Basic Wastewater Terms: What Is Wastewater Reuse?
Although there are many wastewater treatment technologies, their basic principles mainly include separation, transformation, and utilization. [1] Separation refers to using various technical methods to separate suspended solids or colloidal particles from wastewater so that it is purified, or to reduce pollutants to a minimum. Transformation refers to pollutants that are dissolved in water and cannot or need not be 'taken out'; [2] biochemical, chemical, or electrochemical methods are used to convert dissolved pollutants into harmless substances or into substances easy to separate. In short, wastewater treatment should develop pollutants in a direction favorable to control.
Agricultural water use is a major field of wastewater reuse, mainly for crop irrigation, whose quality must meet the 'Quality Standard for Agricultural Irrigation Water' [1-3] [6]. In recent years, various localities have actively explored safe and efficient reclaimed-water agricultural irrigation models. For example, Zhentou Village, Sandu Town, Jiande City, Zhejiang Province, innovatively implemented a 'graded treatment, graded utilization' renovation model for rural domestic wastewater, achieving over 80% resource-utilization rate; through three-stage reuse ('front, middle, end'), treated wastewater is used for farmland irrigation, orchard irrigation, and greenbelt sprinkler irrigation, providing over 10,000 tons of water-fertilizer resources annually, saving about 1.5 tons of fertilizer and reducing electricity costs by more than 6,000 yuan [16] [40]. Ningxia Hui Autonomous Region also explored using reclaimed water for agricultural irrigation through a special implementation plan, to ease water shortages [13].
Environmental water use mainly includes river/lake replenishment and greenbelt irrigation [3]; ecological replenishment has become an important direction of reclaimed-water utilization. For example, in 2024 Beijing's river/lake replenishment used 1.182 billion m³ of reclaimed water, 70.5% of the city's total river/lake replenishment [33] [39]. The treated reclaimed water from Chengdu No. 9 Reclaimed Water Plant is mainly used to supplement ecological base flow to rivers, lakes, and wetlands such as Qingshui River and Xiaojia River Wetland; in 2024 Chengdu's reclaimed-water utilization scale reached 1.25 million tons/day [21]. The treated reclaimed water from Jinan Cuizhai Wastewater Treatment Plant (also called Capital Water) is also widely used for river replenishment, providing solid support for ecological replenishment in dry seasons [7] [17].
According to the national standard GB/T 18919-202x 'Reuse of Urban Wastewater — Classification,' industrial water use mainly includes cooling water, washing water, boiler and heating-network make-up water, process water, product water, etc. [15]. In severely water-short areas, reclaimed water has become the 'second water source' or even the 'first water source' for industrial production [24]. For example, Shanxi Province explicitly designated reclaimed water as the 'first water source' for the province's industrial production, requiring high-water-consumption industries to give priority to it. In 2024, Beijing's industrial reclaimed-water use reached 93 million m³, over 30% of the city's industrial water use. Huaibei City, Anhui Province, incorporated reclaimed water into unified water-resource allocation, building a full-chain 'production–distribution–application–management' system; in 2024 the city's reclaimed-water utilization rate reached 45%, with industrial reclaimed-water use of 34.46 million m³, 32% of total industrial water [22] [37]; Datang Huaibei Power Plant has cumulatively used 180 million m³ of reclaimed water since 2013. Keqiao District, Shaoxing, Zhejiang, invested 380 million yuan in a 150,000-ton/day reclaimed-water project; since commissioning in 2023 it has supplied 1.26 billion tons of water to over 270 enterprises in the Binhai Industrial Park, pushing the district's reclaimed-water utilization rate above 28% and saving enterprises about 25 million yuan in production costs annually [34]. Wuhai Thermal Power Plant in Inner Mongolia saves about 4.8 million m³ of groundwater per year after using reclaimed water [31].
In high-end manufacturing, semiconductor production is the core application scenario for ultrapure water. For example, at SEMICON China 2026, related ultrapure-water preparation and recycling technologies were showcased, with whole-plant water recovery rates up to 90%, contributing to water conservation and sustainable development in the semiconductor industry [47].
Municipal miscellaneous water use includes construction, road sprinkling, car washing, and toilet flushing [3]. In large infrastructure construction, wastewater reuse technology is also applied to protect the ecological environment. For example, in the construction of the Tianshan Shengli Tunnel — the world's longest highway tunnel, opened at the end of 2025 — low-disturbance construction methods and wastewater reuse technology were adopted to maximally protect the Tianshan ecological sensitive zone [46]. In 2024, Beijing had replaced 3 million m² of landscaping water with reclaimed water. Reclaimed water is also used for municipal heating and cooling; for instance, Anyang City, Henan, uses reclaimed-water source heat pumps in some communities for winter heating and summer cooling, and Qingdao's Shibei District uses reclaimed-water source heat pumps for heating/cooling some communities, having used about 3.254 million tons of reclaimed water by the end of 2023 [23].
