The U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Resiliency and Finance Center, in collaboration with the...
Selecting the right system for the application
Commercial softening provides a significant opportunity to well-grounded water improvement professionals. It requires a special skill set, similar to residential applications, but at an elevated and more detailed level. While the troubleshooting and repair methods may be more sophisticated, the basics are the same.
System configuration is more critical and variable, allowing for a wider variety of possible solutions. Choose wisely, as the cost of poor initial system selection will be exacerbated over the life of the equipment. The least expensive option is almost never the best.
It is popular to offer residential systems with multiple media types in a single mineral tank to accomplish a wide range of treatment goals. This is often successful only because the removal capabilities are not critical to the application. For example, adding a small quantity of granular activated carbon to a residential softener system may provide decent chlorine reduction for a short period of time. After the limited capacity of the carbon is exhausted, the chlorine levels will begin to slowly climb until they are the same as the influent levels. This gradual return to the original chlorine levels is not likely to be noticed—after all, the primary function of the system was to provide soft water.
This type of temporary, partial performance level achieved by multi-function systems is not likely to be tolerated in commercial applications. Residential systems are designed primarily for aesthetic improvement, whereas commercial applications are more functional in nature. A laundromat does not install softening equipment so the employees’ skin will feel silky clean, it is installed as a fiscally sound investment.
Time clock-initiated softeners continue to be sold, often as a way to save a few dollars when purchasing the system. A time clock-initiated system uses calculations to predict the number of days the system can provide soft water prior to regeneration. Under typical circumstances, time clock softeners are inefficient and wasteful, so few applications can justify their use.
Despite the clear efficiency advantages of twin alternating and single-proportional brining systems, most residential softeners are still single, standard meter-initiated units. They may require a relatively large percentage of the available softened water to be wasted as a reserve capacity, so from a salt usage standpoint, standard single-meter-initiated systems can be extremely wasteful. The reserve capacity is basically one full day of use, which can approach and, in extreme cases, exceed 50% of the system capacity. When this happens, the meter becomes basically moot as the system will need to initiate regeneration nearly every day. Thus, a system with a hardness capacity of 20,000 gal may require a meter setting of 8,000 gal to accommodate a 12,000-gal daily reserve capacity.
The reserve capacity is designed to maintain softened water from the calculated exhaustion set point until the regeneration actually starts. The regeneration start time is typically around 2 a.m. If we were to ignore the needed reserve capacity by setting our 20,000-gal-capacity system at 20,000 gal, there would be frequent hard water issues. If the meter reached the 20,000-gal setting at noon on a given day, the softener would be out of capacity and providing hard water for the next 14 hours until the regeneration starts at 2 a.m.
There is also the option of immediate regeneration, but the consequence is an ambiguous and varying regeneration start time, resulting in periods of hard water as the system bypasses influent during the regeneration process. Even short periods of hard-water contamination are unacceptable in most commercial applications, thus the required reserve capacity for single-metered systems.
An oversized single-meter-initiated softener may seem like a good compromise based on the initial cost and the amount of space it requires compared to a twin- or multiple-tank system. The larger capacity can somewhat mitigate the reserve capacity objection; however, service flow rates, both high and low, are critical components of sizing a system properly. For typical applications, you will want to keep the service flow rate at about 3 to 15 gal per sq ft of bed area. While occasional peaks exceeding this parameter may be tolerable, consistently exceeding this flow rate may lead to exorbitant pressure drop and accelerated resin degradation. Failing to maintain a service flow rate of at least 3 gal per minute per sq ft of bed area may result in channeling, poor softening performance and lower-than-expected hardness removal capacity.
It is critical to employ the most efficient configuration for each commercial application. This will typically mean the use of twin alternating, multiple-stage-by-flow or another job-specific salt-efficient configuration. In some cases in which low hardness leakage is required, it may be advantageous to use a low-salted softener to perform the bulk of the hardness removal, feeding a high-salted polishing softener. This can provide huge salt savings while still providing the high quality softened water required for a particular application. Whatever the choice, it is crucial to account for the expectations, needs and available space of each particular application.
Twin alternating and other multiple alternating or stage-by-flow systems often offer the most efficient and appealing solution to commercial softening applications. Twin alternating systems will in theory use nearly 100% of the available capacity prior to initiating regeneration. When regeneration is called for, the fully regenerated standby unit is brought online while the exhausted unit is simultaneously taken offline in order to regenerate. You must account for the peak flow required to regenerate the exhausted unit, as this can happen at any time during the day and will draw from the available influent.
Multiple alternating or stage-by-flow systems operate in a similar fashion, but have the ability to maintain more than one unit online at a given time. They may even be able to automatically adjust the number of units online based on the flow demand. This can limit low-flow channeling issues and provide higher service flow rates. Be sure to balance the plumbing to encourage equal flow through each unit. Multi-stage systems use nearly all of the available capacity of each tank, can be highly salt efficient and offer favorable flow rate flexibility.
In a wide range of applications, softeners still rule when it comes to alleviating water hardness problems. A relatively new salt-free, anti-scale technology known as template assisted crystallization (TAC) also continues to perform admirably. While not the perfect alternative for all applications, TAC is gaining acceptance over a broad range of applications requiring scale reduction.
TAC does not backwash or use salt, so no salt tank or drain line is required. It incorporates a simple in-and-out connection, providing relatively high service flow rates from a small footprint. The media need to be replaced on a regular basis, usually every three years. While the replacement TAC media can be expensive, the cost can be significantly offset by the salt, regenerant water and maintenance savings. Keep your eye on TAC and other emerging technologies as the water treatment industry continues to search for more efficient and environmentally friendly ways to improve water for commercial water treatment.