Seasonal variations in nutrient changes during sludge anaerobic digestion combined with thermal hydrolysis and deep...

The quality of sludge is significantly influenced by seasonal variations and specific treatment stages, particularly through changes in nutrient levels and organic matter composition. However, the detailed mechanisms of these effects remain poorly understood. In this study, sludge samples were collected from a large-scale wastewater treatment plant in Beijing during spring, summer, autumn and winter, across each process node of the "pre-dewatering + thermal hydrolysis + anaerobic digestion + deep dewatering". A systematic analysis of key soil nutrients (C, N, P, K, Na, Ca and Mg) was conducted on the collected sludge samples. The results demonstrated the concentration of C, N and P in sludge was higher in autumn, with TOC reaching 29.23 %, TN at 44,470.70 mg/kg and TP at 28,572.89 mg/kg. The concentrations of C, N and Na decreased by 34.9 %, 20.4 % and 12.0 % following treatment. Conversely, the concentrations of P, Ca and Mg increased by 20.4 %, 52.7 % and 39.9 %, respectively. Additionally, the reducible fraction contents of K, Na, Ca and Mg exhibited a general increase of 41.68 %, 14.01 %, 31.30 % and 27.13 %, while their oxidizable fractions increased by 101.03 %, 57.27 %, 101.68 % and 59.38 %. The sludge fertilizer quality index (SFQI) showed that the fertility of the treated sludge increased by 1.6 times This study is the first time to use SFQI to quantify sludge quality and systematically evaluate the impact of sludge management on nutrient dynamics. It provides a scientific basis for sludge land utilization and resource utilization.