Comprehensive evaluation of substrate materials for contaminants removal in constructed wetlands

Journal Pre-proofs Review Comprehensive evaluation of substrate materials for contaminants removal in constructed wetlands Yanting Wang, Zhengqing Cai, Sheng Sheng, Fei Pan, Fenfei Chen, Jie Fu PII: DOI: Reference:

S0048-9697(19)34727-8 https://doi.org/10.1016/j.scitotenv.2019.134736 STOTEN 134736

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Science of the Total Environment

Received Date: Revised Date: Accepted Date:

28 July 2019 12 September 2019 28 September 2019

Please cite this article as: Y. Wang, Z. Cai, S. Sheng, F. Pan, F. Chen, J. Fu, Comprehensive evaluation of substrate materials for contaminants removal in constructed wetlands, Science of the Total Environment (2019), doi: https:// doi.org/10.1016/j.scitotenv.2019.134736

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Comprehensive evaluation of substrate materials for contaminants removal in constructed wetlands

Yanting Wanga, Zhengqing Caia, Sheng Shengb, Fei Panc, Fenfei Chenb, Jie Fua,d,*

a

Department of Environmental Science and Engineering, Fudan University, Shanghai

200433, China b

Huadong Engineering Corporation Limited, Hangzhou 311122, China

c

School of Environmental Engineering, Wuhan Textile University, Wuhan 430073,

China d

School of Environmental Science and Engineering, Huazhong University of Science

and Technology, Wuhan 430074, China

*

Corresponding author:

E-mail address: [email protected] (J. Fu)

Comprehensive evaluation of substrate materials for contaminants removal in constructed wetlands

Yanting Wanga, Zhengqing Caia, Sheng Shengb, Fei Panc, Fenfei Chenb, Jie Fua,d,*

a

Department of Environmental Science and Engineering, Fudan University, Shanghai

200433, China b

Huadong Engineering Corporation Limited, Hangzhou 311122, China

c

School of Environmental Engineering, Wuhan Textile University, Wuhan 430073,

China d

School of Environmental Science and Engineering, Huazhong University of Science

and Technology, Wuhan 430074, China

*

Corresponding author:

E-mail address: [email protected] (J. Fu)

Abstract Considerable number of studies have been carried out to develop and apply various substrate materials for constructed wetlands (CWs), however, there is a lack of method and model for comprehensive evaluation of different types of CWs substrates. To this end, this article summarized nearly all the substrate materials of CWs available in the literatures, including natural materials, agricultural/industrial wastes and artificial materials. The sources and physicochemical properties of various substrate materials, as well as their removal capacities for main water contaminants including nutrients, heavy metals, surfactants, pesticides/herbicides, emerging contaminants and fecal indicator bacteria (FIB) were comprehensively described. Further, a scoring model for the substrate evaluation was constructed based on likely cost, availability, permeability, reuse and contaminant removal capacities, which can be used to select the most suitable substrate material for different considerations. The provided information and constructed model contribute to better understanding of CWs substrate for readers, and help solve practical problems on substrates selection and CWs construction.

Keywords:

Constructed wetlands; Substrates; Water contaminants; Removal capacity;

Scoring model

1. Introduction Constructed wetlands (CWs) are green and engineered wastewater treatment systems, which are designed and constructed to utilize the natural purification processes involving wetland plants, substrates and the associated microbes (Cheng et al., 2018). In 1953, Dr. Käthe Seidel from the Max-Planck Institute of Germany reported the ability of aquatic plants in artificial wetlands to absorb and decompose chemical contaminants, which is the starting point for CWs research (Vymazal, 2011). Since 1990s, a significant progress has been made on the CWs technology by scientists and researchers, and CWs have become international and been widely adopted around the world (Vymazal, 2011). Nowadays, the United States has built more than 10,000 CWs sewage treatment systems, Europe has built more than 8,000 CWs, and China has built 425 CWs (Zhang et al., 2012). In addition to the initial application in treatments of domestic wastewater (K.Kivaisi, 2001) and acid mine wastewater (Wieder, 1989), CWs have extended to be used in purifications of agricultural effluents (Wood et al., 2007), tile drainage waters (Kynkäänniemi et al., 2013), and industrial effluents (Mbuligwe, 2005). According to the wetland hydrology, CWs are typically classified into three types: surface flow (SF) CWs, subsurface flow (SSF) CWs and hybrid systems (Tilley et al., 2014). SF CWs are similar to natural wetlands, with shallow flow of wastewater over substrates

(Fig. 1a). In SSF systems, wastewater flows through the substrates, and

based on the flow direction, SSF CWs could be further divided into horizontal flow (HF) and vertical flow (VF) CWs (Wu et al., 2015). In HF CWs, the wastewater flows

horizontally through the substrates under the surface of the bed planted with macrophytes (e.g., Common reed) (Fig. 1b). The lack of oxygen in HF CWs facilitates the bacterial decomposition of organics in anaerobic conditions (Brix, 1990) and denitrification process (Kröpfelová, 2008), but suppresses the oxidation of ammonia and hence nitrification process (Vymazal, 2011). In VF CWs, wastewater flows longitudinally from the surface into the bottom of the bed planted with macrophytes (Fig. 1c). The intermittent bed draining allows air to refill the bed and provides greater oxygen transfer into the bed, thus producing a strong ability of nitrification (Cooper et al., 1996). However, the denitrification is relativley limited in VF CWs process. To achieve a higher removal efficiency of contaminants, especially for nitrogen, various types of CWs may be combined to form hybrid systems (Hoffmann et al., 2011). Substrates are indispensable parts of CWs, and most of the physical, chemical and biological reactions are carried out in substrates (Yang et al., 2018a). The substrates not only support the growth of wetland plants and provide attachments for biofilms, but also play a significant role in contaminant removal (Wu et al., 2015). The substrate purification mechanisms include physical sedimentation and filtration (Wood, 1995), sorption and burial in substrate matrix (Daneshvar et al., 2017), complexation and precipitation by colloid composition (Kröpfelová et al., 2009), ion exchange (Matagi et al., 1998), gas diffusion in the substrate gap (Vymazal et al., 1998), microbial degradation, transformation and bio-immobilization in the substrate (Stottmeister et al., 2003), and uptake and metabolism by plant root in the substrate (Brix, 1997).

