Rheological measurements as indicator for hydrolysis rate, organic matter removal, and dewaterability of digestate in anaerobic digesters

The relationship between rheological properties and the hydrolysis rate in anaerobic digestion (AD) is poorlyunderstood. This study investigated AD of waste-activated sludge (WAS) in batch digesters of 2.10%, 3.40%,4.10%, and 5.02%TS digester media (WAS mixed with inoculum). The digesters operated below ammonia in-hibition levels and at the same food to inoculum (F/I) ratios, temperature, and mixing. The results indicated thata decrease in the hydrolysis rate was associated with an increase in yield stress, theflow consistency index, andviscoelastic moduli. In addition, the statistical analysis revealed the linear relationships between changes in theflow consistency index and hydrolysis rate as well as the efficiency of organic matter removal, regardless of thetypes and origin of sludge, the F/I ratio, and the seasons when sampling occurred. A linear relationship betweenyield stress and dewaterability was also revealed. Thesefindings indicate that theflow consistency indexmeasurement could be a reliable method for monitoring the hydrolysis stage and organic matter removal inanaerobic digsters.