Buying a Reverse Osmosis (RO) system for an industrial setting – a simple guide

A Reverse Osmosis (RO) system is a highly effective and popular technology used by many manufacturing industries to purify water.   There are many different systems on the market so it can be overwhelming when trying to find one that meets your company’s exact requirements.  If you are currently looking for an industrial RO system and want to know what reverse osmosis is, what to look for in a RO system and what questions you should be asking, then this guide is for you.   

If, after reading our guide, you still have some unanswered questions our team at AllWater Technologies Ltd are happy to help.  They can be contacted on enquiries@allwatertech.co.uk or +44 (0) 1934 751333.    

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Contents: 

What is Reverse Osmosis?

To understand what Reverse Osmosis is you first need to understand osmosis.  In simple terms, osmosis is the movement of a solvent, for example, water through a semi-permeable membrane from a region of lower concentration to a region of higher concentration to achieve balance on both sides of the membrane. This movement occurs naturally without any energy being applied.   To stop this natural process from occurring you need to apply pressure to the higher concentration side of the membrane.  The osmotic pressure is the pressure difference needed to stop this natural flow of the solvent across the semi-permeable membrane.    

For reverse osmosis to occur, you apply a pressure that is higher than the osmotic pressure.  This forces the solvent through the semi-permeable membrane in the opposite direction to that which occurs naturally with osmosis, leaving any impurities behind.  The water passing through the membrane is referred to as the “permeate” (or product) whilst the remainder is referred to as “concentrate” (or reject).   As the RO membrane only allows a minute amount of salts in the feed water to pass through, the conductivity of the permeate is very low, whereas the concentrate is very high in dissolved salts.      

To achieve reverse osmosis in an industrial setting you need to buy a suitable reverse osmosis system.  An RO system is an efficient water filtration system and is usually a self-contained unit.  Using an RO system you can effectively remove many different pollutants and chemicals from water (approximately 95%-99% of contaminents)

How does a Reverse Osmosis System work?

How do I know if my company needs an RO system?

Reverse osmosis technology produces purified water, so it may be suitable for any industry that needs high quality water for any reason.  

There are many reasons why some industries choose an RO system over other technologies.   For example, water purified using a RO system may be used as an ingredient for drugs by pharmaceutical firms.  For the food and beverage industries the purified water might be used in production of packaging materials or as an ingredient itself and for aerospace and automotive companies the water may be used for production and finishing.  For other manufacturing firms, the water may be used for cooling or cleaning products and equipment. 

Basically, if you need water to be of a higher quality than your town’s water supply, then an RO system might work for you.  As there are many different RO systems to choose from, it is worth narrowing your search to companies that have expertise in other industrial water treatment technologies in addition to RO systems as they may recommend a different solution that better suits your requirements. 

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What chemicals and substances can an RO system remove?

RO systems are most often renowned for their ability to remove micro-organisms, but they can also remove a host of other chemicals and substances.   The membrane used in an RO system is a molecular filter and as a result it will remove most contaminants including:

  • Nitrates.  Whilst good for plants nitrates can be harmful to people in high concentrations.
  • Bacteria.  
  • Viruses.
  • Endotoxins.  An endotoxin is a byproduct of bacterial breakdown and like, nitrates, these can be harmful to people.
  • Silica.  Removal of silica is particularly useful for boiler feed water applications.
  • Dissolved salts and minerals. Although dissolved salts and minerals are often beneficial in drinking water, for manufacturing purposes these might lead to deposits on work surfaces that are detrimental to decorative finish or operational performance. 

Whilst an RO system will remove almost everything it will remove some chemicals to a better degree than others. For example, small molecules such as Sodium Chloride are more likely to pass through the membrane than larger molecules such as Sulphates.

We are often asked if an RO system will remove Bicarbonate.  Bicarbonate is unusual because it can pass through an RO membrane depending upon pH.  For example, at lower pH the Bicarbonate exists in the form of carbon dioxide and will easily pass through an RO membrane.   To prevent this from occurring and to ensure that as much Bicarbonate is rejected as possible, then the ideal pH is 8.3. At this pH the Bicarbonate exists as HC03.  If your water has a high Bicarbonate level but a low pH, you have two options.  You can either raise the pH by dosing with a small amount of an alkaline reagent or use an alternative technology such as a degassing membrane to remove the Carbon Dioxide.

