Fenton pre-treatment of secondary sludge to enhance anaerobic digestion: Energy balance and greenhouse gas emissions

Laboratory scale experiments on Fenton pre-treatment (FPT) application to the secondary sludge (SS) were carried out to determine its effect on the anaerobic digestion (AD) efficiency. The effect of FPT on AD was evaluated by computing energy balance and corresponding greenhouse gas (GHG) emissions. Four treatment processes were evaluated: (i) Process 1 − AD of SS (control); (ii) Process 2 − AD of the Fenton pre-treated SS; (iii) Process 3 − Process 2 + Fenton pre-treatment of the digestate produced in Process 2, followed by AD (iv) Process 4 − Process 1 + Fenton pre-treatment of the digestate generated in Process 1, followed by AD. The results revealed that the cumulative methane production was increased by 15% in Fenton pre-treatment compared to the control. The methane production rate in the control (Process 1) was 430 m3 CH4/Mg VS degraded and it was 496 m3 CH4/Mg VS degraded for Fenton pre-treated SS (Process 2). The net energy was increased 3.1 times with Fenton pre-treatment (Process 2) when compared to the control (Process 1). The net energy for Process 4 increased 2.5 times compared to Process 1. The GHG emissions were reduced in Fenton pre-treatment (Process 2) compared to the control (Process 1). The lowest GHG emissions of 0.128 Mg CO2/Mg of total dry solids (TDS) were observed in the Fenton pre-treated sludge (Process 2). Thus Fenton pre-treatment process (mainly Process 2) is a cost-effective, due to energy recovery and decreased GHG emissions.