Discussion on the Treatment of Rural Domestic Sewage in the Water Source Area of the Middle Route of the South-to-North Water Diversion Project—A Case Study of a Village

Abstract

Rural domestic sewage, originating from human activities that involve the extraction and utilization of natural resources, is an inherent component of the ecological cycle in nature. Therefore, its disposal methods should align and harmonize with the laws governing nature’s evolutionary processes. This study conducted a comprehensive investigation on the domestic sewage facilities in representative villages located within the water source protection area of the middle route of the South-to-North Water Diversion Project. Taking Village A’s domestic sewage treatment station as a case study, an analysis was performed to assess its operational status and identify existing issues. The consideration of rural domestic sewage treatment should encompass factors such as the generation and discharge of household wastewater, the characteristics of water quality, discharge regulations, the natural and social environment, as well as post-completion operations and maintenance modes. We also proposed source reduction measures for the reuse of gray water in domestic sewage treatment in Village A, along with integrated treatment approaches involving biochemical treatment, landscape integration, and farmland irrigation for black water. These measures not only achieve effective treatment outcomes but also foster harmonious coexistence between humans and nature. Moreover, they align with the principles of ecological civilization while considering rural revitalization and promoting green agricultural development.

1. Introduction

China’s rural population accounts for 36.11 percent of the country’s total population, amounting to approximately 510 million individuals [1]. The discharge of domestic sewage in these areas poses a significant threat to the rural environment, resulting in severe pollution. According to data statistics from the Ministry of Housing and Urban Rural Development, there was a consistent upward trend in rural sewage discharges in China between 2016 and 2021, culminating in a total discharge volume of 35 billion m³ by 2021 [2]. However, as of the end of 2021, only about 31% sewage treatment efficiency has been achieved. Due to the larger rural population, dispersed living conditions, worse economic conditions, and a lack of environmental awareness in China, the efficient treatment of rural domestic sewage is not possible. This leads to surface water pollution on the one hand and soil and groundwater pollution on the other hand. Furthermore, the issue of rural domestic sewage treatment is primarily manifested in significant fluctuations in both the quality and quantity of sewage, the inadequate efficiency of sewage treatment equipment, insufficient operation and maintenance management practices, as well as poor sustainability. Consequently, addressing the effective treatment of rural domestic sewage has emerged as an urgent concern in China. Therefore, to address the aforementioned issue, this study aims to investigate the current status and challenges of rural domestic sewage treatment and resource utilization, while proposing treatment models and recommendations tailored to China’s specific conditions.

2. Requisite Measures for The Treatment of Domestic Sewage in Rural Areas

2.1. Requisite for Achieving Symbiotic Coexistence between Humanity and the Natural World

The crux of the coexistence between humans and nature lies in ensuring that human activities align with the natural laws governing global evolution, thereby upholding the delicate balance of ecosystems [3]. Failure to do so will inevitably subject humanity to nature’s retribution. To achieve harmonious coexistence between humans and nature, concrete implementation schemes should focus on accelerating the transition of industry and agriculture towards green production; intensifying efforts to prevent environmental pollution; enhancing the diversity, stability, and sustainability of ecosystems; as well as vigorously promoting carbon peak and carbon neutrality [4]. The previously prevalent method for domestic sewage treatment involved the use of microorganisms to decompose waste, which offered advantages such as low operational costs and reduced secondary pollution. However, in recent years, there has been an escalating concern regarding the severity of global warming. The microorganism-decomposing method exhibits a significant drawback in terms of its substantial discharge of greenhouse gas, specifically carbon dioxide. According to statistical data, the carbon dioxide emissions from sewage treatment account for approximately 3% of the global annual total, making it one of the primary sources contributing to greenhouse gas emissions [5]. Rural domestic sewage, which arises from human activities associated with the extraction and utilization of natural resources, constitutes an intrinsic element within the ecological cycle in nature. Therefore, its disposal methods should align and harmonize with the laws governing nature’s evolutionary processes. Rural domestic sewage contains negligible amounts of toxic and harmful substances, such as heavy metals and persistent contaminants, which are unlikely to pose significant risks to the environment and agricultural products. Moreover, organic matter, nitrogen, and phosphorus present in this sewage can serve as essential nutrients for soil and crops. By harnessing these inherent characteristics of rural domestic sewage through appropriate management practices that facilitate its return to nature, it can be effectively utilized as a soil amendment for enhancing agricultural production or as a means of natural landscape afforestation [6]. The first advantage of this treatment model lies in its contribution to ecological restoration. Secondly, it can stimulate agricultural development within the local community. Lastly, and most importantly, it not only mitigates greenhouse gas emissions by preventing carbon dioxide release but also actively absorbs substantial amounts of carbon dioxide, thereby promoting natural photosynthesis and maintaining a harmonious balance between humans and nature.

