Study on the Seismic Stability of Urban Sewage Treatment and Underground Reservoir of an Abandoned Mine Pumped Storage Power Station

Abstract

As coal’s share in primary energy consumption wanes, the annual increase in abandoned coal mines presents escalating safety and environmental concerns. This paper delves into cutting-edge models and attributes of integrating pumped storage hydropower systems with subterranean reservoirs and advanced wastewater treatment facilities within these decommissioned mines. By utilizing the expansive underground voids left by coal extraction, this method aims to achieve multifaceted objectives: efficient energy storage and generation, reclamation of mine water, and treatment of urban sewage. This research enhances the development and deployment of pumped storage technology in the context of abandoned mines, demonstrating its potential for fostering sustainable energy solutions and optimizing urban infrastructure. This study not only facilitates the progressive transformation and modernization of energy cities but also provides crucial insights for future advances in ecological mining practices, energy efficiency, emission mitigation, and green development strategies in the mining industry.

1. Introduction

China’s coal resources have been continuously exploited on a large scale for a long time. According to statistics, the number of abandoned mines worldwide has exceeded 1 million, and by 2030, China is expected to have 15,000 abandoned mines, with the number of closed and abandoned mines increasing daily [1]. After a mine is abandoned, it leaves behind considerable volumes of various forms of underground roadways and chambers. If these are not treated or reused, they can lead to serious water pollution and geological disasters [2]. Therefore, the rational utilization of space and water resources left by abandoned mines to achieve efficient secondary utilization of underground resources has been a long-standing challenge for China’s coal industry.

Since 1960, R.D. Harza proposed the concept of utilizing underground or open-pit mines for building pumped storage power stations [3]. Successively, countries such as Austria, the United Kingdom, the United States, and China, among others, have repurposed abandoned mines for constructing pumped storage power stations [4]. This approach not only positively impacts disaster prevention and control in underground goafs but also contributes to the ecological restoration of mines. Economically, repurposing abandoned underground spaces reduces construction time and significantly lowers investment costs [5]. Simultaneously, urbanization has led to increased discharge of domestic sewage, necessitating measures for sewage recycling to address water shortages and pollution [6,7]. However, challenges persist in sewage treatment, including low inlet water concentration, suboptimal collection rates, and high operating costs, severely impeding sustainable socioeconomic development [8]. “Energy conservation, carbon reduction, and efficiency enhancement” are critical imperatives for urban sewage plants. Various countries, including the United States, the United Kingdom, and Japan, are developing underground sewage treatment plants, which have demonstrated favorable economic and social outcomes [9]. With decreasing costs associated with underground space development, the feasibility of underground sewage treatment plants is growing, aligning with the trend of modern metropolitan facilities moving underground. The construction of underground sewage treatment plants is well suited to China’s conditions of high population density, limited land availability, and climatic diversity. Therefore, as China enhances water environment governance, underground sewage treatment plants are poised for significant global development [10].

This study is the first to systematically investigate a pumped storage power station and an urban wastewater treatment system as a deeply integrated “electricity–water–carbon” coupled micro-scale energy system. It proposes an innovative concept of introducing a multi-energy complementary integrated energy system within abandoned mines to effectively address the high energy consumption of underground wastewater treatment, thereby enhancing operational efficiency, energy savings, and energy security. In response to the vast volume of abandoned underground coal mine space in China, this work innovatively proposes a hybrid development model of “pumped storage + urban wastewater treatment”, breaking the traditional site selection and topographic constraints of conventional pumped storage power plants and enabling the efficient reuse of underground mine space. Based on systematically collected and analyzed hydrogeological data of abandoned mines in China, this study reviews current underground space reuse practices and establishes a “pumped storage–wastewater treatment” integrated utilization model adapted to China’s regional characteristics. This provides a theoretical foundation and engineering approach with practical significance for advancing coal mine ecological restoration, promoting urban green transformation, and achieving the nation’s dual carbon goals.

2. Feasibility Study of the Model System

2.1. Research Status of Pumped Storage Power Stations in Abandoned Mines

The inception of pumped storage power stations in China dates back to 1968 [11]. However, during that era, power shortages were the primary concern within the power system, overshadowing the regulatory potential of pumped storage stations. It was only until 1980 that the expansion of power grids in Guangdong Province, North China, and East China accelerated, exposing the limited load regulation capacity of the existing power system as a bottleneck hindering economic development [12]. Presently, Japan boasts an in-service pumped storage capacity comprising 8.5% of its total installed power supply capacity, while Italy, Spain, Germany, and France range from 3.5% to 6.6% [13,14]. Figure 1 illustrates the evolution of China’s total pumped storage capacity since 2010 and its contribution to the nation’s overall installed power capacity. Despite the rapid growth of new energy installations in China, a substantial gap persists when comparing the proportion of installed capacity with that of developed nations and China’s own pumped storage development targets.

Based on incomplete statistics gathered from public data sources such as Power China and local government websites, Figure 2 illustrates the distribution, scale, and average construction costs of pumped storage power stations in China [15,16,17]. By the conclusion of 2023, 43 pumped storage power stations had been completed and commissioned in China, boasting an aggregated installed capacity of 46.85 GW. Figure 3 provides a geographic breakdown of operational and under-construction pumped storage power stations across various provinces and regions nationwide. This portrayal reveals an uneven development landscape characterized by significant disparities in installed capacity. Currently, pumped storage power station development is predominantly concentrated in economically advanced eastern regions and Central, Northern, and Northeastern China, where thermal power holds sway. Notably, provinces such as Guangdong, Zhejiang, Anhui, Hebei, and Fujian feature prominently in this regard. Conversely, the western provinces, including Xinjiang and Tibet, host only a sparse number of pumped storage facilities. Furthermore, pumped storage resources remain exceedingly scarce in other provinces [18].

