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Trickling
Filter - Trickling filter is an oldest form of aerobic oxidation treatment. It is also called as bacteria bed or percolating filters. It consists of a circular or rectangular bed of stone, gravel or synthetic material with a particle size of 5010cm.
The filter bed is usually 1.5 to 2.0 m in depths and is supported by a layer of larger stones over the underdrains. There is a rotating arm above the surface of filter bed with nozzles. The rotating arm sprays the settled sewage over the filter bed either continuously or intermittently.
The spraying saturates the liquid with oxygen.Within the interstices of the medium the microorganisms grow in the form of a shine or filth. The film is often known as zoogloeal film and consists of bacteria, fungi, protozoa and algae. A newly constructed bed must acquire the zoogloeal film before it can function efficiently.
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Effluent sprayed over the bed slowly trickles through the interstices in the medium which is coated with the zoogloeal film. Actually the liquid flows over the film rather than through it. When the liquid flows over the microbial film the organic materials bind to it and are oxidised. Unlike the microorganisms in the activated sludge, which move along with the sewage, microorganisms in the trickling filter are stationary.
Portions of the film may be sloughed off by the shear forces created by the flow of wastewater. The treated effluent containing the sloughed off biomass leaves the bed via the underdrain into a sedimentation tank. This tank is known as humus tank and the sludge sedimented at the bottom of the tank is called humus sludge.
The highest rate of oxidation takes place in the upper regions of the bed where oxygen is supplied by, natural ventilation. Below this level the rate of oxidation decreases due to the decreasing concentration of organic matter in the liquid phase.
Trickle filters are mostly used for the treatment of industrial wastes, especially those from cannery and food industry.
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