Several studies have been conducted 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. The studies found that a properly sized and operated Demand Initiated Regeneration (DIR) water softener on an efficient salt setting of 3,500 grains per pound of salt or better will not have harmful effects on a septic tank, including the Advanced Treatment Unit (ATU) type.
Reference: Environmental Impact Study: Water Softener Effects on Septic System Performance: Water Quality Research Foundation (2011) Environmental Impact Study. Lisle: WQRF.