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The US annually produces 79 million dry tons of liquid organic waste including sewage sludge.Anaerobic digestion can only reduce the sludge volume by 50% in mass, leaving the other half as a growing waste management and hygienic problem. Hydrothermal Processing (HTP), a set of several chemical digestion processes, could be employed to convert sewage sludge into valuable products and minimize potential environmental pollution risks. Specifically, hydrothermal carbonization and hydrothermal liquefaction have been extensively studied to sustainably manage sludge. Two of the main reasons for this are the high upscaleability of HTP for public waste management and that it is estimated that HTP can recover eleven times more energy from waste products than landfilling. An integration of HTP with anaerobic digestion or recycling the soluble organics (in the HTP aqueous products) into the HTP process could lead to a higher overall rate of energy recovery for municipal sewage sludge.

In this work, leucine and glucose were used as model molecules to investigate the Maillard reaction between proteins and carbohydrates during hydrothermal liquefaction (HTL) of microalgae. The main pathway of the Maillard reaction between leucine and glucose via HTL was related to the reaction of deaminated leucine with cyclic oxygenated compounds from degraded glucose to produce pyrazine derivatives. Gas chromatography–mass spectrometry results revealed that N and O heterocyclic compounds, organic acids, and carbonyls/imine/amine were the main components of both the biocrude oil and aqueous fraction. The optimal reaction temperature, reaction time, leucine/glucose weight ratio, and solid concentration for biocrude oil production are 320 °C, 60 min, 2:5, and 20 wt %, respectively. The obtained biocrude oil (higher heating value of 38.07 MJ/kg), presenting a large amount of pyrazine derivatives (84.2% peak area), yielded 47.6 wt % of leucine and glucose feedstock.

Algal bloom microalgae are abundant in polluted water systems, but their biocrude oil production potential via hydrothermal liquefaction (HTL) is limited. This study proposed a novel process that combined biological (dark fermentation) and thermochemical (HTL) techniques aimed at changing the feedstock characteristics to be more suitable for thermochemical conversion, herein named integrated dark fermentation−hydrothermal liquefaction (DF-HTL). DF-HTL conversion of algae significantly enhanced the biocrude oil yield (wt %), carbon content (mol), energy content (MJ), and energy conversion ratios by 9.8, 29.7, 40.0, and 61.0%, respectively, in comparison to the control. Furthermore, DF-HTL processing significantly decreased the aqueous byproduct yield (wt %), carbon content (mol), nitrogen content (mol), and ammonia content (mol) by 19.0, 38.4, 25.0, and 13.2%, respectively, in comparison to the control. Therefore, DF-HTL reduced the environmental impact associated with disposing of the wastewater byproduct. However, DF-HTL also augmented the nitrogen content (mol) of the biocrude oil by 42.2% in comparison to the control. The benefits of DF-HTL were attributed to the increased acid content, the incorporation of H 2 as a processing gas, and the enhancement of the Maillard reaction, which shifted the distribution of reaction products from the aqueous phase to the biocrude oil phase. This article provides insights into the efficacy of a novel integrated biological−thermochemical processing method with distinct environmental and energetic advantages over conventional HTL that heightens the biocrude oil yield for feedstocks with a high carbohydrate and a high protein content.

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