６７６：家庭雑排水の処理

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Greywater treatment needs complementary measures

A new study has indicated that, although greywater treatment and re-use could produce water savings of up to 37 per cent, it could not comprehensively remove hazardous substances. Complementary measures are needed, such as eco-labelling and regulatory controls.

Greywater is wastewater from all domestic sources except toilets. Greywater typically contains less organic matter and less disease-carrying bacteria and viruses than wastewater that contains toilet waste. Its treatment and re-use for purposes other than drinking, such as toilet flushing and irrigation, could be valuable and cost-effective. When conducted on-site, it could reduce the amount of wastewater sent to municipal wastewater treatment plants (WWTPs). However, there is little research on the nature, levels and fate of pollutants within greywater which makes it difficult to determine the level of risk associated with greywater treatment and re-use.

The study explored on-site greywater treatment using two methods. Firstly, it applied 'geo-chemical modelling' to predict the fate of pollutants identified by the EU Water Framework Directive1 as 'priority substances' or 'priority hazardous substances'. This indicated that the removal of the majority of substances (29 out of 33) is likely to be due to separation of the water from the solid/sludge that was previously suspended in the water. Only benzene, choloroform and dicholoromethane appeared to remain within the water or as a gaseous form.

Secondly, the study considered three scenarios of greywater treatment to estimate potential water savings using Danish water use statistics:

Scenario 1: bathroom greywater is treated on-site using rotating biological contactors (RBC) and re-used for toilet flushing. It was estimated this would produce a maximum saving of 23 per cent of potable water (water that can be used for drinking) and a reduction of 11 per cent in the amount of wastewater going to WWTP.
Scenario 2: both bathroom and laundry greywater, but not kitchen greywater, are treated on site using RBC and re-used for toilet flushing and laundry washing. It was estimated that this would save a maximum of 37 per cent of potable water and reduce the wastewater sent to WWTP by 17 per cent.
Scenario 3: both bathroom and laundry, but not kitchen greywater, is treated using a land based reed or willow bed system. This would reduce the amount sent to the WWTP by 23 per cent, but would not reduce the use of potable water for household purposes.
Some re-use options, such as irrigation, remove the water destined for WWTPs whilst others, such as toilet flushing, merely divert the water temporarily. This temporary diversion reduces the use of potable water but does not have a significant impact on the level of pollutants, particularly if they remain within the sludge that is later transferred to the WWTP.

This suggests that current greywater treatment technologies cannot necessarily be considered effective at preventing the release of household pollutants. Moreover, if the treated water is used for irrigation the pollutants that are not removed in the sludge, such as benzene and chloroform, may be released into the environment.

These results suggest that, alongside developing greywater treatment technologies, attention must be given to minimising pollutant release into water from household sources. These include green procurement, eco-labelling, public information campaigns and regulatory controls.

Source: Donner, E., Eriksson, E., Revitt, DM. et al. (2010) Presence and fate of priority substances in domestic greywater treatment and reuse systems. Science of the Total Environment.