A Critical Review

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Science of the Total Environment 927 (2024) 172155

Available online 2 April 2024

Review

A critical review of characteristics of domestic wastewater and key treatment techniques in Chinese villages

Jing Zhang

^a

, Yungeng Jiang

^a

, Heyu Zhang

^a

, Dan Feng

^b

, Hongling Bu

^c

, Linlin Li

^a^,^*

, Shaoyong Lu

^a^,^*

aState Key Laboratory of Environmental Criteria and Risk Assessment, National Engineering Laboratory for Lake Pollution Control and Ecological Restoration, State Environment Protection Key Laboratory for Lake Pollution Control, State Environmental Protection Scientific Observation and Research Station for Lake Dongtinghu (SEPSORSLD), Chinese Research Academy of Environmental Sciences, Beijing 100012, PR China

bKey Laboratory of Agro-Forestry Environmental Processes and Ecological Regulation of Hainan Province, Hainan University, Haikou 570228, PR China

cSchool of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China

H I G H L I G H T S G R A P H I C A L A B S T R A C T

•Wastewater reuse could alleviate the water shortage.

•Centralized, distributed, and combined rural wastewater treatment techniques are compared.

•Differences between wastewater treatment techniques come from the engineering practice.

•The limitations of constructed wetlands are cold, clogging, and design, operation, and maintenance.

•Constructed wetlands are one of the most popular rural wastewater treatment techniques in China.

A R T I C L E I N F O Editor: Huu Hao Ngo Keywords:

Domestic wastewater Bibliometric analysis Treatment technologies

Wastewater reuse, constructed wetlands

A B S T R A C T

As of 2022, China’s rural sewage treatment rate is only approximately 31 %. Rapid rural development has led to higher demand. However, China’s rural areas are complex and face many problems, such as uneven economic development, population distribution, and water availability. Long-lasting and low-cost wastewater treatment measures are needed for application in rural areas. The quantity and quality of rural domestic wastewater in China were characterized first. Next, the hot topic of domestic wastewater in Chinese villages was confirmed via bibliometric analysis using CiteSpace, and the treatment technologies for rural domestic wastewater were compared. Specifically, the technical status and challenges of the most common technology in rural domestic wastewater treatment, constructed wetlands, were summarized.

* Corresponding authors.

E-mail addresses: [email protected] (L. Li), [email protected] (S. Lu).

Contents lists available at ScienceDirect

Science of the Total Environment

journal homepage: www.elsevier.com/locate/scitotenv

https://doi.org/10.1016/j.scitotenv.2024.172155

Received 21 January 2024; Received in revised form 27 March 2024; Accepted 30 March 2024

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1. Introduction

In recent years, domestic wastewater has been viewed as a resource rather than a waste, in order to alleviate global water scarcity (McCarty et al., 2011). It has been reported that the global volume of wastewater is 359.4 ×10⁹m³/year, of which around 48 % (172.5 ×10⁹m³/year) is discharge directly without any treatment. The volume of wastewater reuse differs between East Asia and Pacific region (11.9 ×10⁹m³/year), North America (9.1 ×10⁹m³/year), South Asia (0.5 ×10⁹m³/year) and sub-Saharan Africa (1.6 × 10⁹m³/year) (Jones et al., 2021).

Rapidly urbanizing country, such as China, India, in which the issue of rural domestic wastewater treatment is a pressing problem that urgently needs to be addressed (Huang et al., 2022; Sharma et al., 2021). In 2021, in rural areas of China, 3.5 ×10⁷m³of wastewater were discharged, while the ratio of wastewater treatment, i.e., treated wastewater volume/total wastewater volume, is still very low at 31 % (The State Council of the People’s Republic of China, 2023). The government increasingly pays attention to environmental protection; thus, four ministries of Ministry of Ecology and Environment, Ministry of Agri- culture and Rural Affairs, Ministry of Housing and Urban-Rural Devel- opment and Ministry of Water Resources, and the National Rural Revitalization Administration of the People’s Republic of China jointly issued the “Action Plan for the Battle of Agricultural and Rural Pollution Control (2021–2025)” on January 19, 2022, stating that “by 2025, the comprehensive utilization rate of livestock and poultry waste will reach more than 80 %, and the treatment rate of rural domestic wastewater will reach 40 %”. The Ministry of Ecology and Environment of the People’s Republic of China (MOE) set the 2025 target at 40 % in a press conference on April 22, 2022. This target is to be met by the tripartite action of the government, the market, and the villagers, and the project will be realized on a county-by-county basis (Xiang, 2022).

Normally, the type of wastewater treatment should be applied to villages according to their circumstances (different lifestyles within counties result in uneven distribution of wastewater quality and quantity) and the limitations of the wastewater treatment techniques. Due to the dispersion of rural settlements, topography and economics, some scholars are attempting to use models to optimize the rural wastewater treatment scheme, such as ALLOWS-GIS model (Afferden et al., 2015), SNIP (Leit˜ao et al., 2005; Eggimann et al., 2015) and the drop/add model, RuST optimization model (Huang et al., 2022). Additionally, some rural wastewater treatment facilities have been built but not used because the local government lacks financial resources and low levels of participation by the farmers who should directly benefit (Wu et al., 2023b). According to surveys performed in Jiangxi Province and Shandong Province, 70 % of farmers were willing to pay for rural domestic wastewater treatment, and the willingness-to-pay level was about 5.7 Chinese yuan/(household⋅month) (Chen et al., 2017; Wei et al., 2022; Xie et al., 2018; Su et al., 2020; Wu et al., 2023b). Currently, the main funding source for the design, building, and maintenance of wastewater treatment is the local government, which slows the popu- larization of rural wastewater treatment. Comprehensively considering the effect of wastewater treatment, the available land area and location, economic accounting, and other issues is one way to popularize rural wastewater treatment facilities. Considering the problem of global warming and the fact that the domestic wastewater treatment industry accounts for a significant proportion of carbon emissions, greenhouse gases emissions from carbon dioxide, methane (100-year global warming potential is about 28 times of CO2) and nitrous oxide (100-year global warming potential is about 265 times of CO2) (USEPA, 1997;

IPCC, 2013; Koutsou et al., 2018), the greenhouse gas emissions of the various wastewater treatment techniques should be taken into account in the further study.

