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Water and Wastewater Reuse

Water scarcity and water pollution are some of the crucial issues in the world. One of the ways to reduce the impact of water scarcity and pollution is to expand water and wastewater reuse.
GEC and IETC jointly published WATER AND WASTEWATER REUSE as an introductory guideline for wastewater reuse strategies both in developed and developing countries.

1. Need for Wastewater Reuse

Access to water supply and improved sanitation is one of key factors in improving health and economic productivity. In order to increase access to water supply the following three elements are especially important.
1) Development of new water sources
2) Prevention of water resource degradation
3) Improvement in efficiency of water consumption
Wastewater reuse contributes to all of above three elements. Wastewater reuse can provide alternative source of water, reduce pollution load to water environment by less discharged wastewater. Moreover wastewater reuse in agriculture and industry enable more efficient water withdrawal for other purpose because freshwater withdrawal for agriculture and industry constitute a large share of global water usage and account for 67% and 20% of total use respectively. Therefore wastewater reuse has a big potential to bring about environmental, economic and financial benefits.

Figure 1 Role of Wastewater Reuse

2. Category of Wastewater Reuse

Wastewater reuse can be categorized by its sources, benefits, applications and required treatments. In this section wastewater reuse is divided by its sources into Gray Water Reuse, Industrial Process Water Reuse and Effluent Reuse. Rain water can also be a source of water reuse, but not covered here. If you need detail information about rain water harvesting, click the button below.

Rain Water Harvesting

(1) Gray Water Reuse

1) Definition of Gray Water

Gray water is defined as "untreated household wastewater that has not come in contact with toilet waste and includes wastewater from bathtubs, showers, washbasins, clothes washing machines and laundry tubs, but does not include wastewater from kitchen sinks or dishwashers or laundry water from washing of materials soiled with human excreta, such as diapers."

Figure 2 Sources of Household Wastewater

2) Outline of Gray Water Reuse

In conventional sewer system, gray water cannot be reused as it is discharged into sewer system as well as black water. In on-site sanitation areas, gray water can be utilized, thereby reducing fresh water consumption, if it is separated from black water.

Gray water is generally reused without pre-treatment for agricultural or landscape irrigation in household scale or in larger scale. It is best to design a grey water system that prevents human contact and the potential for environmental contamination. Gray water reuse should be avoided for irrigation of root crop and edible parts of food crop. It is also needed to prevent ground water contamination.

Figure 3 Outline of Grey Water Reuse

(2) Reuse of effluent from wastewater treatment plants

1) Outline of Effluent Reuse

Reuse of effluent from wastewater treatment plants is possible in a wide range of categories, from agricultural to potable purpose, as shown in Figure 4. Sewerage effluent, in most case, secondary effluent, is recycled with or without treatment to meet specific quality requirement.

Figure 4 Outline of Secondary Effluent Reuse

2) Effluent Reuse for Agricultural and Landscape Irrigation

Agricultural irrigation is crucial for improving the quality and quantity of production. Worldwide, agriculture is the largest user of water; the sector has accounted for 67% of total freshwater withdrawal in the world in 2000. Therefore more efficient use of agricultural water through wastewater reuse is essential for sustainable water management.

Main benefits of effluent reuse for agricultural and landscape irrigation include the followings.

Conservation and more rational allocation of freshwater resources
Reduced requirements for artificial fertilizers and associated reduction in industrial discharge and energy expenditure
Reduction of pollution load to receiving water bodies

Effluent reuse for agriculture should be practiced with good management to reduce negative human health impacts that could be caused by uncontrolled use, so the effluent intended for reuse should be treated adequately and monitored to ensure that it is suitable for the intended use.

Effluent reuse for agriculture needs to be planned with attention to target crops and existing water delivery methods. Excess nitrogen may cause overgrowth, delayed maturity, and poor quality of crops.

Picture.1 Rice farming with treated wastewater

(Source: Kumamoto City, Japan)

3) Effluent Reuse for Urban Applications

In urban cities, recycling system of effluent will play a great roll for controlling water consumption and reducing its pollutant load on the environment, and there are big potentials for introducing wastewater reuse in urban cities with high rate of water usage and discharging wastewater everyday. Most of water used for urban activities need not have the quality as high as that of drinking water. In most cases, secondly treated domestic wastewater followed by sand filtration treatment and disinfections are used for non-portable purposes in urban water reuse, that is, applications for toilet flushing in business or commercial building, car washing, garden watering, park or other open space planting, fire-fighting snow melting and so on. Disinfection can reduce bio film formation as well as public health minimization.

