Preliminary Study on the Treatment Efficiency of Pasteurized Lime Thermal Alkaline Hydrolysis for Excess Activated Sludge and Reduction of Tetracycline Resistance Genes

Abstract

Thermal alkaline hydrolysis is a common pretreatment method for the utilization of excess activated sludge (EAS). Owing to strict environment laws and need for better energy utilization, new methods were developed in this study to improve the efficiency of pretreatment method. Direct thermal hydrolysis (TH), pasteurized thermal hydrolysis (PTH), and alkaline pasteurized thermal hydrolysis (PTH + CaO and PTH + NaOH) methods were used to treat EAS. Each method was compared and analyzed in terms of dissolution in ammonium nitrogen (NH 4 + -N) and soluble COD (SCOD) in EAS. Furthermore, the removal of tetracycline resistance genes (TRGs)and class 1 transposon gene intI1 from EAS was investigated. The NH 4 + -N and SCOD concentrations in EAS treated by PTH were 1.24 and 2.58 times higher than those of TH. However, the removal efficiency of total TRGs and intI1 between the groups was comparable. The SCOD concentration of the PTH + NaOH group was 4.37 times higher than that of the PTH group, and the removal efficiency of total TRGs was increased by 9.52% compared with that by PTH. The NH 4 + -N and SCOD concentrations of the PTH + CaO group could reach 85.04% and 92.14% of the PTH + NaOH group, but the removal efficiency of total TRGs by PTH + CaO was 19.78% lower than that by PTH + NaOH. Thus, to reduce the financial cost in actual operation, lime (CaO) can be used instead of a strong alkali (NaOH), and pasteurized steam at 70°C instead of conventional high-temperature heating to treat EAS. This study provides a reference for the development of alkaline hydrolysis under moderate temperatures along with the removal of TRGs in EAS.