In situ transesterification of wet sewage sludge via hydrothermal process: Biodiesel production and residue utilization

Abstract

Recent studies show the feasibility of choosing sewage sludge as feedstock to produce biodiesel. However, low yield of biodiesel as using wet sludge directly, high cost of the pretreatment of sludge drying, and poor cognition about residues utilization, lead to the urgent need of new processes. In this study, hydrothermal process (HP) was introduced for biodiesel production by in situ transesterification. A favorable condition was determined as methanol dosage of 20 g g−1 dried sludge, and 2% sulfuric acid at 160 °C for 4 h, where the biodiesel yield reached 8.63%. HP also shows the ability to break the distinction of biodiesel yield derived from wet or dried sludge, since a similar yield of 8.71% was obtained when using dried sludge as feedstock. Furthermore, solid and liquid residues from HP could be applied as adsorbent for organic pollutant removal and nutrient substrate for yeast cultivation, respectively. A maximum adsorption capacity of 75.52 mg g−1 for Congo red (CR) was obtained using the solid residue as adsorbent. A 4.6 g L−1 biomass of Candida utili and 35.33% of protein in yeast cell were harvested using the liquid residue as nutrient substrate. This study not only demonstrates the feasibility of applying HP for biodiesel production from wet sludge, but also brings new insights about full utilization of wet sludge.