This review breaks down the information barriers between researchers and wastewater-related workers, such as government officials, businesspeople, and villagers, and provides data and information for global researchers and industry partners regarding the state of rural

domestic wastewater in China. A comprehensive review was developed by surveying the vast amount of literature available in both Chinese and English. First, the hotspots in rural domestic wastewater in China were analyzed through bibliometric analysis using CiteSpace (Chen, 2004).

Second, the quality and quantity of rural domestic wastewater in China were characterized. Third, the research status and available wastewater reuse technologies are summarized, and treatment technologies for rural domestic wastewater are introduced and compared. Finally, the technical status and difficulties of the most common technologies in rural domestic wastewater treatment, constructed wetlands (CWs), are summarized.

2. Characteristics of domestic wastewater in Chinese villages Domestic wastewater can be divided into grey and black water based on its source and characteristics. Black water can be further divided into yellow water and brown water. Yellow water contains urine, brown water is fecal water or manure, and grey water includes kitchen and washing wastewater (Zhang et al., 2015), including dishwashing water, rice-washing water, and slop formed from leftover food and leftover vegetables. Rural domestic washing wastewater includes water used for washing, gargling, and bathing (Yang, 2013). The higher quality of grey water makes it suitable for on-site treatment and reuse for agricultural irrigation, unlike black water (Gross et al., 2007). This distinction has encouraged the separate collection and treatment of grey and black water and has achieved results (Boyjoo et al., 2013). However, the quantitative and qualitative characterization of rural domestic wastewater must be limited by counties due to the differences in the level of economic development and living habits of different counties.

2.1. Quantitative characteristics of wastewater

The total annual domestic water use of China is 17.0 ×10⁹m³in towns and 11.6 × 10⁹m³in villages, according to an investigation conducted by the Ministry of Housing and Urban-Rural Development of China (Ministry of Housing and Urban-Rural Development of the Peo- ple’s Republic of China, 2010). In rural China, grey water production is closely related to work and rest routines, thus, more domestic wastewater is discharged in the mornings and evenings than during the day, and discharge decreases at night, with peak times dominated by dishwashing and washing wastewater (Li et al., 2021b; Liu, 2015).

The average daily water consumption per person in high-income and low-income countries is 193 ±76 L (mean ±standard deviation) and 126 ±59 L, respectively (Shaikh and Ahammed, 2020). In China, the average daily water consumption is 184 L in 2022, with a range of 119 to 287 L (Ministry of Housing and Urban-Rural Development of the Peo- ple’s Republic of China, 2023). This phenomenon of huge differences in water volume reflects the huge gap between rich and poor, and corresponds to the reality of China. In modern rural areas, grey water production can be calculated by multiplying the hours of use and the flow rate from household taps, such as those found in kitchens, washbasins, and bathrooms (Noutsopoulos et al., 2018). However, in underdevel- oped rural areas, a single standpipe is the water source for the entire family, and water is carried to various places for use. This leads to significantly lower water consumption; thus, less grey water is generated in rural areas without washing machines or showers (Sall and Takaha- shi, 2006).

Grey water discharge is very low in areas where water is scarce or remote rural and agrarian areas. In particular, the volume of wastewater water is more concentrated in the flat plains because the distribution of villages is more concentrated, and they are relatively large. The domestic wastewater is usually directly discharged in the immediate vi- cinity or flows into the nearby lakes or rivers along the terrain in villages in the hills and mountains because the topographic variation has promoted scattered villages, which are not conducive to the centralized collection and treatment of wastewater (Liu, 2015).

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2.2. Quality characteristics of wastewater

As living standards improve in rural areas of China, in addition to a significant increase in domestic water use, the increased use of flushing toilets and fertilizers has led to a dramatic deterioration in the water quality of many rural waterways (Guo et al., 2014). Approximately half of the major water pollutants of China originate from villages, i.e., 43 % chemical oxygen demand (COD), 57 % total nitrogen (TN), and 67 % of total phosphorus (TP) (Ministry of Ecological Environment of the Peo- ple’s Republic of China, 2010a).

Grey water contains organic matter, N, P, surfactants, microorganisms, oils and fats, antibiotics, and other contaminants. P and N are relatively lower in bathroom grey water due to the exclusion of urine and feces (Noutsopoulos et al., 2018). Li et al. (2021b) have surveyed the pollutant concentrations of grey water and found it contained a large range of pollutants, of which the COD concentration range was 76–1461 mg/L, the biochemical oxygen demand (BOD) concentration range was 33–296 mg/L, the TN concentration range was 7.14–54 mg/L, ammonia nitrogen concentration range was 1.58–47 mg/L, TP concentration range was 0.3–5.2 mg/L, and the total Escherichia coli counts ranged from 0.6 to 4 ×10⁷(Li et al., 2021b). Compared with black water, grey water has a faster decomposition rate (Prathapar et al., 2005). After one night of storage, the COD, turbidity, and pH of the grey water were reduced by 13 %, 31 %, and 0.5 units, respectively (Chin et al., 2009).

Generally, the pollutant content in black water is higher than in grey water (Xu, 2010). The N and P contents of black water are high, and the pollutant content of water bodies is affected by regional, seasonal, and economic development to different degrees (Liu et al., 2017; Jiang et al., 2015; Yan and Chao, 2022; Cheng et al., 2022). The black water data was collected from the literature and is shown in Table 1. The domestic wastewater water quality indicators include COD, BOD, ammoniacal nitrogen, TP, TN, suspended solids (SS), acidity and alkalinity, and discharge volume (Liu et al., 2017; Jiang et al., 2015; Yan and Chao, 2022; Cheng et al., 2022; Xu, 2010; Li, 2017; Yan, 2008; Cao et al., 2018).

The black water pollutants differ between rural regions. Some show seasonal differences in the amount of wastewater discharged: the amount of water used in summer is larger than in spring (Liu et al., 2017;

Yan and Chao, 2022). The discharged wastewater is mostly alkaline (Li, 2017; Jiang et al., 2015), containing a certain number of SS (Liu et al., 2017; Jiang et al., 2015; Cao et al., 2018), which has a negative impact on the surrounding land and aquatic environment of the countryside.

3. Domestic wastewater treatment techniques in Chinese villages

3.1. Bibliometric analysis of domestic wastewater treatments of Chinese villages

3.1.1. Materials and methods

The China National Knowledge Infrastructure (CNKI) database, which contains 280 million records up to November 20, 2023, was searched for “rural domestic wastewater” and related terms in Chinese.

