Reactors with membrane-grown biofilms: their capacity to cope with fluctuating inflow conditions and with shock loads of xenobiotics

Biofilm reactors containing gas-permeable tubings both for oxygen supply and as support material were used for the treatment of a wastewater spiked with chlorophenols as model compounds for poorly degradable xenobiotics. Objective of this work was to ascertain possibilities to enhance the performance of biofilm reactors in the case of shock loads. Under plug flow conditions, the continuous flow operation of one biofilm reactor (continuous flow biofilm reactor — CFBR) resulted in a stratification of biomass due to concentration gradients. Therefore, the effluent quality of a CFBR deteriorated as a result of break-through events attributed to the displacement of chlorophenols into sections of the reactor with low biomass or with microorganisms not well adapted to these compounds. For comparison reasons, a second, geometrically identical membrane biofilm reactor was operated discontiuously as sequencing batch biofilm reactor (SBBR) to achieve a more homogeneous distribution of biomass. A higher stability of reactor performance became evident for the SBBR, if the reactors were subjected to periodically changing influent concentrations (at an interval of 12 h, i.e. the twofold of the theoretical residence time). It was shown that the sequencing batch reactor technology is very promising to handle strong fluctuations of influent conditions because of the flexibility of the operation schedule. In case of a shock load with up to 110 mg l−1 4-monochlorophenol (MCP), an elimination of 99.5% was obtained by prolonging the SBBR cycle from 6 to 8 h. For the CFBR, it was possible to avoid breakthrough events by recirculation.