In a one stage RO system, the feed water enters the system from one end and exits the other end of the system as either concentrate or permeate water. In a two-stage system the concentrate (or reject) from the first stage turns into the feed stream to the second stage. The permeate water that was collected from the first stage is combined with permeate water from the second stage. Additional stages increase the recovery from the system.

In an RO System an array describes the physical arrangement of the pressure vessels in a 2-stage system. Pressure vessels usually contain 1 to 6 RO membranes. The reject of each stage then becomes the feed stream for the next successive stage. The example of the 2 stage RO system displayed previously is a 2:1 array. This means that the concentrate (or reject) of the first 2 RO vessels is fed to the next 1 vessel.

If an RO system can’t be properly staged and the feed water chemistry allows for it, a concentrate recycle setup can be done. This is where a portion of the concentrate stream is fed back to the feed water to the first stage to help increase the system recovery.

A pass can be seen as a stand-alone RO system. With a double pass RO, the permeate from the first pass becomes the feed water to the second pass, which ends up producing a much higher quality permeate due to it has essentially gone through two RO systems. It also allows the opportunity to remove carbon dioxide gas from the permeate by injecting caustic between the first and second pass. C0₂ is undesirable when you have mixed bed ion exchange resin beds after the RO. By adding caustic after the first pass, you increase the pH of the first pass permeate water and convert C0₂ to bicarbonate (HCO₃^–) and carbonate (CO₃^2-) for better rejection by the RO membranes in the second pass. This process can’t be done with a single pass RO because injecting caustic and forming carbonate (CO₃^2-) in the presence of cations such as calcium will cause scaling of the RO membranes.

In order to prevent fouling, scaling and costly premature RO membrane failure, proper pretreatment using both mechanical and chemical treatments is critical for an RO system as well as frequent cleaning requirements. Below are factors of common problems an RO system experiences due to lack of proper pretreatment.

When contaminants accumulate on the membrane surface effectively plugging the membrane, this process is called fouling. There are many contaminants in feed water that are naked to the human eye and harmless for human consumption, but large enough to quickly foul (or plug) an RO system. Fouling usually occurs in the front end of an RO system and results in a higher pressure drop across the RO system and a lower permeate flow which leads to the need to clean or replace the RO membranes. This is inevitable, fouling will take place eventually to some extent given the extremely fine pore size of an RO membrane no matter how effective your pretreatment and cleaning schedule is. Although, by having proper pretreatment can minimize the need to address fouling related problems on a regular basis.
Fouling can be caused by:

1. Particulate or colloidal mater such as dirt, silt, clay, etc.
2. Organics such as humic/fulvic acids, etc.
3. Microorganisms such as bacteria present one of the most common fouling problems since RO membranes in use today cannot tolerate a disinfectant such as chlorine, therefore microorganisms are often able to thrive and multiply on the membrane surface. They may produce biofilms that can cover the membrane surface and as a result in heavy fouling.
4. Breakthrough of filter media upstream of the RO unit. GAC carbon beds and softener beds may develop an under drain leak and if there’s not adequate post filtration in place the media can cause the RO system to foul.

Analytical tests, can help determine if the feed water to your RO has a high potential for fouling. Mechanical filtration methods are used to prevent fouling of an RO system, the most popular method being the use of multi-media filters (MMF) or microfiltration (MF). In some cases, cartridge filtration will be enough.

When certain dissolved (inorganic) compounds become more concentrated (as previously mentioned in concentration factor section) then scaling may occur if these compounds exceed their solubility limits and precipitate on the membrane surface as scale. A higher pressure drops across the system, higher salt passage (less salt rejection), low permeate flow and lower permeate water quality are results of scaling. An example of a common scale that tends to form on an RO membrane is calcium carbonate (CaCO₃).

Oxidizers such as chlorine will ‘burn’ holes in the membrane pores and can cause irreparable damage leading us to believe that modern thin film composite membranes are not tolerant to chlorine or chloramines. The result of chemical attack on an RO membrane is a higher permeate flow and a higher salt passage (causes a poorer quality permeate water). Which is the reason why microorganism growth on RO membranes tends to foul RO membranes so easily since there is no biocide to prevent its growth.

A piece of the pretreatment scheme should be pre and post RO system plumbing and controls. Damage to the RO membranes may happen if ‘hard starts’ occurs and also if there is too much backpressure on the RO system. Variable frequency drive motors can help start high pressure pumps for RO systems and by installing check valve(s) and/or pressure relief valves to prevent excessive back pressure on the RO unit that can cause permanent membrane damage.

Below you’ll find some pretreatment solutions for RO systems that can help minimize fouling, scaling and chemical attack.

