Post-thermal hydrolysis and centrate recirculation for enhancing anaerobic digestion of sewage sludge

Abstract

Thermal hydrolysis process (THP) is widely used as the pretreatment for sludge anaerobic digestion (AD), but with limited improvement of biogas production, dewaterability, and high energy consumption. In this study, an improved AD configuration based on post-THP treatment (AD-THP) was developed to overcome these limitations. The digested sludge was thermally hydrolyzed and then dewatered directly, the centrate was recirculated for re-digestion with raw sludge. Batch and semi-continuous experiments were conducted to investigate the differences between pre- and post-THP treatment and compare the process performances of three configurations, including conventional anaerobic digestion (CAD), thermal hydrolysis pretreatment (THP-AD), and post-treatment (AD-THP). Results showed that post-THP could further dissolve much SCOD from the digested sludge by disintegrating the recalcitrant organic matter of raw sludge and EPS generated during digestion, resulting in further improvement of methane production. The SCOD solubilization during THP post-treatment was depended on the degradation extent of digested sludge. The main difference between pre- and post-treatment was that the pH changed towards the opposite directions due to the different properties of raw and digested sludge. The maximum specific methane production and volatile solid reduction of the AD-THP system were 344.25 mL CH4/g VSadd and 62% during the CSTR experiments, respectively, significantly higher than those of THP-AD and CAD configurations. In addition, the dewaterability of THP treated sludge was always significantly better than the digested sludge, whether for THP pretreatment or post-treatment. The improved AD-THP configuration could be a promising alternative for the upgrading of sludge treatment.