Reverse Osmosis Pre Filtration

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 pre-treatment. Poorly designed pre-treatment 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.

Pre-treatment of RO systems and cartridge filtration

Pre-treatment 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 pre-treatment of RO systems, used mainly as a guard filter for protecting the membrane from suspended particles. 

Cartridge filter 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. 

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

Products for RO Prefiltration

Siga Filtration can supply filters for both low and high flow rate applications. Please see range overview for initial product selection below: 

  • – 1FU stainless steel cartridge filter housings for low flow rate applications
  • – PORALine PI cartridge filter housings for low flow rate plastic systems
  • – ZXFlow multi or high flow cartridge filter housings for high flow rate applications


1FU Single Cartridge Filter Housing (Threaded Closure)

Compact SS housing, 1" connections. Water, chemicals, CNC, inks, paints, coatings, fine particulate removal. 

acuraLine 1FU single cartridge filter housings are constructed from stainless steel and come with a threaded closure. These are compact units with inline connections for easy installation, suitable for a single cartridge of 5", 10", 20" or 30" length. 1FU housings can be supplied with a fully assembled DP gauge for reliable indication of blocked filter element. An operating pressure of 25 bar, flow capability of 6 m3/h and inline connections for easy installation, make this housing a practical choice for industrial applications.  See datasheet for range of useful accessories for this housing.  Use with removable and cleanable acuraScreen Stainless Steel Element Use with disposable acuraMultiflow Polypropylene spun element for fine particulate removal View our differential pressure measurement solutions

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PORALine PI cartridge filter housing (NSF 42 Certified)

Single cartridge filter housing for standard (64 – 69mm OD) cartridges in lengths of 251 mm or 508 mm . Available in Polypropylene (Opaque) or styrene-acrylonitrile SAN (transparent) sump.

  • Cartridge diameter: 2 1/2″ to 2 7/8″
  • Cartridge size: 251 or 508 mm DOE or 213 internal o ring
  • Material: PP I PP/SAN 
  • operating pressure: 8.5 bar
  • operating temperature: +50°C
  • NSF 42 Certified

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PORALine PIBB/PIBC Big Blue Filter Housing

Large diameter plastic filter housings to suit 10" or 20" 114mm OD big blue filter elements. Our PIBB big blue filter housing comes with a blue sump and our PIBC filter housings come with a clear sump. The extra-large housing allows for greater cartridge capacity, reducing the number of vessels required for high flow-rate applications.

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acuraMultiflow PP Depth Filter Cartridge

acuraMultiflow is a nominal meltbolown depth filter cartridge which provides exceptional performance in clarification and prefiltration applications. These cartridges work by trapping a wide range of particle sizes throughout the fibre matrix. 100% Polypropylene construction provides excellent thermal and chemical resistance, making acuraMultiflow filter cartridges ideal for use in water and chemicals. These filters are also suitable for food contact and can be used in food & beverage applications.  View range of cartridge filter housings Absolute removal required? See acuraPromelt. 

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ZXFlow HF filter housing for high flow elements

High flow filter housing solution for Cryptosporidium removal, fire sprinkler systems, RO prefiltration, fine particulate removal, food & beverage.


ZXFLow HF Series high flow filter housing provides a high flow rate solution with a single large diameter filter element and can be installed horizontally or vertically. These filter housings can be supplied for 20", 40" and 60" high flow elements in single or multi cartridge configuration.  ZXFlow filter housings can be used for water flow rates of up to 60 m3/h with 40” and 90 m3/h with a single 60” HFX high flow filter element. Multi cartridge housings can take 3, 4, 5 & 7 elements and offer flow capability of up to 500 m3/h.   Typical applications: 
  • Cryptosporidium removal
  • Fire suppression (Fire sprinkler systems)
  • Protection of reverse osmosis membranes
  • Protection of UV systems
  • Fine particulate removal from process water
  See compatible HFX High Flow Filter Elements for this housing

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HFX High Flow Filter Element

High flow solution for Cryptosporidium removal, fire sprinkler systems, RO prefiltration, fine particulate removal, food & beverage.

  HFX High Flow filter elements feature a high surface area pleated construction (160mm OD) in lengths of 20”, 40” & 60” for high flow rate or high dirt load applications. A single high surface area design eliminates the need for multi-cartridge vessels and reduces maintenance demand.  Absolute rated Polypropylene or Glass Fibre media provides 99.98% removal of particles. HFX high flow elements can be supplied with a special 'CR' media for the removal of Cryptosporidium.  Typical applications: 
  • Cryptosporidium removal
  • Fire suppression (Fire sprinkler systems)
  • Protection of reverse osmosis membranes
  • Protection of UV systems
  • Fine particulate removal from process water
  View ZXFlow HF Stainless Steel filter housing for HFX elements

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