Maintaining Ion Exchange Resin Based Systemsby

Michael C. KellerSybron Chemicals Inc.

Ion exchange resins use a process based upon ion selectivity. In this method, ionsof greater preference will displace ions of lower preference. For example, in watersoftening, objectionable ions such as calcium and magnesium are exchanged on tothe resin and sodium or potassium ions are released in their place. To regeneratethe bed, a strong regenerant solution must pass through the resin bed. In watersoftening a 5% to saturated solution (~25%) of sodium chloride (NaCl) orpotassium chloride (KCl) is used to regenerate the bed of resin. Because of thehigh concentration of sodium or potassium, hardness is displaced from the resin.In this way, ion exchange resin can be used over and over again. Ion exchangeresin will also act as a filter to the incoming water (this is not recommended).When substances like oil, grease, metal precipitates (iron, manganese, copper,etc.), high molecular weight metals (aluminum, lead, etc.), chlorine, organics(tannins, polymers etc.), silt and sand come in contact with resin, the exchangemechanism can become inhibited. These factors can produce poor water quality,short service cycles, lower flow rates and a decrease the useful life of the resin.

Properly manufactured 8% crosslinked softening resin can have a life expectancyin excess of 20 years when placed in unstressed situations. Anion resins do not lastas long with a life expectancy of 4 - 7 years. When fouling and/or degradation hasoccurred, the resin can either be thrown out or it can be cleaned and put back intoservice. There are many different forms of fouling/degradation that will manifestin or on the ion exchange resin. This article will be dealing with the cleaning andpreventative maintenance of ion exchange systems.

Iron Fouling

Probably the greatest threat to softening resin is iron fouling. Cation resin willexchange ferrous iron Fe+2 for sodium on to its structure. As long as the iron staysin the ferrous form, salt will be able to regenerate the iron off the resin. If there areany oxidizing agents like chlorine and/or oxygen in the water, the ferrous iron Fe+2

can be oxidized to ferric iron Fe+3 (rust). This is known as precipitation and canoccur both on the surface as well as in the internal matrix of the bead. Ferric ironcan not be regenerated from the resin with simply salt. A quick testing procedurefor iron fouling is to obtain approximately 2 oz. (50 gm) of resin and place it into abeaker. Add the same amount of hydrochloric acid (muriatic acid) to the beaker

and let it sit for 2 hours. If the solution is heated, a shorter contact time can beutilized. The acid will turn from clear to a yellow / tea color. The darker the colorthe heavier the iron fouling.

Reducing agents, such as sodium hydrosulfite and sodium bisulfite, can beeffectively used to clean iron fouled beds. One half to one pound of reducing agentcan be added to one gallon of water. This mixture is then immediately poured intothe brine well, or directly into the salt, and will treat one cubic foot of resin. Thesecompounds decompose rapidly and should be used immediately after mixing.Always remember to add the chemical to water (not water to the chemical). Amanual regeneration is performed to clean the bed.

Acids, such as hydrochloric (HCl) also known as muriatic acid, phosphoric acid,and citric acid are commonly used to clean iron fouled resin beds. These acids willgenerally produce better results thAn the hydrosulfite and bisulfite cleaningcompounds. However, precautions must be taken when utilizing acids.Hydrochloric acid is very corrosive. In most instances when HCl comes in contactwith a metal surface, corrosion will occur. When an acid is used, some of theexchange sites on the resin will be converted to the hydrogen form. If the resin isnot fully regenerated back to the sodium form with salt, a low pH can result duringthe first service cycle after the cleaning. A double regeneration with salt willusually prevent a low pH from occurring.

The most effective way to keep iron fouling to a minimum is the use of a salt thatcontain a cleaning agent. There are salts on the market which contain one or moreof the following cleaning agents: Hydrosulfite, Bisulfite, Citric acid andPhosphoric Acid. When salt bearing one of these compounds is used, the resin willbe cleaned during each regeneration. These compounds are not harmful and willhelp to increase the life expectancy of the softening resin.

Aluminum Fouling

Aluminum is another type of metal that can foul softening resin. Municipal watertreatment facilities use aluminum sulfate (alum) as a coagulant. If the alum is overfed, soluble aluminum can enter the distribution system. Although low inconcentration, the aluminum will be exchanged on to the softening resin.Aluminum is trivalent (A1+3) and will not regenerate from the resin efficiently. Aslow decrease in capacity will indicate this type of fouling. Hydrosulfite andbisulfite will not strip aluminum from the resin. An acid is required to clean theresin. Citric, phosphoric or hydrochloric acids are effective cleaning agents. As

stated above, the equipment must be compatible to the agent that would be used inthe cleaning process. Citric and phosphoric acid are less aggressive, making thema better choice in this application. A build of aluminum floc can also occur at thetop of the bed. Acid treatment, along with backwashing and air lancing, will helpto clean the resin.

