The main objective of this study was to evaluate two types of greywater treatment systems; commercial and locally manufactured. These treatment systems were installed at three sites. The greywater treatment systems were evaluated on the basis of treated water quality, and potential Internal Rate of Return (IRR). Additionally, flow meters were installed at two sites to monitor the volume of treated greywater used. Daily and weekly monitoring was conducted to evaluate the amount of greywater produced at these sites. One hundred eighty grab samples were collected and analyzed for twelve weeks. In these analyses several physical, chemical, and biological water quality parameters were selected to evaluate the units. The results showed that all physical parameters which include Dissolved Oxygen, Turbidity, pH, and Total Suspended Solids (TSS), were improved significantly after treatment. However, some parameters did not have significant change like Total Dissolved Solids (TDS) and Electrical Conductivity (EC). Furthermore, excluding BODs measured at a private house, all chemical parameters were within the limits of Omani standards (A) for irrigation. Moreover, Coliform and E-Coli bacteria were used to evaluate disinfection efficiency. The results showed that most samples of untreated greywater were fully contaminated with Coliform and with significant amounts of E Coli. After disinfection significant reduction of Coliform and E-Coli was achieved to satisfy existing standards. It was clear that locally manufactured treatment unit at Al Hail Mosque was better in percentage basis in improvement of dissolved oxygen, decreasing turbidity and reducing BODs when compared with SQU mosque's commercial unit. However, the use of beach sand in the locally manufactured system lead to the addition of some salts to the greywater after treatment. In term of percentage improvement of water quality parameters (DO, Turbidity, TSS, COD and BODs), commercial greywater systems at the house was the best of the three units. However, all treatment units' performance efficiency was enough to satisfy Omani regulation of reuse of wastewater in irrigation. Results showed that significant quantities of greywater were produced at SQU and Al Hail Mosques. On average, they were 46 m and 10.5 m® per week, respectively, based on data from two years of monitoring. On the other hand, estimations of water Is consumption at the house (8 inhabitants) showed that the total volume of greywater produced was between 11.5 m2/ week to 14 m2/week. The financial analysis showed that internal rate of return (IRR) for Al Hail South and SQU Mosque systems after 10 years were 14.9% and 19.06% respectively. IRRs estimated were acceptable for community mosques considering the cost of the units and volume of greywater produced. On the other hand, IRR found for the system at private house was -1.44% which indicated that it was not cost effective to use such systems at house.