Oxidation of Sewage by Activated Sludge

Abstract

It is quite natural that the rate of biochemical oxidation of dilute sewage mixtures, which has been carefully studied,1'2 should be assumed to hold for the biological oxidation devices, such as the activated sludge process. The lack of experimental evidence to the contrary and the fact that the oxygen demand of the sludge itself, excepting the first day, conforms fairly well with the established rates for sewage mixtures has doubtless contributed to this view. The general conception of the ac tivated sludge process is that there is a rapid adsorption of organic matter by the sludge and that the adsorbed material is then oxidized biochemically at a rate approximating that observed for a polluted water or sewage mixture. The conditions under which the organic matter is oxidized in the activated sludge treatment plant and in a bottle are quite different. Under the conditions of the biochemical oxygen demand test, the bac teria are relatively few at the start and multiply until a limiting number is reached. In the activated sludge process, the sewage is added to a material where many bacteria are concentrated in the zo ogleal masses,3 so that the number of bacteria present per unit volume far exceeds that which is ordinarily reached. The activated sludge floe can be considered a bacterial colony in liquid media. Thus a solution containing enough of the dispersed zoogleal bacteria to give it a milky appearance will produce only a small amount of floe, so that with a solution containing 1,000 p.p.m. of floe, there are so many bacteria con centrated in the floe that the actual number present far exceeds that ever observed in the dispersed condition. Under these conditions it is reasonable to suppose that the organic matter might be oxidized at an accelerated rate. Likewise, it might be inferred that slow growing bacterial species, such as nitrifying organisms, would require more time to accomplish a given amount of work than the more active species which utilize carbonaceous materials under the conditions imposed by the biochemical oxygen demand test, where two definite stages of oxida tion are observed. With activated sludge, it may take three weeks to build up an actively nitrifying variety. Once the organisms are present in sufficient numbers, they should oxidize ammonia from the start and the two stages observed under the former condition might proceed simultaneously in the activated sludge tanks, it being understood that the ammonia formed from the decomposition of nitrogenous substances must necessarily be released before it can be utilized. A study of the oxidation of sewage in the presence of activated sludge would furnish valuable information on these possibilities. The work done along these lines has dealt with the oxygen demand of the sludge-sewage mixtures. Grant, Hurwitz, and Mohlman4 com 562