Investigating the effects of inoculum temperature and characteristics on cellulose and sewage sludge biodegradability: A comparative study of three inocula.

The role of inoculum in initiating anaerobic digestion (AD), and accelerating the start-up of anaerobic digesters has been well-documented. However, the effect of aligning the origin temperature of the inoculum with the operational temperature of the new digester remains underexplored. This study investigates how the origin temperature and characteristics of the inoculum affect the kinetics and biodegradability of sewage sludge (SS) and microcrystalline cellulose (MCC) under mesophilic and thermophilic conditions. Three inocula were used: one thermophilic (I1) and two mesophilic inocula (I2 and I3) in six Biomethane Potential tests (BMP) at 37 and 55 °C. Results indicated that inoculum temperature had no significant impact on the BMP values for MCC and SS, regardless of the experimental temperature. However, kinetic analyses revealed that I2 significantly outperformed I1 and I3 under both temperature conditions. This was attributed to I2's more diverse bacterial structure and lower inhibitor concentrations. High alkalinity, ammonia, and volatile fatty acids (VFA), as well as the presence of denitrifying bacteria (41.7 % of total communities in I1) contributed to poor kinetics of I1 and I3, which were unsuitable for mesophilic and thermophilic temperatures, respectively. Alkalinity (correlation with the Simpson index = -0.92, p < 0.05) and ammonia (correlations with Chao and ACE = -0.93 and -0.91, respectively, p < 0.05) were significantly linked to low bacterial diversity, while high VFA levels were strongly associated with poor inoculum kinetics (correlation with degradation kinetics = -0.90 to -0.99, p < 0.05). These findings offer insights into assessing the inoculum suitability based on its characteristics.