Characterization of value-added chemicals derived from the thermal hydrolysis and wet oxidation of sewage sludge
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RESEARCH ARTICLE
Characterization of value-added chemicals derived from the thermal hydrolysis and wet oxidation of sewage sludge Milan Malhotra, Anurag Garg (✉) Environmental Science and Engineering Department, Indian Institute of Technology Bombay, Powai Mumbai 400076, India
HIGHLIGHTS
GRAPHIC ABSTRACT
• 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 ARTICLE INFO Article history:
Received 27 November 2019 Revised 17 June 2020 Accepted 22 June 2020 Available online 13 August 2020 Keywords: Hydrothermal pretreatment Reaction kinetics Reaction pathway Sewage sludge Thermal hydrolysis Wet oxidation
1
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.
Introduction
Waste activated sludge (WAS) is a semi-solid material generated from sewage treatment plants (STPs). The sludge is predominantly suspended organic solids, dissolved compounds and pathogenic organisms such as viruses and bacteria. It is a rich source of valuable macromolecules such as proteins, carbohydrates, and humic substances (Baroutian et al., 2016). The quantity of sludge is likely to increase (particularly in developing
✉ Corresponding author E-mail: [email protected]
© Higher Education Press 2020
nations) with the construction of new STPs and improvement in sewage collection efficiency. The treatment and disposal of sewage sludge a major economic burden on STP operators as it can contribute 30%–60% of the total operational cost (Hii et al., 2014; Xiao et a
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