Comparison of effects of ferric nitrate additions in thermophilic, mesophilic and psychrophilic aerobic digestion for sewage sludge

Abstract The effects of ferric nitrate additions in thermophilic, mesophilic and psychrophilic conditions were investigated in aerobic digestion for sewage sludge. The results showed that the addition of chemical could significantly enhance the volatile solid (VS) removal by shortening 7 days of stabilization time, and increasing 5.52% and 2.12% after 21 days of digestion at 55 °C, 35 °C and 15 °C comparing to the control, respectively. Higher efficiency of removing excessive volatile fatty acid (VFA) was achieved at higher temperature of digestion with ferric nitrate, which resulted in the different orders and changing tendencies of individual VFA. Thus, greater rate of biodegradation of organic substances and higher level of stabilization were obtained for sludge with ferric nitrate at higher temperature, with lower total chemical oxygen demand and soluble chemical oxygen demand. Acceleration of protein disintegration with higher pH, and lower oxidation reduction potential but lower total phosphorus was also acquired by addition of ferric nitrate. VS concentration with chemical is well fitted by an exponential decay function over a temperature range of 15–55 °C, which represented that the improvement of stabilization of sludge with ferric nitrate was almost independent of temperature, but the chemical itself.