Using the organic carbon fractions of the Van soest method to determine compounds responsible for C and N mineralization from sludge amended soils

Abstract

The composition of sludge organic matter is mainly influenced by the origin of wastewater. The biochemical composition of sludge could, however, alter from post wastewater treatment drying techniques. Such changes have a direct effect on the N fertilizer value of sludge because the biochemical composition of sludge dictates its decomposition rate. Therefore, proper understanding of the effect of wastewater treatment methods and post treatment drying techniques on a) the C and N contents as well as organic matter composition of sludge, and b) C decomposition and N mineralization is crucial for N management in sludge amended agricultural lands. The aim of this study was to investigate the effect of selected wastewater treatment and post treatment drying techniques on a) the N, C, and organic matter composition, as well as b) C decomposition and N mineralization of municipal sewage sludge. The study also investigated the compounds responsible for C and N release during a decomposition process. Two consecutive incubation studies (100 days each) were conducted to determine N release, C decomposition and sludge organic matter decomposition using different sludge types namely: Thermally hydrolysed sludge (THS), Activated sludge (Activated), and anaerobically digested sludge. The anaerobically digested sludges were collected from two different drying techniques (concrete beds and earth paddy) as well as two sludge drying depths on beds ( 10 cm and 25 cm). Inorganic N was determined by the steam distillation method. Sludge organic matter composition was analysed using Van Soest method. Total N and total organic C were determined using Carlo-Erba method. A parallel sludge alone incubation study was conducted from which organic compounds degradation and transformation analysis were conducted. The anaerobically digested sludge dried in thin layers of 10 cm (ADS3) had higher total N (4.95%) than similar sludge types dried in thick layers of 25 cm in concrete beds (ADS1= 2.81%N) and in earth paddy (ADS2 = 2.83%N). Large fraction (>60%) of the organic C in all sludge types was in soluble compounds form (SOL). Thermally hydrolyzed sludge (THS) had the highest (90%) SOL. ADS3 and Activated had lower lignin fraction (<3%) than THS, ADS1 and ADS2. Net N mineralization per kg C applied was highest for activated (77 g kg-1 C) and was lowest for ADS2 (23 g kg-1 C). Despite having highest SOL, THS had relatively lower net N mineralization per kg organic N applied (350 g kg-1 organic N) than Activated sludge (449 g kg-1 organic N) mainly due to the relatively higher lignin fraction. It was apparent from this study that the soluble compounds were responsible for about 90% of the organic matter decomposition. The Van Soest method was also successfully used to determine residual compounds at intervals during the incubation study from the sludge alone study but not with the soilsludge mixture.