Jul 26 2014

Desalination in Dare County – More on the Water-Energy Nexus

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In the past forty years, the Dare County has seen tremendous growth. From 1970 to 1980, the permanent population growth rate was 6.7 percent and approximately 5.5 percent in the following decade (Dare County, 2011.) Moreover, the profuse influx of tourist during peak summer months draws the total population to exceed 268,000 people (Dare County, 2011.)  Fluxes in seasonal population coupled with a constant growth rate of permanent residency has lead to drastic increases in fresh water demand since the incorporation of a county wide water management body in 1962 (Hardee, 1989.) Moreover, limited supply of fresh surface water and aquifer resources caused the county to explore brackish water sources to be used in desalination.  In 1988, Dare County opened its first reverse osmosis desalination plant in Kill Devil Hills. I had the opportunity to visit the plant, known as the North Reverse Osmosis Facility (NRO), and speak with Ken Flatt, the director of the Dare County Water Department. Through this experience, I was able to better understand how Dare County continues to evolve their water resource management strategy with attention to energy efficiency.

Please note that all data concerning water and energy efficiency were given by Ken Flatt unless otherwise noted

Energy Basics 

Although the NRO is powered by fossil fuels via grid connection, many of its internal technologies are among the most efficient processes available to desalination facilities. The plant operates with five purification devices, each containing approximately twenty reverse osmosis membranes. Each device is powered by a generator that contains a variable frequency drive. A variable frequency drive makes the generator more efficient by converting AC to DC in two stages. During the second stage, DC is inverted back to AC which allows the generator to regulate frequency and thus speed of the current, allowing the device to increase or decrease the speed based on purification needs (Youtube, 2014.) The membranes have a recover rate of 75 percent, meaning for every one gallon of raw water input that passes through the system, ¾ of a gallon of fresh water is produced. Moreover, each set of membranes contains an energy recovery device to help reduce grid electricity needs for purification. The energy recovery devices utilize the brine (waste water) produced after the raw water passes through the first set of membranes to subsequently power a turbine that boosts pressure in the next membrane chamber (Figure 1.) This is an important facet of energy efficiency when considering that each purification device has a total of 430 square feet of membranes and operate using a pressure between 260-280 pounds per square inch (Figure 2.) At first I was surprised to quantify the operating pressure with such a large unit, but after seeing how thick each membrane was I realized that this is indeed energy efficient (Figure 3.)

Figure 1: Energy Recover Device, the turbine is featured in the bottom right

Figure 1: Energy Recover Device, the turbine is featured in the bottom right

 

Figure 2: Membrane Devices

Figure 2: Membrane Devices

Figure 3: A view of the membrane where filtered water passes through the tube in the middle

Figure 3: A view of the membrane where filtered water passes through the tube in the middle

Since the facility’s first operations in 1989, the market for reverse osmosis membranes has become increasingly more competitive. Consequently, desalination in the Outer Banks has become just as cost efficient as treating the county’s only fresh water aquifer. Furthermore, in the case of decreased water quality, the facility can easily switch out membranes without taking a huge economic toll on operations. The plant draws its water from the Yorktown aquifer, a brackish aquifer that gains salinity content from deep fossil saltwater. The aquifer has naturally large reservoirs of arsenic, which calls for a minimally energy intensive post-desalination treatment process (Vengosh, 2011.) The treated water is passed through vats filled with iron silt that absorb the arsenic. This process adds very little to the energy needs of the facility Lastly, the brine is either discharged into the Atlantic or into the Croatan Sound. The discharge process has negligible environmental impact, as concluded by Duke researchers. This substantially cuts down on the plants energy consumption as it minimizes the need for long distance pipelines or storage facilities.

Political Obstacles for Improving Energy Efficiency

When asked about areas in which the facility can improve in areas of energy efficiency, Mr. Flatt responded by explaining the political hindrances that prevent more cost and energy efficient water management strategies. Rooted in historical economic policies, the towns of Nags Head and Kill Devil Hills resolved to separately deliver their residences with clean, fresh drinking water. As funding difficulties prevented the municipalities from sticking to their goal, Dare County became the primary governmental unit for regulating and treating drinking water (Hardee, 1989.) This interlocal agreement remains today with both Nags Head and Kill Devil Hills buying the water from the county and distributing to residences with separate resources. Mr. Flatt explained that Dare County pumps water from their ground storage units to each municipality’s ground storage units where they bring the water to elevated storage (Figure 5.) Logically, treated drinking water is usually treated and then pumped to elevated storage before direct distribution. Mr. Flatt admits this increases water costs, but claims that “consolidation is unfriendly word around here” seeing that both towns collect revenue from water distribution.

A view of the elevated storage for Nags Head (left) and Kill Devil Hills (right) from Highway 158. The two storage facilities are so close to one another they separated by only a strip mall.

A view of the elevated storage for Nags Head (left) and Kill Devil Hills (right) from Highway 158. The two storage facilities are so close to one another they separated by only a strip mall.

Conclusions

Desalination in Dare County presents overall net benefits to citizens through striving to keep the most up to date and energy efficient technologies.  Although some obstacles to greater efficiency are beyond technical solutions, desalination is the most logical avenue for producing fresh water along the northern outer banks.

 

Special thanks to Ken Flatt and the Dare County North Reverse Osmosis Facility

 

Works Cited

Dare County (2011). Existing and Emerging Conditions. http://www.darenc.com/announce/2011/sec1.pdf

Hardee, J.E. (1989).  Dare Beaches Water Supply: Fresh Pond to Reverse Osmosis. http://www.darenc.com/water/Papers/DareBeachesWaterSupply-BlackVeatch1989.pdf

YouTube. (2014). What is a VFD (Variable Frequency Device/Inverter)?. http://www.youtube.com/watch?v=wDZANW2HeJ8

Vengosh, A. (2011). Rising Salinity in N.C. Coastal Aquifer Traced to Fossil Seawater. http://nicholas.duke.edu/news/rising-salinity-nc-coastal-aquifer-traced-fossil-seawater

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