Optimizing the Hydrothermal Carbonization of Sewage Sludge—Response Surface Methodology and the Effect of Volatile Solids

Tobias Blach,Markus Engelhart

doi:10.3390/w13091225

Abstract

This study focuses on identifying the optimum conditions of sewage sludge hydrothermal carbonization by Box–Behnken Design and on the effects of volatile solids on heating value and on process water load. To get insight into the solid and process water characteristics, we applied the Box–Behnken Design on the hydrothermal reaction temperature (190, 220, 250 °C), reaction time (0.5, 2.25, 4 h) and pH (3.9, 5, 6.1). The response surface of the liquid phase revealed decreasing dissolved organic carbon (DOC) concentrations with increasing temperature from 9446 mg/L (190 °C) to 7402 mg/L (250 °C) at 4 h reaction time. For the same hydrothermal conditions, NH4-N concentration increased from 754 to 1230 mg/L. Reaction temperature was identified as the most important process parameter, whereas reaction time and pH had only minor effects. Moreover, linear coefficients of the models were more decisive than the interrelation and quadratic coefficients. Volatile solids (VS) of the feedstock were found to significantly influence both the load of the process water and the change in heating value of the hydrochars. Process water load increased steadily with higher VS. The heating value only increased with more than around 65–80% VS in feedstock.

Highlights

In municipal wastewater treatment, the main distinction is made between primary sewage sludge (PS), waste activated sludge (WAS), including precipitation sludge from phosphorus removal, and anaerobically digested sludge (ADS)
The main reaction parameters on the hydrothermal treatment of sewage sludge temperature and time were predominantly described by their linear coefficients in the model fits of the BBD
The results of these model fits suggest that intensifying hydrothermal carbonization (HTC) process conditions favor the higher higher heating value (HHV) of the hydrochars and coalification

Summary

Introduction

The main distinction is made between primary sewage sludge (PS), waste activated sludge (WAS), including precipitation sludge from phosphorus removal, and anaerobically digested sludge (ADS). The treatment and disposal of sewage sludge is gaining more and more attention as its components, especially phosphorus and the chemically bound energy, are considered valuable and worth recovering. Current trends towards renewable energies, further optimization of sewage sludge treatment processes and carbon sequestration promote alternative technologies such as the hydrothermal carbonization (HTC), which reproduces the natural coalification of organic matter in the presence of water in a technical application [6]. Both temperature and feedstock significantly determine chemical pathways and the thermochemical breakdown of the biomass structure into lower weight molecules. A review of several studies by Wang et al [7]

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Conclusion