Desalination is a method for producing water for human consumption, irrigation or industrial utilisation. In this study, a reverse osmosis (RO) system for brackish water desalination was theoretically investigated to produce both potable drinking and agricultural water with a lower overall and specific energy consumption. As a case study, the Main Outfall Drain in Iraq is used as the brackish water source. A numerical model based on solution-diffusion theory was developed in Matlab Simulink and used to analyse the design and performance of an RO system. The effect of feed water temperature, pressure, salinity and recovery ratio on the efficiency of the whole RO system was investigated for a wide range of design considerations. The design of an RO system for this application was optimised and economic assessment carried out. Results show that with boosting recovery ratio from 30% to 60%, the specific energy of desalinated water production below 400 ppm was reduced from 2.8 kWh/m(3) to a more economically favourable value of 0.8 kWh/m(3), when utilizing a pressure exchanger as a recovery device. Salt rejection was reduced from 97% to 88% to obtain large quantities of water for irrigation with an acceptable salinity (<1600 ppm), for agricultural use. The reduction in salt rejection is influenced by the feed water temperature and pressure; also the average pore diameter of the RO membrane and in turn determines the reduction in system energy consumption. It was found that the total cost to produce 24,000 m(3)/d of water from a feed salinity of 15,000 ppm and a water quality of <400 ppm would be 0.11 pound/m(3) with a corresponding investment cost of 14.4 pound million for the drinking water, and for irrigation) obtained product <1600 ppm) are 0.9 pound/m(3) and 11.3 pound million. (C) 2016 The Authors. Published by Elsevier B.V.