Water Softener Regenerant Effects on an On-site Wastewater System: A Case Study

May 9, 2022

Analyzing water softener regenerant influent on on-site wastewater systems

About the author:

Frank A. Brigano is industry advisor & principal for Brigano Consulting, LLC. Brigano can be reached at [email protected].

Andrea Scarpino is president & CEO for PVS Associates. Scarpino can be reached at [email protected].


Water softeners have evolved considerably since their inception 100 years ago when synthetic zeolite was first put into a bucket that was hung from a faucet to produce soft water. In the intervening years, water softener technology has moved from buckets to sophisticated systems using state-of-the-art technology to control and monitor the water softening process. Today’s systems, complete with apps for your mobile device, allow for real-time monitoring of the system’s water and salt usage, leak detection and more. Of paramount importance, today’s technologies accurately monitor when a water softener needs to be regenerated thus minimizing the amount of salt and water used per regeneration. This feature saves money for the consumer and protects our environment by reducing the amount of waste minerals released.

Though water softener technologies have advanced significantly, softeners are still being held accountable for their performance in the early days when they were either manual or time clock operated with frequent regenerations using high dosages of salt and significant water usage per regeneration. Many communities regulate the installation of water softeners and other Point-of-Entry (POE) and Point-of-Use (POU) systems. These regulations are primarily driven by discharges, i.e., salt or water, that may no longer be justified due to the advancements made with the electronic driven controls and sensors of today. By presenting field data on real-world systems our industry can change these regulations.

According to the Water Quality Association (WQA), there are 16 U.S. States and one Canadian Province that regulate POU/POE discharges into on-site wastewater systems (WQA Regulatory Database as of January 2022). Water softeners are the primary target for these regulations. Regulations for softener discharges into on-site wastewater systems are typically based on potential issues related to hydraulic overloading of a system, primarily those not sized appropriately for the household; spalling of the cement in the concrete septic tanks due to introduction of salts, either sodium our potassium chloride; sludge buildup in the leaching system when significant levels of iron and manganese are in the source water leading to leach field failure; and groundwater contamination (1).

10 years’ worth of field data collected and stored on a “real-world” water softener (Whirlpool model WHES42) are shown in Table 1. This system, a demand-initiated regeneration (DIR) water softener, was set at 16 grains per gallon (gpg) water hardness, though the well water hardness (Table 2) was found to vary over time to up to 18 gpg. For most of the 10-years, there were two occupants in the home. The system’s data measured the average salt dosage at 2.49 lbs per regeneration with a total of 315,958 gallons of soft water produced at an average daily water usage of 94 gallons. The softener computer (Table 3) estimated 1,314 lbs of salt were used during this time period resulting in an estimated salt efficiency of 3,848 grains/lb of salt. Using the data from Table 1 and calculating these values shows the salt efficiency to be about 4,700 grains/lb of salt. This shows that the homeowner needed only to purchase three 40-lb bags of salt a year.

The discharges from this water softener were into an on-site wastewater (septic) system similar to that shown in Figure 1. The tank for the system was a 1,200-gallon two chamber concrete tank with outlet screen. The homeowner had the septic tank emptied every two years as recommended and cleaned the outlet screen annually. On-site wastewater systems, e.g., a septic system, are typically used in areas without centralized treatment and treat all of the wastewater from a household. Wastewater from the home empties into the first chamber and holds the liquids long enough to allow the solids to settle to the bottom forming sludge and the oil and grease to float to the top as scum. The liquid from the tank (effluent) exits into the drain field, which consists of subsurface perforated piping set in a leach field that allows the effluent to percolate slowly into the soil allowing for discharge to the ground water and evaporation into the atmosphere.

Percolation of the wastewater into the soil results in the natural elimination of harmful microorganisms and nutrients.

Novak and Hogan (2,3) conducted studies on the effects of water softener discharges to on-site septic systems. Their studies showed that sodium could adversely affect the effluent discharges from the septic system with an increase discharge of solids. Laboratory and field data from their research showed that the monovalent (sodium and potassium) to divalent (calcium and magnesium) ion ratio (M:D) on an equivalent basis for the wastewater sent to the septic correlated to septic system performance. These researchers determined that an M:D ratio >5 potentially could have a negative effect on the septic system’s operational performance.