2.2. The Principles of Adapting to Local Conditions and Prioritizing Utilization Should Be Adhered to

When choosing the treatment method of domestic sewage, factors such as the production and discharge of rural domestic sewage, water quality characteristics, rural environmental conditions, discharge requirements, and past construction operation and maintenance practices should be comprehensively evaluated. A straightforward and pragmatic processing approach that is adaptable to local circumstances should be selected, prioritizing resource utilization while considering local conditions and emphasizing the integration of sewage treatment with ecological construction and productive practices. To enhance efforts to reduce domestic sewage sources and utilize resources, a combined cleaning treatment and resource utilization mode should be employed to effectively meet diverse domestic sewage treatment requirements in different types of villages.

2.3. ‘Mulberry Base Pond’ Treatment Mode with Priority Selection

In China, for thousands of years, the ‘Mulberry Base Pond’ farming practice has exemplified the concept that there is no absolute waste; rather, resources are either misplaced or underutilized. The “Mulberry Base Pond” mode represents a prevalent agricultural production system in the Yangtze River Delta and Pearl River Delta regions of China [7]. Its original operational procedure is depicted in the accompanying Figure 1. Currently, this approach has widespread application across various domains due to the following two significant advantages: (1) Enhanced economic benefits. By leveraging the principles of energy and material cycle transformation in Figure 2, as well as symbiotic relationships and mutual nourishment among organisms, it achieves intensive management effects that align with the principle of maximizing yield with minimal input. (2) The biomass ratios of each nutrient level in each link of the “Mulberry Base Pond” system are appropriately balanced, ensuring material and energy input and output equilibrium, thereby enhancing the recycling efficiency of plant and animal resources and maintaining ecological balance [8]. At present, “Mulberry Base Pond” refers to the circular economy approach, which emphasizes the rational design of the industrial chain, so that waste from upstream industries can be used as raw materials for downstream industries and can form a closed loop to achieve no waste discharge.

According to the findings of a study on rural domestic sewage in China, the primary challenge faced by current rural domestic sewage treatment is the small volume of sewage. This results in the inability of professionals to effectively operate and maintain sewage stations after construction, ultimately leading to their dysfunction. Therefore, the primary limitation of rural domestic sewage treatment lies in the lack of sustainability in the current sewage treatment mode. However, the implementation of the “Mulberry Base Pond” method not only offers economic and ecological benefits but also addresses the challenge of achieving sustainable operation in rural domestic sewage treatment. For instance, by effectively harnessing the abundant organic matter, nitrogen, phosphorus, and other essential nutrients present in rural domestic sewage and aligning them with local agricultural fertilizer requirements and water resource availability, it is possible to convert this sewage into organic fertilizer or directly apply it to cultivate high-value-added agricultural products [9]. This approach not only mitigates the contamination of surrounding farmland caused by domestic sewage but also offers economic benefits. The sales revenue generated from high-value agricultural products with significant additional benefits can sufficiently support the recruitment of dedicated personnel for operation and maintenance, thereby ensuring the sustainable development of rural domestic sewage treatment.

3. Common Approaches to Rural Domestic Sewage Treatment

3.1. Discharge after Treatment

The discharge mode generally employs an integrated approach combining principles from physics, chemistry, and biology to effectively treat rural domestic sewage, ensuring compliance with specific emission requirements before discharging into surface water bodies [10]. The design of the treatment process should comprehensively consider factors such as sewage volume, discharge standards, operation and maintenance requirements, as well as the unique characteristics of the local natural environment. Currently, rural domestic sewage treatment encompasses various modes, including distributed collection treatment, concentration collection treatment, and incorporated piping treatment. The treatment process primarily involves the implementation of anoxic–oxic processes (AO), membrane bioreactors (MBRs), coagulation techniques, constructed wetlands, ecological ponds, or a combination of these methods. The majority of rural domestic sewage treatment facilities commonly employ the AO process combined with constructed wetlands. However, the findings suggest that this approach is ill suited for the specific characteristics of rural domestic sewage, resulting in poor treatment effectiveness. The operation and maintenance of these sewage stations pose significant challenges, as they often lack sufficient guarantees. Furthermore, the engineering practice nationwide has demonstrated that the concentration collection treatment discharge mode is unsuitable for small-scale rural domestic sewage treatment due to its high cost and operational and maintenance difficulties, which impede the achievement of effective assurance.

