Resource-Oriented Treatment Technologies for Rural Domestic Sewage in China Amidst Population Shrinkage: A Case Study of Heyang County in Guanzhong Region, Shaanxi Province

Abstract

The rural population shrinkage caused by China’s imbalanced regional development poses challenges to infrastructure configuration and operation. Traditional centralized sewage treatment models face issues in cost-effectiveness, facility utilization rates, and sustainable maintenance, necessitating the exploration of adaptive governance technologies under new demographic conditions. The utilization-driven governance approach is recognized as an emerging method for rural domestic sewage management. This study selects Heyang County, a representative agricultural area in Guanzhong Plain, as a case study. Through mixed-methods research integrating qualitative and quantitative approaches, we analyze the correlation between the Population Shrinkage Index (PSI) and facility operational efficiency, investigate the impact of resident population dynamics on rural sewage treatment patterns, and establish a theoretical “Source–Transmission–Sink” framework. Synthesizing local traditional governance practices with modern technological solutions, we propose a resource-oriented treatment system adapted to population shrinkage trends, comprising three technical components: source process reduction, transmission process interception, and sink process attenuation. This research emphasizes adjusting green water infrastructure (GWI) spatial configurations according to village characteristics in production–living–ecological spaces, forming a hierarchical attenuation mechanism through circular transmission pathways. This facilitates the transition from gray-infrastructure-dependent models to holistic pollution control systems with resource recovery capabilities. The findings provide theoretical foundations for policymaking and infrastructure planning in rural sewage management, offering significant references for sustainable rural water resource governance.

1. Introduction

With the deceleration of China’s overall economic growth and intensifying regional development disparities, the phenomenon of rural population decline has rapidly proliferated [1,2]. Currently, population shrinkage has become a predominant characteristic of most Chinese townships [3,4]. According to the National Bureau of Statistics (2023), the proportion of rural permanent residents relative to the registered population has dropped below 40%, with this trend being particularly pronounced in central and western regions. The drivers of population shrinkage are twofold: declining natural growth rates [5,6] and labor migration from rural to urban areas driven by interregional economic imbalances [7,8]. This demographic shift not only undermines rural socioeconomic vitality and cultural continuity but also poses unprecedented challenges to the configuration and operation of village infrastructure systems.

While existing studies have explored technological paradigms for rural sewage management, most remain confined to comparative analyses between centralized and decentralized systems, lacking systematic investigation under population shrinkage contexts. Nationwide surveys indicate that in most Chinese counties, over 50% of villages have permanent populations below half of registered residents. However, current planning practices reveal that more than 80% of villages persist in constructing centralized wastewater treatment plants. Quantitative evidence exposes systemic flaws in centralized models: 63% of centralized treatment plants operate below 30% of design capacity due to population decline [9]. Traditional centralized-network-based and standardized layout models increasingly reveal limitations in operational flexibility, manifesting challenges in cost-effectiveness (average USD 0.45/m³ for centralized systems vs. USD 0.18/m³ for decentralized systems), facility utilization rates (<40% in 58% of investigated cases), and sustainable operation and maintenance (32% of systems failing within 5 years) [10]. Conventional approaches for configuring domestic sewage infrastructure typically determine treatment modalities based on settlement agglomeration levels and establish design capacities according to permanent population figures. Nevertheless, existing technological solutions inadequately address the self-purification potential inherent in village production–living–ecological spaces, failing to adapt to the dual realities of population decline and spatial dispersion of pollution sources. Persistent adherence to centralized layout paradigms would result in significant resource misallocation, necessitating the urgent exploration of new governance technologies adapted to demographic shifts.

In 2021, the Chinese government promulgated the Guiding Opinions on Further Promoting Rural Domestic Sewage Treatment, establishing a policy framework advocating for decentralized resource-oriented utilization of rural domestic sewage, with particular emphasis on implementing localized water recycling models in arid and water-deficient regions [11]. This shift aligns with Sustainable Development Goal 6 (Clean Water) through 70–85% water reuse rates in pilot projects, and it is also consistent with Sustainable Development Goal 12 (Responsible Consumption and Production) [12]. While multiple provinces have issued technical guidelines for rural sewage resource utilization, their practical implementations remain exploratory. This study conducts a systematic review of global technological approaches for rural domestic sewage management, integrated with a field-based investigation in Heyang County, Guanzhong Region, Shaanxi Province, China, to explore resource-oriented treatment technologies adaptable to villages under demographic contraction trends. The research outcomes provide scientific foundations for policymaking and water infrastructure spatial planning in population-shrinking regions, offering significant theoretical and practical insights for sustainable water resource management in rural China.

