Wastewater Treatment Basic Terminology: What Is Reclaimed Water?
What Is Reclaimed Water?
The term "reclaimed water" originated in Japan. It has several definitions: in wastewater engineering it is called reclaimed water, while in factories it is known as reuse water, generally distinguished by water quality. It mainly refers to non-potable water produced when municipal or domestic sewage is treated to a defined water-quality standard and can be reused within a certain scope.
Water obtained after deep purification of municipal sewage — including advanced-treated secondary effluent from wastewater treatment plants, and centrally treated shower and kitchen gray water from large buildings and residential communities — is collectively called "middle water" (zhongshui). Its quality lies between tap water (upper water) and piped sewage (lower water), hence the name. Reclaimed water utilization is also referred to as wastewater reuse.
In the United States, Japan, Israel and other countries, reclaimed water is extensively used for toilet flushing, landscape and farmland irrigation, road cleaning, car washing, urban fountains and cooling-water make-up. China is a water-scarce country, but dedicated reclaimed-water projects and special funds are still lacking, with only policy guidance; the volume of reclaimed water used in each city depends on its degree of water shortage, and in recent years many experts have called for maximizing its utilization.
Shrinking Water Resources Drive Reuse
Water resources are gradually declining, and droughts have become frequent in recent years. Australia, for example, had experienced a drought lasting as long as 10 years by 2009, causing a sharp drop in domestic water availability. Wastewater reclamation is now regarded as a possible solution to severe water shortages caused by climate change. Especially after the severe drought of 2000–2003, more than 500 municipal wastewater treatment plants in Australia began water reclamation to ease the impact of government water restrictions. Israel has also implemented aquifer storage and recovery programs, storing reclaimed water over multiple years for agricultural irrigation in dry years.
Market Status in China
During the 12th Five-Year Plan period, China planned to invest 30.4 billion yuan to build 26.75 million m³/day of new reclaimed-water treatment capacity, with an investment growth rate of 198.04% and a compound annual growth rate of 24.41%, far above the industry average of 5.30% per year. The national urban reclaimed-water utilization rate was to rise from less than 10% to 15%.
Although the fields and regions of reclaimed-water use in China keep expanding, the overall utilization level remains low. According to the Urban-Rural Construction Statistical Yearbook, China's total wastewater treatment volume in 2022 was 73.8 billion m³; at an 80% utilization rate, the potential of reclaimed water would be 41.2 billion m³ — there is still enormous room for growth.
Technical Feasibility
There is no technical barrier to using reclaimed water in cities: current water-treatment technology can treat sewage to whatever quality standard is required. Impurities in municipal sewage account for less than 0.1% and can be removed by conventional advanced treatment such as media filtration, microfiltration, nanofiltration and reverse osmosis. After pretreatment, media-filtration effluent can meet domestic miscellaneous demands including toilet flushing, green-space irrigation, road washing and general industrial cooling water; microfiltration effluent can meet landscape-water requirements; and reverse-osmosis effluent quality far exceeds tap-water standards.
Economic Advantages
Municipal wastewater resourceization — secondary treatment followed by advanced treatment and reuse as a renewable resource — requires far less capital investment than long-distance water transfer. Data show that treating municipal sewage to miscellaneous-use level costs roughly the same as transferring water from 30 km away, and to high-grade process-water level, from 40–60 km away. The central route of the South-to-North Water Diversion Project transfers more than 10 billion m³ per year with a main-project investment of over 100 billion yuan, a unit investment of about 3,500–4,000 yuan per ton. Many countries therefore regard municipal reclaimed-water use as one of the options for water shortages and an important water-saving path, which is economically very attractive. In China, dozens of reclaimed-water projects have been built, including those at the China World Trade Center and the Baoding Lugang wastewater treatment plant. Practice has proved that wider adoption of wastewater treatment technology is imperative, and reclaimed water as a city's "second water source" is an inevitable development trend.
Compared with seawater desalination, reclaimed water is also much cheaper: municipal sewage contains less than 0.1% impurities that advanced treatment can remove, while seawater carries 3.5% dissolved salts plus abundant organics — more than 35 times the impurity load of secondary effluent — requiring complex pretreatment and expensive reverse-osmosis or flash-evaporation technology. Internationally, desalinated water costs between US$1.1 and US$2.5 per ton, roughly equal to the local consumer price; in China the cost has fallen to about 5 yuan per ton (possibly about 3.7 yuan for large plants) — still far above the reclaimed-water reuse price of less than 1 yuan per ton.
Development History in China
Although China began reusing sewage through farmland irrigation as early as the 1950s, real reuse of advanced-treated sewage in urban life and industry has developed only in recent decades. The Ministry of Construction first included municipal wastewater reuse in the 6th Five-Year Plan key science program, conducting pilot explorations in Dalian and Qingdao. The results showed that sewage could be reused again through relatively simple advanced treatment — a promising water source.
From 1986 onward, municipal wastewater reuse was successively listed in the national key science programs of the 7th, 8th and 9th Five-Year Plans. The 7th Plan project "Water Pollution Control and Municipal Wastewater Resourceization" systematically studied reuse processes, reuse technologies for different targets, and techno-economic policies, covering 14 wastewater plants including Qingdao Yan'an Third Road. The 8th Plan project "Wastewater Purification and Resourceization" carried out engineering trials in Dalian, Taiyuan, Tianjin, Tai'an and Yanshan Petrochemical, producing technical specifications and water-quality standards for reuse in industrial processes, cooling, chemical, petrochemical, steel and municipal landscape applications at a practical, production-verified level. The 9th Plan project addressed integration of reuse technologies and advanced treatment for groundwater recharge. At the "International Symposium on Urban Wastewater Treatment and Resourceization Strategy in the 21st Century," the Ministry of Construction stated that China would fully launch wastewater resourceization projects, strengthen international technical cooperation, and welcome foreign financial institutions and enterprises to invest in China's municipal wastewater resourceization projects — signaling rapidly growing investment and demand.
