Determination of the optimal dosing time of ferric nitrate on disinhibition of excessive volatile fatty acids in autothermal thermophilic aerobic digestion for sewage sludge

Abstract

Ferric nitrate has been proved to be effective on removing inhibition of excessive volatile fatty acids (VFAs) and promote stabilization of sludge in autothermal thermophilic aerobic digestion (ATAD) recently. The dosing time of Fe(NO₃)₃ had a significant impact on performance of Fe(NO₃)₃ on disinhibition of excessive VFAs in the ATAD process. The timings of Fe(NO₃)₃ additions were determined as 3rd day, 6th day, 9th day and 12th day to remove available acetic acid. The lowest concentrations of total VFAs (TVFA) and VFAs but highest microbial activity were found in the digester with Fe(NO₃)₃ dosed on the 6th day (T6). The sludge in T6 achieved stabilization 6 days earlier than that in digester without chemical addition with VS removal of 38.50% on the 15th day and VS removal of 42.74% on the 21st day. The lower TVFA concentration favored the lower NH₄⁺–N and TN contents and improved the microbial activity which contributed to the lower concentrations of SCOD and TP in supernatant.