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Review

Bottleneck Problems and Countermeasures in Operation and Maintenance of Non-Point Source Pollution Ecological Treatment Projects in China

by
Yungeng Jiang
1,2,3,
Jing Zhang
1,*,
Xiaoxin Liu
1,4,
Han Liu
1,4,
Yurui Ma
1,4,
Wanhui Wang
1,4 and
Shaoyong Lu
1,*
1
State 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, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
2
Sino-Danish College, University of Chinese Academy of Sciences, Beijing 101408, China
3
Sino-Danish Centre for Education and Research, Beijing 101408, China
4
College of Life Science, Cangzhou Normal University, Cangzhou 061001, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(1), 9; https://doi.org/10.3390/agronomy15010009
Submission received: 29 October 2024 / Revised: 12 December 2024 / Accepted: 22 December 2024 / Published: 24 December 2024
(This article belongs to the Special Issue Agricultural Water Management and Non-Point Source Contamination Mitigation)
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Abstract

:
At present, non-point source pollution (NPSP) has overtaken point source as the most important source of water pollution in China. Ecological treatment projects (ETP) suitable for non-point source pollution have been widely recommended. However, China’s NPSP prevention system has not yet taken shape, the implementation and management levels are disorganized, and the long-term management and protection of NPSP-ETP remains an urgent problem. This paper focuses on the practical problems encountered in the promotion of ETP in China, and ways to solve these problems in the operation and maintenance process. First, problems encountered in the practice of NPSP-ETP in China are summarized as being caused by a lack of systematic regulation on operation and maintenance. Following this, promising countermeasures to solve these problems are proposed, including establishing an ecological treatment technology system, improving the technology selection assessment system, improving the assessment method in project operation, and establishing a systematic operation and maintenance process. Finally, a novel theory of Monitoring-Assessment-Repair (MAR) for ETP is proposed to solve the systematic bottlenecks in engineering operation and maintenance. Furthermore, the problem of clogging in infrastructure is discussed in detail, to illustrate the concrete operation of MAR theory. Overall, this study clarifies the issues in favor of a long-term mechanism of NPSP prevention in and beyond China.

1. Introduction

An ecological treatment project (ETP) refers to a structure designed to treat sewage by using or simulating natural ecosystems. ETPs are widely used to treat non-point source pollution (NPSP), because they are simple and easy to maintain and manage. ETPs offer the advantages of highly efficient human-engineered technology and rational utilization of resources compared with purely natural ecosystems. Examples of NPSP-ETPs include constructed wetlands (CWs), land infiltration, stabilization ponds, ecological ditches, vegetation buffer zones, lakeside buffer zones, ecological floating beds, ecological submerged dams, ecological revetments, and so forth. China’s urban sewage treatment rate reached 98.11% in 2022, while the rate of rural sewage treatment was 31% in 2021 [1]. Due to the dispersal of wastewater in rural areas, ETPs are particularly effective methods of improving sewage treatment in such areas. However, weak maintenance and management measures and manager inexperience can lead to greatly shortened ETP lifespans, reductions in their operating efficiency, or even inoperability, which means they cannot achieve the expected non-point source pollution control objectives. Therefore, a systematic, complete, and targeted management system for NPSP-ETP must be urgently established.
This paper focuses on the practical problems encountered in the promotion of ETP in China, and ways to solve these problems in the operation and maintenance process. We have (1) summarized the problems encountered in ETP practice, (2) clarified the standards for ecological treatment technology in China, (3) established an ecological treatment technology system appropriate to China, (4) summarized assessment methods for ETPs in various project stages, and (5) proposed and explained a novel theory of maintenance bottlenecks in ETPs. This research clarifies the issues in favor of the establishment and formation of a long-term mechanism of NPSP prevention in China, that can also be relevant to other countries facing the same problems.

2. Practice and Bottlenecks in Ecological Treatment Technology of NPSP in China

2.1. Problems Encountered in Practice

ETPs have been implemented for about 30 years in China, with the first such constructions tracing back to the 1990s. The first ETP in China was the Bainikeng Constructed Wetland (Shenzhen, China), established by the South China Institute of Environmental Sciences at the Ministry of Ecology and Environment in 1990 [2]. Subsequently, in 1991, the first hedgerow project was built on the arid valley slope of the Jinsha River [3], while the first ecological floating beds were built in Beijing, Shanghai, Hangzhou, and Wuxi to control polluted rivers [4]. In 1992, China’s first soil infiltration project was completed [5]. In 1994, an ecological oxidation pond project was completed in Guangzhou [6]. Thus, China’s ecological treatment technologies have been developed over more than three decades. Furthermore, due to China’s status as a major agricultural nation, advanced technologies have been developed for the promotion and advancement of ETPs in China. For example, techniques such as paddy field consumption [7] and ecological ridges [8] have been implemented to address non-point source nitrogen (N) and phosphorus (P) pollution dispersion on large fields, thereby reducing the need for nitrogen and phosphorus fertilizers. Through long-term practice in China, operational experience of ETPs has accumulated, and problems such as clogging, low temperature, equipment failure, and secondary pollution from dead vegetation have been encountered.

2.1.1. Clogging

Clogging is one of the most unavoidable and serious problems in the operation of porous media ETPs [9], such as subsurface flow constructed wetlands (CWs), ecological filters, and soil infiltration systems. Clogging can directly cause hydraulic obstacles and disrupt the ideal laminar flow in ETPs, resulting in spatial inhomogeneity [10]. Wang et al. [11] measured four subsurface flow CW projects (Haihe River Basin, China) using the in-situ falling head method, two of which were seriously clogged. They found that the longer the wetland is in operation, the higher the degree of clogging. Liu et al. [12] measured the distribution of clogging materials in a subsurface flow CW project (Shandong Mata Lake, China) and found that serious clogging occurred within a flow distance of 10 m from the influent and that clogging showed different characteristics in the upper, middle, and bottom layers. Spatial inhomogeneity caused by clogging, including short flows and dead zones, not only leads to the deterioration of hydraulic conditions, but can also evolve into inhomogeneity of microbial growth and local differences in nutrient utilization capacity [13]. Zhou et al. [14] found that short flows in subsurface flow CWs greatly shorten hydraulic retention time in the system, resulting in inadequate degradation of pollutants. The dead zone will cause a loss of local processing capacity in the system and reduce the system’s effective area, while the long residence time will not only lead to a decline in processing capacity but may also generate odor and favor mosquito reproduction. Investigation of CW projects by Cooper et al. [15] shows that neglect of maintenance is considered to be an important cause of clogging, while maintenance actions such as the replacement of substrate are a timely way to alleviate the clogging problem.