2.2. Research Status of Multi-Energy Complementary Comprehensive Utilization

With China’s ongoing adjustments to its energy structure, the proportion of renewable energy sources, particularly wind and solar, has been steadily increasing [19]. However, wind and solar energy exhibit characteristics such as randomness, volatility, intermittency, seasonality, and regional disparities, leading to poor sustainability and stability of energy supply, thereby presenting significant challenges to grid integration [20,21]. To mitigate the impacts of these renewable sources on the power grid, large-scale energy storage solutions are essential [22,23]. Currently, mature energy storage technologies capable of large-scale power storage mainly include pumped storage and compressed air energy storage systems [23,24]. However, traditional ground-based installations for pumped storage and compressed air energy storage require stringent site selection criteria, necessitating abundant high-altitude energy storage space, which is limited in availability. Conversely, abandoned mines, abundant in various European and American countries due to their well-developed mining industries, offer vast underground spaces suitable for energy storage facility construction, making them pioneers in abandoned mine utilization [25,26]. Since the 1960s, the United States, the United Kingdom, Germany, and other European and American nations have conducted extensive research and practical applications concerning abandoned mines, yielding remarkable results. In contrast, China’s exploration into abandoned mine development commenced relatively late, constrained by factors such as complex mine geology and numerous partially closed mines. Initial efforts in this area have primarily focused on industrial experiments, including coal mine gas extraction, underground coal gasification, underground reservoir construction, and gas storage within abandoned mines [27,28]. Therefore, leveraging the hydrogeology of abandoned mines, along with their spatial resource conditions above and below ground, adjacency relationships between mines, and factors such as wind, solar, and other new energy sources, a paradigm of multi-energy complementarity and comprehensive utilization, notably pumped storage power generation within abandoned mines, has started to emerge [29,30,31,32]. The schematic representation of the comprehensive utilization system of abandoned mines is depicted in Figure 4.

2.3. Research Status of Urban Sewage Treatment

In recent years, with the continuous advancement of urbanization, the discharge of urban domestic sewage has continued to grow, and the recycling of urban sewage has become an important measure to solve the problem of population water shortage and water environment pollution. However, the corresponding sewage treatment methods are lagging behind, which seriously restricts the sustainable development of social economy [33,34,35]. It can be seen that strengthening urban sewage treatment is an important way to comprehensively enhance urban functions and improve public services.

Urban wastewater sources in surrounding areas exhibit significant characteristics of scale and centralization. With the acceleration of urbanization, both the quantity and diversity of such wastewater—originating from domestic, industrial, medical, and commercial sectors—have increased markedly, making them an important component of potential reclaimed water resources. Figure 5 illustrates the typical urban wastewater treatment process in China, comprising primary physical treatment, secondary biological treatment, and tertiary advanced treatment [36,37,38]. Primary treatment involves units such as the grid, regulating tank, lifting pump, and primary sedimentation tank. The grid intercepts suspended solids, the regulating tank regulates and homogenizes water quality, and the lifting pump transfers sewage to the primary sedimentation tank for further sedimentation and filtration. Secondary treatment primarily employs microorganisms to remove organic matter, nitrogen, and phosphorus, mitigating CH₄ and N₂O emissions while generating carbon emissions. Effluent from the biochemical tank undergoes secondary sedimentation for mud–water separation; the supernatant, typically containing microorganisms, nitrogen, and phosphorus, can serve as agricultural or irrigation water. The deposited sludge exhibits significant biochemical activity; a portion is recirculated to the biochemical tank via the reflux pump to maximize activity, while the remainder, along with sludge from the primary sedimentation tank, is directed to the biogas tank. Tertiary treatment utilizes adsorption, electrodialysis, reverse osmosis, and similar methods for comprehensive removal of organic matter, nitrogen, and phosphorus from the secondary effluent, yielding a tertiary effluent suitable for industrial miscellaneous reclaimed water.

The current ground sewage treatment plants face various issues including air and noise pollution, wastage of land resources, and high operational costs. Consequently, underground sewage or reclaimed water plants are increasingly favored due to their ability to mitigate these challenges. The development of underground sewage treatment facilities aligns with the trend of modern metropolises moving public infrastructure underground [39,40]. Many countries, including China, the United States, the United Kingdom, and Japan, are actively developing such facilities, yielding positive economic and social outcomes. While underground sewage treatment plants offer benefits, they pose challenges such as increased energy consumption due to the need for ventilation and deodorization systems. The energy demands of underground plants surpass those of equivalent above-ground facilities, necessitating innovative power supply solutions to address this issue urgently [41,42,43].

3. Model Performance Evaluation

To effectively address the dual pressures of environmental degradation and resource scarcity arising from the growing volume of urban wastewater discharge, it is imperative to explore alternative treatment spaces with integrated resource utilization potential. In this context, abandoned mines—owing to their unique underground structures and spatial characteristics—have emerged as promising candidates for wastewater storage and treatment. Therefore, this study systematically evaluates the resource attributes and treatment capacity of abandoned mines to assess their engineering applicability and utilization potential in urban wastewater management.

3.1. Power Generation and Storage Assessment

After a mine is abandoned, a large area of ground and underground space is left unused. This not only reduces the need for extensive dam construction during subsequent development but also results in cost savings. Additionally, the structure of photovoltaic power generation systems is straightforward, comprising only photovoltaic arrays, inverters, and combiner boxes. This simplicity streamlines the process of integrating photovoltaic energy storage devices, facilitating the timely storage and utilization of photovoltaic electricity through pumped storage stations. This approach promotes the efficient consumption of renewable energy while mitigating the environmental pollution associated with photovoltaic energy storage devices, thus contributing to the enhancement of the surrounding ecological environment. Furthermore, the incorporation of photovoltaic power into pumped storage power stations enhances their peak regulation capacity. In summary, utilizing abandoned mines for the construction of wind and photovoltaic pumped storage power stations represents a prudent choice concerning power grid operations, the utilization of renewable energy, ecological preservation, and economic viability.

3.2. Assessment of Sewage Treatment Capacity

The operation of urban sewage treatment plants is directly related to treatment efficiency and costs. In the context of sewage treatment systems for pumped storage power stations, considerations include water treatment processes, carbon emissions, sewage classification, utilization, and available space in abandoned mines. This entails analyzing intake forecasts, pollutant concentrations, and water demand for the following day. Coordination of environmental protection objectives in sewage plant operation involves a comprehensive assessment of energy costs, carbon emissions, equipment maintenance, sewage reuse benefits, and environmental benefits related to water quality improvement while meeting treatment standards and operational constraints [44,45].

