Complete Digestion of Sewage Sludge Using Wet-Oxidation and Subsequently Simultaneous Removal of Residual NH₄⁺ and Volatile Fatty Acids by Biological Treatment

Abstract

Thickened sewage sludge consisting of primary and surplus sludges with a total solids of 4% was treated by wet-oxidation without catalysts. The digestion efficiency of VSS was 94 to 96% at 80 kgf (cm²G) ^-1 and 250°C for holding times ranging from 30 to 120min. However, TOC and NH₄⁺ in the liquid generated by the wet-oxidation pmcess was as high as 3, 845 mg l^-1 and 1, 410 mg l^-1, respectively, even at a holding time of 120 min.The organic matter containing in the wet-oxidized wastewater could be removed in the biological denitrification with the supplement of NaNO₃.The TOC and VFA removal efficiencies in the biological denitrification were 65% and 100%, respectively, at a TOC loading rate of 5.0g l^-1 d^-1.The NH₄⁺ remained in the denitrified wastewater was treated in the biological nitrification after two-times dilution and the NH₄⁺ was oxidized to NO₃^- completely, at the same time the TOC removal efficiency was 36 to 50%.On the basis of these results, NH₄⁺ and organic matter in the two-times diluted wet-oxidized wastewater could be removed simultaneously in a biological denitrification and nitrification process with a recirculation, resulted in reduction of the supplementation amount of NO₃^- and HCO₃^- necessary for denitrification and nitrification, respectively. Moreover, the supplementation of NO₃^- and HCO₃^- could be reduced to zero by an improved process, namely, installing an activated sludge tank between the denitrification and the nitrification reactors.