Comparative study on hydrothermal treatment as pre- and post-treatment of anaerobic digestion of primary sludge: Focus on energy balance, resources transformation and sludge dewaterability

Abstract

Hydrothermal treatment (HTT) has been recognized as a highly efficient technology for organics decomposition and energy/nutrients recovery from waste biomass. Up to now, however, the efficiency of HTT as the pre- and post-treatment of anaerobic digestion (AD) has not been well compared and documented. In this study, the effects of HTT as the pre- (strategy I, HTT + AD) and post-treatment (strategy II, 1st AD + HTT + 2nd AD) of AD of primary sludge were evaluated based on comparative experiments in regards to energy balance, nutrients transformation, and sludge dewaterability. Results show that the optimal HTT temperature was 130 °C for strategy I and strategy II according to the maximum methane production rate (μ) estimated from Gompertz model and the net energy gain (ΔQ) calculated. Although HTT as the post-treatment of AD achieved higher total methane yield and solids reduction, the increment of methane yield was found to be similar through both strategies compared to their control counterparts (no HTT). The decomposition of insoluble organic carbon was also similar via both strategies. Insoluble nitrogen fraction was detected to be the lowest (6.8%) after HTT at 190 °C in strategy II, comparable to that after HTT at 210 °C (∼10.0%) using strategy I. The proportion of bioavailable phosphorus was found to slightly decrease with the increase of HTT temperature in strategy II, probably due to the alkaline pH in the treated digestate. The sludge dewaterability indicated by specific resistance to filtration (SRF) showed a similar trend after AD under the tested conditions of both strategies, which was remarkably improved when HTT was performed at temperatures higher than 170 °C. HTT as the pre-treatment (strategy I) was found to be more energy efficient in comparison to HTT as the post-treatment (strategy II) of AD of primary sludge. In addition, a positive energy efficiency could be achieved when the sludge solids content ≥2.2%.