Water Reuse / Greywater

Water reuse can include reusing water from many sources. Sometimes it is used water from showers or dishwashing, and other times it is managing agricultural runoff. The water does not need filtration if it is for plants (where the water will not contact any edible parts). Although it can be filtered for use in washing water, less often filtered for drinking water, but with the appropriate filtration system, it is possible.

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.

Three processes that support reuse

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.