Study of mechanism of biological foam formation and control at activated sludge wastewater treatment plants
Prof. Ing. Jiří Wanner, DrSc.
Biological foaming in the aeration tanks as well as the subsequent scum production in the secondary clarifiers and anaerobic digesters belong to one of the most widespread troublesome problems experienced in the wastewater treatment plants in all parts of the world. Both nutrient removal plants and the conventional plants suffer from this undesirable phenomenon. Although there are known also another types of foaming (surfactant foaming, denitrification foaming, foaming during start-up), the stable biological foam production has become the subject of the major concern for activated sludge plants operators.
The main objective of this thesis was to study the origin of the biological foaming in wastewater treatment process from both microbial and physicochemical point of view. The main emphasis has been laid on the determination of possible relationships between physico-chemical parameters of activated sludge and biological foam to reveal any mutual correlations and better understanding of mechanisms of biological foam formation.
This study draws from results obtained both from long-term screenings of foam forming filamentous population and physico-chemical parameters of biological samples (hydrophobicity HFB, scum index SI, aeration tank cover) taking from wastewater treatment plants in Czech Republic as well as from heterogeneous lab-scale experiments.
In this thesis it is examined how the physico-chemical characteristics of the solids are related to the foam formation and described how the foaming potential of full-scale plants might be assessed. The relations among activated sludge and biological foam hydrophobicity, scum index, aeration tank cover and filamentous population were evaluated. Individual parameters comparison showed that the presence of foam forming microorganisms identified among other filamentous microorganisms indicates rather an easy distinguished symptom than a reason of foaming problem. Consequently, it was also found that the foaming episodes might be quantified and assessed to a certain degree just based on the knowledge of parameters describing its visual indication (scum index SI, aeration tank cover) and not its cause (hydrophobicity HFB). The incidence of biological foams cannot be predicted in terms of activated sludge hydrofobicity changes at all.
Generally, an adhesion of the hydrophobic material to an air-water interface is considered as a main mechanism of biological foam formation. Therefore, another goal of this thesis was to understand the microbial adhesion process in more detail. MATH assay methodology was critically reviewed with regard to pH and ionic strength. The results of MATH assay were found to be indicative of the surface hydrophobicity solely in case when electrostatic interactions are absent, i.e. close to the isoelectric point values of activated sludge or hydrocarbons. Thus, preliminary surface charge measurement and an isoelectric point determination were recommended as a routine standard part of MATH assay.
The data obtained from rheological characteristic measurement showed a clear relationship between surface charge of sludge suspend solids and viscosity. This relation is controlled by the pH and ionic strength of the suspending fluid and is also associated with the chemical nature of the sludge components surface. It was showed that high values of surface charge cause repulsive interactions in the structure of activated sludge, which results in open structure of flocs, space expansion and the lose of their compact properties. The lowest values of viscosity were obtained in the range of pH close to the isoelectric point where the role of electrostatic interactions was eliminated. The effects of antifoamer and enzymatic agents on the viscosity of biological samples and existence of biological foam were examined. On the basis of series of antifoamer and enzymatic agent treatment batch tests the different background of their action was compared with regards to the viscosity measurement.
The impact of surfactants on foaming potential of activated sludge was tested. Lab-scale foaming tests and full-scale screening of wastewater treatment plant running together with analysis of nonionic surfactants concentration entering the aeration tanks were performed. The result obtained from lab-scale foaming test partly demonstrated a possible positive impact of applied nonionic surfactants on intensity (not stability) of formed foam but only under the assumption that foaming potential of tested activated sludge was already fully established and typical foam forming filaments were present in dominant position. Any significant supporting effect of analyzed nonionic surfactants on the biological foam formation and stabilization under detected surfactant concentration in aeration tank of the real wastewater treatment plant was not observed. Based on the foaming test evaluation it was concluded that the chemical structure of surfactant composition does not affect the foaming potential of activated sludge.
The results of this study raise doubts about current understanding of mechanisms of foam formation. In order to fully establish a complete understanding of the effects involved in the formation of this undesirable separation phenomenon of activated sludge it might be necessary to consider foaming process, in addition to the effects of microbiological population and hydrophobic properties of mixed liquor system, from wider perspective. For example the interactions between microorganisms of activated sludge and the bulk liquid that depend on both nature of cell surface and the physicochemical properties of the surrounding liquid (pH, ionic strength, surface charge, zeta potential of microorganisms and water-air interfaces respectively) are likely to be important, and may provide another partial explanation for biological foaming.