Influence of physiologically relevant parameters on biomass formation in a trickle-bed bioreactor used for waste gas cleaning

Limitation of biomass formation in a mixed culture immobilised in a trickle-bed bioreactor without substantially affecting the biological degradation of organic compounds in waste gas streams was investigated. As carbon source, the industrially relevant volatile organic compounds ethyl acetate and toluene were used. The temporal biofilm composition was investigated by means of transmission electron microscopy of ultrathin sections cut along the film height. Physiologically relevant parameters were varied. In this context the effect of(a) the type of nitrogen source, (b) the concentration of inert salt and (c) limiting the availability of essential nutrients by intermittent trickling was studied. The effect of these parameters on both biomass formation and degradation was expressed in terms of the ratio R which was defined as the fractional inhibition of biomass formation related to the fractional decrease of degradation. Using nitrate as nitrogen source instead of ammonium, R was 0.71, which means that the fractional inhibition of biomass formation was less than the fractional inhibition of degradation. When the concentration of NaCl as inert salt was adjusted to 0.4 M, the R became 1.32, showing that the fractional inhibition of biomass formation was stronger than the fractional inhibition of degradation. Limiting the availability of nutrients by intermittent trickling, the pressure drop fell by 50% whereas the degradation efficiency decreased by 30%. In summary, intermittent trickling and addition of an inert salt were observed to be advantageous unlike the impact of the type of nitrogen source.