By Ruming Pang
Water scarcity limits electricity production in China, but a new power plant in this Liaoning Province coastal city incorporates a twostage RO system with pretreatment membranes from Koch Membrane Systems to capitalize on seawater as an alternative source
Scarcity of water resources impedes economic development in China, both directly and indirectly. A 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 a rapidly growing economy.
To produce electricity, power plants require large volumes of high quality water for boiler make-up and cooling. In most locations, though, no surplus water is available and the government won’t allow power plants to place additional demands on an already scarce water supply. To gain government approval, new power plants must either find alternative sources of water like seawater and wastewater or pay very high fees for surface water or groundwater.
At the new electrical power plant in the Liaoning Province coastal city of Zhuanghe on the Dalian peninsula east of Beijing, 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.
Construction of the Zhuanghe plant was divided into two phases. The first phase, completed in October 2006, provides 1,200 megawatts. When the second phase was to be complete at the end of 2007, the total capacity would expand to 3,200 mW. During the first phase, the plant drew surface water from a reservoir located 19 kilometers away. With completion of the second phase, the plant was to switch over to seawater for all cooling water and boiler makeup needs.
UF makes RO economical
“The Zhuanghe plant has been designed as a model facility to showcase the best available technology,” according to plant manager Mr. Zhang. “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.”
Optimizing SWRO/BWRO processes
The reverse osmosis system at the Zhuanghe plant also has an advantage of being a twostage 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 RO systems performance, and protect them from fouling, an effective pretreatment system is required. Zhuanghe plant officials 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 desalinations plants and other large-scale RO systems in China and around the world.
Hollow fiber UF solution
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. TARGA®-10 UF cartridges from Koch Membrane Systems Inc. (KMS) were chosen for the project after plant officials visited two power plants that employ TARGA cartridges for very similar RO pretreatment applications.
The cartridges employ a proprietary semi-permeable polysulfone hollow fiber membrane successfully deployed in municipal and industrial water treatment plants in China and many other countries for more than a decade. In China alone, TARGA cartridges treat over 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 removal of particulates and larger molecular weight components. These KMS fibers have demonstrated the ability to reduce turbidity to less than 0.1 NTU and SDI to between 1.0 and 3.0, making these UF units an ideal pretreatment step for spiral-wound RO membranes.
Two trains of UF cartridges were commissioned in October 2006, and three additional trains were commissioned in the second half of 2007 for the second phase of the plant’s construction. The trains operate in parallel, and are each equipped with 44 TARGA 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-to-1.0 mg/L. A 100-μm screen web pre-filter, that can be automatically backwashed, is installed before the UF to remove larger particles.
A sustainable future
“The most important reason that ultrafiltration was selected for seawater pretreatment is the high quality of the permeate,” said Mr. Zhang. “The high permeate quality results in fewer RO cleaning passes and ensures longer RO element life. With the TARGA ultrafiltration system, we are able to cost-effectively tap the limitless seawater and avoid draining our scarce surface water resources.”
For over 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.
Author’s Note:
Ruming Pang is regional sales manager in China for Koch Membrane Systems Inc. He’s based in Shanghai. This article is a summary of a paper presented at the 2007 World Desalination Congress in Gran Canarias, Spain. KMS, a Koch Chemical Technology Group LLC company, is based in Wilmington, Massachusetts, USA. Contact: +86 (21) 62674846, info@kochmembrane. com or www.kochmembrane.com
