Water Reuse
In decentralized systems water reuse should be encouraged, as it saves fresh water and alleviates the disposal problem. Also, the reuse of water in close proximity to its source of generation reduces considerably the length of pipeline needed in this process.
One of the most important problems in water reuse is that the required effluent water quality must be maintained at the necessary standard all the time, public health being the primary concern. As it was mentioned previously, the O&M of the treatment facility is the responsibility of the owner and in some cases, the regulating authorities are reluctant to issue permits for reuse of the treated effluent, because they are not able to control the quality of the effluent. Therefore, only very reliable and robust treatment and reuse practices should be recommended.
The most common beneficial uses of the effluent are landscape irrigation and toilet flushing. The last has the advantage that the reuse is constant. Landscape irrigation has seasonal water requirements. In climates with long and cold winter, the reuse for landscape irrigation can be applied only in summer. This requires storage volumes to retain the treated effluents for the period when they are not used; or alternatively a safe disposal method of discharge should be applied.
Figure 13 shows different options for water reuse, suitable for warm climates. If gray water is reused only, treatment facility A is provided and the effluent can be used for: a) toilet flushing, b) surface irrigation or c) subsurface irrigation. The treatment facility B serves the total wastewater flow and is followed by one or several constructed wetlands in series, which can be incorporated in the surrounding garden. The excess water is infiltrated in a soak away.
Figure 14 shows how the wastewater, generated in a building can be reused for green spaces located at suitable locations. Contemporary architecture is recommending roof gardens, which are contributing to a more aesthetic and environmentally friendly urban environment.
Important points
1. As a rule of thumb – in cases where treated effluent might became in direct contact with humans, disinfection is obligatory, as shown in Figure 13 (in this case UV disinfection is suggested).
2. The wastewater treatment system and the water reuse system are connected, meaning you can reuse as much water as you generate as wastewater. Wastewater quantities can be estimated based on the number of inhabitants and an average daily consumption rate; e.g. in case of low number of inhabitants and reuse for irrigation, additional amounts of fresh or rain water would be necessary to maintain the garden.
3. In cases of decentralized water supply and wastewater system (in such cases the water supply source is usually a well or a borehole) strict measures need to be taken for the save and secure treatment and disposal of the wastewater, as the risk to public health is much higher!
Figure 13 On-site Water Reuse options
Figure 14 Green buildings