Water Reuse / Greywater

Water reuse can include reusing water from many sources. Sometimes it is used water from bathing or dishwashing, and other times it is managing agricultural runoff. Reusing waste water is also very popular worldwide. Other names include “water recycling,” “water purification,” “reclaimed water,” “recycled water,” “reuse water,” and “repurified water.”

Water reuse is a well-known concept and part of the 3R process (recharge, retention, reuse). In this way, managing water well is thought of as a "buffer" to insure consistent supplies - A buffer between the user and the bad effects of drought or an unreliable water resource.

Today, reclaimed water applications range from pasture irrigation, where minimal treatment is acceptable, to potable reuse, where extensive treatment is needed to meet drinking water standards. Agricultural irrigation continues to be the major use of reclaimed water in most developing countries.

Three processes are important in managing reuse. The first is management of (non-beneficial) evaporation. Water that evaporates ‘leaves’ the system and can no longer circulate within it. This is an important concept. In some areas, for instance, ‘efficient’ irrigation reduces reusable recharge and results in the evaporation of a higher percentage of the water. This makes less water available for reuse and may jeopardize the water balance. One source of evaporation is from the soil – particularly from depressions and moist stretches. There is a fine balance between keeping good soil moisture (which is also achieved by agronomic practices, shade trees and the like) and avoiding evaporation losses from the soil. In fact, in some areas a reduction in groundwater table (from very high to moderate) reduces such non-beneficial evaporation.

The second process in managing reuse is managing water quality. The possibility for reuse depends on the quality of the water, with different functions putting different demands on the water quality. Water quality management is an important element in buffer management. It entails avoiding the mixing of reusable water with lower quality water, and preventing up-coning or lateral flows from lower quality sources. Ensuring that repeated reuse of water and frequent circulation do not move water quality beyond safe thresholds requires significant effort. The fact that drinking water must be of higher quality than irrigation supplies, suggests the necessary sequence of reuse.

The third element of optimizing reuse is ensuring that water does not move to an area from which it is difficult to retrieve and reuse. The difference between wet and dry buffers is relevant here. Water which is recharged in a dry unsaturated buffer is difficult to retrieve and, though not lost, is difficult to bring back into circulation. When the buffer is saturated, on the other hand, it can be readily retrieved. The wet buffer or saturated zone is where intense hydrological interaction occurs between recharge and reuse, and between surface water and groundwater. In the saturated zone, reuse is rapid as water that seeps away is quickly picked up and circulated again. An important challenge in 3R is to increase the ‘wet water buffers’ and successfully manage the existing uses.

By ponding up groundwater and slowing down lateral movement, retention can create or enlarge such saturated zones. These nuances must be appreciated in order to avoid the assumption that because a basin is a hydrological unit all water related processes in the basin are one and the same.

Reuse around the world

The following analysis of regions for reuse of water is an introduction to this subject. A full report with specific countries highlighted can be found in the Status and Role of Water Reuse reference at the bottom of the page.

Latin America (Central and South America)

More than 80% of the 700 million people in Latin America live in urban areas, making large quantities of treated and untreated wastewater available for reuse, principally for agricultural irrigation. Drivers for water reuse include wastewater availability, seasonal variations in water availability and use, low or no cost of wastewater to farmers, high salinity of many natural waters, and soil and crop benefits associated with organic matter and nutrients in wastewater used for irrigation. Water availability varies dramatically throughout the region, with countries in Central America generally having larger volumes of natural water resources than some countries in South America such as Peru and Chile.

Middle East and North Africa

The drivers for water reuse in developing countries in the Middle East vary but are principally related to population growth, climate, limited water resources, and socio-economic conditions. Agricultural irrigation is the leading use of reclaimed water in the Middle East. Unplanned reuse is common, where untreated or minimally-treated wastewater is discharged into watercourses and subsequently withdrawn by farmers with or without mixing with water that may be present in the rivers. Farmers are often unaware that the irrigation water they draw to irrigate food crops is grossly contaminated; the link between high disease incidence and wastewater is seldom recognized.

In some countries, untreated or partially treated wastewater is preferred over more highly treated wastewater for irrigation due to its low (or no) cost and high organic and nutrient value. In some Muslim countries, the use of wastewater for irrigation has been opposed on religious grounds (i.e., that the water originated from wastewater and is therefore contaminated). This resistance has been relieved in some cases by religious scholars who have issued fatwas (i.e., legal pronouncements issued by religious specialists on specific issues, usually at the request of an individual or judge to settle questions where Islamic jurisprudence is unclear) stating that wastewater reclamation and reuse can result in a “pure” water source of water that is safe and adequate for different applications.

South Africa

South Africa is a semi-arid country with limited water resources, and it is projected that water demand will exceed available supplies around 2020. The main drivers for water reuse in South Africa include: variable and uneven distribution of rainfall; high evaporation rates; low-yielding aquifers; and growing industrial and urban development. In the past, many of the investigations have been directed at potable reuse. Intensive research has been conducted at the Stander reclamation plant in Cape Town and demonstration plants in Pretoria and Athlone, but planned direct potable reuse projects have yet to be implemented and more attention has recently been given to nonpotable reuse.

Far East

Population increase is the main driver for water reuse in most of the Far East, although rapid industrial growth in some areas, such as China, have drastically increased the need for water and at the same time has resulted in gross pollution of many of the existing water resources. Water is scarce in many parts of the Far East, particularly in the northern and western regions of China, India, and Pakistan. Almost two-thirds of China’s 668 cities have limited water resources and 136 cities experience severe water shortages. The tropical countries in the Far East generally have abundant water resources and have limited water reuse.

India and Pakistan

India: Only about 72% of the 17 Mm3 (4.5 billion gallons) of wastewater generated annually in India is collected, and less than 25% of the collected sewage is treated. This has resulted in a high number of waterborne disease cases in the country. Sewage farms irrigating salad crops with low quality wastewater are prevalent even though irrigation wastewater is forbidden. India does provide high levels of treatment of industrial wastewater for reuse. In some cases, reverse osmosis treatment is provided for industrial wastewater, but tertiary treatment of municipal wastewater is rare.

Pakistan: The use of untreated wastewater for the irrigation of all types of agricultural crops is common. About 80% of the urban communities use untreated wastewater for agricultural irrigation, including vegetables. In the City of Faisalabad, for example, more than 2,000 ha (4,900 ac) of agricultural land is irrigated with untreated wastewater. Farmers prefer to use untreated wastewater because of its high nutrient value and, although there is one small wastewater treatment facility in the City, all wastewater used for irrigation is untreated. Wastewater is sold to the farmers; the revenues generated are used to operate and maintain the drinking water and sewage disposal systems.

Water reuse links

Non-Conventional Techniques for Water Reuse in the Mediterranean
Limpopo river awareness kit: Conservation and reuse.
WATER AND WASTEWATER REUSE. UNEP & GEC.
AN EVALUATION OF AGRICULTURAL WATER REUSE PRACTICES IN THE ARID U.S.-MEXICO BORDER REGION—MEXICALI, BAJA CALIFORNIA, MEXICO. Universidad Autonoma de Baja California & Arizona State University.

Acknowledgements

Managing the Water Buffer for Development and Climate Change Adaptation: Groundwater Recharge, Retention, Reuse and Rainwater Storage, Frank van Steenbergen and Albert Tuinhof.
Status and Role of Water Reuse, Global Water Research Coalition.