Waste water treatment: How can we convert dirty water into a resource?
Why not turn a problem into a solution?
Every year the Earth's growing population requires more water while simultaneously producing more waste water. Providing people sustainably with clean water for consumption, industry and agriculture will be one of the major challenges of the century. Part of the solution is seeing waste water as a resource rather than a discharge problem. With the right technology it can become a valuable and sustainable alternative to groundwater and surface water. It can also generate energy and serve as fertiliser. What is required are reliable and efficient pumps.
Waste water can provide more than just water
Waste water can be an alternative to extracting groundwater or surface water. It is available wherever people are living. Recycling waste water uses relatively little energy. Treating waste water to drinking water quality is estimated by the International Water Association to cost between 0.45 and 0.75 US dollars per cubic metre. By way of comparison, the price for seawater desalination for large systems is between 0.50 and 1.80 US dollars per cubic metre, depending on the energy costs and location.
Waste water is much more than just a source of usable water. Phosphorus can be gained from it, for example, which is a key base material for producing fertilisers. It has been forecast for mineral phosphorus resources to become scarce or even run out in the next 50 to 100 years. An estimated 22 % of the global phosphorus demand could be covered by recycling domestic waste water. For phosphorus to be utilised it does not even need to be isolated from the water as fields and green spaces can be directly irrigated by treated waste water.
Energy, too, can be gained from waste water. The aerobic digestion of sludge in waste water treatment plants generates biogas, for example, which usually contains more then 60 % of methane. This, in turn, is used for producing power and heat in combined heat and power plants. The energy stored in waste water in chemical form is clearly exceeded by energy in thermal form. This can be reclaimed with proven and environmentally friendly technologies such as heat exchangers or heat pumps in order to be used for district heating, agricultural greenhouses and for drying sewage sludge.
In dry regions, recycled waste water can be an important alternative to extracting groundwater or surface water. It can prevent aquifers from running dry and add to the diversification of water supply. However, it should never be employed as a sole measure for fighting dry conditions.
Key measures should always focus on enhancing the efficiency of water use and limiting water consumption. Suppliers and municipalities further have to be aware and consider for their planning that water recycling may affect the ecosystem and the water cycle in its own way. If waste water is used for irrigation, for instance, rather than being discharged into rivers and lakes, the water level of these rivers and lakes may fall.
Fit for purpose: The right water for every application
Recycling waste water is not a new technology. New is only how it is being used: Rather than only providing water of drinking water quality, suppliers now offer different types of water of various degrees of cleanliness. Consumers can select the type that best fits their application. This principle is referred to as fit for purpose. The water for fire-fighting or for irrigating a golf course, for example, does not need to be of drinking water quality. Treated waste water is more cost-efficient and sustainable, plus its properties often make it particularly suited for specific purposes. For example, stormwater that has been collected in stormwater drains and treated is ideal for cleaning tasks, car washes and industrial applications as it is low in lime and nutrients. By contrast, domestic waste water is rich in phosphates and nitrates, which makes it a good choice for the irrigation and fertilising of plants.
The treatment processes of waste water prior to being reused hardly differ from those in municipal waste water treatment plants or drinking water treatment systems. The water passes up to four consecutive treatment stages. Depending on the degree of cleanliness it can be used for different purposes. First are the mechanical and biological treatment stages. Water that has been through the first two stages can be used for irrigating parks, for example. The third treatment stage can be chemical treatment, filtering or additional biological treatment. After these stages, the water is suitable for toilet flushing, industrial applications or irrigating edible plants. Water that has passed a fourth stage, such as reverse osmosis, ultrafiltration or activated charcoal filtering, can be used directly as drinking water or for filling artificial lakes or aquifers used for drinking water supply. Disinfection by chlorine or UV radiation is required before the water is used, regardless of the application.
Solutions in practice: Vendée in France
What recycling of waste water looks like in practice can be seen in Europe's unique project in the Département Vendée in the west of France, at the Atlantic coast. KSB is involved in this project, which will be completed in 2024. About 90 % of the drinking water for the Département is supplied by 14 reservoirs with a storage capacity of 56 million m³ of water. Consequently, the water supply strongly depends on rainfall. In dry summer months the Département fell short by up to 8 million m³ of drinking water. This is why Vendée Eau, the public water supplier, started the Jourdain project, a programme for reclaiming waste water.
Some of the water leaving the waste water treatment plant Les Sables-d’Olonne is to be treated by ultrafiltration, reverse osmosis and UV radiation in order to be reused. The water reclaimed in this way is then to be transported via a 27 km pipeline to the Jaunay dam, where it will be discharged into a vegetation zone. This is where it will mix with river water and flow into the artificial Jaunay lake. The lake is where the drinking water station of Le Jaunay extracts its water. The water treatment plant will secure the supply of 150,000 people with drinking water during the summer. KSB has supplied the project with Amarex pumps for extracting waste water as well as Etanorm and Multitec pumps for feeding water into the ultrafiltration and reverse osmosis stages or serving as backwash pumps. One of the reasons the KSB products were chosen is their corrosion resistance as close proximity to the ocean means a high chloride content in the water. This is a challenge the project had to tackle.
