Process development for anaerobic waste water treatment using porous carriers for microbial colonization

A chemostat is well known to be a tool for selecting microorganisms with minimum doubling times during continuous fermentation, which is of special importance for slowly growing microorganisms like those used in anaerobic digestion. By set point control (SPC) of the operation parameters — especially of residence time τ — by means of a process computer, the selection stress can be optimized. A continuous operation very close to wash-out conditions is possible in spite of the microorganisms population changing properties. For anaerobic digestion of acid, a biomass was obtained that could reduce the chemical oxygen demand (COD) by more than 20 kg per kg dry biomass per d. For a high space-time yield, not only a high biomass activity but also a high biomass concentration is needed; therefore, methods for biomass retention or recycling have been developed. A porous glass sponge with free permeable macropores that can be colonized by anaerobic microorganisms proved to be especially useful. Raschig ring-shaped particles of this material were used in fixed-bed loop reactors for the anaerobic digestion of organically high polluted effluents.