2.1.2. Low Temperature

Northeast and northwest China are located in the sub-frigid zone, and the Qinghai-Tibet Plateau is a high-altitude and low-temperature area. In the face of cold winter weather in China, there are ETPs situated in an open environment that is susceptible to climate [16], and free water surfaces, such as in a stabilization pond, freeze easily [17]. ETPs are generally planted with large herbaceous plants, with poor cold resistance, that enter a dormant period in winter or even perish. In winter, the activity of substrate and rhizosphere microorganisms also decreases significantly [18]. Jing et al. [19] investigated six ETPs in northern China and found that the removal rates of N and P in winter were significantly lower than those in summer, suggesting that the bioactivity of nitrifying bacteria was severely inhibited. When the water temperature is lower than 10 °C, the purification effect of a CW decreases significantly, and nitrification gradually stops at 4 °C [20], which is about 94% lower than that of the normal temperature (25%) [21]. Therefore, seasonal variation has become a major aspect that significantly affects nutrient removal [22].
To solve the problem of low temperature, Wu et al. [23] experimented with covering the system with a 0.4 m biomass layer to provide significant system insulation and maintain high processing performance under cold winter conditions. However, the ETP with this additional cover showed unstable pollutant removal performance, due to reduced oxygen transfer inside the filtering bed. By contrast with thermal insulation, a different technology is proposed for the purpose of increasing temperature. Xiang [24] proposed a solar greenhouse warming CW that can assist the CW in heat preservation. Its heat transfer process consisted of a solar greenhouse, heat-insulation bed, and heating pool. Wang [25] proposed a new CW structure using geothermal energy, laying water heating pipes in layers of the CW and raising the internal temperature of the system to 15 °C. This kind of technology is able to rely on the abundant solar and thermal resources in the vast rural areas of northeast and northwest China and the abundant geothermal resources in the Qinghai-Tibet Plateau and has broad application prospects.

2.1.3. Failure of Aeration and Other Equipment

In order to strengthen their pollutant purification capacity and solve the problems of low hydraulic load and anti-impact load, ETPs often include non-ecological artificial facilities or methods, such as aeration devices [26], chemical additives [27], biological enzymes, or degrading bacteria [28], and mechanical harvesting of aquatic plants [29]. However, these strengthening methods often weaken the projects’ advantages of convenient operation and management, and low construction and maintenance costs. A survey of 680 rural sewage treatment facilities around Beijing found that outages were common and that outages caused by equipment damage accounted for 19.7% of the total [30]. However, the cost of machinery for maintaining every 100 m3 CW project was estimated at 50,000 RMB/time [31]. Taking aeration equipment as an example, aeration can increase the dissolved oxygen content in the system, but the daily power consumption of aeration equipment is 0.24~16.52 kW·h [32], which will increase operational energy consumption and cost; this contradiction is especially prominent in rural areas with poor development (Liu and Shen 2015) [33]. In a survey of 311 rural sewage treatment facilities in the areas surrounding Changshu City, 7.1% of sewage treatment facilities were out of operation, and outages caused by equipment power problems accounted for 63.6% of the total [32]. Thus, the addition of enhanced equipment carries enormous operational and maintenance costs to ETPs. Furthermore, there is a lack of information on the extent of outages at present.

2.2. Operational and Maintenance Bottlenecks

Engineering specifications are an important part of effective operational and maintenance management following the adoption of ETPs, and the formulation of standards and specifications is the starting point for the effective control of the ecological treatment of NPSP. Thus, we have collected and summarized the relevant standards for ETPs published and implemented in China (excluding Hong Kong, Macao, and Taiwan) (Table 1). The standard for CWs is the most common type among them. Jiangsu Province has the largest number of relevant local standards and includes most types of ETPs. Meanwhile, Jiangsu Province also has the largest total CW area, as well as the most concentrated area of CWs in China [34]. This is because Taihu Lake basin is the most densely populated and economically developed area in China, which led to its being the location of China’s first serious eutrophication problem [35]. NPSP contributes the most to the eutrophication process of the basin [36]. In order to solve the problem of non-point source pollution treatment in Taihu Lake, Jiangsu took the lead in establishing a number of ETPs [37]. The content of the relevant standards in Table 1 is obviously skewed to the control of the design, construction and acceptance stages, and there is a serious lack of comprehensive state assessment for the later stages of a project. The problems of clogging and low temperature occur repeatedly, pointing to the seriousness of these problems. However, the standards only cover temporary repair measures in detail. Meanwhile, by contrast with the requirements on water quality, there is neither a method for monitoring clogging nor a standard against which to evaluate clogging, which is entirely impractical. The problems in the implementation of ETPs in China are evident on multiple levels. At the standards level, there is a lack of systematic maintenance requirements. At the technical design level, the standards exhibit a lack of any requirement for a complete regional comprehensive assessment system. At the policy management level, the standards lack applicable and comprehensive assessment methods. Finally, at the technical implementation level, there is no technical system targeting NPSP, and monitoring and remediation methods are still at the exploratory stage. In particular, the lack of monitoring methods undermines the operational and maintenance processes, making it impossible to implement effective post-construction maintenance.

3. Solutions to the Bottleneck

In view of the systematic problems in ETP management in China, this paper proposes four solutions. Establishing an ecological treatment technology system for NPSP in China aims to solve the optimization problem of implementing ETPs for NPSP in different watersheds across the country. This system would include establishing a comprehensive assessment index system for NPSP control technology to enable the selection of technologies that are suitable for local design and maintenance, by improving the assessment method for ETPs, and facilitating judgments on project status from a more comprehensive perspective. Finally, for the operation and maintenance stage, MAR theory is proposed to ensure the strict and regular implementation of operational and maintenance processes.

