Biological waste tanks can be shared between several houses and they make use of a bacterial process that breaks down the waste. The managers of these systems are often wary of anything that seems like it might be a risk to this process.
However, a great deal of work has been done, much of it in the USA where the situation is common, which shows that the waste from a water softener does not disturb the process.
There is much confusion regarding salt and water softeners and they are often blamed for problems caused by other factors.
The water softening process converts sodium chloride in the salt to calcium chloride in the effluent from the softener and calcium bicarbonate in the hard water to sodium bicarbonate in the softened water. Common table salt and common baking powder.
In our experience, a problem has never been caused by the water softener effluent.
This document put out by the Environmental Protection Agency in the USA clearly supports this view:
* Onsite Wastewater Treatment Systems Special Issues Fact Sheet 3 – Water Softeners
Our water softeners help you around the home in so many ways. Softened water can prevent limescale build-up in taps, pipes and appliances. Many people think it tastes better than hard water, and it’s much softer on skin.