Field evaluation of capillary UF technology as a pretreatment for large seawater RO systems

RO seawater systems operating on a surface feed water, originating from an open intake source, require an extensive pretreatment process in order to control membrane fouling. Considerations of long-term membrane performance stability have lead to an initial design concept of operation of seawater RO systems at a low permeate flux rate and low permeate recovery. In recent years the nominal performance of composite seawater membrane elements has improved significantly, and in parallel new backwashable microfiltration and ultrafiltration capillary technologies have been introduced commercially. This new membrane technology can be utilized to treat seawater from a surface sources. Use of membrane capillary technology as a pretreatment step can improve quality of the surface feed water to a level comparable or better than the water quality from the well water sources. A better feed water quality enables more effective optimization of operating parameters in the RO systems. Both permeate flux and system recovery rate can be increased considerably without creating membrane fouling conditions. Operation of seawater systems at higher permeate flux and recovery rate results in improved economics of RO seawater desalting. Field evaluation of hybrid membrane systems consisting of UF membrane pretreatment unit and a RO seawater unit was conducted subsequently at two test sites. The first test site was at the Red Sea (Eilat site) and the second test site was on the Mediterranean (Ashdod site). The RO membranes were commercial seawater elements in spiral wound configuration. The UF equipment utilized capillary backwashable elements operated in dead end flow mode. For comparison, a second pilot system consisting of conventional pretreatment and an RO unit was operated in parallel at the above sites. The conventional pretreatment unit included in line flocculation followed by media filtration. The tests were conducted over a period of two years. Raw water quality reflected seasonal changes of composition and weather conditions. The performances of UF and RO equipment were evaluated over a wide range of operating parameters such as recovery and flux rate. Field results were used to project and compare the economics of the seawater RO desalting process using conventional and membrane pretreatment. This paper will describe the experimental procedure and results of parallel operation of an integrated membrane unit and a conventional RO sweater system. The economic analysis of both designs based on local site conditions, will be provided as well.

The desalted water cost in seawater systems is affected by the performance of the RO membranes, the operating parameters and the system configuration. The increase in salt rejection of the current RO membranes provides additional flexibility for process cost optimization through the ability to design and operate the RO system at higher recovery rate. The water cost considerations indicate that in seawater RO systems the optimum recovery rate, with current membranes, is in the range of 50–60%. The recovery rate value corresponding to the optimum water cost depends on feed water salinity and power rate. The field test results of UF membrane pretreatment, tested at two different sites confirm that the membrane pretreatment is a reliable technology capable of providing consistently good quality feed water for RO seawater system independently of the raw water quality fluctuation. The cost of membrane pretreatment is higher than the cost of conventional pretreatment. The application of membrane pretreatment in seawater RO desalting systems is feasible for sites which require very extensive conventional pretreatment or where wide fluctuation of raw water quality can be expected.