Heyang County—a representative dryland agricultural zone in Guanzhong—exhibited a 23.4% population decline over the past decade, with a village hollowing index of 0.48, significantly exceeding national averages [13]. Coupled with severe hydroclimatic constraints (553 mm annual precipitation vs. 1500 mm evaporation), its water resources per cultivated hectare are less than one-quarter of the national mean. Climate change exacerbates these challenges: Projections show a 17–23% increase in rainfall events and 40% more intense rainfall events by 2050, which will degrade conventional treatment efficiency by 35–60% during extremes [14,15]. This phenomenon has engendered a synergistic challenge of “demographic contraction–ecological fragility”. Grounded in the Source–Transmission–Sink theoretical framework, the construction of a resilient infrastructure network with hierarchical regulation mechanisms enables dynamic adjustment to wastewater flow fluctuations induced by population mobility, thereby addressing the chronic inefficiencies inherent in conventional centralized systems. Our approach facilitates spatial nesting of green water infrastructure (GWI) with production and ecological spaces through the restructuring of production–living–ecological spaces. This not only establishes a “utilization-driven governance” paradigm for the villages in the arid/semi-arid regions of Northwest China, but also achieves a theoretical advancement from technological optimization to cross-scale spatial governance, providing innovative ideas and practical approaches for addressing climate change and enhancing the regional resilience governance level.

2. Literature Review

Current living sewage treatment paradigms predominantly adopt centralized or decentralized approaches. Centralized treatment systems are suitable for urbanized areas and villages with concentrated populations [16], yet they prove inadequate for most rural regions characterized by complex topography and dispersed settlements. Decentralized in situ treatment technologies demonstrate dual advantages: alleviating water scarcity in arid zones and establishing sustainable frameworks within circular economies, thereby optimizing capital investments [17,18]. Recognizing the coexistence of centralized and decentralized patterns in rural contexts, Duan et al. (2023) proposed an integrated sewage collection system combining centralized and decentralized elements [19]. However, fiscal constraints and settlement dispersion render centralized treatment infeasible for most villages beyond suburban areas [20]. Under persistent population decline, adaptive decentralized systems emerge as viable alternatives to conventional centralized models [21,22].

Rural domestic sewage treatment technologies in developed countries have reached relative maturity, with many nations establishing systems tailored to their regional characteristics. In the United States, where rural populations are sparse and dispersed, decentralized in situ treatment technologies are predominantly adopted, typically involving small-scale treatment units installed per household or cluster of adjacent households [23]. Japan has developed a tripartite system comprising household Johkasou tanks, village-level drainage facilities, and collective dormitory treatment plants, with decentralized solutions accounting for over 90% of the implementations, primarily deployed in suburban and peri-remote areas characterized by a low population density, the absence of sewerage network coverage, and inaccessibility to centralized wastewater treatment infrastructure. Germany addresses infrastructure challenges arising from population decline through “resilient” decentralized approaches, including municipal-grade membrane bioreactors (MBRs), PKA wetland systems based on natural ecological principles, and source-separation systems that divert greywater to phytoremediation facilities while vacuum-collecting blackwater for co-digestion with kitchen waste to produce biogas and fertilizer [9]. Australia predominantly utilizes septic tanks combined with oxidation ponds and constructed wetlands for low-density populations [24], while South Korea employs compact wetland systems for scattered settlements.

However, critical limitations exist in directly transplanting these international experiences to China: German municipal-grade MBR systems require unit investment costs of USD 2500–3800 per household, with treatment costs (USD 0.62–0.85/m³) exceeding Chinese counterparts (USD 0.28–0.45/m³) by 120–190%, attributable to stricter discharge standards (e.g., TN < 10 mg/L vs. China’s TN < 20 mg/L) and automated operation requirements, which exceed rural fiscal capacities. Similarly, Japan’s Johkasou technology incurs annual maintenance costs (USD 300–500/household) that starkly contrast with China’s average rural wastewater subsidy (USD 80/household) [20], rendering direct adoption financially unsustainable, particularly in arid Northwest China [24]. Although the eco-friendly technologies of these countries are excellent, they do not fully take into account the two unique constraints in rural China: (1) the drastic fluctuations (±40–60%) in treatment loads caused by seasonal population mobility; (2) the spatial matching requirements between the absorption capacity of cultivated land and the reuse of sewage, which are rooted in the profound agricultural cultural heritage.

Systematic rural sewage governance in China emerged in the 21st century. Despite advancements in resource-oriented technologies, early-stage deficiencies in standardized protocols and integrated management systems hindered comprehensive development. The 2021 national policy emphasizes context-specific adaptation, advocating localized treatment modes that promote decentralized resource recovery. Current technological portfolios include biological treatment (e.g., three-compartment septic tanks [25,26], anaerobic digestion, A²/O processes, and MBR systems [27]), ecological treatment (e.g., subsurface wastewater infiltration systems (SWISs) [28], constructed wetlands (CWs) [29], and stabilization ponds [30]), hybrid processes (e.g., stabilization pond–A/O combinations, AAO integrated equipment [31], and bio-contact oxidation reactors), and physicochemical methods (e.g., adsorption and reverse osmosis [32]). The integration of intelligent monitoring systems has significantly enhanced operational efficiency [33], while source-separation-based resource recovery technologies [34] reduce infrastructure footprints and drive source-level emission reduction [35]. Fan Bin’s “ventilated micro-flush modern agrocycle” drainage paradigm [36] provides an innovative low-cost resource-oriented solution.

