At an aggregate washing plant, pit run is washed to remove the finer particles. This process naturally produces a by-product: water laden with fine particles.
In this particular case, there was a need to reduce turbidity and total heavy metal value. The treatment needed to happen on 4,000 gallon per minute which was discharging into a local fish-bearing stream. When the turbidity of the receiving stream reached a high value, production stopped until the levels were acceptable again. As you can imagine, this caused a lot of lost time.