Where Does the Water Come From?
The PSD has multiple sources of water, which allows us to select from the most economical options for our customers and also provides us with resiliency in the face of a natural disaster such as a hurricane. Let’s look at the sources and treatment techniques for the PSD’s water.
Reverse Osmosis (RO) Water Treatment Facility
The PSD’s Reverse Osmosis (RO) Water Treatment Facility provides more than 40 percent of our water supply. Reverse osmosis is a pressure-driven membrane separation process that removes ions, salts, and other dissolved solids and nonvolatile organic compounds. The separation capability of the process is controlled by the diffusion rate of solutes through a barrier and by sieving. The membranes are comprised of synthetic polymers similar to Nylon. The polymers are permeable to water but reject particulate matter such as salts and other minerals. The process produces fresh water and yields a concentrate of the rejected solids.
The PSD’s RO Plant, located across U.S. 278 from Windmill Harbour, began operations in April 2009. It provides customers with 3 million gallons of fresh water a day, about half of the average daily demand for water. In 2013, the PSD began preparations to expand the plant’s production to 4 million gallons a day. The RO Plant’s construction was necessary to replace water supply lost to saltwater intrusion. The plant uses state-of-the-art filtration to produce a very high-quality water. In fact, it’s the same process used to produce many brand-name bottled waters and other beverages. The RO plant is expandable to provide 6 million gallons of water a day. The PSD has the capability to expand the plant in 1-MGD (million gallons per day) increments, thereby allowing the plant’s capacity to increase as it is needed to replace water supply lost as a result of saltwater intrusion.
The RO Plant is fed by three raw water wells drilled into the 600-foot-deep Middle Floridan Aquifer. The wells are pumped at a rate of approximately 1,500 gallons per minute (gpm) and the plant operates 24/7. The groundwater in this aquifer is brackish and requires filtration in the RO Plant. For every 3 MGD of drinking water produced by the plant, 1 MGD of concentrate is diffused into the saltwater background of Skull Creek via a pipeline at the end of Jenkins Island Road. Scientific research and monitoring has shown that the Middle Floridan Aquifer is a very abundant and long-term source of water for the island’s future.
The PSD purchases water from the Beaufort-Jasper Water & Sewer Authority (BJWSA). BJWSA operates state-of-the-art water treatment plants in Chelsea and Purrysburg on the mainland and also utilizes the Upper Floridan Aquifer and Aquifer Storage & Recovery (ASR) wells. Wholesale water enters the PSD’s water storage tanks and distribution lines via a large pipeline located beneath the Intracoastal Waterway. Wholesale water accounts for about 30 percent of our water supply.
Aquifer Storage & Recovery (ASR) Well
The PSD constructed the island’s first-ever Aquifer Storage & Recovery (ASR) well inside Hilton Head Plantation in 2011. The well both stores and withdraws water. During the winter months of low water demand, the PSD takes advantage of a reduced wholesale water rate from BJWSA and stores wholesale water in the Middle Floridan Aquifer using the ASR well. Then, in the summer months when water demand and the wholesale water rate increase, the ASR well is used to withdraw and re-treat 2 MGD of water.
The ASR process involved the PSD injecting 240 million gallons of wholesale water into the 600-foot-deep Middle Floridan Aquifer to build a bubble of fresh water in the brackish aquifer. The fresh water displaces the brackish water and a buffer zone is created at the edges of the fresh water bubble. When the ASR well is used to withdraw water, the water is drawn from the center of the fresh water bubble.
The Upper Floridan Aquifer
The Upper Floridan (FLORA-DAN) Aquifer is a limestone, freshwater aquifer located 50 to 150 feet underground. It is one of the largest aquifers in the world, and stretches from the Beaufort area southward through the Florida Everglades. Groundwater from this aquifer requires little treatment. The PSD utilizes automatic feeders that supply precise amounts of chloramines at our wells to treat the groundwater. The PSD treats Upper Floridan Aquifer water with chloramines directly at our well sites.
Unfortunately, this aquifer has been impacted by saltwater intrusion.