Anoxic granulated biomass and its storage

Laboratory experiments involving shutdown and repeated start-up of a denitrification USB reactor with granulated anoxic biomass were conducted in order to find suitable conditions for a safe storage period of the biomass. Anoxic granulated biomass stored under anaerobic conditions for a half year period at 6°C and for a half month period at 18–20°C retained its activity and granular morphology. Storage of anoxic granules under anaerobic conditions for a half year period at 18–20°C led to the loss of the biomass original activity and a significant portion of the granules disintegrated. Anoxic granulated biomass stored for a one and a half month period under endogenous anoxic conditions at 18–20°C retained its activity and granular morphology. A two month storage under endogenous anoxic conditions at 18–20°C was too long and the shutdown of the reactor had to be followed by repeated anoxic granulation. Minimum loading of the USB reactor with N-NO3 to maintain endogenous anoxic conditions in the sludge bed was in the range of 0.06–0.1 kg of N-NO3 per m3 per day. Restart of the USB reactor can be accelerated by an addition of anaerobic granulated biomass.

Comparison of anoxic granulation in USB reactors with various inocula

Anoxic granulation process with four different inocula was monitored in a laboratory post-denitrification up-flow sludge bed (USB) reactor. Wastewater containing 20 mg L−1 NO3-N and methanol as an organic carbon source was used. Gradual increase of mass volumetric loading (B v) and hydraulic loading (γ) resulted in spontaneous granulation of anoxic biomass both from flocculant activated sludge and from anaerobic granulated sludge. With flocculant activated sludge as the inoculum, anoxic granules sedimentation properties and maximum loadings of the USB reactor depended on the sludge volume index (SVI) of the inoculum. B v,max achieved in the USB reactor with flocculant inoculum from a municipal wastewater treatment plant (SVI = 208 mL g−1) was only 4.2 kg of COD per m3 per day and 0.7 kg of NO3-N per m3 per day. B v,max using flocculant inoculum from an industrial wastewater treatment plant (SVI = 170 mL g−1) was 8.1 kg of COD per m3 per day and 1.35 kg of NO3-N per m3 per day. With anaerobic granulated inoculum (SVI range 8–11 mL g−1), markedly higher loadings in the USB reactor and lower SVI values of anoxic granulated biomass were achieved. Values of B v,max were in the range of 16.1–22.4 kg of COD per m3 per day and of 2.7–3.7 kg of NO3-N per m3 per day (depending on the inoculum and the granulation procedure). It was proved that anaerobic granulated sludge is not just an inoculum, it is also a carrier for new denitrification biomass.