By Rick Andrew
Chloride discharge is a major issue in many water districts. Chloride can be difficult for wastewater treatment systems to treat and it can also cause some ecosystem harm when levels rise too high. When chloride levels in wastewater exceed discharge limits, wastewater treatment plants risk non-compliance, leading to concerns about chloride levels in wastewater. When there are concerns about chloride levels, regulators often begin to focus on water softeners. In fact, we have over the years seen various communities consider and even implement restrictions or bans on residential water softeners.
The reason these communities are focusing on water softeners is that the technology uses salt to regenerate. (This salt has the chemical composition NaCl, where the Cl is chloride.) Chloride that is associated with the regeneration of the softener ends up mainly in the brine, resulting from the softener’s regeneration cycle. This brine may be routed to the area’s wastewater treatment system, ultimately at the treatment plant.
The impact of water softeners on chloride levels
In order to better understand the contribution of chloride in water systems by water softeners, let’s consider an example and look at some calculations. A water softener operating at an efficiency of 4,000 grains of hardness per pound of salt, in water with hardness of 20 grains per gallon, discharges one pound of salt in 200 gallons of water:
4,000 grains of hardness / pound of salt ÷ 20 grains of hardness / gallon of water = 200 gallons of water / pound of salt
With an atomic mass of 35.45, chloride makes up 61 percent of the weight of salt (the other 39 percent is sodium given its atomic mass of 23):
35.45 ÷ (35.45 + 23) = 35.45 ÷ 58.45 = 0.61 x 100% = 61%
So this means that there is 0.61 pounds of chloride added to 200 gallons of water under these conditions, which is 0.0031 pounds per gallon of chloride:
0.61 pounds of chloride ÷ 200 gallons = 0.0031 pounds per gallon chloride
This is equivalent to 372 mg/L of chloride:
0.0031 pounds per gallon chloride x 453,592 milligrams per pound x 0.2642 liters per gallon = 372 mg/L chloride
This level of 372 mg/L chloride is higher than the discharge limit for many wastewater treatment plants. For example, in the Santa Clarita Valley in California, the limit on treated wastewater being discharged from the treatment plant is 100 mg/L of chloride.
Of course, softened water is only a portion of the wastewater stream for a treatment plant. There are many other sources of wastewater, many of which are lower in chloride. In the Santa Clarita Valley, it is estimated that the fresh-water supply itself includes 45 to 85 mg/L of chloride and contributes 45 to 60 percent of the total chloride to the wastewater (source: http://www.lacsd.org/civicax/filebank/blobdload.aspx?blobid=8071). Given that even water softeners that are very salt efficient can contribute chloride to the wastewater at levels significantly higher than the discharge limit, however, it is not difficult to understand why water softeners can become a focus of regulators working with communities and districts faced with chloride discharge issues.
Maximizing softener efficiency
Chloride discharge from ion exchange water softeners cannot be eliminated. It is a requirement of the technology that salt be used to regenerate the ion exchange resin. The efficiency of water softeners, however, is variable and can be maximized through the use of modern, demand initiated regeneration (DIR) water softeners. By using DIR technology, an immediate efficiency is gained over time-clock regeneration softeners. With a time-clock regeneration water softener, the softener will regenerate after the set number of days even if no water has been used since the last regeneration. In these cases where no water is used between regeneration cycles, the ion exchange resin is already regenerated when the next regeneration occurs, so additional salt used for regeneration essentially passes right through the softener and into the discharge. For this reason, NSF/ANSI 44 Residential cation exchange water softeners restricts claims of salt efficiency to DIR water softeners.
Among DIR water softeners, the salt efficiency can vary depending on the design of the system and the amount of salt used for regeneration. NSF/ANSI 44 includes a definition, test procedure and calculation for measuring salt efficiency at a given amount of salt used for regeneration. Essentially, this process determines the grains of hardness that can be removed from 20-grain-per-gallon hard water after the softener is regenerated with a given amount of salt. The standard requires that softeners must remove at least 3,350 grains of hardness per pound of salt at a given salt setting to have a claim of salt efficiency at that setting. The state of California goes one step further and requires removal of 4,000 grains of hardness per pound of salt at a given salt setting for a claim of salt efficiency at that setting. This is why a softener with an efficiency of 4,000 grains of hardness per pound of salt was used for the example above. The other part of the definition of efficiency is that the softener must use no more than five gallons of regeneration water per 1,000 grains of hardness removed.
Best practices by industry
The key in addressing the issue of chloride discharge and concerns about residential water softeners is to do our best to assure that the industry is providing modern, highly efficient DIR water softeners to homeowners in areas where chloride discharge is a concern. NSF/ANSI 44 provides us with a tool to define and measure softener efficiency, as well as the minimum criteria for a water softener to have claims of efficiency. The standard also limits claims of efficiency to modern DIR water softeners. By using this tool provided by NSF/ANSI 44 and taking this step of promoting highly efficient water softeners, we can minimize the impact of water softeners on chloride concentration in wastewater and water districts facing challenges with compliance to limits on chloride discharge into their wastewater.
About the author
Rick Andrew is NSF’s Director of Global Business Development–Water Systems. Previously, he served as General Manager of NSF’s Drinking Water Treatment Units (POU/POE), ERS (Protocols) and Biosafety Cabinetry Programs. Andrew has a Bachelor’s Degree in chemistry and an MBA from the University of Michigan. He can be reached at (800) NSF-MARK or email: Andrew@nsf.org