Recent attempts have been made to develop a thermophilic composting process for organic sludge to not only produce organic fertilizers and soil conditioners, but to also utilize the generated ammonia gas to produce high value-added algae. The hydrolysis of organic nitrogen in sludge is a bottleneck in ammonia conversion, and its improvement is a major challenge. The present study aimed to elucidate the effects of inoculated Neurospora sp. on organic matter decomposition and ammonia conversion during thermophilic composting of two organic sludge types: anaerobic digestion sludge and shrimp pond sludge. A laboratory-scale sludge composting experiment was conducted with a 6-day pretreatment period at 30 °C with Neurospora sp., followed by a 10-day thermophilic composting period at 50 °C by inoculating the bacterial community. The final organic matter decomposition was significantly higher in the sludge pretreated with Neurospora sp. than in the untreated sludge. Correspondingly, the amount of non-dissolved nitrogen was also markedly reduced by pretreatment, and the ammonia conversion rate was notably improved. Five enzymes exhibiting high activity only during the pretreatment period were identified, while no or low activity was observed during the subsequent thermophilic composting period, suggesting the involvement of these enzymes in the degradation of hardly degradable fractions, such as bacterial cells. The bacterial community analysis and its function prediction suggested the contribution of Bacillaceae in the degradation of easily degradable organic matter, but the entire bacterial community was highly incapable in degrading the hardly degradable fraction. To conclude, this study is the first to demonstrate that Neurospora sp. decomposes those organic nitrogen fractions that require a long time to be decomposed by the bacterial community during thermophilic composting.
In recent years, attempts have been made to develop a thermophilic composting process for organic sludge to produce ammonia gas for high value-added algal production. However, the hydrolysis of non-dissolved organic nitrogen in sludge is a bottleneck for ammonia conversion. The aim of this study was to identify enzymes that enhance sludge hydrolysis in a thermophilic composting system for ammonia recovery from shrimp pond sludge. This was achieved by screening useful enzymes to degrade non-dissolved nitrogen and subsequently investigating their effectiveness in lab-scale composting systems. Among the four hydrolytic enzyme classes assessed (lysozyme, protease, phospholipase, and collagenase), proteases from Streptomyces griseus were the most effective at hydrolysing non-dissolved nitrogen in the sludge. After composting sludge pre-treated with proteases, the final amount of non-dissolved nitrogen was 46.2% of the total N in the control sample and 22.3% of the total N in the protease sample, thus increasing the ammonia (gaseous and in-compost) conversion efficiency from 41.5% to 56.4% of the total N. The decrease in non-dissolved nitrogen was greater in the protease sample than in the control sample during the pre-treatment period, and no difference was observed during the subsequent composting period. These results suggest that Streptomyces proteases hydrolyse the organic nitrogen fraction, which cannot be degraded by the bacterial community in the compost. Functional potential analysis of the bacterial community using PICRUSt2 suggested that 4 (EC:3.4.21.80, EC:3.4.21.81, EC:3.4.21.82, and EC:3.4.24.77) out of 13 endopeptidase genes in S. griseus were largely absent in the compost bacterial community and that they play a key role in the hydrolysis of non-dissolved nitrogen. This is the first study to identify the enzymes that enhance the hydrolysis of shrimp pond sludge and to show that the thermophilic bacterial community involved in composting has a low ability to secrete these enzymes.