Water treatment remains a key concern in process industries
The Scottsdale Water Campus drinking water treatment plant in Arizona, USA, currently consists of a 45.4ml/d water reclamation plant and a 190ml/d drinking water treatment plant.
The reclamation plant uses a Memcor polypropylene (PP) membrane system, installed in the late 1990s, as pretreatment to reverse osmosis for treating wastewater that is then used for both aquifer recharge and irrigation. The drinking water treatment plant has been using media clarification to treat surface water from a canal. This plant needed to be expanded because of increased demand for drinking water in the North Scottsdale area. Today, the city of Scottsdale has a population of 240000. In the last 10 years, the population has grown by about 35percent.
For the drinking water treatment plant expansion, the Scottsdale Water Campus considered several different membrane technologies. After a successful 100-day pilot test was conducted at the site, it chose the Memcor CP system for its effluent quality, lower installed cost and ease of expandability. The E6.25m contract was placed by Archer Western Contractors, the city’s general contractor.
The Memcor CP system uses 1.8-metre polyvinylidene fluoride (PVDF) membrane modules that increase the filtrate capacity by over 50percent, compared with earlier generation Memcor membrane modules (Fig.1). The CP system provides greater than four-log removal of Cryptosporidium, Giardia and bacteria, and over 1.5-log virus rejection, regardless of changing feed water conditions. Its compact, modular design allows the system to be easily modified and expanded, and takes up roughly 70percent less space than conventional water treatment systems.
The system expansion at the Scottsdale Water Campus will initially consist of seven Memcor CP units, each containing 324 modules, with the building designed to accommodate a total of ten units when it reaches design capacity in 2009.
RO for thermal power
Meanwhile, the Siemens Industrial Solutions and Services Group has received an order from Slovenské elektrárne in Slovakia to modernise the water treatment system of the Elektrárne Nováky thermal power station at Zemianske Kosto. The new treatment system will increase operational safety and reduce environmental pollution.
To this end, the desalination plant for treating river water is being fitted out with a reverse osmosis (RO) system. This will make it possible for 80percent of the used water to be returned to the water circuit of the power station after treatment. The project is scheduled for completion by the end of this year.
The stock company Slovenské elektrárne (Slovakian electricity works) located in Bratislava is responsible for supplying power in Slovakia and operates two nuclear power stations, 34 hydroelectric power stations and two thermal power stations. The Elektrárne Nováky thermal power station, whose water treatment equipment is now being modernised, generates electricity and delivers hot water and district heating to the cities of Prievidza, Nováky and Zemianske Kosto, as well as to factories in the area.
The power station takes the water it needs to generate steam from a river. Before this water can be used in the power station, it has to be treated and cleaned. For this purpose, Siemens is converting the existing desalination system into a RO system. This requires the installation of two systems. One is a reserve to be used when maintenance work is being done, or if the main system fails. Each system has a flow rate of 80m3/h. The RO systems include automatic metering of chemicals for optimum water treatment. In addition, deposits on the membranes of the water treatment equipment are removed automatically. A measuring instrument automatically detects the pH value and conductivity of the water to make sure it has the right qualities needed for operation of the power station.
Siemens is also modernising the existing equipment for filtering the river water. For example, filters for the first cleaning stage will be equipped with fittings for measuring conductivity, the pH value, turbidity and the water level in the treatment tank.
After modernisation, the filters will work and clean themselves fully automatically. The sludge left over at the end of the water treatment process is neutral and does not require any further processing. Siemens is responsible for planning, modernising and supplying all the equipment, including the power distribution system, on the one hand, and the instrumentation and the automation system, on the other. With the help of an instrumentation and control monitoring system, all relevant water values such as pressure, temperature and flow rate in the entire water treatment process can be kept track of, monitored and, if necessary, altered.
A produced water deal
In a third recent water treatment success, Siemens has won a contract from Cairo-based Engineering for the Petroleum and Process Industries (ENPPI) to supply a dissolved gas flotation (DGF) pump system for use on Saudi Aramco’s Safaniya plant in Saudi Arabia.
The new system will remove high concentrations of free oil from the water stream so that water can be used for subsurface re-injection. The pump works by using a dual-sided impeller that pulls both water and gas into the pump volute. The backside of the impeller has a ‘sub-atmospheric’ zone that pulls vapour from the blanket gas source or other means and allows mixing with the incoming fluid.
As this mixing occurs, the vapour is dissolved into the water creating micro-fine bubbles that break out of solution once a pressure drop is experienced. This pressure drop occurs once the fluids and dissolved gas are flowed across a globe valve prior to entrance into the flotation vessel. Due to the close tolerance between the back vanes of the impeller and the back plate of the DGF pump, the vapour is sheared into micro-fine bubbles piped into a vessel or tank allowing the fine gas bubbles to attach to the oil droplets. As the gas bubble attaches to the oil droplet, the droplet floats to the surface at an accelerated rate.
The capacity of the system will be 850000 barrels of water per day when it comes on line in mid-2008.
This is the first time that such a DGF pump system has been used on a Saudi Aramco project.
