Biocrude extraction from human-excreta-derived hydrochar for sustainable energy and agricultural applications.

Hydrothermal carbonization may be a sustainable sanitary treatment for wet organic waste including human excreta. Human-excreta-derived hydrochar properties differ from those of typical wet biomass due to the formation of a biocrude-like phase at low reaction temperatures. This study characterized the importance of this phase in terms of hydrochar combustion properties and potential agricultural use. Hydrothermal carbonization of raw human excreta was undertaken at 180, 210, and 240 °C, after which the biocrude phase was extracted with dichloromethane. Physicochemical properties, surface-area parameters, combustion profiles, and gas emissions of non-extracted hydrochar, biocrude, and extracted hydrochar were compared. The potential agricultural use of extracted hydrochar was assessed in germination experiments. Biocrude comprised up to 49.5% of hydrochar mass with a calorific value of >60% that of extracted hydrochar. Biocrude combustion properties were better than those of hydrochar, before and after extraction as demonstrated by higher combustion index value (Si). The extracted hydrochar surface area (34.7 m2 g-1) was greater than that of non-extracted hydrochar (<2 m2 g-1), and seeds germinated more readily due to the lower phytotoxin content. Most macro and micronutrients required for plant growth were retained in the extracted hydrochar. The extraction of biocrude from human-excreta-derived hydrochar not only provided a higher-quality fuel with enhanced combustion properties but also improved hydrochar characteristics, suggesting its potential as a soil additive for enhanced plant growth.

Hydrothermal carbonization reaction severity as an indicator of human-excreta-derived hydrochar properties and it's combustion.

Hydrothermal carbonization (HTC) is an emerging technology that may potentially address sanitation problems and energy scarcity. However, the significance of the parameters that govern HTC (e.g., temperature and time) is not fully understood, in particular for human excreta. A simplified coalification model was used to describe the 'strength' of thermal reactions by combining temperature and time into a single parameter, the severity factor. This study is the first to assess the extent to which a severity coalification model can predict the properties of human-excreta-derived hydrochar for a given severity with different combinations of reaction time and temperature. HTC experiments with raw human excreta were undertaken with 50 mL batch reactors at five different severities. Severity was established with different combinations of temperature (180 °C, 210 °C, and 240 °C) and reaction time based on the severity-factor equation. The resulting hydrochars were tested for combustion properties, and the respective gas emission as well as, physicochemical and surface area parameters. Significant correlations were found between severity and yield (R2 = 0.88), carbon content (R2 = 0.85), and calorific value (R2 = 0.90), with the properties being similar for a given severity but varying with different severities. Hydrochar's contact angle increased from 53.1° to 81.3° with increasing SF, while surface area remained low, ranging from <1 to 5.1 m2g-1, with no definite correlation to SF. Combustion profiles for a given severity were generally similar, but the ignition, peak, and burnout temperatures differed between severities. Gram-Schmidt curves indicated that gas emission profiles are similar for a given severity but vary with different severities. The main gases emitted in combustion were virtually identical in all treatments, and included CO2, alkenes (C9, C10), CH4, and H2O. It is concluded that many properties of hydrochar can be inferred from the severity factor.

Hydrothermal carbonization of anaerobic digestate and manure from a dairy farm on energy recovery and the fate of nutrients.

Hydrothermal carbonization (HTC) of raw and anaerobically digested (AD) manure with either water or whey was studied, with the goal of recovering energy and nutrients. Specifically, the impacts of HTC reaction temperature (180-240 °C), solid feedstock, and type of liquid on hydrochar quality and aqueous phase properties were tested. Of the hydrochars produced, the calorific value of whey-based hydrochar was the highest, (19.4 and 16.0 MJ/kg for manure and digestate, respectively). Overall, the net energy gain was higher for HTC of manure with whey (7.4-8.3 MJ/kg dry feedstock) and water (4.4-5.1 MJ/kg) compared to the combined AD-HTC process with whey (4.4-5.3 MJ/kg) and water (2.3-2.9 MJ/kg). Digestate-derived hydrochar contained up to 1.8% P, higher than manure-derived hydrochar (≤1.5%). Using whey as a liquid for HTC increased the aqueous-phase N-P-K concentrations up to 3,200, 410, and 7,900 mg/L, respectively, suggesting its potential use as a liquid fertilizer.