Research into reuse of domestic 'greywater' (non-toilet wastewater) has found that typical treatment systems can significantly reduce overall water consumption. However, non-biodegradable micropollutants will continue to reach municipal wastewater treatment plants (WWTPs), unless a new disposal route is found for the greywater treatment sludge.
There is no legislation in the EU specifically addressing water reuse. Urban wastewater treatment is regulated in the EU by the Urban Waste Water Treatment Directive1, which sets obligations for treatment and defines the levels of quality for effluents released by treatment plants. There is increasing worldwide interest in water recycling technologies as well as treatment of greywater (i.e. waste water with low organic pollutant and pathogen content). However, 'xenobiotic micropollutants', which include heavy metals and organic compounds, can be found in greywater and can increase pressure for WWTPs. If these enter the environment, they can potentially cause human health problems and ecosystem damage. New research conducted under the EU SCOREPP project2 suggests that most greywater treatment systems do not efficiently intercept these pollutants.
Greywater is suitable for decentralised treatment and reuse because of its low organic pollutant and pathogen content. Schemes have already been piloted in water stressed areas, such as the Mediterranean, and codes of practice for both treatment and reuse are developing (e.g. in the UK3). Options for greywater use include spray irrigation and car washing, or lower-risk uses, such as toilet flushing or sub-surface irrigation of non-food crops. Although pathogens are the primary concern, it is also important to understand the dynamics of all pollutants to ensure safe reuse.
The research modelled the use of different combinations of greywater sources (bathroom, laundry and kitchen) and applications (toilet flushing, irrigation and laundry) in a Copenhagen residential building. Treating and reusing all greywater has the potential to reducedthe water eventually sent to the WWTPs by 20 per cent, and replace 43 per cent of the potable (suitable for drinking) water used per person, after introducing recycled greywater as an alternative. However, around a third of the recycled greywater had no identified reuse application, so efficient schemes will be optimised according to reuse requirements. Furthermore, greywater reuse will only approach 100 per cent in new developments or refurbishments.
The researchers also considered the fate of five pollutants known to be present in greywater (benzene, nickel, lead, cadmium and 4-nonylhenol, or 4-NP), which are all listed as priority substances by the Water Framework Directive4 and all resist biodegradation. Metals and 4-NP can accumulate in sludge generated by greywater treatment, whilst benzene is mostly released to the air. Each scenario for greywater use was tested with hypothetical pollutant removal rates of 10 per cent, 50 per cent and 90 per cent. Removal rates depend on the technology used.
However, even when treatment removes a high proportion of micropollutants, lower impact at the WWTP is dependent on finding alternative disposal routes for the greywater sludge, such as disposal to land. This is more likely if greywater separation and treatment is carried out at a community rather than household scale. Treating the greywater sludge therefore remains an issue. Greywater used in toilets goes directly to the WWTP, whilst the sludge is usually also eventually discharged to the WWTP. Sludge can be a useful resource in agriculture, to improve soil condition, but cannot be used if metal concentrations exceed limits set by the Sewage Sludge Directive5.