People take a different approach when discharging water softener backwash. Since it contains a high amount of sodium, disposing it just anywhere or your garden isn’t an option.
Most use their septic tank to dispose of the waste. But if the tank is too small, the salt can kill the bacteria responsible for breaking down the sewage waste. You can use the drain usually used to exhaust rainwater unless that leads to someone else’s property.
A lot of people dig drywell and dispose of their backwash there. Make sure the drywell is within your property line, but distant from home and garden.