210 / | HD | LIQUID PURIFICATION OR SEPARATION |
600 | DF | PROCESSES {14} |
601 | DF | .~ Treatment by living organism {13} |
620 | DF | .~.~ Aerobic treatment {3} |
621 | DF | .~.~.~ Recirculating to prior step {2} |
623 | .~.~.~.~ Of sludge or separated solid {3} | |
624 | DF | .~.~.~.~.~> And returning to or withdrawing from diverse treating zones |
625 | DF | .~.~.~.~.~> Treating outside mainstream |
626 | DF | .~.~.~.~.~> To mainstream oxygenation (e.g., activated sludge, etc.) {2} |