Is RO water acidic?

RO water is generally acidic because any CO2 passing through the membrane forms Carbonic acid which lowers the pH.   However, due to the lack of buffering this pH may seem disproportionate to the actual level of acidity so even the smallest amount of an alkaline reagent such as Sodium Hydroxide will quickly lead to an increase in pH.

Does water temperature affect RO performance?

An RO membrane responds to temperature in a similar way as human skin.   Pores open more in response to high temperatures and close in lower temperatures.  For an RO membrane, high water temperatures cause an increase in both flux rate and in salt passage through the membrane resulting in an increase in permeate conductivity.   With cold water temperatures the flux rate decreases but this can result in an improved permeate conductivity.   The performance on most RO systems is usually expressed at standard feed concentration and temperature with temperature typically being 15 degrees C.  Under extreme low temperature water conditions, it would not be unusual to heat the water to some degree first before contact with the RO system. 

The specific temperature-related permeate output can be calculated according to the following calculation equation:

Permeate output at X º C = Rate output x Correction Factor F

T in º C

Design Temperature

Nominal output = 100%Correction Factor

1.00

-10.96
-20.92
-30.88
-40.84
-50.80
-60.77
-70.74
-80.70
-90.67
-100.64

Is an RO system cost-effective when compared to an alternative technology such as ion exchange demineralisation?

Where the incoming water supply is of low conductivity, ion exchange demineralisation is a more cost-effective technology but if the conductivity of the incoming water is high then an RO system tends to become more cost-effective.   If you are unsure our team at AWT can carry out a cost comparison to determine which technology is the most cost-effective for your specific requirements.   

Are RO systems environmentally friendly?

RO systems are generally considered as offering an environmentally friendly way of purifying water. The process typically removes up to 99% of total dissolved solids (TDS) and does not use any hazardous chemical reagents commonly used in other processes.  However, typically they require more electricity to run than other water purification methods and they can waste a lot of water.   Despite this, for many industries the lack of hazardous chemicals offsets these two negatives.    As other systems used for the production of pure water may use corrosive chemicals, such as Hydrochloric Acid and Sodium Hydroxide, for many companies the benefit of not having to manage the safe use of these hazardous chemicals is an advantage. 

Is pre-treatment of water necessary?  

If you are using a town’s water supply then it is highly likely that you will need to undertake some pre-treatment of the water before passing it through your RO system.  Semi-permeable, RO membranes are sensitive and certain chemicals and substances can cause damage to the RO membranes reducing their shelf-life and efficacy.   In order to reduce problems associated with most potential foulants, it may be possible to reduce the level of recovery on the RO system to 25% or less.  However, this is not a solution for damage caused to membranes by free Chlorine present in town water supplies.

Removing Chlorine from water

As a bare minimum, you will have to remove free chlorine present in the incoming water before passing through your RO system.  Chlorine is a powerful oxidant and is added to town’s water supplies to ensure that water is free of bacteria.  As a powerful oxidant Chlorine attacks RO membranes, breaking them down and increasing the pore size.  As a result, higher levels of salt will be able to move through the membrane resulting in higher permeate conductivity. 

To remove free Chlorine from water for use with smaller RO systems you generally use an activated carbon filter – even ‘under the sink’ RO systems use this method.  For a larger RO system, you might treat the water with a reducing agent such as Sodium Bi-Sulphite (SBS).  This method can offer some capital cost savings as well as reducing both footprint and the amount of water required for the backwashing of carbon filters.

Removing Organics from water

Organics are usually large molecules and they block the pores of RO membranes.  This is also known as ‘plug fouling’. They can be removed from water using either activated carbon filtration or an organic scavenger.  You can use activated carbon filtration to remove both free chlorine and the organics at the same time.

The term ‘organic scavenger’ generally refers to an ion exchange system, which uses anion resins to ‘trap’ the organics.   These ‘trapped’ organics are then removed from the resin by regeneration with brine or a combination of caustic and brine, which you can also heat in certain circumstances to make the process more efficient. 

