Water. desalination + reuse

PROJECTS February-March 2014 | Desalination & Water Reuse | 23 | and turbidity variances all affect the process design and operation of the plant. For this reason, the pretreatment design was extensive, including flocculation tanks, plate settlers and multi-media filtration (MMF). Coagulants, flocculants and pH control chemicals were also incorporated to achieve quality pretreatment. The SWRO facilities in the US Virgin Islands had sufficient space to run pipelines in an efficient manner and construct new buildings that were sized for the equipment and ease of operation. Additionally, the facilities on St. Thomas and St. Croix required only single-stage MMF as pretreatment for the SWRO systems. Coagulants or flocculants were unnecessary, with just an oxidant dosed for bio-growth control in the feed system and a reducing agent required to protect the SWRO membranes. Another technical difference between the three projects was the inclusion of post-treatment at the plant in Point Fortin, Trinidad. Neutralizing the water by remineralizing, along with post- chlorination, was included in the design. The inclusion of post-treatment increases complexity in terms of chemical-dose monitoring, additional instrumentation and controls. This was not included in the US Virgin Islands, as WAPA is ultimately responsible for the neutralization/ remineralization or post-chlorination of the SWRO permeate before distribution. ElECTRiCal POwER SuPPly SCOPE A major technical scope difference between the Point Fortin SWRO facility and the SWRO facilities located on St Thomas and St Croix was the electrical scope. In Trinidad, Seven Seas Water was responsible for the medium voltage (MV ), 12,600 VAC, and low voltage (LV ), 480 VAC, design, installation and operation. In the US Virgin Islands, Seven Seas Water was liable only for the LV, 480 VAC, design, installation and operation at both facilities. This difference is significant when consideration is given to the additional design, equipment, installation and operation required for the MV and LV scope in comparison to LV scope only. Ensuring qualified and specialized individuals and test equipment are both onsite for testing and commissioning the MV properly is a key to a successful MV power up. Operations and maintenance (O&M) on the MV equipment also becomes an important consideration when assembling the team that will ultimately be responsible for plant operations. MOdulaR PhaSEd dESign vS PERManEnT laRgER-SCalE dESign The SWRO plant at Point Fortin was to be brought online in phases, and the design was built in smaller modules to achieve phased water production. The smaller modules and equipment sizes resulted in smaller electrical and mechanical equipment sizes; however, a greater number of pieces of equipment The Randolph E Harley Seawater Reverse Osmosis Facility on the island of St Thomas

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