3.1. Establishing China’s NPSP Ecological Treatment Technology System

In 1972, the United States Environmental Protection Agency (USEPA) first proposed a management method suitable for NPSP control, under the rubric of “best management practices” [38]. USEPA defined best management practices (BMPs) as “any method, measure or operation procedure that can reduce or prevent water pollution, including operation and maintenance procedures for engineering and non-engineering measures” [39]. The principle is to ensure the maximization of nutrient utilization in agricultural production and the minimization of loss through runoff, the protection of farmland resources, and improvements to water quality [40]. The best management measures have achieved remarkable results in NPSP control projects, such as, for example, pollution control in the Mississippi River Basin [41] and aquaculture in Alabama [42]. However, in other examples, such as the Chesapeake Bay watershed project, decades of BMP implementation did not result in significant improvements in water quality for many tributaries [43]. Nevertheless, overall, according to Anna et al.’s research on the effectiveness of BMPs in alleviating diffuse nutrient pollution in agricultural and urban watersheds, 60% of the 94 papers clearly showed that the downstream nutrient concentration or load decreased after the implementation of BMPs [44]. In the actual treatment plan and implementation of NPSP in a particular region of China, a list of NPSP control technologies that can be implemented should be listed by different pollution sources and pollutants, and only subsequent to this should the selection of which engineering technology to use for the NPSP ecological treatment be made. Additionally, the ecological treatment technology for NPSP should be pre-defined. Based on the principle of implementing ecological treatment technology according to local conditions, historical data investigation and long-term fixed-point monitoring should be carried out. After establishing the spatial and temporal distribution characteristics of NPSP in the basin and taking into consideration local environmental carrying capacity, it will be possible to select the appropriate ETP technology for NPSP control in the region, with optimal efficiency, low resource consumption, and ease of construction. On the basis of the BMP technology system, we compiled an ETP technology roadmap suitable for China’s NPSP (Figure 1) to support the comprehensive management and control of NPSP in various river basins across the country.
It is suggested that China should refer to the practices of the United States [45] and Europe [46] to establish a database for the assessment of best management practices, and establish a BMP database suitable for the characteristics of NPSP in China—especially so as to distinguish different environments and watersheds. It is further recommended that assistance should be provided to local governments in identifying and screening ETPs suitable for local governance policy systems according to the requirements of national and local standards, combined with the geographical and climatic environment and spatial and temporal characteristics of pollution in the region, and to clarify the implementation process of NPSP-ETPs. For projects occupying a large area and subject to complex conditions, further research on the optimization of the NPSP monitoring network, or digital simulation of watershed NPSP, can be carried out in combination with GIS technology to identify the source of surface source pollution [47,48]. Then, the projects can be combined with BMP technology roadmap and ETP database to propose a treatment plan suitable for local conditions.

3.2. Selecting a Suitable Ecological Treatment Technology at the Design Stage

At the technical design stage, the ETP is evaluated and screened according to the appropriate evaluation method system. The technology suitable for local future maintenance is prospectively screened, which can effectively increase the ETP’s benefits, save on future maintenance costs, and support in formulating an effective restoration plan. Shen et al. [49] selected three criteria layer indicators, 11 index layer indicators and 11 sub-index layer indicators from the three domains of economy, technology and environment, and used these to construct an evaluation index system for rural domestic sewage treatment technology. The weight of each evaluation indicator was determined by the subjective and objective comprehensive weighting method, combining the chromatography analysis method and the entropy weight method. The hierarchical fuzzy integral model was used to comprehensively assess and sort the rural domestic sewage treatment technology models. Based on this evaluation system, the top ten rural domestic sewage treatment technology models were selected, including three-stage stabilization pond technology. Xia et al. [50] established an evaluation index system for rural domestic sewage treatment technology that takes into consideration technical economy, effectiveness, and suitability. Furthermore, an evaluation method for rural domestic sewage treatment technology based on a fuzzy quality coefficient is established, which emphasizes the applicability of rural domestic sewage treatment. Zhang [51] believed that the rural situation in China is complex, and that the choice of an ecological treatment process should be based on basic local needs and economic conditions. After comparing evaluation methods, combined with the actual situation of the case study area, the AHP (Analytic Hierarchy Process)-fuzzy comprehensive evaluation method was selected to establish an evaluation index system for rural domestic sewage treatment technology in southwest China. Based on the AHP-fuzzy comprehensive evaluation method, Dong et al. [52] established an evaluation index system for farmland NPSP control technology in the Liaohe River Basin and combined it with MATLAB software for comprehensive evaluation. In the fuzzy comprehensive evaluation results, CW technology scored the highest in terms of end-treatment technology, and the ecological ditch method scored the highest in terms of process interception technology. The capacity of the rural ecological treatment process to function beneficially does not lie in the actual treatment technology itself, but rather, critically lies in whether it can align with the actual conditions of local residents, local topography, and other locationally-specific conditions.

3.3. Improving the Assessment Method for ETP at the Operational and Maintenance Stage

Regular assessment of ETPs while operational helps improve project efficiency and adjust operational strategies or repairs in a timely manner. Among the assessment methods in the field of environmental engineering, the AHP is widely used, simple, and easy to understand. It can combine quantitative and qualitative consent assessment dimensions to organize the problem and render it hierarchically, and has good operability. It can also hierarchize complex problems, by dividing the indicators related to decision-making into target layer, criterion layer, and index layer [53]. In addition, the fuzzy comprehensive assessment method is widely used. This method can comprehensively assess a large number of complex factors, and use multiple indicators of information at different scales to evaluate ETPs [54]. Other relatively less-used assessment methods include the life cycle analysis method, the single index assessment method, the grey correlation analysis method, the approximate ideal sorting method, the cost-benefit analysis method, the expert scoring method, and the principal component analysis method [34,51,52].
The body of research on the assessment method for CW technology is relatively mature. Scholars use non-market valuation, life cycle assessment, and other methods to assess CW projects. MacDonald et al. [55] used the non-market contingent valuation method (CVM), which measures the net non-use value, to establish two binary response models with which to estimate and utilize the incremental environmental benefits associated with CWs to control agricultural runoff. Flores et al. [56] used the life cycle assessment (LCA) method to assess the environmental impact of tail water treatment in a CW wastewater treatment plant. Jia et al. [57] used the LCA method to determine the impact of three NPSP control technologies—CW, oxidation pond, and land treatment—on the environment, and compared their contributions to different types of environmental impact. Wang et al. [58] used the AHP to comprehensively assess the five main dimensions of purification efficiency, management status, design level, landscape effect, and economic value and applied this AHP assessment system to 21 major CWs in Linyi. Xue [59] established the AHP model’s application to the comprehensive assessment of aquatic plants in CWs, and comprehensively evaluated and graded the aquatic plants of CW projects in northern Shaanxi according to 17 indicators, including adaptability to low temperatures, saline-alkali adaptability, seasonal changes of plants, leaf esteem value, and flowering period.
There are few studies on the assessment method of other non-point source pollution ETPs. Yang et al. [60] used the AHP to propose a comprehensive benefit assessment index system for sewage treatment land and determine the index weight for ecological projects on sewage treatment land, and used linear weighting to calculate a comprehensive benefit assessment index value. Zhang et al. [61] selected 13 indicators from the domains of ecology, economy, construction comprehensiveness, and landscape comprehensiveness of ecological slope protection and constructed a comprehensive assessment index system for river ecological slope protection in cold regions of northeast China by using AHP and a comprehensive index assessment method. It has a certain reference value for ecological floating beds and other kinds of natural river ETPs to use the surface water eutrophication assessment method, that is, the comprehensive trophic level index (TLI), to investigate the hydrological environment quality. Jin and Li [62] took the riparian vegetation buffer zone of the Wenjin River in Suzhou as the object of their research, and used the multi-index assessment analysis method to select the vegetation type, level, bandwidth, coverage, and continuity. Based on these five indicators, the integrity of the vegetation buffer zone was characterized, and the river water quality was characterized by the pollution load index method (PLI); thus, an assessment system for the riparian vegetation buffer zone was established. Fu et al. [63], taking the lakeside buffer zone of Zhushan Bay in Jiangsu Province as their focus, and its ecological and social impacts as their targets, selected indicators such as water quality purification; water conservation; biodiversity protection; employment provision; and value for scientific research, education, and tourism, and established a comprehensive benefit assessment index system of ecological engineering for the lakeside buffer zone.
The assessment of ETP covers purification efficiency, management status, design level, landscape effect, and other aspects [58]. It is necessary to investigate and collect various forms of data on engineering measures, including hydraulic status, water purification efficiency, biological activity at low temperatures, degree of clogging, plant growth status, passenger flow, and satisfaction. Based on data analysis of the various indicators, the comprehensive status of the ETP is assessed. At present, due to the absence of state assessment, as well as of operational and maintenance systems, the ETP is prone to low temperature declines in treatment capacity and clogging, resulting in hydraulic failure and reduced treatment performance, which entails a significant reduction in the service life of the ETP and a large number of abandoned waste [64]. However, the gap in actual data monitoring is very large at present. Therefore, at the project acceptance stage and during the subsequent operation process, long-term monitoring and detection can effectively provide the basis for judging whether the project can operate stably in compliance with water quality regulations and when basic data indicates the need to implement maintenance measures. It is of great practical significance to establish a database of sewage purification capacity and operational stability, as well as hydraulic and clogging material accumulation parameters. Overall, it is important to provide basic data support for the standardized operational management and supervision of ETPs in China, especially in relation to the urgent need to establish an assessment system for various types of ETP at this point, and to improve the sustainability and utilization rate of ETPs.