A total of 3910 articles in Chinese from 1996 to 2023 were retrieved by excluding notices, news articles, and conference reports that were not relevant to ensuring the accuracy of the data up to April 30, 2023. All

retrieved articles were downloaded using Refworks format, which contains information on title, authors, institutions, categories, countries, year, keywords, abstract, etc. The downloaded documents were then analyzed using CiteSpace 6.2.R2 Basic 64-bit versions (Chen, 2004).

CiteSpace is a visual bibliometric analysis software that analyzes trends and patterns of scientific research from the literature database, in particular to understand the development of a field. Keywords were selected as the node type, and one year per slice was defined as the time slice. Within each time slice, the strength of parameter relationship was calculated on the basis of similarity using the cosine algorithm, calculated by: Cosine(

Cij,fi,fj

)

= ^C̅̅̅̅̅̅̅̅^ij

f_i×f_j

√ , where i and j are respectively two different terms, Cij the number of co-occurrences of i and j, and fi and fj

respectively the number of occurrences of i and j (Yang et al., 2023). The parameter threshold g was set using g-index method, g²≤∑_g

i=1(K×Ci), where g is employed to reflect the impact of an individual independent researcher considering the quantity and quality of his/her output, Ci the number of occurrences of i, K the scaling factor (setting at 15 in this study). We then obtained the frequencies of themes, top 25 keywords with the strongest citation bursts and map of research keywords relating to “rural domestic wastewater”. Emergent degrees were obtained from CiteSpace, which could be used to examine keywords that appear sud- denly and rapidly increase in frequency, providing researchers with the most recent evolution and direction of the discipline. Emergent degrees are calculated as follows: P =_(T ^Z×S

1−T)×|N/4−F|, in which T1 is the current time, T the of first appearance of the objective keyword, Z the media- tional centrality, S the emergence strength, F the keyword frequency, and N the maximum keyword frequency. Finally, based on results from CiteSpace, a further worldwide search in English and Chinese was conducted to obtain relevant studies, summarized in the following section.

3.1.2. Results and discussions

Analyzing the 3190 articles extracted from the CNKI database, the number of articles on this topic has gradually increased since 2004 and exceeded 300 in 2019. Among them, 80.59 % are in the discipline of

“Environmental Science and Resource Utilization”, 79 articles are in

“Habitat Environment”, 33 articles are in “Rural Revitalization”, and the rest are under “Governance Measures”. The most frequently mentioned rural wastewater treatment theme was “constructed wetland”, which appears in 196 articles. This indicates that the CW could be an effective or potential technique in the rural wastewater treatment area that needs to be discussed by researchers and engineers and will be comprehensively reviewed in the last section of this work.

Keywords involving treatment facilities with a frequency of 19 or more were extracted to plot the map of research keywords (Fig. 1a), and the high-frequency keywords included “constructed wetland”, “wastewater treatment”, “suggestion”, “governance”, “rural revitalization”,

“nitrogen and phosphorus removal”, and “status”. Thus, the status quo of rural wastewater, treatment methods, and the discharge water quality are highly valued, and the CW is the most important method explored in the rural wastewater treatment measures. Taihu Lake basin is an important field for studying rural wastewater treatment and serving as a filed demonstration (Jing et al., 2009; Wang et al., 2010). Rural revitalization has become a national strategy and has received attention in China, the United States and Canada (Chen et al., 2021; Liu et al., 2023).

Table 1

Physicochemical characteristics of grey and black water.

Type of

water Amount L/

(person⋅d) Biological oxygen demand

(BOD, mg/L) Chemical oxygen demand

(COD, mg/L) Ammoniacal nitrogen (NH3-

N, mg/L) Total nitrogen (TN,

mg/L) Total phosphorous (TP,

mg/L) Grey

water 20–225 33–296 76–1461 1.58–47 7.14–54 0.3–5.2

Black

water 10–175 93.3–410 200–1500 3.1–770 12.4–800 1.1–29.9

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The top 25 highlighted words were obtained by CiteSpace and are presented in Fig. 1b. The top five keywords with emergent values are

“constructed wetland”, “rural revitalization”, “governance mode”, “new countryside”, and “Taihu Lake basin” which had emergent degrees of 16.14, 10.52, 7.7, 7.51, and 7.48, respectively. This reflects the fact that CW is one of the most referenced techniques in the wastewater treatment, which could be due to its special character that will be detailed in the next section. As there are difficulties in terms of income, education, etc., rural areas need to be cared for and improved, which is known as rural revitalization. The keywords with burstiness lasting until 2023 are

“rural revitalization”, “discharge standard”, “governance status”,

“human settlements”, “treatment mode”, “wastewater treatment”, and

“management system”, with burstiness of 10.52, 4.8, 6.1, 4.48, 7.2, 6.44, and 4.34, respectively. From the burstiness keywords, it was observed that the hotspot of research gradually shifted to the aspects of wastewater treatment and the living environment from the beginning of build the new countryside.

3.2. Rural domestic wastewater treatment techniques

Rural domestic wastewater treatment technologies can be classified into biological, ecological, and combined treatment approaches according to their process characteristics (Zhong et al., 2023), and divided into distributed, centralized and combined methods according to their architectural distribution. Some researchers have classified treatment techniques into three modes: distributed, centralized, and tube (Yang and Wang, 2022). As the ‘tube’ mode is often discussed in the context of municipal domestic wastewater treatment, it will not be dealt with in this study. In this study, distributed, centralized and combined treatments and their advantages and disadvantages, will be introduced and discussed below (Table 2). Commonly used treatment technologies include septic tanks, anaerobic/oxic (A/O), anaerobic/anoxic/oxic (A2/

O), MBR, anaerobic/anoxic biofilms, biofilm/activated sludge, CW, and combined treatments (Guo et al., 2014). CW is one of the most used techniques due to its low price and ecological function.

3.2.1. Centralized wastewater treatment mode

The centralized treatment mode collects terminal wastewater from indoor house pipes and then concentrates the wastewater from a certain area for unified treatment according to distance and collection volume.

The types of wastewater collected are Type I, where black water and grey water are collected uniformly, and Type II, where only grey water is collected. The black water in Type I can be subdivided into the following types: (1) mixture of livestock and human feces and urine; (2) mixture of livestock and human feces and urine after treatment in a three- compartment pond; and (3) human feces and urine. For Type II, treatment with microorganisms is unsuitable due to the low concentration of pollutants in the effluent, especially COD and BOD.

