Reclaimed Water as a City's Second Water Source: Quality Rules, Costs and Real Projects

Every city pipes water in and pipes sewage out, yet only one of those streams gets counted as a resource. Reclaimed water — the cleaned-up version of municipal or domestic sewage that meets a defined quality standard and can be reused within a defined scope — is gradually changing that accounting. The term itself traces back to Japan, where engineers distinguish reclaimed water in the wastewater-engineering sense from factory reuse water, usually by water quality. Purified secondary effluent from treatment plants, plus centrally treated shower and kitchen gray water from large buildings, is often called "middle water": its quality sits between tap water on top and piped sewage below, which is exactly where the name comes from.

Where the water actually goes

In the United States, Japan, Israel and other water-stressed economies, reclaimed water already handles unglamorous but essential jobs: toilet flushing, landscape and farmland irrigation, road cleaning, car washing, urban fountains and cooling-water make-up. China qualifies as a water-scarce country, yet dedicated reclaimed-water projects and special funding channels remain thin — most cities still rely on policy guidance alone, and the volume actually reused depends on how desperate each city's water balance is. Many experts have been pushing for years to maximize utilization before scarcity does it for them.

Drought has a way of accelerating the timetable. Australia lived through a dry spell that had lasted 10 years by 2009, and after the severe 2000-2003 drought more than 500 municipal wastewater treatment plants there began water reclamation to blunt government restrictions. Israel went further, running aquifer storage and recovery programs that bank reclaimed water across wet years and withdraw it for irrigation in dry ones.

The economics are hard to argue with

Municipal sewage carries impurities amounting to less than 0.1% of its volume, so advanced treatment — media filtration, microfiltration, nanofiltration, reverse osmosis — can hit whatever standard a reuse application demands. Media-filtration effluent covers toilet flushing, green-space irrigation, road washing and general cooling duty; microfiltration meets landscape-water requirements; reverse-osmosis product water outperforms tap-water standards.

That ease of treatment translates directly into money. Bringing sewage to miscellaneous-use level costs roughly the same as piping natural water from 30 km away; high-grade process water compares with a transfer from 40-60 km. Contrast that with the South-to-North Water Diversion central route, which moves more than 10 billion m3 per year at a unit investment of roughly 3,500-4,000 yuan per ton. Seawater desalination looks worse still: seawater carries 3.5% dissolved salts plus abundant organics — over 35 times the impurity load of secondary effluent — and international desalinated water runs US$1.1-2.5 per ton, while Chinese plants have pushed costs down to about 5 yuan per ton (possibly 3.7 yuan for large facilities). Reclaimed water trades for less than 1 yuan per ton.

How China built the sector

Farm-land irrigation experiments date to the 1950s, but genuine urban reuse is a younger story. The Ministry of Construction put municipal wastewater reuse into the 6th Five-Year Plan key science program, piloting in Dalian and Qingdao, and simple advanced treatment proved enough to make sewage a promising secondary source. The 7th, 8th and 9th Five-Year Plans kept funding it: the 7th Plan project studied reuse processes and techno-economic policy across 14 wastewater plants including Qingdao Yan'an Third Road; the 8th Plan ran engineering trials in Dalian, Taiyuan, Tianjin, Tai'an and Yanshan Petrochemical that produced production-verified specifications for industrial, cooling, chemical, petrochemical, steel and landscape reuse; the 9th Plan tackled groundwater recharge integration. By the time the Ministry spoke at the International Symposium on Urban Wastewater Treatment and Resourceization Strategy in the 21st Century, the policy signal was explicit — foreign capital was welcome.

Spending followed. During the 12th Five-Year Plan period China planned 30.4 billion yuan of investment to add 26.75 million m3/day of reclaimed-water capacity, an investment growth rate of 198.04% and a compound annual growth rate of 24.41% against an industry average of 5.30% per year, with the national urban utilization rate targeted to climb from under 10% to 15%. Total wastewater treatment volume reached 73.8 billion m3 in 2022, so at an 80% utilization rate the theoretical reclaimed-water potential would be 41.2 billion m3. In 2024 the country's total unconventional water use hit 25.16 billion m3 — up 3.93 billion m3 over 2023 — of which reclaimed water accounted for 21.22 billion m3, up 3.46 billion m3 year on year.

Cooling towers and crop fields: the two flagship uses

Industrial cooling is the most representative application. Beijing Gaobeidian wastewater treatment plant delivers 40,000 tons of secondary effluent per day as cooling-water source for a Huaneng thermal power plant, and also supplies Sanhe thermal power plant and other industrial users. Agriculture is the other big taker. In northern cities, each cubic meter of domestic sewage applied to farmland can lift wheat or rice output by about 0.5 kg, though toxic industrial wastewater has degraded some fields and contaminated groundwater. Beijing alone has formed 300,000 mu of reclaimed-water irrigation zones in Daxing and Tongzhou, uses 230 million m3 of reclaimed water per year in agriculture, and saves 60 million m3 of groundwater extraction annually by substituting reclaimed for fresh irrigation water. With daily treatment capacity up 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 61% utilization rate — reclaimed water has become the city's second-largest water source.

Standards built on health risk, not on technology

Reuse quality standards are mostly derived by a low-technology, low-cost risk-control method: epidemiological research combined with measured pathogen-removal performance across existing treatment trains. Typical requirements read like this: fecal coliform not exceeding 14 MPN/100 mL in any sample, with coagulation, sedimentation, filtration and disinfection after secondary treatment; a stricter 2.2 MPN/100 mL limit with no more than one sample per month above 23 MPN/100 mL; or, over a 30-day window, fecal coliform not exceeding 25 MPN/100 mL in 75% of samples plus COD 20 mg/L on annual average and TSS 5 mg/L in any single sample, backed by filtration and deep disinfection. Variants set total coliform medians of 2.2 MPN/100 mL with different single-sample ceilings, intestinal nematode eggs below 1 per liter, or secondary-treatment-equivalent trains with long-term storage.

What the leaders do differently

Florida, with concentrated urban demand, runs large dual-distribution systems that deliver reclaimed water to golf courses, greening and residential districts at about 40% of the tap-water price. Texas prefers indirect reuse through large-scale aquifer recharge. Japan started in 1955 and builds dual-pipe city centers — one drinking loop, one reclaimed-water loop — with domestic miscellaneous uses absorbing about 40% of reuse volume under very strict management, while most regions channel treated wastewater into clear-stream environmental restoration.

The remaining bottleneck in China is institutional rather than technical: financing, legislation, metering and public confidence all move slower than the treatment technology does. But with processes maturing and equipment costs falling, the second water source keeps getting cheaper to tap — and the cities that build the pipe networks early will be the ones that never run dry.