The effect of co-liquefaction of organic solid waste with primary sewage on hydrochar yield and quality

Abstract

The continuous growth of the human population has increased energy demands globally as much as it has generated massive waste. Global energy demands are currently directed at alternative resources as fossil fuels are depleted, marginalised, and phased out due to environmental regulations. Waste generation and management has also become more intricate due to the rapid expansion and urbanisation taking place globally. The organic fraction of municipal solid waste (OFMSW) has a high energy potential. Due to its high moisture content, hydrothermal liquefaction is a viable option for converting it into renewable fuel sources. Hydrothermal liquefaction also relies on a solvent during the process, where water is usually added as a solvent to the reaction.

The continuous growth of the human population has increased energy demands globally as much as it has generated massive waste. Global energy demands are currently directed at alternative resources as fossil fuels are depleted, marginalised, and phased out due to environmental regulations. Waste generation and management has also become more intricate due to the rapid expansion and urbanisation taking place globally. The organic fraction of municipal solid waste (OFMSW) has a high energy potential. Due to its high moisture content, hydrothermal liquefaction is a viable option for converting it into renewable fuel sources. Hydrothermal liquefaction also relies on a solvent during the process, where water is usually added as a solvent to the reaction.

The comparison between sewage and reverse osmosis water as solvent was made using a batch HTL process at 300˚C and 15 minutes of residence time. The various product yields were determined, and the products were characterised.

Substituting the water with primary sewage proved not to affect the yields of the various products. The hydrochar yields were (30.0 ± 1.7%) using deionised water as a solvent and (30.1 ± 2.6%) using sewage as a solvent. Bio-oil yields were (13.3 ± 1.8%) using deionised water as a solvent and (13.0 ± 0.6%) for sewage as a solvent. The calorific value of the hydrochar was similar, regardless of the solvent used, namely 30.4 ± 1.6 MJ/kg for water and 29.2 ± 0.7 MJ/kg for sewage. However, the ash of the hydrochars did differ as the sewage-derived hydrochar was slightly higher in ash than the hydrochar produced with water as solvent (5.0 ± 0.8 g/100g feedstock) vs (5.1 ± 0.2 g/100g feedstock).

The sewage did not significantly affect the yields of the biocrude oil, hydrochar and aqueous phase products although quality changes were noted namely a higher ash content in the biochar produced with sewage as solvent. Hydrothermal liquefaction allows for the simultaneous treatment of OFMSW and sewage, although secondary treatment of the aqueous phase must be considered as bacteria were still detected. This allows for the generation of renewable energy sources and use case of two waste types, namely OFMSW and sewage.