South Island Public Service District, 
South Carolina

Reverse Osmosis Treatment Plant Expansion

Service Type: 
Project Highlights: 
  • Plant expansion from 1.5 to 3.0 mgd due to recent State limitations on the use of Floridian Aquifer water.
  • Plant expansion maximized the use of the deep, geothermal and brackish cretaceous well supply.
  • Carollo provided permitting, design, and construction-phase services.
South Island RO Treatment Plant
Carollo designed the upgrades and expansion of the South Island PSD’s RO WTP from 1.5 to 3 mgd.

The South Island Public Service District (SIPSD) retained Carollo to provide engineering services related to the expansion of their RO treatment plant in Hilton Head, SC. The SIPSD had been operating their RO treatment plant since 2001 and due to limitations on the use of fresh Floridian Aquifer water imposed by the State, the SIPSD decided to expand the RO plant from 1.5 to 3 mgd, maximizing the use of their deep, geothermal and brackish Cretaceous well supply. The initial process consisted of a plate-and-frame heat exchanger/cooling tower process, followed by RO. The heat exchanger/cooling tower process reduced the Cretaceous well temperature from 122oF to 104oF. The RO process operates at a recovery rate of 80 percent.

Prior to expanding the RO treatment plant, Carollo needed to negotiate a new NPDES permit for discharging additional RO by-product water into the environmentally sensitive Calibogue Sound. Toxicity testing was proven troublesome to the SIPSD, and Carollo first demonstrated to the State that the nature of this failure was due to naturally occurring constituents that are not regulated (i.e., common ion toxicity, or ion imbalance toxicity). Additionally, CORMIX modeling was performed, demonstrating infinite dilution at the point of discharge.

In concert with the permitting effort, Carollo completed the design services related to the expansion of the plant capacity. Additional heat exchanger and cooling tower capacity was required for redundancy. Carollo also evaluated maximizing the use of the existing RO equipment. Plant electrical and mechanical audits were prepared to determine if the existing infrastructure could support the additional capacity.

Carollo prepared designs, specifications, and contract documents for the equipment related to the expanded capacity. Construction-phase services were also provided.

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