Effects of bubble size on air-scoured backwashing efficiency in a biofilter

Abstract

The effect of bubble size on air-scoured backwashing efficiency in a biofilter was investigated using experiments and simple mathematical models based on collapse-pulsing theory. Four types of backwashing experiments were conducted: water alone, water and large bubbles (diameter = about 1 cm), water and small bubbles (diameter = about 0.2 cm), and water containing dissolved air. The third type, water and small bubbles, was found to be the most efficient. Modeling results confirmed the experimental results, indicating that even when the same volume of air was used, smaller bubbles generate higher shear stress on the surface of the medium, resulting in more efficient removal of biomass. In the case of dissolved air, however, micro-bubbles (diameter = 10-100 mu m) are too small and rise too slowly to produce a strong-enough collapse-pulsing effect. This produces less shear stress and ultimately results in poor backwashing efficiency.