WATER & WASTEWATER TREATMENT
PROTECTING REVERSE OSMOSIS PLANTS THROUGH WATER PRETREATMENT
Reverse osmosis membrane material for water filtration
One of the most common types of water treatment, reverse osmosis (RO) is a separation technique that is suitable for a wide range of applications, especially when salts and/or dissolved solids need to be removed from a solution.
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here are several factors to be considered when selecting an RO system: the customer’s capacity requirement (water usage), the daily production capacity of the system, and the per cent rejection for specific contaminants in the source water,” explains Anelia Hough, water treatment consultant at Allmech. RO plants require maintenance and care to ensure they function optimally and to extend their lifespan. “Aside from regular maintenance, the best way to protect an RO plant is by pretreating water, which reduces the strain on the RO membrane – a costly component. It also helps to avoid issues like scaling and biofouling,” she adds.
Common issues “Factors that can affect an RO system’s performance include temperature, operating pressure, back pressure, the equilibrium effect/ TDS creep, per cent recovery and, of
course, the RO membrane’s permeate production and per cent rejection ratings,” says Hough. The most common issues in RO plants include: • F ouling: occurs when contaminants accumulate on the membrane surface, effectively plugging the membrane. There are many contaminants in municipal feedwater that are harmless for human consumption, but large enough to quickly foul (or plug) an RO system. • S caling: particles are deposited on a membrane, causing it to plug. As certain dissolved compounds become more concentrated, scaling can occur if these compounds exceed their solubility limits and precipitate on the membrane surface as scale. Scaling calculations are usually only based on the silicate concentration in the feedwater. • B iofouling: reduces actual membrane performance through microbial generation in a biofilm that forms on the membrane surface.
Reverse osmosis is one of the most common types of water treatment
• Chemical damage: on an RO membrane, this means a higher permeate flow and poorer-quality permeate water. Dosing of oxidant agents, such as chlorine or hypochlorite, can reduce performance and ultimately result in the failure of the RO membranes. Use of aggressive cleaners can also cause chemical damage. • M echanical damage: can happen when a system is pressurised too quickly, damaging the RO membrane elements. One of the clearest signs of damage on an RO membrane is the lack of salt rejection capabilities. There is also very often an increase of permeate flow rate. Pretreatment options “Pretreatment can help to avoid these problems. When selecting a prefilter, M A R/ A P R 2022
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