Effects of reactive oxygen species scavengers on thermophilic micro-aerobic digestion for sludge stabilization

In thermophilic digestion systems, sludge stabilization may be adversely impacted by high concentrations of ammonia nitrogen or the rapid accumulation of fatty acids; however, few studies have focused on the mitigation of the inhibition of reactive oxygen species (ROS). In this study, fulvic acids or tea polyphenols were introduced to a thermophilic digestion system and the effects of ROS scavengers on sludge stabilization were investigated. As fulvic acids or tea polyphenols were added to sludge, they reacted with metal cations, such as Cu²⁺ and Zn²⁺, to form stable complexes that enabled active metal ions to be transported into cells to enhance the oxidase activities. Therefore, the digestion systems presented a lower O₂^•- content compared with that of a control group with no additive. Both fulvic acids and tea polyphenols mitigated the adverse effects of the ROS and enhanced the reduction of volatile solids (VS), however, fulvic acids better facilitated the sludge stabilization. The optimal dosage was 0.3% of the total solids as fulvic acids were added to the sludge every 48 h. The VS reduction in the digester reached 38.2% at 14 d, which was clearly higher than that in the control group. The pathway of ROS scavengers affecting sludge stabilization was proposed, and it may be helpful to gain deeper insight into the characteristics of thermophilic digestion processes as well as the mechanism of sludge stabilization.