3.2. Mode of Resource Utilization

There is no such thing as absolute waste; rather, it is a matter of resources being misplaced or underutilized. The organic materials, nitrogen, phosphorus, and other substances present in rural domestic sewage can become water pollutants when discharged into water bodies [11]. However, if utilized for farm irrigation or land consumption, they can serve as valuable nutrients essential for soil fertility and plant growth. Numerous studies have demonstrated that rural domestic sewage contains trace amounts of hazardous substances, such as heavy metals, which can have detrimental effects on soil quality and agricultural produce when utilized for farm irrigation or land consumption [12]. Furthermore, China’s “Standard for irrigation water quality” (GB 5084-2021) [13] explicitly stipulates that domestic sewage treatment should meet these standards before being deemed suitable for farm irrigation. Therefore, it is evident that the utilization of treated rural domestic sewage for irrigated farmland is feasible. The common modes of resource utilization for treating rural domestic sewage mainly include the following:

(1)

Land utilization

This approach primarily utilizes facilities such as septic tanks and fermentation pools to pretreat rural domestic sewage, subsequently employing it as organic fertilizer for soil enrichment. The primary objective of pretreatment is twofold: firstly, to eliminate easily transmissible viruses and pests present in the sewage; secondly, to enhance the humus properties of the sewage, rendering it suitable for direct utilization as organic fertilizer. The cost of construction and operation for this mode is relatively low; however, it is only suitable for areas with ample farmland or extensive planting areas. For instance, this mode is well suited for the treatment of domestic sewage in situations where residents are scattered, but it may not be appropriate for densely populated residential areas with limited surrounding farmland.

(2)

Irrigation practices in agricultural land

This approach primarily involves enhancing anaerobic facilities for the initial treatment of rural domestic sewage to comply with the requirements of ‘Standard for irrigation water quality’ (GB 5084-2021) [13], followed by discharge into reservoirs and ditches, ultimately serving as irrigation water for farmland. This strategy offers cost-effective construction and operation options; however, it is only suitable for regions with ample farmland or extensive cultivation areas. For instance, it may not be applicable to the treatment of scattered domestic sewage from rural residents in new rural areas characterized by a large population scale and limited surrounding farmland.

(3)

Reformulating the production of organic fertilizer

This mode primarily utilizes waste materials, such as rural domestic sewage, straw, sludge from domestic sewage plants, and livestock manure, to produce organic fertilizer through processes like fermentation, granulation, and stoving. While this approach demonstrates high resource utilization and favorable economic benefits, it is susceptible to generating substantial odors that can significantly impact the surrounding atmosphere and residents’ quality of life. Moreover, the construction and site selection of a factory in the water source protection area of the South-to-North Water Diversion Project pose significant challenges, rendering it unsuitable for implementation.

4. Requisite Considerations for The Selection of Rural Domestic Sewage Treatment Modes

4.1. Characteristics of the Local Environment

The local climate, temperature, topography, and other natural environmental characteristics will significantly influence the selection of rural domestic sewage treatment methods [14]. For instance, due to the low concentration of rural domestic sewage quality, it is well suited for ecological treatment technologies such as constructed wetlands, ecological ponds, and land consumption. These ecological technologies are characterized by their extensive land utilization. In vast rural areas, the presence of unused land provides favorable conditions for the implementation of domestic sewage treatment using ecological technology. In plain regions, rural residents are relatively concentrated, enabling domestic sewage to be collected through a pipe network for subsequent centralized treatment. However, in mountainous and hilly areas, residents are more dispersed, posing challenges for sewage collection and necessitating decentralized treatment as a more suitable approach. Moreover, the rural population predominantly engages in work outside their communities in China, resulting in significantly reduced domestic sewage production during non-holiday periods. However, during holidays such as Spring Festival, a majority of residents return to their rural homes and consequently cause a significant surge in domestic sewage production. Therefore, there is a notable change in rural domestic sewage volume during holiday periods with obvious greater production than non-holiday periods. This factor is important for selecting appropriate modes of rural domestic sewage treatment.

4.2. Relevant Attributes of Domestic Sewage in Rural Areas

4.2.1. Characteristics of Water Quality

The Water Source Protection Area of the central section of the South-to-North Water Diversion Project is situated in China’s central region. The introduction of the water quality of rural domestic sewage in the Technical Guide of China Rural Domestic Sewage Treatment (Central and Southern Region) can be used as a reference for the water quality characteristics of rural domestic sewage in water sources. A relevant investigation indicated that rural domestic sewage exhibits characteristics such as a low concentration of water pollutants, high biodegradability, and susceptibility to degradation by microorganisms. Consequently, it can be categorized as a form of wastewater characterized by uncomplicated water quality that is amenable to facile treatment. Although rural domestic sewage possesses the aforementioned characteristics, if not properly treated and discharged, it can contaminate surface water by exceeding quality standards of receiving bodies, leading to eutrophication and excessive algae growth.

4.2.2. Reproductive Traits

Rural domestic sewage contains negligible amounts of toxic and harmful substances, such as heavy metals and persistent contaminants, which are unlikely to pose significant risks to the environment and agricultural products [15]. Moreover, the organic materials, nitrogen, and phosphorus present in the sewage serve as essential nutrients for soil and crops [16]. These factors establish the prerequisites for the resource utilization of rural domestic sewage. In addition, in comparison to fertilizers, utilizing rural domestic sewage as farmyard manure not only offers numerous advantages such as cost-effectiveness, prolonged fertilizer efficacy, and comprehensive nutritional content but also contributes to land improvement, enhanced soil crumb structure, and the prevention of soil compaction. Additionally, rural areas typically encompass vast agricultural land, facilitating the assimilation of substantial volumes of domestic sewage and thereby providing favorable conditions for its treatment and disposal.

