V WS Envig has been awarded the contract to refurbish an existing desalination reverse osmosis (RU) plant for the Albany Coast Water Board (ACWB) at Bushmans River Mouth, next to Kenton-on-Sea, 100 kilometres east of Port Elizabeth.
Once the refurbishment is completed the ACWB desalination plant will have a throughput of 1800m3 per day, which is the largest desalination plant in South Africa. The project follows the successful installation of desalination plants for the communities of Cannon Rocks and Boknes Strand, which also form part of the Ndlambe municipality.
The contract, awarded in August 2009, involves maintaining a supply of potable water while the existing plant is being refurbished, the supply of a new RO skid, as well as the integration and optimisation of existing plant equipment.
The project also includes the implementation of energy saving devices to improve efficiency and reduce operational costs, as well as the running and maintenance of the plant for a year.
"The upgrade of the desalination plant is an important project, as it serves the Ndlambe Municipality, which in turn services a population of around fifty thousand people," says Gareth Kearns, Project Engineer, VWS Envig.
The project has been challenging as VWS Envig's design team needed to consider means by which to implement modern solutions and technologies into existing plant infrastructure.
"With a greenfield project it's easy to stipulate, design, and build facilities which meet the exact requirements of the equipment being installed," Kearns said.
A brownfield project on the other hand requires that you adapt your equipment and installation techniques to meet the requirements of the existing infrastructure. The VWS Envig team has managed to find solutions which not only meet the water delivery criteria of the contract, but also reduce the energy consumption of the plant." The existing ACWB reverse osmosis plant has been servicing the Ndlambe municipality for well over a decade. As a result, the mechanical and electrical components of the plant have deteriorated over time.
"Since our initial involvement with ACWB, we have found the RO plant to be in a run down state of repair. The plant has been experiencing unacceptable amounts of downtime, as well as running at a recovery rate far below the plants operational specifications," he said.
The plant currently has three reverse osmosis trains, mounted on separate skids, in various states of operational functionality. The supplied solution effectively reduces the total number of skids, while bringing the plant up to its specified delivery rate of 1800 m3 per day.
The solution combines the existing ROl and RO2 trains and places them onto a single skid - named RO12. The new RO12 skid will utilise an existing 160 kW Grundfos pump, as well as an energy recovery device, and a new 15 kW booster pump to force saltwater through the RO membranes.
"The RO12 skid represents a massive energy saving for ACWB. Under the old configuration, the ROl and RO2 trains utilised a 160 kW and a 220 kW Grundfos pump respectively. The RO12 train, by comparison, incorporates the existing 160 kW pump, a new 15 kW booster pump and a new pressure exchanger to supply the RO train's energy requirements.
"A pressure exchanger utilises the wastewater brine stream, which exits the system under high pressure to reintroduce energy into the RO process. Pressure exchangers are highly economical devices, which convert waste energy, in the form of high pressure, back into usable energy with a staggering 96.7% efficiency.
"In a similar fashion, we have introduced a turbine based energy recovery device to the smaller new R03 train. The turbine device, while not as efficient as a pressure exchanger, still manages to convert pressure back into energy at an impressive 56% efficiency," Kearns explains.
The implementation of the energy recovery devices and the pressure exchanger will reduce the RO plant's energy requirements by almost 30 percent, making the plant more economical and cost-effective to run.
In addition to the improved RO trains, VWS Envig is supplying five micron bag filtration to be used for pre-filtration prior to membrane treatment; chemical dosing, utilising Hydrex 4104 - an anti-scalant which prolongs RO membrane life; spiral seawater reverse osmosis membranes and a common clean in place (CIP) system, for membrane cleaning.
Duplex stainless steel 2507 has been selected for the high pressure piping to protect against the corrosive nature of seawater.
The plant is expected to be ready for beneficial operation by the end of March 2010. VWS Envig will then operate and maintain the facility for a thirteen month period under an operations and maintenance contract.
