Water for a Thirsty World: Reverse Osmosis Systems from KSB
Reverse osmosis (RO) has emerged as one of the most effective technologies for extracting high-purity water from salty, brackish or contaminated sources. The production of drinking water from seawater has been the most popular application of this technology, especially in areas such as the Middle East or Western Australia where fresh water can be scarce. (The Economist Magazine reported that during a drought in 2008, the citizens of Perth, Australia were obtaining almost a fifth of their drinking water from RO plants.) This process is also popular on ocean-going vessels, especially cruise ships, where there is a high demand for clean water. Closer to home, RO systems can be useful where there is a need to eliminate dissolved impurities from brackish groundwater for chemical processes or to produce demineralized boiler feedwater. This means that RO water purification systems require a lot of energy and designers of these systems are constantly on the lookout for ways of improving energy efficiency.
About the Reverse Osmosis Process
In a reverse osmosis process, water with dissolved impurities is forced through a special porous membrane which acts as a filter, separating water molecules from other materials. With appropriate membrane materials, the water emerging from an RO system is extremely pure, ready for use as drinking water or for sensitive industrial process.
The pressure needed to sustain a flow of water through the membrane is high – 55 to 80 bar (800 to 1200 psi) for seawater and 15 to 26 bar (225 to 375 psi) for brackish groundwater. This means that RO water purification systems require a lot of energy and designers of these systems are constantly on the lookout for ways of improving energy efficiency.
As pure water flows through the permeable membrane, the concentration of dissolved material upstream of the membrane increases. Because of the physics of the process, as the concentration of dissolved materials increases, the pressure required to keep water flowing through the system increases as well so that more energy is required to drive the pumps. For this reason, practical RO systems don’t attempt to extract all of the water from the incoming stream; rather, water with very high concentrations of dissolved impurities is bled from the circuit, to be replaced with ‘fresh’ seawater or brackish water. This bypass water is at very high pressure, and with suitable equipment, it is possible to recover useful amounts of energy from this stream. Devices such as KSB’s SalTec D® pressure exchangers are designed to effectively recover energy from the high-pressure, high-concentration bypass water. KSB has demonstrated that the combination of highly efficient pumps with effective pressure recovery devices and carefully designed control systems can significantly improve the energy efficiency of RO desalination and water treatment systems.
KSB's RO Technologies
KSB, as one of the worlds leading providers of pumps, valves and related equipment has made a major commitment to RO water treatment. The KSB product portfolio includes high-performance multistage pumps capable of delivering the pressures and volumes needed for desalination of seawater and contaminated groundwater; valves; and specialized pressure recovery units that improve the overall operating efficiency of RO systems. KSB also provides booster pumps that support the pressure recovery systems.
Some of the stars in the KSB RO catalogue include:
HGM-RO pumps: These workhorses can pump up 1,150 cubic metres per hour and can operate pressures ass high as 120 bar (1740 psi). Manufactured from Duplex stainless steel, they are highly resistant to corrosion and feature simple and reliable seals and bearings to reduce maintenance costs.
SalTec DT® pressure exchangers: Designed for large-scale seawater desalination systems, these units recover pressure energy from concentrated brine collected on the upstream side of the RO membrane and use it to pre-pressurize raw seawater entering the system, thereby reducing the overall energy requirements of the system by a significant amount.
Salino® integrated pump pressure recovery units: These are specialized units designed for smaller desalination and water purification plants (e.g. cruise ships, isolated resorts or industrial plants). Combining a high-pressure pump and pressure exchanger in a single, skid-mounted unit, these devices provide outstanding energy efficiency and convenience.