Performance characteristics of a cyclically operated seawater desalination plant in Tajoura, Libya

The Tajoura seawater desalination plant, producing 10,000 m³/d for mostly domestic uses, is Libya’s largest reverse osmosis plant. The plant was designed to operate continuously 355 days per year producing drinking water meeting the WHO standards with TDS concentrations not exceeding 500 mg/l from Mediterranean seawater with an average TDS concentration of 37,700 mg/l. The plant consists of intake works with a decanting basin, seawater pumping station, pretreatment works, RO modules in 2 stages, post-treatment works, product water storage tank, as well as other supporting facilities (laboratory, chemical and other materials storage, etc.). Because the conveyance works from the plant to the town, a few km away, and other facilities and appurtenances had not been completed at the time of the plant’s commissioning, the water supplied was in large excess of the small domestic and non-domestic demands in the plant’s vicinity. Therefore, the plant had to be operated either on continuous basis at a small fraction of its design capacity or cyclically at the design capacity to meet the small domestic and non-domestic demands. For technical reasons, the plant’s management opted for cyclic operation taking advantage of the large product water storage facilities available at the plant site. The objective of the paper is to evaluate the plant’s performance characteristics during its first 8 months of operation. To achieve this objective, design criteria and operational data are presented and unit operations and processes are described. Data were analyzed so that plant operational and performance characteristics were determined. Plant performance characteristics were determined and evaluated based on analysis of daily and monthly variations in selected operational parameters including water temperature, flowrates, pH, electrical conductivity, pressure differentials across membrane stages, recovery and plant availability. Operation and maintenance records were also reviewed with special focus on intake works, reverse osmosis elements (membrane cleaning, element flushing, and sterilization), instrumentation and control, brine disposal, utilities consumption, chemicals consumption, and personnel requirements and training. Product water costs were also estimated based on capital and operational costs including utility and chemicals costs so that a complete evaluation was obtained. Results