Disinhibition of excessive volatile fatty acids to improve the efficiency of autothermal thermophilic aerobic sludge digestion by chemical approach

• Ferric nitrate was introduced to remove excessive volatile fatty acids in ATAD.
• Ferric nitrate dosed digester achieved stabilization 9 days earlier than the control.
• Total VFAs and acetic acid in ferric nitrate dosed digester were lowest.
• The lower Total VFAs favored removal of NH4+-N, SCOD and TP.
• The lower Total VFAs improved the microbial activity.

In this study, we explored a chemical approach to eliminate inhibition of excessive volatile fatty acids (VFAs) in autothermal thermophilic aerobic digestion (ATAD). Ferric nitrate, ferric chloride, potassium nitrate and potassium chloride were employed to demonstrate the combined action of ferric ion and nitrate ion. Supplementation of ferric nitrate in the sludge digestion system resulted in reducing the concentration of Total VFAs (TVFA) by round 5000 mg/L and more than 2000 mg/L of acetic acid at the end of digestion. Lower TVFA concentration contributed to faster sludge stabilization rate and the VS removal of ferric nitrate dosed digester achieved 38.18% after 12 days digestion which was 9 days in advance compared with the stabilization time of sludge in digester without chemicals addition. Lower concentrations of NH4+-N and SCOD in supernatant while higher content of TP in digestion sludge were obtained in digester with ferric nitrate added.