Improvement in the physicochemical characteristics of biochar derived from solid digestate of food waste with different moisture contents

Anaerobic fermentation integrated with pyrolysis for carbon resource recovery from food waste and biogas sludge: Effects of inoculation ratio and pyrolysis temperature

In view of the food waste (FW) as well as its digestate are both the organic sources of municipal solid waste, this study explored the anaerobic fermentation (AF) and following pyrolysis carbonization to co-disposal the two wastes for carbon resource recovery, including short chain organic acid (SCOAs), pyrolysis gas and biochar. Results indicated that both the rate and yield of SCOAs production both increase with the rising ratio of biogas sludge (BS) to FW, enhancing the soluble carbon recovery. The highest SCOAs production of 474.33 mg/g-VS was achieved at the ratio of 2:1 in 72 h. To further utilize the carbon source, the solids from the fermented residue (FR) was pyrolyzed at 400, 600 and 800 °C, respectively. Findings showed that the carbon content in biochar decreases with the increasing pyrolysis temperature, while the carbon in pyrolysis gas exhibits the opposite trend. Integrating the AF and pyrolysis contributed to a carbon recovery about 56.39% when the FW and BS were co-fermented at a 2:1 ratio, followed by its FR was pyrolyzed at 600 °C. Additionally, the biochar prepared under these conditions displayed a specific surface area (SSA) of 313.10 m2/g, along with abundant pore structures and functional groups, indicating its potential applications as pollutant adsorbents and soil amendments. This research offers a new perspective on efficiently recovering high-value carbon sources through the co-treatment of FW and its digestate via AF integrated with pyrolysis.

Hydrothermal-enhanced pyrolysis for efficient NOX reduction and biochar valorization from food waste digestate

Pyrolysis has emerged as a promising technology for valorizing digestate resulting from the anaerobic digestion of food waste. However, the high NOX emissions during pyrolysis limit its application. This study proposed a hydrothermal coupled pyrolysis process to control the element transfer in digestate during biochar production. The efficient reduction of NOX emissions and the improvement of biochar adsorbability were realized. The hydrothermal process reduced the nitrogen content in solid digestate by 49.10 %-81.79 %, thus reducing the NOX precursors in syngas and the N-containing substances in bio-oil. Additionally, the specific surface area and the total pore volume of biochar were enhanced from 25 m2/g to 60-73 m2/g and 0.06 cm3/g to 0.12-0.14 cm3/g, respectively. More defects, oxygen-containing functional groups, and doped Ca on the biochar resulted in a high phosphate removal efficiency of 94 %. The proposed technology provides an efficient and environmentally friendly way to utilize the digestate.

Efficient removal of Cd(II) and Pb(II) from aqueous solution using biochars derived from food waste

Massive amount of food waste has been generated annually, posing a threat to ecological sustainability and the social economy due to current disposal methods. Urgent action is needed worldwide to convert the traditional pathway for treating food waste into a sustainable bioeconomy, as this will significantly benefit food chain management. This study explores the use of pyrolysis to produce different types of food waste biochars and investigates their adsorption capabilities for removing Cd2+ and Pb2+ in aqueous solution. The results indicated that co-pyrolysis biochar from fresh food waste and rice husk (FWRB) exhibited superior adsorption performance for Cd2+ (61.84 mg·g-1) and Pb2+ (245.52 mg·g-1), respectively. Pseudo-second-order kinetics (0.74 ≤ R2 ≤ 0.98) and Langmuir isotherms (0.87 ≤ R2 ≤ 0.98) indicated that the immobilized Cd2+ and Pb2+ on biochars were mainly attributed to the chemisorption, including precipitation with minerals (e.g., carbonates, silicates, and phosphate), complexation with functional groups (-OH), cation exchange (-COO-), and coordination with π-electrons. Furthermore, FWRB demonstrated reduced EC and Na content in comparison to food waste digestate biochar (FWDB) and food waste digestate co-pyrolysis with sawdust biochar (FWSB), with levels of Cd and Pb falling below China's current guideline thresholds. These findings suggested that co-pyrolysis of fresh food waste with rice husk could be applicable to the recycling of food waste into biochar products for heavy metal stabilization in contaminated water and soils.