A variety of materials could be used as substrates for CWs, including natural materials (e.g., gravel and sand), agricultural/industrial wastes (e.g., oyster shell and fly ash) and artificial materials (e.g., activate carbon and ceramsite) (Cheng et al., 2018), and different materials possess different characteristics. Natural materials are materials in nature that can be directly used without processing or basically without processing. Agricultural waste refers to the waste discharged from agricultural production, processing of agricultural products, rural residents' lives and animal husbandry. Industrial waste refers to solid, semi-solid, liquid and gaseous particles and substances that are released in the industrial production activities or that have been discarded or abandoned. Artificial materials are made from natural and/or waste materials and usually have the characteristic of recyclability. Substrate selection is one of the key issues in CWs wastewater treatment (Yang et al., 2018a), and suitable substrates can effectively remove various pollutants, and avoid clogging and improve operation cycle. Problems to be considered for the selection of substrate materials include their source and cost, hydraulic and engineering feasibility, ability to remove contaminants, support for plant growth and microbial adhesion, safety (secondary pollution), substrate plugging, substrate life, and recovery and disposal issues of exhaustion substrate, etc. (Fig. 2). Just because of so many problems to consider, the choice of substrate has become a confusing issue for scientists and engineers. In response to the above problems, it is necessary to find a good method of substrate selection. Therefore, this article reviewed the sources and physicochemical

properties of various substrate materials, as well as their removal capacities for main water contaminants including nutrients, heavy metals, surfactants, pesticides/herbicides, emerging contaminants and fecal indicator bacteria (FIB). Further, a scoring model for the substrate evaluation was constructed based on likely cost, availability, reuse and contaminant removal capacities, which can be used to select the most suitable substrate for different considerations. The overall goal of this work was to provide a simpler, faster, scientific and reliable reference for the selection of substrates in CWs with different purposes. 2. Substrate materials 2.1. Natural materials 2.1.1. Gravel Gravel is a rock fragment whose particle size is larger than 2 mm and smaller than a single soil, and the main chemical compositions are SiO2 and Al2O3. A large number of studies have shown that the existence of gravel in the soil will not only affect the surface roughness and soil physical properties (soil bulk density, soil porosity, soil water conductivity, soil moisture content), but also affect the soil infiltration characteristics. Gravelly soils are widely distributed in the world, and they are relatively easy to obtain, safe and non-polluting. In addition, gravel possesses high permeability with permeability coefficient of 3×10-43×10-2 m/s. Many studies suggested that gravel had worse P adsorption capacity than other materials. For example, Ge et al. (2015) found a higher adsorption capacity of slag (3.15 g/kg) than gravel (0.81 g/kg) in batch tests, and the removal of total phosphorus by slag

was about 20% higher than that of gravel in horizontal subsurface flow (HSSF). However, Chen et al. (2013)) observed that pot gravel had a high total P removal efficiency for 65% in vertical flow stormwater wetland. The feasibility of P adsorption by gravel was also demonstrated in a large-scale CWs system with the removal efficiency ranging from -40% to 40% (Mann and Bavor, 1993). The adsorpiton isotherm of P onto gravel fitted well with Freundlich model (Tang et al., 2010), indicating a heterogeneous surface of gravel with nonuniform distribution of the adsorption sites. Several laboratory researches have been counducted to determine the N adsorption capacity of gravel, and researchers have concluded that the effect of gravel on N adsorption is not very prominent. For example, Zhu et al. (2011)) estimated the maximum N adsorption capacities of gravel to be 769.23 mg/kg using the Langmuir isotherm. However, the field tests have achieved more positive results. During the 2year monitoring, Ge et al. (2015)) observed comparable N removal rates in large-scale horizontal subsurface flow CWs with either gravel or slag as substrate, providing a possible solution for polluted urban river remediation in northern China. There are numerous examples that gravel as CWs substrate has excellent heavy metals removal capacity. Buddhawong et al. (2010)) found that in the CWs with gravel as the substrate could completely remove As and Zn. The Cr and Ni removal abilities of gravel-bed CWs were studied by Yadav et al. (2010)), and results showed that the maximum removal of Cr and Ni reached 98.3% and 96.2%, respectively. Some studies indicated gravel had the potential for water decolorization. Bulc and Ojstršek (2008)) tried to use CWs to treat dye-rich textile wastewater, and results showed that CWs

packed with gravel, sand, and zeolitic could reduce colour by up to 70%. Manios et al. (2002))

found CWs with gravel reed beds achieved a high reduction of E. coli, and

could remove above 3.3 log for E. coli. 2.1.2. Sand Sand is a kind of granular material with a grain size <2 mm, which is rolled by rain or rolled by rock after weathering. It is usually divided into coarse sand (permeability coefficient: 9×10-76×10-3 m/s), medium sand (permeability coefficient: 9×10-75×10-4 m/s) and fine sand (permeability coefficient: 2×10-72×10-4 m/s). The main component of sand is SiO2 with a density of 2.65 g/cm3 (Zuhailawati et al., 2007). The P removal by sand-filled CWs have been widely studied. Greenway (2016)) compared the P removal efficiency of various CW substrate materials, and found sand was the best (94-99%), followed by brickies loam (92%), and then red mud modified sand (86-89%). A long-term full-scale study indicated the subsurface flow CWs with sand substrate could remove 99% of dissolved P within three years (Shannon et al., 2001). Based on the budget anlaysis on sand-bed CWs (Mander et al., 2003), 88.1% of P was removed by sand adsorption, which far outweighed the contributions of plant accumulation and microbial immobilization. However, some type of sand is unsuitable for long-term P retetion in CWs. For example, a pilot study reported masonry sand substrate retained insignificant quantity of P (11.9±21.8 mg/kg) after one year of CWs operation (Forbes et al., 2005). In recent years, several studies about N removal by sand-filled CWs was carried out. A full-scale study on the sand-bed CWs showed that total N removal increased from 60.1% to 93.4% in the first three years of operation

(Shannon et al., 2001). Eturki et al. (2012)) studied the wastewater purification capacity of sand-clays fortified by pebbles, and this reconstituted sand filter gave a N reduction of 65%. Scholz (2003)) indicated sand substrate could effectively remove the heavy metals and play the dominant role for the removal of heavy metals in CWs. Wu et al. (2014)) studied the potential of manganese sand as CW substrate for removal of As(V), and the results showed manganese sand had an As(V) adsorption capacity of 42.37 μg/g. Quiñónez-Díaz et al. (2001) investigated the removal of FIB by a sand-filled CW, and found that >90% of FIB were removed. 2.1.3. Shale Shale is a rock formed by dehydration and cementation of clay. According to different composition, shale is divided into carbonaceous shale, calcareous shale, sandy shale and siliceous shale . The compressive strength of rock mass is 19.61-68.65 Mpa or lower, and the permeability coefficient is 1×10-132×10-9 m/s. Many studies have shown that shale has very excellent P adsorption effect. Drizo et al. (1999)) compared

the P adsorption effects of seven substrates, and results

showed that the fly ash and shale had the best P adsorption capacity, followed by bauxite, limestone and light expanded clay aggregates. Long term experiments showed that shale and bauxite gave the maximum P adsorption capacity of 730 and 355 mg/kg. A one-year pilot study indicated expanded shale can greatly improve the retention of P in CWs, and for a substrate depth of 0.9 m, aerial P retention by shale was 201±98.6 g/m2·y (Forbes et al., 2005). Daothaisong and Yimrattanabovorn (2009)) indicated that