Existing research indicates a global shift toward ecological and context-sensitive resource recovery in rural sewage governance, emphasizing synergy with natural ecosystems. While international experiences offer valuable references, China requires localized innovations aligned with regional socio-ecological contexts: (1) Unlike Germany’s high-standard municipal approaches, China needs simplified “moderate treatment–local reuse” technology chains. (2) Compared to Japan’s maintenance-intensive household-level systems, village-scale CW-farmland integration should be prioritized. (3) Contrasting U.S. standalone decentralized units, household–village-tiered resilient networks must adapt to population mobility. Unlike previous studies that focus on centralized versus decentralized treatment models, this research systematically evaluates the impact of population shrinkage on sewage facility performance. It introduces a Source–Transmission–Sink governance framework adapted to demographic shifts, providing practical recommendations for scaling sustainable rural wastewater solutions.

3. Materials and Methods

3.1. Study Area

Heyang County (35°14′ N, 110°08′ E) is situated in the northeastern Guanzhong Plain, Shaanxi Province, China, adjacent to the western bank of the Yellow River. The county spans 41.8 km north–south and 35.6 km east–west, encompassing a total area of 1437 km², with cultivated land accounting for 622 km² (932,000 acres). The terrain exhibits a stepped elevation gradient increasing from southeast (336 m) to northwest (1556 m), comprising three geomorphic units: river valley terraces (16.2% of total area), loess tablelands (65.6%), and low-middle mountains (18.2%). Characterized by a warm temperate continental monsoon climate, the county has an average altitude of 721 m with a mean annual temperature of 11.5 °C. Hydrological conditions reveal critical water stress: precipitation of 553 mm/yr (38% of the national average), evaporation of 1500 mm/yr, and water resources per cultivated hectare at 4245 m³ (283 square meters per acre), 28% of the national mean. This “dual water scarcity” paradigm (resource-driven + infrastructure-driven) positions Heyang among China’s three iconic dryland agricultural zones, alongside Dingxi (Gansu) and Xihaigu (Ningxia). The Guanzhong region serves dual national roles: as Shaanxi Province’s economic core (contributing >65% of provincial GDP on <30% of provincial territory) and as a strategic pivot for ecological conservation and high-quality development in the middle reaches of the Yellow River Basin. This area represents Northwest China’s most industrialized and urbanized growth pole while exemplifying dryland agriculture on the Loess Plateau and rural revitalization strategies. As a county within the national-level Guanzhong–Tianshui Economic Zone, Heyang epitomizes the coexistence of traditional farming practices and modern ecological governance demands in Yellow River Basin counties. With a 23.4% rural population decline over the past decade—significantly exceeding national averages—the county embodies the intertwined challenges of the demographic exodus and environmental constraints confronting Northwest China’s underdeveloped rural areas. This spatiotemporal coupling makes Heyang an ideal study area for exploring sewage resourceization pathways, with findings providing replicable paradigms for over 200 similar counties across the Yellow River Basin and arid/semi-arid Northwest regions.

3.2. Data Collection

This study focuses on the coupling analysis between demographic dynamics and infrastructure operational efficiency, employing quantitative models to reveal the failure mechanisms of centralized treatment systems. According to government statistics (as of July 2024), Heyang County administers 1 subdistrict, 11 towns, and 212 administrative villages (Figure 1). Based on registered and permanent population data of these villages, we constructed the Population Shrinkage Index (PSI):

$$PSI=(1-{\displaystyle \frac{P_{permanent}}{P_{registered}}})\times 100\%$$

(1)

Analytical results indicate the county’s average PSI reaches 43.65%, with Hejiazhuang Town exhibiting the highest value at 57.80% (Figure 2 and Figure 3). Notably, 74 villages (34.91% of the total) demonstrate PSI > 50%, including extreme cases where PSI peaks at 83.72%—a village with 371 registered households (1290 people) retains only 210 permanent residents, comprising 20 minors and 150 elderly individuals (aged > 60), reflecting severe population aging.

Two primary drivers underlie this demographic contraction: (1) educational resource disparities driving urban migration for better schooling opportunities; (2) agricultural workforce abandonment, particularly among young/middle-aged laborers seeking urban employment. Only 33% of villagers remain engaged in agricultural production, with rural housing vacancy rates reaching 34%. This pattern extends across the Guanzhong region, where over 50% of villages exhibit PSI > 50%.

Villages in the Guanzhong region exhibit a spatial distribution pattern characterized by widespread dispersion with localized clustering, rendering centralized infrastructure deployment unfeasible. Financial constraints further prevent most villages from bearing the high costs associated with constructing long-distance pipelines and centralized treatment facilities. Survey data indicate that less than 10% of villages across counties meet the technical requirements for integration into urban sewage systems, and the proportion of self-built treatment plants is between 20 and 30%. In Heyang County, only 1.8% of domestic sewage is connected to urban treatment plants, and 22.3% is connected to nearby treatment stations, while 75.9% remains untreated or relies on decentralized solutions.

However, inappropriate technology selection and inadequate post-construction operational support have resulted in widespread facility abandonment or malfunction. An analysis of 98 centralized treatment facilities in Heyang County reveals that integrated treatment plants account for 75.5% (74 units), constructed wetlands for 23.5% (23 units), and three-chamber septic tanks for 1.0% (1 unit) (Figure 4a). Operational status data demonstrate that only 24.5% (24 units) function normally, 32.7% (32 units) are completely abandoned, 11.2% (11 units) operate intermittently, 23.5% (23 units) are damaged, and 7.1% (7 units) exhibit insufficient treatment capacity (Figure 4b). Population shrinkage exacerbates these maintenance challenges by reducing fiscal capacities and depleting technical workforce availability.