Industrial Cooling: The Most Representative Use
Secondary effluent from wastewater plants can be applied directly, or after further treatment to higher quality, in industrial processes — the most common and representative use being industrial cooling water. China has conducted extensive trials using secondary or advanced secondary effluent as cooling water, with successful operating cases: the secondary effluent of Beijing Gaobeidian wastewater treatment plant supplies 40,000 tons per day as cooling-water source for a Huaneng thermal power plant, and the plant also supplies Sanhe thermal power plant and other industrial enterprises.
Agricultural Irrigation
In northern Chinese cities, municipal sewage and industrial wastewater have become one of the main irrigation sources for suburban farmland, bringing economic benefits: improved soil structure, more water and nutrients, and higher crop yields — each cubic meter of domestic sewage can increase wheat or rice output by about 0.5 kg. However, irrigation with toxic industrial wastewater has degraded some farmland, reduced yields and contaminated groundwater, soil and produce. The area irrigated with reclaimed water is increasing year by year: 300,000 mu of reclaimed-water irrigation zones have formed in Daxing, Tongzhou and other districts, and the city's annual agricultural use of reclaimed water has reached 230 million m³. At the end of 2006, when the outlet gates of the Xiaohongmen wastewater treatment plant opened, clear reclaimed water flowed into the Liangfeng irrigation canal, bringing reclaimed water to 200,000 mu of farmland in eight towns including Qingyundian, Zhangziying and Caiyu in Daxing District. Replacing fresh water with reclaimed water for irrigation saves 60 million m³ of groundwater extraction every year.
Reclaimed water has now become the city's second-largest water source, widely used for industry, agricultural irrigation, urban greening and toilet flushing. In the past five years, wastewater and reclaimed-water plants such as Lugouqiao, Wujiaicun and Shahe have been built and reclaimed-water pipe networks vigorously extended: daily wastewater treatment capacity rose from 3.29 million m³ in 2008 to 3.95 million m³, the treatment rate from 79% to 83%, and annual reclaimed-water use from 600 million to 750 million m³ — a utilization rate of 61%, both nationally leading.
Health-Risk-Based Quality Standards
Reclaimed-water standards are mostly derived by a low-technology/low-cost risk-control method: based on epidemiological research combined with the pathogen-removal performance of existing wastewater treatment processes, the health risk of reuse is analyzed. National reclaimed-water quality standards such as the US reuse guidelines are formulated this way, relying on reuse experience and confidence in pathogen removal. Typical requirements include:
Fecal coliform not exceeding 14 MPN/100 mL in any sample — meaning no detectable fecal coliform in practice — with coagulation, sedimentation, filtration and disinfection after secondary treatment.
Fecal coliform not exceeding 2.2 MPN/100 mL (no more than one sample per month may exceed 23 MPN/100 mL), with filtration and disinfection after secondary treatment.
Over a 30-day period, fecal coliform not exceeding 25 MPN/100 mL in 75% of samples; secondary treatment plus filtration and deep disinfection; COD 20 mg/L (annual average), TSS 5 mg/L (single sample).
Total coliform not exceeding 2.2 MPN/100 mL (median); coagulation, sedimentation, filtration and disinfection required after secondary treatment.
Total coliform not exceeding 2.2 MPN/100 mL (median) and 23 MPN/100 mL (single sample); coagulation, sedimentation, filtration and disinfection after secondary treatment.
Fecal coliform not exceeding 50 MPN/100 mL in 80% of monthly samples, with a maximum allowable value of 100 MPN/100 mL; intestinal nematode eggs not exceeding 1 per liter; tertiary treatment followed by disinfection.
Total coliform not exceeding 2.2 MPN/100 mL in 50% of samples and 12 MPN/100 mL in 80% of samples; secondary treatment or equivalent (e.g., long-term storage) followed by disinfection.
International Practice
In Florida, given its concentrated urban water use, the basic model is non-potable reuse: large-scale dual-distribution systems supply reclaimed water to golf courses, urban greening and residential districts at about 40% of the tap-water price. Texas, following its own water traditions and hydrogeology, adopts an "indirect reuse" model with large-scale aquifer recharge of treated wastewater.
Japan started reclaimed-water use as early as 1955. Dual-pipe systems are common in large Japanese cities — one drinking-water system and one reclaimed-water system ("reclaimed water loop"), the latter famous for delivering reclaimed water for domestic miscellaneous uses, accounting for about 40% of reuse volume. Japanese reclaimed water is mainly used for urban miscellaneous purposes, industry and agricultural irrigation under very strict management. Recharge is mainly through river channel replenishment of groundwater, and a subsurface capillary infiltration system has recently been developed for groundwater recharge. Most regions use treated wastewater for "clear stream restoration," making water-environment restoration and protection the focus of reuse.
Latest National Data
In 2024, China's total unconventional water use reached 25.16 billion m³, an increase of 3.93 billion m³ over 2023, of which reclaimed water accounted for 21.22 billion m³, up 3.46 billion m³ year on year.
Further development of reclaimed-water technology and expansion of urban reclaimed-water use are essential requirements for solving water shortages and controlling water pollution, and the foundation of a circular economy. Reclaimed-water treatment and application is a large, complex, systematic and long-term undertaking that requires coordination of institutions, legislation, administration, management, education, publicity, technology and finance. With processes advancing and equipment and materials constantly innovating, the safety and reliability of reclaimed-water supply will keep improving while treatment costs continue to fall — a broad prospect for this "second water source."
