Investigation of the shape change property of bio-aggregates and biofilms and its effect on mass transport

Abstract

In engineered or natural systems, microorganisms are normally suspended in the form of aggregates. Generally, the capacity and performance of wastewater treatment are often controlled by the rate of mass transfer into the aggregates. Conventionally, molecular diffusion is considered to be the dominate mechanism of substrate transport into the aggregates. However, microbial aggregates have highly porous and fractal structure that would permit internal flow through their interior. In addition, the soft and flexible structure of the aggregates would allow change of their shape by the external shear force in the bioreactors. This shape deformation, e.g., compression or expansion of the aggregates, would enhance the material exchange between the internal pores and the bulk fluid, which however has not been previously addressed. The present study was conducted to investigate the importance of shape deformation to the overall mass transport of microbial aggregates, including activated sludge flocs, aerobic granules and suspended biofilms. A particle imaging velocimetry (PIV) system was employed to record the shape change details of microbial aggregates in a sheared fluid. The laser induced fluorescence (LIF) technique was used to track the transfer of fluorescent dye into or from the aggregates during their shape change. The PIV results provide a clear evidence of the shape change or deformation of the aggregates when they were subject to the external force. The degree of shape change increased as the fluid shear intensity increased. For activated sludge flocs, the rate of mass transport driven by deformation could be up to 30 times and twice higher than molecular diffusion and internal permeation, respectively. Deformation induced transport accounted for over 50% of the overall mass transport for large bio-flocs. Suspended biofilms, such as aerobic granules and GAC-based biofilms, are also highly porous, soft and flexible. PIV images revealed the deformable feature of the granules and moving biofilms in a turbulent fluid environment, especially during collisions. The dye release rate from the granules was linearly related to the fluid shear intensity. A higher shear rate induced frequent collisions between granules, increasing the rate and degree of the shape change. For GAC-based biofilms, the LIF results also indicated that shape deformation induced by frequent collisions is an important mechanism of material transport, particularly for large molecular pollutants with a low diffusivity. Based on the theoretical analysis and experimental work, a comprehensive model has been developed for the mass transport of microbial aggregates, accounting for molecular diffusion, internal permeation and shape deformation in a sheared fluid. The results show that the deformation induced mass transport can be one and two orders of magnitude higher than internal permeation and molecular diffusion. For the transport of macromolecular solutes into large-sized aggregates, the importance of deformation is even more significant. In addition, computational fluid dynamics (CFD) was applied to simulate the mass transport process for individual granules with shape deformation. The CFD simulation results also confirmed the experimental finding that shape deformation of aerobic granules can greatly enhance the fluid permeation and material transport through the interior of the granules.