3.3 How does a trickling filter work?

A wastewater distribution system, which can be either fixed or rotating, is responsible for distributing the wastewater over the filter bed. The rotating distribution system is designed to distribute the wastewater uniformly by using two or more arms equipped with nozzles as outlets for discharging the wastewater evenly across the filter bed. The rotation of these arms is achieved by the jet action produced by the nozzles. In some cases, an electric motor may also be used to drive the motion of the rotor.

Both distribution systems, fixed and rotating, alternate water supply to promote air circulation.

The supplied wastewater passes through a filter media, where biofilm is formed on the surface; within this biofilm, the microorganisms oxidise the organic matter in the wastewater to carbon dioxide while generating new biomass. To keep the microorganisms alive, the filter media must be kept wet; hence, effluent recirculation is applied to prevent the drying out of the biofilm layer during low-flow conditions.

The thickness of the biofilm can reach up to 10 mm, depending on the operating conditions. As the biofilm thickness increases, it creates anaerobic conditions at the filter surface. At this point, the biofilm enters an endogenous respiration phase due to the limited supply of organic matter to the inner depths of the biofilm. Microorganisms can no longer attach to the filter media, causing the biofilm to detach due to gravity and incoming wastewater. This is called sloughing.

The underdrain system collects the treated effluent and sloughed biomass in a trickling filter. Additionally, it allows for air circulation to ensure proper functioning. The underdrain system should be designed to carry a maximum wastewater flow for which the trickling filter is designed without exceeding half of its capacity.

Air is supplied to the filter bed through a natural air draft. If the influent wastewater temperature is lower than the ambient temperature, the air will flow downward through the trickling filter bed. Conversely, the air will flow upward if the wastewater temperature exceeds the ambient temperature.

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