3.4. Proposing the Theory of Systematic Operation and Maintenance (MAR)

If the aim is to implement operational and maintenance management measures fully, the measures described in the preceding sections still lack the requisite systematicity. On the one hand, researchers have tried to further optimize and improve the applicability and reliability of various ETP designs. On the other hand, through their investigations and the appeal to relevant standards and policies, the researchers have also improved the options for dealing with the dilemmas raised when running ETPs in their later stages of operation. How to implement effective maintenance and management of the project is not only a technical problem and a policy problem, but also a cognitive problem and a procedural problem. In this regard, this paper proposes the “Monitoring-Assessment-Repair” (MAR) theory of ETP operation and maintenance (Figure 2). The three dimensions (monitoring, assessment, and repair) of MAR are necessary and indispensable in the operation and maintenance of ETPs. Monitoring provides managers with data on the operational status of the project. The wider the scope of monitoring, the timelier data detection can be, and the more favorably it can function as a basis for quantifying the comprehensive status of the project. Assessment is a scientific judgment on the operational status of the project. Managers would be in a position to dispense with subjective judgments, base their evaluations on reliable evidence, and contribute to the implementation of national systematic management. Repair is the ultimate intervention. Effective repair enables the project to maintain healthy and stable operation over a long period of time. The three together constitute a complete circuit of the operation and maintenance process.

4. MAR Solutions to Clogging Problems: An Example of the Application of MAR

At present, there is no complete implementation of the MAR approach, so this paper takes the solution of the clogging problem found in CW projects as an example through which to explain the specific operation of MAR.

4.1. Monitoring Link

The technology for monitoring clogging is still in the developmental stage, and none of the existing monitoring methods is recognized as standard and generally implemented. Among the current research methods used in this area, one type is the conventional method, based on hydraulic principles. The hydraulic indicators monitored not only represent the ETPs’ hydraulic health condition but also represent the degree of solid accumulation within them.
Typical representative methods include the drainage method [65], the pulse tracer test [66], and the permeability coefficient method [67]. These methods often only deliver qualitative judgments about whether Projects are clogged and have obvious destructive impacts, making them sub-optimal for meeting the needs of in-depth research on clogging. However, due to their simple principles, easy operation, and price advantages, this type of method can still be useful as a conventional clogging research method [68] and is more easily promoted for a wide range of situations. Specifically, the drainage method is a traditional method that can only be applied at the laboratory scale, and its extremely low cost make it the most common method for characterizing clogging. The dye tracer test was initially limited by conditions and was only used as a simple, inexpensive, and intuitive flow observation technique [69]. It has since evolved with the development of imaging technology and computer image processing technology, and can be used to observe flow and particle distribution [70], visually displaying the process of clogging formation. However, due to the requirement of matrix transparency in image processing technology, the new technology is only applied at a laboratory scale. The permeability coefficient method is based on Darcy’s law and measures hydraulic conductivity (Ks), which can characterize the permeability of ETPs. Currently, multiple in-situ measurement methods have been developed [71], mainly divided into the constant head method [72] and the falling head method [73], both of which are matrix insertion methods. The in-situ permeability coefficient method can ignore the damage in ETPs, but caution is needed at the laboratory scale.
Other promising methods come from the field of geophysical exploration. In recent years, to meet the increasing demand for engineering monitoring, the field of non-point source pollution has evolved as a new research and development trend by incorporating advanced detection technologies from other disciplines into monitoring ETPs’ operational status (Table 2). These methods can locate, quantify, and visually monitor clogging, but their accuracy, reliability, adaptability to different regions, and limitations with regard to clogging detection are unknown. There are still many aspects to be studied in the field of geophysical exploration technology applied to the monitoring of ETP clogging. For example, the time domain reflectometer (TDR) has been used to detect underground voids and soil moisture [74]. When this is applied to monitor the degree of clogging in ETPs, there is a problem of measurement deviation caused by high water content in the clogging material [34]. Electrical resistivity tomography (ERT) has excellent non-destructive imaging effects and large-scale monitoring capabilities for engineering applications [75]. ERT has been the most extensively studied of the various monitoring methods, and has been preliminarily applied in experiments on CWs in China [17], Spain [76], Italy [77], and Tunisia [78]. Matos et al. [79] applied ground penetrating radar (GPR) to CWs, and successfully monitored the distribution of clogging in full-scale CWs through electromagnetic wave velocity images. Hughes-Riley et al. [80] applied nuclear magnetic resonance (NMR) technology to CW monitoring, and developed and continuously studied a permanent in-situ probe-NMR sensor that can detect dynamic changes in CW clogging in a non-destructive and real-time manner [81]. X-ray computed tomography (X-CT) is a non-destructive imaging technique that observes the overall three-dimensional structure of CWs as a whole through cross-sectional scanning imaging [82]. Unlike other geophysical methods, X-CT can only image small objects and can be applied at the laboratory scale, which presents difficulties for the project scale. However, Martinez-Carvajal et al. [83] combined X-CT with frequency domain electromagnetic measurements (FDEM) to successfully monitor the Montromant CW project (Rhône, France).
MAR requires operational staff to consider what methods are available and how to choose them during operation and maintenance. The first step is to select a suitable clogging monitoring method tailored to local conditions, according to (1) type of ETP, (2) scale of ETP, (3) moisture content of ETP, and whether saturation with water is required, (4) seasonal restrictions, (5) matrix size, and whether it is easy to insert, (6) depth, (7) frequency of monitoring, and whether it is automatic or not, (8) cost/price. Operatives can refer to Table 2 to appraise this.