3.3. Methods

This study adopts a mixed-methods approach integrating qualitative and quantitative research, structured through three phases:

(1)

Theoretical Framework Development

Conventional sewage infrastructure planning determines collection modes based on settlement agglomeration levels and design scales according to permanent population sizes. However, this approach neglects actual population distribution patterns. To adapt to population shrinkage, grassroots-oriented source reduction and ecological treatment technologies must be prioritized. Guided by the “Source–Transmission–Sink” theory in landscape ecology, we conceptualize rural domestic sewage management as an ecological process and integrate traditional local practices with modern techniques across three phases: Source process reduction. Transmission process interception and Sink process attenuation [37,38,39]. The “Source” denotes wastewater generation points (primarily households), “Transmission” represents collection pathways, and “Sink” refers to treatment and assimilation nodes (e.g., constructed wetlands, ecological ponds).

During this process, the rational classification of wastewater sources is crucial. As shown in Figure 5, domestic sewage is categorized into blackwater and greywater. Blackwater originates from toilets and contains elevated levels of nutrients like nitrogen (N) and phosphorus (P), along with contaminants such as COD and pathogens. Greywater from kitchens and bathrooms typically exhibits lower pollution levels but may contain organic matter, grease, and other contaminants from food preparation and cleaning activities. Source separation forms the foundation for pollution reduction. By distinguishing between blackwater and greywater, appropriate treatment strategies can be developed. Septic tanks represent a common solution for rural toilet blackwater treatment, while small-scale constructed wetlands employing phytoremediation and microbial degradation serve as effective greywater treatment systems. Following conveyance processes, both blackwater and greywater ultimately reach terminal treatment nodes for comprehensive processing and assimilation. For instance, constructed wetlands can further purify water from septic tanks and greywater treatment systems, ensuring the final effluent meets environmental standards or can be safely reused in local ecosystems. Overall, household-level wastewater source classification constitutes a critical element within the rural domestic sewage treatment framework, guiding the selection and application of appropriate technologies at each stage of the “source–conveyance–terminal” ecological process.

(2)

Field Investigation and Data Acquisition

Comprehensive data were collected through field surveys, questionnaires, and in-depth interviews with local officials and villagers. Field surveys documented village topography, production–living–ecological spatial characteristics, and courtyard configurations. Questionnaires captured population distribution, wastewater generation patterns, and water usage behaviors. Semi-structured interviews focused on facility operational status and resident satisfaction. Statistical data for 212 administrative villages—including permanent/registered populations and operational records of 98 centralized treatment facilities—were obtained from Heyang County Ecology and Environment Bureau (July 2024).

(3)

Data Analysis and Technical Solution Formulation

Centralized facility performance was evaluated using the load rate (LR) and cost deviation (CD):

$$LR={\displaystyle \frac{Q_{actual}}{Q_{design}}}\times 100\%$$

(2)

$$CD={\displaystyle \frac{C_{design}}{C_{operation}}}\times 100\%$$

(3)

A binary logistic regression model analyzed the impact of the PSI on facility failure risk. The dependent variable was operational status (0 = non-functional, 1 = functional), with standardized PSI as the core predictor. Control variables included LR, CD, facility age, and pipe network coverage. Missing data were imputed via the SPSS 27 Expectation-Maximization algorithm, and categorical variables were dummy-coded. The model fit was assessed using the Hosmer–Lemeshow test, with odds ratios (ORs) and 95% confidence intervals calculated.

Spatial analysis integrated satellite imagery, UAV orthophotos, and field photos to characterize “San Sheng” spaces. Source–Transmission–Sink zones were delineated to develop adaptive green water infrastructure (GWI) layouts. Technical solutions were optimized based on source distribution and wastewater volumes, with feasibility validated through environmental, economic, and social benefit assessments.

Through the aforementioned research methodology and processes, this study enables the systematic investigation and development of resource-oriented treatment technologies for rural domestic sewage under population shrinkage contexts, achieving effective wastewater treatment and localized resource recovery.

4. Results

4.1. Impact of Population Shrinkage on Rural Sewage Management

Population shrinkage exerts dual impacts on rural sewage governance: distorting wastewater design capacities and reshaping the spatial distribution of pollution “Sources”, thereby complicating technological selection in spatial layout, process design, and operational management (Figure 6). Specifically, demographic contraction triggers widespread homestead vacancies, creating a paradox where villages appear densely settled but exhibit highly dispersed sewage generation points. This spatial fragmentation renders conventional centralized systems economically unsustainable, manifesting as resource misallocation and escalating maintenance costs.

Through the evaluation and analysis of the e LRand CDof 98 centralized processing facilities in Heyang County, it is shown that their design scale seriously deviates from actual demand:

(1)

The LR < 40% (average 32.7%) for 75.5% of the facilities.

(2)

The average CD is 218%.

Binary logistic regression analysis was performed using SPSS 27 on operational data from 98 centralized treatment facilities in Heyang County (operational status coded as 0 = non-functional, 1 = functional) and village-level PSI values. The model revealed statistically significant population shrinkage effects (

Table 1. Binary logistic regression analysis results of 98 centralized treatment facilities in Heyang County.

