Using the modified pine wood as a novel recyclable bulking agent for sewage sludge composting: Effect on nitrogen conversion and microbial community structures

•Modified pinewood was an effective bulking agent in sewage sludge (SS) composting.•Addition of modified pinewood reduced ammonia release and reduced nitrogen loss.•Modified pinewood addition increased the duration of the thermophilic phase.•Sodium hydroxide-modified pinewood has a good potential for recycling. This study investigated the effect of a recoverable sulphuric acid and sodium hydroxide-modified pinewood (MOP) as a bulking agent during sewage sludge and sawdust composting (MOPC), with a control experiment using unpretreated pinewood (UNP; UNPC) as the bulking agent. Results show that addition of MOP effectively promoted the degradation of organic matter during composting. The maximum temperature increased by 1.50 °C and the high temperature period (T > 50 °C) of composting was extended 4 days longer than the control experiment. Furthermore, MOP addition reduced the loss of nitrogen by 9.40%. High-throughput sequencing analysis showed that the bacterial communities in the UNPC and MOPC treatments were significantly different. Pseudoxanthomonas was the dominant bacteria during the thermophilic and cooling phases of the MOPC treatment. In addition, the recycling efficiency of the UNP and MOP was 99.18% and 99.37%, respectively.