Rheological behavior of the sludge in a long-running anaerobic digestor: Essential factors to optimize the operation

• A real-time online torque monitoring system was established.
• The rheological behavior of sludge in a long-running digestor was revealed.
• Variations of different rheological parameters over time were described in detailed.
• Contents of TSS and EPS have close relationships with the limit viscosity of sludge.
• Thixotropic index in a long running anaerobic digestor was analyzed.

This work investigated the rheological behavior of sludge in a long-running digestor and established a timely approach for monitoring the torque information of an anaerobic process. With multi-methods, the rheological behavior of sludge within the digestor were revealed, which indicated that: (1) the on-line torque sensing approach was an efficient method to obtain timely torque signals, which could give a reliable prediction to the variation in the rheological behavior of sludge, and further provide useful information to monitor the operation of an anaerobic digestor; (2) the thixotropic index of sludge in the digestor decreased continuously along with the operational time, which showed a weakening tendency in the interaction among sludge particles; (3) both the content of total suspended solids (TSS) and extracellular polymeric substances (EPS) had close relationships with the limit viscosity of sludge, but TSS showed a positive impact, while, the impact of EPS was negative.

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