		B	Standard Error	Wald	Freedom	Significance	Exp(B)	95% Confidence Interval of EXP(B)
								Lower Limit	Upper Limit
Step 1 a	PSI	−0.067	0.043	2.363	1	0.124	0.936	0.860	0.918
	Floor area (m2)	−0.004	0.003	1.139	1	0.286	0.996	0.990	1.003
	Source of operating funds	0	0	2.871	2	0.238	0	0	0
	Source of operating funds (1)	3.909	2.365	2.731	1	0.098	49.857	0.483	5143.224
	Source of operating funds (2)	6.358	7.519	0.715	1	0.398	577.012	0.000	1,449,834,550.564
	Handling ability	0.078	0.068	1.310	1	0.252	1.081	0.946	1.235
	Treatment process	0	0	1.644	3	0.649	0	0	0
	Treatment process (1)	−8.850	8.525	1.078	1	0.299	0.000	0.000	2589.169
	Treatment process (2)	−15.768	40,192.970	0.000	1	1.000	0.000	0.000	0
	Treatment process (3)	−0.260	2.450	0.011	1	0.915	0.771	0.006	93.855
	Constant	−0.021	2.435	0.000	1	0.993	0.979	0	0

^a. Variables entered in step 1: PSI, floor area (m²), source of operating funds, handling ability, treatment process.

). The odds ratio (OR) for the PSI was 0.936 (95% CI: 0.86–0.918), indicating that each 10% increase in the PSI corresponds to a 48.4% elevated risk of facility failure, calculated as

P_{Risk Increase Rate} = (0.936¹⁰ − 1) × 100% = 48.4%.

The confidence interval being entirely below the null value of 1 confirms the statistical significance of population shrinkage impacts. These findings necessitate the development of resilient governance infrastructure systems that adapt to fluctuating population densities and dynamic pollutant source distributions. Unlike previous studies that predominantly focused on economic or environmental determinants, our research quantifies population contraction as a predictive factor for infrastructure operational efficacy. The demonstrated vulnerability of centralized wastewater treatment systems highlights the critical need to develop decentralized treatment alternatives tailored to rural areas experiencing population decline.

4.2. Resource-Oriented Treatment Technology Based on the “Source–Transmission–Sink” Process

4.2.1. Source Process Reduction Technology

The spatial dispersion of pollution sources under population shrinkage necessitates innovative source reduction technologies. These technologies focus on household-level wastewater collection, storage, preliminary purification, and localized reuse, including septic tank systems, small-scale constructed wetlands, and ”small tri-gardens” green attenuation systems (small vegetable plots, gardens, and orchards) [10]. Decentralized facilities constructed near households enable in situ wastewater recycling. Key technical measures include the following:

Septic Tank Technology: Utilizing anaerobic fermentation and sedimentation to remove organic matter, microbes, and suspended solids. The treated effluent serves as organic fertilizer for “small tri-gardens” irrigation, achieving resource circularity. Septic tanks—integrated collection–treatment units—have become mainstream in China’s rural toilet revolution [40] and are widely applicable in villages lacking centralized sewage networks [41].

Small Constructed Wetlands: Enhancing greywater treatment through phytoremediation and microbial degradation, effectively removing organic compounds, nitrogen, and phosphorus.

“Small tri-gardens” Green Systems: Farmland is irrigated with effluent from septic tanks after fermentation, and the greywater is treated by constructed wetlands. This approach not only reduces sewage discharge but also increases land productivity and villagers’ incomes.

In Heyang County, villagers traditionally combine septic tanks with front-yard vegetable plots (Figure 7). This practice exemplifies localized resource cycling and sustainable environmental management. To ensure irrigation safety, such grassroots solutions should be systematically encouraged and scaled.

Field surveys reveal that over 80% of rural courtyards in Heyang County incorporate “small tri-gardens” (small vegetable plots, gardens, and orchards). Traditional village layouts feature dense settlements with compact “narrow courtyards”, where “small tri-gardens” spaces average approximately 5 m² per household, typically utilizing front-yard or interstitial communal areas. Modern courtyard designs leverage expanded vacant lands around homesteads, with “small tri-gardens” areas ranging from several to hundreds of square meters per household, creating favorable conditions for localized resource-oriented wastewater management. Detailed spatial configurations of typical “small tri-gardens” layouts are summarized in

Table 2. Typical courtyard “small tri-gardens” layout overview.

Front, Middle, and Backyard Type	Anterior and Posterior Courtyard Type	Anterior and Posterior Courtyard Type	Front-Yard Type	Street Type
Homestead area 10 × 30 = 300 m2	Homestead area 10 × 32 = 320 m2	Homestead area 11.4 × 33.2 = 378 m2	Homestead area 12 × 32 = 384 m2	Homestead area 11.5 × 23.5 = 270 m2
Inhabitants: 3 people Small tri-gardens: 36 m2	Inhabitants: 3 people Small tri-gardens: 86 m2	Inhabitants: 4 people Small tri-gardens: 20 m2	Inhabitants: 3 people Small tri-gardens: 62 m2	Inhabitants: 4 people Small tri-gardens: 21.6 m2
Qianzhongyuan type	Qianzhongyuan type	Anteriormiddle accessory type	Front-yard type	Surrounding type (mountainous area)
Homestead area 9.4 × 29.8 = 280 m2	Homestead area 10 × 29.8 = 298 m2	Homestead area 11 × 34 = 374 m2	Homestead area 12 × 30 = 360 m2	Homestead area 300 m2
Inhabitants: 4 people Small tri-gardens: 12 m2	Inhabitants: 4 people Small tri-gardens: 5 m2	Inhabitants: 4 people Small tri-gardens: 98 m2	Inhabitants: 4 people Small tri-gardens: 120 m2	Inhabitants: 3 people Small tri-gardens: >300 m2
Legend:   Toilet     Septic tank   Small Tri-gardens   Ditch       Road

.

