Digestion activity of thermophilic bacteria isolated from ash-amended sewage sludge compost

Previous experiments had shown that co-composting sewage sludge with coal fly ash resulted in a decrease in microbial activity during the thermophilic phase. Therefore, attempts were made to utilize isolated dominant thermophilic bacteria from ash-amended sludge compost to enhance the decomposition of organic matter in digestion tests involving mixtures of sewage sludge and coal fly ash. Cultures of three Bacillus species, i.e., B. brevis, B. coagulans, and B. licheniformis were inoculated into sewage sludge amended with 25% coal fly ash at cell densities ranging from 10⁵ to 10⁷ CFU g^-1 dry sludge, and were incubated for 10 days in aqueous suspension The digestion test showed that an inoculation level of ≥10⁶ CFU g^-1 dry sludge was suitable for achieving an acceptable rate of digestion of ash-sludge mixture, as indicated by the significantly higher evolution of CO₂ compared to the control receiving no inoculation. Weight loss and contents of soluble organic carbon, protein, and amino acids were lower in ash-sludge mixture with bacterial inoculation. Hence, all the three bacilli were able to decompose the organic matter in ash-sludge mixture faster than that of the control. Among the three bacilli, B. brevis was less efficient in decomposing the organic matter in the ash-sludge mixture than the other two bacilli at an inoculation rate of 10⁶ CFU g^-1 dry sludge, but no significant difference was noted among the three bacilli at an inoculation rate of 10⁷ CFU g^-1 dry sludge. It can be concluded that the three bacilli all exhibited the ability to improve the decomposition of organic matter in ash-sludge compost.