4.2. The Assessment Dimension of MAR

Research on the assessment of CW functioning is still in its initial stages in China, and there is a development bottleneck due to the lack of CW project assessment, making it difficult to obtain effective feedback on these ETPs. There have been some studies on clogging assessment, but they all have experimental status and have not been widely used in practice. The assessment of ETPs can be divided into assessment indicators and assessment methods. Assessment indicators are a type of clogging problem that are only used to assess the degree of clogging or the stage of clogging development. Wang et al. [71] assessed the degree of clogging in four CW projects (Haihe River Basin, China) using a clogging coefficient based on Ks, dividing the four projects into three levels: severe clogging, partial clogging, and non-severe clogging. Liu et al. [17] assessed the Zhulong River CW (Shandong, China) with two monitoring methods (ERT and the water displacement method) and one indicator (clogging matter fraction, v/v), assessing the degree of clogging and the differences between sections at different depths.
The other type of approach is a comprehensive assessment of operations and maintenance, which includes aspects such as congestion or management level. A comprehensive assessment should include non-quantitative aspects, such as maintenance costs and equipment status, in addition to the degree of clogging.
The assessment method proposed by Xu et al. [88] is detailed and comprehensive. They proposed a comprehensive assessment index system for CWs based on the combination of qualitative and quantitative analysis, drawing from AHP and the comprehensive index method, including site topography and climate suitability, design reliability, operation effect and management, cost analysis, ecosystem service value, social benefits, and comprehensive scoring of CWs, that was applied in Dezhou (Shandong, China). The assessment method developed by Liu et al. [89] specifically mentioned clogging management. This comprehensive assessment system weights various indicators of the operational status of CWs, from aspects of management level and the degree of functional implementation. The indicators cover the stability of inflow water quality, plant management, water quality monitoring, clogging management, equipment management, winter insulation, water quality status, effluent quality compliance rate, pollutant reduction rate, operation and treatment cost, operation cost, ecosystem service value, product service, regulation and maintenance service value, leisure, scientific research, and education value. Based on this, a comprehensive assessment index system for the operational state of CWs was proposed, and a comprehensive assessment indicator was constructed using a linear weighting method.
MAR requires that staff should not only pay attention to the degree of clogging but should also conduct comprehensive assessments intermittently over a long period of time. The dimensions of clogging monitoring and clogging repair are not isolated. It is necessary to score the method selection, degree of implementation, and implementation effect through the assessment dimension, as well as to summarize the problems. This conclusion is then fed back to the monitoring dimension and repair dimension so as to adjust, give feedback again, and re-adjust. The assessment dimension of MAR reflects its systematic and dynamic nature, while the monitoring and repair dimensions do not exist in isolation.

4.3. Repair Dimension

The research and applications pertaining to clogging repair are the most mature of the three MAR dimensions. These can be roughly divided into replacement of the substrate, speculative industrial preparations, and biological pore increasing.
Regular replacement and renovation of the substrate are the most direct means to effect physical recovery. By regularly replacing the surface substrate of wetlands, the problem of wetland clogging can be effectively prevented [90]. However, for large-scale projects, the amount of substrate replacement and refurbishment required is extensive, time and economic costs are high, and the already-constrained project maintenance costs make this method difficult to popularize. The impact of this too-direct measure on the microbial community structure of the system cannot be ignored, and how the eliminated substrate will be dealt with also needs to be considered.
Because inorganic clogging substances are difficult to remove, and considering the important role of organic clogging substances on water flow, the efficacy of methods for reducing organic clogging substances has come to dominate the research in this area. Clogging removal measures of adding industrial preparations have the advantages of simple operation, short treatment time, and obvious effects. The investment cost of traditional chemical reagents is lower than that of biological reagents, but there is a certain risk to the ecological environment of the system. Acid and alkali adsorbents, such as hydrochloric acid (HCl) and sodium hydroxide (NaOH), and oxidative bacteriostatic agents, such as sodium hypochlorite (NaClO) and hydrogen peroxide (H2O2), have shown good effects in improving clogging conditions in studies [91,92]. Biological reagent clogging removal measures are less destructive than physical and chemical measures, but some reagents are expensive, which is not conducive to large-scale promotion. Biosurfactants, such as sodium dodecyl sulfate (SDS), rhamnolipids (RLs), and citric acid (CA), also play a role in dissolving biofilms and disrupting biologically clogged structures [90]. Hydrolytic enzymes, such as α-glucan amylase and β-glucan, can effectively catalyze and hydrolyze protein and polysaccharides in bio-clogging substances [93]. Although these methods have excellent treatment efficiency and only slight negative effects on the system, their high cost has meant the use of these biological repair measures remains at the laboratory level, and there is no large-scale application practice for engineering.
The biological pore-increasing method is summarized by the author as naturally increasing porosity by utilizing the bioactivity of organisms. It can be divided into the introduction of earthworms and root-hole technology. Researchers use earthworms to form new water flow channels when they transit through sludge [94], as well as to improve the overall permeability of the filler [95]; they also observe that earthworms can ingest blockages [96], which has obvious conversion and reduction effects on clogging, no negative impact on water quality, and is extremely low cost. However, the survival of earthworms is susceptible to environmental factors such as dissolved oxygen and matrix roughness [3]. Future research needs to address the survival rate of water earthworms and the long-term maintenance of the system with permeability coefficient following earthworm introduction. Root pore technology can effectively change the macropore structure of the subsurface layer [11], which is an important technical advance to improve the self-sustaining operation ability of ETPs. However, the impacts of its implementation only manifest slowly over time.
MAR requires operatives to give priority to what methods are available and how to choose methods before implementing clogging repairs in a project. The key is to select the appropriate clogging repair method, with reference to the following points: (1) allowable repair time, (2) past repair frequency, (3) inflow load, (4) geographical location, and whether convenient for transportation, (5) safety impact on effluent and surrounding environment, (6) maintenance cost, and (7) difficulty in disposing of repair waste.

