Production of High Purity Water From Seawater

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Production of High Purity Water From Seawater

 

The Diablo Canyon Power Plant at Avila Beach in California
utilizes seawater for both cooling water and makeup water for steam generation.
Ionics, Inc., Watertown, Mass., designed and built and now operates a complete
water treatment system serving the high-purity water needs of this power plant.
Over the past eight years, the seawater treatment section has demonstrated
excellent long-term performance as a result of strong design, consistent
maintenance and qualified operators.

 

This article mainly discusses operating experience of the
seawater desalination section of the water treatment plant. However, there also
is some discussion of the rest of the integrated membrane system for
high-purity water production at Diablo Canyon Nuclear Station, operated by Pacific
Gas and Electric.

 

This entire system has operated reliably since installation
and start-up with very few technical problems. Remarkably, the seawater
membranes never have required cleaning or replacement.

 

The following discussion and data will highlight the design
and operation of this successful water treatment scheme.

Plant Background

The Diablo Canyon water treatment plant was commissioned in
March 1992. The main technical feature of this water treatment plant is that
the bulk of the water destined for the power plant steam boiler comes from the
Pacific Ocean.

 

Seawater is used at Diablo Canyon Power Plant (DCPP) to cool
down the waste heat generated by the twin nuclear reactors. The large intake
pumps draw millions of gallons of water per day, and a tiny fraction of this is
sent to the biolab. (The biolab is a large facility that maintains samples of
marine life to demonstrate their health and viability in the nuclear power
plant environment.) Seawater overflow from the biolab feeds the seawater
treatment system.

 

The seawater reverse osmosis (SWRO) facility generates a
maximum product flow of 450 gpm from two parallel units. Permeate product from
the seawater site is pumped to a set of twin man-made reservoirs, which have a
combined capacity of six million gallons. Flow to these reservoirs is
supplemented by seasonally available well and creek water that is pH adjusted
and chlorinated. Water is drawn from these reservoirs into the makeup water
treatment system (MWTS) where it is further processed for either power plant
steam makeup or drinking and domestic water. Figure 1 illustrates the water
treatment system.1

 

The makeup water treatment system combines ultrafiltration
(UF), electrodialysis reversal (EDR), double-pass RO, vacuum degasifier and mixed-bed ion exchange to produce high-purity water. Blended water also is fed to another RO system that produces potable water for plant drinking and domestic water.

 

Normal demand is approximately 180 gpm, but the MWTS quickly
can ramp up to 600 gpm to meet higher demand.

Description of Seawater Treatment Plant

The seawater treatment system consists of the following
train of unit operations (as shown in Figure 2).

 

                Lift
pumps.
Seawater feeds into a 6,000-gallon
tank. A set of custom-made 8-inch stainless steel strainers feeds a set of
three lift pumps—Worthington Model D-1012 50 hp pumps. These send the
water to the primary filters.

 

                Primary
filters (dual media).
A set of five dual
media (DM) filters each are 8 feet in diameter with 63 inches of media height.
The layers are gravel, coarse sand, fine sand and anthracite. Polyelectrolyte
and ferric sulfate are injected at this point for in-line coagulation. Flow is 4.0
gpm/sq. ft.

 

                Secondary
filters (multimedia).
A set of four
multimedia (MM) filters also are 8 feet in diameter with 63 inches of media
height. The layers are gravel, coarse sand, fine garnet, fine sand and
anthracite. Both the dual and MM filters are monitored for pressure drop.
Backwashing of the filter is initiated manually. For the MM filter, there is an
additional criterion for the SDI15 to be less than 3.0. If the SDI15 is greater
than 3.0, an additional backwash may be initiated. Flow is 4.5 gpm/sq. ft.

 

                Ultraviolet
(UV) lamps.
This set of three UV units
operates at 254 nm. Dosage is 30,000 µw sec/sq. cm. Two of three are used
at one time for full flow. Each has a flow capacity of 520 gpm.

 

                Cartridge
filters.
After Hypersperse AF200 antiscalant
injection, the flow continues to a set of four cartridge filter units, each
with a capacity of 440 gpm. Cartridge filter rating is 5 microns. Anti-scalant
is added as a requirement of RO process design to prevent scaling inside the RO
system. The criterion for cartridge filter replacement is pressure drop across
the filters.

