Two Temporary Reverse Osmosis (RO) Water Treatment Plants, designed and built by ACWA Emirates for the Consolidated Contractors Company (CCC), are providing high quality drinking water for construction workers at the Khursaniyah and Khazna (KPF) Project site camps in Saudi Arabia.
To provide secure, cost-effective water supplies, not only for the Saudi projects, but for future use on other construction sites, the Client demanded that both plants be capable of providing high operational flexibility, long-life and low maintenance. CCC recognised ACWA's considerable experience in applying some of the very latest membrane technologies to a variety of international water treatment projects and of the Company's practical and responsive approach to process contracting.
ACWA RO processes are designed to achieve the maximum possible membrane life at relatively low cost. All pumping systems on the feed side of these installations have corrosion resistant wetted parts and where possible, are selected for high efficiency and low running speeds. Where appropriate, process tanks are resistant to seawater or chemical attack and are constructed either from glass-fused-to-steel bolted panels or GRP. In addition, all high pressure pipework is fabricated from duplex stainless steel.
At the Khursaniyah and Khazna sites, high volumes of raw water containing a cocktail of calcium, magnesium, sodium, potassium, barium, strontium, iron, bicarbonate, sulphate, chloride, boron and silica, are pumped to the treatment processes from deep wells in the local area. The ACWA treatment processes use three x 33% of RO and were installed with modular, common pretreatment. Pretreatment includes dual media filters for the removal of iron, manganese and suspended solids, together with cartridge filters and acid antiscalant dosing.
Each plant was designed so that the filtration rate at full production is 9.17m3/m2/hour through a 1 Metre deep filter bed. It is possible, however, to operate the pretreatment process with one or two RO streams in operation and by doing so, greatly enhance filtration efficiencies.
Pretreatment
Media Filtration
At each site, raw well water from two 250m3 storage tanks is pumped into a common manifold by four filter feed pumps (operating one duty per RO stream/one standby). The manifold feeds the media filters, enabling any pump to operate with any of the RO systems. It is possible for the operator to adjust the flow initiation setting to satisfy prevailing on site conditions.
The media filter system comprises six vertical epoxy-lined filter vessels operating in parallel - five on duty, one on standby. Each vessel incorporates a manually operated butterfly valve for backwashing and rinsing, using pumped water from a 100m3 storage tank. Two positive displacement air blowers, one duty and one standby in manual mode, are installed to break up compaction and prevent "channelling".
Dosing
In order to inhibit scaling of the RO membranes, antiscalant is dosed into the feed water prior to the cartridge filters at 0.3 Litres per hour per RO. Should the feed water need correction to 6.6pH at this stage, it is dosed with sulphuric acid at 0-10 Litres per hour by two dosing pumps operating (one duty, one standby). The dosing pumps incorporate variable speed motors which are adjusted manually according to the pH target set by the operator.
When one or more of the RO streams have to be 'mothballed' a Bisulphate dosing system with two pumps (duty/standby), doses the appropriate streams at 0-10 Litres per hour.
Cartridge Filtration
To provide protection and extend the life of the downstream RO membranes, raw water is passed through a cartridge filtration system incorporating four vertical filter housings, each containing 20 x 40" elements (rated at 5 Microns nominal). Following cartridge filtration, the Silt Density Index (SDI) of the treated water is measured and at any time during plant operation is maintained below 5.0 units.
The RO System
Three high-pressure multistage centrifugal pumps with a Duplex stainless steel fluid end, are installed to feed the RO system. The pressure varies inversely with the water temperature and directly with the degree of membrane fouling. The pump/motor capacities are based on the operating pressure necessary to treat feed water at 35-40 degrees C.
The inlet flow/pressure to the system is controlled by a manual throttling valve on the pump discharge. This is adjusted manually as the temperature varies and/or the membranes become fouled, to regulate the quantity of seawater entering the membranes.
Each of the three reverse osmosis streams - installed individually with high pressure pumps - incorporate thirteen pressure tubes, each containing 7 RO membranes in series. Each stream was designed to produce 69.4 m3/hr of permeate at the design feed conditions of <500mg/l TDS. Under normal operating conditions, this is usually <250mg/l TDS.
As each RO stream is fully independent, it is possible to isolate any of the three streams from the system without effecting the performance of the other streams.
Recovery Control
The three membrane systems are designed to operate at 50% permeate recovery as a proportion of the feed-water and are PLC controlled via an actuated valve installed on the concentrate-to-waste line. The valve is adjusted by the PLC to set and maintain the required permeate flow rate.
Sampling facilities are also provided to enable the quality of permeate in individual pressure vessels to be manually checked. The quality of permeate is also checked in every membrane by use of a probing system - a facility which uses sampling tubes, which are inserted along the length of each vessel. An integral system is installed to clean a complete RO stream or individual stacks of tubes within a stream.
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© Press Release 2009
