To soften or not to soften? That is the question...when you're on a septic system. Hard water is a problem for many, but one that's easily remedied by a water softening system. Assumptions in prior decades caused a number of counties and states to become concerned about the effects of water softeners on septic systems.
A lack of definitive research even made it possible for some areas to enact legislation preventing water softeners from being used with a septic system. To help clear things up, Water Quality Association (WQA) sponsored research into the issue in the 1970s at the University of Wisconsin - Madison and the National Sanitation Foundation (NSF). These studies are widely cited to this day, and their results borne out by later research.
But before we get into the potential problems, it's helpful to know how both systems work. If you aren't familiar with septic system operation, this article should get you up to speed.
How Water Softeners Work
Hard water is caused by an excess of naturally-occurring minerals - namely calcium and magnesium - that come in the form of positively-charged ions. A typical water softener utilizes a resinous "bead" material with a negative ionic charge. When hard water moves through the softener, calcium and magnesium ions bond to this resin, and are removed from the water.
Clearly, this softening process can't go on forever - the resin can hold only so much. This is why softeners have a "regeneration cycle", where the resin is flushed with water and a brine solution. Even though the hardness ions have a stronger charge than sodium or potassium, they're no match for the concentration of sodium/potassium ions in the brine. Sodium/potassium ions bully the hardness ions out of the resin, discharging them along with the remaining brine and water into the septic system.
Studies have addressed three important concerns with this discharge:
1. It is commonly known that bacteria are threatened if their surroundings have too much or too little salt. It was feared that the higher concentration of salt in the effluent or softened water would be harmful or fatal to the tank's bacterial action.
Result: No negative effect on either aerobic or anaerobic bacterial action was observed. In some cases, the effect of softened water on bacteria was actually beneficial. The normal salt content found in unsoftened effluent is less than ideal for bacterial growth. The addition of sodium to the system was found to bring the bacterial environment closer to the optimal range.
2. Another concern was that the discharge flow rate during regeneration would introduce too much heavy, salt-laden water too fast. This could overload the tank, and/or disrupt the settling of solids, forcing solid waste and effluent into the drain field before bacterial action could be completed, and before water was clarified.
Result: The discharge volume (of about 50 gallons per regeneration) did not overload the tank, and was on par with other household appliances like dishwashers and clothes washers. The influx of dense water did not appear to have an effect on settling and stratification inside the tank, though it should be noted that this point has been disputed over the years. Additional recent research sponsored by WQA appears to support the original conclusion.
3. Finally, it was feared that the salt brine produced by the softener would hamper the drainage field's ability to absorb water, and affect bacterial populations. This assumption was based on agricultural studies of irrigation systems with a high sodium content.
Result: The increased sodium in the tank's discharge was shown to have no detrimental effect on a normal drain field. Some soil conditions actually benefited from it. When the softener's calcium-rich regeneration backwash emptied into the septic system, the discharge could actually improve the soil's percolation (gypsum, a high calcium mineral, has long been used to increase the porosity of clay soils).
These studies - and others since - show that a normally operating water softener should work just fine with a well-maintained septic system (or aerobic treatment system). Dingy clothes, spotted dishes, pipes choked with buildup: all can be things of the past without risking the health and performance of your septic system!