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3.3.5        Water reuse

A perhaps less-recognised benefit of waste water treatment is that it enables others to reuse this precious resource. Cleaning the water allows use for other human activities, avoiding the abstraction from other, possibly non-renewable sources, such as groundwater.

The return of suitably-clean water is also important for aquatic life, ensuring there is sufficient water in lakes and rivers for life to persist. In parts of Europe, particularly the south, climate change is predicted to lower river discharge levels by up to 40% under a 3 ^oC temperature rise scenario (EEA, 2021).  The WFD and Biodiversity Strategy (EC, 2020) consider ecological flows, i.e. the amount of water required for the aquatic ecosystem to continue to thrive and provide the services we rely upon (EC, 2016), but Member States are in early stages of implementation (EEA, 2021).

Water reuse has become a key part of water resources management in countries suffering highest water stress. Where water resources are abundant or less stressed (CSI 018), waste water reuse is driven by other factors, e.g. conservation of groundwater resources, reduction of costs; the precautionary principle.  The primary use of reused water is in irrigation for agriculture, with the modelled potential for reuse shown in Figure 3.4.  Other uses are in irrigation of urban space, groundwater recharge and river flow improvement. 

Indirect reuse is the reuse of treated waste water which is placed into a water body such as a lake, river, or aquifer, and then some of it retrieved for later use. Direct reuse of treated waste water refers to the introduction of treated waste water via pipelines, storage tanks, and other infrastructure directly from a water treatment plant. For example, the distribution of treated waste water to be used directly in agricultural irrigation.

The quality requirement of reclaimed water is dictated by the final use (agricultural, industrial, urban, environmental, recreational) and this determines the treatment technology to be applied. Waste water treatment plants producing reclaimed water may be equipped with an advanced treatment composed of different technologies (e.g. coagulation/flocculation, filtration, ultrafiltration, reverse osmosis, disinfection) which determine investment, maintenance, and operating costs.