Compared with conventional wastewater treatment systems, the pumped storage-based wastewater treatment system incorporates a three-stage sedimentation unit upstream of the equalization tank. This configuration allows for the removal of large suspended solids through natural sedimentation, resulting in predominantly colloidal turbidity in the water entering the equalization tank. Such pretreatment creates favorable conditions for subsequent coagulation processes. Moreover, the enclosed environment of the abandoned mine provides extended hydraulic retention time, which facilitates the thorough degradation of organic pollutants.

The wastewater treatment capacity of the abandoned mine was estimated based on its effective underground storage volume, as well as the feasible water exchange rate. The effective volume (V_eff) was calculated using the mine cross-sectional area and usable height, adjusted by a porosity coefficient, as shown in Equation (1). Assuming an annual circulation frequency of 5–8 times, the annual and daily treatment capacities were derived accordingly.

$$V_{eff}=A_{mine}\times H_{mine}\times \varphi$$

(1)

where A_mine is mine horizontal section area (m²); Hmine is the mine elevation (m) that can be used for processing; φ is the pore/filling ratio (effective water filling rate, 0.3–0.6, depending on the mine structure).

Energy consumption during wastewater treatment was estimated using a typical energy intensity of 0.5 kWh/m³, encompassing aeration, pumping, and filtration processes. Thus, the total daily energy consumption (E_total) was expressed as:

$$E_{total}=Q_{day}\times e$$

(2)

where Q_day is the daily treatment volume and e is the energy intensity per cubic meter of treated water.

In terms of operating costs, the total cost (C_total) was estimated using a unit treatment cost (c) that includes energy, chemical reagents, labor, and maintenance. Typically, this value ranges between 0.2 and 0.5 USD/m³ depending on local economic conditions and treatment technology [46,47,48].

4. Integrated Technology Model of Urban Sewage Treatment by a Multi-Energy Complementary Abandoned Mine Pumped Storage Power Station

4.1. Multi-Energy Complementary Power Generation Mode of Abandoned Mines

The abandoned mine pumped storage power station consists of two main components. The first part resembles a traditional pumped storage power station, operating on the principle of utilizing surplus power from the grid during periods of low demand to drive water turbine units in the lower reservoir [49,50]. This water is then pumped into the upper reservoir for energy storage. Conversely, during peak electricity demand, water from the upper reservoir is released to turn hydro-generator units in the lower reservoir, converting gravitational potential energy into electrical energy. The second part incorporates photovoltaic and wind power generation systems.

The structure of the pumped storage power station is depicted in Figure 6, illustrating the underground framework utilized for the construction of the pumped storage system and the ground structure for the wind photovoltaic power generation system [51,52]. The underground structure primarily comprises four components: the upper reservoir, the lower reservoir, the underground plant, and the waterway system. Leveraging the significant height difference between the underground mine void and the upper roadway void, the underground roadway and void serve as the lower reservoir, while the upper shaft roadway void is utilized as the upper reservoir. The plant accommodates various equipment, such as pump generators, turbines, generators, and related controls, and can be constructed within underground roadways and spaces. The waterway system includes a channel and a tailwater channel connecting the upper reservoir and the lower reservoir, with the main pipeline of the pumped storage power station being typically situated in the mountain apart from the inlet and outlet. The construction of surface wind photovoltaic power stations primarily involves utilizing a large area of abandoned land in abandoned mines and selecting suitable areas for the installation of photovoltaic arrays and wind power generation. Solar panels, brackets, inverters, and box-type transformers are arranged in the photovoltaic array, with the azimuth angle being determined by the terrain, slope, and inclination angle of the bracket installation in the abandoned mine [53].

4.2. Sewage Treatment Mode of Abandoned Mine Pumped Storage Power Stations

Ensuring the long-term stable operation of wastewater treatment systems is one of the key issues for the sustainable application of abandoned mines in urban wastewater management. To maintain operational stability, the mine-based treatment system is equipped with facilities such as equalization tanks and secondary and tertiary reservoirs. These components not only satisfy the requirements for water collection and storage but also offer flexibility in regulating inflow volumes, enabling dynamic monitoring and intelligent adjustment of water quality, quantity, and equipment status [54], as shown in Figure 7.

Given that mine structures have been dormant for extended periods, reactivation may lead to risks such as seepage or support structure failure. Regular inspections and maintenance of shaft walls are therefore essential, with reinforcement measures such as shotcreting, inner lining, or waterproof membranes being applied as necessary. Furthermore, a pretreatment stage—comprising coarse screens, grit chambers, and pH adjustment units—should be implemented to reduce the impact of high-concentration pollutants on the mine’s water environment. Special attention must be paid to preventing the direct discharge of oily, toxic heavy metal-containing, or strongly acidic/alkaline wastewater, which may contaminate residual groundwater resources. Operating biochemical treatment facilities in underground spaces also requires reliable oxygen supply and temperature control systems to prevent efficiency degradation due to micro-environmental fluctuations. In addition, wastewater often carries suspended solids, inorganic salts, and microbial communities that can form deposits or biofilms on pipeline surfaces, pumps, and tank walls during prolonged operation. This can lead to clogging, corrosion, or reduced system performance. To mitigate these effects, corrosion-resistant materials (e.g., stainless steel, HDPE) should be used for key components, complemented by periodic flushing, chemical cleaning, or backwashing systems to reduce maintenance costs and extend service life.

In this study, we systematically compared the performance of underground wastewater treatment systems based on abandoned mines with that of conventional surface treatment plants across several key performance indicators, as summarized in

Table 1. Comparison of performance indexes of different sewage treatment systems.

