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.

Advanced near-zero waste treatment of food processing wastewater with water, carbon, and nutrient recovery.

A near-zero waste treatment system for food processing wastewater was developed and studied. The wastewater was treated using an anaerobic membrane bioreactor (AnMBR), polished using an outdoor photobioreactor for microalgae cultivation (three species were studied), and excess sludge was treated using hydrothermal carbonization. The study was conducted under arid climate conditions for one year (four seasons). The AnMBR reduced the total organic carbon by 97%, which was mostly recovered as methane (~57%) and hydrochar (~4%). Microalgal biomass productivity in the AnMBR effluent ranged from 0.25 to 0.8 g·L-1·day-1. Nitrogen (N) and phosphorous (P) uptake varied seasonally, from 18 to 45 mg·L-1·day-1 and up to 5 mg·L-1·day-1, respectively. N and P mass balance analysis demonstrated that the process was highly efficient in the recovery of nitrogen (~77%), and phosphorus (~91%). The performance of the microalgal culture changed among seasons because of climatic variation, as a result of variation in the wastewater chemistry, and possibly due to differences among the microalgal species. Effluent standards for irrigation use were met throughout the year and were achieved within two days in summer and 4.5 days in winter. Overall, the study demonstrated a near-zero waste discharge system capable of producing high-quality effluent, achieving nutrient and carbon recovery into microalgae biomass, and energy production as biogas and hydrochar.

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.

Facile activation of sludge-based hydrochar by Fenton oxidation for ammonium adsorption in aqueous media.

Lately, there has been a growing interest in converting low-cost biomass residuals, including wastewater sludge, into char-like materials for various applications. In this research, ammonium (NH4+) adsorption and desorption potential of hydrochar activated via Fenton oxidation were systematically investigated. Hydrochar was prepared from domestic wastewater treatment plant sludge and activated by Fenton oxidation using different H2O2 concentrations, H2O2/Fe2+ ratios, and activation times. The activated hydrochars (AHs) were characterized by ATR-FTIR, high-resolution XPS, BET specific surface area, and SEM, and their NH4+ adsorption capacity was analyzed. The NH4+ adsorption isotherms and kinetics, adsorption in the presence of competing ions (with and without humic acid), and NH4+ desorption were investigated. The results show that following hydrochar activation, the acidic groups' concentration and the BET surface area increased, but the morphology remained essentially unchanged. It was also found that the activation occurs within a few minutes when using a relatively low concentration of reagents, and without extensive post-treatment steps. The NH4+ adsorption onto AH at equilibrium fitted the Langmuir isotherm model, with a maximum adsorption capacity of 30.77 mg g-1, and the NH4+ adsorption kinetics fitted the pseudo-second-order model. NH4+ adsorption in the presence of competing ions decreased by up to 33 ± 3%. NH4+ desorption experiments demonstrated that NH4+ recovery can reach 33 ± 5% with ultrapure water and 67 ± 2% with 2 M KCl. The results of this study indicate that Fenton oxidation is a promising alternative for hydrochar activation, and can be used as an adsorbent for NH4+ remediation in wastewater treatment processes.

Characterization and utilization of hydrothermal carbonization aqueous phase as nutrient source for microalgal growth.

This study investigated the feasibility of using hydrothermal carbonization (HTC) aqueous phase as an alternative nutrient source for microalgae cultivation, and the microalgae cultivation capability to treat this complex medium to a level enabling its reuse or discharge. HTC of activated sludge was optimized in terms of the energy content of the solid hydrochar and the nutrient content of the HTC aqueous phase adequate for microalgal growth. Growth rates of Coelastrella sp. and Chlorella sp. in the HTC aqueous phase based growth medium and a control medium (mBG11) were similar, indicating that the HTC aqueous phase does not inhibit the microalgae growth. Nitrogen and phosphorus concentrations were reduced by >90% and dissolved organic carbon by 80% after 6 days of cultivation, resulting in water quality suitable for reuse or discharge. This study confirms the microalgae high potential in a circular bio-economy to valorize wet bio-waste streams from various treatment methods.