shale was a potential substrate in CWs for N removal. The adsorption efficiencies of shale for NH4-N and NO3-N were 52.9-69.0% and 57.0-72.0%, respectively. 2.1.4. Soil/sediment Soil is a major component of the Earth’s ecosystem, and forms pedosphere. Soil particles comprise of minerals (clay, silt and sand) with a density of 2.6-2.7 g/cm3. The permeability coefficient of soil/sediment is 6.0×10-76.0×10-6 m/s. When the soil is broken down by processes of weathering and erosion, the soil particles will be washed away by the water, flow into the river, lake and ocean, and form the sediments by sedimentation (Fernandez et al., 2003). Numerous researches have been conducted in recent years, and demonstrated that soil/sediment in CWs exhibited excellent P sorption capacity. Farahbakhshazad et al. (2000)) found that vertical upflow wetland system in Piracicaba, Brazil could removal 93% P and the effective P removal was observed in the top soil layer which had a high surface adsorption area. The ability of peat soil to retain PO4-P was determined by adsorption isotherms in both laboratory and field tests (Heikkinen et al., 1995; Walbridge and Struthers, 1993), and the contents of oxalate-extractable Fe and Al in peat soil could positively influence the ability of peat to retain P. Litaor et al. (2003) found that a relatively low degree of P saturation (DPS <15%) was

observed in

histosols, while a high DPS (>30%) was observed in many of the hydromorphic organomineral soils. D'Angelo (2005) observed that late successional (LS) soils had three times greater capacity to remove and retain soluble inorganic P than early successional (ES) soils, which was mostly due to higher amounts of amorphous Al oxides (oxalate-

extractable) in LS soils. Reddy et al. (1998) indicated the P retention by stream sediments and Wetland soils was strongly correlated with contents of amorphous and poorly crystalline forms of Fe and Al, which explained 87% of the variability in P retention. Besides of the mineral compositions, the orgnaic matter in soil/sediment also play an important role for the P removal (Gilbin et al., 2000). Different results were achieved by researchers about N removal capacities of soil/sediment. Zhu et al. (2011) reported the maximum NH4-N adsorption capacities of paddy soil and red soil were 588.2 and 555.6 mg/kg, respectively. Zhang et al. (2017) indicated that 98.8% of NH4-N and 88.3% of total N (248.6 mg/L) were removed from low-strength swine wastewater by a pilot-scale SF CW using sediment as the substrate, and vegetation of the macrophytes (e.g., Myriophyllum aquaticum) would increase the N removal of CWs by enhancing the sediment microorganisms responsible for N cyclings. Llorens et al. (2015) reported a mean removal efficiency of 52% for total N could be achieved using a hybrid constructed soil filter. For the N removal mechanism in soil-filled CWs, studies indicated microbial nitrification/denitrification played the dominant role, and adsorption onto soil and plant uptake played the secondary role (Abe et al., 2014). Therefore, promoting the microbial activity in soils is an effective way to enhance the N removal in CWs, such as modulating the favorable pH for microbial growth (Yin et al., 2016). Many studies indicated soil substrate in CWs can effectively adsorb heavy metals, which played the most important role for the removal of heavy metals. For example, Mohammed and Babatunde (2017) indicated that up to 91%, 91% and 89% of Cr, Cd

and Pd respectively could be removed by soil adsorption process in VF CWs. Palmer et al. (2015) measured the retention efficiencies based on monitoring of peatland inflow and outflow waters and the results indicated that retention of As, Sb and Ni was generally good (up to 95%) but temporarily variable. Adsorption experiments indicated the high potential of peat soils to adsorb As, Ni and especially Sb. Ncibi et al. (2017) indicated that soil-filled CWs were suitable for removal of surfactants, and the removal efficiency of nonionic surfactants was higher than that of anionic surfactants. Nowadays, removal of pesticides/herbicides from water by soil/sediment based CWs

has become a hot topic, and many studies on this have been carried out.

Passeport et al. (2011) indicated wetland sediment and forest soil had the most important potential for pesticides adsorption. Vallée et al. (2014) found that the presence of plants in CWs could increase soil/sediment sorption capacity for pesticides, which probably because the presence of plants increases water retention time and pesticide residues, then prolongs the contact time between soil/sediment and pesticides. Besides of the soil sorption and plant accumulation, microbial degradation was demonstrated to play an important role for the removal of herbicides in SF CWs with soil (Chen et al., 2017). For the removal process of pesticides across soil/sedimentfilled CWs, retention time is a critical factor influncing the final removal efficiency (Gregoire et al., 2009). Studies demonstrated CWs with soil/sediment substrate could effectively remove pharmaceuticals (Hussain et al., 2012), dyes (Davies et al., 2006), halogenated flame retardants (HFRs) (Chow et al., 2017), and many other organic contaminants. Hussain

et al. (2012) studied the removal efficiency of three antibiotics of ionophore group in two SF CWs with different substrates: one with a sandy clay loam soil and another with a sandy soil. Sandy soil condition showed higher removal efficiency in comparison to sandy clay loam soil. More water could immerse in sandy soil than in clay soil, which provides more interaction between pharmaceuticals and substrate, resulting in greater pharmaceuticals sorption onto sandy soil than clay soils. Numerous studies have shown that CWs can effectively remove FIB from wastewater (Dorsey et al., 2010; Morató et al., 2014), and SSF CWs have a better removal capacity than SF CWs (Wu et al., 2016b). Hydrology and wetland studies generally assume the removal mechanism of FIB in CWs is binding to soil particles in the water and immobilizing as soil particles sedimentation (Waller and Bruland, 2016). Predation by copepods (Song et al., 2008) and zooplankton (Chudoba et al., 2013) is an another important mechanism for the removal of bacteria in CWs. In addition, root exudates/rhizosphere microbes may also adversely impact the survival of fecal microorganisms (Chandrasena et al., 2017). A lot of studies have been conducted and recommended soil or sediment as a good substrate for FIB removal in CWs. For example, Thurston et al. (2001) reported that in the CWs with soil substrate receiving secondary sewage effluent, total coliforms were reduced by an average of 98.8%, FIB by 98.2%, and coliphage by 95.2%. 2.1.5. Zeolite Zeolite is formed from the reactions between volcanic rocks and ash layers with alkaline groundwater, and it is also crystallized in post-depositional environments in

shallow marine basins. The skeleton structure of the zeolite is relatively porous with a specific gravity of 2.0-2.2, and the dehydrated cavity can be as large as 47-50%. Rich micropore and macropore of zeolite determines its high sorption capacity (sum of absorption and adsorption) for contaminants. Besides, it has good characteristics of ion exchange, catalysis, stability, chemical reactivity, reversible dehydration, and conductivity. Zeolite has been widely used in the treatment of wastewater and landfill leachate because of its low price, easy to obtain, large specific surface area, welldeveloped pore channel, good ion exchange capacity, and good removal capacity on heavy metals, ammonia and other contaminants (Wen et al., 2018). Zeolite is well-known as a substrate used in CWs, while various studies have proved that its P adsorption capacity is low. For example, Zhang et al. (2006)) demonstrated that the removal efficiency of phosphate and hydrogen ions of shale was better than that of zeolite, which was due to the main component of shale was CaCO3. Dai and Hu (2017)) studied the P adsorption capacities of four substrate materials and revealed zeolite had a moderate adsorption capacity. Stefanakis et al. (2009)) used fineand coarse-grained clinoptilolite in CWs. The average removal rate of total P by the two kinds of zeolites was 56.8% and 40.5%, respectively. The mechanistic study carried out by Cheng et al. (2013)) indicated the adsorption of P onto zeolite was more apt to electrostatic attraction or ion-exchange, while other active adsorption materials (e.g., sponge iron) was more apt to chemical combination. The ability to adsorb P could be improved by modifying zeolite or combining with other substrates (Zhang et al., 2014).