Performance Index	Traditional Surface Sewage Treatment Plant	Underground Treatment System Based on Abandoned Mines	Comparative Result
Occupied area	High (about 5−10 m2/person)	Very low (basically no new surface area)	The reuse of underground space and the release of surface space are especially suitable for the areas with limited land resources around the city.
Construction cost	Medium to high (1.5–3 million CNY/10,000 m3/d)	Medium (depending on the needs of mine transformation)	The mine infrastructure can be reused, and the cost of civil engineering can be reduced, but the cost of transformation depends on the mine conditions.
Operation energy consumption	High (0.3–0.6 kWh/m3)	Medium (rely on gravity flow to reduce energy consumption)	The energy consumption of drainage can be reduced by using the vertical water level difference of the mine.
Temperature fluctuation	Obviously (5–35 °C, need additional adjustment)	Stable (underground temperature 12–18 °C)	The underground constant temperature is conducive to the stability of the flora and reduces the energy consumption of temperature control.
Flocculant dosage	Conventional (15–25 mg/L)	Decrease (10–15 mg/L)	Reduce the use of chemical agents with the help of a slow settling environment and stable reaction conditions
Hydraulic retention time	Short (6–12 h)	Prolonged (12–48 h)	The large volume of mine space is conducive to the realization of longer biochemical reaction time.
Treatment of water quality stability	Affected by temperature and water fluctuation	High (constant temperature, small hydraulic disturbance)	The large volume of mine space is conducive to achieving longer biochemical reaction time.
Urban sewage response capacity (sudden increase flow)	Medium (expansion or diversion required)	High (can be used as a temporary reservoir)	The mine has strong water storage capacity and certain buffering and storage functions.
Environmental impact	Need to manage noise, odor, landscape, etc.	Small underground closed operation)	The underground system basically does not affect the surface environment, and the environmentally sensitive areas have more advantages.

. As shown in Table 1, the deep sections of abandoned mines exhibit excellent thermal insulation properties, maintaining a stable year-round temperature between 12 and 18 °C. This consistent temperature range is beneficial for sustaining microbial activity and community stability within biochemical reaction systems, thereby reducing operational uncertainties caused by temperature fluctuations. The constant thermal environment also eliminates the need for extensive energy input for inflow heating during winter months, effectively lowering operational energy consumption.

Furthermore, the enclosed underground environment allows for extended hydraulic retention times, which promotes the thorough degradation of organic pollutants. Coupled with low underground flow velocities and minimal water body disturbance, this setting enhances the sedimentation of particulate flocs. Consequently, the dosage of flocculants can be appropriately reduced, leading to decreased chemical costs and a lighter burden for subsequent sludge treatment. In terms of handling sudden hydraulic surges, abandoned mine systems also offer significant advantages. The large storage capacity of the mine can serve as a buffer during urban stormwater events or sudden wastewater inflows, effectively balancing treatment loads. In contrast, conventional surface-based treatment systems are often constrained by limited basin volumes and spatial layout, making them less resilient to hydraulic shocks. While mine-based systems require specialized operational protocols and maintenance practices—such as structural safety assessments and routine inspections—the development of intelligent monitoring and automated control technologies is progressively enhancing their manageability and operational viability.

To further assess the engineering suitability and feasibility of utilizing abandoned mines for pumped storage systems, this study summarizes key design parameters based on existing engineering experience, technical standards, and the specific characteristics of underground spaces (

Table 2. Reference table of main design parameters of pumped storage system in abandoned mines.

Parameter Type Check	Parameter Name	Design Value	Analysis of Influencing Factors
Water parameters	Effective head	100–600 m	Depending on the height difference between the upper and lower libraries
	Flow rate	10–100 m3/s	Depending on the energy storage capacity and the number of units to determine
Pipe parameters	Inlet/Outlet pipe diameter	1.5–5 m	Matched with the design meteor and flow velocity
	limit of velocity	2.5–5 m/s	Ensure economic flow rate and reduce head loss
Reservoir parameter	Upper/Lower reservoir capacity	104–10° m3	Meet the peaking/frequency modulation operation requirements
	Anti-seepage performance of reservoir	K < 1 × 10−7 cm/s	Ensure closure and avoid water leakage
Pump/Power Generation Equipment Parameters	Pump turbine efficiency	85–93%	According to the model and operating conditions
	Adjustable running time	4–8 h	Depending on the power grid regulation demand and water storage capacity setting
Other environmental requirements	Minimum depth of waste mine	≥100 m	Ensure effective bit difference and capacity safety
	Stability of rock mass	RMR ≥ 60 (moderate to good)	Ensure the safety and reliability of the reformed roadway structure

). During system operation, maintaining a reasonable flow velocity (2.5–5 m/s) is essential for minimizing energy losses. The selection of an appropriate hydraulic head (100–600 m) and reservoir capacity directly determines the system’s power output and load-shifting capability.

The abandoned mine should have a minimum depth of 100 m to ensure sufficient hydraulic head. Additionally, adequate rock mass stability is required to withstand the high-pressure flow conditions. Through the optimization of these parameters and system configurations, stable operation of pumped storage facilities within mine environments can be achieved, thereby providing a reliable foundation for the integration of urban wastewater treatment and renewable energy systems.

4.3. Integrated Operation Mode of Abandoned Mine Pumped Storage Power Stations

Based on the ground space resources, water resources, surrounding wind energy conditions, and photovoltaic conditions of the abandoned mine, a multi-energy complementary development and utilization design scheme suitable for pumped storage and sewage treatment can be proposed, as illustrated in Figure 8.

4.4. Analysis of the Comprehensive Utilization Benefits of Abandoned Mine Pumped Storage Power Stations

With the aim of “increasing the proportion of non-fossil energy” in China, a target of over 1.2 billion kilowatts of wind and solar installed capacity by 2030 has been proposed. This necessitates concurrent improvements in energy storage systems and security measures. It is anticipated that pumped storage capacity will exceed 62 million kilowatts by 2025, and production capacity is expected to reach about 120 million kilowatts by 2030 [55,56,57]. The growing integration of wind and solar power into the grid escalates the demand for pumped storage in the power system. However, economic considerations significantly impact the feasibility of constructing pumped storage facilities in abandoned mines, hence necessitating comprehensive cost-benefit analyses in the early stages to assess their viability and economic rationale.

