DISPOSAL OF WASTES TO LEACHFIELD
From the NH Department of Environmental Services Fact Sheet WD-DWGB-2-12:
There is often concern that septic tanks or disposal fields will be harmed by brine waste. Studies by the Water Quality Association (WQA) indicate that waste brine and purged contaminants do not injure leach fields or septic tanks. This WQA report is available from DES. If concern remains, a separate dry well can be constructed on the user’s property to dispose of the brine waste from the regeneration of the softener. There is no state regulation of this brine waste disposal system.
As long as the septic system is sized properly for your home, there should be no issue with the additional water load on the septic system. Some septic companies advise homeowners that the contents of the backwash harms the septic system, but they only offer anecdotal evidence to back up their claims. In fact, there is some evidence that any residual sodium in the backwash is actually helpful to the septic environment. As alternatives to the septic system; a dry-well can be dug for the backwash or, in some cases, the backwash can be emptied directly to ground through an outside wall.
See this informative link about this long debated issue.
For the Advance 6700 Softener:
Follow these instructions