Wastewater reuse yields significant comprehensive benefits [41]. Economically, it directly reduces water costs for enterprises and municipalities [10] — e.g., the Keqiao reclaimed-water project saves enterprises 25 million yuan/year [34], Wuhai Thermal Power saves annual water fees [31], and Suining, Sichuan, saves about 1 million yuan/year in water costs via reclaimed-water reuse [10]. Environmentally, it greatly reduces freshwater extraction and wastewater discharge and improves the water environment [41] — e.g., Feicheng, Shandong, through its 'wastewater treatment + wetland purification + reclaimed-water recycling' system, will reduce groundwater extraction by 21 million tons/year at full operation [8], and the Keqiao project has cumulatively cut COD discharge by over 2,300 tons and total nitrogen by over 1,100 tons [34]. Socially, it helps raise public water-conservation awareness, build a water recycling system, and support sustainable socioeconomic development [13] [18]; at the national level, a clear target has been set: by 2025, reclaimed-water utilization in water-short cities at prefecture level and above should exceed 25%, and in the Beijing–Tianjin–Hebei region exceed 35%, to foster a systematic, safe, eco-friendly, and economical wastewater-resource utilization pattern [18-20].
Wastewater-reuse treatment technologies and processes, based on secondary treatment, mainly achieve water-quality improvement and safe reuse through advanced combined processes, membrane separation, ecological purification, and smart management [1-3] [6].
Common advanced-treatment combinations include the 'pretreatment + UF + RO' dual-membrane method, e.g., the Dongying Chemical Park reclaimed-water project [45]; and the 'A²O integrated oxidation ditch + wetland purification + UF/RO' model, e.g., the Shandong Feicheng model [8]. In addition, conventional advanced treatment represented by 'mechanical mixing — small-grid reaction tank — inclined-plate settler — Type-D filter — chlorination disinfection' is also widely used, e.g., at Ningbo Beilun Yandong WWTP [5] [12].
Membrane separation is the core of advanced treatment; ultrafiltration (UF) separates particles and soluble matter, reverse osmosis (RO) performs deep desalination [10]. The MBR process integrates biological treatment with membrane separation for efficient solid–liquid separation, e.g., at Jinan Cuizhai WWTP and Beijing Huaifang Reclaimed Water Plant [7] [17].
Ecological purification uses constructed wetlands and other eco-engineering to further purify compliant effluent — e.g., Feicheng's Kanghui River Wetland and Anyang's tail-water wetland park — where aquatic plants and microbes synergistically adsorb and degrade residual pollutants [8] [27].
In industry, through electrochemically enhanced descaling, membrane filtration concentration, and collaborative wastewater treatment (e.g., using brewery wastewater as a biomass carbon source for WWTPs), graded and cascaded reuse of wastewater is achieved within enterprises or parks.
On this basis, industry experts have conducted extensive research and improvement.
Shaoxing's Keqiao District built an integrated 'industrial clustering + wastewater treatment + reuse' reclaimed-water system [42]. In 2022 it invested 380 million yuan to launch a 150,000-ton/day project, blending treated domestic wastewater with river-network water as raw water for an industrial water plant. From commissioning in March 2023 to 2025, it cumulatively used 1.26 billion tons of reclaimed water, serving over 270 enterprises in the Binhai Industrial Park and pushing the district's reuse rate above 28% [34].
Wuhai City, Inner Mongolia, reached a 94% reclaimed-water utilization rate in 2023, collecting and using nearly 25 million m³. It built a point-to-point reclaimed-water recycling system from WWTPs directly to enterprises. Most reclaimed water from the city's 2 municipal and 4 industrial-park WWTPs is used for industrial production, achieving 100% use of reclaimed water for circulating cooling in thermal power plants [31].
Beyond this, multiple factors must be considered in practical engineering applications.
(2) Develop efficient and low-cost reuse treatment technologies. Sound reuse treatment technology is the guarantee for further development of wastewater reuse; common reuse treatment methods include:
Also, from an industry-development perspective, market demand is driving technological progress.
However, further promotion of wastewater reuse still faces multiple challenges: first, high technology cost, complex processes, and large capital needs for construction and operation [10] [24]; second, insufficient public awareness and acceptance of reclaimed water; third, policy support and standards systems need improvement, and market mechanisms and incentive policies are not yet sound [10] [24] [29]; fourth, supporting infrastructure (e.g., reclaimed-water pipelines) is difficult and costly to build, especially in built-up areas [21] [24].
Looking ahead, wastewater reuse will show the following trends: first, toward refinement, intensification, and digital-intelligence, using big data and AI to build smart management platforms that improve water-use efficiency and management [29]; second, continuous expansion of application fields, deepening from traditional industrial cooling, municipal miscellaneous use, and ecological replenishment into agricultural irrigation and regional recycling systems [27] [30]; third, greater emphasis on systematic planning and full-chain management, fostering a safe, systematic, and economical resource-utilization pattern of 'sewage collection — purification — ecological storage — distribution and use' [27].