5. Perspectives

(1)
Formulating and improving the norms of operation and maintenance of ETPs in China.
To solve the problem of operation and maintenance of NPSP-ETP, improved standards are needed that can alleviate the bottleneck problem. The implementation of a monitoring and assessment method for NPSP-ETP must rely on the formulation of standards able to alleviate the management bottleneck.
(2)
Accelerating the construction and promotion of an ETP assessment system.
China’s ETP assessment system is in its nascent stages. In the process of its normalization and adaptation to requirements, although there have been multiple studies that have attempted to construct suitable assessment methods, examples of implementation and feedback are lacking. Thus, there is a current need for an assessment system that is adaptable to the differing conditions in various regions and is strongly pragmatic.
(3)
Developing reliable and feasible new monitoring methods and repair technologies.
At present, the monitoring method best suited to urgent problems such as clogging, low temperature, and breakdown of mechanical equipment remains at the research and development stage. Cooperative interdisciplinary research needs to be carried out on the accuracy, repeatability, and application conditions of new technologies, so as to determine universally feasible technologies as soon as possible. This needs to be combined with new monitoring methods and repair techniques to solve technical bottlenecks.

Author Contributions

All authors contributed to the study’s conception and design. Material preparation, data collection, and analysis were performed by Y.J., X.L., H.L., Y.M. and W.W. The first draft of the manuscript was written by Y.J., J.Z. and S.L. commented on and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

The research leading to these results has received funding from the National Key Research and Development Program of China (2021YFC3201505); and the National Natural Science Foundation of China under grant number (42207154).

Conflicts of Interest

The authors declare no conflict of interest.

