ReviewGlobal development of various emerged substrates utilized in constructed wetlands
Graphical abstract
Introduction
Constructed wetlands (CWs) are engineered systems that have been designed and constructed to enhance the natural processes for wastewater treatment (Vymazal, 2011). CWs are particularly suitable for rapidly growing small- (population less than 200,000) and medium-size (population between 200,000 and 500,000) cities (Zhang et al., 2009). As have been well recognised as low-cost, sustainable, robust, and efficient engineered systems (Carvalho et al., 2017), CWs have been globally applied for treating wastewaters ranging from municipal to industrial and from urban to agriculture along with treating stormwater runoff, leachates and acid mine drainage and for sludge dewatering since 1960s (Doherty et al., 2015). Substrates, also known as media, support matrix/material, filling material, are one of the major components in CWs. They have been widely acknowledged to play a significant role (as carrier for biofilm development, as medium for wetland plant growth and as adsorbent for pollutant immobilization) in CWs (Wu et al., 2015), especially for the removal of non-biodegradable pollutants like organic xenobiotics (Dordio and Carvalho, 2013) and toxic metals (Allende et al., 2011, Hua et al., 2015).
Traditional substrates, such as soil, sand, and gravel, were thought mainly fulfilling the functions of supporting plants in CWs with marginal function on nutrient (especially P) and some specific pollutant removal. Systems using conventional substrates may be confronted with several problems, such as low removal performance and clogging (Wang et al., 2010, Zhu et al., 2011). These issues present a challenge to traditional substrates and inspire the development of alternative substrates in CWs. Over the last 10 years, intensive studies have been focused on pursuing cost-effective and efficient substrates in order to increase the treatment capacity or to minimize the clogging problem. Novel emerged substrates like oyster shell (Park and Polprasert, 2008), tire chips (Chyan et al., 2013), construction wastes (Yang et al., 2011, Shi et al., 2017), light weight aggregates/light expand clay aggregates (LWAs/LECA, commercially known as Filtralite) (Calheiros et al., 2008, Albuquerque et al., 2009), have been tested. Our group has focused on applying dewatered alum sludge (drinking water treatment residual) as main substrate in CWs over the last 10 years (Zhao et al., 2009, Xu et al., 2017a, Xu et al., 2017b).
Considering a wide range of substrates studied, a number of reviews on substrates used in CWs were done in recent years (Ballantine and Tanner, 2010, Wu et al., 2015). However, most reviewed substrates are classified based on their origin and categorized as natural substrates, industrial substrates, and man-made substrates. Few researchers reviewed substrates only aimed at specific pollutants. Vohla et al. (2011) studied substrates for P-retention. Dordio and Carvalho (2013) summarized substrates for organic matter removal, while Saeed and Sun, 2012a, Saeed and Sun, 2012b evaluated the influence of wetland substrates for the removal of organics and nitrogen. It is therefore highly desirable to comprehensively review the new development on substrates regarding state of the art CW technology. The objectives of this review were to: 1) identify the last 10 years’ development of emerged substrates employed in CWs; 2) elucidate the unique properties of various substrates so that they can be better applied into CWs for different contaminant removal based on their unique properties; and 3) highlight the application of industrial “wastes” as substrates to emphasis the sustainable and cost-effective characteristics of the CWs.
Section snippets
Selection of substrates
Substrate is a crucial component of CWs as it plays in integrated role of physical, chemical and more importantly biological functions to eliminate (including filter, trap, adsorb, biodegrade) the pollutants. Careful selection of substrates can lead to significant enhancements of the CW efficiencies. On a practical level, cost and local availability are the two fundamental factors determining the selection (Dordio and Carvalho, 2013). More significantly, the physical (e.g., particle size,
Categories of substrates
Substrates in CWs are traditionally classified into natural material, industrial by-products, and artificial/man-made products based on their sources (Tsihrintzis, 2017, Vohla et al., 2011, Wu et al., 2015). In this review, brief recall of the conventional substrates was given prior to the special focus on the newly emerged substrates which are classified based on their main mechanisms for pollutant removal in CWs.
Abundance & cost of using various substrates
CWs are generally typified as cost-effective technology due to minimal costs for construction, operation and maintenance. The total capital cost (construction and operation/maintenance cost) of an ecosystem consisting of integrated ponds and CW system is about half of the conventional activated sludge process (Zhang et al., 2009). The operation cost of gravel and sand based CWs is only 0.12–0.25 €/m3 (Gkika et al., 2014, Tsihrintzis, 2017), which are much lower than the convention biological
Conclusions
Emerged substrates are comprehensively reviewed and presented as ion-exchange substrates (specialized in ammonium removal, e.g., biochar), P sorption substrates (rich in Ca/Fe/Al, e.g., red mud), and electron donor substrates (efficient for denitrification, e.g., rice straw). Reuse of waste materials (like alum sludge, tyre chips) as alternative substrates are recommended in terms of the benefit in waste disposal and reuse. However, further studies on avoiding clogging of the reviewed
Acknowledgements
This work was supported by Irish Research Council, Republic of Ireland (GOIPD/2017/1367), Natural Science Foundation of Anhui Province, China (No. 1808085QE144), and Natural Science Foundation of China, China (41472047, 41702043, and 41772038).
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