Water softening is a process in which water flows through a bed of resin to exchange the hardness ions, calcium and magnesium, for sodium ions. When the resin has reached its capacity for holding hardness ions, the water softener initiates a regeneration cycle. During this cycle, a sodium chloride brine solution flows through the resin and effectively reverses the process by exchanging sodium ions for hardness ions, and flushing the hardness ions down the drain.
1. What Is Resin?
Resin is the ion exchange media used commonly in water softening applications. The most widely used resin in the industry is polystyrene-type gel resin. This resin has a very porous, skeletal structure and each bead ranges in size from 0.3-1.2mm, containing approximately 45% moisture. The building blocks of this type of resin are Polystyrene and Divinylbenzene (DVB). To better understand the function of the resin bead and the failure mechanisms associated with it, consider the following analogy where the spherical sponge represents polystyrene and the elastic bands represent DVB.
Several elastic bands are wrapped around the sponge that is compressed more and more with each elastic band that’s added. The “bead” becomes stronger and more compact with this process known as “crosslinking”. Crosslinking varies from 2-20% DVB content, but the most commonly used in softening applications are 8% and 10%. The 10% crosslinked resin offers up to 50% longer life and 10% additional capacity than the 8% crosslinked resin. A higher degree of crosslinking leads to a decreased bead size and therefore a greater number of beads allowed per cubic foot of resin. More beads per cubic foot effectively allows for more functional groups to attract hardness ions, resulting in a greater capacity.
2. How Long Does it Last?
Contrary to popular belief, resin does not last forever. Throughout the life of a water softener, resin is under constant attack from hydraulic shock, oxidation, osmotic shock, general attrition, fouling and more. Resin manufacturers often use ten years as a general rule for expected lifetime, but this can change significantly depending on the conditions to which the resin is subjected.
3. What are the Failure Mechanisms?
There are many failure mechanisms associated with the resin bead and the following are brief descriptions of some of the most common:
- Water Hammer (Hydraulic Shock)
The sudden interruption of high pressure water flow causes the resin beads to “slam” against the side of the tank and will lead to cracked and/or broken beads. Avoiding fast acting solenoid valves in the system design is advisable to minimize this risk.
- Oxidation (Chlorine Attack)
Consider chlorine attack to be analogous to “snipping away” the elastic bands around the spherical sponge causing the bead to lose strength, swell, and retain higher moisture content. The number of ion exchange sites will remain unchanged with swelling but the resin beads now occupy a larger volume within the tank. This can lead to cracked and/or broken beads.
- Osmotic Shock
By nature, resin beads swell and contract as they exhaust and regenerate. As time goes on, these beads will eventually crack and/or break. The expansion rate during the backwash stage of regeneration is a function of flow rate and incoming temperature. As such, the osmotic stress is greater in low temperature, high backwash-rate systems.
- Resin Attrition
An increased pressure drop across the resin bed can often be attributed to a high percentage of cracked and/or broken beads. Broken bead particles tighten the bed surface by filling the void spaces with bead particulate. “Fines” will eventually leave the system in the backwash stage of regeneration and a reduced capacity in the softener will be observed due to the decrease in exchange sites.
- Metal Fouling
Oxygen is introduced with brine during every regeneration cycle so iron fouling is to be expected in any system with elevated iron levels in the incoming water. This iron oxide precipitant cannot be removed by regular salt regeneration and effectively plugs up resin exchange sites that would otherwise be available for softening, resulting in reduced capacity.
4. What Can You Do?
First and foremost, understanding the incoming water source is critical for predicting the life of the resin and properly maintaining the system. The incoming water should be tested prior to installing a new water softener system and the type of resin should be carefully selected. After the system is installed, the following are recommendations and available options to consider:
- Log Books: Daily hardness levels, water meter readings, flow rates and inlet/outlet pressure readings should be recorded on a regular basis for each softener. Monitoring these trends is a great way to identify when the system performance is declining and additional testing, cleaning, and/or replacing resin should be considered.
- Resin Testing: Core samples can be obtained from the resin bed and analyzed for total capacity, moisture content, percent broken and more.
- “Topping Up”: Some degree of resin attrition and resin loss is to be expected in any system. Oftentimes, a simple “top-up” of the tank is advisable to increase overall capacity.
- Chemical Additive: In extreme cases, a chemical additive such as Eldon Water's Resinklenz Fe should be considered. This regeneration aid prevents iron from precipitating into iron oxide, improving performance and extending the life of the resin significantly.
- Chemical Cleaning: In heavily fouled systems, resin can be cleaned using a low pH solution such as Citric Acid. In chemical cleanings, uniform exposure and contact time is critical.
- Filtration: Pre-filtration can be considered in cases to remove iron and other foulants from entering the softener system and fouling the resin.
- Resin Replacement: Though the initial cost for higher strength, higher crosslinked resin is greater, it may be the better option in the long-run and the economics should be considered.
Water softeners are a critical piece of equipment in many industries where failures can quickly lead to costly breakdowns. Considering resin to be a maintenance item rather than the commonly forgotten about “beads inside that tank” is a great start to improved management of your water system.