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REVERSE OSMOSIS WATER TREATMENT PROCESS
Wells
The
source of the feed water is from the Mid Yorktown Aquifer.
Two wells are drilled to a depth of 395 feet below the
surface and are screened from the 300 foot depth to the
bottom. Both well pumps are 6 inch, two stage submersible
turbine pumps. Well #1, at the water treatment facility, is
driven by a 25 horsepower motor capable of delivering 500
gpm and Well #2, at the elevated tower site, is powered by a
50 horsepower motor delivering with a 600 gpm capability.
They are positioned 80 feet below the surface of the well.
Pre-Treatment
These
well pumps bring the brackish raw water to the plant for
treatment through PVC plastic pipe. The raw water contains
approximately 1,310 parts per million (ppm) total dissolved
solids. This is approximately 4% of the salinity of sea
water which contains about 35,000 ppm total dissolved
solids.
The raw water is injected
with scale inhibitor which is Pre Treat Plus 0100 by King Lee Industries. It is
fed at 3.5 ppm and is added before the membranes to minimize the formation of
any precipitant on the membrane surface.
The water, after scalant
addition and mixing, is filtered using 40 inch long polypropylene wound
cylindrical cartridge filters. The cartridge filters are sized at a nominal 5.0
micron and last up to a year depending on the amount of water processed.
The Reverse Osmosis Treatment
Once pre-treated, the water becomes feed
water for one of two Reverse Osmosis units which are called
Trains. Each of the two Trains consists of a feed header,
long tubes called pressure vessels (PVS), interstate and
concentrate manifolds and a concentrate control valve. In
addition, there are various gauges and meters installed for
analysis. Train #1 has 18 PVS tubes each housing six 40 inch
long membrane elements in series. Train #2 has 14 PVS tubes
each housing six 40 inch long membranes.
The
water enters the first 10 PVS where about 50% of the total product (permeate) is
separated from the feed. The remaining PVS tubes squeeze out the remainder of
the product in the second stage of treatment. The plant runs on a 77% to 80%
product recovery. That is, a little over three quarters of the feed water to the
units eventually becomes product water.
About
10% of the total water produced is raw water mixed into the permeate to add
alkalinity and some hardness, as well as to enhance some stability to this
aggressive, low total dissolved solids permeate created through the RO process.
Together with the raw water blend, the two units are capable of producing
approximately 1,180,400 gallons per day of treated water.
The
plant utilizes Toray TMG20-430 membranes. These are 8 inch spiral wound brackish
water membranes. They produce a 97% salt rejected permeate product. The average
feed pressure to the units is 140 psi. This is accomplished through the use of
horizontal centrifugal Patterson pumps with 125 hp motors. These motors are
adjusted with variable speed drive controllers.
Post Treatment
After
exiting the RO units, the permeate mixes with the raw water
prior to the addition of the post treatment chemicals. These
chemicals are chlorine for disinfection at around 4 parts
per million (ppm) dosage, Fluoride for dental care at 1.0
ppm, Sodium Hydroxide for stabilization and corrosion
control and Zinc Orthophosphate also for corrosion control.
The finished water contains about 200 ppm of total dissolved
solids. The blended water contains around 8 ppm total
hardness. After mixing, the treated finished water goes
directly to a 1.0 MG water ground storage reservoir.
Waste Discharge
The 20%
to 23% of the feed water that is not product becomes a waste
stream in which all of the solids removed in the Reverse
osmosis process are disposed. This stream is discharged into
the sound at Blackmar Gut. This waste water contains
approximately one-third the salinity of its receiving stream
and is under the scrutiny of the Department of Water Quality
by way of an NPDES permit.
Computer Control
The entire process is completely controlled by an Allen Bradley Programmable
Logic Controller (PLC). It controls pre-treatment, processing the water
through RO units, and post-treatment fully automated with alarm subsystems
for operator intervention if need be.
Summary
The major
operating cost is the electricity used to generate the
needed RO pressure to push the feed water through the RO
membranes.
Although this process is more
expensive than conventional treatment, it produces drinking water from a
brackish saltwater source that is otherwise non-potable and has allowed us, on
the Atlantic Coast, the ability to keep up with customer demand in an area
sparse in freshwater sources.
Page last updated July 21, 2011 |