4.5. Comparison of Investment Cost Between New Pumped Storage Power Stations and Conventional Pumped Storage Power Stations

4.5.1. Cost Source Analysis

Economic viability is one of the key factors in assessing the feasibility of converting abandoned mines into pumped storage systems. Compared with the construction of conventional pumped storage power stations, utilizing existing mine spaces can significantly reduce total investment across multiple cost dimensions. To more intuitively illustrate the differences in cost structures between the two systems, this study presents a comparative analysis of the major cost components of traditional and mine-based pumped storage systems in

Table 3. Comparison of the investment cost of different pumped storage power stations.

Cost Items	Traditional Pumped Storage System	Pumping Sauce Energy System of Abandoned Mine	Analysis of Cost Advantages
Underground space development and excavation	High: necessary to build new upper and lower reservoirs and diversion tunnels, and the amount of excavation is large.	Low: direct use of existing mine roadways and goafs.	Save 50–70% civil engineering cost.
Foundation and slope treatment Slope and foundation treatment	High: large-scale stable edges are required.	Low: the original roadway has a certain bearing capacity of rock mass.	The engineering quantity of slope and foundation treatment is greatly reduced.
Upper reservoir construction	High: site selection, land acquisition and new construction of large-scale storage reservoir.	Optional: can be combined with the existing mining subsidence area or transformation of small reservoirs.	Save part or all of the civil construction investment of the upper reservoir.
Installation of piping system and pumping equipment	Medium: standard installation cost.	Medium: similar to the traditional system.	The cost of major equipment is similar.
Land requisition and environmental restoration	High: wide range of land acquisition, strict environmental approval.	Low: abandoned mine stock resources, land use pressure is small.	Avoid land acquisition and additional ecological compensation.
Sewage pretreatment and transportation system construction	Not involved or high (if new dedicated sewage system is required).	Medium: need to transform access to municipal sewage or industrial wastewater system.	The initial investment is controllable, which is conducive to the recycling of water resources.
Operation, maintenance, and safety management	Medium: routine inspection and equipment management.	Medium to high: it is necessary to strengthen underground stability monitoring and gas/settlement control.	The initial safety investment is slightly higher, but the long-term stability is controllable.
Estimate total cost (relative)	100% (baseline).	About 60–70% (relative to the traditional system).	expected to save 30–40% of the initial construction cost.

.

Traditional pumped storage power stations often require extensive underground excavation, reservoir construction, slope stabilization, and land acquisition—projects that typically account for more than 50% of the total construction cost. In contrast, abandoned mine-based systems can take advantage of pre-existing mine shafts, tunnels, and goafs, which inherently possess the basic spatial conditions for water storage and diversion. This substantially reduces civil construction volumes and eliminates much of the initial excavation and space development costs. Additionally, as mines are usually located in degraded resource areas, the land is already available for redevelopment, thereby lowering the burden of land acquisition, environmental permitting, and regulatory approvals.

In terms of pumping equipment and pipeline systems, both systems share similar technical configurations, and equipment investments remain at comparable levels. However, mine-based systems require enhanced operation and maintenance efforts focused on underground structural stability, ventilation, drainage, and gas monitoring, which results in a moderate increase in operational expenditure.

Overall, under the premise of meeting basic geological conditions and energy access requirements, mine-based pumped storage systems can reduce initial construction costs by 30–50%. This provides a practical and economically viable solution for establishing clean energy peak-shaving infrastructure in urban fringe areas. The integration of reclaimed urban wastewater resources with the redevelopment of abandoned mine spaces exemplifies a green, low-carbon, and resource-efficient approach to infrastructure development.

4.5.2. Economic Construction Costs

The construction of conventional pumped storage power stations is restricted by various factors such as topography, land occupation, engineering costs, and ecological environmental protection, especially in the cities of the North China Plain, where finding natural high-drop terrain conditions suitable for surface pumped storage power stations is difficult. However, abandoned mines, resulting from resource exploitation, perfectly meet this site selection condition. After mineral resource extraction, vast underground spaces are formed. The rational utilization of this space for the new pumped storage power station’s ground, lower reservoir, and equipment storage warehouse not only effectively reduces the construction costs of surface and underground reservoirs but also provides natural elevation differences and ample water resources. This creates favorable conditions for potential energy and geographic features for the new pumped storage power station to treat urban sewage, thereby realizing efficient space resource utilization.

The transformation of abandoned mines into urban wastewater treatment facilities involves several cost components during engineering implementation, including structural modification of mine shafts, installation of wastewater treatment equipment, construction of storage and conveyance systems, and auxiliary infrastructure setup. Firstly, the repair and reinforcement of basic mine structures—such as shaft cleaning, support maintenance, anti-seepage treatment, and upgrades to power supply and ventilation systems—typically cost approximately 200–400 CNY/m². Based on a representative renovation area of 10,000 m², the total structural modification cost is estimated at CNY 2–4 million. Secondly, wastewater treatment units commonly employ well-established processes such as moving bed biofilm reactor (MBBR) and anaerobic–anoxic–oxic systems. For a treatment capacity of 5000 m³/day, the cost of complete equipment procurement and installation is approximately CNY 4–6 million, translating to CNY 800–1200 per cubic meter of treatment capacity. In addition, storage tanks, reuse reservoirs, and conveyance systems connecting to municipal sewage pipelines must be constructed, with an estimated investment of CNY 1–2 million. At the operational preparation stage, additional facilities—including automation control, monitoring systems, and emergency power supplies—are required, with associated costs of CNY 0.5–1 million. Therefore, for a wastewater treatment capacity of 5000 m³/day, the total initial investment for converting an abandoned mine into a treatment facility is estimated to range between CNY 7.5 and 13 million. This estimate is preliminary and subject to variation depending on factors such as the structural characteristics of the mine, wastewater properties, and specific treatment standards [58,59].

4.5.3. Water Resources Costs

Regarding water resources protection, traditional pumped storage power stations experience significant evaporation and leakage due to their primary working cycle being located on the surface. In contrast, abandoned mine pumped storage power stations are mainly underground, which largely avoids water evaporation caused by direct sunlight or harsh environmental conditions, thus reducing the waste of water resources. Additionally, a steady stream of sewage input ensures the stable operation of the pumped storage power station [60].