 

                SWRO
high pressure pumps.
A set of three pumps
accepts and pressurizes the filtered water. Controlled by variable frequency
drives (VFDs), these pumps are 450 hp, each with 500 gpm capacity. Typical
operating feed pressure is 800 psi.

 

                Energy
recovery turbines.
Reverse-turning
turbines accept the brine reject flow. This reduces the load on the feed pump
thus reducing the power consumption of the SWRO system.

 

                RO
element array.
The array design is a
12-vessel first stage feeding a nine-vessel second stage. The first stage has
three additional spare vessels, and the second stage has one additional spare
vessel. However, the design has been maintained at the 12:9 staging with seven elements
in each vessel. The RO elements are Filmtec SW30-8040HR. These are operated at
45 percent recovery with 99 percent or higher

salt rejection.

                Product
tank and transfer pumps
. RO permeate goes
to a 10,000-gallon tank. A set of three 40 hp product transfer pumps sends the
water to the MWTS reservoirs.

 

                RO
reject.
RO waste reject flow is sent to a
backwash tank. This backwash water is used for a high-salinity flush for the DM
and MM filters. The high salinity helps to reduce bacteria viability by osmotic
shock. The availability of this backwash water reduces the need for additional
feed water pumping.

 

The number of units used depends on plant demand. All three
RO feed pumps and energy recovery turbines are rotated so that each operates
the approximate same amount of time. This also is true of the two main RO
units, A and B. A similar philosophy applies to the UV units, lift pumps and
product transfer pumps that are rotated automatically.

Makeup Water Treatment System

The MWTS consists of a blend of seawater permeate and MM
processed creek water that is drawn from man-made reservoirs with lift pumps to
feed the five triple membrane trailers (TMT).2 Each TMT is capable of producing
120 gpm. The TMT consists of ultrafiltration for pretreatment, electrodialysis
reversal for dissolved solid reduction and RO for further reduction of ionic
materials.

 

The RO permeate from each TMT is fed to a second pass of RO,
housed in another trailer referred to as the RO polish trailer. The five RO
polish trains reduce the conductivity to approximately 1 µs/cm. The RO
polish reject streams are recycled back to the RO feed in the TMT to achieve
higher water recovery.

 

The RO polish permeate is fed to a 600 gpm vacuum
degasifier. The vacuum degasifier reduces the dissolved oxygen to less than 10
ppb (parts per billion). The degasifier effluent is pumped to an ion exchange
trailer consisting of two stages of 40 3.6 cu. ft. portable vessels. The ion exchange trailer also contains a silica monitor, resistivity instrument, total organic carbon and pH meters to assure high purity water specifications. Critical water quality specifications for high purity water are 5 ppb silica, 20 ppb oxygen, 50 ppb TOC and above 18 MegOhm/cm for resistivity.

 

In addition to supplying 600 gpm of ultrapure water for the
plant, the MWTS has a 100 gpm potable water system. This system consists of a
separate RO unit with a carbon bed and MM pretreatment. After the RO unit, the
flow continues to a calcium bed. Calcium hydroxide and sodium hypochlorite
injection is used for bacteriological control. Carbon dioxide also is injected
to adjust the pH to 8.5 for a noncorrosive water.

 

While this ultrapure water makeup system design was state of
the art in the early 1990s when it was conceived, a current design (or plant
upgrade) would most likely include electrodeionization technology to reduce the
frequency of ion-exchange bottle change-out.3

Seawater RO Performance

Performance was tracked according to a number of operating
parameters: salt rejection, normalized salt passage, normalized product flow
and differential pressure.

 

                Salt
rejection.
After 3,000 days of operation,
salt rejection has been maintained at the target of 99 percent. The average
salt rejection actually has increased a few tenths over this period of time due
in part to fouling effects. No spikes in performance are noted. SWRO product
quality averages 400 µs/cm conductivity. A graph of salt rejection vs.
time is shown in Figure 3.

