Biochar production from organic wastes is a promising way to achieve both waste minimization and resource recovery and relies on the pyrolysis process fixing carbon in solid-state. As a result, the fixation of carbon reduces the greenhouse gas emission, while the produced biochar can be effectively reused as a resource in the agricultural industry, building sector, water treatment facilities, etc. However, biochar production from waste activated sludge is limited due to high energy requirements and low product quality. A “seawater-in-sludge” approach was first proposed in this thesis to modify the biochar properties produced from sewage sludge. The biochar yield was found to be increased by 10% when more alkali and alkaline earth metals (AAEM) species were incorporated into the sludge m...[ Read more ]

Biochar production from organic wastes is a promising way to achieve both waste minimization and resource recovery and relies on the pyrolysis process fixing carbon in solid-state. As a result, the fixation of carbon reduces the greenhouse gas emission, while the produced biochar can be effectively reused as a resource in the agricultural industry, building sector, water treatment facilities, etc. However, biochar production from waste activated sludge is limited due to high energy requirements and low product quality. A “seawater-in-sludge” approach was first proposed in this thesis to modify the biochar properties produced from sewage sludge. The biochar yield was found to be increased by 10% when more alkali and alkaline earth metals (AAEM) species were incorporated into the sludge matrix from seawater. The nitrogen-sorption isotherm indicated a higher specific surface area of biochar (480.3 m²/g) obtained from the minerals loaded sludge at high temperatures, showing activation effects. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) revealed that minerals activation increased the graphitization degree of the carbon structure and decreased the surface functional groups. This lead to lower resistance and a higher capacitance of 113.9 F/g of the carbon electrodes (60% higher than the control groups without seawater). Despite the merits of AAEM cations on biochar surfaces, the roles of them during sludge pyrolysis remain unclear. Granulation of conventional flocculent sludge is a natural enrichment of AAEM cations and the formed granular sludge would exhibit distinct pyrolysis behaviour from flocculent sludge. In such contexts, this study investigated the process kinetics and mechanisms of both flocculent and granular sludge in the second part. Granular sludge devolatilized from 210℃ was more thermally stable than flocculent sludge whose devolatilization started from 170 ℃. However, the activation energy required for granular sludge decomposition was around 20% lower than that of flocculent sludge. Detailed characterization revealed that the catalytic effect of AAEM species originating from the seawater inside granular sludge was responsible for the lower activation energy requirement and the alteration of the reaction pathways. The incorporated AAEM cations could act as activation agents and catalysts during the sludge biochar production process but how they entered the sludge remains unclear. In the third part of this thesis, cationic interactions between AAEM species in seawater and sludge polymer were understood at the mechanism level. Both Ca²⁺ and Mg²⁺ were concentrated mostly in the extracellular part of the sludge and a surface complexation model could be used to explain the uptakes of these metal cations by sludge polymer. Granular sludge showed the highest Ca²⁺ and Mg²⁺ uptake of 48.7 and 25.5 mg/g, respectively. The uptake of Ca²⁺/Mg²⁺ was not affected by the increased Na⁺ concentration but competition between Ca²⁺ and Mg²⁺ for surface sites was observed in multi-element systems. The total Gibbs interaction energy analysis proved that the dominant role was Lewis acid-base interaction and the higher affinity for Ca²⁺ over Mg²⁺ can be attributed to the closer linkage between the carboxyl group and Ca²⁺ than Mg²⁺.