Chinese Power Plant Chooses UF Membranes for Seawater RO Pretreatment

Recent industrial and urban growth in China has led to an explosive increase in energy demand. While this is good news for the country's power producers, a continual decline in freshwater supply has put undue pressure on the industry. With the existing freshwater supply being depleted at a rapid pace, the government is placing restrictions on its use and charging increasingly higher fees. Clearly, future industrial growth will depend upon the country's ability to find alternative sources of water.

One solution is to use desalinated seawater for industrial applications, thereby reducing the demand on groundwater and surface water supplies. 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 76,800 m3/day (just over 20 million gallons per day) of feedwater for the power plant. This plant will be the first in China to generate 1,000 megawatts per generator, and is also the country's first ultra-super critical, coal-fired plant.

Challenges of Seawater

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 cleanings, 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.

Fortunately, membrane filtration offers a simplified solution.

The Membrane Solution

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 percent of the time, turbidity less than 0.1 NTU, and Total Suspended Solids (TSS) less than 0.5 mg/L.

The ultrafiltration membrane system will provide 76,800 m3 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 0.5 mm 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 533 m3 per hour. The UF system piping is designed for a future capacity of 640 m3 per hour 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. In the event that 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 minimized, floor space requirements are substantially reduced, and future expandability is much easier to accommodate.

Benefits of Immersed Membrane Pretreatment

A major benefit of ZeeWeed UF membrane filtration is that the membrane provides a physical barrier to particulate matter, therefore 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 pressurized 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 a compelling choice 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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