5. Comprehensive Economic Benefits of Multi-Energy Complementary Utilization of Abandoned Mines

5.1. Power Generation Efficiency

Under the integrated utilization of multiple energy sources such as solar and wind energy in abandoned mines, it is estimated that 1 MW of photovoltaic (PV) power generation typically requires a land area of approximately 2.00 to 2.67 hectares (hm²). Given that the upper reservoir of the system occupies 0.75 hm², the available area can accommodate the construction of a PV power station with an installed capacity of approximately 0.33 MW. Assuming an actual system efficiency of 70% and 10 h of daily operation, the estimated annual electricity generation would be approximately 8.4 × 10⁵ kWh [61,62]. The above estimation is based on the following assumptions:

• Geographic location and solar irradiance: It is assumed that the PV panels are installed in a representative geographic region with typical solar radiation conditions.
• Efficiency of PV panels: The calculation assumes the use of commercially available monocrystalline silicon PV modules with a conversion efficiency of 18%. This reflects the current mainstream technology in the PV market. Additionally, it is assumed that the panels operate under stable full irradiance conditions without significant efficiency degradation.
• Installation angle and orientation: The panels are assumed to be installed at the optimal tilt angle of 30°, facing the direction of maximum solar exposure. This angle is suitable for most regions and is designed to maximize solar energy capture and improve generation efficiency.
• System losses: A system loss factor of 10% is assumed, accounting for inverter losses, cable losses, and other balance-of-system inefficiencies. Therefore, 10% of the gross generated electricity is deducted to obtain the net usable output.

Furthermore, the photovoltaic power generation is estimated using Equation (3):

$$E=P\times H\times \eta \times (1-L)$$

(3)

where E is the annual electricity output (kWh), P is the installed capacity (kW), H is the number of effective sunshine hours per year, η is the panel efficiency, and L is the system loss rate.

5.2. Effluent Treatment Benefits

Sewage, encompassing various organic, inorganic, and microbial substances generated from urban living and industrial activities, poses significant challenges in treatment due to its complex composition. Conventional sewage treatment processes involve extensive use of electrical equipment and high power consumption, leading to substantial direct production costs and contributing significantly to overall energy consumption in water treatment plants [63,64,65]. The utilization of pumped storage in abandoned mines presents a sustainable solution, enabling a green and efficient production cycle encompassing power generation, sewage treatment, and tap water supply. Proper treatment of urban industrial and domestic wastewater in these facilities yields significant circular economic benefits. Additionally, repurposing the land surrounding abandoned mines for the construction of solar photovoltaic power stations facilitates self-consumption and electricity production, leveraging clean energy sources to achieve sustainable energy practices. Moreover, sewage treatment in abandoned mines mitigates carbon dioxide, ammonia, and chlorine emissions, effectively reducing air pollution levels [66].

5.3. Environmental Benefits

Thermal power plants emit significant amounts of harmful gases, including carbon dioxide, sulfur dioxide, and nitrogen oxides. In comparison, abandoned mine pumped storage power stations emit substantially fewer harmful gases while generating the same amount of power. Consequently, deploying abandoned mine pumped storage stations allows for a reduction in the construction of thermal power plants of equivalent capacity, thereby greatly mitigating nitrogen oxide emissions. Leveraging its environmentally friendly attributes, renewability, and low energy consumption, pumped storage technology enhances the energy mix, fosters sustainable national economic development, and promotes the creation of a green, low-carbon society [67,68,69].

6. Selection and Analysis of Potential Sites of Abandoned Mines

6.1. Application Potential of Abandoned Mines as Urban Sewage Treatment Sites

The application of abandoned mines in urban wastewater treatment demonstrates significant potential, particularly in the areas of resource reutilization, environmental mitigation, and technological integration. On the one hand, abandoned mines, as existing underground spatial resources, can substantially reduce the construction costs of wastewater treatment facilities. Most mine structures exhibit high stability, with favorable sealing and isolation properties, making them suitable for primary sedimentation, flow regulation, and even certain ecological treatment functions. Additionally, as many abandoned mines are located in suburban areas, they offer convenient access to urban sewage networks, creating favorable conditions for the development of decentralized wastewater treatment systems. On the other hand, these mines also provide positive contributions to environmental regulation. During flood seasons, they can serve as temporary wastewater retention basins, alleviating pressure on surface discharge and treatment infrastructure. Their capacity for groundwater level regulation also helps prevent groundwater backflow. Furthermore, mine voids can be integrated into pollutant retention and ecological purification systems, enabling synergistic effects between pollution control and ecological restoration.

In terms of technological integration, abandoned mines exhibit strong potential for co-development with other engineering applications, such as geothermal energy utilization, pumped storage hydropower, and carbon sequestration. They can also accommodate modern biochemical processes such as moving bed biofilm reactor (MBBR) and anoxic–oxic treatment to achieve efficient wastewater treatment. Moreover, they may be incorporated into urban water reuse systems to enhance the overall efficiency of urban water recycling.

In summary, abandoned mines offer an innovative, resource-efficient, and environmentally friendly pathway for urban wastewater treatment.

6.2. Analysis of the Limitation of Abandoned Mines as Urban Sewage Treatment Sites

Although this study proposes an integrated utilization model of abandoned mines for urban wastewater treatment and pumped storage hydropower generation—with strong theoretical innovation and practical application potential—its applicability remains limited under varying mine conditions.

First, differences in geological conditions are a critical factor affecting the broader implementation of this approach. Some abandoned mines are located in fault-dense zones, regions with fractured rock strata, or karst-developed areas, where poor structural stability makes it difficult to meet the sealing and load-bearing requirements for long-term operation. In such areas, the use of underground space for wastewater treatment or water storage poses risks such as shaft collapse, leakage, and even the induction of geological hazards, necessitating targeted geological reinforcement and anti-seepage measures, which significantly increase renovation costs. Second, the complexity of the mine structure can hinder engineering layout and operational efficiency. Mine spaces are typically distributed across irregular and multilayered levels, with numerous abandoned roadways and shafts. Designing an optimal layout for wastewater treatment units and pumped storage equipment, along with the routing of pipelines and maintenance access, presents considerable engineering challenges. Moreover, many mines that have been sealed for extended periods may contain accumulated water or sediment, making their cleanup and reuse difficult and labor-intensive. Third, groundwater flow conditions directly affect the containment performance and environmental safety of the treatment system. In mines with strong groundwater dynamics or high aquifer connectivity, wastewater may migrate through fractures or unsealed shafts, posing risks of groundwater contamination. Applications in such regions require thorough assessment of seepage pathways, installation of multi-layer anti-seepage barriers, and enhanced long-term water quality monitoring. Lastly, from an energy system perspective, not all abandoned mines are suitable for constructing pumped storage power stations due to limitations in hydraulic head and storage capacity. Mines with shallow depths or insufficient vertical drop are economically unviable for such applications. Conversely, while deeper mines may offer the necessary elevation difference, they often demand higher capital investment and operating costs.