Today, people recognize the significance of wastewater as a water source. In fact, a reuse system simulates nature's water cycle through engineering. Against this backdrop, reclaimed-water utilization is seen as a key measure to ease water supply–demand conflicts, safeguard aquatic-ecological security, and implement an all-round conservation strategy — the city's 'second water source' [19-20] [32]. Meanwhile, the naming shift from 'sewage treatment plant' to 'water purification plant' to 'reclaimed water plant' profoundly reflects the conceptual shift from end-of-pipe treatment to resource recycling [21].
Also, from an industry-development perspective, market demand is driving technological progress.
Sewage irrigation is risky. Due to inadequate treatment or underestimation of long-term irrigation risk, China has many lessons from sewage irrigation — e.g., after over 20 years of sewage irrigation in Shenyang's Zhangshi irrigation district, 2,500 hm² of farmland was polluted with severe cadmium contamination (5–7 mg/kg in paddy soil); near-suburban Tianjin saw 23,000 hm² of farmland polluted by sewage irrigation; near-suburban Guangzhou polluted 2,700 hm² of farmland by sewage irrigation and about 13,333 hm² by applying contaminated sludge, 46% of cultivated land; a mid-1980s survey of a Beijing sewage-irrigation district showed about 60% of soil and 36% of brown rice were polluted.
According to surveys, Beijing's industrial water accounts for about 25% of total city water use, with great potential for conservation. Facing acute freshwater shortages and rising water prices, besides recycling their own wastewater to raise reuse rates, industrial enterprises increasingly value municipal wastewater reuse. Industrial water-quality requirements vary greatly by use; the higher the requirement, the higher the treatment cost. Ideal reuse targets are cooling water and low-quality process water (washing, ash sluicing, dust removal, direct cooling, etc.). When considering whether a process can use reclaimed sewage, the required water quality must be met and the costs of reuse and treatment calculated for maximum economic benefit.
Mainly used for construction, road sprinkling, car washing, and toilet flushing. It is estimated that if all 2 million-plus vehicles in Beijing used reclaimed water for washing, it would save nearly the monthly domestic water of 13,000 households every day. Reclaimed-water reuse must pay special attention to hygiene to avoid harming consumers' health. Also, reclaimed water should contain no pathogenic bacteria, be clean, odorless, and non-toxic, and meet suspended-solids requirements for its use.
In recent decades, continuous drought caused over-extraction of groundwater, forming a subsidence funnel over 2,500 km² in Beijing, seriously affecting surface ecosystems and aquifer safety. Injecting secondary-treated municipal wastewater underground develops a new water source together with the original groundwater after flowing a certain distance. This both prevents land subsidence from over-extraction and uses soil self-purification to improve return-water quality, supplying industry and domestic miscellaneous use directly. Aquifer recharge demands very high water quality; before recharge it must undergo biological treatment (including nitrification and denitrification) and effectively remove toxic organics and heavy metals; if recharge water fails to meet requirements, the aquifer will be polluted.
Meanwhile, the related supporting processes and equipment are also continuously optimized and upgraded.
The MBFB (Membrane Biological Fluidized Bed) process for wastewater reuse, on top of compliant discharge, further reduces COD, NH₃-N, turbidity, etc., via a biological fluidized bed and ceramic-membrane separation system. It can be directly reused, or serve as pretreatment for RO desalination, replacing the lengthy sand filtration, security filtration, and UF train; lower organics greatly extend RO membrane life and cut reuse cost. The inorganic ceramic membrane separation system is the world's first inorganic membrane separation system dedicated to wastewater treatment, and compared with other organic and inorganic membranes it offers high flux, backwashability, and fully automatic operation.
In the MBFB reactor, powdered activated carbon (PAC), having adsorbed large numbers of microbes, becomes biological activated carbon (BAC), so PAC not only adsorbs and enriches small-molecule organic pollutants but also adsorbs and protects microbes, adsorbs dissolved oxygen, enables microbial decomposition of small organics under locally high pollutant and DO concentrations, and undergoes biological regeneration. Under intense fluidization, mixing, mass transfer, and shear, PAC, microbes, DO, and pollutants efficiently decompose micro-polluted small organics.
(1) PAC's adsorption and enrichment of small organics: PAC enriches pollutants into local high-concentration zones, favoring microbial growth and decomposition of micro-polluted small organics;
Also, from an industry-development perspective, market demand is driving technological progress.
(4) Microbial decomposition of small organics: the MBFB process uses PAC to adsorb and enrich microbes, pollutants, and DO; through PAC's protection of microbes, microbes efficiently use trace organics as substrate and DO as electron acceptor to decompose organics in micro-polluted water, achieving deep purification;
Notably, the technologies and standards in this field are also continuously developing and improving.
The Di'er DECLEAN inorganic ceramic membrane system developed by Seattle Environmental Technology (USA), based on ordinary ceramic-membrane research and upgraded with high-tech, reduces membrane fouling and greatly raises flux, effectively overcoming the two biggest barriers to inorganic ceramic membranes in water treatment (high cost, low flux) and making their application possible.
