Study of microbiological aspects of foam formation on aeration basins at wastewater treatment plants
Prof. Ing. Jiří Wanner, DrSc.
This work is focused on study of influence of various measures on formation or on the contrary on preventing creation of biological foams on activation tanks of wastewater treatment plants, on sedimentation properties of activated sludge that are aggravated by excessive appearance of filamentous microorganisms and on development of microbiological composition of activated sludge after taking these measures.
The theoretical part summarizes the present knowledge on microorganisms present in activated sludge, mechanism of bioflocculation, identification and classification of filamentous microorganisms, furthermore brief information about the most commonly appearing filamentous bacteria in the conditions of Czech Republic, separation properties of activated sludge, possible problems and evaluation of these properties, the ways of regulation of filamentous bulking and foaming. The end of this section summarizes information concerning influence of fat and grease on composition of activated sludge biocenosis and the effect of enzyme additives that decompose them.
Five objectives of this work had been appointed, these being observing the occurrence of biological foams, appearance and composition of filamentous bacteria species and sedimentation properties of activated sludge and how these would be influenced by presence of fats in wastewater, various technological measures, e.g. change of aeration system, complete exchange of activated sludge etc., furthermore to study the effect of various enzyme additives available in the market on these parameters and finding the optimum dosing and finally to examine the influence of addition of defoaming agents based on physico-chemical principle.
Section called Methods and Materials gives description and schemes of installations where the particular experiment were held, applied analytical methods and techniques of microscopy and used experimental materials.
Section Results and Discussion contains detailed description and discussion over experimental results, this section is once again briefly summarized and evaluated in chapter Conclusions, where there is also assessment of meeting the work’s objectives.
The following facts result from the Conclusions:
- In all experiments with one exception, which was carried out under completely special conditions, difficulties with biological foam’s occurrence and growth of filamentous microorganisms’ appearance were noticed in periods when fats were present. No relationship of fats’ presence in wastewater and sludge volume index was found.
- In all cases when mechanical aeration was replaced by fine bubble system, increased problems with biological foam occurrence arose after the change. When a device rippling the level in activation tank was installed, decrease of difficulties with foam was observed.
- In both of the experiments when all activated sludge in wastewater treatment plant was completely exchanged, tendency to return to the original composition of activated sludge was observed after certain time. The original species of filamentous bacteria gradually took place of the imported ones.
- In five of the eight experiments reduction of biological foam layer was observed during dosing of enzyme additives. In three of the six experiments decrease of sludge volume index was noticed and in the same number of experiments appearance of filamentous bacteria lowered when enzyme additives were applied. The addition of these agents had no effect where wastewater entering the plant was intensively diluted and the retention time in activation tank was short. Negative consequences occurred after high doses of enzyme additives were applied.
- One week after start of dosing physico-chemical defoaming agent biological foam totally disappeared. Filamentous bacteria originally accumulated in the foam moved into activated sludge underneath, where their increased appearance led to quite rapid deterioration of sludge flocks’ structure. No influence of dosing on sludge volume index was noticed. After the end of agent adding biological foam quickly formed again, structure of activated sludge flocks in activation tank gradually started to create once again.