The A/O process is a treatment process in which anaerobic and aerobic treatments are successively connected (Li, 2022). The effectiveness of the A/O process (anaerobic–aerobic process) is limited;

therefore, the technology can only be used as a pretreatment in conjunction with other techniques. For example, in septic tanks and biofilm combined with the activated sludge and A/O processes, the effluent is used for irrigation or directly discharged into surface water.

The combination of the anaerobic and ecological treatment processes and the combination of CWs achieves the simultaneous removal of N, P, Fig. 1. Research characteristics of domestic wastewater of Chinese villages using the search for “rural domestic wastewater” and related literature in Chinese in the China National Knowledge Infrastructure (CNKI) database. (a) Map of research keywords relating to “rural domestic wastewater” obtained using CiteSpace. Each ring represents the year of publish of the article (rings from inside to outside indicate years from far to near) and the width of ring reflects the number of articles. Lines between keywords indicate the correlation between them. The map was plotted with keywords appearance times greater than 19 and with search terms such as

“domestic wastewater”, “village” and “rural wastewater” removed. (b) Top 25 keywords with the strongest citation bursts obtained using CiteSpace. The “year” corresponds to the first appearance of the keywords, and “began” and “end” correspond to the length of time the keywords have been hotspot, and “strength” reflects the emergent strength. In the time span, the width of lines signifies the emergent strength of the keywords and the red the hotspot.

Table 2

Classification of treatment techniques for rural domestic wastewater.

Mode Description

Tube Rural areas within 3 km of urban areas, with a concentrated population, and terrain and construction conditions that meet the requirements for transporting wastewater to wastewater treatment plants.

Centralized Rural areas where a single village or adjacent villages are less than 2 km apart so that a pipeline network can be constructed for unified collection and processing, can be divided into single village concentration and contiguous concentration.

Distributed Rural areas where factors such as dispersed households and limited terrain conditions make it difficult to collect and handle wastewater in a unified manner.

Combined High concentration of pollutants or/and wastewater load.

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and BOD. The advantages of the A/O process are (1) high efficiency and simplicity; (2) low construction and low operating costs; (3) high degradation efficiency of pollutants in the anaerobic process; and (4) high intensity load shock resistance (Zheng et al., 2019; Wu et al., 2007).

MBR is a treatment technology that combines the traditional activated sludge method with membrane separation technology. MBR in- tercepts activated sludge microorganisms through the action of the membrane, improves the concentration of activated sludge, and strengthens the effect of biological treatments. Furthermore, the membrane is good at removing SS and has a strong resistance to shock loading (Mulder, 2001), so MBR has been extensively studied for grey water treatment (Li et al., 2021a). MBR treatment technology allows grey water to flow into the regulation pool and enter anaerobic, anoxic, and aerobic tanks sequentially. Finally, the treated water is discharged from the aerobic tank. The wastewater that does not meet the discharge standard flows directly into the anoxic tank through the aerobic tank and then flows into the aerobic tank again after treatment to yield the final effluent. MBR has the advantages of shock load resistance, effective pollutant reduction, low land occupation, and low sludge production (Paul et al., 2018; Wang et al., 2022), but also has the disadvantages of high cost, membrane contamination, and high energy consumption (Wu et al., 2013).

The operating principle of biological contact oxidation is filling many carriers in the reaction chamber, making microorganisms adhere to the carriers and form a biofilm, and purifying domestic wastewater through biodegradation (Parde et al., 2021). Biological contact involves collecting wastewater into a reservoir, which flows into a regulating tank, into a biological contact oxidation tank, purifying the wastewater through microbial degradation, and then flowing into a sludge settling tank. Then, the treated wastewater is discharged directly. The substrate that cannot be discharged from the sludge settling tank enters a sludge thickening tank for sludge transport, while the upper layer of liquid flows back into the regulating tank and repeats the process (Chen et al., 2022). This technology has the advantages of both activated sludge and biofilm methods and can effectively treat wastewater, reduce energy consumption, and deal with sludge output. In addition, the process is simple and easy to operate and manage, requiring few equipment and easy maintenance (Zheng et al., 2019; Jin et al., 2020).

The sequencing batch reactor (SBR) process is an intermittent activated sludge process with discontinuous working conditions. Waste- water enters the SBR reactor intermittently and periodically and circulates sequentially through different treatment processes or func- tional states in each cycle (Zheng et al., 2020). The SBR process can treat wastewater batch by batch, and the effluent quality is stable and high.

First, the wastewater is allowed to settle, and the wastewater enters the catchment tank after settling for water storage. Then, aeration and sedimentation are performed through the SBR reactor, and the treated wastewater enters the disinfectant tank, from which it can be directly dispatched. The bottom sludge dewatering is conducted in the SBR reactor. Last, the sludge is directly transferred out of the system (Lamine and Bousselmi, 2007; Hesham et al., 2017). The facility is simple and does not require a sludge return process, reducing the land needed. SBR is flexible, and the system performance can be improved by imple- menting different operation modes (Boon, 2007).

3.2.2. Distributed wastewater mode

The distributed treatment mode is related to the centralized treatment mode of wastewater. The term mainly refers to in situ wastewater treatment to meet discharge standards. Distributed wastewater treatment uses anaerobic organisms for wastewater treatment, and its facilities are relatively small in scale and treatment range. These systems are mainly constructed in areas where the population is dispersed (Wu et al., 2014).

The rural population in China is sparsely distributed, so wastewater treatment technologies must have low construction requirements, low running costs, high treatment efficiencies, and be easy to manage. Thus,

the septic tank is a widespread technology in China. Septic tanks are the most common primary treatment technology (Song et al., 2018), and the overflows and leaks that always occur are harmful to human health and the environment. In the past, septic tanks were used to convert wastewater into agricultural fertilizer that was returned to the farm directly.

Stricter environmental regulations require farmers to find water treatment methods other than septic tanks.

The biogas digester is a method of energy utilization that uses straw as a raw material and can treat pollution. Its advantages are ease of construction, small land occupation, easy maintenance, low energy consumption, good environmental benefits, and great improvement of the rural ecological environment. The disadvantages of the biogas digester include unstable effluent and incomplete removal of N and P.

The centralized supply model is the future for biogas development in rural China as it can be improved easily once the constraints of the small family scale unit are removed (Chen et al., 2014).