Zeolite is rich in micropores and mesopores and therefore has strong N adsorption capacity especially ammonia . In recent years, many researches investigated the N adsorption onto zeolite and recommended zeolite as an ideal CWs substrate to enhance the N removal in CWs (Cyrus and Reddy, 2011; Zou et al., 2012). Pucher et al. (2017)) investigated the effect of zeolite layer with different depths on removal of NH4-N. Results indicated that the increase of zeolite layer in CWs could significantly reduce the amount of NH4-N in wastewater. Laboratory-scale experiment carried out by Liu et al. (2014)) showed that the N removal efficiency of zeolite-based CWs was better than that of other substrates filled CWs, which was because of the competitive properties of zeolite (micropore volume: 61.2 mm3/g, specific surface area: 16.6 m2/g, and cation exchange capacity: 4.3 cmol/kg). Peng et al. (2012)) indicated zeolite substrate adsorption played a dominant role for the removal of NH4-N in CWs in low temperature (3-7.5 °C), while both nitrification and zeolite adsorption made contribution in high temperature (20-25

°C). A lot of mixed substrates of zeolite and other materials

showed high N sorption capacities (Fang et al., 2016; Mojiri et al., 2016). For example, Singh et al. (2014)) found the mixture of natural zeolite and gravel as the CWs substrate showed a high removal efficiency of NH4-N (78%). Pálfy et al. (2017)) reported a sandzeolite mixture filter could removal 52% of N. Research conducted by Shi et al. (2017)) showed that the combination of steel slag and zeolite was better than two separate substrates for N removal.

Zeolite has shown a high potential for removal of heavy metals as CWs substrate. A laboratory-scale study investigated by Allende et al. (2011)) indicated that zeolite could significantly remove As, Fe, Mn, Cu and Zn. 2.1.6. Limestone Limestone is a sedimentary rock which is often composed of the skeletal fragments of marine organisms such as coral, foraminifera and molluscs. The main component of limestone is calcium carbonate, and its density is about 2.65-2.80 g/cm3. The permeability coefficient of limestone is 1×10-96×10-6 m/s. Limestone are widely used as raw materials for building, as white pigment in paints, as a chemical feedstock for the production of lime, and as a soil conditioner (Flores et al., 2018). Li et al. (2012)) investigated the influencing factors of limestone adsorption with a series of batch experiments including temperature, pH, limestone size, background electrolyte and organic acids. Based on the results, they concluded that limestone was suitable for P removal as CWs substrate in wastewater advanced treatment. However, Drizo et al. (1999)) showed that the P adsorption capacity of limestone was moderate, and suggeted mixing limestone with other materials (e.g., zeolite) to improve the P removal efficiency. There are relatively limited studies for the application of limestone substrate in CWs for N removal. Srinivasan et al. (2008)) used a combined adsorbent of limestone and zeolite and achieved a removal rate of 70% for NH4-N. The heavy metal removal capacities of gravel, cocopeat, zeolite and limestone were investigated by Allende et al. (2012)), and limestone showed the best removal capacity.

2.1.7. Volcanics Volcanics belongs to magmatic rock (igneous rock). It is a extrusive rock formed from magma condensed from the crater to the Earth’s surface. Volcanics is among the most common rock types on Earth’s surface, and it has been estimated that volcanics covers about 8% of the land surface. Volcanics is porous and has a good hydraulic conductivity (Liu et al., 2013a). The permeability coefficient of limestone is 8×1093×10-4

m/s. It is composed mainly of SiO2, Al2O3, Fe2O3, MnO, MgO, Na2O, K2O,

and CaO (Ramos et al., 2015). The P adsorption capacity of volcanics was very poor, and sometimes, volcanics could be used as an ingredient of substrate mixture to enhance treatment performance of CWs (Zhao et al., 2016). Liu et al. (2014) demonstrated that the N adsorption capacity of volcanics was low. However, as mentioned above, the mixture of zeolite, expanded vermiculite, and volcanics showed good N adsorption efficiencies (Zhao et al., 2016). 2.1.8. Minerals Minerals are the elements and their chemical compounds that are produced and developed in a variety of geological processes, and are relatively stable under certain geological and physico-chemical conditions. A mineral has a relatively fixed chemical composition and a definite internal structure, which is the basic unit for the composition of rocks and ores. The widely used minerals in CWs include calcite (CaCO3), dolomite (CaMg(CO3)2), hornblende ((Ca,Na)2-3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2), hematite

(Fe2O3),

apatite

(Ca10(PO4)6(OH,F,Cl)2),

wollastonite

(CaSiO3),

vermiculite

(Mg0.7(Mg,Fe,Al)6(Si,Al)8O20(OH)4·8H2O) and pyrite (FeS2). Calcite was demonstrated by Dong et al. (2008) to be the best adsorption substrate for P at various temperatures and pH. They reported the removal of total P in the effluent was 94.3% in hybrid CWs with calcite substrate. Phosphorus uptake by calcite was also studied in both batch and continuous systems by Molle et al. (2003). In batch tests, calcite showed attractive sorption capacity, however, some limitation in retention capacity and effluent quality were pointed out in open reactor experiments. CWs with river sand and dolomite (10:1, w/w) as substrate were studied on phosphate removal by Prochaska and Zouboulis (2006), and results indicated that CWs could remove more than 45% of initially applied phosphate. Tang et al. (2010) indicated the maximum P adsorption capacity of hornblende was 153.1 mg/kg with the particle size in the range of 0.5-1.0 mm, and the adsorption data could be well explained by Langmuir isotherm model. Grüneberg and Kern (2001) reported the P adsorption capacity of hematite was 140 μg/g, and the adsorption of P was mainly dependent on the formation of amorphous ferrous hydroxides under anaerobic condition. Apatite is an effective and sustainable substrate for P removal from wastewater. Dong et al. (2008) indicated calcite was the best adsorption substrate for N compared with other substrate materials, and the removal efficiency reached to 68.4% in the CWs using calcite as the substrate. However, the other minerals including dolomite shwoed modest N removal capacity. Some researches were conducted to determine if minerals could be used in heavy metal removal. Chowdhury (2017) found that pyrite was the main adsorbent for As in

