Treating Sub-Sea Water
The two-step process for sub-sea water
treatment is pretreatment to remove suspended solids followed by desalination to
remove dissolved solids in the raw water
(Figure 2). Specifically, there are two stages of pretreatment. First, the raw water is
pumped into a clarifier. After reacting with
flocculants, such as polymerized aluminum
chloride (PAC), the suspended solids in the
water are reduced to less than 5 mg/l. In
the second stage, the water is fed into the
second stage of pretreatment, starting with
two multimedia filters (MMF) in series to
further remove residual impurities, such as
suspended solids, colloids, and organics so
that the effluent’s Silt Density Index (SDI)
is less than 5. Before the filters, 2 mg/l to
5 mg/l of sodium hypochlorite (NaClO) is
added to control bacteria growth.
However, if there is not enough budget,
or if there is limited space for water treatment equipment, hollow fiber ultrafiltration
(UF) may be another effective option in the
second-stage pretreatment to protect the RO
membranes from suspended solids, bacteria,
and nondissolved organics. The SDI of effluent from UF can reach less than 3 mg/l.
After the second stage of treatment (
filtration or UF), sodium bisulfite (NaHSO3)
and an antiscalent are added to remove any
residual free chlorine and to avoid scaling
of the RO membranes. The water is then filtered to 5 microns, typically in a cartridge
filter, and then pumped to the RO units for
desalination.
In a typical RO unit, 95% to 98% of salt
and other solved organics, colloids, and bacteria are removed in the reject water. If the
water is for potable use, two or three stages
of single-pass RO are used to obtain water of
the specified quality. If higher purity water
is required for industrial purposes such as
boiler water, then a two-pass RO or a two-pass RO followed by IX mixed bed and/or
continuous electrodeionization (CEDI) will
be required.
As an example, Yuhuan Water Treatment
Corp.—located in Yuhuan Town, Wenzhou
City, Zhejiang, China—owns and operates a
desalination potable water plant. The town’s
sub-sea water TDS is about 2,800 mg/l to
3,800 mg/L at a temperature ranging from
12C (54F) to 17C (63F). During one 101-
hour operating period, the water feed TDS
changed from 5,500 µS/cm to 7,000 µS/cm
(measuring TDS based on conductivity rather than weight). The TDS of the RO permeate
or product water also changed from 120µS/
cm to 300µS/cm. Here, the RO permeate
water meets the requirement of the Chinese
Standards for Drinking Water Quality of less
than 1,000 mg/l TDS. The U.S. TDS limit for
2. Removing water hardness. A typical sub-sea water desalination treatment process.
Source: Zhejiang Hydrotechnics, June 2006
Pretreatment ank Raw water boost pump NaCIO dosing
Underwater
pump
PAC
dosing
Clarifier
Mechanical
filter
Demineralized
water tank
1st pass and
3-stage reverse
osmosis
High-
pressure
pump
Cartridge
filter
( 5 micron)
NaHSO3 &
antiscalant
dosing
potable water set by the Environmental Protection Agency is 500 mg/l.
RO Makes Power Plant
Makeup Water
The Huarun Power Plant (HPP) entered commercial service in 2007 with two pulverized coal boilers each producing 260 tons/hr
(573,000 lb/hr) of steam used to generate 360
MW in a pair of like-sized steam turbines.
The plant is located in Huangge Town in the
Nansha Development Zone, an important hub
port at the Pearl River Delta south of Guang-zhou City, Guangdong Province, China.
HPP’s makeup water comes from the
Xiao HuLi River, which is close to the
South Sea entrance, hence the river water’s
TDS, chloride, and conductivity increase as
a result of seawater refill during the dry season. An RO system was supplied by Nanjing
Zhongdianlian Co. designed to produce the
necessary boiler makeup water for the two
power generation units from the available
sub-sea water supply.
In Table 2, the TDS record of Xiao HuLi
River’s water shows wide TDS variation, from
80 mg/l to 14,000 mg/l in 2006, and chemical oxygen demand (COD) that ranged from
4.92 mg/l to 39. 1 mg/l. The boiler makeup
water specification requires water quality
of 15 MΩ/cm at 25C (equal to 0.067µmhos/
cm), which is more rigorous than the normal
boiler makeup water standard (0.15 µmhos/
cm); hence, a two-pass RO and CEDI or IX
mixed bed was required for this water treatment system (Table 3).
Table 2. Xiao HuLi River’s aver-
age water quality record from
2004 to 2006. Source: Dow Water &
Process Solutions
Measurement
pH
Temperature (C)
Total hardness (mmol/l)
COD (Mn), mg/l
BOD5 (mg/l)
TDS (mg/l)
Silicon (mg/l)
Cl–(mg/l)
Dissolved oxygen (mg/l)
SO42- (mg/l)
Ca2+ (mg/l)
Na+ (mg/l)
Mg2+ (mg/l)
Data range
6.92–7.69
15–35
0. 3–46
4.92–39.1
2. 58–10
80– 14,000
2–13
9–7,000
3. 27–4.78
19–307
21. 2–62. 44
2–7,000
3. 31–65. 25
Table 3. Boiler makeup water re-
quirements of Huarun Power Plant.
Source: Dow Water & Process Solutions
Constituent
Hardness (µmol/l)
Sodium (µg/l)
Resistance (MΩ/cm @
25C) ≥ 15
SiO2 (µg/l) < 15
Requirement
≈0
<100