The water purification process of a stabilization pond is similar to the self-purification process (Wu et al., 2007). Stabilization ponds have low construction and operating costs, simple maintenance, easy operation, no need for aeration devices, low cost, and are suitable for villages with unused rivers, abandoned reservoirs, and ponds. The disadvantages of stabilization ponds are the long hydraulic retention time, low treatment efficiency, and the need for a large amount of unused land for the stabilization ponds (Sun, 2022).

3.2.3. Comparison of rural domestic wastewater techniques

Wastewater treatment modes can be divided into centralized and distributed. A centralized treatment mode can handle a wide range of wastewater, has reliable operation, and reduces water requirements and costs, but it has higher initial construction and operation costs than the distributed treatment mode. Distributed treatment mode facilities require less construction, have lower operating costs and simpler operation, and can reuse wastewater, but their treatment range is smaller, the facilities are distributed, and the facility occupies more land than the centralized treatment (Li, 2022). The different rural wastewater treatment techniques are compared in Table 3.

Wastewater treatments can also be categorized as biological, ecological, or combined (biological +ecological; biological +biological; ecological +ecological) based on the treatment mechanism. Zhong et al. (2023) have summarized the classical wastewater treatment processes as MBR, three-chamber septic tank, multi-layer soil stratification system, biological filter, biological contact oxidation + CW system, anaerobic digestion tank +CW system, and other combined biological and ecological treatment methods (Zhong et al., 2023).

MBR +A2/O can remove N and P effectively, with a low sludge volume and high shock resistance (Yang et al., 2021). The technique of modified septic tanks is effective in removing SS, COD, and BOD but has a weak ability to remove N and P (Moreira and Dias, 2020; Pishgar et al., 2021; Singh et al., 2019). The multi-soil-layering system set up by Xu et al. (2022) has a high hydraulic load, low cost, easy maintenance, and high adaptability (Luo et al., 2014). Water discharged from the multi- soil-layering can be reused (Bo and Wen, 2022) and provide nutrients, such as N and P, to the soil (Ahmed et al., 2010).

Li (2022) has set up a distributed domestic wastewater treatment model with three characteristics: (1) the anhydrous conditioning hydrolysis +CW +stabilization pond wastewater treatment process is easy to maintain, the effluent water quality is good, but the wastewater treatment capacity is low; (2) conditioning hydrolysis +CW +biological oxidation of the treatment process occupies a smaller amount of land, the limitation for the quality and quantity of water is low, and the wastewater treatment capacity is high, but the disadvantages are the high cost of preliminary construction and operation, and complex system operation; and (3) the regulation of hydrolysis +stabilization pond treatment process, the preliminary construction and operation costs are low and the construction and maintenance are convenient, but the treated water quality is not high, and the treatment is easily affected by

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the climate.

Chen et al. (2022) have discussed the integrated rural wastewater treatment modes commonly used in Japan, the United Kingdom, New Zealand, and other countries, such as the purification tank process used in Japan, comprised of an anaerobic filter, a contact oxidation tank, a sedimentation tank, and a disinfection tank. In the United Kingdom, biological contact oxidation integrated treatment is used, comprising a primary sedimentation tank, filter bed (air injection or rotary sprinkler irrigation), and a secondary sedimentation tank. The integrated wastewater treatment system of New Zealand is based on ecological treatment and integrated anaerobic and aerobic treatments, composed of a septic tank wastewater treatment and a soil infiltration system.

The three wastewater treatment processing modes summarized by

Guo et al. (2014) are the most representative (Fig. 2). Mode I is the combination of septic tank +SBR/biofilm/MBR/A2/O, mode II is the combination of A/O +ecological treatment, and mode III is aerobic ecological treatment +septic tank/aerobic biofilm/CW. Mode I is the most widely used technique in China because of the high water quality achieved after treatment, which can be used for irrigation or directly discharged into surface water. In addition, mode I has small land occupancy, although the cost is high. Thus, mode I is mostly used in areas with poor land resources. Mode II is rarely used in China because of its low treatment capacity, easily interrupted operation, and substandard water discharge. Mode II can only be used for small-capacity wastewater treatment. Mode III is normally used in areas with strict drainage water quality standards.

Table 3

Comparison of rural wastewater treatment techniques.

Indicators A/O A2/O Ecological ditch Constructed wetland Pretreatment-

anaerobic tank- constructed wetland

Enhanced

pretreatment-aeration stabilization pond Treatment

mode Centralized Centralized Distributed Distributed Combined Combined

Application Organic wastewater with relatively stable water quality and quantity and good biodegradability.

Domestic wastewater is centralized in villages and has good biodegradability.

It is suitable for rural areas with relatively good ditch systems between farmlands, but flood discharge and narrow ditches should not be used.

It is suitable for rural areas where land is available and the concentration of suspended solids in incoming water is low.

Rural areas with

abundant land Rural areas in alpine regions and rural areas with a large amount of farmland to absorb treated wastewater.

Treatment

effect Good effluent quality,

general TN, TP removal Stable effluent quality, medium TN, TP removal, TP effect is slightly better than A/O

The effluent water quality is medium, and the effect of intercepting agricultural runoff is good.

Good effluent quality The quality of effluent water is good, TN, TP removal effect is medium, TP effect is slightly better than A/

O

The effluent water quality is good, and the treatment effect of TN and TP is relatively low.

Pollution load High High Low Relatively high Low Low

Land

occupation Small Medium Small Small Large Large

Architectural

complexity Medium Medium Easy Easy Easy Easy

Impact load

resistance Strong Strong Weak Weak Weak Weak

The level of automation control

Low requirements for automation control, only simple controls to meet the operating requirements

Automatic operation

without control Automatic operation without control

Daily operation Medium Medium Easy Easy Easy Easy

Construction cost (Chinese yuan/m³)

13,000–16,000 14,000–17,000 2000–9000 3000–15,000 5000–12,000 5000–10,000

Operating cost (Chinese yuan/m³per year)

1.2 1.2 0.04–0.10 0.05–0.15 0.1–0.2 0.05–0.2

Advantages Low hydraulic load, small footprint, simultaneous removal of nitrogen and

phosphorus, high organic matter degradation rate, and good sludge settling performance

Better phosphorus removal than A/O, more stable effluent water quality

Simple structure, general quality of effluent water, low construction cost, no or low energy consumption, operation cost saving, and simple maintenance management.

Simple structure, good water quality, low construction cost, no or low energy

consumption, low operating costs, easy maintenance and management.

Good heat preservation, high hydraulic load, good organic matter removal, the operation is less affected by climate, good sanitary conditions, simple operation and management.