 

                Normalized
salt passage.
A slow decline in normalized
salt passage is noted up until approximately 1,500 days of operation. After a
plateau of 400–500 days of operation, the data shows an incipient
increase in salt passage. The gap in data is where this parameter was not
calculated. A graph of salt passage vs. time is shown in Figure 4.

 

                Normalized
product flow.
This flow was very steady
until the point where the normalized salt passage showed an inflection point.
The product flow began to decline slightly at this point. This is an indication
of fouling and loss of product flow leading to a point where a membrane
cleaning will become necessary. A graph of normalized product flow vs. time is
shown in Figure 5.

 

                Differential
pressure.
The differential pressure shows
a minor adjustment at about 1,200 days of operation. After that adjustment,
differentials were steady until 2,100 days of operation where both stage one
and stage two differentials began to migrate upward, also an indication of flow
but steady fouling. The second stage appears to be fouling more quickly than
the first stage. A graph of differential pressure vs. time is shown in Figure
6.

 

These long-term performance results appear to be nearly
identical for both RO units. Minor differences are attributed to factors such
as off-line time and temperature.

 

Energy consumption also was very stable at 17 kw hr/kgal at
the seawater sites as seen in Figure 7. Energy users include style="mso-spacerun: yes"> the lift pump, UV lamps, RO pumps,
metering pumps, auxiliary equipment  and product transfer pumps. Energy  use of the SWRO HP pumps only is approximately 15 kw hr/kgal.

 

Online availability for the SWRO unit has been 100 percent.
At least one high-pressure SWRO pump always has been available for service.
Over the entire 3,000 days of unit operation, there were only 16 days in which
full capacity (two RO pumps and units) was not available. This represents a
99.5 percent full capacity availability.

 

SWRO permeate comprises the bulk of the available water in
the twin reservoirs. This water then is drawn from the reservoirs and further
treated for both drinking and domestic or water supply for power plant steam
generation.

 

Overall, the water treatment system consistently has
produced the full volume of in-spec water required by the power generation
needs. The seawater treatment RO system consistently has produced high-quality
product since start-up and commission. In addition, the SWRO membranes have not
required a chemical cleaning since start-up.

 

This successful long-term performance of the SWRO system is
a result of several factors. These include careful attention and maintenance of
MM filters and bacteria growth in the RO pretreatment train. Important factors
are as follows.

 

                Six
parts per million (ppm) of ferric sulfate and one ppm of polyelectrolyte are
injected into the feed stream prior to the dual media filters. Bacteria counts
as colony-forming units (CFU) are very low leaving the media filters.

 

                Chemical
pumps feeding the pretreatment train are calibrated and verified daily.

 

                Media
filter pressure drop and rate

 

of increase of pressure drop is very carefully monitored. If
there is a storm and more frequent backwash is required, the filters are taken
off automatic operation and backwashed manually as required on a 24-hour basis.
These filters are backwashed with RO concentrate. The high salinity helps to
keep bacteria growth in check.

 

                When
an RO line is shut down, the membranes are flushed with RO permeate. This
presents a lethal concentration gradient to ambient bacteria populations since
the RO permeate salinity level represents an extreme drop in salt
concentration.

 

                UV
treatment also is a key to success. Bacteria counts as CFU are near zero
downstream of the UV unit.

 

                Silt
density indexes (SDIs) also are carefully monitored. This is done in
conjunction with observation of ocean conditions—swell frequency as an
indicator of depth of swell and, therefore, ocean bottom turbulence. Anticipation
of plankton blooms and plant shutdown also is critical.

 

These careful maintenance procedures are critical in maintaining the good functioning of the RO system.

 

The long, stable operating run with steady salt rejection
indicates the excellent performance of this SWRO system.

 

This is an indication of an appropriate design and
specification of pretreatment, SWRO array, membrane type and materials of
construction.

 

The design also was appropriate to the feed source and
production requirements. The extremely long period of operation without
requiring chemical cleaning of the RO elements also is a testament to accurate
design. Long-term high performance can be attributed in part to steady and
timely maintenance of all equipment by well-trained and responsible operators.
This includes efforts to combat seawater corrosion, motor maintenance, pump
maintenance, periodic inspection and other preventative maintenance issues. style='mso-tab-count:1'>  

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