The rôle of enzymes in activated sludge and sewage oxidations

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Abstract
Evidence is presented that the most important factor in biol. oxidations of sewage is a series of catalyzed oxidation-reduction reactions determined by bacterial enzymes, present in either living or dead bacterial cells or liberated by them into the fluid of the reacting system. Crude sewage, sterilized by filtering through a Seitz bacterial filter, by heat, or by various "disinfectants," does not usually absorb oxygen. Auto-claved sludge does not absorb oxygen, even on vigorous aeration. Addition of a seeding of crude sewage, a suspension of washed sludge, a suspension of washed bacterial cells (of several species) or a suspension of bacteria-free protozoal cells, induces the oxidation of sterile sewage or sterile sludge. Some species of bacteria effect the oxidation of sterile sewage rapidly; others slowly. The 2 species of protozoa examined proved less active than most bacteria. A mixed suspension of Ps. fluorescens and Polytoma uvella caused no more rapid oxidation of the sterile sewage than the sum of the rates obtained when the bacterium and the protozoon were allowed to act separately. Urethane (5%) inhibits oxidation of sewage and sludge. Cyanide (M/200 to Ml 10,000 KCN, neutralized, or the vapor from solid KCN) inhibits the uptake of oxygen by crude sewage and by sludge but the inhibition is not complete and usually lessens with time, presumably due to the alteration of the cyanide. Bacteria carefully treated with formaldehyde, although unable to proliferate on the usual nutrient media, can induce slow oxidation of sterile sewage. Sewage sterilized by ultra-violet will still absorb oxygen on shaking with air provided the irradiation is not too severe. Several originally identical suspensions of Ps. fluorescens, irradiated with u.-v. for different periods of time so that the number of viable cells differed considerably from suspension to suspension, oxidized sterile sewage at rates largely independent of the proportion of viable cells present. Oxidation of the constituents of sewage and of sludge depends generally upon the presence of certain oxidative enzymes (de-hydrogenases and oxidases) of micro-organisms. These enzymes may be effective whether the organisms are alive or dead, provided that the method of killing ha3 not destroyed the enzymes. Although protozoa possess enzymes that can oxidize the constituents of sewage, most of the oxidation is caused by bacterial enzymes, the bacteria being both far more numerous and generally more active. Oxidations of sewage depend mainly on the presence of bacterial enzymes, which themselves may be associated with either living or dead cells. The enzymic activities of living and dead cells have not been directly compared, but the activity of the living proliferating cell for certain oxidations is probably greater than that of the dead cell. The dead cell is important in oxidations by activated sludge as the sludge is likely to contain a large proportion of dead bacterial cells many of which are probably enzymically active.