Digestion Activity of Thermophilic Bacteria Isolated from Ash-Amended Sewage Sludge Compost

Previous experiments had shown thatco-composting sewage sludge with coal fly ash resultedin a decrease in microbial activity during thethermophilic phase. Therefore, attempts were made toutilize isolated dominant thermophilic bacteria fromash-amended sludge compost to enhance thedecomposition of organic matter in digestion testsinvolving 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 sludgeamended with 25% coal fly ash at cell densitiesranging from 105 to 107 CFU g-1 drysludge, and were incubated for 10 days in aqueoussuspension. The digestion test showed that aninoculation level of ≥106 CFU g-1dry sludge was suitable for achieving an acceptablerate of digestion of ash-sludge mixture, as indicatedby the significantly higher evolution of CO2compared to the control receiving no inoculation.Weight loss and contents of soluble organic carbon,protein, and amino acids were lower in ash-sludgemixture with bacterial inoculation. Hence, all thethree bacilli were able to decompose the organicmatter in ash-sludge mixture faster than that of thecontrol. Among the three bacilli, B. brevis wasless efficient in decomposing the organic matter inthe ash-sludge mixture than the other two bacilli atan inoculation rate of 106 CFU g-1 drysludge, but no significant difference was noted amongthe three bacilli at an inoculation rate of 107CFU g-1 dry sludge. It can be concluded that thethree bacilli all exhibited the ability to improve thedecomposition of organic matter in ash-sludge compost.