Hydrothermal Carbonization of Sewage Sludge: Optimization of Operating Conditions Using Design of Experiment Approach and Evaluation of Resource Recovery Potential

Abstract

Scientific disposal of sewage sludge (SS) is a tedious task worldwide. Landfilling of the SS after centrifugation may also have adverse impacts on the environment. Therefore, the present study investigated the efficacy of hydrothermal carbonization (HTC) as pretreatment for centrifuged sewage sludge to improve resource recovery from the waste. The HTC conditions were optimized using design of experiments (DOE) approach. The runs were performed in a high-pressure reactor at temperatures of 160-240°C for 1-3 h duration without supply of air. The optimum HTC temperature and duration were 200°C and 1 h, respectively. Under these conditions, the hydrochar (HC) had higher heating value of ~13 MJ/kg. Its properties were found similar to the lignite or peat coal based on the change in atomic ratio of elements. The HTC pretreatment resulted in substantial reduction in carbon and oxygen due to reactions including decarboxylation, dehydration and oxidation (due to oxygen present in reactor headspace). The process wastewater (PWW) recovered after HTC at optimum conditions contained highest concentration of recoverable value-added compounds, humic acid (HA) and PO 4 3- -P (concentration = 20 g/l and ~0.5 g/l, respectively). Moreover, significant quantity of biogas was generated from the PWW. Hence, hydrothermal pretreatment can be a viable option for SS which promotes improved resource recovery.

Keywords: biochemical methane potential, Centrifuge sewage sludge, hydrothermal carbonization, Mass balance, resource recovery