microbe-mediated adsorption process in CWs. Kaplan and Knox (2004) found that apatite addition to sediment resulted in the significant reduction of Cd, Co, Hg, Pb, and U concentrations in porewater. 2.2. Agricultural/industrial wastes 2.2.1. Alum sludge Alum sludge is a waste product from the drinking water treatment plant when aluminium salts

are employed as the coagulants. The content of aluminum in alum

sludge is ~30% based on dry weight (Babatunde and Zhao, 2007). Although the most prevalent final disposal of the dewatered alum sludge is landfill, the reuse of the alum sludge is attracting more and more attention (Zhao et al., 2009a). The idea of reuse alum sludge as CWs substrate lies in its characteristics of abundant Al content and easy, local and huge availability. Aluminum sludge itself can form a precipitation of aluminium hydroxide, which was used to adsorb and precipitate phosphorus and nitrogen. At the same time, colloidal precipitation of aluminium hydroxide also increases the porosity of alum sludge, and provides sufficient and persistent permeability. In recent years, several researches have been carried out and demonstrated the high nutrients adsorption capacity of alum sludge as CWs substrate. For example, Zhao et al. (2009b)) conducted a batch test of P adsorption onto alum sludge. Result showed that alum sludge (dry solid) had excellent P adsorption capacity of 14.3 mg/g at pH 7.0, and lower pH was more conducive to adsorption. The column test with alum sludge media achieved P removal efficiency of 91.9% under the hydraulic loading of 0.36 m3/m2·d. Zhao et al. (2011) also conducted a pilot test on CW system with alum sludge

as the main substrate, and results showed that the average removal rates of total P and inorganic P per month were 75-94% and 73-97%, respectively. In a column test with alum sludge media, Zhao et al. reported a high removal efficiency of 76.5% for total N (Zhao et al., 2009b). In another pilot test on CWs with alum sludge as substrate, Zhao et al. (2011) found the average removal rates of total N and NH4-N per month were 1178% and 49-93%, respectively, indicating that NH4-N could be effectively reduced by alum sludge. 2.2.2. Oyster shell Oysters are salt-water bivalve molluscs that live in marine or brackish habitats. Oysters have many uses: some kinds of oyesters can be consumed by humans, some oysters produce pearls within the mantle, and translucent shells of some windowpane oysters can be for decorative objects. Oyster shell contains high concentration of CaCO3 (>95% by weight) (Cheng et al., 2018). Many studies have shown that oyster shell has excellent effect on P adsorption. During the adsorption test carried out by Wang et al. (2013a), oyster shell showed the best phosphate removal efficiency. The authors furtherly carried out a pilot test to treat the wastewater by a VF CW using oyster shell as the substrate and achieved a very significant removal of P. For the P removal mechanism, adsorption and precipitation played the greatest role, and speciation analysis indicated adsorption of P onto oyster shell was dominated by active Ca (Liu et al., 2013a). Yam et al. (2013) indicated that oyster shell was an effective N adsorbent in CWs. In Park and Polprasert’ work (2008),

the integrated CW system packed with oyster shells as filtration media showed a high efficiency in removing N. 2.2.3. Woodchip Woodchip is sawdust and shavings powder left in wood processing. It is mainly used to make paper, fuel, lightweight filling material, or wood based panel. As reported, wood chips would leach out organic compounds and provide increased organic concentration in the wastewater. Besides, wood chips would have short system life, rapidly losing their permeability. Recently, some researchers indicated applying woodchip to CWs as substrate could enhance operation performance (Robertson, 2010). Chen et al. (2013)) constructed woodchip-based VF CWs and achieved a high total N removal efficiency (40%). Fatehi-Pouladi and co-workers designed woodchip bioreactors based on the CWs principle and conducted a long term study to treat greenhouse effluent (Fatehi-Pouladi et al., 2019). The bioreactors exhibited excellent N removal capacity, and planting with Typha angustifolia could increase the denitrification rate. Gottschall et al. (2016)) applied woodchip bioreactors to treat drainage from agricultural production systems, and evaluated the removal efficiencies for a suite of veterinary antibiotics. Results showed woodchip bioreactors had a very high removal efficiency for all the antibiotics. 2.2.4. Plant waste Plant waste refers to the by-product generated during the plant processing, such as coconut dust, coconut shell, cocopeat, cattails leaves, oil palm shell, reed leaves, sedge shoots, bark mulch, rice husk, and straw. Due to rich organic carbon, porous structure

and light weight, many plant wastes can be applied to CWs as the substrate to enhance the treatment performance. Karunarathna et al. (2007) indicated that the P removal rate of CWs with coconut dust as the substrate was 98%. Chen et al. (2010) investigated the P removal efficiency by the dried straw of water hyacinth. The straw showed a rapid reduction of total P, and the adsorption efficiency at saturation was about 36%. Karunarathna et al. (2007) demonstrated that the removal efficiency of total N and NO3-N by CWs with coconut powder as substrate was significantly higher than that of sand-based CWs. Allende et al. (2011) conducted a lab-scale study and indicated that cocopeat could significantly remove As, Fe, Mn, Cu and Zn. Sharainliew et al. (2011) found cattails (Typha angustifolia) leaves was a feasible, cheap and environmentally friendly substrate for Pb(II) adsorption. Batch adsorption test showed that cattails leaves with the optimized dosage of 0.6 g could effectively remove 86.04% of Pb(II) at equilibrium from a 100 mL Pb(II) solution of 25 mg/L. Chong et al. (2013) found that oil palm shell had the adsorption capacity of 1.756 and 3.390 mg/g for Cu(II) and Pb(II), respectively. Parallel batch experiments conducted by Camilo et al. (2013) indicated bark mulch bioreactors were capable of retaining the herbicides atrazine or bentazone, and the removal of herbicides were based on adsorption to bark mulch and degradation processes. Saba et al. (2015) studied the decolorization in CWs, and adsorption process could remove 50% dye with rice husk as substrate. Weragoda et al. (2010) indicated applying coconut shell as substrate in CW system could increase E. coli removal.

