Impacts of projected changes in feed-water salinity on the City of Cape Coral Florida north brackish-water reverse osmosis desalination plant operation

Successful design of brackish-water reverse osmosis (BWRO) desalination systems requires detailed knowledge concerning the source aquifer system to assure that future water quality changes induced by pumping do not adversely impact operations. The City of Cape Coral, Florida has successfully operated two BWRO systems with a combined capacity of 113,632 m3/d (South Plant: 68,182 m3/d, North Plant: 45,455 m3/d) with the North facility being operated for the past 11+ years. An investigation of the salinity changes in the production wells that provide feed water to the North BWRO plant shows that all wells have an increasing salinity based on the standard conceptual model of pumping-induced upwards recharge from the underlying aquifer. The blended average feed water quality has historically increased in dissolved chloride concentration from about 900 to slightly over 1,000 mg/L, which is a total dissolved solids (TDS) change from 1,913 to 2,072 mg/L. This change is within the general range predicted by pre-design groundwater modeling. The projected dissolved chloride value at the 20-year point is 2,007 mg/L (3,860 mg/L TDS) for the low-salinity wells based on current trends. The BWRO membrane process was designed to treat feed water with up to 4,000 mg/L of TDS or a dissolved chloride value of 2,080 mg/L. Based on the evaluation of the changes in dissolved chloride concentration in all the production wells, the facility will be able to operate below the maximum treatable salinity over the next 20 years which will allow any necessary process upgrades to be made in a planned manner. This is an example of good coordination between planning, plant design, and operation of a BWRO facility where increases in feed water salinity with time are known to occur.