|
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. Each of the well pumps are Goulds (6
inches in size), two stage submersible turbine and are driven with 25 horsepower
Franklin submersible motors. They each deliver up to 500 gallons per minute.
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 first pre-treated with 93% sulfuric acid to depress the
raw pH of the water from about 8.0 down to 7.6. The acid is required for
pre-treatment to minimize the potential for calcium carbonate scaling of
the membrane surface and to increase the carbon dioxide content of the
permeate for stabilization of the finished product in post-treatment.
The raw water is also injected with scale inhibitor which is Hypersperse
AS 120 by Hercules Betz Dearborn. 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 chemical 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 from 2 to 3 months depending
on the amount of water processed.
The Reverse Osmosis Treatment
Once pre-treated,
the water becomes feed water at the RO units. Each of the two units consists
of a feed header, 14 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. These 14 PVS
house six 40 inch long membrane elements in series each. The water enters
the first 10 PVS where about 50% of the total product (permeate) is separated
from the feed. The remaining 4 PVS squeeze out the remainder of the product
in the second stage of treatment. Our plant runs on a 75% product recovery.
That is, 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. Each unit produces 450,000 gallons per day of permeate
that is blended with 50,000 gallons of raw water per day for a total of 500,000 gallons per day.
The plant utilizes Dow Chemical Filmtec 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 190 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 a corrosion inhibitor zinc orthophosphate also for corrosion
control. The finished water contains about 200 ppm of total dissolved solids.
The blended water contains around 15 ppm Total Hardness. After mixing,
the finished water goes directly to a 1.0 MG product ground storage reservoir.
Waste Discharge
The 25% 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. It represents
25.3% of the total cost to produce the water.
Although this process is more expensive than conventional treatment
($0.542 per thousand gallons compared to $0.275 per k gallons last year
at our 5.0 MGD Ion Exchange WTP) it produces a drinking water from a brackish
saltwater source 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 November 2,
2001 |
