You rarely see any mention about cleaning the car wash pit when hearing about the wonderful opportunities owning a car wash provides. The pit will tend to need more attention in the rural areas as more mud will get in there but regardless you will need to clean it out and isn’t the most fun.
The most common way to clean the pit is to shovel it out, dry and then taken to the local landfill where they accept it. Dry material is not considered hazardous but in a wet form it is. Some areas are more difficult to dispose this mud as they have concerns about levels of petroleum hydrocarbon levels. While many studies have been performed and dirt samples are deemed to be non-hazardous as the levels are low, good luck getting long standing laws repealed.
Two types of shovels to make the job easier are a sewer shovel and a product called the handi-clam pit shovel. It has parts like a clam shell that come together when using a lever on the top of a long extension. Everybody has their personal favorite.
One of the benefits to the mud is that some of the same chemicals (most notably phosphates) that are in fertilizers are also in detergents. You may be able to sell it or just use it for your landscaping.
If the manual removal isn’t for you look to hiring septic tank cleaners. The costs are pretty high as opposed but you can’t argue against the time savings. Check to see if any are in your area who specialize in car wash pit cleaning.