A Coastal Upgrade
Due to rapid growth in the Burnt Store Corridor, Charlotte County Utilities in southwest Florida is expanding its existing Burnt Store reverse osmosis (RO) water treatment plant (WTP) from 1.13 million gal per day (mgd) to 3.61 mgd. The primary objective of the Burnt Store WTP expansion is to maximize the treatment capacity of the WTP within the existing footprint. Additional goals include: providing high-quality drinking water and meeting the future water demands in the Burnt Store service area.
The Burnt Store WTP currently uses two 0.5-mgd RO skids to treat brackish groundwater, with total dissolved solids (TDS) between 2,000 and 2,700 mg/L supplied from five production wells. Prior to RO membrane desalination, the water is injected with scale inhibitor and then passes through 1-micron cartridge filters for prefiltration. Approximately 5% of the raw water is bypassed around the RO membranes and blended with RO permeate.
The existing RO system operates at a 70% permeate recovery. After desalination, the RO permeate and bypass water enter a 1.25-mgd-capacity degasification tower, where hydrogen sulfide and carbon dioxide are removed. The degasified water is then treated with chlorine for primary disinfection and sodium hydroxide for pH adjustment. The stabilized water passes through two 0.5-mgd ground storage tanks and is then pumped to distribution. Concentrate from the RO process is disposed of down a deep injection well located on site.
As part of the WTP expansion, two new production wells will be installed to supply additional water to the plant. Three new 0.75-mgd RO skids will be installed in the existing RO building beside the two existing 0.5-mgd skids. In addition to a scale inhibitor, sulfuric acid will be injected into the raw water for pretreatment and the permeate recovery will be increased from 71% to 80%. The amount of bypass water will also be increased to 10% to maximize the capacity of the expanded plant while meeting all water quality goals.
Two new cartridge filters and degasification towers will be added to the expanded WTP. One 0.5-mgd ground storage tank and three new high-service pumps will be installed to increase the peak hour capacity of the WTP to 7 mgd. Aqua ammonia will be added to the finished water to form a chloramine residual within the distribution system. A new 9.5-mgd deep injection well was recently constructed to provide disposal of the additional concentrate from the expanded RO process and future upgrade of the water reclamation and WTP facilities.
The Florida Department of Environmental Protection’s redundancy requirements are satisfied for all equipment within the expanded WTP, including the RO system. The goal of the new RO train’s design is based on the system’s flexibility to provide capacity for two design flow conditions while maintaining operating parameters within manufacturer’s recommended limits.
The membrane projections completed for the various membrane types were based on providing three new RO membrane trains configured in a two-stage (concentrate staging) array. The goal of the new RO train design is based on the system’s flexibility to provide capacity (i.e., permeate flows) for two design flow conditions, which are as follows:
1. Normal operation, with an overall treatment capacity of 0.75 mgd at 80% permeate recovery and an average system flux of approximately 13.5 gal per sq ft per day (gfd).
2. Redundant operation, with an overall treatment capacity of 1 mgd at 80% permeate recovery and an average system flux of approximately 18 gfd.
The redundant operation is based on meeting the overall WTP rated capacity of 3.6 mgd with one of the largest RO trains taken out of service. The new train’s capacity increase to provide additional flows for the redundant operation is based on the existing RO system’s current rated capacity of 0.50 mgd.
The existing Hawthorn well water quality will permit a portion of the raw feedwater to be bypassed around the RO system and combine the permeate to obtain a blended product water TDS concentration of approximately 350 mg/L. The bypass groundwater will not require treatment prior to blending with RO permeate and post-treatment processes. Approximately 10% (percent of finished water flow) bypass water will be blended with RO permeate during normal operation.
Currently, a small-diameter deep injection well is used to dispose of the RO concentrate. The capacity of the existing deep well was not sufficient to handle anticipated concentrate flows of up to 1 mgd as the RO plant is expanded. Therefore, a new large-diameter deep injection well was constructed to accommodate the higher concentrate flows as the WTP is expanded. In addition to RO concentrate, treated water from an onsite wastewater plant will also be sent to the deep injection well.
At this time, it is anticipated that the combined peak flow at buildout, from both the drinking and wastewater plants, will be around 9.5 mgd. Concentrate from the WTP will flow into a deep injection well pump station and then be pumped down the injection well.
The Burnt Store WTP expansion is currently under construction and is scheduled to be completed by the summer of 2009.