Swro Design and Energy Recovery Part 2: Hydropro Design

Design Requirements
Traditionally Hydropro has always put the needs of the customer into the forefront of its company philosophy. By doing this, Hydropro has always stayed abreast of the latest advancements in technology in the water treatment field. In this case, mostly because of the remote location (nearly everything, including fuel for the diesel generators, is delivered by ship), the most important customer needs were associated with conserving energy and maintaining reliability. Availability of replacement parts was also a major concern due to the remote location and the lead-time required to ship items to the island. Another concern Hydropro had to address was ease of operation and ease of maintenance, as the remote island of Ebye did not have any skilled RO plant operators. The end result would incorporate all these requirements to produce a reliable supply of potable water from a seawater source for the citizens of Ebye.

In the original RFP, KAJUR requested twin 75,000 gpd SWRO units (expandable to 100,000 gpd) designed for a seawater feed of 45,000 mg/l TDS. The proposal presented by Hydropro was for two Seawater Reverse Osmosis Water Treatment units each designed to produce 75,000 gallons per day. Permeate water was projected to be of less than 300 mg/l TDS based on feed water from seawater wells with a maximum TDS of 50,000 mg/l and an SDI of less than 3. Each unit was designed to be easily expandable to a daily capacity of 100,000 gallons by the addition of one pressure vessel containing seven seawater membranes. All instrumentation, piping, valves, headers and pumps were pre-sized to accommodate the expansion.

Each proposed SWRO system consisted of four pressure vessels containing seven membrane elements each arranged in a single, one-pass array. With the expansion, the system would consist of five pressure vessels in a single staged array. Each system was designed to operate at a 30-40% recovery rate, with a maximum trans-membrane (feed to product) pressure of 1100 psi at a feed water TDS of 50,000 mg/l. With a feed water TDS of 46,000 mg/l, the trans-membrane pressure was projected to be approximately 900 psi at startup and 950 psi after three years of operation.

System Design
The final, installed 100,000 gpd Hydropro design consisted of the following major components and unit operations for each SWRO unit:

• Sand and Particulate Filters: Two HYDROPRO Tubular filter units Model STF5M2-400- PVC/150 each consisting of one PVC housing with a 150-micron wedge wire PVC screen for the removal of sand and particles, with automatic purge valves
• Micron Filters: Three heavy-duty filter housings constructed of FRP/PVC and built to ASME Code X, the housings are Eden Model 24EFC each accommodating six (6) 40" long five micron polypropylene cartridges
• RO High Pressure Booster Pumps: Two high pressure feed booster pumps Grundfos Model BM 17-27R (installed in series) - horizontal centrifugal, multi-stage construction of 904L Super Austenitic Stainless Steel, each driven by a 35 HP submersible type motor rated at 460V/60Hz/3Ø utilizing a Soft start motor starter and VFD RO Low Pressure Booster Pump: One booster pump Grundfos Model BM 30-4R - horizontal centrifugal, multi-stage type of 904L Super Austenitic Stainless Steel, driven by a 7.5 HP submersible type motor rated at 460V/60Hz/3Ø controlled by a variable frequency drive
• Membrane Modules: One FRP construction structural frame, five pressure vessels of FRP construction rated at 1200 psi operating pressure, 35 Thin Film Composite membrane elements ¬ 8" x 40", 2205 DUPLEX SS headers for feed and concentrate and Sch. 80 PVC for the permeate headers and low pressure feed, suction and concentrate piping, Allen- Bradley PLC SLC 5/04 based control system - installed in a NEMA 4X enclosure with system switches lights etc. installed on the panel door
• Chemical Feed Systems: One anti-scalant dosing system and one chlorine dosing system
• Freshwater Flush/Membrane Cleaning System

The system skid was designed and fabricated for a compact footprint due to limited installation space and to allow for shipping both units in a single container.
The entire system was pre-assembled as much as possible to minimize field services.