To maximize the sludge degradation efficiencies during anaerobic digestion, pre-ozonation of raw activated sludge and/or post-ozonation of anaerobically digested sludge were used in combination with a reactor operated without solid withdrawal (closed loop operation scheme). Out of the two studied configurations, the configuration with the post-ozonation of digested sludge was found to be superior with respect to degradation efficiencies, lower accumulation of total volatile solids (TVS) and lower required ozone dose. The TVS concentration in the reactor with post ozonation was found to be around 26 ± 2 g/l at an average TVS loading of 0.6 kgTVS/m3.d. The maximum TVS removal efficiencies were observed to be around 85% with average specific gas recoveries of 0.36 l CH4/gTVS fed. The ozone dose of 3% on the recycle sludge was found to be sufficient as it was observed that the higher ozone doses of 6% neither improved the TVS reduction efficiency nor reduced the TVS concentration in the reactor. The ozone requirements were estimated at around 0.054 gO3/gTVS removed or 0.045 gO3/gTVS fed. The soluble COD concentrations from the reactor ranged from 500-875 mg/l with negligibly small amount of VFA. The average COD in the centrate of the digested sludge was around 3000 mg/l suggesting the presence of colloidal COD in the reactors.

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