Fig 16.4 shows the section through a trickling filter. It essentially consists of a masonry or R.C.C. tank which is circular or rectangular in plan. Coarse aggregate of impervious nature is filled in this tank which acts as filtering media. The under-drainage system is provided in the bed to collect the Aggregate effluent. Fig. 16.4 (a) shows the under-drainage system provided in the trickling filters.
The Contact Media:
The most common filter media of trickling filters i. crushed rock, but large gravel anthracite coal, blast furnace slag, broken bricks, clinker, cinder ceramic and other such materials can also be used. Whatsoever type of media may be used, it must be weather-resistant and strong enough.
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Before filling the media, it must be washed with water and dried to remove the dirts and other impurities. The size of the media should be between 30 mm to 80 mm. The surface area decreases with the increase in size, but it offers less resistance to the flow of sewage during sprinkling.
The size of the aggregate should be kept same from top to bottom. But the larger size aggregate may be kept in the bottom side so that the film of solids deposited at the surface may be easily washed. The bottom material of aggregate should be done carefully and it should be hand packed upto a height of about 25 cm above the under draws.
Filter Depth:
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Experimentally, it has been observed that the concentration of remaining impurities in sewage decreases as it passes downward in the filters. Therefore, the amount of work of removing impurities decreases. To avoid this reduction in the effectiveness of the filter, it is better to make the filter shallow and recirculate the effluent through the filter. In high rate filters, the depth of tank varies from 1.0 to 1.8 metres in high rate filters and in low rate filters from 2.0 to 3.0 metres.
The active surfaces of the filtering media are worked at the same time on the impurities present in the various stages of the purification.
Under-Drainage System:
The under-drainage system of trickling filters, consists of v- shaped or half round channels, cast in the concrete floor during its construction. These drains are covered by the concrete blocks. Few types of concrete blocks are shown in Fig. 16.4. The remaining floor is given a slope of about 1 in 100 to 1 in 50 during its construction, towards the drains, for the collection of the treated sewage. The compressed air is also passed through these under-drainage lines.
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The under-drainage system should be self-cleaning and easily accessible during cleaning. This under-drainage system should also be made open to the atmosphere. Vertical pipes are fixed at some places for supplying the compressed air at a velocity of 60 cm/see.
Ventilation:
If the filter has been properly designed, sufficient natural ventilation will be there causing the increase of efficiency of the filter. This natural ventilation is induced by the difference in temperature in the air and the sewage. With the increase in the rate of dose, more quantity of air is required. At the temperature difference of about 6°C, the air gets displaced at a rate of 0.3 m3/m2/sec, of the filters bed surface.
When the temperature difference is less than 1.9°C, there is no movement or displacement of the air. The air requirement depends on the rate of application of sewage on the filter and its strength. Usually 6°C temperature difference is sufficient to supply the required quantity of air in the sewage.
ADVERTISEMENTS:
The inlet openings to underdrains should have a combined, unsubmerged area equal to 15% or more of the surface area, for the free passage to the air. The channels of the underdrains should also be designed not to flow more than half full. If the forced ventilation is provided at the rate of 0.3 m3/m2/sec. The under drainage system must be air-tight and the effluent channel should be provided with trap.
Distribution System:
The sewage can be sprayed on the surface of the trickling filters from fixed or moving sprays. In the fixed sprays, the sewage is sprinkled through stationary nozzles. Now a days the fixed sprays are not used and only moving sprays are used.
Revolving distributors have four or two horizontal arms of the pipe having perforations or holes. These arms rotate about a central hollow shaft. The rotating arms usually remain 15 to 25 cms. above the top surface of the contact media, due to which the sewage gets fully aerated during its distribution on the filtering media.
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The distribution arms are rotated by the electric motor or the back reaction on the arms by the sprayed sewage, at 2 R. P. M. for small units and 1/3 to 1/2 R.P.M. for large units. The head of30 to 80 cm of sewage water is required to rotate the arms, if they have to work on the counter reaction of spray only.
Sometimes travelling distributors are used for spraying the sewage on the trickling filters. These units consist of perforated horizontal through or pipe supported from a truss, bridging the rectangular trickling beds. These trusses can be moved to and fro on the rails on opposite filter walls. But these types of travelling distributors are not commonly used in practice.