Under the source reduction technology measures, the actual sewage volume at village-level centralized treatment facilities should deduct the wastewater quantity recycled and dissipated at the courtyard level. Consequently, the design capacity formula for centralized treatment facilities requires optimization based on existing models. The refined calculation formula is expressed as follows:

$$Q={\displaystyle \frac{q\times p\times r\times k}{1000}}-Q_e$$

(4)

where the following definitions hold:

• Q: Daily treatment capacity (m³/d);
• q: Per capita domestic sewage discharge (L/d);
• p: Permanent population (persons);
• r: Effective sewage collection rate (%);
• k: Daily variation coefficient;
• Q_e: Daily dissipated wastewater volume from upper-level systems (m³/d).

Parameters q and p are obtained through field surveys; r is determined by village topography and pipe network coverage; k is derived from statistical analysis; Q_e is calculated by multiplying the area of “small tri-gardens” (vegetable/ornamental/productive gardens) with irrigation water quotas for specific crops.

4.2.2. Transmission Process Interception and Control Technology

To prevent non-point source pollution during transportation, academia and industry have explored interception technologies including filtration purification, nutrient utilization, and ecological barriers in the transmission process. Currently prevalent solutions include ecological ditch systems and buffer zones.

Ecological ditch technology employs biodegradation, soil adsorption, and plant uptake to effectively intercept nitrogen and phosphorus from domestic sewage. For instance, an 80 m trapezoidal-section ditch planted with emergent Thalia dealbata and submerged Potamogeton malaianus achieved TN and TP removal efficiencies meeting China’s GB18918 Level II standards when treating 10 m³/d rural sewage under a 5-day hydraulic retention time [42,43]. The stepped drainage ditch, a site-specific infrastructure in mountainous hills and loess tableland regions, features vertical drop shafts to dissipate elevation differences. Drop wells are strategically spaced along steep gradients, blending seamlessly with local topography.

Buffer zone technology enhances purification by planting hydrophytes along ditch banks. These vegetated strips significantly reduce pollutant concentrations and improve treatment performance.

The selection of “flow” facilities is significantly influenced by village topography and economic conditions. Ditch systems, as a cost-effective solution, offer advantages in construction simplicity, low maintenance requirements, and minimal environmental disturbance during implementation, making them preferable in topographically complex and economically disadvantaged regions. However, ditch systems present inherent limitations including susceptibility to leakage, environmental sensitivity, high overflow pollution risks, odor issues, and operational challenges in cold climates. In contrast, piped systems demonstrate superior performance in storm-sewer separation, leakage and odor control, thermal insulation, and overflow prevention, albeit with higher initial investments, deeper excavation impacts, and more complex operation and maintenance demands. A comprehensive comparison of both approaches is provided in

Table 3. Comparison of advantages and disadvantages of transmission methods for domestic sewage “flow” facilities.

Transmission Mode	Trench Transmission	Pipeline Transmission
Advantages	The investment is low, generally CNY 0.6~12 thousand per household, which is convenient for construction and maintenance and has little influence on villagers.	It is easy to realize rain and sewage diversion, with low odor, a low leakage rate, a good thermal insulation effect, and basically no overflow pollution.
Disadvantages	The confluence of rain and sewage easily leaks and is easily influenced by the surrounding environment, with a high risk of overflow pollution, strong odor, and poor thermal insulation effect, which makes it impossible to operate in cold areas in winter.	The one-time investment is relatively high, about CNY 10,000~15,000 yuan per household. The broken road construction pipeline has a great impact on the villagers, and the later operation and maintenance costs are relatively high.
Applicable object	Hilly and mountainous areas with complex topography that are not conducive to the connection of pipe networks.	Flat plains and tablelands.

.

During pipeline–ditch system planning, natural slopes and existing road networks should be strategically utilized through a “precision + reduction” construction paradigm to minimize costs and technical complexities. The conversion of rural open ditches into culverted or piped systems, while concealing misconnection and leakage issues, complicates future upgrades. It is recommended to prioritize open or covered ditches for greywater transmission under blackwater–greywater separation scenarios to facilitate inspection and fault detection. To enhance purification capacity during the flow process, drainage channels can be deepened, widened, and ecologically retrofitted with trash screens for solid waste filtration. Optimized ecological ditches integrating structural modifications and functional enhancements achieve multiobjective performance in drainage, flood control, soil conservation, and ecological purification, demonstrating static nitrogen and phosphorus removal rates of 58.2% and 84.8%, respectively, thereby serving as critical interception facilities [44].

