Phosphorus recovery from municipal sludge-derived hydrochar: Insights into leaching mechanisms and hydroxyapatite synthesis.

Hydrothermal liquefaction has the potential to exploit resources from municipal sewage sludge. It converts most organics into a liquid biofuel (biocrude), concentrates P in the solid residue (hydrochar), and consequently enables its efficient recovery. This study thoroughly evaluated the effects of extraction conditions on P and metal release from hydrochar by nitric acid. Among assessed factors, acid normality (0.02-1 N), liquid-to-solid ratio (5-100 mL/g), and contact time (0-24 h) had positive effects while decreasing eluate pH (0.5-4) improved leaching efficiencies of P and metals. Notably, eluate pH played a dominant role in P leaching and pH < 1.5 was crucial for complete extraction. P and metal leaching from hydrochar have strong interactions and their leaching mechanism was identified as product layer diffusion using the shrinking core model. This suggests that the leaching efficiency is susceptible to agitation and particle size but not temperature. Using 10 mL/g of 0.6 N HNO3 for 2 h was considered the best extraction condition for efficient P leaching (nearly 100%) and minimization of cost and contaminants (heavy metals). Following extraction, adding Ca(OH)2 at a Ca:P molar ratio of 1.7-2 precipitated most P (99-100%) at pH 5-6, while a higher pH (13) synthesized hydroxyapatite. The recovered precipitates had high plant availability (61-100%) of P and satisfactory concentrations of heavy metals as fertilizers in Canada and the US. Overall, this study established reproducible procedures for P recovery from hydrochar and advanced one step closer to wastewater biorefinery.

New insights on mercury abatement and modeling in a full-scale municipal solid waste incineration flue gas treatment unit.

Modeling approaches are generally used to describe mercury transformations in a single step of flue gas treatment processes. However, less attention has been given to the interactions between the different process stages. Accordingly, the mercury removal performance of a full-scale solid waste incineration plant, equipped with a dry flue gas treatment line was investigated using two complementary modeling strategies: a thermochemical equilibrium approach to study the mercury transformation mechanisms and speciation in the flue gas, and a kinetic approach to describe the mercury adsorption process. The modeling observations were then compared to real-operation full-scale data. Considering the typical flue gas composition of waste incineration facilities (high concentrations of HCl compared to Hg), it was found that a process temperature decrease results in better mercury removal efficiencies, associated with a higher oxidation extent of Hg in HgCl2, and the enhancement of the sorbent capacity. Improvements can also be attained by increasing the sorbent injection rate to the process, or the solid/gas separation cycles. An empirical correlation to predict the mercury removal efficiency from the main operating parameters of dry flue gas treatment units was proposed, representing a useful tool for waste incineration facilities. The presented modeling approach proved to be suitable to evaluate the behavior of full-scale gas treatment units, and properly select the most adequate adjustments in operating parameters, in order to respect the increasingly constraining mercury emissions regulations.