Investigations into Integrated Mild Acid Pre-treatment and Pyrolysis for Biosolids Conversion to Biochar and Value-added Chemicals

Biosolids (stabilised sewage sludge) are the final solid residues of the wastewater treatment process. While substantial quantities of biosolids are traditionally reused in agricultural land, the increasing concerns with emerging contaminants and stringent environmental regulations may restrain the land application route. Pyrolysis is a promising thermal treatment technique for upcycling biosolids into a range of products (biochar, bio-oil, and syngas) with the potential for contaminant destruction and resource recovery. However, the suitability of biosolids as pyrolysis feedstock can be limited by their composition, typically containing 20–50 wt% inorganic matter, and the conventional ‘single pot’ pyrolysis approach may not be suitable for processing highly heterogeneous materials such as biosolids. Therefore, the research presented in this thesis explores an integrated approach to biosolids processing aiming to improve biosolids grade for land application and pyrolysis conversion to biochar and value-added chemicals. The proposed integrated process includes (i) mild acid pre-treatment of biosolids for the selective removal of heavy metals (HMs) and alkali and alkaline earth metals (AAEMs) without degrading the organic matter, (ii) closed-loop hydrometallurgical treatment process towards the production of least contaminated grade biosolids, and (iii) fluidised bed pyrolysis and co-pyrolysis of pre-treated biosolids for the production of biochar and platform chemicals. The first investigation involved a carefully designed box-Behnken optimisation study of the sulfuric acid pre-treatment process through which 3% (v/v) H2SO4, 25 ℃, 600 rpm, and 30 min were identified as the optimum pre-treatment conditions for selective metal extraction with least alterations of the organic matrix. Under this condition, the total HMs concentration and ash content were reduced by 80% and 50%, respectively, accompanied by a ~10% increase in volatile matter and a 66% increase in fixed carbon content. Despite these changes in the proximate compositions of the biosolids, the ultimate properties were largely similar, suggesting that the pre-treatment process was mainly selective to inorganic removal through demineralisation mechanisms. Extensive physicochemical characterisations jointly support the conclusion that the organic structure of the biosolids was not remarkably altered following the acid pre-treatment under the conditions investigated. The kinetic parameters of the process were derived through the shrinking core models, and the mixed kinetic model comprising product layer diffusion and surface chemical reaction best described the acid pre-treatment process. The dominating process mechanism was the acid dissolution of metal-containing components and the ion exchange of metal cation with H+ from H2SO4. The findings from the first study were used to launch an investigation into the feasibility of a hydrometallurgical process for the removal and recovery of valuable HMs ions from biosolids towards the production of the least-contaminant grade biosolids. Sulfuric acid was the most suitable lixiviant for metal extraction, and the process was remarkably influenced by the solid-to-liquid ratio, as indicated by the stoichiometry of the metal desorption process. For the first time, the continuous recycling of the leachate stream was explored to advance the acid leaching process for metal extraction. The extraction performance of the spent leachate stream was only attractive at 5% solids loading. The rapid build-up of Fe(III) concentration, increase in solution pH beyond 2, and the subsequent precipitation of Fe3+ limited the recyclability strength of the leachate stream at 10% solids loading. Following the concentration of the leachate stream via recycling, the dissolved metals were recovered in staged alkali precipitation and biochar adsorption. Adding H2O2 prior to NaOH staged precipitation efficiently purified the leachate stream with the recovery of Al/Fe/P rich metal sludge at the first stage, and the second stage precipitation had an overall recovery of 50-95% Zn/Mn/Cu rich metal sludge. The process flow sheet was developed, mass balances were performed, and the fate of organic nutrients and PFAS in the process streams were assessed. The treated biosolids produced through the hydrometallurgical process developed in this work were limited only by Cu concentration to become C1-grade biosolids (C1-grade being the least contaminant grade biosolids as prescribed by Victoria EPA biosolids guidelines), and the organic nutrients were modestly preserved. The last investigation in this thesis was to study the pyrolysis and co-pyrolysis of acid-treated biosolids and understand the impact of pre-treatment on product distribution and product properties under a wide range of temperatures (300–700 ℃) in a fluidised bed reactor. Pre-treatment improved organic matter devolatilisation at all temperatures to increase bio-oil yield and slightly decrease biochar yield relative to the raw biosolids. However, demineralisation inhibited gas production due to the inferior catalytic effect of ash components in treated biosolids. Notably, at 700 ℃, treated biosolids biochar had higher surface area (107 m2/g), calorific value (15 MJ/kg), fixed carbon (35 wt%), and organic carbon retention (66% dry ash-free) compared to the raw biosolids biochar with surface area (56 m2/g), calorific value (9 MJ/kg), fixed carbon (20 wt%) and organic carbon retention (50%). Pre-treatment also improved the microporous structure development of the biochar and substantially decreased the HMs total and bioavailable concentration by at least 60% relative to the raw biosolids biochar. The chemical components distribution in the bio-oil was impacted by pre-treatment, and anhydrosugars, phenolics, furans, and aromatic hydrocarbons were enhanced by pre-treatment through the inhibition of AAEMs-catalysed dehydration, fragmentation, and crosslinking pyrolytic reactions. Hydrogen yield in the pyrolysis gas was promoted by the passivation of AAEMs activities and dehydration reactions of the acidic metal sulfates salt. Blending biosolids with wheat straw and their treated variants further enhanced the pyrolysis platform to switch between gas profile and bio-oil chemical value while improving biochar physiochemical and textural properties. Notably, combining pre-treatment and co-pyrolysis strategies drastically reduced the HMs concentration and bioavailability in the biochar. However, biosolids pre-treatment alone gave the highest HMs reduction and stability during pyrolysis. The research in this thesis has demonstrated through comprehensive experimental investigations that biosolids pre-treatment is a viable technique for improving biosolids quality for unrestricted land application and for pyrolysis conversion to value-added products through the co-removal of HMs and ash-forming elements.