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Agronomy 15 00009 g001
Figure 1. BMP technology system of non-point source pollution ETP in China. The system for ecological treatment of agricultural NPSP comprises processes from 1 to 5, while the urban system comprises processes from 2 to 5 (excluding process 1).
Figure 1. BMP technology system of non-point source pollution ETP in China. The system for ecological treatment of agricultural NPSP comprises processes from 1 to 5, while the urban system comprises processes from 2 to 5 (excluding process 1).
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Figure 2. NPSP-ETP operation and maintenance process (MAR) concept map. (Picture of “Standard” used the cover of “Technical specification of constructed wetlands for wastewater treatment engineering” from China. Picture of “Comprehensive assessment index” refers to Zhang et al. [61]).
Figure 2. NPSP-ETP operation and maintenance process (MAR) concept map. (Picture of “Standard” used the cover of “Technical specification of constructed wetlands for wastewater treatment engineering” from China. Picture of “Comprehensive assessment index” refers to Zhang et al. [61]).
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Table 1. Relevant standards for ETPs issued and implemented in China (excluding Hong Kong, Macao and Taiwan) pertaining to: horizontal subsurface flow constructed wetland (HFCW), vertical subsurface flow constructed wetland (VFCW), and surface flow constructed wetland (SFCW).
Table 1. Relevant standards for ETPs issued and implemented in China (excluding Hong Kong, Macao and Taiwan) pertaining to: horizontal subsurface flow constructed wetland (HFCW), vertical subsurface flow constructed wetland (VFCW), and surface flow constructed wetland (SFCW).
Name and Number of the StandardObjectPublishing DepartmentRelease TimeDelivery TimeMajor Design ParametersWater Intake StandardPollution Reduction Load g/(m2·d)Divide the Region or NotWith Project Assessment Method or NotWith Clogging Management or NotWith Low Temperature Management or NotWith Equipment Management or Not
Technical specification of constructed wetland sewage treatment project (HJ 2005–2010)Constructed wetlandMinistry of Environmental Protection of the People’s Republic of China17 December 20101 March 2011HFCW:
area < 800 m2		HFCW:
BODs ≤ 80 CODCr ≤ 200
SS ≤ 60 NH3-H ≤ 25 TP ≤ 5		-NNYYN
VFCW:
area < 1500 m2, length ratio < 3:1, length 20~50 m, hydraulic slope 0.5~1%		VFCW:
BODs ≤ 80 CODCr ≤ 200 SS ≤ 80 NH3-H ≤ 25 TP ≤ 5
SFCW:
length ratio 3:1–5:1, water depth 0.3–0.5 m, hydraulic slope < 0.5%		SFCW:
BODs ≤ 50 CODCr ≤ 125
SS ≤ 100 NH3-H ≤ 10 TP ≤ 3
Technical guide for water purification of constructed wetland (Environmental Protection Office Water Body Letter [2021] 173)Constructed wetlandMinistry of Ecological Environment of the People’s Republic of China14 April 2021-HFCW:
area < 2000 m2 length ratio < 3:1 length 20~50 m hydraulic slope 0~0.5%		-HFCW:
1 ≤ CODCr ≤ 10		YNYYY
VFCW:
area < 1500 m2 aspect ratio 1:1~3:1 water depth 0.8~2.0 m		VFCW:
1.5 ≤ CODCr ≤ 12
SFCW:
area < 3000 m2 aspect ratio > 3:1 water depth 0.3~2.0 m		SFCW:
0.1 ≤ CODCr ≤ 5
Natural sewage treatment
Engineering Technical Specifications (CJJ/T 54-2017)		Constructed wetlandMinistry of Housing and Urban Rural Development of the People’s Republic of China23 March 20171 September 2017SFCW:
area < 3000 m2		-SFCW:
1.5 ≤ BOD5 ≤ 3.5		NNYNN
HFCW:
area < 800 m2		HFCW:
4 ≤ BOD5 ≤ 6
VFCW:
area < 1500 m2		VFCW:
5 ≤ BOD5 ≤ 7
Technical specification for constructed wetland wastewater treatment (DG/TJ08-2100-2012)Constructed wetlandShanghai Urban and Rural Construction and Transportation Committee10 April 20121 June 2012SFCW:
Length 20 m–50 m aspect ratio 3:1~5:1 water depth 30 cm~60 cm bottom slope 0.1–0.5%		--NNNNN
HFCW:
Length 0.5 m hydraulic slope 0.5~1.0%
aspect ratio 3:1~4:1 length < 50 cm
Technical specification for rural domestic sewage constructed wetland treatment project (DB11/T 1376-2016)Constructed wetlandBeijing Municipal Bureau of Quality and Technical Supervision22 December 20161 April 2017HFCW:
area < 1000 m2 aspect ratio 1:1~3:1
VFCW:
area < 1000 m2		--NNYNN
Technical Guide for Water Purification Engineering of Constructed Wetland (DB 37/T3394-2018)Constructed wetlandShandong Provincial Bureau of Quality and Technical Supervision17 August 201817 September 2018HFCW:
area < 2000 m2 aspect ratio < 3:1 length 20 m~50 m
water depth 0.6 m~1.6 m hydraulic slope 0~0.5%		-HFCW:
0.5 ≤ CODCr ≤ 10		NNYYN
VFCW:
area < 1500 m2 aspect ratio 1:1~3:1
water depth 0.8 m~2.0 m		VFCW:
0.5 ≤ CODCr ≤ 10
SFCW:
area < 3000 m2 aspect ratio > 3:1 water depth 0.3 m~2.0 m		SFCW:
0.2 ≤ CODCr ≤ 5
Technical specification for constructed wetland wastewater treatment (DGJ32/TJ112-2010)Constructed wetlandJiangsu Provincial Department of Housing and Urban-Rural Development2 December 20101 January 2011-COD ≤ 200 mg/L
SS ≤ 80 mg/L		HFCW:
CODCr ≤ 16
VFCW:
CODCr ≤ 20		NNNNN
Zhejiang province technical specification for constructed wetland treatment project of domestic sewage (Zhejiang Environmental Industry Association
[2015]14)		Constructed wetlandZhejiang Environmental Protection Industry Association31 December 20151 January 2016SFCW:
area < 3000 m2		-SFCW:
2.5 ≤ CODCr ≤ 4		NNYNN
HFCW:
area < 2000 m2		HFCW:
6 ≤ CODCr ≤ 10
VFCW:
area < 2000 m2		VFCW:
10 ≤ CODCr ≤ 12
Technical specification for tail water constructed wetland project of domestic sewage treatment plant (DB34/T 4384-2023)Constructed wetlandAnhui Provincial Department of Ecological Environment1 March 20231 April 2023SFCW:
length 20–50 m
aspect ratio 3:1~5:1
water depth 30~60 cm bottom slope 0.1–0.5%		--NNYYN
HFCW:
length 0.5 m
hydraulic slope 0.5~1.0%
aspect ratio 3:1~4:1
length < 50 cm
Technical specification for constructed wetland project of tail water discharged from sewage treatment plant (DB41/T 1947-2020)Constructed wetlandHenan Provincial Department of Ecological Environment21 January 202021 April 2020SFCW:
water depth 30~60 cm
aspect ratio 3:1~5:1
bottom slope < 0.5%		-SFCW:
0.2 ≤ CODCr ≤ 5		NNYYN
HFCW:
area < 800 m2
water depth 80~140 cm
aspect ratio < 3:1
length 20~50 m		HFCW:
0.2 ≤ CODCr ≤ 5
VFCW:
area < 1000 m2
water depth 80~140 cm
aspect ratio < 3:1
length 20~50 m		VFCW:
0.2 ≤ CODCr ≤ 5
Technical specification for water purification engineering of constructed wetland (DB13/T 5184-2020)Constructed wetlandHebei Market Supervision Administration Bureau25 March 202025 April 2020SFCW:
area < 3000 m2
aspect ratio 3:1~5:1
water depth 0.3 m~0.6		-SFCW:
3 ≤ CODCr ≤ 8		NNYYY
HFCW:
area < 2000 m2
aspect ratio 3:1~10:1		HFCW:
11 ≤ CODCr ≤ 16
VFCW:
area < 1500 m2
depth > 0.5 m		VFCW:
13 ≤ CODCr ≤ 18
Technical specification of artificial wetland in plateau lake area (DB53/T 306-2010)Constructed wetlandYunnan Provincial Bureau of Quality and Technical Supervision16 March 20101 July 2010-CODCr ≤ 30 mg/L
BOD5 ≤ 10 mg/L
SS ≤ 40 mg/L
TN ≤ 5.0 mg/L
TP ≤ 0.5 mg/L		SFCW:
1.8 ≤ BOD5 ≤ 5		NNYNN
HFCW:
5 ≤ BOD5 ≤ 8
VFCW:
5 ≤ BOD5 ≤ 8
Hydrolysis acidification-Technical specification for unpowered wastewater treatment engineering of constructed wetland (DB44T 1995-2017)Constructed wetlandGuangdong Provincial Bureau of Quality and Technical Supervision10 May 201710 August 2017HFCW:
