Water recycling means reusing treated wastewater instead of drinking-quality water for purposes such as agricultural and landscape irrigation, industrial processes, toilet flushing, environmental enhancement, and replenishing of depleted groundwater aquifers. Converting waste water into usable, even potable, water can happen by many processes, which may involve treating it to remove malignant life forms and pollutants. This process can vary from very simple waste treatment pools to very complex detoxification plants.
In many industries much of the water used for cooling and other processes does not need to be of drinking quality. A large proportion of water initially withdrawn for industrial purposes can be recycled several times before it is finally disposed of. The efficiency of water technology can be increased further by such means as totally integrated water-recycling systems. In several industries such as iron and steel and even mining, it is now economically feasible to recycle water. The paper and pulp industry, which has long had the reputation of being one of the largest consumers and polluters of water, is yet another sector where successful efforts have been made to recycle water after use. For the manufacturing industries, the cost of water on average is only about 3% of total costs. Because of the low cost, incentives for using water more efficiently must come from strict water allocations, stringent pollution control requirements or through water-pricing policies. In the industrial world, greater attention has recently been paid to re-using and recycling treated waste water. This practice, however, is not yet universal. Re-use of waste water has been advocated mainly for non-potable purposes, such as agricultural irrigation, cooling, and industrial in-plant recycling.
Developing countries are well and probably even better placed than old industrial countries to take advantage of new recycling technologies, because installing water efficiency and pollution controls into new plants is much cheaper than retrofitting old ones. Some of the technologies available are capable of reducing water use and waste water flows by up to 90%. Information on these technical options should be disseminated systematically. Technology transfer thus could contribute to alleviating water supply and pollution problems in the emerging industrial countries. While the scope for waste-water re-use so far is relatively small in developing countries, since many of them do not have sewerage systems that collect the used water, there is large scope for building new industries with water recycling systems. Moreover, domestic waste water could be collected and used after treatment for agricultural purposes.
The cost of recycling is higher than naturally found water.
Some forms of recycling are hazardous to health.