Water Reuse

Content Table

Water Reuse

Reusing and recycling water

Water is an increasingly scarce resource for much of the World's population. Global warming, an increasing world population and generally increasing incomes mean the demand for water is likely to rise further. If current trends continue, we will use 40% more water by 2024 than we do now. In industrialised countries much of the water used for non-potable purposes such as industrial applications, toilet flushing and irrigation, is unnecessarily treated to potable-water standard. We have to carefully treat wastewater to protect water resources and ensure water is available for abstraction. We need to take care not to over-abstract water from rivers and aquifers but instead to reduce abstraction to within sustainable limits.

Reusing and recycling water conserves existing water resources, reduces the need for new fresh water and, because it is no longer discharged to the environment, reduces contamination and degradation of freshwater resources. Previously degraded sources can be reclaimed (referred as water reclamation) for water supply. Wastewater can be recycled/reused as a source of water for a multitude of activities such as agriculture, aquifer recharge, aquaculture, fire fighting, flushing of toilets, snow melting, industrial cooling, parks and golf course watering, formation of wetlands for wildlife habitats, recreational impoundments, and essentially for several other non-potable requirements. Potential reuses of wastewater depend on the hydraulic and biochemical characteristics of wastewater, which determine the methods and degree of treatment required. While agricultural irrigation reuses, in general, require lower quality levels of treatment, domestic reuse options (direct or indirect potable and non-potable) reuses need the highest treatment level. Level of treatment for other reuse options lie between these two extremes.

Agricultural irrigation has, by far, been the largest reported reuse of wastewater. Irrigation reuse of wastewater can be applied to agricultural crops, woodlots and pastures, or landscape and recreational areas.

Benefits and risks of water reuse in agriculture
Use of reclaimed water is an attractive option and entails a number of benefits for several reasons, including:
• Reduction of surface water pollution commonly resulting when wastewater is discharged into the environment;
• Postponement of potentially more costly water supply augmentation projects (i.e. storage, transfer and/or desalination schemes);
• Nutrients in treated wastewater can reduce the need for applying chemical fertilizers, thereby reducing costs and potentially adverse effects associated with fertilizers;
• Savings of high quality freshwater since reclaimed water can be further used in industrial cooling; landscape irrigation; fire protection; and toilet flushing beyond irrigation.
• Greater water security given the stability of wastewater supply.

Nevertheless, a number of risks are associated with water reuse and warrant specific attention in agriculture. Such potential risks include:
• Health risks to agricultural workers resulting from fields irrigated with untreated or inadequately treated wastewater.
• Health risks to consumers of agricultural goods produced from untreated or inadequately treated wastewater.
• Contamination of soils and plants through introduction of chemicals found in inadequately treated wastewater.
• Ground and surface water pollution from infiltration of contaminated irrigation water.

Industrial reuse of reclaimed wastewater represents major reuse next only to irrigation in both developed and developing countries. Reclaimed wastewater is ideal for many industrial purposes, which do not require water of high quality. Often industries are located near populated area where centralized treatment facilities already generate reclaimed water. Depending on the type of industry, reclaimed water can be utilized for cooling water make-up, boiler feed water, process water etc. Cooling water make-up a majority of industrial operations and represents the single largest water usage.

Indirect reuse and non-potable reuse/recycle are already viable options for many parts of the world. Adequately treated wastewater meeting strict quality criteria, can be planned for reuse for many non-potable domestic purposes. Non-potable reuse leads to both a reduction water consumption from other sources, and a reduction in wastewater flow rate. So, non-potable reuse schemes can avoid adverse environmental consequences associated with conventional water sources and wastewater disposal systems. Non-potable domestic reuse can be planned either within single households/buildings, or on a larger-scale use through a reticulation system meant only for use for non-potable purpose.

By segregating the "gray" sullage from "black" toilet wastes, the potential for reuse with minimal treatment within the household significantly increases. There are several different schemes for reusing gray water at the household levels. A dual reticulation system is the wastewater reuse concept for urban areas where a centralized sewage collection system is in place, on a large scale. This system supplies treated wastewater to houses, and commercial/official/shopping complexes through a separate water supply network, to be used primarily for toilet flushing, and irrigation of lawns. Thus, households will have two water supply lines, one for potable and human-contact use purposes, and the second for non-potable, non-contact uses such as toilet flushing, use in the yards and gardens etc.

On the other hand, direct potable reuse (i.e. introducing reclaimed water directly into the water distribution system) is not considered a safe practice until one understands more about the possible risks of trace levels of organics and the reliability of barriers to pathogens.

Indirect potable reuse of treated wastewater may occur unintentionally, when wastewater is disposed into a receiving body of water that is used as a source of potable water supply. Treated wastewater which meets the criteria for potable reuse except for total dissolved solids, is diluted by water from other sources to meet this criteria, and used for potable purposes. Another planned indirect potable reuse can be through groundwater recharge of treated wastewater. Use of treated wastewater for artificial groundwater recharge is increasing as a way to treat and store effluent underground for subsequent recovery and unrestricted reuse.