The capture and release of biomass in a high voidage fibrous microstructure: Mechanisms and shear stress levels

This paper is concerned with the capture and release of flowing biomass in high voidage fibre networks, and the work is relevant to a number of areas including filtration and biomass growth within scaffold structures. Experiments were carried out using a micro scale visualisation flow cell and a laboratory scale filter with similar hydrodynamic conditions and these apparatus were used to establish the dominant modes and mechanisms of deposition during steady flow, and also the efficacy of oscillatory fluid flow for the re-suspension of deposited biomass. Numerical simulation was used to elucidate the flow characteristics through a 'clean' fibre network. Quantitative image analysis of in situ confocal laser scanning microscope data from the flow cell was conducted, obtaining both the local void fraction and surface area exposed to flow as the filtration progressed. The laboratory scale filter apparatus gave the overall filtration efficiency and pressure difference data, and also provided model parameters to estimate the filter coefficient X as a function of the deposit level in the filter. By combining these model parameters with the image analysis results, it was possible to obtain the deposit profile in the filter bed as the filtration progressed and also the average shear stress acting on the biomass deposit as a function of fluid drag. The experimental results showed that the high voidage fibre network was an effective way of capturing biomass, and that filter ripening, steady-state capture and breakthrough are all present during the course of a filter run. During biomass filtration, the average shear stress acting on the biomass started at approximately 1.0 Pa and gradually rose to a peak of 6.5 Pa, which was the maximum sustainable by the deposit. Thereafter, a reduction in the shear stress indicated rearrangement of the deposit to zones further downstream in the filter. Subsequent high intensity oscillatory fluid flow was partially effective at removing the biomass deposit both in the micro flow cell and the laboratory filter experiment, with shear stress levels during this high intensity oscillatory flow starting at approximately 65 Pa with a fully loaded fibre network, and decreasing to 7.5 Pa as the filter medium approached a 'clean' state. (c) 2005 Elsevier B.V. All rights reserved.