(Initial page layout) Impact of UV radiation dose, suspended solids and organic matter on its efficiency to remove pathogens from greywater Amit Gross *, Diana Ferrando Chavez *, Yuval Alfiya **, Yael Gilboa ** and Eran Friedler ** * Zuckerberg Institute for Water Research, Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev Sede Boqer Campus 84990 Midreshet Ben-Gurion, Israel (E-mail: amgross@bgu.ac.il; ferrando@post.bgu.ac.il) ** Faculty of Civil & Environmental Engineering, Technion - Israel Inst. of Technology. Technion, 3200 Haifa Israel (E-mail: eranf@tx.technion.ac.il; alfiya@tx.technion.ac.il; ygilboa@technion.ac.il) Abstract Onsite greywater (GW) reuse is receiving growing interest as a measure to alleviate water scarcity. In spite its advantages, GW even after treatment often contains pathogens that have to be inactivated. An increasingly popular disinfection method for onsite installations is the use of low- pressure UV irradiation. Yet, inadequate water quality might reduce UV efficiency, specifically the presence of elevated particulate matter and organic substance. In this study, we tested low- pressure UV disinfection of GW under a range of TSS and BOD 5 concentrations in a controlled collimated beam laboratory setup, and in a flow through UV reactor commonly used in full scale onsite GW treatment systems. In the laboratory, treated GW were used as is or subjected to increase concentration of either TSS or dissolved organic matter (measured as BOD 5 ) or a combination of both. These samples were exposed to three UV doses applied by a collimated beam setup, and the inactivation efficiency of fecal coliforms (FC) was recorded. The results indicate that TSS reduced the UV disinfection efficiency more than the presence of BOD 5 . Further, the reduction in UV efficiency was more pronounced at the lower UV doses, in which FC removal was reduced as TSS concentration was higher than 50 mg/L. However, as UV dose increased, the negative influence of TSS reduced. Based on the laboratory results a multiple linear regression model was developed, that correlated between the removal of FC with applied UV dose, TSS and BOD 5 concentrations, and FC concentrations in the GW (before irradiation). The model was validated against results from the flow-through reactor. Finally, the model was used to suggest a conversion factor between the laboratory and flow-through reactor experiments. Keywords Greywater; disinfection; UV; suspended solids; BOD; flow through UV reactor INTRODUCTION Onsite reuse of greywater (GW), which consists of domestic wastewater excluding the stream generated by the toilet, has been recognized as an important facet in addressing water supply shortage. For example, Beck et al. (2013) estimated that greywater consumption in a US household average 114 L/person/day. Friedler (2004) claimed that using GW may reduce domestic water use by up to 50%, while using graywater for household toilet flushing alone can reduce urban water consumption by 10–20%, which consequently can result in various environmental and economic benefits. Moreover, using GW in dry, water-stressed areas might be the only way to sustain public and private gardens year-round (Gross et al., 2015) as well as onsite agriculture. Local, decentralized systems used for treatment and reuse of GW from individual and clusters of homes, are an attractive option as they minimize the need for additional distribution infrastructure (Winward et al., 2008). Greywater reuse standards often require to lower indicator bacteria concentrations (e.g. fecal coliforms) to levels ranging between 3-1000 cfu/100 mL which in practice means that disinfection is needed (WHO, 2006; Franke et al., 2013). The most common ways for onsite GW disinfection is chlorination and low-pressure UV irradiation which is the focus of this study (Fenner and Komvuschara, 2005; Benami et al., 2015; Ekeren et al., 2016). UV disinfection inactivates microorganisms by a photochemical reaction that damages their