Water reuse refers to the process of recovering water that has already been used for one purpose and treating it to a standard appropriate for a second purpose; typically, though not exclusively, without return to the natural environment. In principle, any type of wastewater, whether domestic, municipal or industrial can be considered for reuse. The terms 'reused water', 'reclaimed water' and 'recycled water' can all be used to refer to this process.
The History of Water Reuse
The practice of recycling previously used water for secondary purposes has a long history. Early evidence of agricultural irrigation with wastewater can be found in Crete and Greece dating as far back as the early Bronze Age (3000 BC). Roman systems for harvesting rainwater from rooftops for domestic uses have also been discovered in cities such as Pompeii (6BC-7AD). The reuse of wastewater as a nutrient source in aquaculture has been practised for hundreds of years around the world. Vestiges of fishponds purposely build to receive nutrient-rich wastewater diverted from abbey latrines dating from the 13th century have been discovered in Germany and France. In the 19th century, unintended wastewater reuse also took place in various countries through the operation of sewage farms originally engineered for sanitation purposes, where wastewater was applied to land to benefit from its fertilising value.
However, it is only from the beginning of the 20th century that planned water reuse started to be seen as an opportunity to increase the efficiency of water use and limit the abstraction of freshwater from rivers and aquifers. For instance, early developments of engineered water reuse systems for irrigation are reported from the 1920s in California. In the 1950s, Japan started reusing wastewater for industrial applications. From the 1960s, crop irrigation with reclaimed wastewater was becoming common practice in Israel and other Mediterranean countries, while in 1968 the city of Windhoek in Namibia implemented the first advanced direct wastewater reclamation system to augment potable water supplies. In 1984, the Tokyo Metropolitan Government pioneered the use of reclaimed water for toilet flushing in 19 high-rise buildings in the Tokyoite Shinjuku district. Since the 1980s, thousands of water reuse projects have been developed around the world, and it is estimated that more than 200 water-recycling schemes are in operation in the European Union.
What are the benefits of water reuse?
Water scarcity, as defined by the United Nation Environment Program, "occurs when the amount of water withdrawn from lakes, rivers or groundwater is so great that water supplies are no longer adequate to satisfy all human or ecosystem requirements, resulting in increased competition between water users and other demands". Threats to the availability of water are an issue in many parts of the world and are not limited to arid areas. Indeed, water scarcity along with the deterioration of freshwater quality is the result of a combination of factors that include not only droughts but also population growth, urbanisation and industrial development.
In Europe, there are significant pressures on water resources. A useful indicator which is used to measure the pressure on fresh water resources is the water exploitation index (WEI), which represents the ratio of the total freshwater abstraction of a country to its total long-term average freshwater resource. A WEI above 20 % implies that a water resource is under stress and values above 40 % indicate severe water stress and clearly unsustainable use of the water resource. As reported by the European Environment Agency (EEA) in 2012, water stress issues mainly occur in southern European countries such as Cyprus (WEI of 45%) and eastern and south-eastern regions of Spain (WEI> 40%). However, central and northern European river basins located in Belgium, the UK and western France are extremely water stressed not only due to climate change but also due to high population densities and industry development.
Although water reuse is already practiced in a large number of European countries with the majority of applications in the agricultural sector (see picture below), the EC has highlighted the need to further develop strategies that will encourage the use of alternative water sources to help balance water supply and demand, and promote sustainable water management.
Water reuse in Europe (Demoware)
Water reuse is seen as an important water management option to help balance water supply and demand, while preserving freshwater sources and protecting the quality of the natural environment.
The use of reclaimed water offers various benefits, among which are:
Reducing pressure on freshwater resources;
Ensuring a long-term balance between water supply and demand;
Reducing or preventing pollution associated with the discharge of wastewater to sensitive water bodies;
Providing opportunities for energy and nutrient recovery;
Enhancing the environment (e.g. creation of wetlands, maintenance of attractive recreational spaces in regions prone to droughts);
Improving water supply efficiency and hence reducing the cost of water services;
Contributing to the stability of local economies by providing a constant, reliable and controllable water supply;
Lowering industrial production costs by making more efficient use of water resources.
Offering opportunities to store water for use during periods of high demand or to help alleviate floods.
Wetlands enhancement using reclaimed water, Belgium (Photo Credit: KLC)
How is water recycled and for what purposes?
Water for reuse can be obtained from a wide variety of sources, including municipal wastewater treatment plants, industrial premises, and household sources such as from bathrooms and kitchens.
A wide variety of technologies can be used to treat water for reuse. The level of treatment applied will depend on the quality of the source water and the quality of water required for the particular reuse application. Public health protection is always of primary concern in any water reuse scheme. The technologies used for water reuse are based on the same principles as conventional water and wastewater treatment technologies and involve a range of physical, biological and chemical treatment processes. These are either applied individually or in combination to remove pollutants and achieve the desired level of water quality. Typical treatment options include depth filtration systems (e.g. sand filters), membrane systems (e.g. micro- and ultrafiltration, reverse osmosis), disinfection (e.g. chlorination, ultra-violet light), and advanced oxidation processes (e.g. ozonation).
Globally, the most common water reuse application is agricultural irrigation. In Europe, Spain is a leader in that domain. In 2008, the Spanish Ministry of the Environment reported a reclaimed water production of nearly 450 million cubic metres with 71% being used for agricultural purposes. More specifically, the treatment and reclamation scheme in place in the Murcia region is reported to treat annually approximately 110 million cubic metres of urban and industrial wastewater which allows environmental restoration of the Segura River while providing 100 million cubic metres of tertiary treated wastewater for direct and indirect agricultural irrigation.
In the United Kingdom, the Flag fen Water recycling plant in Peterborough is a well-known water-recycling scheme for industrial reuse. At this site, up to 1,600 cubic metres of final effluent from a municipal wastewater plant is treated daily using a number of membrane processes to produce up to 1,200 cubic metres of ultra pure water for use at the local power station for steam generation. The implementation of this scheme has allowed the power station to reduce its total water use by 11%, while also reducing operating and chemical disposal costs.
In China, water recycling for urban uses is well illustrated at the Beijing Capital International airport. This system, in place since the 2008 Olympic games, treats municipal wastewater effluent using ultrafiltration and reverse osmosis. With a total daily treatment capacity of 10,000 cubic metres, this recycling scheme helps to meet the water needs of the international airport, frequented by approximately 20,000 people per day. It supplies highly treated water for toilet flushing in the premises of the airport and other urban uses (i.e. car washing, street cleaning, irrigation of green spaces etc.)
What are the challenges associated with developing water reuse schemes?
Despite the obvious benefits associated with water reuse, several regulatory, social and economic challenges remain to be addressed before more widespread implementation of water reuse schemes in Europe will be possible. For instance, while some European countries such as Spain have developed their own regulations for water reuse which stimulate the implementation of schemes, many European states do not yet have such supportive regulation. Further efforts are also required to better explain the benefits of reusing water in order to stimulate public, commercial, and government enthusiasm for water reuse. This will not only favour the implementation of new projects but will also support the development of financial incentives for reuse schemes. Indeed, effective business models are a critical element of successful water reuse projects. Finally, although many innovative treatment technologies for water reuse have been developed and implemented worldwide, more consistent methods are needed to select technologies adapted to specific reuse applications.