Water. desalination + reuse

DWR FebMarch 2015

Water. Desalination + reuse

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February-March 2015 | Desalination & Water Reuse | 15 | projects _________ David H Paul president, David H Paul, Inc. Editor's note: IDE Technologies (IDE) designed and operates three of the largest seawater reverse osmosis desalination plants in the world each with a cost per cubic metre of produced water among the lowest globally. This it accomplished despite a fourfold variability in the electricity price and demanding regulatory limits on boron, chloride and sodium concentrations. The key to their performance is demonstrated in the extraordinary aspects of the design, operation and personnel at the Hadera plant. WHEN ISRAEL'S Ashkelon seawater reverse osmosis (SWRO) plant came on line in 2005 it was the largest reverse osmosis desalination plant in the world. The same title went subsequently to the Hadera facility in 2010 and then the Sorek plant in 2013 – both also in Israel. Plant managers and lead engineering personnel from these three SWRO plants recently took part in training at the Hadera plant. While the three plants have many similarities they have also significant differences. But the operation, design and personnel of the Hadera SWRO plant are, in many ways, outstanding and unique. UniqUe OPeratiOn Electricity is the single biggest operating cost for any RO plant. This is especially true for a SWRO plant due to the high osmotic backpressure of the feed water. The total dissolved solids (TDS) of the Mediterranean seawater that feeds the Hadera plant can be as high as 41,000 mg/l. So to minimize the total water cost of the finished water produced, everything that can possibly be done to reduce the cost of electricity is required. The most challenging part of optimizing electricity use according to price is variation in wholesale power prices depending on the season of the year, day of the week, and hour of the day. For example, the highest electricity fees for industrial customers are charged in July and August when the lowest rate ¬– the base rate – is from 21:00 to 07:00 the next morning on weekdays (Sunday-Thursday) and for 24 hours a day on the weekends (Friday and Saturday). The highest rate – the peak rate – occurs in July and August between 10:00 and 17:00 on weekdays. The hours just before and after the peak rate are charged an intermediate rate called the shoulder rate. Typical electricity prices in July and August are a base rate of 0.08 US$/kWh; a shoulder rate of 0.12 US$/kWh and a peak rate of 0.28 US$/kWh. So it typically costs nearly four times as much to produce the same amount of water during peak-rate hours as during base-rate hours. One way to reduce electricity costs would be to operate only during base rate hours. This would be nine hours a day during the weekdays and 24 hours a day during the weekends during the months of July and August. But issues arise when RO units and pretreatment and post treatment systems are shut down, such as the need to flush the RO units with permeate upon shutdown. And there is a period required after shutdown and startup to get pretreatment, RO and post-treatment systems stabilized. Plus there is the ever- growing need for finished water. All of these issues preclude operating only during base-rate times. The solution adopted at Hadera was to determine total water costs, given all the existing variables and needs, and implement the most cost-effective way of operating the plant. This resulted in operating the SWRO plant hour by hour at flow rates that minimized costs. During periods of the lowest electricity rates, the plant operates at full capacity turning out produced water at 20,000 m 3 /h. During the highest electricity rate periods the plant might operate at only 7,500 m 3 /h – 38% of maximum flow rate. Particularly demanding regulatory requirements for the finished water from this plant are: a boron level that cannot exceed 0.3 mg/l; a chloride level below 20 mg/l; and a sodium level not to exceed 30 mg/l. Behind these stringent limitations is the use of the finished water for irrigation of crops, including citrus crops that are sensitive to these chemical species. Other parameters for the finished water are: • turbidity: up to 0.5 Nephelometric Turbidity Units • Hardness: 80-120 mg/l as calcium carbonate • Alkalinity: 80 mg/l minimum as calcium carbonate • pH: 7.8 – 8.5 • Residual chlorine: 0.1-0.5 mg/l. Desalination with distinction

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