A reverse osmosis desalination unit is proposed to desalinate seawater. The pressure required to overcome the osmotic pressure and initiate the reverse osmotic process is provided by utilizing the mechanical potential energy results from the difference in heads between a high level column of seawater and a low level column of purified water. A mathematical model is proposed to simulate the proposed unit behavior under steady and transient conditions. The effect of different operating and design conditions on the purified water production rate is investigated.