When the M:D ratio was <5 the on-site wastewater system performance was not negatively affected. This correlation was shown also to influence the biochemical oxygen demand (BOD), chemical oxygen demand (COD), and other measured parameters. Novak and Hogan demonstrated that M:D ratios of <5 were generally achieved with a DIR water softener. These researchers also showed that septic effluent characteristics were often favorable with a minimal amount of excess sodium versus systems where softener regenerant wastes were diverted. Further, Novak and Hogan showed that grease flocculation and anaerobic digestion were not affected by the sodium level.

WQA and the National Onsite Wastewater Recycling Organization (NOWRA) issued an operational guidance document (4) for water softener use in homes served by conventional septic and drainage systems. This guidance recommends the use of a “screening tool” that calculates the M:D ratio for the proper operation of a softener on an on-site septic system. The screening tool (spreadsheet) takes input from the mono- and divalent cations measured for the household water, plus uses average cation values for wastes from typical households along with the softener salt efficiency setting to calculate the M:D ratio. The spreadsheet is available on the WQA website (www.wqa.org). Data inputted for the subject system calculates an M:D ratio of 1.8 which is within the range for a properly operating septic system.

The biological and chemical makeup of the subject septic system is shown in Table 4. This analysis was from a grab sample collected from the depths of the second tank of the septic system taken at the end of this study. Unfortunately, this sample was the only effluent sample taken over the 10-year period of operation nor were there any comparison control data from an on-site septic system with similar water quality either without receiving water softener regenerant waste or with a water softener having an M:D ratio of >5. The BOD and microbial data from the subject system indicate a properly operating system. However, without comparison data to the appropriate control systems, it is hard to draw a conclusion. The BOD data from the subject system when compared to that reported by Novak and Hogan are in the same range as measured in their systems with a similar M:D ratio.

Physical data show the water softener regenerant influent to the subject on-site septic system not to have an ill effect. At the conclusion of the 10-year period, the homeowner had the septic system pumped. Photos 1 and 2 show the open access cover to the first compartment of the septic tank with the scum layer. Photo 3 is a view of the second tank compartment showing the outlet manifold with filter. Photo 4 clearly shows the unblemished concrete wall of the first tank compartment. The fact that the concrete wall is not spalled or damaged in any way after 10-years of operation with an operating DIR water softener demonstrates that properly operating DIR water softeners do not have a structural damaging effect on septic tanks. It should be noted that the septic pumping technician documented the system was “functioning properly.”

Field data from water softeners operating in the real-world can be a terrific asset to our industry demonstrating that DIR softeners do not have adverse effects to on-site wastewater systems nor adversely impact municipal wastewater treatment plants with excessive salt discharges. Today’s modern DIR water softening systems with advanced onboard electronics, remote monitoring and data collection are a valuable source of operational data for our industry. Our Industry should harvest these data to reverse regulatory activities that were based on outdated technology. Collection of data from today’s DIR softeners and other POU/POE systems potentially could save a consumer thousands of dollars in installation costs by not having to install an alternate on site means to dispose of regenerant waste. Real-world data from operating on-site wastewater systems could also validate the data collected to date by researchers showing properly operating and maintained DIR water softeners with on-site wastewater systems can work together in harmony.

Water softening technology has advanced considerably since our industry’s inception and will continue to advance even more rapidly in the coming years. It behooves our industry to continue to advance technology and to gather real-world operational data from both municipal and private water sources that can improve the practices and operation of our industry’s people and equipment. Data, such as those presented from this random, non-scientific study, can be useful to advance and promote our technologies.


  • Private drinking water in Connecticut Publication No. 29: hardwater-softeners facts and issues. (https://portal.ct.gov/-/media/Departments-and-Agencies/DPH/dph/environmental_health/private_wells/29HardwaterSoftenersFactsandIssuespdf.pdf)
  • Novak, J.T. and P. Hogan. 2013. “Changes in Septic Tank Effluent due to Water Softener Use”. (https://www.nowra.org/Customer-Content/www/CMS/files/Resources/2013_Report_FINAL.pdf).
  • Hogan, P.L. 2012. “Changes in Septic Tank Effluent due to Water Softener Use”. Master of Science Theses Virginia Polytechnic Institute and State University. Blacksburg, VA.
  • WQA/NOWRA. “Guidance for the Use of Water Softeners and Onsite Wastewater Systems on Individual Properties”. May 2013. (https://www.nowra.org/Customer-Content/www/CMS/files/Partners/2013_NOWRA_WQA_GuidanceDocument.pdf).

Authors’ Note: The authors wish to thank Ecowater, LLC for retrieving data from the onboard computer memory of the subject water softener for this case study and the Water Quality Research Foundation (WQRF) for providing the “screening tool” spreadsheet for the calculation.

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