Greywater is domestic wastewater excluding the toilet and kitchen streams. Interest in greywater reuse as potential alternative water source for alleviation of water scarcity has risen significantly in the last decade. Nevertheless, its reuse may pose health risk. Legionella genus is a cause of waterborne infections via inhalation of contaminated aerosols. The two main greywater reuse options are toilet flushing and garden irrigation, both producing aerosols of inhalable range. Thus, Legionella presence should be considered when assessing the health risk associated with greywater reuse. In order to assess this risk, four domestic water systems were monitored for a year. Sampling included the supply water and greywater at different treatment stages: untreated (RGW), treated (TGW) and chlorinated greywater (TGW+Cl). Legionella concentrations ranged from 1.0*101 to 2.8*104 (cfu/ l) in potable water and averaged 1.4*104, 7.8*102 and 2.0*102 (cfu/l) in RGW, TGW and TGW+Cl, respectively. The potable water, presented a seasonal pattern with relatively low Legionella concentrations in autumn, winter and spring and an increase during summer. Greywater samples presented an almost inverse pattern of relatively high Legionella concentrations in spring, and a decrease in summer. A statistically significant reduction in Legionella concentrations was found along the greywater treatment process. No significant difference was found between Legionella concentrations in potable water and the final treated greywater. These findings indicate that reusing greywater without treatment would elevate the health risk associated with Legionella, while reusing treated greywater would exhibit risk that is very similar to the risk associated with using potable water.
