What Is a Trickling Filter?

A trickling filter is a type of biological filter in which water continuously runs over a porous surface, such as peat moss, plastic or rocks, and a thick layer of biological slime manifests to capture pollutants. The name is a misnomer, because there technically is no filtering or straining done, but the slime is able to collect and reduce the amount of waste in the water. A trickling filter is usually used before more advanced machine-operated forms of purifying wastewater. The three parts of trickling filters are the filter medium, the distribution system and the underdrain system, which combine to help purify the water.

Wastewater is constantly produced by an array of human actions, such as using the restroom or washing dishes. This water needs to be purified so it can go back into the system and be used again for other water needs. A trickling filter is one of the first purifying processes that wastewater meets, and it works by producing slime and trickling water.

Wastewater first touches the filter medium, a porous substance that is usually cheap to purpose and has a large surface area. If the material clogs easily or is brittle, it will not be suitable as a filter medium, because water will be trapped or will destroy the filter’s surface. When the wastewater runs over this surface, the surface will begin to accumulate slime from all the pollutants. After some time, the slime will be thick enough that it will block oxygen from penetrating the slime layer. This allows the slime to absorb additional pollutants.

A distribution system is the second part of building a trickling filter, but it is the first part water encounters. Distribution systems carry the water from different sources to the filter medium. An effective distribution system is able to uniformly distribute the wastewater over the medium’s surface. This means sprinklers are commonly used, so the water can sprinkle in all directions.

After the water passes through the slime, it encounters the third part of the trickling filter, the underdrain system. There are normally two arms for this system. One arm is able to take water after it has passed through the slime, so it can go on to additional treatment from machines or other methods. The other arm takes excess slime — and all the pollutants it carries with it — that travels down the filter.