length ratio 2:1~3:1
water depth > 1 m		CODCr 80~200 mg/L
BOD5 20~60 mg/L
SS 150~250 mg/L
NH3 8~20 (20~60) mg/L
TP 1~10 mg/L		-NNNNN
SFCW:
water depth 0.3~0.5 m
hydraulic gradient 0.05~0.1%
Technical specification for construction of rural domestic sewage constructed wetland treatment project (T/CSF 007-2022)Constructed wetlandChinese Society of Forestry11 November 202211 November 2022HFCW: l/w > 3:1, depth 0.3~0.6 mHFCW:
BODs ≤ 80 CODCr ≤ 200
SS ≤ 60 NH3-H ≤ 25 TP ≤ 5		-YYYYY
VFCW: l/w = 3:1~10:1, length 20~50 m, depth 0.6~1.6 mVFCW:
BODs ≤ 80 CODCr ≤ 200 SS ≤ 80 NH3-H ≤ 25 TP ≤ 5
SFCW: l/w = 1:1~3:1, depth 0.8~2.0 mSFCW:
BODs ≤ 50 CODCr ≤ 125
SS ≤ 100 NH3-H ≤ 10 TP ≤ 3
Technical specification for treatment of scattered point source sewage by constructed wetland (DB33/T 2371-2021)Constructed wetlandZhejiang Provincial Administration for Market Regulation22 September 202122 October 2021HFCW: l/w > 3:1, depth 0.3~0.5 m, area < 3000 m2HFCW:
BODs ≤ 80 CODCr ≤ 200
SS ≤ 60 NH3-H ≤ 25 TP ≤ 5		HFCW:
6 ≤ CODCr ≤ 10		NNYYN
VFCW: l/w = 3:1~5:1, depth 0.4~1.6 m, area < 2000 m2VFCW:
BODs ≤ 80 CODCr ≤ 200 SS ≤ 80 NH3-H ≤ 25 TP ≤ 5		VFCW:
10 ≤ CODCr ≤ 12
SFCW: l/w = 1:1~3:1, depth 0.4~1.6 m, area < 2000 m2SFCW:
BODs ≤ 50 CODCr ≤ 125
SS ≤ 100 NH3-H ≤ 10 TP ≤ 3		SFCW:
2.5 ≤ CODCr ≤ 4
Technical specification for biological filter wastewater treatment engineering (HJ 2014-2012)Ecological filterMinistry of Environmental Protection of the People’s Republic of China19 March 20121 June 2012High load ecological filter: overall height 2.0~4.0 mtotal alkalinity (CaCO3/NH3-N) > 7.14
BOD5/TP > 17.0
BOD5/CODCr > 0.3
BOD5 < 500 mg/L		-NNNNY
Tower ecological filter: diameter 1.0~3.5 m diameter: height 1:6~1:8
thickness of filter material 8~12 m
Ecological aerated filter: sectional area 50~100 m2
Design specification of sewage stabilization pond (CJJ/T 54-1993)Stabilization pondMinistry of Construction of the People’s Republic of China6 May 19931 January 1994Anaerobic pond: water depth 4~5 m, slope inside the pond 1.5:1~1:3, slope outside the pond 1:2~1:4, facultative lagoon: depth 1.0~2.0 m aspect ratio 3:1~4:1, slope inside the pond 1:2~1:3, slope outside the pond 1:2~1:5BOD5 < 30 mg/L
COD < 120 mg/L
30 < SS < 60 mg/L
organic acid concentration < 3000 mg/L
sulfate concentration < 500 mg/L		-NN-NN
Aerobic pond: aspect ratio 3:1~4:1, slope inside the pond, 1:2~1:3, slope outside the pond 1:2~1:5, aerated lagoon: water depth 2~6 m
Technical specification for construction of nitrogen and phosphorus ecological interception ditches for farmland runoff (DB3025/T157-2008)Ecological ditchSuzhou Quality and Technical Supervision Bureau of Jiangsu Province1 March 20091 March 2009The section of the canal is isosceles trapezoid, the upper width is 1.5 m, the bottom width is 1.0 m, and the depth is 0.6 m. The height of the dam is 0.5 m, The total length of the dam is 0.6 m and the total width is 1.25 m
The slope coefficient of the permeable dam slope is 1:1~1:2.5		NH4+-N
0.17–1.23 mg/L
NH4+-N
0.67 mg/L		-NN-NN
Design standard for irrigation and drainage engineering (GB 50288-2018)Ecological buffer zoneMinistry of Housing and Urban Rural Development of the People’s Republic of ChinaNovember 202115 December 2021The width of buffer zone
of good vegetation
land > 30 m
Width of desert land buffer zone > 50 m
Width of rock-type land buffer zone > 50 m		--NN-NY
Ecological floating island (floating bed) plant planting technical regulations (DB42/T 1417-2018)Ecological floating bedHubei Provincial Bureau of Quality and Technical Supervision11 September 201810 December 2018The side length of the ecological floating bed is 1~5 mCODMn ≤ 2
CODCr ≤ 15
BOD5 ≤ 3
NH3 ≤ 0.15
TP ≤ 0.02
TN ≤ 0.2		-NN-N-
Technical guidelines for lakeside ecosystem construction and stability maintenance (DB32/T 4045-2021)Ecological submerged damJiangsu Market Supervision Administration Bureau3 June 20213 July 2021Gentle-slope type (gradient < 25°) submerged dam
abrupt slope-type (gradient 25~35°) submerged dam		--NN-NN
Technical specification of green concrete ecological slope protection (DB41∕T 2231-2022)Ecological slope protectionHenan Market Supervision and Administration Bureau13 January 202212 April 2022Compressive strength—10~15 N/m2
settlement rate—0
Porosity—25~35%
percentage of green coverage ≥ 90%		--NN-NN
Table 2. Technical characteristics of monitoring method.
Table 2. Technical characteristics of monitoring method.
Monitoring MethodProximate ObjectObjective IndexTesting PositionTesting FormTesting RangeTesting FrequencyApplication Cases
Descending water head methodHead lossPermeability coefficientin situinsertion1~34 m-[84]
Constant head methodHead lossPermeability coefficientin situinsertion2~32 m-[71]
Drainage methodDrainage volumePorosityin situnon-destruction<2 m-[85]
Pulse-tracer methodTracer concentrationHydraulic retention timein situnon-destruction1~34 m-[77]
Dye tracer-image processingPixel valueVelocity distributionin situdestruction<0.6 m-[14]
Filtration-evaporation-burning methodVolatile solidAccumulation degree of solids in poresectopiadestruction--[17]
TDRThe propagation velocity of high-frequency electromagnetic pulse along the probeQualitative description of the water content of the matrixin situinsertion1~20 mreal-time[86]
ERTThe difference in apparent resistivity between mediaQuantity and distribution of matrix and voidsin situinsertion4~34 mreal-time[17]
GPRPropagation velocity and dielectric constant of electromagnetic wave in a mediumThe distribution of entities and voids reflected by the energy attenuation imagein situnon- destruction1~24 m-[79]
NMRThe spectrum generated by the resonance frequency in nuclear magnetic resonanceThe degree of blockage and the relative amount of biomass and particlesin situinsertion<0.5 m or >4 mreal-time[81]
X-CTX-ray absorption of matterThe changes in hydraulic conditions such as dead water area and short flow area in the systemectopiadestruction<0.1 m-[87]
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Jiang, Y.; Zhang, J.; Liu, X.; Liu, H.; Ma, Y.; Wang, W.; Lu, S. Bottleneck Problems and Countermeasures in Operation and Maintenance of Non-Point Source Pollution Ecological Treatment Projects in China. Agronomy 2025, 15, 9. https://doi.org/10.3390/agronomy15010009

AMA Style

Jiang Y, Zhang J, Liu X, Liu H, Ma Y, Wang W, Lu S. Bottleneck Problems and Countermeasures in Operation and Maintenance of Non-Point Source Pollution Ecological Treatment Projects in China. Agronomy. 2025; 15(1):9. https://doi.org/10.3390/agronomy15010009

Chicago/Turabian Style

Jiang, Yungeng, Jing Zhang, Xiaoxin Liu, Han Liu, Yurui Ma, Wanhui Wang, and Shaoyong Lu. 2025. "Bottleneck Problems and Countermeasures in Operation and Maintenance of Non-Point Source Pollution Ecological Treatment Projects in China" Agronomy 15, no. 1: 9. https://doi.org/10.3390/agronomy15010009

APA Style

Jiang, Y., Zhang, J., Liu, X., Liu, H., Ma, Y., Wang, W., & Lu, S. (2025). Bottleneck Problems and Countermeasures in Operation and Maintenance of Non-Point Source Pollution Ecological Treatment Projects in China. Agronomy, 15(1), 9. https://doi.org/10.3390/agronomy15010009

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