2.2.5. Fly ash Fly ash is a fine ash captured from the flue gas after coal combustion, and is the main solid waste discharged from coal-fired power plants. Its main oxide compositions include SiO2, Al2O3, FeO, Fe2O3, CaO, and TiO2. Fly ash particles are porous and honeycomb, with large specific surface area and high adsorption capacity. The particle size ranges from 0.5 to 300 μm. Guo et al. (2014) used the mixture of fly ash and soil in CWs as substrate to remove P from wastewater. The results showed that when the mass ratio of fly ash and soil was 3:7, the treatment effect was the best, and the removal rate of total P was 97.58%. Removal of different forms of P was investigated by Liu et al. (2013b) using fly ash brickbat in CWs. Results revealed that the removal of soluble active P was accomplished by adsorption onto the substrate with a removal rate of over 97%. Wendling et al. (2013) demonstrated that fly ash could effectively remove organic and inorganic N. The potential neoformation of Al, Fe and Mn oxide/hydroxide minerals, such as Fe(OH)3, 𝛼-FeOOH, 𝛼-Fe2O3, Fe8O8(OH)6(SO4)•nH2O, in fly ash promotes the sorption of nutrients and DOC. Guo et al. (2014) found the removal rate of total N and NH4-N were 57.52% and 93.78%, respectively across the CWs using the mixture of fly ash and soil as the substrate. 2.2.6. Slag Slag includes blast furnace (BF) slag and steel slag. BF slag is a by-product of blast furnace ironmaking process, which contains Si (30-40%), CaO (30-40%), Al2O3 (5-15%), and Fe2O3 (0.02-0.45%) (Haynes et al., 2011). BF slag is a hard crystalline

material and its mineralogy is dominated by calcium aluminosilicate mellilite group including akermanite (Ca2MgSi2O7) and gehlenite (Ca2Al(AlSiO7)). Steel slag the byproduct of steel-making process, which contains Fe2O3 (42-84%), CaO (25-55%), SiO2 (9-18%), Al2O3 (0-3%), and MnO (1-6%) (Geiseler, 1996). Steel slag has a crystalline structure with the main mineral phase being wustite (FeO), and the other phases include dicalcium silicate, tricalcium silicate, and dicalcium ferrite. BF slag and steel slag have been demonstrated to be an excellent adsorbent for P by numerous researches. Mann (1997) observed that steelworks by-products (i.e., BF slag, steel slag and fly ash) exhibited high P adsorption capacities (>380 mg/kg), and the P adsorption capacities of slags were closely related to the active Ca and Mg. In batch experiments conducted by Korkusuz et al. (2007), BF granulated slag showed the highest P adsorption capacity than other filter materials due to its higher content of Ca and porous structure. Paul and Anderson (2011) evaluated the P removal efficiencies of BF slag and cement clinker in long-term flow cell experiments, with gravel as a control medium. As a result, BF slag and cement clinker showed nearly 100% P removal efficiency, while the control substrate gravel only achieved 50% removal efficiency. Blanco et al. (2016) studied the P removal by basic oxygen furnace (BOF) steel slag. In column experiments, BOF steel slag achieved the P removal efficiency of higher than 99%, and P removal capacity of 3.1 mg/g. The P removal mechanism was mainly calcium phosphate precipitation, which depended on the release of Ca2+ and OH- from the BOF steel slag. Zhu et al. (2011) compared several substrates for N adsorption capacity, and BF slag exhibited medium adsorption capacity.

Andreomartínez et al. (2017) found applying BF slag to CWs as adsorption substrate would be beneficial for total N removal. Andreomartínez et al. (2017) found that HF CWs filled with BF slag could effectively remove E. coli with a removal of 96.9±1.7%. 2.2.7. Construction waste Construction waste refers to the dregs, residual materials, abandoned bricks, silt, and other wastes produced in the process of construction, laying, demolition and repair of various buildings, structures and pipelines. Utilization of construction waste is a promising way to reduce its disposal cost. Wang et al. (2013a) screened P-removing substrates for use in CWs treating swine wastewater, and indicated broken bricks were suitable substrates for P removal. Zhou et al. (2010) studied the nutrients removal from secondary effluent across the CWs with foamed insulation bricks as the substrate. Results showed that the removal rate of total P reached 75%, while the removal rate of total N was only 30%. 2.3. Artificial materials 2.3.1. Ceramsite Ceramsite is a round, oval or irregular ceramic particle, with the bulk density of 300-900 kg/m3, and particle size of 5-25 mm. Ceramsite is produced by ceramic production of raw materials by high temperature calcination. Ceramsite can be used in wastewater treatment and CWs due to its high adsorption capacity, stability and affinity to microbial biofilm biomass (Wu et al., 2016a).

He et al. (2010) compared several substrates for nutrients adsorption in CWs, and fly ash ceramsite showed the largest adsorption amount of P, but a medium capacity for N. The P adsorption onto ceramic sands were studied by Wang et al. (2013b), and the Langmuir equation could well describe the experimental data of adsorption isotherm. Cheng and co-workers prepared a sustainable and efficient ceramsite substrate for P immobilization in CWs by adopting coal fly ash and waterworks sludge as the main materials, and oyster shell as the additive (Cheng et al., 2018). Static adsorption experiments indicated the theoretical maximum P adsorption capacity of as prepared ceramsite reached up to 4.51 mg/g, and the the dynamic column experiments filled with ceramsite medium achieved as much as 90% of the P removal with an HRT of 12 h. Yuan et al. (2015) used bentonite, metallic iron, and activated carbon to prepare ceramisite, and the as-prepared ceramisite achieved a high Pb removal efficiency (> 99%). 2.3.2. Activated carbon Activated carbon is an amorphous carbon obtained by processing. It has a large specific surface area, and has a good adsorption capacity for gas, inorganic or organic matter and colloid particles in the solution. The properties of activated carbon are stable, high mechanical strength, acid resistance, alkali resistance, heat resistance, insoluble in water and organic solvents, and reusable. It has been widely used in various fields, such as chemical industry, environmental protection, food processing, metallurgy, pharmaceutical refining and so on (Dias et al., 2007). At present, modified activated carbon materials have been widely used in the fields of sewage treatment and air

pollution prevention and control, and become more and more attractive in the field of environmental pollution control (Kyzas et al., 2015). Dai and Hu (2017) evaluated the P adsorption of several CWs substrates, and indicated the P adsorption efficiency of activated carbon was not very good. A pilot investigation indicated the biologically-active granular activated carbon was an effective method to remove NH4-N with an average removal of 60.87±29.69% (Fu et al., 2017). 2.3.3. Synthetic fiber Synthetic fiber is made by chemical processes, as opposed to natural fiber. Synthetic fiber is made from synthesized polymers of small molecules. Synthetic fiber has advantages of high strength, light quality, good elasticity, water resistant and stain resistant. Chen et al. (2013) found synthetic fiber had a relatively good P removal rate of 75%. However, synthetic fiber was poor in ammonia removal (Chen et al., 2012). 2.3.4. Cement clinker Cement clinker is a semi-finished product obtained by using limestone and clays as main raw materials in an appropriate ratio, buring to partial or full melting, and then cooling. Cement clinker is usually of diameter 3-25 mm and dark grey in color. The main components of cement clinker are alite, belite, aluminate, and ferrite. Cement clinker is primarily used to produce cement. Cement clinker has been studied and showed excellent P removal capacity. For example, Calder et al. (2006) investigated capacity of cement clinker to remove P in a

