Increasing thermal drying temperature of biosolids reduced nitrogen mineralisation and soil N2O emissions

Previous studies found that thermally dried biosolids contained more mineralisable organic nitrogen (N) than the raw or anaerobically digested (AD) biosolids they were derived from. However, the effect of thermal drying temperature on biosolid N availability is not well understood. This will be of importance for the value of the biosolids when used to fertilise crops. We sourced AD biosolids from a Danish waste water treatment plant (WWTP) and dried it in the laboratory at 70, 130, 190 or 250 °C to >95 % dry matter content. Also, we sourced biosolids from the WWTP dried using its in-house thermal drying process (input temperature 95 °C, thermal fluid circuit temperature 200 °C, 95 % dry matter content). The drying process reduced the ammonium content of the biosolids and reduced it further at higher drying temperatures. These findings were attributed to ammonia volatilisation. The percentage of mineralisable organic N fraction (min-N) in the biosolids, and nitrous oxide (N₂O) and carbon dioxide (CO₂) production were analysed 120 days after addition to soil. When incubated at soil field capacity (pF 2), none of the dried biosolids had a greater min-N than the AD biosolids (46.4 %). Min-N was lowest in biosolids dried at higher temperatures (e.g. 19.3 % at 250 °C vs 35.4 % at 70 °C). Considering only the dried biosolids, min-N was greater in WWTP-dried biosolids (50.5 %) than all of the laboratory-dried biosolids with the exception of the 70 °C-dried biosolids. Biosolid carbon mineralisation (CO₂ release) and N₂O production was also the lowest in treatments of the highest drying temperature, suggesting that this material was more recalcitrant. Overall, thermal drying temperature had a significant influence on N availability from the AD biosolids, but drying did not improve the N availability of these biosolids in any case.