I think maybe you don't need to the final cold rinse. Does that mean your nappies are being rinsed 3 times?
I do a pre rinse then a regular wash cycle. Also the pooey ones get a hose down with the little squirt before the dry pail.

I have done the traditional style soaking of flat nappies and that uses more water. Do you use liners? They can catch quite a bit of poo. Also try not to be too antsy about the nappies coming out stain free from the wash as the sun is terrific at bleaching out the stains naturally.