WATER TREATMENT
Sub-Sea Water Treatment System
Provides Reliable Supply for the
Huarun Power Plant
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 vir-
tually unlimited supply of boiler-quality water.
By Echo Zhao, Dow Water & Process Solutions; Yu Fangbing, Huarun Thermal Power Plant; and Yasushi Maeda, Dow Water &
Process Solutions
The rapid development of an industrial- ized China has quickly increased the de- mand 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).
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
10
Table 1. Water quality definitions by TDS level. Source: China Water and Rivers
Commission, June 2000
Water type
Freshwater
Marginal water
Brackish
Saline
Hypersaline
TDS range
Less than 500 mg/l (good quality)
500– 1,500 mg/l (over 1,000mg/l may have excessive scaling, corrosion and
unsatisfactory taste)
1,500– 5,000mg/l
More than 5,000mg/l
More than 50,000mg/l
1. Processes for treating water. Main desalination processes. Source: Dow Chemical
Distillation
Seawater RO
membranes
20,000
8,000 50,000
Brackish water RO membranes
Low energy BW RO membranes
Reverse osmosis (RO)
2,000
12,000
50
50,000
Electrodialysis
300
10,000
Ion exchange
600
Raw water salt concentration (mg/l)
100,000
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 salin-
ity 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 re-
generation 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 indus-
trial desalination plants (Figure 1).
