Reverse osmosis on open intake seawater: pre-treatment strategy

Pre-treatment of seawater feeding reverse osmosis (RO) membranes is a key step in designing desalination plants. The pre-treatment process must be adapted to the seawater quality to be treated (wells, open intake, etc.), especially when treating surface seawater with highly variable quality. After a general presentation of different pretreatment options in relation to the seawater quality, this paper is focussing on two case studies, two open intake seawater pre-treatment upstream reverse osmosis desalination. The first site is located in the Gulf of Oman (Indian Ocean), the second in the Persian Gulf. The pre-treatment uses different technology strategies, conventional pretreatment (coagulation and direct filtration on dual media filters) and innovative technologies (high rate dissolved air flotation, ultrafiltration and microfiltration) according to the water quality. The parameters taken into account for the water quality characterisation are the suspended solids, turbidity, fouling tendency, organic matters and algae content. This paper presents the pre-treated water quality achieved by the two types of pre-treatment and discusses potential impacts on RO hydraulic performances.

The pilot testing during 9 months on the Gulf of Oman, which was paramount to optimise the pre-treatment stage and the chemical regime in relation to the seawater quality improves the optimisation of the process. The results were used for the commissioning of the full industrial plant, a 37.5 MGD open seawater intake desalination plant located in Fujairah. Preliminary full-scale plant results demonstrate that the foreseen performance and water quality are being achieved. In the Persian Gulf, on a rather bad surface seawater intake, the pre-treatment including dissolved air flotation, direct filtration or ultrafiltration, gives good results in terms of turbidity, algae and hydrocarbon removal, leading to a reliable SDI far bellow 3 value. These pilot tests improve the knowledge of surface seawater treatment upstream RO and demonstrate the reliability and robustness of such treatment facing variable surface seawater.