4.2.3. Characteristics of Discharge

The quantity of domestic sewage discharges in rural areas is a negligible amount, with individual discharge rates typically below 50 L daily. This can be attributed to two main factors: firstly, rural residents tend to be more frugal, resulting in lower water consumption and shower frequency compared to urban residents; secondly, the prevalence of low-rise houses in rural areas often accompanied by flower or vegetable gardens surrounding the premises. The gray water generated by residents in their daily activities (primarily wastewater from laundry, bathing, etc.) is predominantly utilized for plant irrigation or dust suppression through sprinkling, while only a small portion of black water (mainly used for toilet flushing) requires collection and treatment [17]. Another notable characteristic of rural domestic sewage discharge is the significant fluctuation in emission levels, with lower rates observed during non-holiday periods compared to holiday periods. The characteristics of rural domestic sewage, such as minimal discharge rates and increased significant fluctuations in quantity, make it unsuitable for the conventional mode of urban domestic sewage treatment. Therefore, alternative approaches focusing on resource utilization or simplified and convenient treatment methods should be considered [18].

5. The Challenges and Current Status of Treating Rural Domestic Sewage

In order to gain insights into the current status of domestic sewage treatment in China, a comprehensive investigation was conducted on rural domestic sewage treatment practices in five representative villages located in Henan Province. The detailed research findings are presented in

Table 1. Current status of domestic sewage treatment in the surveyed villages.

Village	Process Mode	Sewage Station Design Treatment Scale	Daily Actual Processing Capacity	Emission Standard	Sewage Station Treatment Technology	Operation Status and Problems
A	Pretreatment (septic tank) + gravity pipe network collection + sewage station treatment after discharge	200 m3/d	20–30 m3/d	The primary standard of “Henan Province Rural Domestic Sewage Treatment Facility Water Pollutant Discharge Standards” (DB41/1820-2019)	Aerobic + constructed wetland + ecological pond	Pipe network leakage, less water collection; The operation and maintenance personnel lack professional knowledge, and the operation and maintenance of aeration and mud drainage are unreasonable; Wetlands and ecological ponds are paralyzed and idle; The water supply is substandard.
B	Pretreatment (septic tank) + gravity pipe network collection + sewage station treatment after discharge	500 m3/d	200 m3/d		Contact oxidation + constructed wetland	Operation and maintenance personnel lack professional knowledge, and aeration fans and other equipment are damaged due to lack of maintenance; Wetland and ecological pond paralysis; The water is not up to standard.
C	Pretreatment (septic tank) + gravity pipe network collection + sewage station treatment after discharge	---	---		Anaerobic tank + constructed wetland	Pipe network is blocked; Wetland blockage and paralysis; The process is too simple; The water is not up to standard.
D	Pretreatment (septic tank) + gravity pipe network collection + sewage station treatment after discharge	10 m3/d	5 m3/d		Anaerobic tank + constructed wetland	Wetland blockage and paralysis; The process is too simple; The water is not up to standard.
E	Discharge after septic tank treatment	---	---		---	The process is too simple.

. The domestic sewage of the surveyed villages undergoes preliminary treatment in septic tanks before being conveyed to the sewage collection pipe network. The sewage then flows through the pipe network and is directed towards sewage treatment stations by gravity. Following further treatment at these stations, it is discharged into surrounding ditches or water bodies. According to the Environmental Management Regulations of Henan Province, it is mandatory for all rural domestic sewage concentrated treatment devices to comply with the discharge standards specified in “Henan Province Rural Domestic Sewage Treatment Facility Water Pollutant Discharge Standards” (DB41/1820-2019) [19]. The specific data regarding these standards are presented in

Table 2. DB41/1820-2019 control project maximum permissible discharge concentrations of water pollutants. Unit: mg/L (except PH) [19].

SN	Pollutant	Primary Standard	Secondary Standard	Tertiary Standard
1	pH	6~9
2	SS	20	30	50
3	COD	60	80	100
4	NH3-N	8 (15)	15 (20)	20 (25)
5	N	20	—	—
6	P	1	2	—
7	Oil	3	5	5

Note(s): COD: the amount of oxidant required to be released by the oxidative decomposition of organic matter in water. Note: The value outside the brackets is the control requirement for water temperature ≥12 °C, and the value inside the brackets is the control requirement for water temperature ≤12 °C.

