A Mechanism of Reducing Methane Production During Sewage Sludge Composting by Adding Urea

The study of the effect of the mechanism of urea addition to sewage sludge and sawdust-composting substrates on methane production is still limited. In the present study, the systematic investigation of the effect of urea addition (0.18, 0.9 and 1.8 kg) on methane production is discussed through the dynamics of physical properties, enzymes, and the microbial community during composting. The results showed that high urea addition (1.8 kg) suppressed methane production, with a lower rate and a shorter duration of warming in the thermophilic phase, but significantly enhanced cellulase activity, urease, and peroxidase, and promoted the degradation of organic carbon, as well as the loss of nitrogen. A high addition of urea stimulated the growth and reproduction of <i>Sinibacillus</i>, <i>Pseudogracilibacillus</i>, <i>Sporosarcina</i>, and <i>Oceanobacillus</i>. The random forest model indicated that the top six independent determinants of CH₄ emissions were <i>Methanobacterium</i>, temperature, organic matter (OM), <i>Methanospirillum</i>, and NH₄⁺-N. Furthermore, structural equation modeling displayed that NH₄⁺-N, O₂, and pH were the main physicochemical properties affecting CH₄ emissions. <i>Methanobacterium</i>, <i>Methanosarcina</i>, and <i>Methanosphaera</i> were the main archaea, and <i>Bacillaceae</i> were the main bacteria affecting CH₄ emissions. This study provides new insights and a theoretical basis for optimizing urea addition strategies during composting.