The formation of extensive, biological foams or scums on the liquid surfaces of domestic activated sludge plants was investigated. An extensive survey of the problem in Queensland and Australia was carried out; the dominant organisms in the foam were isolated and identified; field scale experiments that studied the control of the problem were done; the physiology and pathogenicity of the dominant organisms in the foam were investigated; and the intrinsic reasons for foam formation were explored in the laboratory.
Foaming in activated sludge plants was found to be a significant problem in Australia and in incidence was more prevalent than bulking, the other biological problem of activated sludge. Greater than 90% of plants in Queensland have experienced biological foaming episodes, whilst in 38% of the plants the problem was continuously present. No mutually inclusive feature about the activated sludge plants could explain the formation or presence of the foam. The only feature that was common to the 8% of plants that had not experienced foaming was that they were all organically and hydraulically underloaded. Of the non-foaming plants, 75% were mechanically aerated and were operated at a sludge age of greater than 10 days. Foaming was poorly controlled in Queensland. Surface chlorination of the foam to kill the organisms was the most successful control strategy. Most activated sludge parameters were identified by different plant personnel as being crucial in foam formation, however, long sludge age, high mixed liquor suspended solids and high or low dissolved oxygen in the mixed liquor were those most frequently mentioned.
Nocardia amarae and a newly described actinomycete, Nocardia pinensis were the dominant organisms isolated from the foam. Micromanipulation was used to isolate the actinomycetes. This was found to be the most efficacious method for these slow growing, morphologically distinctive organisms when isolation from the milieu of mixed liquor is required. N. amarae was isolated from 45% of plants in Australia that submitted samples, whilst both N. amarae and N. pinensis were isolated from 5.2% of plants and N. pinensis alone from 7.0%. A total of 69% of plants had a scum present on the day that the sample was collected. The dominant organisms in these scums were actinomycetes in 90% of cases while "Microthrix" alone dominated in the scum in one plant and was present with N. amarae in another.
Plants that contained N. pinensis in the scum were found to be operated at long sludge ages. The scums that were associated with these organisms were normally paler in colour than those associated with N. amarae.
Trials that investigated the control of biological foams by the return of anaerobic digester products to the aeration tank, by altering the rate of return of sludge from the bottom of the secondary sedimentation tank, or by proprietary product additions were initiated. Although alone none of the strategies effectively controls the scum, they may assist in control in some circumstances.
A limited amount of information about the physiology of the scumming organisms was obtained. Although they are slow growers on laboratory media when compared with some other bacteria, their .growth rate in the activated sludge plant is unknown. Nocardia amarae has a (specific growth rate of 0.1 h-1 on acetate and a yield of 0.3-0.5 g cells per g acetate consumed. Many technical problems need to be overcome before chemostat methodology can be accurately applied in the culture of N. amarae. This is in common with the mycobacteria where problems encountered have a similar basis.
The actinomycetes that were isolated from scums on activated sludge plants did not cause pathogenic responses in mice, unlike a recognised pathogen, N. asteroides. The scumming organisms could not be recovered from the autopsied animals.
Foaming and surface tension determinations carried out on pure cultures of N. amarae grown in defined media showed that a surfactant was released by the organisms and that the bacterial cells were responsible for stabilization of laboratory scale foams. The nitrogen source for growth did not affect the foam or surfactant production, however, the carbon source for growth produced growth form-dependent changes to the foam but did not affect surfactant production. When the growth produced an evenly turbid culture in the liquid medium, foam stabilization occurred. When the growth appeared as large colonies in the slightly turbid medium, foams were poorly stable. Although N. amarae can grow on cooking oil and a hydrocarbon, the stabilization of foams is not dependent upon these carbon sources being present.
The cells of N. amarae are hydrophobic and a surfactant is produced in culture. These are properties which suggest an explanation for the mechanism of foam formation based on adsorptive bubble separation theory. Surfactants at certain low levels and hydrophobic particles aid the formation of stable foams. Attempts were made to alter the nature of the nocardial cells to prevent their transport into the bubble phase of the foams. Several types of hydrophilic particles were added to the cultures prior to aeration in the laboratory foaming apparatus, and success was achieved with the 2:1 expanding lattice clay, montmorillonite. It was concluded that a salt-dependent, reversible bacterium-montmorillonite complex formed that conferred hydrophilicity to the otherwise hydrophobic bacteria. This property prevented the bacteria from entering and stabilizing the foam phase. The clay concentration that elicited this response was about 100 µg/ml.
Although a method of control of biological foams on the surface of the activated sludge plants was not discovered, the initial steps to this end have been made. The physiology of the organisms needs to be investigated to ascertain the reason for the selective advantage the actinomycetes have in the activated sludge plant.