. Although the treatment scale of all sewage stations is smaller, not exceeding 500 m³/d, it still surpasses the volume of sewage discharged during non-holiday periods, thereby adequately meeting the daily treatment demands. The sewage stations in different villages employ various treatment technologies, with the predominant approach being a combination of biochemical technology and constructed wetland treatment technology. This integrated approach is theoretically capable of meeting the water pollutant discharge standards for RDS treatment facilities in Henan Province, as specified by the emission requirements outlined in DB41/1820-2019 [19]. Moreover, a few villages’ sewage stations employ overly simplistic technology. For instance, the sewage station in Village E solely relies on septic-tank treatment for direct discharge, which evidently fails to meet the required standards of wastewater treatment.

Based on the findings of our field investigation, it is evident that a majority of sewage stations in rural areas manifest significant issues. These include substantial leakage or blockage within the collection pipeline network, malfunctioning or damage to treatment devices, inadequate sewage treatment technology, and even instances where some sewage stations remain idle due to insufficient maintenance. The sewage treatment effect is evidently insufficient to meet the discharge requirements stated in the “Water Pollutant Discharge Standards for Rural Domestic Sewage Treatment Facilities in Henan Province” (DB41/1820-2019) [19] or fails to achieve the effluent standard of the device.

6. The Current Status, Challenges, and Enhancement Strategies of Domestic Sewage Treatment in a Rural Setting

6.1. The Current Status and Challenges of Sewage Pipe Network Collection in Village A

The registered population of Village A is 750, whereas the actual resident population ranges from 100 to 200 people. Consequently, there exists significant volatility in the volume of domestic sewage produced by residents in Village A. According to the “Technical Guidelines for Rural Domestic Sewage Treatment in Central and Southern Regions (Trial)” compiled by the Ministry of Housing and Urban-Rural Development of the People’s Republic of China, Village A has a sewage production volume that can reach 60 m³/d during peak holiday periods, while it is approximately 15 m³/d during non-holiday periods. Each household’s sewage undergoes initial treatment within an individual septic tank before being discharged into the communal sewage collection pipe network.

The rural domestic sewage collection methods employed in Village A are well documented; however, the process of sewage collection is relatively intricate, primarily due to an excessive number of non-essential transfer pools, resulting in escalated construction costs. Furthermore, according to managers of sewage treatment facilities, the construction of the sewage network has been ongoing for over a decade and has exhibited issues such as leakage and blockages. Currently, during non-holiday periods, the average daily inflow of sewage into the treatment station is only 3–4 m³, significantly lower than the actual production of 15 m³. This discrepancy may be attributed to four potential factors. Firstly, inadequate solid–liquid separation in septic tanks and transfer pools leads to a substantial influx of floating debris and solid materials into the pipe network, resulting in blockages. The second issue pertains to the absence of manholes in the pipe network, resulting in elevated air pressure and blockages of suspended solids within the system, thereby impeding sewage flow. The third issue concerns broken pipes that lead to sewage leakage. Fourthly, an unreasonable slope design of the pipe network causes the siltation of sewage within it. All four reasons are plausible; however, further investigation is necessary to determine a specific cause.

6.2. Current Status and Challenges of Wastewater Treatment Facilities in Village A

6.2.1. Current Status of Sewage Treatment in Village A’s Wastewater Treatment Plant

According to the site visit, it has been observed that there is no presence of livestock and poultry farming among the residents in Village A. Therefore, in order to assess the water quality of rural domestic sewage in Village A, a reference can be made to the data provided in the “Technical Guidelines for Rural Domestic Wastewater Treatment in China (Central and Southern China)”. The specific data are presented in

Table 3. Reference table for water quality range of rural domestic sewage in central and southern China. Unit: mg/L.

Main Index	pH	SS	COD	BOD5	NH3-N	N	P
Recommended value range	6.5~8.5	100~200	100~300	60~150	20~80	40~100	2.0~7.0

.

According to local residents’ accounts, the domestic sewage treatment stations in Village A were constructed in around 2010. During that time, the design of the treatment effluent water quality met the first-level requirements of the “Integrated wastewater discharge standard” (GB8978-1996) [20]. The technological process for designing the sewage treatment station is illustrated in Figure 3.

During non-holiday periods, the daily inflow of sewage at the station does not exceed 4 m³. Due to limited water intake and the absence of automated control devices, part-time personnel are required to visit the station once a week for approximately two hours to initiate operation of the lift pump and aerator fan. The current paralysis of this sewage station can be attributed to the non-utilization of the constructed wetland and clearwell, resulting in a decline in effluent quality below acceptable standards.

