Phosphorus recovery from aqueous product of hydrothermal carbonization of cow manure

Efficient reclamation of phosphorus from wetland biomass waste via liquid-recirculated hydrothermal carbonization and precipitation

Hydrothermal carbonization (HTC) for the recovery of phosphorus (P) from biomass wastes has attracted considerable attention, while migration of P to the liquid phase greatly weakened P recovery efficiency and elevated the environmental risk. Therefore, a systematic scheme was proposed in this work to accomplish the complete reclamation of P from wetland plant (Ceratophyllum demersum) through coupling liquid-recirculated HTC mediated by H2O or H2SO4 with precipitation, and the migration and speciation of P during this process was determined by P K-edge X-ray absorption near edge structure, 31P nuclear magnetic resonance, and the modified sequential extraction. The P concentration in the liquid phase increased with the recirculation of HTC process water, and reached up to 550.6 mg L-1. >98.1 % of P in the recirculated liquid products was recovered in the forms of hydroxyapatite and struvite with the HTC mediums of H2O and H2SO4, respectively, without the addition of exogenous metals. In addition to the production of P compounds, P-enriched hydrochar was simultaneously obtained during this process. The HTC medium and liquid recirculation had profound impact on the hydrochar characteristics and the transformation of P. Hydroxyapatite and magnesium phosphate were the dominant P species in the hydrochars derived from H2O-mediated HTC, while FePO4 and other Ca-P species [Ca3(PO4)2 and Ca(H2PO4)2] dominated the P compounds in the H2SO4-mediated hydrochar. These results suggest that coupling liquid-recirculated HTC and precipitation could be a promising strategy for P reclamation, which could provide new insights into the P recovery from biomass waste.

Comprehensive assessment of cow manure hydrothermal treatment products for land application and energy recovery

In this study, cow manure was hydrothermally treated in a 2-litre reactor for 1 h at temperatures between 100 °C and 260 °C. Both the raw manure and the solid and liquid products of the hydrothermal treatment were characterized to understand the fate of the inorganic elements and to assess the suitability of the products for land applications and energy recovery. Satisfactory elemental balances were obtained for the organic and most inorganic elements and indicated that most inorganic elements were incorporated into the solids with lower solubility, with the exception of potassium and sodium, which were mostly solubilized in the process water; calcium and chlorine were also solubilized to a lesser extent in the process water. Elemental composition and surface functional groups showed that hydrochar produced within the hydrothermal carbonization range (180-260 °C) seemed better suited for utilization as a soil amendment than raw cow manure. The potential for energy recovery lies in the anaerobic digestion of the process water, from which higher methane yields can be obtained than from raw cow manure. Lower temperatures in hydrothermal carbonization are considered a compromise for the safe land applications of cow manure, energy recovery from the process water, and enhanced dewaterability. These findings can help to eliminate bottlenecks in the upscaling of cow manure hydrothermal treatment and promote the circular bio-economy.

Carbon Sequestration Potential of Manure-Derived Hydrochar Aided by Secondary Stabilization

Hydrothermal carbonization (HTC) is a process that produces a carbon-rich solid from wet organic materials through the application of heat and pressure. Carbonized solids, previously correlated to long-term soil stability, may be considered for carbon sequestration through incorporation into soil. Chars produced by pyrolysis are known for exceptional stability in soil, but pyrolysis is expensive when applied to wet biomass, such as manure. Chars produced from manure by HTC show considerably improved potential for carbon sequestration relative to untreated manure, although not as great as that of chars produced by pyrolysis. This study focuses on producing and evaluating chars by HTC paired with pyrolysis and different methods of chemical oxidation for long-term carbon sequestration in soil. It is shown that a two-step process of pyrolysis following HTC produces a char that outperforms those produced by either individual process (HTC or pyrolysis) in carbon yield, carbon content, and, more importantly, soil carbon sequestration potential. It was found that acid-catalyzed HTC followed by pyrolysis resulted in a char with a 13% increase in carbon yield, a 51% increase in carbon content, and an atomic O/C ratio 64% smaller than the char produced by conventional pyrolysis.