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NORTH REVERSE OSMOSIS (NRO) WATER TREATMENT PROCESS

Wells

Four additional wells were drilled in 2005 and are 420 feet deep and 8 inches in diameter. Two out of the four have 60 HP Franklin motors which pump up to 550 GPM at 215 feet of head. The other two have 50 HP Franklin motors which pump up to 550 GPM at 190 feet of head.

Pre-Treatment

The scale inhibitor presently in use is a King Lee Pretreat Plus product used at 3.5-4.0 ppm added before the membranes also 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. They are replaced when the pressure drop exceeds 10 pounds per square inch.

The Reverse Osmosis (RO) Treatment

Once pre-treated, the water becomes feedwater at the RO units. Each of the three units consists of a feed header, 35 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 35 PVS house six membranes in series each. The water enters the first 25 PVS where about 50% of the total product is separated from the feed. The remaining 10 PVS squeeze out the remainder of the product in the second stage of treatment. Our plant runs on a 78% product recovery. That is, approximately three quarters of the feed water to the units eventually becomes product water. About 7% 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 999,360 gallons per day of permeate that is blended with 72,000 gallons of raw water per day for a total of 1.07 MGD per RO Unit.

We use Hydranautics membranes Model 8040-LH4-CPA3, 8-inch, spiral wound, brackish water membranes. They produce a 97% salt rejected permeate.

The average feed pressure to the units is 260 psi. This is accomplished through the use of 8 inch by 6 inch submersible, 6 stage, vertical turbine Afton pumps with 250 hp U.S. motors. Our facility is the first of its kind where the feed water pumps are adjusted with variable speed drives (Siemens) to conserve energy.

Arsenic Removal System

The product from the RO treatment has a small amount of chlorine added to achieve a 0.3 mg/L residual and then is routed through six arsenic removal vessels. These vessels are arranged in two sets of three. The two sets are in series. The vessels are 12 feet in diameter; they are filled with 40 inches of granular ferric hydroxide that the water flows down through. The arsenic is adsorbed by the ferric hydroxide media. The arsenic vessels were put on line in November 2004 in an effort to meet the new Federal MCL (maximum contaminate level) for arsenic of 10 µg/l (micro grams per liter) that came into effect in January of 2005. That level was an 80% reduction from the old standard of 50 µg/l. The arsenic occurs naturally in the earth's crust and is found at about 40 µg/l in the NRO well water. The RO process removes about half of the arsenic from the raw water which leaves about 14 to 20 µg/l arsenic in the permeate water that feeds the arsenic removal vessels. The ferric hydroxide media will be removed and replaced under an EP A approved process when it no longer has the ability to remove arsenic down to the required limit of 10 µg/l. At the time of this writing, October 2006, the arsenic continues to be removed to below 1 µg/l or ten times lower than the standard requires. We have been very pleased with the performance of the arsenic removal process.

Post Treatment

After exiting the arsenic removal vessels, the product mixes with 7% raw water blend prior to the addition of the post treatment chemicals. These chemicals are chlorine for disinfection at around 5.0 parts per million (ppm) dosage, Fluoride for dental care at 1.0 ppm, Sodium Hydroxide for stabilization and corrosion control at 18 ppm and a Zinc orthophosphate also for corrosion control at 4-5 ppm. The finished water contains about 375-400 ppm of Total Dissolved Solids with 180 ppm being Chlorides and 90 ppm Sodium.

Waste Discharge

Computer Control
 
The production process is controlled by Allen -Bradley Programmable Logic Controllers   (PLC). These PLCs control pre-treatment, processing the water through RO units and arsenic removal vessels, and post-treatment, and is fully automated with alarm subsystems for operator intervention.

The Distribution System is controlled by Schneider Electric Momentum PLCs. These PLCs control tank levels, wells, and turns pumps on and off  to meet demand. It is fully automated with alarm subsystems for operator intervention. 

The HMI (Human Machine Interface) software used is GE Fanuc's Proficy HMI/SCADA iFix.


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 our energy requirements cost only $0.134 per thousand gallons produced, at 2.981 kilowatt hours per thousand gallons, this electrical cost represents 30.5% of the total cost to produce the water.

Although this process is more expensive than conventional treatment, 0.439 per thousand gallons compared to 0.294 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 October 2006

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