Effect of continuous aeration on bacterial oxidation of organic matter

Abstract

Growth of bacteria in an infusion of flax under quiescent conditions resulted in the formation of an acidic liquor with an appreciable content of volatile acids. When a similar infusion was aerated by a continuous current of air, the liquid tended to become alkaline and the content of volatile acids was much smaller and in some cases negligible. Aeration reduced the polluting strength of the liquor, as measured by oxygen absorbed from permanganate and by the biochemical oxygen demand, by much larger amounts than did growth under quiescent conditions.Experiments with infusion in which the natural flora was allowed to grow, and with sterile infusion inoculated with pure strains of different types of bacteria, showed that the general effect of aeration was to alter the metabolism of the bacteria in the direction of more complete oxidation of the substrate. Balance sheets for carbon showed that the organic carbon lost from the culture was accounted for by the CO2evolved. Thus carbohydrates, which under anaerobic conditions were fermented to organic acids, neutral volatile compounds, and gases, were oxidized, when the conditions were sufficiently aerobic; to CO2and water.The magnitude of the effects observed depended largely on the nature of the organisms present, and partly on the strength of the infusion in which they grew. With pure cultures ofBact. coli, Bact. aerogenesandBacillus subtilis,and with the natural mixed flora of the flax, aeration at moderate rates in bottles for 3–5 days reduced the value for oxygen absorbed from permanganate by 43–52 %, and the biochemical oxygen demand by 26–92 %, indifferent experiments. The organic carbon content of the infusion was reduced by 30–31 % byBact. aerogenes,byBacillus subtilisand by the mixed flora. With streptococci and with a strain ofAchromobacteriumthe effects observed were very small. Aeration at higher rates with diffused air in small open tanks reduced the organic carbon content of a flax infusion by 50 % in about 80 hr., and of a beetroot infusion by 50 % in about 60 hr. Sugar was destroyed during the aeration and disappeared rapidly from the flax infusion in the early stages.The work described in this paper was carried out in connexion with an investigation of retting of flax and disposal of waste waters, made, as an extramural research, for the Ministry of Supply as part of the programme of the Water Pollution Research Board of the Department of Scientific and Industrial Research. The paper is published by permission of the Chief Scientific Officer, Ministry of Supply, and of the Department.