Abstract: Abstract:Converting excess sludge to acetic acid is considered a promising and feasible path. This process not only achieves the objectives of controlling pollution and reducing sludge volumes, but also efficiently explores the resource of organic substances in sludge. Anaerobic microorganisms involved in the fermentation of acetic acid have inherent requirements for trace metal elements, but this is not often reported in literature. The effects of Fe²⁺, Co²⁺, Cu²⁺, Zn²⁺ and Mn²⁺ on the production of acetic acid were investigated in this study. The experimental results showed that these metal elements had the potential to enhance the production of acetic acid, with the influence as follows:Co²⁺>Mn²⁺>Fe²⁺>Zn²⁺>Cu²⁺. However, the production of acetic acid decreased obviously while the mass fractions of Fe²⁺, Co²⁺, Cu²⁺, Zn²⁺ and Mn²⁺ were more than 0.1000%, 0.0060%, 0.0020%, 0.0040% and 0.1000%, respectively. Response surface was then employed to optimize the mixing doses of various metal elements for acetic acid production in the case that the mass ranges were set as 0.0400%-0.1500%(Fe²⁺), 0.0020%-0.0080%(Co²⁺), 0.0005%-0.0060%(Cu²⁺), 0.0005%-0.0100% (Zn²⁺) and 0.0400%-0.1500%(Mn²⁺). The optimal mixing conditions were 0.0900% of Fe²⁺, 0.0900% of Mn²⁺, 0.0050% of Co²⁺, 0.0050% of Zn²⁺ and 0.0035% of Cu²⁺. Under these conditions, the concentration of acetic acid increased by 61.8% and reached up to 3452 mg/L. Moreover, the result of fitting between the model prediction (3475 mg/L) and the experiment under the optimal conditions came up to 99.34%, which indicated that the established polynomial regression model was feasible.