Abstract
A hydrothermal pre-treatment has been developed to improve sewage sludge quality or to produce low nitrogen biocrude via hydrothermal liquefaction (HTL) in a subsequent step. The mild hydrothermal pre-treatment (150 °C) step was performed with deionized water, sulfuric acid (0.5 M), or citric acid (0.5 M) to solubilize nitrogen containing compounds in the aqueous supernatant. Downstream, the residual solid material was liquefied with the addition of sodium carbonate via hydrothermal liquefaction (350 °C). The pre-treatment with citric acid transferred up to 66.7 wt. % of nitrogen into the aqueous supernatant, while 62.0 wt. % of carbon was recovered in the solid. Due to the pre-treatment lipids retained in the sewage sludge solid, which increased the favored biocrude yield up to 42.9 wt. % and the quality evaluating value H/Ceff ratio significantly to 1.48. Multi-method characterization of the resulted biocrude samples showed a lower concentration of N-heterocycles, while long-chain aliphatics and free fatty acid are increased.
Highlights
Maillard reactions form Nheterocycles, while amidation of fatty acids leads to long-chain amides. These findings propose a pre-treatment where proteins, carbohydrates and their Maillard products are solubilized in an aqueous supernatant and subsequently removed
Sodium carbonate, dichloromethane, deuterated chloroform, hexane, hexadecanol, phenol, pentadecane, and FAME mix were obtained from Merck (Darmstadt, Germany)
The results show that the municipal sewage sludge (MSS) has an organic content of 70.0 wt. %, which decreased to 61.9 wt. % by hydrothermal pre-treatment with DW or increased to 75.3 wt. % by treatment with citric acid (CA), while SA had no notable effect
Summary
Sewage sludge is generated from wastewater treatment plants, demand urgent management and proper disposal. More than 10 million tons of sewage sludge were produced in the European Union (EU) in 2010, with a rising amount due to population growth, urbanization and higher regulations on the wastewater effluent [1,2]. Common disposal methods are the use in agriculture, incineration or landfill [3]. The sludge management remains a challenging issue for the member states as the European legislations provided with the Directive 2018/851/EC a priority hierarchy in waste management, starting from prevention, to preparing for reuse, to recycling, to energy recovery, and to disposal [4]. The limit levels for pathogens, metal and nutrient application rates to land are set and new issues arise with micro-plastics and pharmaceutical residuals [5,6,7]
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