There are over 20 million households in the United States currently using on-site septic systems, and that number appears to be growing. We get many questions from our customers as to what effect (if any) water softener discharge has on a these systems. With such a wide range of information and opinions available, we felt that it was appropriate to examine some of the studies on this topic, and try to accurately present an overview of current understanding.
The Septic System and Water Softening Process
Septic system function is very straightforward. Your homes plumbing is piped into a in-ground storage tank. When wastewater enters this tank, the heavier solids settle to the bottom. Bacteria present in the storage tank digest the solids, breaking it down to a liquid. After this process is complete, relatively clear water is discharged from the primary tank into a second holding tank or distribution box. Water then re-enters the surrounding soil through a drainage field consisting of perforated underground piping.
The water softening process is accomplished by a chemical cation exchange that replaces the calcium and magnesium in your water with a equivalent number of sodium or potassium ions. During the softening process, your household water passes through the resin bed, and the magnesium and calcium contained in the water are removed. A given sized resin bed has a fixed capacity to remove hardness before it needs to be regenerated to full capacity in order to continue to provide softened water (for example, one cubic foot of resin has the ability to remove 32,000 grains of hardness from your water). When the resin bed is nearing exhaustion, the control valve washes the resin bed, and draws salt containing solution from the brine tank through the resin. As the salt contacts the resin bed, the process of ion exchange occurs, and the magnesium and calcium (hardness) that was collected in the bed during operation is washed to drain. After a final rinse to remove the excess salt, the resin bed is again ready to provide softened water.
The concern of discharging water softeners into an on-site septic system arises out of a belief that sodium salts used by water softeners during the regeneration stage - or the increased amount of water entering into the system - may be harmful and possibly cause septic systems to fail. Although there is no scientific data available that supports harmful effects, there have been many investigations into the potential for problems to occur.
The Effect of Sodium Salt on Septic Systems
Common knowledge supports that higher levels of sodium salt can have a direct impact on bacterial life forms. For instance, most bacteria usually found in fresh water ecosystems would be unable to live in a high salinity environment like an ocean. For this reason, concern was generated that septic systems that rely so heavily on bacterial action may be effected by high concentrations of sodium.
These concerns seem to be unwarranted. First, a typical residential sized water softener discharges between 40 and 70 gallons of water per regeneration. Through much of the regeneration process, fresh water is discharged, containing no salt at all, so the total concentration of salt is very dilute. However, during some stages of regeneration, the sodium concentration can reach as high a 5,000 to 10,000 ppm for brief periods of time.
To see if this level of sodium effected microorganisms typically found in aerobic on-site septic systems, a study was performed that exposed these microbes to a worst case scenario of 10,000 ppm brine solution. The study concluded that "there were no statistically significant differences in the metabolic activity of the microbial community", and that it was "unlikely that failures in domestic water treatment system are the result of exposure to the brine from home water softeners." (1)
Other studies indicate that the effect of putting softened water into septic system can actually be beneficial. There is a very low amount of sodium contained in softened water. For every grain of hardness removed, approximately 8 ppm (parts per million) of sodium is added. Although some naturally occurring water sources have very high sodium levels, softened water normally has a slightly elevated sodium level vs. untreated hard water. While this concentration is normally insignificant at typical hardness levels, these higher sodium levels are more in the optimal range for septic system bacterial growth, and can promote bacterial development.(2,7)
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.
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