Power Plant Taps the Sea with Ultrafiltration

Written by Ruming Pang, Koch Membrane Systems, Inc.

Pumps & Systems, October 2007

The scarcity of water resources directly and indirectly impedes economic development in China. The lack of potable water directly impairs the health and well-being of millions of people and restricts agricultural and industrial activity. The shortage of water also causes a secondary problem by constraining the ability to expand electrical power production to meet the needs of the rapidly growing economy.

To produce electricity, power plants require large volumes of high quality water for boiler make-up and for cooling. In most locations, however, no surplus water is available and the government will not allow power plants to place additional demands on already scarce water supply. To gain government approval, new power plants must either find alternative sources of water like seawater and wastewater or they must pay very high fees for surface water or groundwater.

At the new electrical power plant in the Liaoning Province coastal city of Zhuanghe, seawater was a natural choice. The plant supports the region's thriving economy, which is driven in large part by the governing sub-provincial city of Dalian, a prosperous industrial center with the third largest port in China and the country's northernmost ice-free seaport.

The construction of the Zhuanghe plant has been divided into two phases. The first phase, completed in fall of 2006, provides 1200-MW. When the second phase completes at the end of this year, the total capacity will expand to 3200-MW. During the first phase, the plant drew surface water from a reservoir located 19-km away. Once the second phase is completed, the plant will switch over to seawater for all cooling water and boiler makeup needs.

Ultrafiltration Makes RO Economical

"The Zhuanghe plant has been designed as a model facility to showcase the best available technology," according to Mr. Zhang, the plant manager. "The first large saltwater desalination plant in China was commissioned in 1999, and it has since been well-established that reverse osmosis is an incredibly economical process, with lower operating costs and a smaller footprint than thermal distillation."

The reverse osmosis (RO) system at the Zhuanghe plant also has the advantage of being a two-stage process that can be tailored to meet different requirements for particular applications. Only the first stage, seawater reverse osmosis (SWRO), is required for the cooling water used by accessory equipment. Boiler makeup requires higher quality water that must also pass through the second stage, a brackish water reverse osmosis (BWRO) system.

To optimize the performance of the RO systems and to protect them from fouling, an effective pretreatment system is required. The Zhuanghe plant chose ultrafiltration (UF) as the pretreatment solution because UF occupies a small footprint and provides higher permeate quality as compared to conventional pretreatment systems. Fully automatic control and relatively low investment costs were also important factors in the selection of UF technology.

UF pretreatment for RO systems is an increasingly common combination in desalination plants and other large-scale reverse osmosis systems in China and around the world.

Beijing Lucency Enviro-Tech Co., Ltd., one the largest providers of industrial and municipal water filtration systems in China, was responsible for designing and installing the UF pretreatment system.

The Zhuanghe plant decided on the type of UF cartridges to use after plant officials visited two power plants that employ the cartridges for similar RO pretreatment applications.

These cartridges employ a proprietary semi-permeable polysulfone hollow fiber membrane that has been successfully deployed in municipal and industrial water treatment plants in China and many other countries for more than a decade. In China alone, these cartridges treat more than 500,000-m3/d (132-mgd) of water from a variety of sources.

The hollow fibers are true ultrafiltration membranes, with a nominal molecular weight cut-off of 100,000 daltons, resulting in the removal of particulates and larger molecular weight components. These fibers have demonstrated the ability to reduce turbidity to less than 0.1-ntu and SDI to between 1.0 and 3.0, making it an ideal pretreatment step for spiral RO membranes.

Two trains of UF were commissioned in October 2006, and three additional trains will be commissioned by the end of 2007 for the second phase of the plant's construction. The trains operate in parallel, and are each equipped with 44 cartridges having a total capacity of 232-m3/h per train.

Prior to the UF, the seawater is pretreated with coagulation and sedimentation, both performed in the same tank. FeCl3 or Poly FeSO4 is used as coagulant, and if needed, anionic PAM is used as coagulant aid. To control biological growth, sodium hypochlorite is added to the feed of the sedimentation tank. The controlled free chlorine before the UF is about 0.3-mg/l to 1.0-mg/l. A 100-μm screen web pre-filter that can be automatically backwashed is installed before the UF to remove the large particles.

A Sustainable Future

"The most important reason that ultrafiltration was selected for seawater pre-treatment is the high quality of the permeate," explains Mr. Zhang. "The high permeate quality results in fewer RO cleaning passes and ensures longer RO element life. With our ultrafiltration system, we are able to tap the limitless seawater cost-effectively and avoid draining our scarce surface water resources."

For more than two millennia, the Dalian region has derived fame and fortune from its strategic coastal location. Now, with UF and RO technology, the sea will support the region's growth in yet another way: by providing a sustainable supply of water for the new electrical power plant.

Ruming Pang is the regional sales manager, Greater China for Koch Membrane Systems Inc. Shanghai Office. Koch Membrane Systems, 850 Main Street, Wilmington, MA 01887-3388, 888-677-5624, Fax: 978-657-5208, http://www.kochmembrane.com/.

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