Solid state anaerobic digestion of food waste and sewage sludge: Impact of mixing ratios and temperature on microbial diversity, reactor stability and methane yield

Biofuel production for circular bioeconomy: Present scenario and future scope

In recent years, biofuel production has attracted considerable attention, especially given the increasing worldwide demand for energy and emissions of greenhouse gases that threaten this planet. In this case, one possible solution is to convert biomass into green and sustainable biofuel, which can enhance the bioeconomy and contribute to sustainable economic development goals. Due to being in large quantities and containing high organic content, various biomass sources such as food waste, textile waste, microalgal waste, agricultural waste and sewage sludge have gained significant attention for biofuel production. Also, biofuel production technologies, including thermochemical processing, anaerobic digestion, fermentation and bioelectrochemical systems, have been extensively reported, which can achieve waste valorization through producing biofuels and re-utilizing wastes. Nevertheless, the commercial feasibility of biofuel production is still being determined, and it is unclear whether biofuel can compete equally with other existing fuels in the market. The concept of a circular economy in biofuel production can promote the environmentally friendly and sustainable valorization of biomass waste. This review comprehensively discusses the state-of-the-art production of biofuel from various biomass sources and the bioeconomy perspectives associated with it. Biofuel production is evaluated within the framework of the bioeconomy. Further perspectives on possible integration approaches to maximizing waste utilization for biofuel production are discussed, and what this could mean for the circular economy. More research related to pretreatment and machine learning of biofuel production should be conducted to optimize the biofuel production process, increase the biofuel yield and make the biofuel prices competitive.

Temperature-dependent transformation of microbial community: A systematic approach to analyzing functional microbes and biogas production

The stability and effectiveness of the anaerobic digestion (AD) system are significantly influenced by temperature. While majority research has focused on the composition of the microbial community in the AD process, the relationships between functional gene profile deduced from gene expression at different temperatures have received less attention. The current study investigates the AD process of potato peel waste and explores the association between biogas production and microbial gene expression at 15, 25, and 35 °C through metatranscriptomic analysis. The production of total biogas decreased with temperature at 15 °C (19.94 mL/g VS), however, it increased at 35 °C (269.50 mL/g VS). The relative abundance of Petrimonas, Clostridium, Aminobacterium, Methanobacterium, Methanothrix, and Methanosarcina were most dominant in the AD system at different temperatures. At the functional pathways level 3, α-diversity indices, including Evenness (Y = 5.85x + 8.85; R2 = 0.56), Simpson (Y = 2.20x + 2.09; R2 = 0.33), and Shannon index (Y = 1.11x + 4.64; R2 = 0.59), revealed a linear and negative correlation with biogas production. Based on KEGG level 3, several dominant functional pathways associated with Oxidative phosphorylation (ko00190) (25.09, 24.25, 24.04%), methane metabolism (ko00680) (30.58, 32.13, and 32.89%), and Carbon fixation pathways in prokaryotes (ko00720) (27.07, 26.47, and 26.29%), were identified at 15 °C, 25 °C and 35 °C. The regulation of biogas production by temperature possibly occurs through enhancement of central function pathways while decreasing the diversity of functional pathways. Therefore, the methanogenesis and associated processes received the majority of cellular resources and activities, thereby improving the effectiveness of substrate conversion to biogas. The findings of this study illustrated the crucial role of central function pathways in the effective functioning of these systems.

