Coquina Coast plumps for land-based desalination

An interesting comparison between land- and ship-based desalination systems has been revealed in the Final Recommended Project Report submitted by consultants Malcolm Pirnie for the Coquina Coast Seawater Desalination Alternative Water Supply Project.

While every case is site and use specific, the Coquina report seems to indicate that for some shallow continental coastal areas, ship-based desalination may not be very competitive.

The Coquina Coast project is investigating the feasibility of a seawater desalination facility to be located in St Johns, Flagler or Volusia County on the east coast of Florida, USA. A group of municipal suppliers led by the St Johns River Water Management District have now concluded Phase 1 of the project, to evaluate alternatives and provide conceptual engineering.

Because the facility is anticipated to serve as a base-load facility, existing supplies would be used to make up the difference between additional future average day and future maximum day demands. The two "build out" capacities evaluated in Phase 1 were 50 MGD (189,000 m³/d) and 80 MGD (303,000 m³/d), depending on how many districts eventually agreed to be supplied.

The primary objective of Phase 1 was to conduct a comparative evaluation of land-based and vessel-based desalination, and make a recommendation regarding the applicability of each as a solution for seawater desalination for the group of suppliers. After listing the advantages for ship-based desalination (speed, land costs, ocean conditions etc), the Pirnie report says "None of the conditions above exist for this project."

The report calculates that the number of vessels required to ultimately provide 50-80 MGD was "a concern". Of 19 potential vessel-based desalination providers contacted, only six were able to provide vessels that could produce at least 5 MGD (19,000 m³/d), which was the minimum threshold considering the potential for short-term supply needs.

Only three (Water Standard, Subsea Infrastructure and SeaStar Energy) were likely to be able to provide up to 50 MGD from a single vessel. The others would require perhaps 5-10 vessels, to supply 50 MGD or more.

Cost estimates were provided by two vessel-based desalination providers. One estimated a cost of US$ 1.0-2.0/m³, the other a cost of less than $2/m³. However, the local bathymetry is such that a vessel would need to be sited at least 4.8 km offshore to reach a depth of 12 m and up to 8.0 km or more to reach a depth of 20 m.

The local requirement for intake and outfall pipes to be buried rather than laid on the seabed (as suggested by the suppliers) would impact the cost. Based on this analysis, it was assumed that the cost for vessel-based desalination was at least US$ 2/m³.

By comparison, the costs for the land-based alternatives were generally in the range of US$ 3.70-4.25/1,000 gal (US$ 0.98-1.12/m³) with the highest-cost option estimated at $4.50/1,000 gal (US$ 1.19/m³).

For this and other reasons, the report recommended a land-based facility. A fuller article on this project will appear in D&WR's February/March issue.

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