In summary, the geological structure, hydrogeological conditions, and spatial layout of abandoned mines directly influence the technical feasibility and economic viability of this integrated approach. Therefore, future application and promotion should be based on comprehensive mine-specific adaptability assessments, with customized engineering designs tailored to local geological conditions, to ensure safe operation and efficient resource utilization.

7. Research on the Seismic Mechanism of Underground Reservoirs in Abandoned Mines

During operation, the underground reservoir of a coal mine is constantly subjected to dynamic loads, such as mining activities near the working face, roof collapses in goaf areas, mine-induced seismic events, and regional earthquakes, all of which pose threats to the safety and stability of the reservoir’s underground roadways. Therefore, studying its seismic performance is essential to ensure the safety of coal mine underground reservoir projects.

7.1. Influencing Factors of Earthquake-Induced Underground Reservoirs in Abandoned Mines

Research into the mechanisms of reservoir-induced earthquakes in China commenced in the 1980s. It was discovered that under specific conditions, reservoirs can alter seismic activities in their vicinity. Factors such as the structural geology, hydrogeological characteristics, tectonic stress accumulation, fault distribution, pore water pressure, and the physical and chemical effects of reservoir water on rock composition are crucial considerations for earthquake occurrence [70,71,72].

Based on the aforementioned research, it is evident that the seismic activity in underground reservoirs of abandoned mines is intricately linked to water presence within the reservoir. Whether the impoundment of the reservoir can induce seismic activity hinges upon the environmental conditions present in the dam area. Current understanding suggests that these conditions can be broadly categorized into two groups: reservoir water infiltration and seismic tectonic conditions. Geologically, permeability represents a static factor predominantly determined by rock properties and their arrangement, while seismic conditions denote dynamic factors reflecting the contemporary tectonic activity of a region.

7.2. Permeation Conditions

Water in the Earth’s crust exhibits permeability and capillary effects. Reservoir impoundment penetrates the original discontinuous micro-fractures in the reservoir bedrock, altering the stress state of the rock and inducing liquid flow and fracture deformation. This process significantly enhances permeability with increasing pore water pressure. Furthermore, the change in groundwater level due to the water body’s forced load on the rock mass in the reservoir area markedly augments the elastic stress of the reservoir foundation. This not only alters the stress state of the bedrock but also elevates pore pressure, modifying the frictional resistance of fracture surfaces. Consequently, the shear strength of weak structural planes is reduced, leading to rock mass displacement—a necessary condition for reservoir-induced seismicity. Clearly, a pathway for reservoir water infiltration must exist. As fracture channels gradually develop to depth, stress corrosion and elastic deformation of the reservoir base cause local stress imbalances, resulting in microseismic activities within the reservoir area [73,74].

7.3. Conditions of Earthquake Occurrence

The power source generated by a reservoir results from the combined influence of the original stress field in the rock body before impoundment and the additional stress field post-impoundment. As reservoir water permeates the ground, it releases strain energy through weak surfaces, triggering induced seismic events. These weak surfaces, known as seismic structures or active faults, serve a dual function in reservoir-induced earthquakes: they act as pathways for reservoir water infiltration and as sites for induced seismicity. Thus, active faults constitute an indispensable environmental factor in water-induced earthquakes [75].

Figure 9 illustrates the seismic factors influencing the reservoir and their interrelations. It is evident that the occurrence of reservoir-induced earthquakes post-impoundment depends not only on reservoir-specific factors such as water depth, capacity, and layout but also on surrounding environmental variables and their collective conditions. Therefore, a thorough analysis of these factors is crucial for making informed assessments regarding the likelihood of reservoir-induced seismicity.

7.4. Analysis of the Seismic Stability of Underground Reservoirs in Abandoned Mines

The stress analysis illustrated in Figure 10 reveals the pressures acting on the roadway wall within the underground reservoir of the abandoned mine. This analysis demonstrates that the roadway wall experiences a composite influence, including overlying rock pressure, water storage pressure, lateral pressure from surrounding rock masses in the goaf, and the impact force from water body displacement due to expansive rock mass movement, seismic activity, or mine-related tremors. Therefore, ensuring the safety and stability of the roadway under such unique working conditions is imperative [76,77,78].

The mechanical model depicted in Figure 11 elucidates the displacement and progressive failure dynamics of the roadway when subjected to seismic activity.

From Figure 11, it is evident that seismic loading induces stress concentration at the reservoir bottom, as indicated by the above mechanical analysis. Seismic loading may exert significant influence on the structural stability of mine shafts, the evolution of rock mass fractures, and the operational safety of underground facilities in abandoned mines. However, due to the current lack of detailed site-specific geological structures, rock mechanical parameters, and seismic response data, the necessary conditions for conducting comprehensive quantitative seismic response simulations are not yet available. Therefore, this study primarily focuses on theoretical discussion and planning-level recommendations regarding seismic impacts, without including dynamic modeling or numerical validation processes. Future research should incorporate field-based geological survey data and apply methods such as dynamic finite element analysis and discrete element modeling to develop integrated seismic response models. These models should aim to systematically evaluate the deformation characteristics, damage mechanisms, and safety margins of retrofitted mine systems under representative seismic scenarios.