Evaluating the potential of different bioaugmented strains to enhance methane production during thermophilic anaerobic digestion of food waste

Bioaugmentation technology for improving the performance of thermophilic anaerobic digestion (TAD) of food waste (FW) treatment is gaining more attention. In this study, four thermophilic strains (Ureibacillus suwonensis E11, Clostridium thermopalmarium HK1, Bacillus thermoamylovorans Y25 and Caldibacillus thermoamylovorans QK5) were inoculated in the TAD of FW system, and the biochemical methane potential (BMP) batch study was conducted to assess the potential of different bioaugmented strains to enhance methane production. The results showed that the cumulative methane production in groups inoculated with E11, HK1, Y25 and QK5 improved by 2.05%, 14.54%, 19.79% and 9.17%, respectively, compared with the control group with no inoculation. Moreover, microbial community composition analysis indicated that the relative abundance of the main hydrolytic bacteria and/or methanogenic archaea was increased after bioaugmentation, and the four strains successfully became representative bacterial biomarkers in each group. The four strains enhanced methane production by strengthening starch, sucrose, galactose, pyruvate and methane metabolism functions. Further, the correlation networks demonstrated that the representative bacterial genera had positive correlations with the differential metabolic functions in each bioaugmentation group. This study provides new insights into the TAD of FW with bioaugmented strains.

Anaerobic co-digestion of sewage sludge pretreated by thermal hydrolysis and food waste: gas production, dewatering performance, and community structure

Anaerobic co-digestion can effectively break the limitations of mono-digestion. However, there are still some problems such as long residence time, unsatisfactory methane yield, and unstable performance for co-digestion of sewage sludge (SS) and food waste (FW). Therefore, the SS in the reactor treating co-digestion of SS and FW is considered to be pretreated by thermal hydrolysis. In this work, the anaerobic co-digestion of SS of thermal hydrolysis pretreatment (THP) and FW significantly improved the stability, methane production of the digestive reactor, and dewaterability of the digested sludge. The R6 obtained the most cumulative methane production (315.76 mL/g VS). In addition, compared to R3, the cumulative methane production and maximum methane production rate of R5 increased by 9.93% and 14.56%, respectively. The dewaterability of R4, R5, and R6 was improved, while the dewatering performance of the R3 decreased to a greater extent. The results of the kinetic model fitting were consistent with the experimental results. Among them, the hydrolysis constants (Kh) of anaerobic co-digestion of THP-SS and FW were 0.121, 0.130, and 0.114 d-1, respectively, which were higher than those of other groups. And the estimated lag time (λ) of co-digestion was also lower than that of mono-digestion groups. Microbial community analysis indicated that the bacterial diversity and richness of anaerobic co-digested groups of THP-SS and FW were enhanced, while the methanogens with acetoclastic pathway became the main methanogenic microorganisms. This work provides essential information on anaerobic co-digestion containing different THP-SS contents.

Pilot-scale two-phase anaerobic digestion of deoiled food waste and waste activated sludge: Effects of mixing ratios and functional analysis

Anaerobic co-digestion of deoiled food waste (dFW) and waste activated sludge (WAS) can address the challenges derived from mono-digestion of FW. In the present study, a pilot-scale methanogenic bioreactor of a two-phase anaerobic digestion system was developed to explore the impact of dFW/WAS volatile solids ratios on the overall performance, microbial community, and metabolic pathways. Besides, the tech-economic of the system was analyzed. The results showed that the degradation efficiency of soluble chemical oxygen demand (SCOD) was more than 84.90% for all the dFW/WAS ratios (v/v) (1:0, 39:1, 29:1, 19:1 and 9:1). Moreover, the dominant genus of bacteria and archaea with different ratios were Lactobacillus (66.84-98.44%) and Methanosaeta (53.66-80.09%), respectively. Co-digestion of dFW and WAS (29: 1 in v/v ratios) obtained the highest yield of methane (0.41 L CH₄/L_added) with approximately 90% of SCOD being removed. In the pilot-scale experiment, the co-digestion of FW and WAS makes positive contribution to reusing solid waste for improving solid management.