Regulation of fungal communities during pig manure composting

The role of protein shell (PS) amendment in altering the fungal community during pig manure (PM) composting was investigated. Six different dosages of PS based on the dry weight of PM (0 %, 2.5 %, 5 %, 7.5 %, 10 %, and 12 %; T1-T6, respectively) were mixed with wheat straw to make the initial feedstock and composted for 42 days. The results showed that Ascomycota, Basidiomycota, and Giomeromycota were the most abundant phyla in all treatments. However, the relative abundance of Giomeromycota was the highest in the control treatment, although a substantially greater population was observed in all treatments. Genus abundance declined steadily from T1 to T6; however, T4 and T6 had smaller populations. Correlation analysis also suggested that T6 amendment increased the overall fungal dynamics and organic matter degradation. Thus, T6 was more efficient to enhance the overall fungal population and dynamics with considerable network connections among all the analyzed parameters.

Optimizing adsorption performance of sludge-derived biochar via inherent moisture-regulated physicochemical properties

Understanding the impact of abundant inherent moisture in sewage sludge on the physicochemical properties and adsorption applications of sludge-derived biochar (SDB) contributed significantly to promoting economical sludge reuse. The moisture (0-80%) contributed to the development of micropore and mesopore in SDB at 400 °C, resulting in a maximum increase in specific surface area (SSA) and total pore volume (TPV) of SDB by 38.47% (84.811-117.437 m2/g) and 92.60% (0.0905-0.1743 m3/g), respectively. At 600/800 °C, moisture only facilitated mesopore formation, while was exacerbated with increasing moisture content. Despite reduction in SSA during this stage, TPV increased by a maximum of 20.47% (0.1700-0.2048 m3/g). The presence of moisture during pyrolysis led to an increase in the formation of 3-5 thickened benzene rings and defective structures in SDB, along with more C=O, O-C=O/-OH, pyrrole N, pyridine N, and thiophene. As a result, moisture (40%/80%) increased the maximum adsorption capacity (76.2694-88.0448/90.1190 mg/g) of SDB (600 °C) for tetracycline, mainly due to enhanced pore filling effect and hydrogen bonding induced by improved physicochemical properties. This study offered a novel approach for optimizing the performance of SDB adsorption applications by manipulating the sludge moisture, which is critical for practical sludge management.

Digestate-derived carbonized char and activated carbon: Application perspective

The flourishment of anaerobic digestion (AD) on waste treatment emphasizes the importance of digestate valorization, which plays an essential role in determining the benefits provided by the AD process. The perception of digestate gradually shifts from waste to products to realize the concept of circular economy and maximize the benefits of digestate valorization. This review first outlined the current status of digestate valorization, focusing on thermal-chemical methods. The novel valorization methods were then summarized from the recent research, illustrating prospects for digestate valorization. Limits and perspectives are finally addressed. Methods for preparing digestate-derived activated carbon and impurity effects were elucidated. Inherent mineral content/inorganic impurity could be a niche for downstream use. High surface area and well-developed pore structure are essential for satisfying downstream use performance, but they are not the only factors. Digestate char applications other than use as an energy fuel are suggested.

Boosted photocatalytic performance of OVs-rich BiVO4 hollow microsphere self-assembled with the assistance of SDBS

In this study, monoclinic phase bismuth vanadate (BiOV₄) photocatalyst with unique hollow microsphere morphology was successfully prepared by a hydrothermal method in the existence of sodium dodecyl benzene sulfonate (SDBS). The prepared photocatalysts were characterized by X-ray diffraction (XRD), scanning electron (SEM) and X-ray photoelectron spectrometer (XPS) and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). Experimental results show that SDBS definitely changes the microstructure of BiVO₄, which is allocated to the template role of SDBS in the preparation process. Moreover, the hydrothermal treatment time is also of crucial importance in affecting the structure and morphology of the photocatalysts, and the optimal hydrothermal treatment time for the formation of hollow microsphere is 24 h. Furthermore, the feasible growth mechanism for hollow microsphere was elaborated. Enriched oxygen vacancies (OVs) are introduced into BiOV₄ prepared with SDBS, largely elevating the separation efficiency of photo-generated charges. Under visible light irradiation, the photocatalytic activities of BiOV₄ for destruction of rhodamine (RhB) were evaluated. The photocatalytic degradation rate constant of RhB on the 3SBVO is 2.23 times of that on the blank BiOV₄ as the mass ratio of SDBS/BiOV₄ is 3 %. Photocatalytic degradation mechanism of BiVO₄ toward detoxification of organic pollutants was presented.

