The design below is the basic design that some recyclers use for a ‘closed-loop’ pressure washing system.
Make sure you have a plumber or engineer design the pipe and pump size.
The goal is to avoid discharging water to the ground or the environment.
A Septic tank makes a good settling tank – oil and sludge in settling Tank needs to be pumped out occasionally. filters can be simply absorbent pads that absorb the oil and antifreeze in water. If possible, design a system so that water from the pressure washing area drains using gravity to the holding tank. Extra water will need to be added to the holding tank when water levels get low (water will be lost to evaporation). Aerate the water in the holding tank using a large aquarium air pump to avoid smell.