Percival P. Cassidy wrote:
We are thinking of installing a water softener, and the most
convenient place to discharge the drainage water is into the sump,
from which a pump sends the water into the back yard.

Would the effluent from a water softener be harmful to grass and other
vegetation?

Perce
2 to 4 pounds of salt (sodium chloride or potassium chloride) are
dissolved into the incoming water and flushed over the resin to remove
iron,magnesium, calcium, and several other metals from the surface of
the resin.

This effluent is both very salty AND very hard water with lots of
dissolved minerals in it. It will KILL vegetation that it comes in
contact with over the short term unless the discharge happens ONLY when
heavy rainstorms are in progress.

Only municipal sewer systems are capable of handling this effluent safely.

This effluent is potentially harmful to septic tank systems as well, so
many rural communities where septci systems get wide usage ban water
softener effluent and Reverse Osmosis effluent from septic systems. Two
reasons for this, very hard water discharge to septic, and large volumes
of water discharged to septic system. Both are bad (mostly lots of water).

Its even recommended in many if not most septic systems that clothes
washing and showers are spread out over several days. i.e. don't spend
Saturday mornign washing several loads of clothes and towels.