Biofilm detachment mechanisms in a liquid‐fluidized bed

Abstract

Bed fluidization offers the possibility of gaining the advantages of fixed-film biological processes without the disadvantage of pore clogging. However, the biofilm detachment rate, due to hydrodynamics and particle-to-particle attrition, is very poorly understood for fluidized-bed biofilm processes. In this work, a two-phase fluidized-bed biofilm was operated under a constant surface loading (0.09 mg total organic carbon/cm(2) day) and with a range of bed height (H), fluid velocities (U), and support-particle concentrations (C(p)). Direct measurements were made for the specific biofilm loss rate coefficient (b(s))and the total biofilm accumulation (X(f)L(f)). A hydrodynamic model allowed independent determination of the biofilm density (X(f)), biofilm thickness (L(f)), liquid shear stress (tau), and Reynolds number (Re). Multiple regression analysis of the results showed that increased particle-to-particle attrition, proportional to C(p) and increased turbulence, described by Re, caused the biofilms to be denser and thinner. The specific detachment rate coefficient (b(s)) increased as C(p) and Re increased. Almost all of the 6, values were larger than predicted by a previous model derived for smooth biofilms on a nonfluidized surface. Therefore, the turbulence and attrition of bed fluidization appear to be dominant detachment mechanisms.

Detection of Ammonia-oxidizing Bacteria (AOB) in the Biofilm and Suspended Growth Biomass of Fully- and Partially-packed Biological Aerated Filters

“…Reynolds number (Chang et al 1991) In this study, since the medium is fixed, there is no attrition effect. Therefore turbulence effect could be the major factor that increases the detachment pressures, and caused the biofilm to become denser and thinner.…”

“…In a study to evaluate the essential role of hydrodynamic shear force in the formation of biofilm, Liu and Tay (2002) pointed out that biofilm density quasi-linearly increases with the increase of shear stress. Chang et al (1991) discovered that the medium concentration and the turbulence indicated by Reynolds numbers, significantly affected biofilm density and thickness of a fluidized bed biofilm reactor. In this type of reactor, increasing medium concentration can be associated with increasing attrition due to particle-to-particle contacts and increasing turbulence correlates flow fluctuations that could create forces normal to the biofilm, i.e.…”

Biomechanical Analysis of Infectious Biofilms

“…Reactor geometries are designed to expose high surface areas of the biofilm to flow, such as porous media [18,99] or fibres [47,94], and biomass effluent and other quantities measured.…”

“…The inter-polymeric bonds that provide the matrix with its mechanical resilience and structural integrity must either be separated from cell surface proteins, enzymatically lysed, or overcome by tensile forces deriving from the external fluid shear [40], before cells can escape the biofilm envelope. Erosion of single cells, multi-cellular clusters (which maintain their resistance to antibiotics [36]) and sudden large-scale sloughing events also result from fluid shear [23], limiting biofilm thickness [18].…”

“…Reactors maximise the area of contact between microbes and fluid by growing the biofilms on highly porous supports, but this entails the risk of overgrowth leading to reduced flow rates and eventual blockage. Quantifying biomass effluent from complex biofilms grown in a glass-bead reactor in terms of a spatially-averaged model has revealed design principles that could be exploited to control growth and detachment [18]. Spatially-extended models reveal a complex biomass distribution on the scale of reactor pores, with reduced nutrient availability and biofilm growth downstream from clogged pores [52,93].…”