What is a Oxidation ditch?
Oxidation ditch: Ring-shaped extended-aeration activated sludge reactor
An oxidation ditch is an activated sludge reactor shaped as a ring or oval channel in which mixed liquor circulates continuously, driven and aerated by brush rotors, disc aerators or surface aerators, with a long hydraulic retention time of 18 to 30 hours and a sludge age of 15 to 30 days. Because it runs as extended aeration it needs no primary clarifiers, produces a well-stabilised sludge and can remove nitrogen by creating anoxic zones along the loop, which makes it the workhorse of small and medium plants between about 0.5 and 10 MGD. Klarifi’s aerial imagery review identifies 1,496 US plants with oxidation ditches (carrousels) across 50 states.
Why small utilities choose it
The ditch tolerates swings in flow and load, needs little operator attention, and the long sludge age means less sludge to handle. The trade-off is energy: keeping a large volume moving and aerated around the clock is expensive, so replacing brush rotors with fine-bubble diffusers and adding dissolved-oxygen control are common upgrade projects.
Nutrient removal in a ditch
Oxygen is highest just after each aerator and falls along the channel, creating an anoxic stretch where bacteria use nitrate instead of oxygen and release nitrogen gas. With the right controls a ditch removes most of the nitrogen without a separate tank, which is why many ditch plants meet ammonia and total-nitrogen limits that lagoons cannot.
Recognising one from the air
Oxidation ditches show as looped channels, often two or more side by side, with aerators visible as white water. Klarifi counts them for every plant, which is how the directory lists ditch plants by state even where the EPA process data is missing.