Simple structure, no sludge treatment, good water quality, low construction cost, no or low energy

consumption, low operation cost, easy maintenance and management.

Disadvantages Low phosphorus removal

rate Reactor volume is

larger than A/O, energy consumption is higher than A/O, and cost is slightly higher than A/O

Low load, large land occupancy, treatment effect fluctuates with season.

High pollution load, pretreatment before wastewater entry, large land occupation, lower treatment effect in winter, part of the area will produce odors and breed mosquitoes

Slightly higher construction cost, complicated maintenance and management, and lower oxygen content in the system.

Low pollution load, pretreatment before entering wastewater, large floor space needed, treatment effect fluctuates with the season, high concentration of pollutants in the pond will produce odors and breed mosquitoes.

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3.3. Wastewater reuse 3.3.1. Black water reuse

According to the Food and Agriculture Organization of the United Nations (2019), the global population will increase by about 14 % in 6 years, and consequently, the demand for nitrogen fertilizer will increase by nearly 6 %. Black water is rich in nutrients and has a high pollutant concentration, which is more difficult to treat and can easily lead to eutrophication of water bodies. However, eutrophication could be pre- vented if the black water could be recycled and reused, bringing ecological and economic benefits. Yellow water can make foliar fertilizer by returning it to the field after simple fermentation or reusing it for toilet flushing after treatment. Brown water can be fermented to produce fertilizer or anaerobically digested to produce biogas or generate power to achieve resource utilization.

Black water biological treatment technology can be categorized into aerobic and anaerobic methods according to the oxygen demand of the active microorganisms. Aerobic wastewater treatment technology can remove more than 90 % of the organic matter in black water. No odors are produced during the process, oxidation is complete, and the end products are CO2 and H2O. Compared with anaerobic wastewater treatment technology, the aerobic microorganisms that dominate aerobic wastewater treatments decompose organic matter at a much higher rate; thus, the reactor volume needed for aerobic wastewater treatment is smaller (Robinson and Maris, 1983; Sundaresan and Philip, 2008;

Abbas et al., 2015; Gander et al., 2000; Çeçen and Aktas¸, 2004; Müller et al., 1995).

However, a disadvantage of aerobic wastewater treatment is that the high concentration of organic matter in black water requires a large amount of oxygen to support aerobic decomposition, accompanied by huge energy consumption (Zhao et al., 2011). In addition, volatilization in the aerobic wastewater treatment process loses a large amount of ammonia. In summary, the advantages of black water aerobic treatment technology are the short treatment cycle, 5–7 d, and the complete degradation of the organic matter. The hydraulic retention time of black water in the aerobic reactor of membrane bioreactor (MBR) is only 8 to 15 h. The black water can be treated in the aerobic reactor using the aerobic composting cycle.

The anaerobic biological treatment technology is one of the most popular black water treatment methods, such as the upflow anaerobic sludge bed (De Lemos Chernicharo and Von Sperling, 2005; Vlyssides et al., 2009), anaerobic membrane bioreactor (Wen et al., 1999; Ueda,

2000), expanded granular sludge bed (Zhang et al., 2008; Chan et al., 2009). Briefly, under anaerobic or anoxic conditions, the anaerobic microorganisms convert the organic matter in the black water into CH4

and CO2 to obtain biogas, fertilizers, and other high value-added products. Compared to other treatment technologies, the advantage of anaerobic biological treatment is the ability to recover biogas energy and obtain high value-added products, such as organic fertilizers, especially with a high carbon, nitrogen, phosphorus, and potassium recovery rate. However, anaerobic technology has shortcomings, such as poor operational stability, incomplete organic matter degradation, long operation cycles, and risky operation and maintenance processes.

This technology is suitable for rural and mountainous areas where it is difficult to build sewerage networks, and it can realize the harmless and resourceful treatment of black water in situ. A source-separated ecological sanitation system has been developed and used in Sneek in the northern part of the Netherlands, where black water generated by community residents is collected and transported by vacuum for anaerobic treatment. In a 32-household anaerobic treatment application, 7.6 kg of nitrogen and 0.63 kg of phosphorus were recovered daily, accounting for 69 % and 48 % of the theoretically possible values, respectively (Zeeman and Kujawa-Roeleveld, 2011).

Yin et al. (2016) established a continuous stirred reactor system, and it was used to anaerobically digest black water from the septic tanks of the campus of the University of Science and Technology, Beijing, and then to inactivate pathogens by thermal pretreatment. The time required to completely inactivate the pathogens (i.e., coliform bacteria, Salmo- nella, and Streptococcus faecalis) was proportional to the total solids (TS) content of the black water, and the maximum biogas yield of black water was 453.21 L/(kg⋅d) from the continuous stirred reactor process at a fermentation temperature of 37 ±1 ^◦C.

In practical engineering applications, a combination of anaerobic–aerobic technology is widely used, e.g., in CW systems. Paulo et al.

(2013) constructed a system combining black water evaporation with a CW treatment system. The black water is evaporated and concentrated for anaerobic digestion, and the methane liquid produced after collecting the biogas is discharged into the CW with the lavatory wastewater.

This system operated stably for 400 d. The average removal rates of COD and BOD in the anaerobic digestion unit were 45 % and 80 %, respectively. The COD removal rate was up to 90 % after treatment in the vertical flow CW. The content of Escherichia coli and other pathogens in the effluent of CW was lower than the standard for water reused for irrigation. Prosser and Sibley (2015) reported that PPCPs could Fig. 2. Rural domestic wastewater treatment modes. I, II, and III represent different treatment modes. Mode I combines septic tank +SBR/biofilm/MBR/A2/O, mode II is the combination of A/O +ecological treatment, and mode III is aerobic ecological treatment +septic tank/aerobic biofilm/CW.

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accumulate in edible tissues of plants such as soybeans, carrots, and tomatoes, but the accumulated residues pose a de minimise risk to human health. The removal efficiency of antibiotics and antibiotic resistance genes was related to the substrate and hydraulic residence time of CW (Chen et al., 2016). Considering the biological risk of ARGs and pathogens, Zhang et al. (2023) cautiously suggested that CW could not be used alone to treat wastewater.

However, black water should be reused more cautiously, as the effluent from black water after treatment may contain high levels of heavy metals. For instance, the reuse of black water sludge is currently prohibited in the Netherlands due to high residual concentrations of Cu and Zn (Tervahauta et al., 2014).

