23 CLARIFIERS, CONTACT TANKS FOR THE REMOVAL OF IRON AND MANGANESE FROM WATER, DEMINERALIZERS, DEAERATORS, AERATORS, DEGASIFIERS, DEALKALIZERS, CONTACT SEWAGE DISPOSAL PLANTS AND CHEMICAL FEEDER PUMPING UNITS, INCLUDING, AS COMPONENT PARTS THEREOF, GAS AND HYPOCHLORIDE CHLORINATION EQUIPMENT, CHEMICAL FEEDERS AND FEEDER ACCESSORIES