Prefiltration of Reverse Osmosis Membranes using Cartridge Filters

Increasing population, contamination of natural water sources and stringent water quality and treatment regulations have made water purification and production one of the biggest challenges of our time.

Reverse Osmosis is a popular technology for desalination and water purification but has also found use in many other industries like boiler feed water production in power generation, water production for pharmaceutical or beverage use, military use, humanitarian work and ultrapure water in semiconductor industry.  All technologies come with their limitations and reverse osmosis is no different, premature failure of reverse osmosis membranes due to fouling can be very costly.

What is RO membrane fouling & scaling?

Membrane fouling is a common problem and starts when substances in water build up over time inside and on the surface of RO membrane, gradually reducing flux. Contaminants can block the membrane pores or adsorb on the surface. Fouling is usually progressive and can be controlled to a degree with appropriate pretreatment. Poorly designed pretreatment however can cause a rapid increase in the rate of fouling and even contribute to fouling.

Fouling contaminants

RO membrane fouling contaminants can be categorised as

Particulate/Scale – Clay, silt, particles, Calcium, Magnesium, Barium, Aluminosilicates

Biological/Microbiological – Bacteria, slime, algae

Organic – Oil, n-Hexane, humic acid, fluvic acid, protein

Other materials – coagulants, detergent, biocides

For the purpose of this article we will focus only on removal of suspended solids by cartridge filtration.

Method for measuring fouling potential

The fouling potential of feed water is determined by a method called Silt Density Index (SDI) which calculates the rate of plugging of a 0.45 micron microfiltration membrane at constant applied pressure of 30 psig.  SDI index of less than 3.5 is usually required to minimise the fouling potential.

Another useful parameter is turbidity as it mainly results from suspended material, which can contribute to fouling of RO membranes. NTU value of 0.5 is recommended for RO feed water.

Pretreatment of RO systems and cartridge filtration

Pretreatment of RO systems has been studied in great detail and usually includes some of the following steps depending on application, source and quality of feed water:

Straining

Chlorination

Clarification

Biocides

Media filtration

pH adjustment

Scale inhibitor

Cartridge filtration

Cartridge filtration

Cartridge filtration is a mature technology and usually the last step in pretreatment of RO systems, used mainly as a guard filter for protecting the membrane from suspended particles. Cartridge filters can also aid in reducing biofouling by trapping some bacteria, larger than rated pore size, however that is not the intended purpose of cartridge filtration and any microbiological contaminants would have been treated by using biocides in earlier pretreatment.

Cartridge Filter size and changeout frequency for RO prefiltration

5 micron filter cartridges are more commonly used but depending on application requirements, this can be as low as 1 – 3 micron.  Changeout is usually carried out once the differential pressure starts to rise and reaches a critical level, usually between 1.5 – 2barg. It is recommended that filter cartridges are changed out every three months.

acuraPromelt and acuraMultiflow filter cartridges by Siga Filtration are excellent depth filter cartridges and provide RO membrane protection at 1 – 5 micron by trapping dirt throughout the depth of media. Cartridges are constructed from 100% Polypropylene material using thermal bonding process which eliminates the need for binders.  These cartridges provide exceptional performance in removing particles carried over from media filters, colloids, organics and sediment from feed or pretreated water.

acuraPromelt and acuraMultiflow filter cartridges can be supplied in a range of end cap styles and will retrofit filter cartridges by other manufacturers.