3.3.2. Grey water reuse

He et al. (2019) have reported that more than 50 % to 80 % of the wastewater produced by residents is grey water. The concentration of organic matter in grey water is low and can be discharged or reused following appropriate technical treatment. Treated grey water can be used for many purposes, including farmland irrigation, toilet flushing, and landscape irrigation. Frey water treatment methods can be divided into physical, chemical, biological, ecological, and integrated technologies (Li et al., 2021b).

Physical treatment technologies include filtration and adsorption methods. Adsorption technology is often applied in combination with other technologies due to its low treatment capacity. Filtration technology removes organic matter through physical interception and adsorption, which are effective and environmentally friendly. Dalahmeh et al. (2012) have compared the effects of filtration with pine bark, activated carbon, polyurethane foam, and sand on the treatment of grey water. Pine bark and activated carbon removed the most BOD, with removal rates of 98 % and 97 %, respectively. The removal rates for surfactants and TP also reached more than 90 %, and the grey water filtered by pine bark and activated carbon met the irrigation water standard.

Chemical treatment technology mainly degrades organic matter through chemical reactions, such as catalysis and oxidation. Tony et al.

(2016) have used the Fenton process to treat grey water. When the pH was 3, the concentration of Fe³⁺was 40 mg/L, the concentration of H2O2 was 200 mg/L, and the reaction time was 15 min; the removal rate of COD reached 95 %.

Biological treatment technologies mainly achieve wastewater purification through the adsorption, oxidation, and degradation of pollutants using activated sludge. MBR combines the traditional activated sludge process with membrane separation technology. Fountoulakis et al. (2016) have used a submerged membrane bioreactor to treat grey water in situ. The system ran continuously for 1 year. The COD, SS, and anionic surfactant concentrations decreased from 466, 95, and 37 mg/L to 59, 8, and 8 mg/L, respectively, and the removal rates reached 87 %, 92 %, and 80 %, respectively. The effluent met the international standard for toilet flushing water.

Ecological treatment technology has been widely studied and applied for rural domestic wastewater treatment due to its good performance, low cost, and convenient maintenance. Comino et al. (2013) have constructed a combined vertical and horizontal flow CW reactor (design scale 50 L/d) to treat grey water. The COD removal rate of the reactor with vegetation was 95 % higher than without vegetation.

For the treatment of grey water, ecological treatment technologies such as CWs and ecological filters have great advantages in performance, operation, and maintenance. For areas with high effluent standards, the use of ecological and physical integration technology to treat domestic grey water can meet the treatment standards, and the construction, operation, and maintenance costs are relatively low; thus, they have great potential for application in rural areas.

3.4. Research and application status of constructed wetlands in China 3.4.1. Development of constructed wetlands in China

According to the latest literature analysis, CWs are one of the most frequently used techniques for rural domestic wastewater treatment (Zhong et al., 2023). CWs are complex ecosystems containing physical, chemical, and biological processes in which substrates (including soil), aquatic plants, animals, and microorganisms act synergistically (Vymazal, 2022), and include free-surface flow, subsurface flow, floating, and hybrid CWs. CWs have the significant advantages of low- cost construction and operation and high pollutant removal capacity.

The first CW was established in England in 1903, and the first CW started to operate in China in 1990. Excluding Taiwan, Hong Kong, and Macau, the total area of CWs in China has exceeded 67,000 km², mainly in eastern China (National Bureau of Statistics of China, 2019), which is highly developed with a dense population and a mild and pleasant climate. Zhang et al. (2021) have reported that the CW density in each province has similar distribution patterns. CWs constructed in the last three decades in the Chinese mainland are mainly for effluent from wastewater treatment plants (Zhang et al., 2021).

CWs are the current key ecological pollution reduction measure promoted by the MOE of China. Industry standards, such as Technical Specification of Constructed Wetlands for Wastewater Treatment Engineering (Ministry of Ecological Environment of the People’s Republic of China, 2010b) and Technical Specification for Natural Wastewater Treatment En- gineering (Ministry of Housing and Urban-Rural Development of the People’s Republic of China, 2017), were released in 2010 and 2017, respectively, to guide the design and application of CWs. With increasing environmental protection efforts in China and the need for CWs to treat the low-pollution effluent from upgraded wastewater treatment plants, as well as to further purify minimally polluted river water, Technical Guidelines for Water Purification in Constructed Wetlands was released by the MOE of China in 2021 (Ministry of Ecological Environment of the People’s Republic of China, 2021). These technical guidelines detail the design, construction, acceptance, operation, and maintenance of CWs. It provides guidelines for the low-temperature ambient and long-term operation of CWs for the treatment of wastewater from rural domestic wastewater and the treatment of surface- source pollution.

3.4.2. Obstacles and limitations to sustainability of constructed wetlands After more than 120 years of research and application, the technology of CW is relatively mature and ideally suited to rural wastewater treatment, but challenges still remain for applying CWs, such as clogging (Peng et al., 2012), cryogenics, and adaptive design (Wu et al., 2023a), which will be detailed below.

3.4.2.1. Cold temperatures. CWs are usually planted with large herba- ceous plants, which have poor cold resistance and go dormant or even wilt and die in winter. Microbial activity also decreases dramatically in winter (Fleming-Singer and Horne, 2002). The main pollution removal mechanisms of a CW are substrate adsorption, plant absorption, microbial degradation, and their synergistic effects that purify water. Thus, the wastewater treatment efficiency of a CW decreases dramatically in winter when the temperature is low. A large number of studies have found that the treatment efficiency of a CW decreases significantly when the water temperature is lower than 10 ^◦C, and the nitrification tends to stop gradually at 4 ^◦C (Richardson et al., 2004), which is about 94 % lower than that at room temperature (Xiang et al., 2009). Hence, seasonal variation significantly affects nutrient removal (Varma et al., 2021).

For better application of CWs considering the temperature conditions, China is divided into five zones (i.e., frigid region, cold region, hot-summer and cold-winter region, hot-summer and warm-winter region, and mild region) in the Technical Guidelines for Water Purification in

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Constructed Wetlands (Ministry of Ecological Environment of the Peo- ple’s Republic of China, 2021), based on the daily and monthly average temperatures, supplemented by the cumulative number of days with an average temperature less than 5 ^◦C or higher than 25 ^◦C. In addition, the pre-design and post-operation and maintenance management parame- ters of CWs were standardized in these zones based on different temperatures. This categorization ensures the sustainable utilization of CWs.

