Effect of Interstage Hydrothermal Treatment on Anaerobic Digestion of Sewage Sludge: Speciation Evolution of Phosphorus, Iron, and Sulfur

Abstract

Anaerobic digestion (AD) with interstage hydrothermal treatment (HT) (i.e., AD−HT−AD) is an emerging technology for energy and nutrient recovery from sewage sludge. Yet, systematic understanding on the speciation evolution of nutrient phosphorus (P) and other redox-sensitive elements iron (Fe) and sulfur (S) during AD−HT−AD is still missing. This study investigated the coupled changes of P, Fe, and S speciation during AD−HT−AD of sewage sludge using complementary chemical extraction, microscopy, and spectroscopy analyses. Results indicated that after the first-stage AD a significant fraction of Fe was present as vivianite in the AD solids due to the microbial reduction of strengite and other Fe(III) species. During HT of the anaerobically digested solids, less vivianite but more strengite formed in the HT hydrochars with increasing HT temperature. Partial auto-oxidation of vivianite by H 2 O at high HT temperatures (155 and 185 °C) induced the formation of strengite. For the second-stage AD, more vivianite formed in AD solids as compared to that in their corresponding hydrochars from higher HT temperatures (155 and 185 °C). Regarding S speciation, both AD stages and HT favored the formation of iron sulfides. Results from this study suggest that the cycling of both P and S is strongly coupled with Fe chemistry. This study sheds light on the reaction mechanisms during AD−HT−AD of sewage sludge and the nutrient recycling and reclamation options for sewage sludge management.

Phosphorus recovery from municipal sludge-derived ash and hydrochar through wet-chemical technology: A review towards sustainable waste management

“…From a waste activated sludge with 3.6% Fe and 2.4% Al by weight, AlPO 4 was the major form (43% of total P) in hydrochar (treated at 225 • C for 4 h), whereas both aluminum phosphate (Al-P: AlPO 4 and alumina-adsorbed, 38% of total P) and ferrihydrite-adsorbed P (27% of total P) became the majority in hydrochar derived from digested sludge with a much higher Fe content (9.1%) [104]. Another study observed the dominance of strengite (FePO 4 ⋅2H 2 O, 42% of total P), Al-P (AlPO 4 and aluminaadsorbed, 34% of total P), and Ca-P (hydroxyapatite and octacalcium phosphate, 21% of total P) in hydrochar obtained at 185 • C for 4 h from digested sludge [105]. It was also found that higher temperature (>155 • C) promoted the oxidation of Fe 3 (PO 4 ) 2 to FePO 4 in hydrochar [105].…”

“…Another study observed the dominance of strengite (FePO 4 ⋅2H 2 O, 42% of total P), Al-P (AlPO 4 and aluminaadsorbed, 34% of total P), and Ca-P (hydroxyapatite and octacalcium phosphate, 21% of total P) in hydrochar obtained at 185 • C for 4 h from digested sludge [105]. It was also found that higher temperature (>155 • C) promoted the oxidation of Fe 3 (PO 4 ) 2 to FePO 4 in hydrochar [105].…”

Comparative assessment of pre‐ and inter‐stage hydrothermal treatment of municipal sludge for increased methane production

“…For the AD‐HT‐AD configuration, in order to obtain pre‐digested sludge, the first batch AD was carried out in a 9‐L glass reactor with a liquid volume of 6 L, which contained (initial values): sludge mixture (see Table 1), 10 g volatile solids (VS)/L; anaerobic inoculum, 1 g VS/L; secondary wastewater effluent, 4.15 L; and NaHCO 3 , 1.4 g/L. It should be noted that instead of pre‐reduced medium, secondary wastewater effluent was used because the digestates at the end of the incubation were used to assess the speciation of several elements (nutrients and metals) in a companion, parallel study without any interference from the medium (Wang, Zhang, Liu, et al, 2020; Wang, Zhang, Patel, et al, 2020; Wang, Zhang, Jung, et al, 2021). For the same reason, a low VS‐based inoculum‐to‐substrate ratio (ISR) of 1:10 was chosen to avoid over diluting the sludge mixture.…”

“…Secondary wastewater effluent was then added to reach a total liquid volume of 400 ml. As mentioned above, instead of pre‐reduced medium, secondary wastewater effluent was used to avoid any interference from the medium as the digestates at the end of the incubation for the BMP tests I and II were used in a companion, parallel study which assessed the speciation of several elements (nutrients and metals) (Wang et al, 2021; Wang, Zhang, Liu, et al, 2020; Wang, Zhang, Patel, et al, 2020). Preliminary experiments showed that substituting secondary wastewater effluent for medium in BMP tests did not significantly affect the rate and extent of methane production (data not shown).…”

Impacts of aluminum‐ and iron‐based coagulants on municipal sludge anaerobic digestibility, dewaterability, and odor emission

“…It is known that THP offers much better hydrolysis of organic matter such as protein (Bougrier et al, 2008; Hii et al, 2014; Jeong et al, 2016; Sapkaite et al, 2017) and thus increases ammonia and alkalinity (Penaud et al, 1999). For coagulants‐laden sludge, THP may release Fe 3+ through the decomposition of Fe 3+ ‐organic complex (e.g., Fe 3+ ‐protein) during thermal hydrolysis, leading to Fe 3+ precipitation with phosphate (Wang et al, 2020). This may result in different contaminnent removal characteristics and alter the inhibitory effects of aluminum‐/iron‐based coagulants on subsequent anaerobic digestion and sludge handling.…”