Coupling hydrothermal carbonization with anaerobic digestion: an evaluation based on energy recovery and hydrochar utilization

Document Type : Research Paper


1 Doctoral School of Environmental Sciences, University of Szeged, H-6724 Szeged, Dugonics ter 13, Hungary.

2 Gábor Dénes College, Fejér Lipót u. 70, 1119, Budapest, Hungary.

3 Department of Process Engineering, Faculty of Engineering, University of Szeged, H-6725 Szeged, Moszkvai krt. 9, Hungary.


This work evaluates the effect of hydrothermal carbonization (HTC) as a pretreatment and post-treatment technique to anaerobic digestion (AD) of dairy sludge. HTC's effect on AD was evaluated based on energy recovery, nutrient transformation, and hydrochar utilization. The first approach was executed by performing HTC under a range of temperatures before mesophilic AD. HTC optimal pretreatment temperature was 210 °C for 30 min residence time. HTC pretreatment significantly increased the methane yield potential by 192%, the chemical oxygen demand removal by 18%, and the sludge biodegradability during AD by 30%. On the other hand, the application of HTC after AD (post-treatment) increased the total energy production, i.e., in addition to methane, a hydrochar with a caloric value of 10.2 MJ/kg was also obtained. Moreover, HTC post-treatment improved the steam gasification performance of the AD digestate. From the fertilizer quality point of view, HTC implementation generally boosted the concentrations of macro, micro, and secondary nutrients, suggesting its suitability for use as a liquid fertilizer. Overall, the findings of the present study indicate that if bioenergy production were the main target, HTC post-treatment following AD would lead to the most promising outcomes.

Graphical Abstract

Coupling hydrothermal carbonization with anaerobic digestion: an evaluation based on energy recovery and hydrochar utilization


  • Coupling hydrothermal carbonization (HTC) pretreatment with anaerobic digestion (AD) increased methane production by 192%.
  • HTC improved fuel quality and sludge biodegradability of dairy sludge.
  • A positive net energy of 4.28 kWh/kgsludge was obtained by HTC pretreatment.
  • HTC post-treatment to AD resulted in higher net energy gain (5.2 kWh/kgsludge).
  • HTC post-treatment improved steam gasification performance of AD digestate.


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