Research has been performed to improve the wastewater treatment efficiency at low temperatures, such as the introduction, breeding, and selection of cold-resistant macrophytes, including Carex aquatilis (Yates et al., 2016), inoculation with psychotropic microorganisms and cold- resistant benthos (Ji et al., 2020b), and the use of insulating materials, such as adding a heat-preserving greenhouse or plant mulch (Ji et al., 2020a).

3.4.2.2. Clogging. Subsurface flow constructed wetlands (SSCWs) are the optimal choice for land-scarce areas due to their high purification efficiency per unit area. However, with lengthy operation time, the SSCW is prone to clogging, which decreases hydraulic conductivity or even causes a dead zone or short flow, seriously shortening the CW service life (from decades to years) (Wu et al., 2023a; Nivala et al., 2012) and reducing the pollutant removal efficiency. CWs are generally constructed for 10 years of use; however, over 50 % of SSCWs will have different clogging problems within 5 years (Cao, 2021). About half of the 355 SSCWs in the United States and the United Kingdom have shown different degrees of clogging within 5 years (Cooper et al., 2008). Dotro and Chazarenc (2014) surveyed the operation of CWs in several countries, including France, the United States, the United Kingdom, Belgium, Italy, and Denmark. They concluded that clogging occurs when CWs are operated for long periods under a certain intake loading condition. The clogging problem and its severity in CWs, especially SSCWs, is a technical challenge to be solved in the industry.

The causes of clogging in CWs can be categorized into physical, chemical, and biological effects, among which physical and biological effects are considered the main causes of clogging in CWs. Physical and biological effects are more common and have more serious conse- quences (Vymazal et al., 2021; Wang et al., 2021; Lin et al., 2019).

Biological clogging in SSCWs is mainly caused by reduced hydraulic conductivity induced by biofilms, especially the extracellular polymers (EPSs) (Xia et al., 2014). EPSs can undergo complexation reactions, further aggravating the clogging (Thullner, 2010; Hua et al., 2013; Nan et al., 2020; Zhou et al., 2020b). The multiphase substances in the wetland are spatially distributed; the clogging substances are scattered and form in the substrate area of the subsurface wetland (Zhou, 2021), which brings great technical difficulties for their detection. On the one hand, researchers need to locate the clogging area and understand its microstructure to support more in-depth research in clogging law exploration and clogging mitigation technology development (Mao et al., 2020). On the other hand, the CW wastewater treatment project needs to track and monitor the operation status of the entire wetland during the operation process to provide reliable data for improving operation control and alternative predictive models (Wu et al., 2023a;

Ye and Li, 2009). Therefore, there is an urgent need to develop a clogging monitoring technique for use in CWs.

An SSCW is a complex ecosystem with many bacteria, fungi, and algae, all of which can produce biofilms and subsequently form clogs (Tang et al., 2023). A biofilm is a membrane-like structure composed of microorganisms and the natural organic matter they secrete, such as EPS, ferromanganese oxides, and humus. In SSCWs, as the degree of clogging increases, a large number of microorganisms and EPS accumulate on the surface of the substrate, forming a biofilm, and the EPS further forms inert organic (humus and polysaccharides) and inorganic matter, which eventually becomes the clogging layer. Stoodley et al.

(2022) have classified biofilm growth into five stages: (i) initial cell attachment, (ii) the generation of firm “irreversible” EPS, (iii) the early

development of the biofilm structure, (iv) the maturation of the biofilm structure, and (v) the separation of individual cells from the biofilm.

Baveye et al. (1998) have classified CW bioclogging according to the status of the EPS as follows: (i) EPS-induced period, (ii) clogging period, and (iii) clogging development period (Fig. 3).

Biofilms have a three-dimensional structure that can serve as an in- ternal microbial attachment medium and help microorganisms defend themselves against adverse external environmental factors. The formation and spreading of biofilm are regulated by the quorum sensing (QS) system (Connell et al., 2014; Dickschat, 2010). QS is mediated by exogenous signaling molecules that accumulate during microbial cell growth and induce gene expression when a threshold concentration is reached (Huang et al., 2016). Diffusion coefficient, matrix porosity, and distance between cells are factors that affect the transmission of signaling molecules (Zhang, 2019). The biofilm production of Australian red algae (Delisea pulchra) can be inhibited by furazone (Manefield et al., 2002). QS signaling molecules extracted from anaerobic sludge induced a 2.25-fold increase in the bioproduction of Chlorella sorokiniana (Das et al., 2019).

EPS consists of water, polysaccharides, proteins, lipids, and nucleic acids, and 50 %–90 % of the organic carbon in biofilms is in the EPS. EPS provides biofilms with stability and adhesion, cell aggregation, cell–cell interactions and lateral gene transfer, water retention, and adsorption of organic/inorganic molecules and acts as a source and sink of excess energy and nutrients. The negatively charged surface of EPS chelates positively charged metal ions and microalgae. Numerous studies have shown that both EPS and biofilms are responses of microorganisms in response to adverse external environments, and their formation is also regulated by QS signaling molecules. Low temperature induces microorganisms to secrete more EPS (Ji et al., 2020b), and the content of EPS gradually decreases as the SSCW deepens (Zhou et al., 2020a). Soluble EPS in CWs planted with plants contains a large amount of tryptophan, which then generates indoleacetic acid and promotes the plant root system to extend to the EPS-enriched area (Cheng, 2021). QS signaling molecules promote the secretion of EPS by algae, which provides more space for bacteria to grow (Wyss, 2013). QS signaling molecules can either cause bacteria to secrete more EPS to impede the growth of other bacteria or secrete less EPS to promote growth and division, depending on the application scenario. Interfering with or blocking QS may slow down biofilm formation, which in turn provides theoretical support and pre-exploration for delaying or preventing the clogging of CWs and prolonging the lifespan of SSCWs and is of great practical significance.

Once clogging occurs in SSCW, it can only be alleviated by removing the clogged packing material or backwashing. Some studies have shown that adding oxidizing agents, hydrolytic enzymes, microorganisms, biosurfactants, and the release of wetland animals can alleviate clogging, but the required cost is high, or the effectiveness is slow, and most potential solutions are in the research stage.

3.4.2.3. Sustainable design, operation and maintenance.Due to the imbalance of social development, there are differences in the pollutants found in the wastewater in different regions of China. Improving the water quality of water in rural areas is an urgent need (Zhang et al.,

Fig. 3.Biological clogging process of constructed wetland (Lin et al., 2019).