This study was undertaken to investigate the effect home ion-exchange water softeners may have on the performance of onsite septic tanks. A laboratory study was set up with columns that contained solids collected from operating septic tanks. Wastewater was added with varying levels of sodium, as well as calcium and magnesium, modeling the periodic addition of regenerant from softeners operating at different salt dosing ranging from high efficiency to very poor efficiency. To reinforce the column experiments, data were obtained from private septic tanks to determine the effluent quality from septic tanks which did or did not receive the regenerant from softeners.
The data indicate that the use of efficiently operated water softeners (at or above ~3000 gr/lb salt efficiency) improves septic tank performance, while the use of very inefficient home softeners (at or below ~1000 gr/lb salt efficiency) may have a negative effect on solids discharge to the drain field. 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.