Image analysis procedure to derive bubble size distributions for better understanding of the oxygen transfer mechanism

Oxygen transfer in biological wastewater treatment represent the most energy demanding step of aerobic processes. Submerged aerators are designed to maximize the available surface for exchange by generating as fine bubbles as possible. Potential effects of process water characteristics on bubble sizes are too often not considered in wastewater treatment, while, liquid properties are known to have important effects on bubble size distribution (BSD) and transfer capabilities. The use of appropriate and consistent imaging tools is of crucial importance for detecting size changes in the distribution of bubbles. The bubble size distribution (BSD) image analysis tool is implemented in the Python environment, with the aim of creating a package under an open software licence. The BSD image analysis tool provided in this paper was specifically developed for measurements of bubbles in clean water and transparent surrogate liquids mimicking the characteristics of activated sludge. A selection of image processing techniques was implemented in this package in the Python environment, aimed at characterizing changes in BSD due to variations in liquid medium compositions that specifically occur in biological reactors. Given the consistency of the results in the different conditions tested and the ability to detect effects of hydrostatic pressure and variation in air flow and fluid characteristics on bubble sizes, the BSD image analysis tool proved to be a potential help in unravelling the dynamics behind oxygen transfer in wastewater applications.