Changes in Septic Tank Effluent Due to Water Softener Use

Abstract

The compatibility of home water softeners and septic tanks is of concern for the on-site wastewater treatment community. Research has shown that high sodium levels in activated sludge plants can lead to deflocculation and poor effluent quality. Therefore, it is logical to assume that high sodium levels that result from the exchange of calcium and magnesium for sodium in home softeners could give rise to poor effluent quality from septic tanks, leading to shortened lives of drain fields. Additionally, the release of regeneration discharges to the septic tank might further damage performance. This study was undertaken to investigate the effect home ion-exchange softeners have on septic tank performance. A column study was set up and varying levels of sodium were added to wastewater influent and these were fed to columns that contained solids collected from operating septic tanks. In addition, slug influent solutions, which mimic regeneration flow, with varying amounts of excess sodium were investigated. To reinforce the lab column experiments, data were obtained from private septic tanks to determine the effluent quality from septic tanks both diverting and receiving the regeneration flow. Also utilized were graduated cylinder experiments, where the effect of sodium on grease flocculation was determined, and batch anaerobic digestion studies, which determined the effect sodium has on the production of gases and the degradation of solids. The study showed that the addition of sodium to septic tanks is likely to impact the effluent quality of sewage discharged from a septic tank to a drain field. The common way of measuring ion concentrations for comparison in this study was to obtain the monovalent to divalent ratio (M/D Ratio). This is simply the concentration of the sodium ions in solution divided by the concentrations of magnesium and calcium, on an equivalent weight basis (all other monovalent and divalent ions were negligible). Slug solutions of high levels of salts (Septic Tank Effluent M/D = 11), mimicking regeneration wastes from water softeners with an inefficient regeneration cycle, increased the effluent solids, COD and BOD. However, if the regeneration wastes contained the same amount of calcium and magnesium, but a smaller amount of sodium (Septic Tank Effluent M/D = 5), the negative effect on these effluent characteristics was greatly lessened. In an optimum case with a regeneration solution containing a minimal amount of excess sodium (Septic Tank Effluent M/D = 3), the effluent characteristics were often actually more favorable than in similar situations where the regeneration wastes were diverted (Septic Tank Effluent M/D = 2). The case studies reinforced these data, showing that sodium concentrations correlated with an increased discharge of solids to the drain field. The studies on grease flocculation as well as anaerobic digestion suggest that these processes are not affected by the sodium level. Overall, it appears that the use of home softeners with septic tanks may have an effect on solids discharge to the drain field and the level of impact will depend on the level of hardness in the water, whether the regeneration waste is discharged to the septic tank, and the amount of excess sodium present in regeneration wastes.