UC Berkeley

The formation of a substantial foam layer has severe impacts on the overall digestion process that is responsible for the major portion of waste stabilization. An understanding of dynamics of foam produced by three primary causes - dispersed gas bubbles, surface-active materials, and hydrophobic compounds - provides insight into the prevention of the foam layer and into the control of the foam stability. To better understand the mechanisms in three-phase foam dynamics, we developed a systematic methodology to characterize foam drainage behavior using electrical conductance measurements.

With no sludge conditions, all tested sodium dodecyl sulfate (SDS) foams exhibit a node-dominated drainage regime with high mobility at bubble surfaces. Drainage regime under similar foaming conditions was consistent with existing drainage studies using other measurement techniques in the literature.

The drainage of sodium dodecylbenzenesulfonate (SDBS) surfactant solution, a commercial form of linear alkylbenzene sulfonate (LAS) that is most frequently found in anaerobic sludge, was studied in detail by several approaches. Two complementary methods of macroscopic drainage investigations (forced and free drainage) were conducted to gain confidence in its validity. The experimental data can be fitted using a power law with an exponent of 1/3 for forced drainage and of 1.0 for free drainage. These data indicate the following drainage behavior: mobile bubble surfaces, causing plug-like flow within Plateau borders, thus dissipation mainly occurs inside the nodes.

With the drainage studies for a variety of aqueous surfactant solutions, experimental and theoretical studies to include wastewater sludge are important for understanding the stability of three-phase foams, and therefore foaming in anaerobic digesters. The drainage behavior of sludge-containing foams was characterized by our developed method. The presence of anaerobic sludge at total solid (TS) concentrations of approximately 2.5-2.8% induced a transition from the node-dominated regime to a Plateau-border (PB)-dominated regime. This apparent transition was verified in both forced and free drainage experiments. These drainage studies supported a more detailed estimation of foam stability in relation to its structure.

In summary, our developed method using the electrical conductance measurements was applied to understand the key aspects of foam stability required for prediction and control of foaming in anaerobic digesters. A systematic methodology for assessing foam dynamics was proposed and discussed in two- and three-phase foams.