Microflora of aerobic activated sludge acclimated to high lipid levels
In this study, the main microorganisms involved in lipid biodegradation in activated sludge sewage treatment plants were isolated and their action on triglycerides and on fatty acids was investigated. Six different sludge samples were studied. An activated sludge of the usual type (A) was used as reference. At the same site, activated sludge was adapted to higher lipid levels, through separation of fats reaching the sewage treatment plant, followed by their addition in higher proportion to activated sludge. In experiment B, adaptation took place in a « BIOMASTER® G » reactor, which is a new process for the elimination of fats by an aerobic treatment. Experiments C and D used the same original sludge plus lipid mixture, but with the addition of commercial bioadditives containing lipolytic bacteria. Finally, microorganisms isolated from two other systems, located at two other sites, were also studied (experiments E and F). The control activated sludge, A, did not contain any strongly lipolytic microorganisms. On the other hand, in the new system at equilibrium, after adaptation, bacteria with a strong lipolytic potential such as Acinetobacter calcoaceticus and, Pseudomonas putida were isolated. The use of bioadditives at the start of the incubation did not afford any notable advantage, since at equilibrium microorganisms contained in these additives were not reisolated. However, other microorganisms with high lipolytic power (activity higher than 0.1 μmole acid liberated per minute and per ml of culture broth) were isolated, such as Pseudomorns sp., Aeromonas hydrophila and Staphylococcus sp. The genus Pseudomozas was almost always detected, regardless of the acclimated sludge studied. Conversely, Gram positive bacteria were virtually absent. All Gram negative isolates were more or less capable of degrading fatty acids with various chain lengths. The only Gram positive isolate was inhibited, even killed, by short and medium chain fatty acids. In the present study, this might contribute to the paucity of Gram positive bacteria in the adapted sludges. The main genera encountered, Pseudomonas, Acinetobacter and Aeromonas, were able to perform both lipolysis (liberation of fatty acids) and the subsequent oxidation of the liberated fatty acids. The natural enrichment of the lipolltic microflora of activated sludges in the process leads to an acclimated microflora, able to completely biodegrade lipids.