6.2.2. The Current Issues Pertaining to the Sewage Station in Village A

In the past, rural domestic sewage was subjected to minimal treatment requirements and lacked any specific regulations. Therefore, the design of sewage station collection and treatment facilities is both reasonable and feasible. However, after more than 13 years of operation, numerous malfunctions have been observed in various facilities and devices within the sewage station, resulting in a significant decline in treatment efficiency. Furthermore, with the establishment of the South to North Water Diversion Water Source Protection Area, there has been an increasing demand for sewage treatment in the water source region, particularly following the implementation of various environmental protection standards and regulations such as “Henan Province Rural Domestic Sewage Treatment Facility Water Pollutant Discharge Standards” (DB41/1820-2019) [19] and Regulations on Protection of drinking water source of South-to-North Water Diversion Project in Henan Province. The effluent from rural domestic sewage treatment facilities in the water source area must comply with the first-level Henan Province Rural Domestic Sewage Treatment Facility Water Pollutant Discharge Standard (DB41/1820-2019) [19] before being discharged, as the existing treatment facilities and their operation and maintenance fail to meet the current environmental management requirements in the water source area. The existing concrete problems are primarily manifested in the following aspects:

(1)

The absence of automatic control renders the operation impractical

The aeration fan and lifting pump, for instance, lack automatic control equipment such as time and space switches, relying solely on manual operation for activation and shutdown. Furthermore, the sewage station currently lacks dedicated personnel for its operation and maintenance, resulting in a limited operational period. On the one hand, this operational approach results in a reduction in the hydraulic retention time (HRT), leading to the premature discharge of organic and other pollutants from the biochemical tank before complete degradation, thereby compromising treatment efficacy comparable to direct discharge. On the other hand, it also leads to an insufficient aeration duration for the contact oxidation tank’s aerator, resulting in compromised aerobic biochemical performance. Additionally, repeated burnout of the lifting pump is caused by its lack of an installed liquid level control switch.

(2)

Insufficient infrastructure for the disposal of sludge

As is commonly acknowledged, a crucial prerequisite for the biological removal of phosphorus from wastewater is the proper discharge of sludge from the biochemical system. However, it is worth noting that none of the structures in the sewage treatment station, such as regulating pools, contact oxidation tanks, and sedimentation tanks, are equipped with sludge discharge facilities [21]. This deficiency significantly undermines the biological phosphorus removal efficacy of the sewage treatment system and contributes to sludge aging within the biochemical tank. The sludge generated from the treatment of rural domestic sewage does not contain any substances harmful to soil. Moreover, it is abundant in organic matter, nitrogen, and phosphorus—essential nutrients for plant growth. Therefore, this sludge can be utilized as an organic fertilizer or soil amendment.

(3)

Requisite operation and maintenance of sewage treatment station are insufficient

The lack of maintenance in the clear water pool and constructed wetland, shown in Figure 4, has resulted in the proliferation of algae. Both systems are currently inactive and non-functional, significantly impacting the local environmental landscape.

(4)

Inadequate sewage treatment process

Constructed wetlands, for instance, are primarily employed for the esthetic enhancement of the landscape and necessitate regular sludge processing; thus, they are suitable for implementation in warmer regions [22]. Nevertheless, the frigid temperatures and prolonged duration of winter in the source water of the South to North Water Diversion Project result in plant mortality and desiccation within constructed wetlands, thereby exerting an adverse impact on the local scenery.

(5)

Reconsideration of the treatment capacity in sewage treatment station design

According to Section 6.1, the jurisdiction of Village A is introduced, revealing that the per capita sewage production in this area is estimated at 80 L/day. Even during peak holiday periods, the theoretical maximum sewage production would not exceed 60 m³/d, and on non-holidays, it is only about 15 m³/d. These quantities are significantly lower than the design scale of the village’s sewage treatment station. Consequently, this situation hampers nutrient supply for microorganisms in biochemical tanks while also leading to increased construction and operation costs for the sewage treatment station.

(6)

Reckless selection of facilities and excessive energy consumption during operation

According to the on-site inspection, it was found that the flow volume of the lifting pump and aeration rate of the aeration fan at Village A’s sewage treatment station significantly exceeded the actual demand, resulting in excessive energy consumption.

(7)

Other problems

The aforementioned issues are prevalent in the sewage treatment station of Village A, and similar problems can be observed in other rural domestic sewage treatment stations within water source areas. Furthermore, there are two prominent issues in rural domestic sewage treatment within water source areas. Firstly, the design of sewage stations often overlooks unfavorable factors such as significant fluctuations in water quantity. This leads to the overloading of sewage treatment facilities during holidays when there is an excessive production of rural domestic sewage, while abnormal operation occurs during non-holidays due to insufficient production. Additionally, a majority of sewage stations experience equipment faults and malfunctions, such as damaged aeration fans and lifting pumps.

6.3. Revisiting the Etiology of Existing Challenges in Treatment Facility Management

According to the investigation findings on rural domestic sewage treatment facilities within the water source project area of the South-to-North Water Diversion Water Source Protection, the majority of these facilities were constructed circa 2010. The prevailing governance model involves treating rural domestic sewage as waste and discharging it after the required standards are met. This treatment concept and scheme were deemed sufficient to meet environmental requirements at that time. The rural domestic sewage treatment process, facilities, and device configuration can generally comply with designed discharge standards during normal operation and maintenance. However, these surface-level reasons only partially explain the issues surrounding rural domestic sewage treatment status in the water source project area. The underlying cause lies in an unreasonable selection of the rural domestic sewage treatment mode, which manifests itself through three key aspects.

