From Discharge to Drinking Water
The village of Bald Head Island is a picturesque island community located off the mouth of the Cape Fear River in coastal North Carolina. The village is renowned for its views and slow pace of life—no cars are allowed on the island, and access is only by ferry from the mainland. Bald Head Island strives to protect the delicate environmental balance of its beautiful island home while providing for the needs of both residents and visitors.
A Potable Water Treatment Challenge
For potable water supply, the village treats groundwater from a semi-confined aquifer via 16 production wells located at depths ranging from 55 ft to 65 ft below the surface. The water from the wells is piped to the water treatment facility, where it is treated to remove dissolved contaminants using multiple reverse osmosis (RO) treatment units. Following RO treatment, the water is disinfected, aerated and stored for potable water use.
The village had used two RO systems purchased in the late 1990s, but due to discharge limitations and the volume of brine reject, the water treatment plant had to be shut down. Normally, supplemental water was purchased from the mainland; however, the village now was forced to rely on this supply.
McKim & Creed, Wilmington, N.C., was hired to develop a solution whereby the village could produce potable water using its existing wells without violating the National Pollutant Discharge Elimination System discharge permit, and eliminate the need to purchase water from outside sources. The village needed to move quickly to minimize the cost of purchasing water from the mainland.
The Solution: A Secondary Recovery RO
Knowing the island’s water demands were increasing, McKim & Creed considered adding an additional 40 gal per minute (gpm) from existing wells for treatment. The two existing RO units treated an influent flow of 200 gpm at 70% recovery, producing about 140 gpm of clean water and 60 gpm of brine reject. By adding a secondary recovery RO system, this brine reject could be treated for potable water and the amount of brine waste volume could be reduced. Because the RO effluent requires blending to achieve target hardness, the additional 40-gpm capacity of the existing wells also could be used in the final effluent capacity.
A packaged RO system (the Siemens Vantage Series M83 system) was selected to treat the 60-gpm reject stream blended with 40 gpm of well water. This pre-engineered, preassembled system minimized installation and startup time, as only the membrane element installation had to be done on site. The membranes are Dow LE-400 low-energy thin-film composite membranes; these were selected because they are optimized to produce the desired effluent quality upon product blending, while reducing the required booster pump feed pressure.
The new secondary recovery system was installed in series behind the two existing primary units in an existing structure. Many of the existing systems such as controls, compressors and chemical feeds, were reused or modified for use with the additional RO unit. Also, relocation of the discharge line was required. The controls of the reject recovery unit were interfaced with the existing SCADA/control system using an Anybus X-Gateway module. This module is used to facilitate communication between systems that use different industrial networks. This was very desirable to the village, as it allowed a seamless integration with existing equipment without modifying its SCADA system.
More Water, Less Waste
The village was able to return the water treatment plant to operational status by February 2009, just five months from the project bid date. The new system produces almost 50% more water (210 gpm vs. 140 gpm) to the island and reduces reject discharge by 50% (30 gpm vs. 60 gpm). The overall system recovery is approximately 90%.
As reported by the Bald Head Island Conservancy’s Winter 2009 newsletter: “Only the reject water from the RO system will be lost from the island. Through the ingenuity of design and detailed understanding of the extent and yield of the water resource, it is expected that Bald Head Island will set a precedent and become a model of sustainability of freshwater resources for barrier and other oceanic islands.”
The enhanced drinking water treatment plant serves the 2,800 seasonal visitors, residents and businesses on the island. The design capacity will meet the village’s growth needs for the foreseeable future. The village’s budget for the project was $1.6 million, and the actual cost of the project was $990,000, allowing the village to return the difference to its general fund.