SF CWs post-treatment filter, and cement clinker showed a good P removal capacity, keeping an outflow dissolved P concentration of 0.72±0.20 mg/L. Paul and Anderson (2011) found the P removal efficiency of BF slag and cement clinker in long-term flow cell experiments could reach up to nearly 100%. 2.3.5. Recycled concrete Recycled concrete refers to the new concrete which is made up of waste concrete blocks, mixing with cement and water. Recycled concrete can partially or totally replace natural aggregates such as sand and stone. Recycled aggregate are already used in all countries in various applications of civil engineering works, as road pavement materials, subbasements, soil stabilization, improvement of subground, production of concrete of many categories. Molle et al. (2003) investigated the P uptake of recycled concrete in both batch and continuous systems. Recycled concrete showed excellent P sorption capacity. However, recycled concrete was sensitive to strong dissolution, leading to rapid precipitation of P, but led to high conductivity and pH values in treated water. 2.3.6. Modified clays Various modified clay products develop including expanded clay, calcined clay, Filtralite P® and so on. Filtralite P® is high quality filter media, manufactured from expanded clay material, and used for filtration and purification of waste and effluent water for dispersed settlements. The material is a commercially available product with a high pH (>10) and a high Ca and Mg content. It has a grain size in the range 0.5-4

mm, effective porosity of 40%, particle porosity of 68%,

and bulk density of 0.55

g/cm3 (Ádám et al., 2007). Drizo et al. (1999) determined the P adsorption capacity of seven substrates, and concluded that the adsorption effect of light expanded clay on P was not very good. However, other studies demonstrated the good performance of expanded clay and calcined clay on P adsorption. White et al. (2011) observed that the average P adsorption capacity of crude calcined clay reached up to 497 mg/kg. Jesus et al. (2014) found that expanded clay showed excellent N adsorption capacity, and could adsorbed 0.31 mg/kg of NH4-N after 7 days of contact. The P adsorption capacity of Filtralite P® was evaluated by Ádám et al. (2007) in both batch and column experiments. In column tests, the removal rate of P in primary wastewater was 91%, and in secondary wastewater was 54%. Jenssen et al. (2010) investigated the removal of FIB by saturated filters containing Filtralite P®, and found FIB in effluent met the European bathing water quality criteria. Ferreira et al. (2017) evaluated the capacity of CWs to remove three emerging organic contaminants. The presence of light expanded clay aggregates showed higher removal (around 61–97%) of lipophilic compounds (oxybenzone and triclosan) than the hydrophilic compound (caffeine; around 19–85%). 2.4. Interaction with plant/microorganism Many studies showed that the presence of plant could effectively improve adsorption of contaminants onto substrate materials in CWs, especially for nutrients and pesticides/herbicides. This probably because the presence of plants increases water

retention time, which prolongs the contact time between substrate materials and contaminants, then the adsorption of contaminants onto substrate materials in CWs are improved. In addition, the presence of plants changes the pH of the system, which is possibly more conducive to adsorption of contaminants onto substrates. For example, Wu et al. (2014) conducted batch tests, and demonstrated the planted units exhibited better performance in P-sorption capacity compared to the unplanted ones. Mbuligwe (2005) found that the overall treatment efficiency of the vegetated CW units was more than twice as high as that of the unplanted bed. On average, the bed vegetated with coco yam plants performed better (7.6%) than the one planted with cattail plants. Drizo et al. (2000) found that the presence of plants can increase the adsorption capacity of shale for P and N. Martín et al. (2013) noted that a VF CW could remove more P and N when planted with reeds. Yang et al. (2002) noted that the effect of plant litter was also a significant mechanism affecting nutrient removal in the SF pattern soil-bed wetland systems without harvest. Abe et al. (2014) indicated that N was removed mainly by denitrification and secondarily by adsorption to soil and by plant uptake. Shannon et al. (2001) also found that N removal in the wetland treatment cells might continue to improve as the density of wetland vegetation increased. Papaevangelou et al. (2016) noted that planted CW units showed better performance for Cr removal than the unplanted ones. A et al. (2017) indicated that the emergent plants would be helpful for decreasing the dredging soil depth for the final removal of heavy metals. Vallée et al. (2014) demonstrated that the presence of plants could increase sorption capacity of soilsediment substrate for pesticides. Chen et al. (2017) reported that plants could enhance

the removal of chloroacetanilide herbicides in CWs, especially the floating hydroponic root mat. Rogers and Stringfellow (2009) found that sorption for pesticides to whole plant stems (Kd = 570-1300 L/kg) was more than 10 times higher than to soil (Kd = 4071 L/kg). Chen et al. (2017) demonstrated that all three planted systems were effective in removing herbicides with removal efficiency being >92% after operating for 9 days. Tee et al. (2009) indicated that planted wetland units performed better than the unplanted ones in the removal and mineralization of phenol. Thomas (2003) noted that in CW system, planted with Phragmites australis in gravel media, greater linear alkylbenzene sulphonate (LAS) removal was observed in the spring coinciding with the plant growth season. Studies conducted by Sehar et al. (2016) highlighted the presence of vegetation/plants in CWs could achieve better FIB removal. Microorganisms in substrate have also been found to improve the contaminants removal in CWs. For example, Chowdhury (2017) demonstrated that microbes could promote the adsorption of As by iron sulfide. Saba et al. (2015) noted that augmentation of microorganisms in CW systems had improved dye removal efficiency to 90%. 3. Evaluation and recommendation The theme of this article is to review the removal capacity of various substrates for contaminants, and to compare their cost, availability, permeability, reuse and safety to provide reference for future research and application on CWs. Here a scoring model is set up based on various properties and characteristics, to score the substrates. Then the score is used to evaluate the quality of substrate, and high score indicates an excellent substrate. In the model, the score comes from the sum of the scores of each

parameter. The scoring indicators are as follows: Contaminant removal capacity: low = 0, medium = 1, high = 2; Availability: low = 1, medium = 2, high = 3; Likely cost: low = 3, medium = 2, high = 1; Permeability: low = 0, medium = 1, high = 2; Reusability: difficult = -1, neutral = 0, beneficial = 1. The results are shown in Table 1. Generally, the natural materials (mean score = 12.25) and agricultural/industrial wastes (mean score = 12) got higher scores than artificial materials (mean score = 10). This suggests natural materials and agricultural/industrial wastes have advantages in cost and effectiveness compared with artificial materials. Both soil/sediment and gravel got the highest score of 19. However, the composition of soil/sediment varies greatly due to the complex sources. In addition, there may be secondary pollution issue to use contaminated soil/sediment. Thus, when they are used as substrate, multiple factors should also be considered. Gravel is one of the most used traditional substrates in CWs due to its abundance, low-cost, and retention for various contaminants (Yang et al., 2018a). Moreover, gravel has the high potential of permeability and reusability, which provides it more advantages over soil/sediment as CWs substrate. The recommended gravel sizes for CWs is 8-16 mm (IWA, 2001). The other substrate materials with high scores include plant waste (score = 16), woodchip (score = 13), sand (score = 12), limestone (score = 12), oyster shell (score = 12), slag (score = 12) and ceramsite (score = 12). Because different substrates have different contaminats removal capacities, specific substrates should be selected considering the contaminants types. For example, for sewage with high E. coli and suspended particles, peat soil may be used as the CWs

