Inoculation enhances directional humification by increasing microbial interaction intensity in food waste composting

Two-stage inoculation with lignocellulose-degrading microorganisms in composting: Enhanced humification efficiency and underlying mechanisms

In this study, lignocellulose-degrading microbes were added to cattle manure and bagasse co-compost through initial- and two-stage inoculations. A comparison was made between the effects of the two inoculations on compost humification parameters, and an investigation was conducted into the dynamic succession of the microbial community, microbial interactions, and amino acid metabolism to uncover the underlying mechanisms. The results showed that two-stage inoculation increased the humus (HS) and humic acid (HA) contents to 86.59 mg/g and 25.80 mg/g, respectively, and achieved a germination index (GI) of 128.77%. At the genus level, it stimulated the growth of Corynebacterium, Thermobifida, and Aspergillus during the high-temperature period, and Luteimonas, Pseudomonas, Actinomadura, and Rhizopus during the maturity period. Two-stage inoculation increased the stability of the bacterial network and microbial cooperation within the fungal network. Additionally, from the cooling to the maturity period, it boosted ten amino acid synthesis pathways. In conclusion, two-stage inoculation is an effective method to promote the maturation and stabilization of co-compost.

Differential contributions of microbial necromass to humic acid during composting of organic wastes

Microbial necromass is a crucial source of stable organic matter in composting, yet its role in the humification process remains poorly understood. This study aims to explore the contribution of microbial necromass carbon (MNC) to humic acid (HA) formation during the composting of sewage sludge (SS), kitchen waste (KW), and pig manure (PM), and to examine the involvement of fungal communities in microbial necromass humification. The results show that fungal necromass carbon (FNC) consistently contributes more to MNC than bacterial necromass carbon (BNC), with FNC accounting for over 60% of MNC across all treatments. KW exhibited the highest accumulation of FNC (4.09-98.92 g/kg), and its MNC contribution to total organic carbon was 23.63%, significantly higher than sewage sludge (5.57%) and pig manure (7.47%). The carbon-to-nitrogen (C/N) ratio was found to be a critical factor influencing microbial growth, necromass accumulation, and HA formation, with a lower C/N ratio promoting faster microbial turnover and enhancing MNC contribution to HA. The analysis also revealed that Ascomycota dominated the maturation phase, with a significant role in driving humification, especially in KW. Structural equation modeling confirmed that FNC and BNC are directly influenced by the C/N ratio, which in turn affects HA formation This study enhances our understanding of microbial necromass dynamics and its contribution to humic substance formation, providing valuable insights for improving compost quality and optimizing composting strategies for enhanced carbon sequestration.

Enhancing aerobic composting of food waste by adding hydrolytically active microorganisms

The biomass of native microorganisms in food waste (FW) suitable for accelerated composting is initially low and requires time for adaptation. Adding of efficient hydrolytic microorganisms should be able to enhance compost-specific microbial activity, adjust microbial community structure, and potentially hasten FW biodegradation. This study aimed to identify bacterial and fungal strains with growth characteristics suitable for accelerating FW composting. Over 7 weeks, FW was composted in a pilot-scale test, either inoculated at the start or on day 28 with three different mixtures of 10 autochthonous Bacillus and Penicillium spp. strains known for their high hydrolytic activity. The effects of inoculation were assessed by measuring the rate of carbon dioxide (CO2) and ammonia (NH3) production and also the increase in temperature due to spontaneous exothermic activity of the enhanced microbial population degrading FW. Inoculation with Bacillus spp., particularly B. amyloliquefaciens and B. subtilis, at the beginning of composting increased CO2 production nearly 3-fold while maintaining stable ammonia production and temperature. The high concentration of Bacillus relative to native FW microorganisms led to dominant fermentation processes even in the presence of oxygen, resulting in moderate heat release and elevated production of volatile organic compounds. Introducing Penicillium spp. at a later stage (day 28) increased CO2 production nearly 2-fold, along with higher NH3 levels and temperature. These findings highlight the significance of inoculation timing and microbial composition in regulating metabolic pathways during FW composting degradation, offering insights for designing effective microbial formulations for composting.

Evaluation of the quality of products from multiple industrial-scale composting treatment facilities for kitchen waste and exploration of influencing factors

The aerobic composting process is extensively utilized to manage kitchen waste. Nonetheless, the variability in the quality of compost derived from engineering practices which significantly hinders its broader industrial application. This work investigated the final products of kitchen waste compost at multiple industrial-scale treatment facilities utilizing three distinct aerobic composting processes in a bid to explore key factors affecting compost quality. The quality evaluation was based on technical parameters like seed germination index (GI), and limiting factors such as heavy metal content. The results showed that most of the compost products failed to meet the established standards, with GI being the primary limiting indicator. Furthermore, maturity assessments suggested that compost with low GI exhibited reduced humification could not be recommended for agricultural use. The investigation delved into the primary determinants of GI, focusing on risk factors such as the oil and salt of kitchen waste, and the microbial community of the humification driving forces. The results indicated that products with low GI had higher oil and salt content and a relatively simple microbial community. A thorough analysis suggested that excessive levels oil and salt were potential influencing factors on GI, as they stimulated the activity of acid-producing bacteria like Lactobacillus, suppressed the activity of humification-promoting bacteria such as Actinomarinales, and influenced the decomposition and humification processes of organic matter and total nitrogen, thereby affecting product quality. The findings provide valuable insights for improving kitchen waste compost products for agricultural applications.

