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STUMPY
POINT REVERSE OSMOSIS
WATER TREATMENT PROCESS
Wells
Stumpy Point Well #1 is the remote well located
approximately 1,000' from the plant site. It is drilled to a total depth
of 200'. The well casing is Schedule 80 PVC six inches in diameter with
a slot PVC screen from 170' to 190'. Stumpy Point Well #2 is the
well located behind the water plant. It is drilled to a total depth of 230'. The well casing is Schedule 40 PVC four inches in diameter with a slot
PVC screen from 170' to 190'. Each
of the well pumps is a Goulds, single stage submersible pump that is driven
by a 5 horsepower
Franklin submersible motor. They each deliver up to 70 gallons per minute.
The pumps are positioned 88' below the ground level.
Pre-Treatment
The two submersible well pumps bring the brackish
raw water to the plant for treatment through PVC piping.
A scale inhibitor is fed to the raw water to minimize the formation of
any precipitant on the membrane surface. The scale inhibitor in use at the
Stumpy Point Plant is Spectraguard/SC used at 6.0 - 7.0 ppm.
The water, after chemical addition and mixing, is filtered using one 5.0 micron
cartridge filter that will last from
6
to 9 months depending on the amount of water processed.
It will be replaced when the pressure drop exceeds 10 pounds per square
inch.
The Reverse Osmosis (RO) Treatment
 Once pre-treated, the water becomes feed water at
the RO unit. The unit consists of a
feed header, 3 long tubes called pressure vessels (PVS), interstage and
concentrate manifolds and a concentrate control valve.
In addition, there are various gauges and meters installed for analysis.
These three PVS house four membranes in series each.
The water enters the first two PVS where part of the total product is
separated from the feed. The
remaining pressure vessel squeezes out the rest of the product in the second
stage of treatment. This plant
operates on approximately 60% product recovery.
Better than one half of the feed water to the units eventually becomes
product water. The unit produces
60,000 gallons per day of product water.
The membranes used in the process are Aqua Pro eight inch TW-8040 membranes.
The average feed pressure to the units is 180 psi.
This is accomplished through the use of a Baldor pump with a 25 hp motor.
Post TreatmentAfter exiting the RO
unit, post treatment chemicals are added to the product water.
These chemicals are sodium hypochlorite for disinfection with a 1.0 part
per million (ppm) residual of free chlorine in the 10,000 gallon finished water
tank, fluoride for dental care at 1.0 ppm, sodium hydroxide for stabilization
and corrosion control and a zinc orthophosphate also for corrosion inhibition at
1-2 ppm. The finished water contains
about 40 ppm of total dissolved solids with 20 ppm being chlorides and 15 ppm
sodium. Calcium chloride is used to bring the total hardness up on the finished
water. The total hardness is two (2) before adding calcium chloride and
twenty-five (25) after post treatment. This helps with corrosion control
along with the other post treatment chemicals.
Waste DischargeThe 40% of the feed
water that is not product becomes a waste stream in which all of the dissolved
minerals removed in the Reverse Osmosis process are disposed. This stream is discharged into the
Stumpy
Point
Bay. The Bay is under the scrutiny of
the NC Division of Water Quality by way of an NPDES permit.
Computer Control
There is one SCADA (Supervisory Control and Data
Acquisition) Workstation HMI (Human Machine Interface) located at the Stumpy
Point Facility. It will be the primary point of operator interface at the
facility. It will communicate directly to the Modicon PLC (Programmable Logic
Controller) via Ethernet. An additional SCADA Workstation is provided for the
Skyco Facility. It is identical to the primary station but also includes the WIN
911 Alarm Notification Software. The processor located at the Dare County Skyco
Water Plant will be used for monitoring during hours that the Stumpy Point Plant
is unmanned. Page last updated February 24,
2004
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