I appreciate this is an old topic, but someone may ask the question again.
For sterilising and cleaning an ion exchange (resin type) water softener Citric Acid is the best cleaner. This is available from most softener specialist shops at about £5 for 100gm, which is extortion! The same chemical is about £6 per kg from your local brew shop or any on line wine or beer making supplier. Clean out all the remaining salt in the bin then wash with clean sponge to remove any stains. Dry out with clean cloth. Add about 100g of citric acid to a litre of hot water in a jug, and stir until dissolved. Add to clean salt bin with another litre of cold water, then perform manual regeneration, using the citric acid solution alone. When completed add salt to bin and repeat a normal, manual regeneration. This procedure will not only clean and sterilise, it will improve the effectiveness of the resin. Some people say never use chlorine bleach to sterilise the softener, as in the USA chlorine is said to destroy the resin. However, Culligan in GB advise thet a cup of bleach is added to the salt in the bin if softener starts to smell. I always use just Citric Acid, dosing about every 6 months. You takes your pick
I hope this late posting helps someone who is looking for an answer to this question.