Impacts of Feedwater Quality Change on the Oldest Continuously Operated Brackish-Water Reverse Osmosis Desalination Plant in the United States

Jeffrey L Pearson,Michael Hegy,Thomas M Missimer

doi:10.3390/w13192654

Abstract

Brackish groundwater is abundant in many coastal zones of the world. The water can be economically treated with low-pressure reverse osmosis. A key issue is the stability of the feedwater pumped from groundwater systems. Commonly, groundwater solute-transport models are used to evaluate the long-term changes in salinity with time that impact brackish-water reverse osmosis (BWRO) desalination system process design. These models are run to assess changes over a 20- to 40-year period. The City of Cape Coral, Florida operates two regional BWRO facilities with the South Plant being the oldest continuously operated system in the world. This facility has a capacity of 68,182 m3/d and can treat raw water with a total dissolved solids (TDS) concentration up to 4000 mg/L. Two solute transport models were constructed to evaluate future salinity change in the groundwater source. The first model conducted in 1991 produced a range of probable changes with a high, most probable (mid), and low range. Actual data confirm the low range of the model produced an accurate result (within 15%) and that the 4000 mg/L threshold would not be exceeded until beyond 2031. The second modeling effort conducted in 2014 suggested that the 4000 mg/L TDS threshold would be reached in 2018, which did not happen. The use of real data and regression analyses for all wells suggests that the 4000 mg/L TDS concentration will not be exceeded until after 2060. Once the TDS threshold is reached, the plant would require a process change to allow treatment of higher TDS water. The current analysis shows that plant process design modification would not be required for up to 40 years into the future. The standard conceptual model assuming predominantly upward recharge during pumping was accurate with the addition of an enhanced zone of leakage caused by a fracture zone or a fault. A key issue that contributed to the success of the facility was the use of groundwater solute transport modeling prior to the final design of the membrane process during plant expansions.

Highlights

Limited supplies of freshwater occur in many regions of the world, wherein rapid population growth in combination with the occurrence of sparse water resources necessitate the development and use of alternative water sources, in coastal areas [1]
Well data were obtained from the City of Cape Coral Utilities Department South Wellfield operating reports
The sole source of drinking water for the City of Cape Coral, Florida is provided by three brackish-water reverse osmosis (BWRO) desalination facilities

Summary

Introduction

Limited supplies of freshwater occur in many regions of the world, wherein rapid population growth in combination with the occurrence of sparse water resources necessitate the development and use of alternative water sources, in coastal areas [1]. Where the quality of water in a saline-water aquifer has a salinity of less than about 8000 mg/L, it can be economically treated using the brackish-water reverse osmosis (BWRO) desalination process [3]. In southern Florida, the use of BWRO desalination technology has become common with about 40 facilities currently being operated [4,5]. The City of Cape Coral (City) in southwestern Florida currently utilizes low-pressure reverse membranes for desalination of brackish from the Lower Theosmosis City of Cape. Florida use between the quality of the raw water supply with its variation in time and the[17,18,19]. Process brackish water extracted from close to the top of the Floridan Aquifer System design of the desalination plant. Most of the plants located in Florida use.

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