Low-carbon emitting university campus achieved via anaerobic digestion of canteen food wastes

University campuses of China accommodate over 30 million students and consume a large amount of fossil fuel energy, leading to high carbon emission. Implementation of bioenergy (e.g. biomethane) is one of promising ways to mitigate emission and foster low-carbon emitting campus. Biomethane potential from anaerobic digestion (AD) of food waste (FW) in 2344 universities of 353 cities of mainland China have been estimated herein. Results have shown that 1.74 million tons of FW are discharged from campus canteens annually, that can generate 195.8 million m³ biomethane and reduce 0.77 million ton CO₂-eq. Wuhan, Zhengzhou, and Guangzhou are the top three cities having the most biomethane potential from campus FW, accounting up to 8.92, 7.89, and 7.28 million m³ year^-1, respectively. Technical challenges and solutions have been summarized and discussed such as FW purity, accumulation of ammonia and fatty acid, foaming, and plant site selection. Low-carbon campuses are supposed to be achieved by using bioenergy, like biomethane, in appropriate ways after resolving technical and management challenges.

Sustainable management of food waste; pre-treatment strategies, techno-economic assessment, bibliometric analysis, and potential utilizations: A systematic review

Food waste (FW) contains many nutritional components such as proteins, lipids, fats, polysaccharides, carbohydrates, and metal ions, which can be reused in some processes to produce value-added products. Furthermore, FW can be converted into biogas, biohydrogen, and biodiesel, and this type of green energy can be used as an alternative to nonrenewable fuel and reduce reliance on fossil fuel sources. It has been demonstrated in many reports that at the laboratory scale production of biochemicals using FW is as good as pure carbon sources. The goal of this paper is to review approaches used globally to promote turning FW into useable products and green energy. In this context, the present review article highlights deeply in a transdisciplinary manner the sources, types, impacts, characteristics, pre-treatment strategies, and potential management of FW into value-added products. We find that FW could be upcycled into different valuable products such as eco-friendly green fuels, organic acids, bioplastics, enzymes, fertilizers, char, and single-cell protein, after the suitable pre-treatment method. The results confirmed the technical feasibility of all the reviewed transformation processes of FW. Furthermore, life cycle and techno-economic assessment studies regarding the socio-economic, environmental, and engineering aspects of FW management are discussed. The reviewed articles showed that energy recovery from FW in various forms is economically feasible.

Anaerobic digestion of hydrothermally pretreated dewatered sewage sludge: effects of process conditions on methane production and the fate of phosphorus

The hydrothermal pretreatment (HTP) characteristics and the fate of phosphorus (P) and anaerobic digestion (AD) performance of dewatered sewage sludge (DSS) were investigated at different hydrothermal conditions. The maximum methane yield reached 241 mL CH₄/g COD when the hydrothermal conditions were 200 °C-2 h-10% (A4), and the yield was 78.28% higher than that without pretreatment (A0) and 29.62% higher than that of the initial hydrothermal conditions (A1, 140 °C-1 h-5%). Proteins, polysaccharides, and volatile fatty acids (VFAs) were the main hydrothermal products of DSS. 3D-EEM analysis revealed that tyrosine, tryptophan proteins, and fulvic acids decreased after HTP, but the content of humic acid-like substances increased, and this phenomenon was more noticeable after AD. Solid-organic P was converted into liquid-P during the hydrothermal process, and nonapatite inorganic P was converted into organic P during AD. All samples achieved positive energy balance, and the energy balance of A4 was 10.50 kJ/g VS. Microbial analysis showed that the composition of the anaerobic microbial degradation community changed as the sludge organic composition was altered. Results showed that the HTP improved the anaerobic digestion of DSS.

Supplemental Sewage Scum and Organic Municipal Solid Waste Addition to the Anaerobic Digestion of Thickened Waste Activated Sludge: Biomethane Potential and Microbiome Analysis

