APPLICATION OF A BIOFILM SUSPENSION REACTOR FOR ACTIC ACID REMOVAL
KWOK WAI KIAT
KWOK WAI KIAT
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Abstract
In this report, removal of acetic acid was studied using three laboratory-scale biofilm airlift suspension (BAS) reactors. The influences of operational conditions (substrate loading rates and carrier concentrations) on the performances of the BAS reactor and the structures of biofilms were studied. Biofilm morphology was examined via image analysis through which the pictures were taken and the statistics of the biofilm particles were quantified. Beside substrate loading rate and carrier concentration, all other operational conditions like hydraulic retention time, reactor geometry, air flow rate, etc. were kept constant in all the experimental runs. The structures of the biofilms were investigated at substrate loading rates of 5, 10, 15, and 20 kg COD/m3d with basalt concentrations of 60 g/l, 150 g/l, and 250 g/l. In total 12 runs were performed.
100 % conversions of acetic acid up to a loading rate of 20 kg COD/m3 .d were obtained during the entire three series of experiments. In all cases, relatively 'steadystate' conditions were achieved between 10 to 30 days after changing to a new COD loading. The system seemed to be relatively unstable at a loading rate of 20 kg COD/m3 .d which led to the formation of filamentous biofilm structures, increased biomass hold-up, and biomass washed out from the reactor. Smoother (less protuberances), denser, thinner and compact biofilms were observed as biomass surface production rate decreased and/or carrier concentration increased in the reactor. However, at a given carrier concentration, there were no specific trends for biofilm density and biofilm shape as volumetric substrate loading was increased. The highest biofilm density and the lowest biomass yield were found to be 67 g/1 and 0.12 g biomasslg acetate respectively. These values were obtained from the experiments employing the highest amount of carrier concentration. Contrary to the general believe, biomass detachment rate was found to decrease as carrier concentration increased. These observations confirm that biofilm structure is very much governed by the biofilm surface loading (proportional to biomass surface production rate, when biomass yield is constant) and shear rate (abrasion rate of particles). When shear forces are relatively high only a patchy biofilm will develop, whereas at low shear rates the biofilm becomes highly heterogeneous with many pores and protuberances. In the case of a right balance, smooth and stable biofilms are obtainable.
Keywords
Acetic acid, biofilm airlift suspension reactor, heterotrophs, biomass surface production rate, biofilm density, biofilm thickness
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Date
1997
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Thesis