Anaerobic co-digestion of hydrothermal liquefaction of aqueous phase with municipal sludge and dewatering centrate

Abstract

For integrating hydrothermal liquefaction (HTL) of sludge treatment to municipal wastewater treatment plants, this study demonstrated the feasibility of thermophilic (55 °C) anaerobic co-digestion of liquid product of HTL (HTL aqueous) with municipal sludge and its dewatering centrate. Seven semi-continuously fed digesters were operated for a total of 420 days. To obtain an optimal mixing ratio of the HTL aqueous with other co-substrates, a wide range of parameters were tested on influent and effluent of the digesters with 0.0%–43.3% of the chemical oxygen demand (COD) loading rate of the co-digesters contributed by HTL aqueous. HTL aqueous was generated from mixed sludge (20% total solids) at 350 °C, 170 bar, and 20 min retention time. Thermophilic co-digesters with HTL aqueous contributing to 12.0%, 23.1%, and 43.3% of the total COD load were fully inhibited after 42, 29, and 15 days, respectively, while co-digesters with 1.21% and 2.74% of total COD contribution by HTL aqueous operated without any inhibition over 180 days. Biogas production in the co-digester with 4.89% COD contribution from HTL aqueous was also inhibited after 95 days of operation. This co-digester experienced a complete process failure for 42 days, followed by 218 days of partial (76%) steady inhibition before fully recovered. This indicates that thermophilic cultures can acclimatize to inhibitors in HTL aqueous to a certain extent. High levels of nitrogen heterocyclic compounds, such as pyridines and pyrazines, in HTL aqueous were determined to be the cause of inhibition in co-digesters fed with high volume of HTL aqueous. This study demonstrated the feasibility of valorizing HTL aqueous from mixed sludge in thermophilic co-digesters with a COD contribution as high as 4.89% from HTL aqueous. • Thermophilic co-digestion fed HTL aqueous (at 350 °C/20 min), mixed sludge, centrate. • Co-digesters fed 12–43.3% of COD load by HTL aqueous had process failure in 42 d. • Co-digesters fed 1.21 and 2.74% of COD load from HTL aqueous had no inhibition sign. • Co-digester fed 4.89% of COD load by HTL aqueous had acute inhibition but recovered. • HTL aqueous N-heterocyclic compounds were determined to be the cause of inhibition.

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