Filterable Solids

Ion exchange resin can act as a physical filter. Although not recommended for thisapplication, particles of dirt, precipitated metals (rust), silt, etc. can be filtered outby a resin bed. Cation resin can filter particles down to a size of approximately 40microns. Finer mesh resin systems can filter even finer particles. If these particlesare not backwashed from the system, increased pressure drop and channelling canoccur, resulting in poor system performance.

Most ion exchange resin systems require a 10 - 15 minute backwash. Lowerbackwash times can be used on systems that have prefiltration for suspendedfoulants in the influent water. The bed should be expanded a minimum of 50%.Note: There are exceptions to this rule. If the bed is already experiencing pressuredrop and channelling, an extended and/or increased backwash flow can be utilizedto clean the bed. If the bed is severely fouled an air lance may be required to breakup the bed.

Oil and Grease

Oil and grease that enter a water treatment system can be filtered out to somedegree by the ion exchange system, whether it is an anion or a cation. Thesesubstances will coat the resin causing short service cycles and poor product waterquality. Dirt particles and broken beads will also cling to the oil and grease,causing increased pressure drop and channelling.

A non ionic surfactant (any detergent that is not cationic or anionic) can be used toclean a cation or an anion resin. Agitating the resin while it is immersed in a warmdetergent solution will enhance the cleaning process.

Organic Fouling

Tannins in water can foul anion resin. This large organic molecule will passthrough a softening resin without any detrimental effect. Tannins have a weaklynegative charge that will allow them to exchange on to anion resin. Once on theanion resin it will tend to migrate in to the inner matrix of the bead and become

very difficult to remove. Tannins will block exchange sites causing short servicecycles.

The use of soda ash along with the brine has been effective at cleaning organicallyfouled anion resin. By increasing the pH of the brine the tannins become moresoluble. The bead will also swell, allowing better elution of the tannins from theresin. In industrial applications, a solution of brine and caustic (sodium hydroxide)can be used. Caustic is very corrosive and is not recommended for residentialwater treatment. If one of these alkaline brine solutions is not effective and theresin is going to be disposed, chlorination can be used as a last ditch effort to cleanthe resin. Place 4 oz. of a household bleach in the brine well and pass it throughthe anion resin bed until a chlorine odor is detected in the effluent. Theregeneration cycle should then be interrupted for 2 hours, after that time theregeneration can be completed. A second regeneration should then be performedto be certain that the bed is free of bleach. This should be used as a last resort.Chlorination will degrade the resin.

Bacteria

Most waters treated by municipal water treatment use some type of disinfection.Disinfected water will usually not contribute to bacterial growth in an ion exchangesystem. Untreated water can cause a bacterial build-up in ion exchange systems.Regeneration of the resin will usually keep the bacterial growth in check. On thoseoccasions when bacteria are still a problem, the unit can be treated with a standardhousehold bleach. This treatment is hard on the resin but it is a necessary evil.The treatment method is the same as treating an organically fouled resin (previousparagraph).

Oxidation

The last topic to be discussed has to do with resin degradation and not fouling.Oxidizing agents like chlorine in water will attack resin. There is no resin on themarket that is totally chlorine tolerant, but there are resins that are more resistant.Standard softening resin is 8% crosslinked; the greater the crosslinking the greatertolerance the resin has for chlorine. Chlorine attacks the skeletal structure of theresin, causing the resin to breakdown. A rule of thumb that can be used is for each1 ppm of chlorine that is found in the influent water, the resin’s life will be reducedin half. Standard softening resin that is in constant contact with 1 ppm chlorinewill have a life expectancy of 7 to 10 years. It should be noted that applicationsthat operate at high temperatures, or waters that contain metals like copper (acatalytic reaction can occur between the oxidant and the metal), will degrade the

resin at a faster rate. If resin turns to powder when rubbed between the indexfinger and thumb, it is an indicator the resin has been oxidized by chlorine. Insevere cases the bed will compact, causing increased pressure drop and channeling.There is no procedure to restore the resin to a usable condition. Chlorine or anyother oxidant should be removed prior the ion exchange system. When excessiveoxidation has occurred, the resin must be replaced.

Correct cleaning and preventative maintenance procedures will help to preventequipment down time, It is also important to know the limitations of a given watertreatment process and stay within the guidelines. Following these ideas willdecrease service calls, increase the viability of the resin based system, as well asincrease customer satisfaction.