MULTIMEDIA FILTRATION (MMF)

A Multi-Media Filter is used to help prevent fouling of an RO system, it typically contains three layers of media consisting of anthracite, sand and garnet, with a supporting layer of gravel at the bottom. The larger (but lighter) anthracite coal will be on top and the heavier (but smaller) garnet will remain on the bottom. The filter media arrangement allows the largest dirt particles to be removed near the top of the media bed with the smaller dirt particles being retained deeper and deeper in the media. Allowing the entire bed to act as a filter allowing much longer filter run times between backwash and more efficient particulate removal. A well-operated Multi-Media Filter can remove particulates down to 15-20 microns and a Multi-Media Filter that uses a coagulant addition (which induces tiny particles to join together to form particles large enough to be filtered) can remove particulates down to 5-10 microns. To get a better idea of how much this is, the width of a human hair is around 50 microns. When the Silt Density Index (SDI) value is greater than 3 or when the turbidity is greater than 0.2 NTU, a multimedia filter is suggested. The above guidelines should be followed to prevent premature fouling of RO membranes. It is key that a 5 micron cartridge filter is placed directly after the MMF unit in the event that the under drains of the MMF fail. It will help prevent the MMF media from damaging downstream pumps and fouling the RO system.

MICROFILTRATION (MF)

Microfiltration (MF) has a pore size of only 0.1-10µm which helps it be effective in removing colloidal and bacteria matter. It is also helpful in reducing the fouling potential for an RO unit. The “hollow fiber” type of membrane is most commonly used even though membrane configuration can vary between manufacturers. Usually, the water is pumped from the outside of the fibers, and the clean water is collected from the inside of the fibers. Microfiltration membranes used in potable water applications usually operate in “dead-end” flow, where all of the water fed to the membrane is filtered through the membrane. A filter cake that must be periodically backwashed from the membrane surface forms. Recovery rates are normally greater than 90 percent on feed water sources which have fairly high quality and low turbidity feeds.

ANTISCALANTS AND SCALE INHIBITORS

Antiscalants and scale inhibitors, are chemicals that can be added to feed water before an RO unit to help reduce the scaling potential of the feed water and increase the solubility limits of troublesome inorganic compounds. By increasing the solubility limits, you are able to concentrate the salts further than otherwise would be possible and therefore achieve a higher recovery rate and run at a higher concentration factor. Antiscalants and scale inhibitors work by interfering with scale formation and crystal growth, the use and the correct dosage depends on the feed water chemistry and RO system design.

SOFTENING BY ION EXCHANGE

To help prevent scaling in an RO system, a water softener can be used by exchanging scale forming ions with non-scale forming ions. Very much like the MMF unit, it is important to have a 5 micron cartridge filter placed directly after the water softener in the event that the under drains of the softener fail.

SODIUM BISULFITE (SBS) INJECTION

Adding sodium bisulfite (SBS or SMBS), to the water stream before an RO at the proper dose you can remove residual chlorine because it’s a reducer.

GRANULAR ACTIVATED CARBON (GAC)

GAC’s main purpose is to both remove organic constituents and residual disinfectants (such as chlorine and chloramines) from water. GAC is made from coal, nutshells or wood. Activated carbon removes residual chlorine and chloramines by a chemical reaction that involves a transfer of electrons from the surface of the GAC to the residual chlorine or chloramines. The end result of the chlorine or chloramines ends up as a chloride ion that is no longer an oxidizer. There is a disadvantage of using a GAC before the RO unit, it being that the GAC will remove chlorine quickly at the very top of the GAC bed. Leaving the remainder of the GAC bed without any biocide to kill microorganisms. A GAC bed will absorb organics throughout the bed, which is potential food for bacteria, so eventually a GAC bed can become a breeding ground for bacteria growth which can pass easily to the RO membranes. A GAC bed is able to produce a very small carbon fine under some circumstances that have the potential to foul an RO.

RO DATA TRENDING AND NORMALIZATION

The heart of the RO system are the RO membranes and certain data points that include the system pressures, flows, quality and temperature need to be collected to determine their health. Water temperature is directly proportional to pressure, as the water temperature decreases it becomes more viscous and the RO permeate flow will drop as it requires more pressure to push the water through the membrane. When the water temperature increases the RO permeate flow will also increase. Performance data for an RO system must be normalized so that flow variations are not interpreted as abnormal when no problem exists. The normalized flows, pressures and salt rejection should be calculated, graphed and compared to the baseline data. For example, when the RO was commissioned or after the membranes were cleaned or replaced. This will help troubleshoot any problems and also determine when to clean or inspect the membranes for damage. Data normalization helps display the true performance of the RO membranes, when the normalized change is +/- 15% from the baseline data then you need to take action. If this is not followed, then RO membrane cleanings may not be very effective at brining the membranes back to near new performance.

RO MEMBRANE CLEANING

RO membranes require periodic cleaning, from 1 to 4 times a year, all depending on the feed water quality. Use as a guide, if the normalized pressure drops or the normalized salt passage has increased by 15%, then it is time to clean the RO membranes. Another case maybe if the normalized permeate flow has decreased by 15% then it is also time to clean the RO membranes. These RO membranes can be cleaned in place or have them removed from the RO system and cleaned off site by a service company that specializes in this service. Offsite membrane cleaning is more effective at providing a better cleaning than onsite cleaning skids. RO membrane cleaning involves low pH cleaners and organics to remove Scaling and as for colloidal and biofouling are treated with a high pH cleaner. Cleaning RO membranes is about using the appropriate chemicals, and other factors involved such as flows, water temperature and quality, properly designed and sized cleaning skids and many other factors that an experienced service group must address in order to properly clean RO membranes.