As above there is also something called Actizyme you can get in Woolies etc, it is little pellets of enzymes or something that will eat away at anything that is down the drain causing odors. Seems to work quite well to keep the drains unblocked and smell free, and isn't any nasty chemicals.

I also have a Bokashi bin, you put all food waste in it along with some sawdust stuff that has got microbes in it, and it decomposes it and produced this juice - the juice smells a little but is also good for cleaning drains etc as it has same enzymes as the Actizyme.