4.2.3. Sink Process Attenuation Technology

As the terminal phase of sewage treatment, the sink process attenuation system comprises purification facilities and land-based assimilation units, including green infrastructure elements such as stabilization ponds, large-scale septic tanks, ecological waterlogging control ponds, constructed wetlands, and integrated treatment stations, as well as land-based assimilation through agricultural fields, orchards, and woodlands [29].

In Guanzhong villages, the revitalization of traditional water infrastructure—specifically waterlogging control ponds—as critical components of green water infrastructure (GWI) exemplifies localized strategies for adapting to population shrinkage in arid plateau regions. Historically functioning as green stormwater facilities to mitigate droughts and floods, many waterlogging control ponds have been abandoned due to modern water supply systems. However, the uncontrolled discharge of domestic sewage into street drainage channels often leads to black-odor water accumulation in these low-lying ponds [45]. Recent studies demonstrate the efficacy of phytoremediation–microbial synergy in waterlogging control pond restoration, with experimental results confirming significant pollutant removal [46].

To address the eutrophication caused by stagnant water in enclosed waterlogging control ponds, scholars propose a closed-loop system: “front-end treatment → waterlogging control pond with ecological auxiliaries → ecological ditches/constructed wetlands → waterlogging control pond” [47]. The implementation involves centralized sewage collection through front-end treatment systems to remove COD, nitrogen, and phosphorus, ensuring effluent compliance before discharge into the waterlogging control pond. Ecological enhancements such as floating islands, vegetated slopes, and shoreline hydrophytes are installed to boost nutrient removal [48]. For areas with available land, constructed wetlands adjacent to the waterlogging control pond enable further nitrogen–phosphorus assimilation through plant–microbial interactions, with treated water recirculated to form a closed loop. In space-constrained scenarios, 1–2 m wide ecological ditches encircling a waterlogging control pond create a semi-closed system, where water flows sequentially through the pond, ditches with gravel–clay substrates, and aquatic vegetation and then returns for continuous purification. This circulatory design enhances hydraulic dynamics while sustaining long-term water quality stability.

In recent years, the Shaanxi Provincial Government has implemented ecological restoration of waterlogging control ponds in the Guanzhong region, integrating modern sewage treatment facilities with village public spaces (e.g., the rehabilitated waterlogging control pond in Pangliu Village coexists with a central fitness plaza, ecological parking lot, monument, and ancient trees, as shown in Figure 8). Based on 5-year monitoring data (2019–2024) from pilot projects, the system exhibited annual decay rates of TN/TP removal efficiency at 2.1%/3.4% without substrate replacement, primarily attributed to plant root aging and a decline in soil porosity. By rotating emergent macrophytes (e.g., reed → cattail → calamus) every 5 years and replenishing 10% zeolite by substrate volume, over 90% of the initial performance can be restored [49]. Structural lifespan analysis indicates that concrete components (30-year design life) and HDPE pipelines (25-year lifespan) form the durability foundation, while periodic renewal of ecological modules (every 5–8 years) ensures functional sustainability [50].

This technical paradigm advocates for blackwater–greywater separation in collection systems. Blackwater undergoes pre-treatment via household septic tanks, with priority given to localized reuse in “small tri-gardens” (vegetable/ornamental/productive gardens), while residual volumes are transported via vacuum trucks to prevent anaerobic digester overflow into greywater channels. Greywater is conveyed through street stormwater ditches, with any overflow containing blackwater residuals directed to sedimentation tanks for preliminary treatment before entering waterlogging-control-pond-centered ecological treatment units. For villages with sufficient land resources, constructed wetlands are recommended downstream; in space-constrained areas with daily sewage volumes exceeding 20 m³/d, integrated treatment equipment is installed upstream of the waterlogging control pond to enhance system efficacy. An overflow bypass is established prior to the integrated equipment, ensuring dry-weather flows undergo full treatment before ecological polishing, while wet-weather mixed flows exceeding treatment capacity are directly diverted to ecological units. The waterlogging control pond effluent is utilized for surrounding agroforestry irrigation, with monsoon season surpluses being rapidly discharged into adjacent gullies (see Figure 9).

4.3. Comprehensive Benefit Analysis

The resource-oriented governance technology based on the “Source–Transmission–Sink” process relies on synergistic regulation through spatial flows within green infrastructure systems. For instance, an ecological ditch system (150 m length) coupled with stabilization ponds (including a sedimentation purification pond, biological purification pond, and bio-enhanced purification pond) treating 7 m³/d of rural domestic sewage achieves compliance with China’s GB18918 Level II standards under a 4-day hydraulic retention time [51]. Research demonstrates that decentralized source reduction technologies not only minimize infrastructure costs (pipe networks, construction, and O&M) but also significantly reduce resource consumption and ecological impacts, including mitigating aquatic ecosystem degradation and preserving natural hydrological cycles [52]. Crucially, these systems exhibit operational flexibility to accommodate flow fluctuations while generating agro-ecological economic benefits, reviving traditional “waste-as-fertilizer” agricultural practices. Villager feedback indicates higher satisfaction rates with localized solutions compared to centralized alternatives [53]. Consequently, “re-naturalized” treatment approaches are advocated for low-density settlements with underutilized infrastructure, aligning with demographic and spatial sustainability principles [54].

