Power to seawater RO in China
The growth of industry and rising cost of water in China has led one energy company to opt for desalination for boiler feed water and drinking water at its power plant in the coastal province of Zhejiang. Technical specialist Angela Amalfitano and Kenneth Lam, managing director for China, Taiwan and Hong Kong, at Canadian membrane manufacturer Zenon Environmental, explain the plan.
The use of desalinated seawater for industrial applications is one such solution. Huaneng Power International, one of China's largest independent power producers, is doing just that.
Located in Zhejiang Province, the Huaneng Yuhuan Power Plant will draw seawater from the East China Sea. After desalination, the treated water will be separated into two streams for use at the plant: boiler feedwater and potable water. Any surplus potable water will be sold to the community.
Given the high levels of impurities in seawater, and the detrimental effects of fouling on RO membranes, effective pretreatment is vital. The company is turning to immersed membrane technology for cost-effective, high quality pretreatment of seawater for its reverse osmosis (RO) desalination system.
Zenon ZeeWeed® immersed ultrafiltration (UF) membranes will be used in the Yuhuan Power Plant, opening in 2007. The Yuhuan plant is a significant project, for a number of reasons. The ZeeWeed system is one of the largest UF pretreatment installations of its kind, producing 76,800m3/d of feedwater for the power plant. This plant will be the first in China to generate 1000MW per generator, and is also the country's first ultra-super critical, coal-fired plant.
Challenges of pretreatment
While RO is a common and effective method of desalination, it requires aggressive pretreatment to ensure stable operation. At a minimum, RO membranes require a feedwater turbidity of less than 1.0 NTU, and a silt density index (SDI) of less than 4. If the feedwater quality does not meet these requirements, the RO membranes will foul rapidly.
Fouled membranes suffer from reduced efficiency, leading to higher operating costs, more frequent cleaning, and increased membrane replacement costs. Therefore, proper pre-treatment is considered paramount to protect expensive RO systems and to ensure efficient operation.
Varying turbidity poses a significant challenge to seawater desalination at this site. The seawater turbidity varies throughout the year, and can change by one or two orders of magnitude within the hour. This volatility in raw water characteristics can make it very difficult to produce consistent, high quality feedwater for RO systems.
Conventional pretreatment methods typically incorporate a complex system of coagulation, flocculation, media filtration, and cartridge filtration, combined with chemical addition. Such methods, however, must be continuously optimized to deal with fluctuating influent characteristics.
System upsets are common with such an approach, and can lead to increased power consumption, more frequent chemical cleanings, reduced RO membrane life, a variety of pressure problems and reduced plant availability. The resulting costs of conventional pretreatment can be prohibitive.
The pretreatment system at Yuhuan Power Plant will consist of enhanced coagulation clarifiers and membrane tanks. Six trains of Zenon ZeeWeed 1000 hollow fibre membranes will be used. The pretreated RO feedwater quality will be exceptional, with SDI less than 2.5 90% of the time, turbidity
less than 0.1 NTU, and Total Suspended Solids (TSS) less than 0.5mg/L,
The UF membrane system will provide 76,800m3 per day of high quality feedwater to the RO system, which represents a recovery rate of 95%. Influent will first be pumped into four clarifiers where enhanced coagulation will help to remove settleable organic and floatable solids. The clarified water will then flow by gravity through 0.5mm punched-hole screens and into the UF feed distribution channel.
From the distribution channel, the clarified water will flow to six individual membrane trains, each providing an average day flow of 533m3/h. The UF system piping is designed for a future capacity of 640m3/h per train, to allow for possible expansion.
The UF membrane cassettes will be immersed directly in the process tanks. A low-pressure vacuum applied to the membranes will draw filtrate (permeate) through the microscopic pores and into the hollow fibres. The membranes form a physical barrier against suspended particles and colloidal materials. Rejected particles remain in the process tank and are periodically removed by backwashing.
Turbidity meters and particle counters will continuously monitor the quality of the ZeeWeed permeate. This ensures that each train will operate within the design parameters. If turbidity or particle counts rise, an alarm will notify the operator so that appropriate action can be taken.
The ZeeWeed® membrane filtration system is designed for simplicity of operation. All normal plant operations are automatically controlled by the PLC system. For example, there are no daily operations that require manual operation of valves. The system design philosophy is to substantially reduce the potential for problems associated with operator error.
By using immersed membrane technology, the pretreatment process at Yuhuan is significantly streamlined. As a result, operational costs are minimised, floor space requirements are substantially reduced, and future expandability is much easier to accommodate.
A major benefit of UF membrane filtration is that the membrane provides a physical barrier to particulate matter, so that high quality water is produced regardless of the seawater turbidity. Membrane technology represents a superior method of RO pretreatment, providing stable and consistent feedwater quality and substantial resistance to system upsets.
UF membranes also offer high performance in a smaller footprint than granular filter media or pressurised microfiltration systems. Zenon membranes are configured in modular cassettes that are ideal for retrofitting or expanding plants that are currently using conventional pretreatment systems.
The high surface area offered by UF membranes can ease space constraints during expansions or retrofits, and provide savings in land acquisition and capital costs. These benefits, combined with ease of cleaning, higher RO membrane operating flux, and reduced downtime, maintenance, and operating costs, make ZeeWeed immersed UF membrane technology ideal for RO pretreatment.
UF pretreatment for RO is increasing worldwide for many applications such as seawater desalination, industrial processes, and wastewater reuse. ZeeWeed membranes are a proven RO pretreatment solution in over 60 sites around the world, many operating in very harsh industrial environments.
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