Effect of Prestage Hydrothermal Treatment on the Formation of Struvite vs Vivianite during Semicontinuous Anaerobic Digestion of Sewage Sludge

Abstract

Anaerobic digestion (AD) with prestage hydrothermal treatment (HT) is an emerging strategy for the sustainable management of sewage sludge and recovery of valuable resources. Both struvite and vivianite can be precipitated during AD process and serve as two important phases for phosphorus (P) and/or nitrogen (N) recovery. Yet the effect of HT conditions on the phase selection and precipitation of struvite vs vivianite in the subsequent AD process remains unclear. This study investigated the evolution and mineralization of P and N in sewage sludge during AD with prestage HT using complementary chemical extraction and X-ray spectroscopy characterizations. Influencing factors such as sludge type and HT temperature were explored. For raw sludge with high molar ratio of Mg/Fe (5.9), HT did not induce the formation of struvite in the HT-derived hydrochars, because the acidic pH of HT slurries was similar to that in the raw sludge. Struvite was observed during the AD of low temperature (90 and 125 °C) HT slurries due to the reaction of Mg-phosphate phases with high concentration of NH 4 + at pH > 7. For HT slurries prepared at 155 °C, the pH was always below 7 during the subsequent AD process, preventing struvite precipitation in the AD solids. For raw sludge with low molar ratio of Mg/Fe (1.1), HT at 90 and 125 °C induced the formation of vivianite in the hydrochars, as compared to the formation of strengite at 155 °C HT. In comparison with the corresponding hydrochars, more vivianite was precipitated during the subsequent AD via microbial reduction of Fe(III) species. Results from this study suggest that the formation of struvite or vivianite in the final AD solids is highly dependent on the Mg/Fe molar ratio and/or the pH of AD suspensions. Meanwhile, both HT and AD also strongly affected N speciation evolution in sewage sludge. Along with the release of large amounts of ammonia, HT favored the accumulation of quaternary-and pyrrole-N in the hydrochars, while only pyrrole-N was enriched in the AD solids. This study sheds light on the recycling and reclamation of P and N during HT-AD of sewage sludge.

Transformation Kinetics of Phosphorus and Nitrogen in Iron-Rich Sewage Sludges during Hydrothermal Treatment and Recovery of Nutrients from Process Water

“…HT at high temperature (i.e., 225 °C) and long treatment time (i.e., 4.5 h) favored the recovery of ammonia from the process water. Struvite can serve as an excellent slow-release P fertilizer for enhancing crop yield . The recovered ammonia as (NH 4 ) 2 SO 4 solution is also an efficient fertilizer.…”

“…The spectra were fitted using the software Thermo Avantage and deconvoluted into the following peaks/species: 399.5 ± 0.1 eV (amine-N), 399.9 ± 0.1 eV (amino-N), 400.4 ± 0.1 eV (pyrrolic-N), 401.4 ± 0.1 eV (quaternarytype nitrogen functionalities; quaternary-N), 51 and 402.5 ± 0.1 eV (inorganic N). 9 The N 1s XPS data processing was described in Wang et al, 52 giving the best fitting accuracies with 3% standard deviation at a 95% confidence level.…”

“…Struvite can serve as an excellent slow-release P fertilizer for enhancing crop yield. 52 The recovered ammonia as (NH 4 ) 2 SO 4 solution is also an efficient fertilizer.…”

Enhanced Phosphorus Recovery as Vivianite from Anaerobically Digested Sewage Sludge with Magnetic Biochar Addition

“…The existence of vivianite in the AD process of sludge has been demonstrated in a number of studies [5,38]. Little is known, however, about the kinetics of vivianite formation due to the difficulty in vivianite quantification.…”

“…The further hydrolysis of glucose will produce active furfural derivatives (e.g., 5-hydroxymethylfurfural (5-HMF)), which can reduce Fe(III) to Fe(II) [42]. At high HT temperature, these intermediates can further form hydrochar-like materials with low reduction ability for Fe(III) [38]. As a result, less vivianite was formed in the AD solids after HT pretreatment at 135 and 185 • C than that at 90 • C. It is noteworthy that in the HT-pretreated AD sludge at 135 and 185 • C, MBC addition significantly increased the relative abundance of total vivianite from 39.35% to 53.28% and from 31.45% to 50.24%, respectively, much higher than that in M1T1.…”