(1)

Mistakenly attempting to apply the sewage treatment methods used in urban areas to rural settings without considering the unique operational and maintenance demands of small-scale facilities. Compared with urban domestic sewage treatment, rural domestic sewage treatment has fewer types of pollutants in sewage, a lower sewage concentration, and less treatment capacity, generally not more than 30 m³/d [23]. While it may be possible to adopt similar treatment processes, it is crucial to ensure that construction standards are met, devices are complete, and manual and online testing instruments are available for complex operation and maintenance procedures. Adequate staffing with high levels of professionalism is also necessary for successful rural domestic sewage treatment operations.

(2)

The water source project area did not adequately consider the integration of rural domestic sewage production and discharge, water quality, and the local rural natural and social environment at the sewage station. For instance, it failed to account for the limited capacity of small-scale rural domestic sewage systems that cannot be operated and maintained by full-time personnel. Additionally, it overlooked the significant fluctuations in sewage volume caused by large population flows in rural areas. Consequently, there was a mismatch between the designed treatment volume and daily production volume. Furthermore, it neglected to consider the long and cold winters in the water source project area when selecting constructed wetlands as a treatment method. This failure to coordinate treatment factors also disregarded important aspects such as leveraging fertility resources from domestic sewage for agricultural production needs in rural areas [8].

(3)

The concept of sewage treatment is antiquated. The contemporary approach to environmental protection extends beyond merely meeting the standards for terminal treatment, but rather prioritizes reducing wastewater generation or resource utilization at the source, ultimately opting for terminal treatment [24].

To summarize, the prevailing issues concerning rural domestic sewage treatment in the water source project area primarily stem from outdated treatment concepts and the inappropriate selection of treatment methods, rather than deficiencies in treatment facilities and equipment.

7. Proposed Enhancement and Refurbishment Plan for the Domestic Sewage Treatment Station in Village A

7.1. Rehabilitation Plan for Pretreatment of Sewage Prior to Entering the Pipeline Network

Considering the distribution and living habits of residents in Village A, along with the ongoing construction of domestic sewage collection facilities, we propose implementing the following rectification measures:

(1)

Incorporate infrastructure for the harnessing of gray water resources

According to the on-site investigation, the residential houses in Village A are very concentrated, and almost every household has installed showers and washing machines, which produce a large amount of sewage, but the types of pollutants in the sewage are few and the concentration is very low, called gray water. On the contrary, sewage mixed with feces and urine discharged from toilets contains a high concentration of organic matter and microorganisms, called black water, which is produced in smaller quantities. Therefore, the gray water generated by activities such as bathing, laundry, and washing vegetables accounts for a large proportion of the total domestic sewage in rural areas. Gray water can be reused after a simple treatment, or even used directly, for example, for greening or dust prevention. Implementing separate collection systems for black water and gray water can be considered along with adopting reuse measures for gray water, including toilet flushing, greening residential areas, and dust prevention through watering. The specific renovation plan is illustrated in Figure 5 below; this approach can potentially reduce the sewage production volume by 40%, thereby leading to cost savings in subsequent sewage station construction and operation.

(2)

The transit pool will be converted into a biofilm anaerobic tank and a grid tank

The utilization of existing transit pools can be optimized by converting them into dual compartments, incorporating biofilm fillers in the first compartment and implementing a fine grid at the outlet of the final compartment. Ultimately, these transit pools can be transformed into a collaborative grid pool with biofilm anaerobic treatment capability. This approach not only enhances solid–liquid separation efficiency and prevents pipeline blockages but also improves biochemical sewage degradation performance to alleviate subsequent sewage treatment burden.

(3)

Rehabilitate the impaired pipeline infrastructure

According to the management personnel of sewage treatment facilities, the constructed sewage pipeline network has been in operation for over a decade. However, during non-holiday periods, the average daily inflow of wastewater into the station is only 3–4 m³/d, which is significantly lower than its designed capacity of 15 m³/d. This suggests that there may be issues with leakages or blockages within the pipeline network and remedial actions such as repairing damaged pipelines are necessary.

7.2. Improved Measures for the Domestic Sewage Treatment Station in Village A

The process flow chart after the renovation of the sewage treatment station in Village A is shown in Figure 6; the specific transformation includes the following aspects:

(1)

Incorporating biofilm filters into the existing regulating pool to enhance the anaerobic biochemical processes within the pool.

(2)

Replacing the filter in the original biological contact oxidation pool can enhance its biochemical efficacy, thereby improving the overall treatment efficiency.

(3)

Rehabilitating and enhancing the outlet weir of the original vertical sedimentation pool to mitigate the discharge of suspended sediments and floating debris into the downstream lotus pond.

