Reuse: Water Supplies
Content Table
Defacto Water Supply Use
A basic tenet of public water supply practice is that water supplies should be drawn from the best available source.
Some cities are fortunate enough to draw their water supplies from protected catchments from which human activities have been excluded. The vast majority of cities, however, have to rely on open catchments which contain urban populations and/or extensive farming and grazing activities. As a result, most urban water supplies require water treatment plants which include water filtration and disinfection processes to ensure that the supply conforms to drinking water guidelines.
A 1980 study by the US Environmental Protection Agency, Wastewater in Receiving Waters at Water Supply Abstraction Points, examined how much recycled water is present in surface water supplies. The study included 525 cities larger than 25,000 people. Of 80 million people served by these supplies, about one-third were supplied from sources that contain from 5% to 100% recycled water during low flow periods. Similar examples of water supplies containing a significant proportion of recycled water can be found in most countries.
This defacto use of recycled water in water supplies generally occurs without problems and without public concern. Defacto water supply use is sometimes called unplanned reuse.
Planning to augment water supplies with recycled water entails no greater risk than the existing defacto occurrences. Planned augmentation provides an opportunity to design the treatment processes and risk management systems to ensure high standards of water quality for consumers.
Recharging of Groundwater
Planned recharge of groundwater aquifers with recycled water is becoming increasingly common . The practice is sometimes called aquifer storage and recovery or managed aquifer recharge.
Aquifer recharge can take place by
- Surface Spreading: Recycled water is placed in spreading basins and percolates down through the overlying soil into the aquifer. Percolation through the soil acts as a natural filter. Some nutrient removal may also be obtained by management of the spreading process
- Direct Injection: Recycled water is pumped directly into the aquifer. Direct injection is being used increasingly to prevent seawater intrusion into coastal aquifers.
Los Angeles has been recharging its aquifers by surface spreading at the Montebello Forebay project since 1962. It is estimated that recycled water recharge provides 20% to 30% of the water supplies drawn from the aquifers. The recycled water is filtered, clarified and chlorinated before spreading.
Orange County in California has been directly injecting recycled water into coastal aquifers as a seawater barrier since 1976. Orange County has recently installed new dual membrane treatment units to expand the capacity of the plant.
Augmenting Rivers and Water Reservoirs
The use of recycled water to augment river flows and reservoirs makes downstream water supplies more reliable during droughts. Also, recycled water flows generally improve in-stream environmental conditions in low flow periods.
Flows into Lake Tegel in Berlin have been supplemented by recycled water discharges since 1948. Water supply is drawn from Lake Tegel through infiltration wells in the bank of the Lake.
Since 1978, recycled water from the treatment plants in the Upper Occaquan district have been used to augment flows into the Occaquan Reservoir which supplies about 1 million people in North Virginia. The recycled water is normally about 15% of inflows but may be up to 90% of inflows during drought periods. The Upper Occaquan treatment process includes lime clarification, filtration, activated carbon filters, ion-exchange and disinfection.
Singapore has commenced augmenting its surface water reservoirs with recycled water (NEWater). The NEWater treatment plants use dual membrane processes to produce water purer than drinking water. The NEWater treatment plants can now supply 30% of Singapore’s freshwater needs.
Augmenting Water Supply Systems
Direct use of recycled water to augment urban water supplies entails pumping water into the distribution system or blending with the conventional supply at the water treatment plant. The recycled water needs to be treated to drinking water standards or better, including advanced treatment processes to remove trace organics and chemicals.
The City of Windhoek in Namibia has employed direct use of recycled water to supplement its limited surface and groundwater supplies since 1968. The City added new advanced treatment process units to the treatment plant in 2002 to increase capacity to 21,000m3/day. The expanded plant supplies up to 35% of the drinking water supply in normal periods and up to 50% in dry periods.
Related Articles
Augmenting Rivers and Reservoirs
Augmenting Water Supply Systems
Recent Papers on Augmenting Water Supplies
References
Asano, T. et al (2007) Water reuse: issues, technologies and applications, McGraw-Hill New York, ISBN-13: 978-0-07-145927-3, Part 4: Water Reuse Applications,
- Chapter 22: Groundwater Recharge,
- Chapter 23: Surface Water Augmentation,
- Chapter 24: Direct Reuse.