Insights into the influence of digestate-derived biochar upon the microbial community succession during the composting of digestate from food waste

The by-product from the anaerobic digestion of food waste (FW) called the digestate (DFW) needs proper disposal because of its high environmental burden. Composting can transform DFW into a nutrient-containing soil improver via a series of microbial metabolic activities. However, the long composting time and high amount of ammonia emission are the key concerns of DFW composting. In the present study, the effect of DFW-derived biochar (BC-DFW) on microbial succession and its involvement in nitrogen transformation and humification during DFW composting were investigated. The results indicated that the BC-DFW accelerated bacterial and fungal evolution, and the bacterial diversity was augmented by increasing the amount of BC-DFW. In particular, Cryomorpha, Castellaniella, Aequorivita, and Moheibacter were enriched by the addition of BC-DFW, thereby enhancing the degradation of organic matter and nitrogen transformation and increasing the germination index. The group with 25% BC-DFW contained a higher relative abundance of Cryomorpha (2.08%, 2.47%) than the control (0.39%, 1.72%) on days 19 and 35 which benefited the degradation of organic matter. The group with 25% BC-DFW quickly enhanced the growth of Nitrosomonas, thereby accelerating the conversion of ammonium-nitrogen to nitrate-nitrogen and reducing the phytotoxicity of the composting product.

Evaluation of digestate-derived biochar to alleviate ammonia inhibition during long-term anaerobic digestion of food waste

The feasibility of using food waste anaerobic digestate-derived biochar (FWDB) to mitigate ammonia toxicity in an anaerobic digester was evaluated. The optimal conditions for preparing and adding the activated FWDB were explored using response surface experiments, and the long-term effects of adding activated FWDB on digester performance under optimum conditions were verified in semi-continuous experiments. The results showed that the optimal preparation and addition conditions for activated FWDB were pyrolysis temperature of 565 °C, particle size of 0-0.30 mm, and dosage of 15.52 g·L-1. During the long-term operation of the digesters, when the total ammonia nitrogen (TAN) concentration was higher than 2000 mg·L-1, the control and experimental digesters showed deteriorated reactor performance. Volatile fatty acids in the control digester accumulated to 20,306 mg·L-1 after the TAN concentration increased to 3391 mg·L-1, the methane yield decreased to 31 mL·g VS-1, and the digester experienced process failure. In contrast, the experimental digester with added activated FWDB only suffered a slight short-term accumulation of acetate and a slight decline in methane yield. This may be attributed to the adsorption of NH4+/NH3 by activated FWDB, which reduced the TAN concentration in the anaerobic digestion (AD) system and mitigated ammonia toxicity. Microbial analysis and metagenome predictions demonstrated that the community richness, diversity, and evenness, as well as the abundance of acetogens and related key genes (ACSM1, paaF, and acdA) were higher in the experimental digester than in the control digester. This study provides a closed-loop AD enhancement strategy by pyrolysis of digestate and in-situ supplementation into the digester.

Mechanism of digestate-derived biochar on odorous gas emissions and humification in composting of digestate from food waste

Emissions of odorous gases and prolonged composting duration are the key concerns in the composting of digestate from food waste (DFW). In this study, different amounts of biochar derived from DFW (BC-DFW) were introduced in the composting process of DFW to decrease the emissions of ammonia (NH₃) and volatile sulfur compounds (VSCs) and composting duration. The addition of BC-DFW increased the temperature and germination index during DFW composting. The group with 25% BC-DFW exhibited a 30% smaller composting duration. Significant amounts of NH₃ and VSCs emissions were observed in the initial phase of DFW composting. Dimethyl disulfide (DMDS) was a prominent contributor to the odor associated with VSCs. The addition of BC-DFW facilitated the adsorption of NH₃ and VSCs, and the corresponding contents decreased by 5-21% and 15-20%, respectively. Moreover,the BC-DFW accelerated the transformation of ammonium-nitrogen (NH₄⁺-N) to nitrate-nitrogen (NO₃^--N), thereby alleviating the NH₃ volatilization. The addition of 25% BC-DFW minimized the NH₃ emission and enhanced the generation of humic-acid-like matter, thereby promoting humification. Therefore, the addition of 25% BC-DFW was optimal for promoting the degradation of organic matter and humification and odor emission reduction (e.g., NH₃, DMDS).