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

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The Tajoura seawater desalination plant, producing 10,000m3/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 500mg/l from Mediterranean seawater with an average TDS concentration of 37,700mg/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 from the evaluation have lead to several important conclusions concerning both the water supply problem and the plant operation; the holistic approach to water supply and distribution proved invaluable with special stress on contractor prequalification, project financing and coordination. Plant availability was very low (37%) while production and recovery rates were constant (4914m3/d and 29.2% respectively). Fouling was nonexistent. Coagulation and flocculation were not needed as the feed water SDI was consistently below 3. Pressure differentials across the first and second stages were practically constant at 0.82 and 4.6bar for the first and second stages, respectively as was the product quality with conductivities consistently below 255 μS and TDS concentrations markedly below 500mg/l. Unit product costs were high as a result of the low availability and high chemical costs especially those used for sterilization. Recommendations were made regarding upgrading of the plant performance and improving the design of new plants. Cyclic operation, although not planned, has demonstrated its viability as a practical, although not the optimum, solution to a rather difficult, unforeseen problem. Real field data have been generated which can be applied to similar situations or for the prediction or optimization of design and operation of reverse osmosis desalination plants. The need for proper characterization of feed water and for pilot plant data prior to full-scale plant design was also demonstrated as certain unit operations, namely coagulation and flocculation, proved unnecessary. Finally, the significance of proper personnel recruiting and training to the plant sustainability was demonstrated. Consideration of this fact has made the Tajoura SWRO plant one of Libya's best operated plants despite its high degree of sophistication and cyclic mode of operation.