Oct 07, 2019

Multi-Tank Water Softening Systems Provide Savings

This article originally appeared in Commercial Water Fall 2019 as "Sustainable Savings"

In a multi-tank system, only one of the four tanks is needed to soften water when use is low.
In a multi-tank system, only one of the four tanks is needed to soften water when use is low.

Over the past decade, sustainable building technologies have transitioned from trendy concepts into project staples. Advancements like electrochromic glass, greywater plumbing systems and green roofs are just a few of the many ways that advancements are improving the performance of buildings. 

An often overlooked opportunity to improve a building’s sustainability is the water softening system. Many commercial softeners use decades-old technology that waste water and flush unnecessary amounts of salt into our wastewater systems. If a building or project has a large single-tank softener that was sized to service the peak or critical flow rate, a new technology called responsive flow may be beneficial.

Chart 1 shows ideal 24-hour water usage in a hotel. Right: Chart 2 shows simulated daily water use in a hotel.
Chart 1 shows ideal 24-hour water usage in a hotel. Right: Chart 2 shows simulated daily water use in a hotel.

Smaller Tanks, Less Waste

Responsive flow technology, also referred to as progressive flow, is a new way of using multi-tank systems (MTS) to minimize the amount of salt and water used to service commercial buildings. MTS systems with responsive flow technology use a series of smaller tanks to replace large, single-tank softening systems. MTS systems service a building’s peak and minimum flow rates without wasting salt or water, and without causing inaccurate metering or channeling in the resin. Channeling occurs when the flow rates through a softener are too low to effectively allow the ion exchange process between the raw water and the softener resin. When channeling occurs, hard water follows a narrow path, or channel, through the resin and exits the softener still hard. This is a problem for buildings that rely on the softener as pre-treatment for applications, such as boiler systems, and for buildings that require consistent high-quality water like hospitals. 

Responsive flow technology for softeners is similar to the advancements and benefits of tankless water heaters. Why pay to heat and hold hot water when you do not need it? Why not just heat it as you need it? The same logic explains the key benefit of responsive flow technology: softened water as you need it. 

Commercial multi-tank softeners with responsive flow technology continuously bring tanks on and offline in response to the water demands of the building. This responsive design allows MTS systems to use up to 50% less salt and regeneration water than conventional single-tank softening systems.

Chart 3 shows simulated water use at a 40-room hotel. Right: Chart 4 compares water consumption for multi-tank and twin alternating systems.
Chart 3 shows simulated water use at a 40-room hotel. Right: Chart 4 compares water consumption for multi-tank and twin alternating systems. 

Commercial Application

An MTS system with responsive flow technology could have an impact on the softening performance in a hotel. Consider a hotel with 40 rooms and one sink, toilet and shower per room; two public washrooms; three water closets with flush tanks; one urinal; one service sink; one utility room; and one laundry machine. Note these specifications have been developed for purposes of comparing calculations between a MTS and a single-tank softener. They are not based on the water use or operating features of an actual hotel.

In an ideal world, buildings would have consistent water usage every hour of the day, as seen in Chart 1. However, in reality, water usage fluctuates throughout the day, as seen in Chart 2. In this case, we know that the hotel uses 3,200 gal of water each day and that the water is approximately 20 grains hard. The hotel also needs soft water 24/7. Based on those parameters, there are two different approaches available:

  • A conventional twin alternating system, using 7-cu-ft tanks and 2-in. valves. This system has a minimum channelling flow rate of 4.8 gpm and a minimum flow rate to run the meter of 6 gpm. 
  • A responsive flow triplex system using 5-cu-ft tanks and 1.25-in. valves. This system has a minimum channelling flow rate of 3.5 gpm and a minimum flow rate to run the meter of 1 gpm. 

As you can see in the hotel water usage chart (Chart 3), when water usage is at its lowest, a twin alternating system will not properly measure the water or prevent channelling. A responsive flow system can prevent channelling at a lower flow rate and can measure flow rates at as low as 1 gpm.

Now, let us see how the systems perform in the simulated hotel. In Chart 4, the responsive flow MTS system is able to use a reduced salt setting of 6 lb per cu ft to soften the water, whereas the twin alternating system needs to use a 15 lb per cu ft salt setting to soften the water. The water used for each regeneration cycle also is greatly reduced, from 66.5 gal per cu ft of resin in the twin alternating system to 35.8 gal per cu ft in the MTS system.

Chart 5 shows projected annual savings for a multi-tank system versus a twin alternating system.
Chart 5 shows projected annual savings for a multi-tank system versus a twin alternating system.

Operating Savings

Chart 5 breaks down the projected annual operational savings of a MTS versus a twin alternating commercial softening system. In this case, the MTS system uses nearly 40% less salt and nearly 20% less water than a twin alternating system. The hotel owners would save $130 and 9,812 gal of water each year. They also would save $946.08 and 3,500 lb of salt, for a total operational savings of $1,076.07 per year. These salt and water savings may seem modest in the context of a single hotel, but consider the potential savings across the U.S. hospitality industry. According to the American Hotel and Lodging Assn., there are more than 54,000 hotel properties in the U.S. If each hotel saved an average of 9,812 gal of water and 3,500 lb of salt per year, the hotel industry would save 530 million gal of water and 189 million lb of salt each year. While the actual salt and water savings would vary by hotel, this simplified example highlights the broader impact that can be made by taking advantage of technologies that reduce salt and water waste in commercial water conditioning.

In every building project, there are unique needs that must be considered when choosing the right mechanical systems. Factors like installation footprint; problems associated with hard water slippage; the importance of flow rate metering accuracy; the capital investment and projected annual costs of a system; and the importance of sustainable, environmental solutions in the building’s design all need to be thoroughly examined and considered. If sustainable design is a criterion of your next project, consider the benefits that a MTS softening system could provide. Saving salt, water and money benefits everyone. 

About the author

Tyler Gamble is commercial product applications manager for Canature WaterGroup. Gamble can be reached at [email protected] or 306.790.4428.

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