The Effect of Increased Water Volume from Water Softeners on Septic Systems
While the volume of water discharged by a water softener during the regeneration process will vary depending on water pressure, pipe diameter, and softener design, water softeners typically use 40 to 70 gallons of water during regeneration.  Questions have been raised as to what effect this increased volume of water (hydraulic load) may have on septic system performance.  Did the flow rate and volume of water softener entering the system during water softener regeneration adversely effect the normal settling process, and allow solids to enter the drain field?

Investigations by NSF International determined that the additional volume of water due to softener regeneration is not sufficient to cause any harm to septic tank systems.  In fact, water softener regeneration flow rate and volume is lower than the typical discharge from an automatic washing machine.(3)  Further, old style “time clock” based water softeners regenerated as set on a calendar basis, normally one to two times per week - the newer generation of metered (on demand) water softeners measure household water consumption, and only regenerate as needed – this further reduces the amount of water discharged into the septic system.

The Effect of Water Softener Salt Discharge on Percolation in Septic Drain Fields

There has been some anecdotal evidence that sodium added to septic systems by a water softener may have negative effects on the ability for the drain field soil to percolate properly.

Many studies have been performed to investigate this issue (2,4,6), and seem to indicate that water softener regeneration discharge does not interfere with drain field percolation, but may actually improve percolation in finely textured soils.  This may be due to a higher calcium concentration found in softener regeneration discharge.  When this “calcium rich” water enters the drain field, it sometimes increased the porosity of the soil (dependent on the type of soil present), improving the percolation process.  To further support this finding, gypsum, another calcium rich material, has long been used for this same purpose.

Conclusions and Recommendations

The results of our review are corroborated by a fact sheet published by the Environmental Protection Agency(5), that points out the following:

a) High concentrations of calcium and magnesium in the softener backwash water have no deleterious effect on the biological function occurring in the septic tank and may, in some cases, be helpful.

b) The additional volume of wastewater generated is added slowly to the wastewater stream, and does not cause any hydraulic overload problems.

c) Soil structure in the soil absorption field is positively affected by the calcium and magnesium ions in water softener effluent.

Based on a detailed evaluation of information and studies performed in this area to date, we conclude that water softener regeneration discharge does not negatively impact common on-site aerobic septic systems.  Slightly elevated levels of sodium in  regeneration discharge do not seem to effect septic system microorganisms, and may indeed benefit the septic process.

Other matters seem to play a much more important role in proper septic tank function.  You should always minimize the amount of household cleaners that enter the waste stream.  Whenever possible, reduce the use of soap and harsh chemical cleaners, and consider “natural” products containing low levels of phosphates.  If you already have a water softener installed, remember that you can reduce the amount of soap use substantially – by as much as 50% to 75% - soap no longer has to work as hard to remove the minerals already removed by a water softener. Avoid using a garbage disposal with an on-site septic, as minimizing these types of organic wastes will reduce the load on the system.  And finally, reduce when possible the amount of water flowing into the system from showers, baths, washing machines and dishwashers.

Work in this area is on-going, and a symposium discussing this issue is scheduled for March, 2006 in Chicago.  A joint task force has been established between the Water Quality Association (WQA) and the National Onsite Waste Recycling Association (NOWRA) to plan and oversee joint activities and investigate any future findings.  Failures of specific on-site septic systems will also be examined by this task force in order to determine the cause.

In conclusion, and after reviewing a range of scientific studies to gain a present-day understanding of this issue, it appears that owners of onsite septic systems can continue to enjoy the benefits that softened water provides, without concerns of system damage.