Insights into thermal hydrolyzed sludge liquor - Identification of plant-growth-promoting compounds

This study proposes a new path to utilize thermal hydrolyzed sludge (TH sludge) as fertilizer given high value chemical compounds that can promote plant growth were identified in the liquid fraction of TH sludge (TH liquor). Together with micro- and macro-nutrients released/synthesized during thermal hydrolysis, the feasibility of using TH liquor as organic fertilizer was evaluated. Besides high contents of N, P and K, total free amino acids (FAAs) and plant-growth-promoting FAAs (including glutamic acid, leucine and cystine) also presented in high concentration (4.98–6.48 and 1.12–2.73 g/100 g) in the TH liquor. For the first time, phytohormone compound, indole-3-acetic acid, was observed and the content was the highest in TH liquor with 165 °C treatment (165 °C TH liquor). Meantime, 165 °C TH liquor did not have negative impact on the growth of soil microbes, and this product, instead, demonstrated stimulating effect on the plant growth. These results suggest that 165 °C TH liquor has a great potential to be an organic fertilizer. The remaining solids of TH sludge could be converted to valuable biochar. The holistic approach of using TH liquor as organic fertilizer and producing biochar could realize nearly zero-waste discharge in sludge management.