Characterization of value-added chemicals derived from the thermal hydrolysis and wet oxidation of sewage sludge

Highlights

• Hydrothermal treatment can greatly improve resource recovery from sewage sludge.

• tCOD removal during WO was ~55% compared with ~23% after TH.

• TOC solubilization during hydrothermal treatment followed first-order kinetics.

• Solids and carbon balance confirmed loss of organics during thermal hydrolysis.

• Reaction pathways for thermal hydrolysis and wet oxidation are proposed.

Abstract

We evaluated the effect of hydrothermal pretreatments, i.e., thermal hydrolysis (TH) and wet oxidation (WO) on sewage sludge to promote resource recovery. The hydrothermal processes were performed under mild temperature conditions (140°C–180°C) in a high pressure reactor. The reaction in acidic environment (pH= 3.3) suppressed the formation of the color imparting undesirable Maillard’s compounds. The oxidative conditions resulted in higher volatile suspended solids (VSS) reduction (~90%) and chemical oxygen demand (COD) removal (~55%) whereas TH caused VSS and COD removals of ~65% and ~27%, respectively at a temperature of 180°C. During TH, the concentrations of carbohydrates and proteins in treated sludge were 400–1000 mg/L and 1500–2500 mg/L, respectively. Whereas, WO resulted in solids solubilization followed by oxidative degradation of organics into smaller molecular weight carboxylic acids such as acetic acid (~400–500 mg/L). Based on sludge transformation products generated during the hydrothermal pretreatments, simplified reaction pathways are predicted. Finally, the application of macromolecules (such as proteins), VFAs and nutrients present in the treated sludge are also discussed. The future study should focus on the development of economic recovery methods for various value-added compounds.

Graphical abstract

Keywords

Hydrothermal pretreatment / Reaction kinetics / Reaction pathway / Sewage sludge / Thermal hydrolysis / Wet oxidation

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Acknowledgements

The first author thanks to the Ministry of Human Resource and Development (MHRD), Government of India for providing fellowship to carry out the presented research work. The support from Sophisticated Analytical Instrument Facility, IIT Bombay is highly appreciated for helping in ultimate analysis of sludge samples. We are thankful to Professor Andrew S. Hursthouse, Professor of Environmental Geochemistry, University of West of Scotland, Paisley, Scotland, UK for his valuable suggestions on the manuscript.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11783-020-1305-2 and is accessible for authorized users.

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