Theoretically, the maximum hardness-removal capacity of a water softener can be calculated from the grains of hardness of the water to be softened, the volume of resin, and the resin capacity expressed in grains/gallon.The actual capacity of a water softener may be impacted by many factors:

• Makeup water quality and variability
• Softener settings
• Regeneration process
• Mechanical operation
• Resin quality and volume

When troubleshooting softener capacity issues, it is important to take all these factors into consideration.

Makeup Water
Not only does one need to know the hardness of the water to be treated, but the consistency or variability of the quality of this water must be determined, too. This is often overlooked. What is the worst-case scenario of how hard the water may be? Many municipalities have multiple water sources with differing water quality. Some cities have a water source that changes from one season to the next. Rain, snow melt, drought, and even road salt can all affect the water quality causing it to change over time or quite rapidly. If the worst case or highest hardness of the incoming water is not determined, hard water upsets may occur before the softener goes into regeneration.

Higher Hardness  =  Lower Softener Capacity

The following table shows an example where the city has two water sources with the hardness ranging from 34-169 ppm. That’s almost 5 times the hardness from the lowest to the highest, and it is from the same water source!

Softener Settings
When the softener nears its total capacity to remove hardness, the unit must be regenerated. This involves a series of steps where the resin bed is backwashed, regenerated, and rinsed. (Refer to the 4 Steps to Softener Regeneration infographic.)

Regeneration has many factors that can directly impact a water softener’s capacity. Questions to consider include:

• Customized Settings: Assuming the supplier’s preprogrammed setup is correct without verifying the data inputs (which often happens) can lead to hard water leakage and/or excessive costs. It is important to customize the softening capacity to your unique needs. Also, was it setup to regenerate after achieving maximum (100%) softening capacity or was there a safety factor built in? Typically, softeners are setup to achieve around 90% of total softening capacity before regeneration to avoid hardness leakage. Building in a safety factor is always a good idea because resin does degrade and foul over time. But too much of a safety factor would increase overall operating costs. The design safety factor needs to protect the systems at a minimum cost.
   
• Makeup Variability: Does the softener capacity setpoint properly take into account any variability in the makeup water? As mentioned previously, makeup water quality can change for a variety of reasons. Should you setup your water softener for worst-case scenario or change the settings seasonally? When the water softener is programmed for the highest expected hardness, the softener will regenerate more frequently, which could be more often than necessary when the makeup hardness level drops. If the softener is not properly programmed for this variable scenario, hard water may pass through. During lower hardness levels, water and salt will be wasted if the same regeneration set points are utilized.
   
• Salt Dosage: How many pounds of salt per cubic foot of resin were used in the softening capacity calculation? Is this consistent with the actual salt usage? Salt is used in the regeneration process to remove the hardness ions (calcium, magnesium) from the resin to be sent down the drain and prepare the ion exchange resin to soften more water. Having the proper salt dosage is critical to an effective regeneration and to efficiently utilize salt. Using the incorrect salt dosage in your calculations can cause you to over or under-estimate how much water your softener can soften. How much salt is programmed into the regeneration cycle can affect the total cost of operation. Using 15 pounds of salt per cubic foot yields 30,000 grains or 2,000 grains per pound of salt. Reducing the salt consumption to 10 pounds per cubic foot reduces the capacity to 25,000 grains but the efficiency goes up to 2,500 grains per pound of salt. The softening capacity is reduced by 16.7%, but the salt usage is reduced by 33.3%.

There are a lot of questions to consider that can affect water quality, costs, and throughput. It would not be wise to just assume the softener was set up properly; however, arbitrarily changing softener settings without having all of the necessary data is ill-advised.

Regeneration Process
Even when all of the above data is known, soft water is not guaranteed. The softener regeneration itself still needs to be analyzed through an elution study. This ensures there is sufficient brine available, the brine is drawn at the correct rate, and the brine concentration through the resin during the regeneration cycle is at the desired levels for the specified period of time. If any of these are out of specification, the hardness may not be forced off of the resin during regeneration or excessive salt and water costs may be incurred. A lot can be learned from an elution study as demonstrated in the graphs below.

Mechanical Operation
How the water softener mechanically operates can greatly impact its softening capacity. If flow rates are too high or even too low through the softening vessel (6-12 gpm/ft²), hard water may leak through for various reasons.

Channeling is a term used to describe when water takes the path of least resistance through a softener resin bed. When this happens, only part of the resin is being used for softening and can become exhausted prematurely. Causes of channeling can include fouled resin and damaged/plugged underdrains.

All or part of the water flow may short circuit around the resin bed itself causing hard water leakage from the softener. Leaking valve nests, damaged internal piping, and leaks at the distributor tube can each lead to short circuiting.

During the regeneration process, backwash flowrates must be sufficient to typically expand the resin bed by 50% for around 10 minutes or more. The temperature of the backwash water may change drastically between summer and winter. The lower the temperature, the denser the backwash water is, and the higher the resin bed will expand. If expanded high enough, resin can be lost down the drain, which effectively reduces total softening capacity of the unit. The resin manufacturer’s specifications should always be consulted for recommended backwash flowrates. Resin beads found in the drain may be a sign the backwash flowrate is too high.

The Resin
Softener resin is typically strong-acid cation media that removes the hardness ions from the water by exchanging sodium ions for primarily calcium and magnesium ions. The volume of resin in the softener may not be what one expects. There are no guarantees. When the resin was first loaded into the softener, some may have remained in the bags if they were not rinsed because the resin tends to stick to the plastic bag, especially in the corners. Some resin may have been spilled as it was poured into the small opening in the top of the softener, and bags can get torn during shipment. As previously mentioned, resin can also be lost during backwash if there are mechanical issues.

Microscopic View of Used Resin

Determining the quantity of resin is necessary, but knowing the condition of the resin is equally as important. The resin beads can crack or break over time as the resin bed is expanded at higher flow rates during the regeneration cycle. When resin is exposed to high concentrations of brine during the brine draw regeneration step, the beads can crack and break due to osmotic shock. Cracked and broken resin beads have the same exchange capacity but become smaller and lighter and can be lost during backwash. The backwash and brine draw cycles cannot be skipped to prevent physical damage because they are integral steps of the regeneration process to keep the resin clean, properly distributed, and ready to remove more hardness.

The ratio of sodium to calcium hardness can impact a resin’s ability to soften water. An extremely high Na/Ca ratio in the raw water can lead to increased leakage of hardness ions.

Iron and bacteria can readily foul resin which reduces the exchange capacity. Over time chlorine can degrade the resin’s crosslinking causing it to break apart or swell. Resin samples may be sent to a lab for periodic analyses to determine its physical condition, fouling, capacity, and moisture content (see example analysis report below).

Conclusion
A water softener can be critical to the success of many water treatment programs. To be successful, the system needs to be properly designed, programmed, and operated. A failure in any of these key areas can lead to poor results.

Routine monitoring and testing of water softeners is critical to any water treatment program’s success. If the water treatment program is designed for soft water, problems such as scaled heat exchange surfaces can occur rapidly if the softener passes hard water. Contact Chem-Aqua today for softener questions and support.

Examples of Scaled Heat Exchange Surfaces

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