Two birds with one stone: The multiple roles of hydrothermal treatment in dewatering municipal sludge and producing value-added products

Phosphorus release and realignment in anaerobic digestion of thermal hydrolysis pretreatment sludge - Masking effects from high ammonium

Waste activated sludge (WAS) is a significant phosphorus (P) repository, and there is a growing interest in P recovery from WAS. Typically, the commercial technology for treating WAS involves thermal hydrolysis pretreatment (THP) coupled with anaerobic digestion (AD). However, there is ongoing debate regarding the transformation and distribution of P throughout this process. To address this, a long-term THP-AD process was operated in this study to comprehensively investigate P transformation and distribution. The results revealed that a substantial biodegradation of dissolved organic nitrogen (DON) raised the pH of the digestate to 8.3 during the AD process. This increased pH facilitated the dissolution of Al, leading to a reduction of 6.92 mg/L of NaOH-P. Simultaneously, sulfate reduction contributed to a decrease of 11.04 mg/L of Bipy-P in the solid. However, the reduction of Bipy-P and NaOH-P in the solid did not result in an improved P release to the supernatant. Conversely, a decrease of 23.60 mg/L P in the aqueous phase was observed after anaerobic digestion. The disappeared P was primarily precipitated with Mg and Ca, driven by the increased pH, and it contributed to the increase of HCl-P in the solid from 107.80 to 144.52 mg/L. These findings were further confirmed by results obtained from scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and solid-state 31P nuclear magnetic resonance (NMR) spectroscopy. This study provides valuable insights into the mechanisms of P transformation during THP-AD process that is nearly opposite from conventional AD system.

Effects of chitosan and rice husk powder on thermal hydrolysis-anaerobic digested sludge conditioning: Dewaterability and biogas slurry fertility

To enhance the dewaterability of anaerobic digested sludge and to make full use of the biogas slurry. This study set up five sludge conditioning methods: polymeric ferric sulfate, polymeric aluminum chloride, cationic polyacrylamide, chitosan, and chitosan combined with rice husk powder. Their effects on the dewaterability of thermal hydrolysis-anaerobic digested sludge, bacterial community, and biogas slurry fertility were studied to find a non-toxic and non-risk dewatering technology for the environment and biogas slurry. Compared with that of the control group, moisture content, normalization capillary suction time, and specific resistance to filtration were reduced by 12.8%, 97.7%, and 82.9%, respectively. Chitosan enlarges the sludge flocs and forms complexes with proteins, disrupting the structure of the extracellular polymeric substances, thereby exposing more hydrophobic groups and reducing the hydrophilicity of the sludge. The subsequent addition of rice husk powder enhances the adsorption of hydrophilic substances and provides a stronger drainage channel for the sludge. In addition, the biogas slurry obtained by this conditioning method used as a fertilizer increased the dry weight and fresh weight of corn seedlings by 59.3% and 91.0%, respectively. And the total chlorophyll content increased by 84.6%. Pearson's correlation analysis showed that chitosan and rice husk meal had no toxic effect on the biogas slurry compared to the other three flocculants. The results showed that the combined treatment of chitosan and rice husk powder resulted in the best dewaterability. Overall, chitosan combined with rice husk powder is a green dewatering technology with great potential for anaerobic digested sludge dewatering and biogas slurry recycling.