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 have serious adverse impacts on the environment. The present study investigated efficacy of hydrothermal carbonization (HTC) as pretreatment for the centrifuged sewage sludge with an aim to improve resource recovery. In this study, both solid and liquid fractions recovered after the HTC were subjected to the detailed analysis for examining the opportunities for recovery of value-added chemicals and biogas. The HTC conditions were optimized using design of experiments (DOE) approach. The runs were performed in a high-pressure reactor at 160–240 °C temperatures and total pressures of 6.1–34.2 kg/cm2 for 1–3 h duration without supply of air. The optimum HTC temperature and duration were found to be 200 °C and 1 h, respectively. Under these conditions, the dried hydrochar (HC) had higher heating value of ~13 MJ/kg. Its properties were found similar to that of the sub-bituminous coal based on the atomic ratio of elements. The HTC pretreatment resulted in substantial reduction in carbon and oxygen due to decarboxylation, dehydration and oxidation (due to oxygen present in reactor headspace) reactions. The process wastewater (PWW) recovered after HTC at the optimum conditions had high concentration of the recoverable value-added compounds such as humic acid (HA) and PO43--P (concentration = 20 g/l and ~0.5 g/l, respectively). Moreover, significant quantity of biogas could be generated from the PWW. Hence, hydrothermal pretreatment can be considered a potential option for SS valorization.