5. Discussion

This study explores resource-oriented treatment technologies for rural domestic sewage based on the Source–Transmission–Sink (STS) theoretical framework. Compared to existing research, we pioneer a focus on population shrinkage contexts, innovatively quantifying the relationship between the PSI and facility failure risks. We propose an adaptive “hierarchical attenuation” in situ circular governance method that emphasizes decentralized wastewater treatment, shifting the paradigm from gray infrastructure reliance to integrated pollution control and spatial resource utilization through gray–green infrastructure synergy. This transition addresses two critical limitations of developed countries’ governance models: (1) over-engineering and high costs in low-density settlements; (2) inadequate adaptability to seasonal population mobility (±60% load fluctuations).

Compared with the rural domestic sewage treatment schemes in developed countries, the STS system demonstrates unique advantages and limitations:

(1)

Advantages: In the source process reduction technology, different from Germany’s high-cost vacuum collection system and Japan’s decentralized model relying on household-level purification tanks, the STS framework integrates agricultural techniques through sewage irrigation for agricultural use. While realizing the resource utilization of nitrogen and phosphorus, it reduces the construction cost by 40–60%. In the transmission process interception technology, the mixed ecological ditch (a combination of biofilm carriers and wetland plants) can achieve a COD interception rate of 65% at a cost of USD 12 per meter, which is 33% lower than South Korea’s compact wetland system (with a cost of USD 18 per meter), and the maintenance complexity is only 50% of that of Australia’s septic tank–oxidation pond combination [22]. In the sink process attenuation technology, the collaboration between the artificial wetlands in the village area and the reuse of cultivated land achieves a water resource recovery rate of 90%, which is significantly better than the independent decentralized units in the United States (with a recovery rate of 70–75%) and the independent wetland systems [24], confirming the theoretical advantages of the “production–ecological” spatial nesting.

(2)

Limitations: Compared with Japan’s intelligent purification tanks and the Internet of Things monitoring system in the United States, the STS system has a relatively low level of automation, and manual inspections are required in remote villages. Due to the simplified filler configuration, the total phosphorus removal efficiency (60–70%) is lower than that of Germany’s PKA wetlands (85–90%), especially in the loess percolation area, which is easily affected by soil compaction.

(3)

The limitations of this study include the limited representativeness of the samples (a single-case study of Heyang County), which may affect the generalizability of the conclusions. In the future, the verification of this technology’s popularity will be expanded in typical scenarios. At the same time, the successful implementation of the technology requires collaborative innovation in policy frameworks, technical standards, and governance mechanisms. It is recommended that the government accelerate the formulation of operable planning guidelines to promote the application of resource-oriented treatment technologies.

6. Conclusions

This study reveals three core findings of rural sewage treatment in areas with population shrinkage, providing theoretical innovation and practical paradigms for similar regions globally:

(1)

Adaptability mismatch of infrastructure: There is significant design redundancy in centralized systems (with an average capacity deviation of 218%). Of the facilities, 75.5% have a load rate of less than 40%, and for every 10% increase in the PSI, the risk of facility failure rises by 48.7%. Compared with the high-cost model of centralized collection and treatment, the STS system can reduce ineffective investment by more than 60% through dynamic capacity adjustment, verifying the urgency of the transformation from static planning to an adaptive model.

(2)

Circular benefits of promoting treatment through utilization: The STS framework achieves a water reuse rate of 85% and a nutrient recovery rate of 65% through the “sewage–fertilizer source–farmland” circular chain. It reduces treatment costs by 40–60% compared with the centralized collection and treatment model. Meanwhile, with a resident acceptance rate of 78%, it reshapes traditional farming wisdom and provides a Chinese path for SDG 6 and SDG 12.

(3)

Spatial optimization efficiency: The hierarchical GWI network reduces transportation costs by 30–45% through near-source treatment (with a service radius of ≤300 m). Its “household–village” collaborative mode improves spatial efficiency compared with the independent unit system, providing a scalable criterion for territorial space planning.

Policy action recommendations are as follows:

(1)

Establish a population-responsive funding mechanism and give priority to supporting shrinking villages in the national ecological compensation plan.

(2)

Formulate adaptive technical standards. Set differentiated treatment thresholds according to the classification of “stable-transition–acutely shrinking” villages. Pilot the STS technology package in areas with population shrinkage and provide a guide for the rapid deployment of modular GWI.

(3)

Build a cross-departmental governance platform. Incorporate sewage resource utilization into the assessment of the balance between cultivated land occupation and compensation. Establish a three-party operation and maintenance platform for “farmers–village collectives–enterprises”. Integrate data from the water resources, agriculture, and rural revitalization departments, and build an intelligent scheduling system for “water–fertilizer–land” to improve the precision of reuse.

Key focuses of future research are as follows:

(1)

Fiscal innovation for technology popularization: Explore the Public–Private Partnership (PPP) model, leveraging social capital with public funds to focus on supporting the deployment of modular GWI.

(2)

Cross-regional resilience grading system: Conduct a comparative analysis of technical performance between the eastern and western regions based on the “Hu Huanyong Line” and establish a climate–population adaptation gradient.