Every liter of water that is reused reduces the amount of water drawn from surface water or groundwater supplies. When communities practice water recycling, they substitute engineered treatment, or purification, of water for the processes that occur naturally during the hydrologic cycle. Treated wastewater can be used for a variety of purposes, including industrial processes, irrigation, recharging groundwater supplies, restoring wetland ecosystems, or even drinking water. In general, the process of treating wastewater to make it safe for reuse is called water reclamation. The more rigorous the treatment, the wider range of uses to which recycled water can be put (Figure 6.25).

Santa Rosa, California, is a city of 170,000, about an hour’s drive north of San Francisco and 50 kilometers (31 miles) inland from the Pacific Ocean. The heart of the Sonoma Wine Country, the region around Santa Rosa has a Mediterranean climate, with cool rainy winters and warm dry summers. During the dry summers in the area, the flows of rivers and streams around Santa Rosa drop to very low levels. Because environmental regulations do not permit Santa Rosa to discharge its wastewater into rivers at a volume of more than 1% of river flow, the city had to find another way to handle its wastewater in summer: recycling (Figure 6.26).

The Santa Rosa water reclamation facility provides tertiary treatment of its wastewater, including disinfecting the water by exposing it to ultraviolet light to kill any pathogens that it may contain. This reclaimed water, which is used for a variety of purposes, meets rigorous standards set by the California Department of Health for full contact with the human body, irrigation of food crops and landscaping, and consumption by livestock. Some of the water is used to irrigate 2,590 hectares (6,400 acres) of land, including pastures, vineyards, vegetable fields, and urban landscapes. During the moist winter months when irrigation is unnecessary, Santa Rosa discharges reclaimed water into a tributary to the nearby Russian River and into storage reservoirs.

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In 2003 Santa Rosa began pumping water through a pipeline to the Geysers steam field for injection into steam wells to generate electricity from geothermal energy (see Figure 10.17, page 306). In 2008 nearly 50,000 m³ of recycled water was delivered each day to the Geysers steam field, enough to generate sufficient electricity to supply 100,000 households. The water recycling program in Santa Rosa is one of many in California, where the use of recycled water has grown rapidly (Figure 6.27).

The country of Namibia occupies some of the driest regions of southwest Africa. Most of its territory lies within the Namib Desert, one of the most spectacular deserts on Earth (Figure 6.28). Because of its arid climate and perennial water shortages, Namibia has had to be innovative in its use of its limited freshwater supplies. The capital, Windhoek, was founded on a natural spring, but it is a rapidly growing city whose size has increased five-fold from just over 60,000 to 300,000 between 1970 and 2008 (Figure 6.29). Windhoek obtains its water primarily from local aquifers and reservoirs that trap the occasional flows of nearby intermittent rivers, which flow only during heavy rainstorms.

When local water supplies started to dry up in the 1960s, Windhoek began to work seriously on plans for the sustainable management of its local water. The nearest abundant source of freshwater is the Okavango River, 650 kilometers (400 miles) north of the city. As an alternative to expensive, long-distance water diversion, Windhoek turned to water reclamation and recycling to help alleviate its water shortage. The first water reclamation plant began producing clean, recycled water in 1968. The capacity of that first reclamation plant was 1.7 billion liters of water per year. Upgraded in 2002, the plant can now produce 7.7 billion liters of reclaimed water per year. Like the city of Santa Rosa, Windhoek puts its reclaimed water through a system of tertiary treatment, including disinfection, and tests it to make sure it is free of disease-causing bacteria, viruses, and toxic chemicals. But Windhoek goes beyond Santa Rosa in using reclaimed water as drinking water. During drought periods driven by periodic El Niños, reclaimed water can supply up to 30% of domestic water needs.

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Drinking treated wastewater sounds unappealing, but treated wastewater has long been released into rivers by one upstream city only to enter the drinking water supply of the next city downstream. For instance, wastewater from Dallas and Fort Worth, Texas, ends up in the Trinity River, which feeds Houston’s water supplies.

Worldwide, industrial water use exceeds that of municipalities (see Figure 6.8, page 164), suggesting significant opportunities for water reclamation and reuse by industries. In fact, many industries are doing just that, including the paper, textile, and food and beverage industries. Their primary goal is to save money, but an important by-product is reduced pressure on water resources. For instance, the Bear Republic Brewing Company, a beer brewery in water-stressed California, recently partnered with Cambrian Innovation, a bioenergy and water provider. Breweries use large volumes of water to produce beer and generate substantial amounts of solid and liquid waste. Traditionally, breweries separated their solid wastes, consisting mainly of spent grains, from liquid waste. They give the spent grains to farmers to feed their livestock, thus avoiding waste disposal fees.

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The liquid waste, however, is less desirable. It contains large concentrations of dissolved organic compounds, which is costly to treat on site or to dispose of through a waste treatment facility. Now, Cambrian Innovations has a method to convert this liquid into two components: methane gas and clean water. The gas can be burned for electricity. The water can be used to clean brewery equipment. The result is a savings of water, energy, and money. Similar opportunities for water reclamation and reuse are being exploited in many other industries.