substrate. For wastewater with high content of heavy metals, limestone and zeolite are preferred as the substrate. For eutrophic waters with high P content, slag or shale is suitable as the substrate. The choice of substrates should follow the principles of easy access, high efficiency, low cost, safety and innocuity. When designing CWs, designers should first screen local materials with high contaminants removal capacities, which could not only improve the ability of CWs to purify the sewage and reduce the cost, but also extend the service life of ecological projects. In addition, because of the great differences in permeability coefficient of different materials, substrates should be chosen according to the wetland designs. For example, SF CWs can select soil as the substrate. Sand, slag or their mixture with soil could be used in SSF CWs, because the wetlands need substrate with high permeability coefficient. Due to complementary effects between various substrates, the combination of different materials could greatly improve the treatment performance of CWs. For example, the zeolite-gravel mixture substrate could simultaneously remove both P and N (Singh et al., 2014). Moreover, there are also synergistic effects between some substrates, and combining them together will achieve better removal effect than a single application. The research on the decontamination effect and influencing factors of the combined substrate will be an important direction for the future research on CWs. Furthermore, the research and development of new substrate materials with good retention capacity, long-lasting performance and cheap cost will also be the focus of CWs substrate research in the future.

4. Conclusions Yang et al. reviewed the research progress of some emerging substrates in CWs (Yang et al., 2018b). Other two papers reviewed the phosphorus removal capacity of various substrates (Ballantine and Tanner, 2010; Vohla et al., 2011). So far, there is no comprehensive review on all kinds of substrate materials and especially evaluation on the removal capacities for various contaminants. This review has documented various properties, e.g., cost, availability, permeability, reuse, safety and removal capacity for contaminants, of 22 substrate materials in CWs. Their removal capacities for main water

contaminants

including

nutrients,

heavy

metals,

surfactants,

pesticides/herbicides, emerging contaminants and FIB were comprehensively described. This paper summarizes more literature and content, especially more comprehensive understanding of various substrates than the previous literature on a single pollutant removal capacity. In addition, a scoring model for substrate evaluation based on cost, availability, reuse and contaminant removal capacities was established to provide important reference for the selection of substrates in CWs under different conditions, which was not available in previous literature. Further, this model not only advances the value of previous research, but also provides an important reference method for such research. Generally, natural materials and agricultural/industrial wastes got higher scores than

artificial

materials,

which

demonstrated

that

natural

materials

and

agricultural/industrial wastes had advantages in cost and effectiveness compared with artificial materials. Overall, this review provided important reference and guidance for

the selection of CWs substrates, which could improve the construction and operation of CWs for wastewater treatment. Further, to a certain extent, the results are only directions for future work due to the wide variety of methods applied in CWs and substrates research. Acknowledgements This work was supported by POWERCHINA HUADONG Science and Technology Project (KY2018-01-05, KY2016-02-04), National Water Pollution Control and Treatment Science and Technology Major Project (2017ZX07602-002), China Postdoctoral Science Foundation (2018M641927), Shanghai Pujiang Program (17PJ1400900), and National Natural Science Foundation of China (41701541, 91851110). References A, D., Oka, M., Fujii, Y., Soda, S., Ishigaki, T., Machimura, T., et al., 2017. Removal of heavy metals from synthetic landfill leachate in lab-scale vertical flow constructed wetlands. Sci. Total. Environ. 584-585, 742-750. Abe, K., Komada, M., Ookuma, A., Itahashi, S., Banzai, K., 2014. Purification performance of a shallow free-water-surface constructed wetland receiving secondary effluent for about 5 years. Ecol. Eng. 69, 126-133. Ádám, K., Krogstad, T., Vråle, L., Søvik, A.K., Jenssen, P.D., 2007. Phosphorus retention in the filter materials shellsand and Filtralite P—Batch and column experiment with synthetic P solution and secondary wastewater. Ecol. Eng. 29, 200208.

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Fig. 1. Diagrammatic sketches of three types of CWs: (a) surface flow CWs, (b) horizontal flow CWs and vertical flow CWs. This figure is modified from literature (Tilley et al., 2014).

ry

l

Fig. 2. Issues needed to be considered for the selection of substrate materials.

Table 1. Scores of substrate materials for application in CWs based on the likely cost, availability, reusability and contaminant removal capacity Likely

Availabi

cost

-lity

Medium

High

Permeability

Reusability

Contaminant removal capacity P

High

Beneficial

High

N

Medium

Heavy

Surfac

Pesticide/

Emerging

Fecal

metal

-tant

herbicide

contaminant

indicator

High

High

High

High

Score

R

19

A

al

l

hi

an Low

High

Medium

Neutral

Low

Medium

High

High

12

al

l

ch

D

ca Low

Low

Low

Beneficial

High

Medium

8

al

l

w

w Low

High

Medium

Difficult

High

High

High

High

Medium

High

High

19

D

se Medium

Medium

Medium

Beneficial

Medium

High

High

11

A

m

hi Low

High

Medium

Beneficial

Medium

Medium

High

12

al

l

A

ob

al

l

D

co

al

l

A

A

re

D

in Medium

High

Medium

Beneficial

Low

Medium

High

Medium

High

Beneficial

High

Medium

8

al

l

al

Low

9

A

ca

an

ial

ltur

D Low

High

Low

Beneficial

High

High

11

ut

re

D

tr

le

m

re

ve Low

High

Medium

Beneficial

High

High

12

e

ltur

en

m

pr

pr

D

ad

de

ty

co Low

High

High

Beneficial

High

High

13

ial

A

or

ab

ca

he

de

D

la Low

High

High

Difficult

High

Medium

Medium

High

High

Medium

16

e

ial

A

re

e

ltur

A

A

w Low

Medium

High

Difficult

High

High

10

A

ca

D

of Medium

Medium

High

Beneficial

High

Medium

High

12

A

ca

re

sa

D

of

to

to

ial

ial

Low

High

Medium

Neutral

High

Medium

10

ad

su

m

pl

D

co Medium

Medium

High

Beneficial

High

Medium

High

12

al

ial

w

im

st

m

st

D

pr

en Medium

Medium

High

Beneficial

Low

Medium

8

ac

re

ad

D Medium

Medium

High

Beneficial

High

Low

9

el

D Low

High

Medium

Beneficial

High

10

A

ad

D Low

High

High

Neutral

High

10

al

ial

A

lig

al

ial

A

m

al

ial

A

re

al

ial

A

A

pr

D

co

va

w High

Medium

Medium

Beneficial

Medium

High

High

al

Medium

11

A

ad

D

fo Score derived by addition as follows: Contaminant removal capacity: low = 0, medium = 1, high = 2; Availability: low = 1, medium = 2, high = 3; Likely cost: low = 3, medium = 2, high = 1; Permeability: low = 0, medium = 1, high = 2; Reusability: difficult = -1, neutral = 0, beneficial = 1

Graphical abstract

Highlights 

Different kinds of substrate materials available in the literatures are summarized.



Source/property and contaminant removal capacity of substrates were described.



A scoring model for the substrate evaluation was constructed.



This review is helpful for substrates selection for constructed wetlands design.