Dynamic of organic matter fractions during sewage sludge and bulking agent composting

【Objectives】 The study for transformation of organic matters, total organic carbon, and total nitrogen in sewage sludge is of considerable importance. The objective is to improve the biological availability of compost and promote its land-application process.【Methods】 The composting experiment was conducted with forced aeration and turning in the factory, and the overall work lasted for 18 days. The pile was composed of sewage sludge, bulking agent (mushroom and saw-dust mixture), and compost in mass ratio of 6∶3∶1. Samples were taken at different times during the composting process. Temperature in real-time, C/N ratio, pH, moisture, and organic matter degradation were determined throughout the period. In particular, the transformations of water soluble organic matter, hemicellulose, cellulose, and lignin were monitored. 【Results】 During the progress of the experiment, windrow temperature rapidly reached up to 71.3℃ and lasted for 5 days, then declined to the normal level gradually. Meanwhile, the pH of windrow showed the same trend with temperature, ranging from 6.0 to 7.5. In addition, moisture continuously reduced from 60.7% to 51.4%. Properties of the windrow were within the range of composting standard after period work. Total organic carbon content descended in the whole period, whereas total nitrogen content decreased by the end of thermophilic phase and then increased gradually. In the initial phase, four organic fractions in the windrow were distributed as follows: water-soluble fraction cellulose hemicellulose lignin. After the compost process, the distribution changed to: cellulose water-soluble fraction lignin hemicellulose. The water-soluble fraction and hemicellulose reduced from 39.5% and 20.1% to 27.9% and 14.4%, respectively. Cellulose content reached 29.5% from 21.8%, and lignin content was stable in the period. According to the results, compost experiment reached the standard requirement. This result indicated that water-soluble organic matter and hemicellulose were the main energy and material sources to microbe. Contents of water soluble organic matter, hemicellulose reduced by 38.6% and 38.8%, respectively. The reduction of cellulose and lignin were 11.7% and 18.5%, respectively. Finally, the mass decrement of the pile was 9.8%. Water-soluble organic matter was mainly degraded in the thermophilic phase, and 65.5% of reduction occurred in this phase. However, hemicellulose was mainly degraded in the stable phase, and 69.1% of reduction occurred in this phase. At the same time, a small amount of cellulose and lignin were degraded in thermophilic phase because of their complex structure. Organic carbon was transformed to inorganic carbon in the whole period. Ammonia nitrogen was lost in the thermophilic phase, whereas nitrate nitrogen accumulated in the stable phase. 【Conclusions】 The composting process rendered the sewage sludge, stabilized and reduced. Moreover, the process increased the biological availability of sludge, which promotes the feasibility of the land application of sludge.