River deltas experience extreme seasonal changes in water quality that perplex conventional water treatment systems. Industrial development in China, including new power plants, has spurred the development of desalination processes that have tamed these brackish water sources to provide a virtually unlimited supply of boiler-quality water.
The rapid development of an industrialized China has quickly increased the demand for pure water. In fact, the demand for industrial pure water a decade ago was 1,139 million m3 (300,892 million gallons) per year, and the Chinese government predicts that 1,839 million m3 per year will be required by 2030. The rising demand for industrial water has overstressed conventional supplies, pushing China to develop desalination technologies to meet its quickly increasing pure water needs. Today, China is a leading user of water desalination technology on a grand scale.
River water supplies in the delta regions of China, Vietnam, and Bangladesh and island nations like Singapore and Malaysia have decidedly inconsistent quality. It is common for river delta water to have a total dissolved solids (TDS) content that fluctuates from 80 mg/l to 12,000 mg/l due to the seasonal seawater refilling. This water is generally categorized as high-salinity brackish water (Table 1).
Table 1. Water quality definitions by TDS level. Source: China Water and Rivers Commission, June 2000
Given the interest of these delta regions in developing marginal water resources, a new category of supply water was established. The high-salinity water supplies that lie between freshwater (less than 500 mg/l) and standard seawater (35,000 mg/l) is called "sub-sea" water.
The salinity of the rivers in this part of Asia is highly seasonal. River water salinity is affected by rainfall, especially near the seashore, because of water runoff. However, in the dry seasons, the river water level will fall only to be refilled by seasonal sea or ocean tidewater that pushes up the river’s average salinity. The typical salinity range of a sub-sea water resource is between 2,000 and 8,000 mg/l. Another characterization of sub-sea water: high chloride levels due to refilling that further complicates the water treatment processes.
These salinity fluctuations render traditional ion exchange (IX) water treatment processes incapable of stable and reliable operation. Typical IX processes have a very narrow TDS operating band of 10 mg/l up to perhaps 600 mg/l. Reverse osmosis (RO) technology has successfully treated brackish water with salinity in the range of 1,500 mg/l to 5,000 mg/l with high salt rejection (SR). IX also suffers from other operational drawbacks, such as using acid and caustic chemicals for resin regeneration and post-treatment of those wastes, more water for regeneration, and a large area of plant floor space. As a result, RO is now the favored technology in 90% of Chinese industrial desalination plants (Figure 1).
1. Processes for treating water. Main desalination processes. Source: Dow Chemical
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