Abstract
The adoption of prolonged solid retention times during the biological treatment of urban wastewaters is a well-known strategy to reduce sewage sludge production. However, it also results in the production of a biological sludge with low percentages of biodegradable organic matter, also characterized by high humification degrees, which may hamper the anaerobic digestion treatment aimed at sludge stabilization. To accelerate the hydrolytic stage, the application of microaerobic conditions during the anaerobic digestion of low-biodegradable sewage sludge was investigated in this study. In particular, six bio-methanation tests of a real sewage sludge were carried out, introducing air in the bioreactors with doses ranging between 0 and 16.83 L air/kg VSin d, in order to evaluate the air dosage that optimizes the biomethane production and organic matter degradation. Notably, the lower air loading rates investigated in this study, such as 0.68 and 1.37 L air/kg VSin d, led to an increase in methane production of up to 19%, due to a higher degradation of total lipids and proteins. In addition, these microaerobic conditions also resulted in a decrease in the sludge humification degree and in lower volatile fatty acid accumulation.
Highlights
Accepted: 31 August 2021Sewage sludge (SS) is largely produced worldwide, and is the residue of the biological, chemical, and physical treatment of wastewater
The SS used in this study was collected twice from the pre-thickening unit of a municipal wastewater treatment plants (WWTPs) located in Nola (Italy)
bio-methanation tests (BMTs) were conducted in triplicate by dosing different air volumes (0–200 mL) and air loading rates (ALRs) (0–16.83 L air/kg VSin d) (Table 2) for 30 days in order to investigate their effect on the anaerobic digestion (AD) process
Summary
Sewage sludge (SS) is largely produced worldwide, and is the residue of the biological, chemical, and physical treatment of wastewater. WWTP operators often apply a long (>20 days) solid retention time (SRT) to the conventional activated sludge (CAS) system, with the aim of minimizing the amount of excess sludge transferred to the sludge streamline. This strategy results in the production of SS with a low percentage of biodegradable OM and a low volatile solids (VS) to total solids (TS) ratio (VS/TS). AD is the traditional method used in WWTPs to reduce the biodegradable fraction of SS (stabilization) and to produce energy through the partial conversion of OM into methane (CH4 ). Bio-methanation tests (BMTs) were performed at laboratory scale to investigate the effects of microaeration on biomethane productivity and the sludge degradability of municipal SS with a lowbiodegradable OM
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