(4)

The transformation of the large-scale constructed wetland into evergreen shrubs, along with the installation of a sludge discharge pipe and solenoid valve connecting the sedimentation tanks to the shrubs, enables automatic opening of the sludge discharge pipe valve based on a predetermined schedule. The sludge accumulated in the sedimentation tanks is regularly gravity-fed to various points for irrigation through the evergreen shrubs [25]. This approach offers several advantages: ① The esthetic appeal of the landscape is enhanced by low-maintenance evergreen shrubs. ② It facilitates sludge utilization from sedimentation tanks, eliminating the need for additional facilities such as storage and dewatering systems. ③ Furthermore, it enhances phosphorus removal efficiency and effluent quality at sewage treatment plants while reducing the frequency of subsequent lotus pond dredging [26].

(5)

The transformation of the original clear water pool into a lotus pond involves breeding ornamental fish and incorporating nano oxygenation pipes to enhance water oxygenation. The required air for oxygenation can be supplied by enhancing the existing aeration fan air transmission network [27]. This measure serves to improve the water quality of the lotus pond and prevent hypoxia-induced mortality in ornamental fish.

(6)

The outlet control valve of the lift pump is installed to regulate the inflow rate into the contact oxidation pool, ensuring gradual and continuous water intake as well as maintaining an appropriate hydraulic retention time within the contact oxidation pool.

(7)

Incorporating an automated control cabinet to regulate the operational duration of electrical equipment, such as lift pumps and aeration fans, ensuring compliance with dissolved oxygen concentration and hydraulic retention time requirements in the contact oxidation tank [28]. Additionally, this guarantees the uninterrupted operation of all facilities even in unattended scenarios.

(8)

Installing liquid level control switches in the regulating tank and contact oxidation tank to regulate the operation of the lifting pump, thereby preventing drainage from the regulating tank and potential burnout of the pump [29]. Additionally, this measure effectively mitigates sewage overflow from the contact oxidation tank.

(9)

Incorporating an application-based remote control system to facilitate the remote monitoring and management of all electrical equipment’s operational status, ensuring their uninterrupted functionality even in unattended scenarios.

(10)

Elevating the water level of the lotus pond to facilitate efficient network flow for irrigation purposes in the surrounding farmland, ensuring a continuous supply of clear water [30]; installing a polyethylene (PE) irrigation pipe network from the lotus pond to the adjacent farmland and strategically positioning irrigation switch valves along the pipeline to enable rotational irrigation across different plots.

7.3. Feasibility Analysis of the Enhancement Plan for the Local Wastewater Treatment Facility in Village A

Compared with before the transformation, the sewage treatment method after the transformation has the following advantages:

(1)

By fully utilizing the existing sewage structures, including regulation tanks, contact oxidation tanks, sedimentation tanks, and clear water tanks in the sewage station, this approach offers advantages such as reduced earthwork excavation requirements and lower reconstruction costs [31].

(2)

The operation of power equipment solely encompasses aeration fans and lifting pumps, characterized by their minimal energy consumption and low failure rate, thereby ensuring the uninterrupted functioning of the sewage station.

(3)

The need for full-time on-site maintenance during runtime is unnecessary; regular equipment checks and maintenance suffice. Sludge dewatering and disposal are not required. The operation and maintenance procedures are straightforward and convenient, ensuring the sustainable treatment of rural domestic sewage.

(4)

The full utilization of all sewage as valuable resources enables the simultaneous achievement of threefold objectives: environmental pollution control, landscape beautification, and green agricultural production.

8. Conclusions

This study conducted a comprehensive investigation on domestic sewage facilities in representative villages in the water source area of the middle route of the South-to-North Water Diversion Project, focusing on the operational status and existing issues of Village A’s domestic sewage treatment station. The findings revealed that rural domestic sewage treatment facilities face challenges such as inadequate design, insufficient operation and maintenance, excessive effluent discharge, and frequent malfunctions. These problems arise from the inappropriate adoption of urban domestic sewage treatment methods, which are not sustainable for rural areas. Therefore, it is crucial to consider various factors including domestic sewage production and discharge in water source areas, water quality characteristics, discharge requirements, natural and social environments, as well as post-construction operation and maintenance methods when designing rural domestic sewage treatment systems. The chosen treatment mode should promote harmonious coexistence between humans and nature while supporting rural revitalization and green agricultural development in line with the concept of ecological civilization. As a priority recommendation for this purpose, the ‘Mulberry Base Pond’ treatment mode is suggested. Additionally, this study proposes source reduction measures for gray water reuse in Village A’s domestic sewage treatment along with combined approaches involving biochemical treatment, landscape viewing utilization, and farmland irrigation for black water to achieve effective wastewater management. It needs to be emphasized that the treatment of rural domestic sewage should be adapted to local conditions and should give priority to resource utilization, and discharge after treatment should be the last choice.