Promotion effects of ultrasound on sludge biodegradation by thermophilic bacteria Geobacillus stearothermophilus TP-12

• We isolated a bacterium which can secrete thermophilic proteolytic enzyme.
• The sludge hydrolysis by thermophilic bacteria was enhanced by ultrasound.
• The content of dissolved organic matter also could be increased simultaneously.
• The ultrasound parameters were optimized by RSM.

Sludge biodegradation using thermophilic bacteria is a promising method for sludge treatment. In order to further enhance the efficiency of sludge reduction and hydrolysis, low-frequency ultrasound was used to promote this process. We isolated a thermophilic strain that is effective in secreting extracellular protease to hydrolyze sludge. Then the key ultrasound parameters were selected using the response surface methodology method. After 12 h treatment using thermophilic bacteria with short-time ultrasound promotion, volatile suspended solids (VSS) reduction ratio was achieved 32.8%, which is 41.4% higher than that without ultrasound promotion. Meanwhile the contents of soluble chemical oxygen demand (SCOD), protein and carbohydrate were increased by 20.2%, 16.8% and 15.9%, respectively. The composition of dissolved organic matter of sludge products evaluated by excitation-emission matrix spectroscopy demonstrated the promotion effect and eliminated the possibility of the direct sludge degradation caused by ultrasound treatment. Low-frequency ultrasound could effectively promote the thermophilic bacteria hydrolysis to achieve higher sludge biodegradation ratio without directly degrading the raw sludge. The promoted process of sludge biodegradation can further reduce the environmental risk and make sludge to be more readily usable.