My 2 stage bay pits are such that I have to shovel them out about every 3-4 weeks. Smell in the summer has been a problem since we bought the wash....To help remedy this lately, after I shovel the mud out of the pit and drain the water, I spray straight bleach on the pit walls and in the remaining bit of water. I use a typical garden sprayer and use about 1/2 gallon per bay. Its amazing how much this helps and keeps the smell down till the next cleaning time. It also helps with the smell in the outside holding tanks.
Theres no way I can drink that much coffee, so the bleach will be enough for now.