Sewage scum (SS) is collected from sedimentation tanks in wastewater treatment plants (WWTPs). Despite its huge biogas potential, there is limited information on its potential as a co-substrate and microbial ecology, especially during anaerobic co-digestion (ACo-D) of the organic fraction of municipal solid waste (OFMSW) and thickened waste activated sludge (TWAS). In this biomethane potential (BMP) study, the bioenergy yield achieved by the supplemental addition of SS and OFMSW to TWAS was investigated, along with the microbial ecology. Compared with the digestion of TWAS alone, which produced 184.6 mLCH4 gVS−1, biomethane yield was enhanced by as much as 32.4–121.6% in trinary mixtures with SS and OFMSW, mainly due to the positive synergistic effect. Furthermore, a mixture of 40%SS + 10%TWAS + 50%OFMSW produced the highest biogas yield of 407 mLCH4 gVS−1, which is proof that existing WWTPs can produce additional energy by incorporating external bioresources, thereby reducing greenhouse gas emissions. Modified Gompertz and logistic function estimates showed that methane production rate improved by as much as 60% in a trinary mixture compared with the digestion of TWAS alone. The genus Methanosaeta, capable of generating methane by the acetoclastic methanogenic pathway among all the archaeal communities, was the most prominent, followed by hydrogenotrophic methanogen Methanospirillum.

Effect of time and temperature of pretreatment and anaerobic co-digestion of rice straw and swine wastewater by domesticated paddy soil microbes

Rice straw and swine wastewater are abundant, easy to obtain, and inexpensive biomass materials. Anaerobic digestion of rice straw and swine wastewater effectively regulates the carbon-to-nitrogen ratio and also improves methane production efficiency. The dense lignocellulosic structure, unsuitable carbon-to-nitrogen ratio, and light texture of rice straw hinder its application in anaerobic digestion. Effective pretreatment technologies can improve degradation efficiency and methane production. Our study is the first to apply domesticated paddy soil microbes to enhance the efficiency of hydrolytic acidification of rice straw and swine wastewater at varying temperatures and times. The results show that the highest total organic carbon (1757.2 mg/L), soluble chemical oxygen demand (5341.7 mg/L), and organic acid concentration (4134.6 mg/L) appeared in the hydrolysate after five days of hydrolytic acidification at 37 °C. Moreover, the use of hydrolysate produced 13% more gas and reduced the anaerobic digestion period by ten days compared to the untreated control. This suggests that using domesticated paddy soil microbes as a pretreatment might be a sustainable and cost-effective strategy for improving the degradation efficacy and methane production from lignocellulosic materials.

Anaerobic co-digestion of food waste and sewage sludge in anaerobic sequencing batch reactors with application of co-hydrothermal pretreatment of sewage sludge and biogas residue

The effect of pretreatment technologies and reactor types on conversion efficiency and operating costs of anaerobic co-digestion of food waste and sewage sludge were investigated by 300-day continuous experiments. The volatile solids (VS) removal efficiency increased from 61% to 77% with the application of co-hydrothermal pretreatment of sewage sludge and biogas residue. Deep dewatering reduced the volume of hydrothermally pretreated biogas residue by 85%. When continuous stirred tank reactors (CSTRs) were converted to anaerobic sequencing batch reactors (ASBRs), vS removal efficiencies increased by 6%, attributed to a 1.4-1.6-fold increase in solids retention time (SRT). The bottom drainage of mineralized sludge every 40 days increased ASBR stability. Firmicutes and Methanosphaera dominated the bacterial and archaeal communities, respectively. Operating costs decreased by 14.9 US$/metric ton feedstock by applying ASBRs. Compared to CSTRs, ASBRs achieved higher organic matter conversion efficiency, smaller volume of biogas residue, and lower operating costs.

Integrated system of anaerobic digestion and pyrolysis for valorization of agricultural and food waste towards circular bioeconomy: Review

Agricultural and food waste have become major issue affecting the environment and climate owing to growing population. However, such wastes have potential to produce renewable fuels which will help to meet energy demands. Numerous valorization pathways like anaerobic digestion, pyrolysis, composting and landfilling have been employed for treating such wastes. However, it requires integrated system that could utilize waste and promote circular bioeconomy. This review explores integration of anaerobic digestion and pyrolysis for treating agricultural and food waste. Proposed system examines the production of biochar and pyro-oil by pyrolysis of digestate. The use of this biochar for stabilizing anaerobic digestion process, biogas purification and soil amendment will promote the circular bioeconomy. Kinetic models and framework of techno-economic analysis of system were discussed and knowledge gaps have been identified for future research. This system will provide sustainable approach and offer carbon capture and storage in form of biochar in soil.

