processes are generally directly and/or indirectly influenced by the different loading rates, temperatures, soil types, operation strategies and redox conditions in the wetland bed (Biederman et al., 2002, Stein et al., 2003, Stein and Hook, 2005, Yang et al., 2011). Given the fast urbanization and the land protection for crop production, natural passive CWs cannot be fully promoted because of the large area requirement. The number of research groups that study how these factors perform in the contaminant removal in CWs has dramatically increased in recent years. Similarly, the volume of knowledge and information published in international journals and books on minimizing the influences of these factors and possible solutions suggested to improve the treatment performance has increased considerably. Better understanding of the intensified removal processes responsible for water treatment has expanded concurrently with CW usage and has led to a great variety of designs and configurations, such as aerated subsurface-flow CWs (Nivala et al., 2007, Nivala et al., 2013b), baffled subsurface-flow CWs (Tee et al., 2012), and combinations of either various types of CWs (Vymazal, 2013) and/or with other technologies, to enhance the performance of CWs for wastewater treatment [e.g., microbial fuel cell (MFC) and electrochemical oxidation] (Grafias et al., 2010, Yadav et al., 2012) (Fig. 1).
The main objective of this paper is to review and discuss the recent developments in CW technology considering a wide range of expanded designs, configurations, and combinations with other technologies for the enhancement of wastewater treatment, mainly targeted on the removal of nitrogen and organic matter. By this study, new ideas should be inspired.
Section snippets
Effluent recirculation
Effluent recirculation has been proposed by various authors (Sun et al., 2003, Arias et al., 2005, He et al., 2006a, He et al., 2006b) as an operational modification to improve the effluent quality of CWs (Table 1). The concept of this method consists of extracting a part of the effluent and transferring it back to the inflow of the system. The main goal of effluent recirculation is to enhance aerobic microbial activity through the intense interactions between pollutants and micro-organisms,
Circular-flow corridor CW
The application of CWs has been increased in the last decades due to its cost-effectiveness and efficiency. However, some operational problems arise if the conventional subsurface flow wetlands were directly used for the treatment of high-strength wastewaters, such as the inhibition of high influent concentration ammonium on plants and deficiency of oxygen for large amounts of organic matter degradation. Considering the fact that the partial recirculation of treated wastewaters within wetlands
Supply of electron donors to enhance the removal of selected inorganic oxygenated anions
The nitrite and nitrate in domestic sewage are easily reduced by microorganism to N gas and leave the wastewater. However, oxygenated inorganic anions, such as sulfate (SO42−), can also be reduced, which can be technically applied for heavy metal precipitation as the insoluble sulfides. Other industrial chemicals, such as chlorate, perchlorate, chromate, and dichromate, that contaminate effluents, surface waters, and groundwater can also be reduced and detoxified by microorganisms (Kosolapov
Specific soil material selection for microbial biofilms establishment
Different substrates also influence the establishment of microbial biofilms and the microbial community structure within complex wetland ecosystems, as well as the treatment performance. A porous matrix, such as expanded clay, provides a greater surface area for treatment contact and biofilm development. Calheiros et al. (2009) investigated the bacterial communities in the CWs with different soil materials, i.e., two types of expanded clay aggregates (FiltraliteMR3-8-FMR and Filtralite
Thermal insulation in cold climate
Although a variety of removal mechanisms, including filtration, precipitation, volatilization, adsorption, and plant uptake, have been well documented (Vymazal, 2007, Kadlec and Wallace, 2009), the removal of most pollutants in CWs primarily caused by microbial activity has been a cornerstone of the technology (Faulwetter et al., 2009). The processes, such as sedimentation and decantation, important in particulate organic matter removal are mostly unaffected by low temperature conditions (
Conclusions
The intensified removal of organic matter and nitrogen in CWs is generally directly and/or indirectly influenced by many factors, including the temperature, soil material types, operation strategies, and redox conditions in the wetland bed. The present knowledge can be summarized as follows:
- (1)
The use of recirculation to enhance the removal performance in CWs depends on many factors, including CW types and influent loads. In most vertical-flow and integrated CWs, the effluent recirculation
Acknowledgments
This work was supported by the grants from “ The China National Natural Science Fund (51308536),” “ Chinese Universities Scientific Fund (2013XJ003),” and the Sino-Danish Center for Education and Research.” We greatly appreciate the critical and constructive comments from the anonymous reviewers, which helped to improve this manuscript.
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