Figure.5 Scheme of Area Recycling System in Shinjuku, (Tokyo, Japan)

(Source: Tokyo Metropolitan Government)

4) Effluent Reuse for Environmental Water Enhancement

Effluent reuse is applied actively for the environmental enhancement such as augmentation of natural/artificial stream, fountain, and ponds in parks. In metropolitan areas, as a result of urbanization, its surface is covered with concrete building and tarmac road, and water retention capacity has been getting smaller. In addition, stormwater is rapidly drained and discharged to the river/sea to prevent flooding, and this makes the water environment in city very poor.

Restoration of perennial stream or pond using reused water in the cities has been practiced in many places, and it contributes to revive an aquatic life and brings a comfortable urban space and scenery.

When treated wastewater is used for water augmentation in water channel, the proper quality guideline has to be considered on the assumption that humans contact with the reused water, so sufficient disinfection has to be carried out by chlorination or UV irradiation etc. In addition to the hygienic aspect, removing of nutrient including nitrogen or phosphorus should be implemented since it may cause an algal blooming which deteriorates the appearance of the stream. In case the restoration of aquatic flora and fauna in the stream is taken into views, ozone or UV disinfection is more recommendable instead of using chlorination since it generate by-product in low level and show the minimum residual effect.

Picture.2 Meguro River (Before)	Picture.3 Meguro River (After)

(Source : Tokyo Metropolitan Government, Japan)

5) Effluent Reuse for Industry

Secondary effluent can be reused for industry with or without treatment and disinfection to meet intended purposes. Detail explanation will be made at (3) Reuse of Industrial Process Water.

6) Effluent Reuse for Groundwater Recharge

Groundwater recharge has been used to prevent declining of groundwater level and to preserve the groundwater resource for future use. Compared to conventional surface water storage, aquifer recharge has many advantages such as negligible evaporation, little secondly contamination by animals, no algae blooming and less costly because no construction of pipelines is required etc. For other benefits, it protects groundwater from saltwater intrusion by barrier formation in coastal regions and control or prevents land subsidence. Groundwater recharge has been widely applied in United States.

An example of groundwater recharge : http://www.cottonwoodwater.org/indirect_potable_reuse.htm#top

7) Potable Reuse of Effluent

Growing populations and increasing constraints on the development of new water sources have spurred a variety of measures to conserve and reuse water over the last two or three decades. As part of this trend, augmentation of potable water supply with highly treated municipal wastewater was introduced in some countries. Potable reuse of wastewater is made possible by improved water treatment technologies and water analysis techniques to detect microbial and chemical contaminants.

A typical treatment process for potable reuse is described below. At first, secondary effluent is treated by flocculation, dissolved air floatation clarifier, sand filtration, activated carbon treatment and membrane filtration. Next it is treated by reverse osmosis and disinfection process, such as chlorination, ozonation and UV irradiation.

The reclaimed water is used for surface water augmentation or aquifer injection(Indirect Reuse). In some cases it is directly utilized to supplement drinking water systems. (J. Haarhoff 1996, PUB 2003 a)

An example of direct potable reuse : http://www.pub.gov.sg/NEWater

(3) Reuse of Industrial Process Water

1) Benefits of Industrial Water Reuse

Industrial water use accounts for approximately 20% of global freshwater withdrawals. So industries should be encouraged to invest in better water efficiency, more recycling and management for freshwater conservation. Industrial water reuse has the following specific benefits, in addition to reduction in water consumption and pollution load to the environment.

- Potential reduction in production costs from recovery of raw materials in the wastewater
- Less permitting and administrative burden from the reduction in wastewater toxicity and volume
- Heat recovery and reduced impacts from high temperature effluent to the ecosystem

2) Outline of Industrial Process Water Reuse

Industrial Process Water Reuse can be through internal recycling and cascading use of industrial process water. It can also be utilized for heating in production process or making hot water for domestic use through recovering its thermal energy.

Figure 6 Cost reduction potential from wastewater reuse

3) Concerns of Industrial Process Water Reuse

Water quality concerns for industrial water reuse include scaling, corrosion, biological growth, and fouling. These concerns are often interrelated one another. These problems can be prevented by reduction of dissolved suspended solids, salts, ammonia, phosphorous and residual organics through treatment like flocculation and filtration.

4) Example of Industrial Process Water Reuse

(Recycling of Part Wash and Rinse Water)
The automated process utilizes various membrane technologies, as shown in the flow diagram below. For example, oil and dirt are separated from the rinse water by an ultra filtration membrane. The detergent is then removed from the water by reverse osmosis, and reused in the washing process. The ion-exchange resin can be used to treat water to be recycled as rinse water as needed.

Figure 7 Flow diagram of washing process

(UF: Ultra filtration, RO: Reverse Osmosis, AC: Activated Carbon, IE: Ion Exchange resin)

The environmental and economic benefits of this process are significant. For example, the volume of water usage and discharge is 0.5% of the previous process. The running cost is one-tenth of an alternative, adsorption treatment that uses activated carbon and ion-exchange resin. The maintenance requirement is once a month or less. (Asahi Engineering, 1999)

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