The bacteriology of activated sludge

Abstract

Homogenization of activated sludge greatly increased the apparent bacterial count obtained by plating on a solid medium, and for this reason facilitated the isolation of the predominant flora by liberating the bacteria from the interior of the sludge flocs. The ordinary domestic cream-making machine was found to be quite effective for this purpose.The high counts obtained indicated that there are probably several hundred thousand million bacteria per gram of dry matter in activated sludge.Aeration of sewage was found to result in an increase in the total count of bacteria, and a high count was maintained for a considerable time. After aeration for several days fairly rapid and apparently fairly complete separation of the sludge occurred on allowing the liquid to stand, but the supernatant liquor contained large numbers of bacteria. When a fresh quantity of sewage was added to the small quantity of sludge which settled and a second period of aeration commenced, the bacteria were found to associate more quickly with the sludge than they had during the first aeration. Coliform bacteria and spore-forming aerobes formed only a small fraction of the total number of micro-organisms concerned in these changes, and moulds and yeasts never accounted for a large proportion. The predominant bacteria did not produce acid from glucose.Various media were tested in investigating the bacteriology of activated sludge. None showed any superiority to nutrient agar. Coliform bacteria and aerobic spore-formers were encountered in negligible numbers. The morphological and physiological characters of seventy-one strains of the predominant bacteria were determined. The majority were Gram-negative rods with no action on carbohydrates and were members of the generaAchromobacterium, Chromobacterium,andPseudomonas.Those isolated from activated sludge soon after its formation from aerated sewage were found to be non-proteolytic, but a proteolytic flora was established after the sludge had been built up for a period of 4 weeks. The characters of the predominant bacteria indicate that they are water types derived from the water supply or the drainage water which enters sewage. Intestinal bacteria appear to be unimportant.The experiments described in this paper were carried out as part of the programme of the Water Pollution Research Board of the Department of Scientific and Industrial Research, and the results are published by permission of the Department.

Fluctuations in Populations of 3 Vorticella Species from an Activated‐Sludge Sewage Plant

“…Five species were present: Chromobacterium prodigiosus (Serratia marcescens) , Bacillus megaterium; 2 were probably Flauobacterium solare and Achromobacter geminum, and the 5th could be identified only as Pseudomonas sp. The 4 identified species have been recorded in activated sludge by other workers (1,28,32). Monoxenic cultures of V .…”

An Evaluation of Procedures for Enumerating Bacteria in Activated Sludge

“…The increases in count after homogenization given here may be compared with those of Allen (1944), Gayford & Richards (1970), Davey & Richards (1970), Williams, Forster & Hughes (1971), and Williams, Stafford, Callely & Hughes (1970), which extend from less than a doubling up to 2 orders of magnitude, with averages of sets of observations between 3.7 (Williams et al, 1971) and 34 (Allen, 1944) times the initial counts. The actual increase obtained seems to depend not only on floc aggregation and etlicacy of homogenization but also on the numbers and relative viabilities of freely suspended bact,eria and floc bacteria and, in single observations, on the variability associated with counting colony forming units in an unhomogenized and highly contagious population.…”

From Total Suspended Solids to Molecular Biology Tools—A Personal View of Biological Wastewater Treatment Process Population Dynamics

“…Two investigations were important in dispelling these myths. Allen (1944) vigorously homogenized activated sludge (with a cream maker) before enumerating the microorganisms in it.…”