Removing metals from water

Most town water supplies are free of heavy metals.   However, if your RO system is operating on water sourced from a borehole then it is possible that metals such as Iron and Manganese may be present in the water.   These metals can precipitate in concentration at the membrane surface causing it to ‘foul’ . To remove Iron and Manganese you need to use a specific filter that incorporates a media, which typically includes Manganese Dioxide.   This acts as a catalyst to oxidise the Iron and the Manganese causing them to precipitate so that they can be filtered and removed prior to contact with the RO membrane.

Removing sediment from water

Turbidity is measured in Nephelometric Turbidity Units (NTU). The design criteria for most RO membranes are that they must have a Fouling Index (FI) less than 3NTU.   If the NTU is higher than this, it will be necessary to undertake additional filtration, otherwise sediment, if present, has the potential to block both the RO membrane feed spacer and the membrane itself.  The ideal pre-filtration in this situation is ultra-filtration although this is dependent upon the level and the nature of the solids present.  Other technologies that can be used as an alternative include flocculation, settlement and sand filtration.  

Scaling and RO membranes

Scaling most commonly refers to the formation of Calcium and Magnesium Sulphate, Carbonate and Bicarbonate salts which precipitate on concentration at the RO membrane.   To prevent scaling you typically have two choices.  You can either use ion exchange softening or you can dose with an anti-scalent.   Ion exchange softening uses a cation resin which exchanges the Calcium and Magnesium ions for Sodium, producing salts which have a higher solubility and will not precipitate on concentration at the RO membrane.  Once the softener resin becomes exhausted it can be regenerated using brine.

You would generally choose to dose with an anti-scalent for larger RO systems as there are benefits to be had with regard to capital cost, footprint and on the volumes of water and brine needed for regeneration of an ion exchange softener.    However, ion exchange softening is considered by many to be a much simpler technology and it is less likely to lead to catastrophic scaling of RO membranes if it starts to fail.   Typically as softener resins age, capacity may be reduced leading to hardness passing towards the end of the service cycle which results in a gradual ‘fouling’ of the RO membranes.  By contrast, failure of an anti-scalent dosing system results in acute failure of the RO membranes as the scale builds up quickly.    All AWT anti-scalent dosing systems incorporate a flow monitoring system, which stops the RO system from running once a low flow is detected before complete failure can occur.    

How do I know what RO System to choose? 

There are two main factors you will need to consider when looking to buy an RO system.

What size of RO system do I need?

First and foremost, you need to determine the flow rate you need. When sizing an RO system the flow rate referred to is normally the permeate flow rate and not the feed flow to the RO unit.

What quality of water do I need?

Another consideration when buying an RO system is the quality of water you are looking to achieve.  An RO membrane, when new, will reject greater than 99% of all Total Dissolved Solids (TDS).  However, with time, you may see the level of salt rejection drop to 97% or lower.      If a high level of salt rejection is important for you, you may wish to consider a Twin Pass RO system.   A Twin Pass RO system is where you have two RO systems working in series, one after the other.  The permeate from the first RO (‘the first pass’) is then treated through the second RO (‘the second pass’) in order to achieve this higher level of salt rejection.   To further improve quality, you may consider caustic dosing or membrane degassing of the first pass permeate. This further improves the rejection of Bicarbonates.  Other technologies to consider post RO treatment include electro deionisation or ion exchange polishing.

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How do I compare different RO systems?

If you are trying to compare different RO systems and the cost implications of each choice, there are several questions that you should ask potential suppliers.

What flux rate do I need? And how many membranes are included within your price?

Flux rate refers to the rate at which the permeate passes through the RO membrane.  Manufacturers of RO membranes publish a maximum flow rate for their particular membranes when operated under a fixed set of parameters.   Flux rate is measured as litres per metre square per hour (lmh).   It is always worth checking that the flux rate specified by the manufacturer of the RO system matches the flux rate recommended by membrane manufacturers.    Membrane life span is dramatically reduced if these do not correlate.   When comparing offers from various RO suppliers, we would advise that you always ask about the flux rate and find out how many membranes are included within the price of the RO system.

Will your RO system support the integration of additional pumps if needed and if so, how many?

