Copper stressed anaerobic fermentation: biogas properties, process stability, biodegradation and enzyme responses

The effect of copper (added as CuCl₂) on the anaerobic co-digestion of Phragmites straw and cow dung was studied in pilot experiments by investigating the biogas properties, process stability, substrate degradation and enzyme activities at different stages of mesophilic fermentation. The results showed that 30 and 100 mg/L Cu²⁺ addition increased the cumulative biogas yields by up to 43.62 and 20.77% respectively, and brought forward the daily biogas yield peak, while 500 mg/L Cu²⁺ addition inhibited biogas production. Meanwhile, the CH₄ content in the 30 and 100 mg/L Cu²⁺-added groups was higher than that in the control group. Higher pH values (close to pH 7) and lower oxidation-reduction potential (ORP) values in the Cu²⁺-added groups after the 8th day indicated better process stability compared to the control group. In the presence of Cu²⁺, the degradation of volatile fatty acids (VFAs) and other organic molecules (represented by chemical oxygen demand, COD) generated from hydrolysis was enhanced, and the ammonia nitrogen (NH₄⁺-N) concentrations were more stable than in the control group. The contents of lignin and hemicellulose in the substrate declined in the Cu²⁺-added groups while the cellulose contents did not. Neither the cellulase nor the coenzyme F₄₂₀ activities could determine the biogas producing efficiency. Taking the whole fermentation process into account, the promoting effect of Cu²⁺ addition on biogas yields was mainly attributable to better process stability, the enhanced degradation of lignin and hemicellulose, the transformation of intermediates into VFA, and the generation of CH₄ from VFA.