Trickling Filter

It falls into the category of attached growth process. Trickling filters take advantage of the rapid adsorption of organic material on microbial slime layer. These slime layers consists of active microorganisms - a balanced flora of bacteria, fungi, algae and protozoa. Higher animals such as fly larvae, insects, snails and worms may also be present. Settled influent to be treated is passed down through a packed bed countercurrent to a flow of air as shown in figure . Older plants employ rock, slag or other low cost materials to provide a support with large specific surface area and a high void volume. Typically, filter depths vary from 1.0m to 2.5m.Microorganisms attached toTrickling Filter the packing matrix adsorb oxygen from the up flowing air and organic matter from down flowing influent. This way the organic matter in the influent is metabolized and its BOD gets reduced. Organic matter is adsorbed by the biological film also called as slime layer. The organic matter is degraded at the surface of the film by aerobic microorganisms. Gradually the thickness of the slime layer increases, the organic matter is metabolized before it can reach the microorganism near the packing matrix. As a result these microorganisms deep in slime layer die and loose ability to adhere to the packing matrix. The liquid then washes the slime off the matrix and a new slime layer starts growing. This process of slime layer removal is called sloughing.

Schematic diagram of Trickling Filter

The trickling filter usually consists of a cylindrical tank. The tank is packed with a bed of stone or special plastic packings. The underdrains are provided at the bottom of the bed for collecting the treated wastewater and any biological solids that have become detached from the packing matrix. The underdrains also serve as a porous structure for air recirculation. The trickling filter is always followed by secondary sedimentation tank to remove the suspended matter from the treated effluent. Modern plants use various types of low mass per unit volume, high specific surface area plastic, medium either in sheet form or randomly packed.Plastic packing depths upto 12m have been used.