You need to consider whether you also need to invest in some peripheral pumps to make your RO system work effectively.  If you need to boost the water pressure to your RO system, you will need a raw water pump.  If you need to distribute the permeate from your RO system to points where it will be used you will need a distribution pump.   Most manufacturers of RO systems offer a standard control system as part of their package, which does not always allow for the integration of these additional pumps.   In this scenario you will need to buy an ancilliary control panel in addition to the RO system.   At AWT all our Reverse Osmosis systems (above 500 lph range) include the optional facility for integrating and controlling up to an additional four pumps. 

What do you charge for Clean in Place (CIP) connectivity?

In order to extend the life of your RO membranes, particularly on larger RO systems, you periodically need to carry out specialist cleaning of the RO membranes in situ (or ‘clean in place’).   At AWT we include CIP connectivity as standard and at no extra cost on all RO systems with a flow rate greater than 500 litres per hour.   Many RO suppliers will charge extra for this service.   

Can water quality be tested from individual membranes?

We would always advise that you ask this question of any RO system supplier because the ability to test water quality from individual membranes will help you save money in the future when it comes to maintaining your equipment and to fault finding should a problem arise.   This is particularly important for larger RO systems. 

At AWT our RO systems are provided with a sample port on each RO membrane vessel.  This allows our engineers to test the permeate quality from individual RO membranes, an essential tool when fault finding.   Consequently we are often able to pinpoint exactly which membranes are not performing as well as they should.  This feature on our systems saves money as you may just need to replace an individual membrane rather than a complete set.

What instrumentation do you offer?

With a new RO system, as a bare minimum, you should be able to measure the conductivity of the permeate.  Ideally this should be temperature dependent as temperature can impact both permeate flow and conductivity.  It is also helpful to have the functionality to measure feed pressure after the RO high pressure pump as any increase in pressure can be an indication of poor performance.   It is also worth checking if you are able to monitor flow on the permeate concentrate and concentrate recycle lines. Often the facility to monitor flow on the concentrate recycle is not always included in the price but this facility can play a significant role against membrane fouling and can extend the life of the RO membranes.

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Do I need a duplex RO system?

In most industrial settings it is unusual to see a duplex RO system being used because, unlike ion exchange systems, there is no requirement with an RO system for regeneration and therefore not the same level of downtime.  In fact, RO systems work most efficiently when they are running for an extended period of time.   You should only consider a duplex RO system if your RO requirement is process critical. Most companies using RO systems choose a single RO system but keep critical spares on site.  With any RO system, duplex or single, you will need to consider the need for downtime for any CIP or emergency maintenance.   

Do RO systems need regular servicing?

Generally speaking, RO systems require less maintenance than some other water treatment technologies provided that they are looked after properly.   The condition of the membranes is critical to how effectively an RO system functions so they do require some regular cleaning and disinfecting.  Knowing your typical flow rate and the average concentration of contaminants needing to be filtered can help you work out how often you need to clean and disinfect.  

Typically we would expect an RO membrane to last in excess of three years.  However, the actual life span depends upon use, the efficiency of pre-treatment, and regularity of cleaning.    In terms of the servicing of your RO system, the frequency will depend upon the complexity of the system you have in place, how strategically important the system is to your business operations, and how efficient your pre-treatment is.   If you need advice on this, our team is happy to help.  

What spares should I keep?

Most standard RO systems only require a single high-pressure pump and therefore this piece of kit is often considered a critical spare.  Other critical spares might include key instrumentation such as conductivity meter and cell.  It is not usual to keep spare RO membranes on site, although you may want to consider this if your RO system is critical to your operations.

What are Cleaning in place (CIP) systems ?

Cleaning in Place (CIP) involves circulating cleaning products around the RO membranes in order to remove foulants.  There are two stages to CIP and one is undertaken directly after the other.

An alkali clean is used to remove organics.  An acid clean is used to remove inorganic substances such as Calcium and Magnesium scaling or fouling by Iron and Manganese.  In addition to these two stages, if it is at all suspected or known that bacterial fouling has occurred, then a specialist bactericide should also be used.

This process should always be undertaken by a specialist or someone who has been properly trained to carry out the procedure.   An expert will instantly recognise when you need to change the cleaning solution and will be able to check that the cleaning has been effective.   He or she will also be able to ensure that RO downtime for the CIP is as minimal as possible and that the RO system is working at the optimum level.  

