Enhanced humic-like acid formation from waste activated sludge during alkaline thermal hydrolysis - Dual role of reducing sugars.

Recovering humic-like acids (HLAs) from waste activated sludge (WAS) using alkaline thermal hydrolysis (ATH) offers a promising route for simultaneous sludge reduction and resource recovery. However, the limited content of readily reactive organic compounds in WAS constrains HLA yield. In this study, fruit waste (FW) as a carbohydrate-rich waste was introduced as a co-substrate to enhance HLA formation. Results showed that HLA yield increased by 130.3%-188.8%, reaching a maximum of 0.53 ± 0.02 g/g volatile solid (VS) of initial WAS at a WAS-to-FW VS ratio of 1:3 under moderate ATH conditions (pH 12.0, 130 °C, 1 h). Structural analysis showed that the generated HLA was characterized by a low degree of aromatic condensation and a high abundance of branched aliphatic side chains. Fructose and glucose accounted for the majority of the total reducing sugars released from FW. Model reactions using bovine serum albumin and representative reducing sugars demonstrated that reducing sugars can directly contribute to HLA formation through Maillard-type reactions. More importantly, the additional HLA formation was observed during ATH treatment of WAS and FW mixture, implying that reducing sugars may play a broader role in promoting organic matter transformation and subsequently enhancing HLA formation. Overall, this work elucidates a saccharide-assisted humic-like acid formation mechanism and proposes an effective strategy for coupling WAS treatment with the valorization of sugar-rich biowastes.