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The basic difference between the wastewater-activated sludge process and the action involved in a wastewater-trickling filter should be understood. In the case of a trickling filter process, the bacterial film coating the contact material is stationary and likely to become clogged after some time. In the wastewater-activated sludge process, the finer suspended matter of sewage itself contains the bacterial film, which is kept moving because of the constant agitation. The so called sludge floc contains active, free-loving organisms which are being continuously swept through the sewage and which, in their search for food and work, oxidize the organic matter present in the sewage in a much more efficient way. As a result, the efficiency of wastewater-activated sludge plants is higher than that of wastewater-trickling filters. Other advantages are:

(1) Lesser land area is required.

(2) The operating head is also comparatively less. As such, little or no pumping is needed.

(3) Higher degree of treatment. The effluent produced is clear, sparkling and non-putrescible. BOD removal is 80 to 95 percent and coliform (bacteria) removal is 90 to 95 percent.

(4) Greater flexibility of treatment permitting a control over the quality of effluent desired.

(5) Freedom from odor or nuisance as the process operates under water.

The disadvantages are:

(1) Relatively high cost of operation and construction.

(2) Greater skilled attendance is required because of the large mechanical equipment involved. This may make the process unsuitable in case of small cities.

(3) It is more sensitive to change in the quality of influent. Any sudden increase in the strength or volume of which (say due to sudden discharge of strong trade effluent) may adversely affect the operation of the plant. Also the presence of synthetic detergents especially in the case of air-diffusion plants produces foaming difficulties.

(4) Difficulty in handling large amounts of sludge produced. Sludge bulking is a common trouble, which does not allow light, fluffy sludge to be easily removed by settlement.

The important characteristics in the choice of the process are local conditions (availability of land, filter media etc.), cost, nature and strength of sewage, and quality of effluent required.

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