7.5. Reinforcement Scheme of Underground Reservoirs of Abandoned Mines

As early as the 1990s, feasibility studies on the use of abandoned coal mines for underground compressed air energy storage (CAES) were initiated in the Ruhr region of Germany. In recent years, China has also launched several pilot projects utilizing abandoned mine spaces for energy storage. For example, provinces such as Shanxi, Anhui, and Guizhou have explored the transformation of mined-out coal areas into pumped storage systems, achieving a dual integration of energy utilization and environmental remediation. Addressing the engineering safety of abandoned coal mines is critical for ensuring the long-term structural integrity of such energy storage systems. MASON et al. [79] emphasized that under high-water pressure conditions, the shaft walls must possess surrounding rock strength equivalent to Class II or above. To enhance load-bearing capacity, reinforcement techniques such as shotcreting, rock bolt support, and concrete lining are essential. Furthermore, Masoud, Wang, and Guo et al. [80,81,82] highlighted the importance of implementing impermeable membranes, polymer sealing agents, and multi-point stress monitoring systems to prevent leakage and mitigate rock creep deformation, thereby ensuring system integrity and controllable structural response.

When the seismic performance of the underground reservoir in an abandoned mine fails to meet safety production standards, seismic reinforcement treatment becomes necessary. Existing engineering experience suggests three main methods for reinforcing underground reservoirs: replacement, enhancement, and pressurization [83,84]. While these methods can enhance seismic resistance, they may still fall short in improving the seismic performance of abandoned mine reservoirs. Therefore, the following reinforcement schemes are proposed to further enhance the seismic resilience of such reservoirs.

(1)

When repurposing abandoned mines for hydropower stations, unused roadways must be backfilled to optimize space utilization. Typically, materials such as flocculating agents, plugging agents, clay, stone, cement, and sawdust are used for backfilling. Additionally, plastic concrete may be added to better accommodate rock mass deformation, significantly reducing wall stress and preventing cracking.

(2)

To ensure that the strength and permeability of the rock mass beneath the reservoir meet standards, anti-seepage measures are implemented on the mine roadway floor. Primarily, cement grouting is employed for the anti-seepage treatment of the reservoir foundation. In areas where erosion is detected on roadway rock walls or hollowed bottom plates, the affected sections are chiseled until fresh concrete is exposed, after which concrete soil is reapplied to enhance reservoir wall stability.

8. Conclusions

Currently, research and engineering practices related to pumped storage power generation from abandoned mines, underground reservoir treatment and construction, and mine sewage treatment in China remain in an exploratory and preliminary stage. Systematic theoretical methodologies and technical specifications have yet to be established in these areas, highlighting the need for widespread attention. Advancements in theory, technology, engineering demonstration, and promotion within these domains hold the potential to not only facilitate effective energy storage and power generation but also foster new growth opportunities within China’s renewable energy sector. This paper proposes a comprehensive construction and operation scheme for abandoned mines, leveraging the functionalities and characteristics of pumped storage power stations and sewage treatment, leading to the following conclusions and policy implications:

(1)

The underground reservoir of a pumped storage power station constitutes a vast system with multiphase and multi-physics coupling, encompassing factors such as the stability of surrounding rock, reservoir capacity, and groundwater dynamics. This study introduces an integrated approach, beginning with the design phase of three types of power stations—pumped storage, wind power, and photovoltaic—to effectively propose a combined technology involving mine sewage treatment plants and pumped storage power generation. This approach simultaneously addresses multiple objectives, including mine water storage, power generation, sewage treatment, and new energy development. The model presents favorable conditions for the establishment and utilization of multi-energy complementary systems, demonstrating feasibility in constructing pumped storage power stations and sewage treatment plants within a multi-energy complementary framework. Such initiatives contribute to subsidence control, ecological balance maintenance in mining areas, and offer far-reaching implications for modern and ecologically sustainable mining, energy efficiency, emission reduction, and green development initiatives.

(2)

The reuse of abandoned mine resources serves as the cornerstone for transforming resource-depleted regions. Simultaneously, by repurposing abandoned mines for the construction of pumped storage power stations and utilizing both above- and below-ground reservoirs for urban sewage treatment, this approach breaks through traditional construction modes and land use concepts associated with “abandoned mine construction pumped storage power stations”. This innovative model integrates photovoltaic and wind power stations with pumped storage systems, addressing not only the functional demands of sewage treatment plants but also achieving secondary development goals for abandoned mines. It offers fresh perspectives for establishing environmentally friendly, low-carbon, energy-saving, and sustainable underground sewage treatment plant models in abandoned mine areas.

(3)

Underground reservoir-induced earthquakes are contingent upon a specific combination of environmental factors, namely infiltration and seismic conditions. Furthermore, these factors are intricately linked and essential in the seismic induction process. Introducing the concept of the safety factor for coal mine reservoirs, this study theoretically analyzes the seismic safety performance of coal pillar roadways and proposes reinforcement schemes. Additionally, it underscores the need to explore the impact of various seismic wave types on the safety of underground reservoir roadways in coal mines. Moreover, the presence of sewage in the underground reservoir poses a threat to the structural integrity of coal pillar roadways, necessitating a comprehensive safety evaluation of their stability under sewage storage conditions.

(4)

Research on utilizing abandoned mines to construct underground reservoirs for pumped storage power stations is in its nascent stages, with a lack of experience in design and construction. Leveraging abandoned mines for this purpose, particularly in treating urban sewage, offers potential benefits for ecological restoration and urban sustainability. However, challenges arise due to the intricate nature of underground spaces and the complexity of urban sewage composition. Notably, hydraulic issues in underground reservoirs are prominent. It is recommended that subsequent research actively address these hydraulic challenges while exploring related fields, thereby offering scientific guidance and ensuring the safe construction of underground reservoirs for pumped storage power stations.

(5)

After a mine is abandoned, there often remain abundant usable resources, but a lack of awareness regarding reuse persists. Pumped storage power plants have proven to be sustainable, cost-effective energy storage solutions that hold great potential in advancing various renewable energy sources. Relevant departments should establish and improve abandoned mine management organizations and devise reuse plans prior to mine closure. Meanwhile, mining enterprises should effectively leverage technical reserves and strategies to facilitate the reuse and multifunctional utilization of abandoned mines, promoting environmental restoration and economic recovery in mining areas.