Current development and perspectives of anaerobic bioconversion of crop stalks to Biogas: A review

As one of the most abundant biomass resources, crop stalks are great potential feedstock available for anaerobic digestion (AD) to produce biogas. However, the specific physical properties and complex chemical structures of crop stalks form strong barriers to efficient AD bioconversion. To overcome these problems, many efforts have been made over the past few years. This paper reviewed recent research in the evolving field of anaerobic bioconversion of crop stalks and was focused on three critical aspects affecting AD performance: various pretreatment methods and their effects on the improvement of crop stalk biodegradability, determination of specific AD operation parameters for crop stalks, and development of AD technologies. Finally, recommendations on the future development of crop stalk AD were proposed.

Anaerobic digestion characteristics and key microorganisms associated with low-temperature rapeseed cake and sheep manure fermentation

In this study, gas production from mixed anaerobic fermentation of rapeseed cake and sheep manure at low temperature (15.2-17.8 °C) was investigated in Qinghai rural household biogas digesters to understand the temporal dynamics of key microbial populations involved in fermentations. Different raw material ratios resulted in significantly different effects on biogas yields and microbial community compositions over 40 days. When the dry weight ratio of sheep manure to rapeseed cake was 1:2, the highest level of cumulative gas production was observed (122.92 m³·t^-1). Firmicutes, Proteobacteria, Bacteroidetes, and Actinobacteria were the dominant bacterial phyla among the 29 digester samples (total relative abundances > 79.23%), followed by Synergistetes (4.09-10.7%). Lactobacillus was the most abundant genus in the biogas digesters with high rapeseed cake contents (average relative abundances: 14.68%), while Peptoniphilus exhibited higher abundances (12.69%) in the mixed treatments. In addition, unclassified Synergistaceae abundances (6.64%) were positively associated with biogas production variation among treatments. Bacteroides (5.74%) and Pseudomonas (5.24%) both accounted for larger proportions of communities in the digesters that used more sheep manure. Methanomicrobiales (66.55%) was the most dominant archaeal group among digesters, with Methanogenium (41.82%) and Methanoculleus (16.55%) representing the main gas-producing archaeal genera; they were more abundant in biogas digesters with higher sheep manure contents and higher rapeseed cake contents, respectively. VFAs and pH were the main factors associated with differences in microbial communities among the 29 samples. Specifically, VFA concentrations were positively correlated with Lactobacillus, Methanoculleus and Methanothrix abundances, while pH was positively correlated with Bacteroides, Pseudomonas, and Methanobacterium abundances.

Stepwise alkaline treatment coupled with ammonia stripping to enhance short-chain fatty acids production from waste activated sludge

To recover resource from waste activated sludge (WAS) is of great significance. This study proposed a promising way, i.e., stepwise alkaline treatment coupled with ammonia stripping, to remarkably enhance short-chain fatty acids (SCFAs) production from WAS anaerobic digestion. The maximal production of SCFAs, with the value of 323 mg COD/g volatile suspended solid, was obtained with first initial pH = 10 adjustment followed by second initial pH = 10 adjustment on third day coupled with ammonia stripping. Mechanistic studies showed that solubilization of both extracellular polymeric substances and cells could be accelerated by stepwise initial pH = 10 adjustment. However, without ammonia stripping, the activities of either acidogens or methanogens could be inhibited by free ammonia formed under alkaline conditions; positively, anaerobes related to SCFAs production were enriched with ammonia stripping. Moreover, the proposed strategy can simultaneously achieve nitrogen and carbon recovery, providing some solutions for the carbon-neutral operation of wastewater treatment plants.