To improve the efficacy of CIP it can be beneficial to heat the solution to a suitable level, albeit not high enough to cause damage to the membranes.   The person carrying out CIP should also monitor the strength and condition of the cleaning chemicals in case it is necessary to either top up the solution strength or to replace it.

When designing a CIP system for an RO system, it is important to consider a number of factors: flow velocity through the membrane modules, the ability to filter any debris removed from the membranes during cleaning and finally the time required to heat the CIP solution in order to minimise downtime.

It is recommended that RO membranes are cleaned when one or more of the parameters listed below is applicable:-

  • The normalized permeate flow drops > 10%
  • The normalized salt passage increases 5-10%
  • The normalized pressure drop (feed pressure minus concentrate pressure) increases 5-10%

What is a Membrane Autopsy?

Having undertaken CIP, if it is not then possible to recover RO membrane performance and, if you want to explore the cause of recurrent membrane fouling, it may we worthwhile carrying out an RO membrane autopsy.   This is a destructive process during which the membrane is taken apart and the various components that make up the membrane are studied for signs of damage. The overall autopsy many include a combination or all of the following tests:

  • Visual inspection and weighing of the element
  • Vacuum test to check integrity
  • Performance test and normalization to manufacturer’s standard conditions
  • Open the membrane, visual inspection with light microscope
  • Fujiwara test to check for damage by halogens (e.g. Chlorine)
  • Wet chemistry tests to analyse organic/inorganic content of foulants
  • Biological activity test using ATP and dip slides
  • Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray analysis (EDX) and Chromatic Elemental imaging (CEI) of samples of foulant to confirm foulant make-up and identify suitable cleaners to test
  • Dye test coupons of membrane for passage where appropriate
  • Cell test coupons of membrane with cleaners where appropriate
  • Final report of findings including photographs and SEM/EDX/CEI images

How do I know if my RO membranes are fouled?

The following could all be indications of membrane fouling: a reduction in permeate flow rate, an increase in feed pressure and /or an increase in permeate conductivity.  However, because water temperature can have a dramatic impact on flux rate and permeate quality, this should always be checked and excluded first.  Normalization is a method used, particularly on larger RO systems, to provide a baseline for membrane performance.   This considers initial commissioning values against current operational parameters such as any change in temperature or feed concentration.

Are there any disadvantages to an RO system?

As already mentioned one of the major limitations of using reverse osmosis in water purification is the amount of water that is wasted during the process. This is the water that does not pass through the membrane and is generally referred to as the concentrate as it retains all the dissolved solids that have been retained in the feed water.  This may be as high as 25% on a standard RO system but can be reduced by employing additional treatment stages or dependent upon the concentration of the feed water. 

Are there more efficient RO systems available?

With the advent of nano-filtration membranes, it is now becoming more common to introduce a recovery stage on the RO concentrate enabling up to 90% recovery.  On AWT higher recovery RO systems, the permeate from the concentrate recovery stage is fed back to the feed on the primary RO, which improves the RO permeate quality by reducing the conductivity on the influent feed.  This is achieved without an increase in power consumption.  Nano-filtration membranes are similar to RO membranes but they have slightly lower rejection for mono-valent ions.  Whilst the resulting permeate quality is not quite as high with nano-filtration membranes, they do operate at lower pressure thus ruling out the requirement for an additional booster pump stage.   Rejection of multi-valent ions such as Sulphates and Nitrates remains high.

Why are RO systems often used for Pharmaceutical applications?

In the design of pharmaceutical systems it is necessary to provide a minimum of two barriers that can reject bacteria.  RO membranes are considered a suitable barrier and therefore one or sometimes two RO membrane stages are often employed when designing pharmaceutical RO systems.   The ability to clean RO membranes with bactericidal chemicals helps ensure bacterial integrity is maintained and in some circumstances “crevice free” membrane housings are also used in order to help reduce areas where viable bacteria can hide and proliferate. 

With more recent advances heat sanitisable membranes have been developed which can tolerate water temperatures high enough to destroy bacteria without damaging the membranes.

We hope that this guide answers most of the questions you might have in relation to reverse osmosis systems for industrial settings.  We would always recommend that you seek professional advice before placing any order for a RO system.  Sometimes RO is not always the best solution for businesses looking to purify water and it pays to take the time to ask a water treatment expert for guidance on what will work best to meet your specific requirements.  

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