Monitoring respiration and biological stability during sludge composting with a modified dynamic respirometer

A static electrolytic respirometer was adapted to work under dynamic conditions using an internal airflow recirculation system that passed air through sludge/straw solid mixtures. Airflow reduced oxygen transfer limitations and increased the maximum respiration rate and kLa values, indicating that the observed value may have been close to the actual biodegradation rate. Airflow caused sludge drying, so the sample moisture was controlled by air humidification. To apply the optimised respirometric technique, a pilotscale composting process was developed. Some commonly used respiration indices (RI24, AT4) were used to measure the final compost stability. RI24 indices without airflow were underestimated during the thermophilic composting stage. Once the easily biodegradable carbon was consumed, the static and dynamic RI24 indices were nearly identical. Because of the dynamic procedure, the respiration rate was likely controlled by the biochemical reaction and not by the mass transfer. The respiration indices indicated that the final compost was unstable.