The Scale-Up and Limitation of Physical Oxygen Transfer in Rotating Biological Contractors

Abstract

Three different sized laboratory scale Rotating Biological Contactor (RBC) units were constructed to study the controlling factors of physical oxygen transfer. Physical parameters were identified and evaluated by the volumetric oxygen transfer coefficient (KLa) from non-steady state clean water tests. The controlling factor was found to be the liquid film flow rate per unit volume and the function could be practically expressed as w1.5 D0.5 S−1. The turbulene effect on the free water surface and subsurface volume becomes comparatively less significant as the disk size increases. The data indicated that the physical oxygen transfer contribution to the biological treatment oxygen requirement became less significant than oxygen transfer through other mechanisms as the disk size increases.

Model for oxygen transfer in rotating biological contactor

“…Kim and Molof (1982), assumed that oxygen transfer only occurred from liquid film entrained onto the RBC disk, and used Eq. (5) suggested by Zeevalkink et al (1978) to compute liquid film thickness (d), which was further used to compute volume renewable number.…”

“…Kim and Molof (1982) assumed that oxygen transfer occurs only through the liquid film flowing on the disc during the air-exposed cycle and the physical variables of RBC were incorporated in the term volume renewal number. The liquid film thickness on the disc was computed by Zeevalkink et al (1978) model.…”

“…Various authors have done experiments and provided physical oxygen transfer data (Kim and Molof, 1982;Bintanja et al, 1975;Zeevalkink et al, 1979). Also, few researchers have attempted to develop empirical/mathematical relationship for estimation of oxygen transfer in a rotating biological contactor.…”

“…However, the presence of attached biomass on disc surface causes a significant increase in the amount of oxygen transport. Kim and Molof (1982), suggested that the possible mechanism for the enhancement is due to direct oxygen absorption by the microorganism during the air exposure of the discs, whereas, Zeevalkink et al (1979), suggested that enhancement was due to increased oxygen diffusion in the liquid film induced by the oxygen consumption in the biofilm. The estimation of enhancement factor has been done by several researchers (Poalini, 1986;Radwan and Ramanujam, 1995;Boumansour and Vasel, 1998).…”

“…The first approach uses the relationship between oxygen transfer coefficient and the Reynolds number (turbulence), Froude number and the immersion factor and the second approach is based on the surface renewal theory and focuses on the relationship between oxygen transfer coefficient and liquid film volume (volume renewal number) over the disc. In the present work the concept of volume renewable number similar to that suggested by Kim and Molof (1982), was used to develop a generalized model termed as modified Kim and Molof model for estimation/computation of the overall oxygen transfer coefficient (K L a) for RBC. The model is applicable for a wide range of operating conditions, that is, when oxygenation by surface renewal of the liquid film on the RBC disc at high revolutions per minute (RPM) is determinant (penetration theory (Higbie model) is applicable) and also at low RPM when tank surface area significantly contributes toward oxygenation.…”