Decentralised treatment with reused wastewater to save drinking water also helps to reduce the amount of wastewater. Moreover, some wastewater treatment systems can easily be integrated into site layouts, and environmental measures can be made visible as a result.
The possibilities for reusing treated wastewater depend largely on the amount and quality of the wastewater available. Essentially, two possibilities exist: treatment with reused greywater and with blackwater. Greywater is all wastewater except wastewater from flushed toilets, i.e., wastewater from personal hygiene (showers, baths and sinks), kitchens, laundry, cleaning, etcetera. Blackwater is all wastewater, i.e., greywater plus toilet flushings. The designations ‘black’ and ‘grey’ come from the colour of the water: greywater is somewhat grey in colour, while wastewater that carries faecal matter turns black after a short while.
In non-residential buildings, the amount of greywater available is generally limited. In such cases, therefore, treating blackwater yields a considerably larger volume of reusable water. Another advantage of blackwater treatment over greywater treatment is that no connection to the wastewater sewer is required. Drawbacks to blackwater treatment are the treatment sludge that is produced in the treatment process and that has to be removed and a certain level of unwillingness to reuse wastewater based on people’s perceptions. For example, Singapore uses membrane filtration to treat all its wastewater to drinking water quality, yet that water is not used as drinking water for the last reason given. Although treated wastewater is generally of slightly poorer quality than rainwater, it is clear and odourless and well-suited for flushing toilets, for water art and for irrigation. As a rule, it contains a higher proportion of nutrients, which reinforces its suitability for plants. If both rainwater and treated wastewater are used, therefore, it is better to use rainwater for the higher-grade purposes.
Several publications exist that describe the installation of these ‘other’ water systems. Useful information can be obtained from the Flemish Environment Agency (Vlaamse Milieumaatschappij) and Vlario in the Dutch-language publications Waterwegwijzer and Vademecum Praktisch afkoppelen van hemelwater. Those publications offer practical advice about installing these systems and contain useful information about the various elements in the systems. VMM, 2010
Use of treated wastewater
Like precipitation, treated wastewater can be used for low-grade purposes in buildings. The treated water can be retained in reservoirs or ponds. As the inflow of treated water is relatively constant (as is the inflow of wastewater), less buffering volume need be created in the reservoir.
In the Netherlands, all buildings are required by law to be connected to the sewer system. Reusing water treated in decentralised facilities requires dispensation from the mandatory mains connection.