No, for functioning and properly sized septic systems, water softeners pose no adverse effects, and may be beneficial.
Several studies have been made to determine the exact nature of water softener recharge waste effluents and their effects on private sewage disposal systems. These studies evaluated three major areas, all dealing with the effect of effluents developed during the recharge of household water softeners. First, it was important to study the effect of dissolved salts in softener recharge effluents on biological action in septic tank systems. These studies demonstrated that recharge effluent from water softeners had no deleterious effect on the biological action in a septic tank and that the recharge waste effluents may actually stimulate biological action.
Second, it was felt important to assess the hydraulic effect of the volume of water softener waste water. These studies demonstrated that the volume of recharge effluent from a water softener is less than that of present day automatic clothes washers. The amount of waste effluent developed by a typical household water softener during recharge is about 50 gallons containing calcium, magnesium, and sodium chlorides. The frequency of recharge is dependent on water hardness, water usage, and recharge salt dosage.
The last area of study concerned the effect of softener recharge effluents on soil percolation in septic system drain fields. This portion of the study is important since much of the literature on irrigation contains references to the adverse effects of high sodium water on soil structure and permeability, particularly in clay-type soils. The study concluded that there was an important difference between water softener effluents and sodium effluents, which has an important bearing on soil percolation and permeability.
The important difference is that water softener effluents contain significant amounts of calcium and magnesium and thus are not really sodium effluents alone. Calcium and magnesium counteract the effect of sodium and help maintain and sustain soil permeability, even in susceptible clay-type soils. Thus, it appears that water softener recharge effluent brine will not affect biological digestion, hydraulic load, or leach field permeability in a septic tank system.
However, if the leach field is composed of swelling clays, permeability will be reduced regardless of the presence of water softener effluent. Moreover, calcium and magnesium contained in recharge effluents actually increased soil permeability. Salts in the waste effluent from recharge of water softeners created no hydraulic conductivity or percolation problems in a properly designed septic tank seepage field. In fact, it was found that soil percolation was increased by water softener recharge effluents, as compared to soil receiving household sewage effluents without the addition of effluents from the recharge of water softeners. In other words, lower hydraulic conductivity (HC) might result if regeneration or recharge wastes from water softeners were not allowed to enter the septic tank seepage field. In this case, the beneficial effects of calcium and magnesium would be lost. This would occur if the regeneration wastes were not discharged to the septic system, but to a dry well, roadside ditch, or other point.
One study was conducted by soil scientists at the University of Wisconsin and dealt solely with anaerobic septic tank systems. The other study, conducted by the National Sanitation Foundation, dealt solely with aerobic tank systems. Conclusions reached in this study were as follows: Water softener regeneration wastes demonstrated no adverse effects on home aerobic waste water treatment plant performance, even when stressed by loading at a use rate simulating ten persons (twice the average use rate). There was no difference in performance between days in which the plant received regeneration wastes and days in which it did not.