Sustainable control of invasive plants: Compost production, quality and effects on wheat germination

Invasive plant species pose significant ecological threats worldwide, affecting the stability and biodiversity of local ecosystems. As a result of their control, a considerable amount of plant biomass is produced, which can be used to produce various value-added products. Five different composts were prepared from three invasive plant species found in Latvia - Reynoutria japonica, Solidago canadensis, Lupinus polyphyllus. The stages of composting have been investigated and recommendations for process optimization have been made based on the quality characterization of the final compost. The quality of the prepared invasive plant biomass composts has been evaluated based on the main plant nutrient concentration, humic substance concentration, and mineral contents. The allelopathic lupin alkaloid concentration throughout the composting process has been evaluated and shows a consistent reduction. Obtained compost quality complies with the EU regulations for fertilizing products and soil amendments thus it can be considered equivalent to industrially produced compost and vermicompost. Seed germination tests confirm that compost prepared from invasive plants is suitable for plant growth and comparable to commercial composts. Based on pilot-scale composting results, recommendations for invasive plant composting have been suggested.

Composite additives regulate physicochemical and microbiological properties in green waste composting: A comparative study of single-period and multi-period addition modes

Composting additives can significantly enhance green waste (GW) composting. However, their effectiveness is limited due to the short action duration of a single-period addition. Therefore, this study proposes that multi-period additive modes to prolong the action duration, expedite lignocellulose degradation, reduce composting time, and enhance product quality. This study conducted six treatments (T1-T6), introducing a compound additive (BLP) during the mesophilic (MP) and cooling periods (CP). Each treatment consistently maintained 25% total BLP addition of GW dry weight, with variations only in the BLP distribution in different periods. The composition of BLP consists of Wbiochar: Wlactic acid: Wpond sediment in a ratio of 10:1:40. Specifically, T1 added 25% BLP in CP, T2 added 5% in MP and 20% in CP, T3 added 10% in MP and 15% in CP, T4 added 15% in MP and 10% in CP, T5 added 20% in MP and 5% in CP, and T6 added 25% in MP. In this study, composting temperature, pH value, electrical conductivity, total porosity, the contents of lignin, cellulose, hemicellulose, and nutrient, scanning electron microscopy images, germination index, and the successions of different bacteria and fungi at the phylum and genus levels were detailed. Results showed T4 achieved two thermophilic periods and matured in just 25 days. T4 enhanced lignocellulose degradation rates (lignin: 16-53%, cellulose: 14-23%, hemicellulose: 9-48%) and improved nutrient content. The above results, combined with correlation analysis and structural equation model, indicated that T4 may promote the development of dominant bacteria (Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes) by regulating compost physicochemical properties and facilitate the growth of dominant fungi (Ascomycota and Basidiomycota) by modulating nutrient supply capacity. This ultimately leads to a microbial community structure more conducive to lignocellulose degradation and nutrient preservation. In summary, this study reveals the comprehensive effects of single-period and multi-period addition methods on GW composting, providing a valuable basis for optimizing the use of additives and enhancing the efficiency and quality of GW composting.

Investigating inhibiting factors affecting seed germination index in kitchen waste compost products: Soluble carbon, nitrogen, and salt insights

The seed germination index (GI) serves as the principal determinant that impedes the integration of aerobic composting products into agricultural lands. The current research work predominantly focuses on exploring the correlation between physical and chemical indicators of the compost products and GI, neglecting the fundamental cause. This study systematically analyzed the composition of GI aqueous extracts from compost products derived from kitchen waste under various composting methodologies, with nitrogen, carbon, and inorganic salt as critical factors. The analytical work concluded that acetic acid, formic acid, and ammonium were the inhibitory factors influencing GI. Validation experiments introduced inhibitory factors, yielding a functional relationship formula depicting GI variations due to a single influential factor. This study conclusively identified acetic acid as the primary constraint, establishing that its inhibitory concentration corresponded to 70 % GI stands at 85 mg/L. This study will provide guidelines for the future research on enhancing aerobic composting techniques.

Mechanism underlying effects of cellulose-degrading microbial inoculation on amino acid degradation and biosynthesis during composting

Amino acids are essential organic compounds in composting products. However, the mechanism underlying the amino acid metabolism during composting remains unclear. This study aims at exploring the impacts of inoculating cellulose-degrading microbes on amino acid metabolism during composting with mulberry branches and silkworm excrements. Cellulose-degrading microbial inoculation enhanced amino acid degradation by 18%-43% by increasing protease and sucrase activities and stimulating eight amino acid degradation pathways from the initial to thermophilic phases, with Enterococcus, Saccharomonospora, Corynebacterium being the dominant bacterial genera, but stimulated amino acid production by 54% by increasing sucrase and urease activities, decreasing β-glucosidase activities, and stimulating twenty-two amino acid synthesis pathways at the mature phase, with Thermobifida, Devosia, and Cellulosimicrobium being the dominant bacterial genera. The results suggest that cellulose-degrading microbial inoculation enhances amino acid degradation from the initial to thermophilic phases and biosynthesis at the mature phase, thereby improving the quality of organic fertilizer.

The bioaugmentation effect of microbial inoculants on humic acid formation during co-composting of bagasse and cow manure

The effective degradation of recalcitrant lignocellulose has emerged as a bottleneck for the humification of compost, and strategies are required to improve the efficiency of bagasse composting. Bioaugmentation is a promising method for promoting compost maturation and improving the quality of final compost. In this study, the bioaugmentation effects of microbial inoculants on humic acid (HA) formation during lignocellulosic composting were explored. In the inoculated group, the maximum temperature was increased to 72.5 °C, and the phenol-protein condensation and Maillard humification pathways were enhanced, thus increasing the HA content by 43.85%. After inoculation, the intensity of the microbial community interactions increased, particularly for fungi (1.4-fold). Macrogenomic analysis revealed that inoculation enriched thermophilic bacteria and lignocellulose-degrading fungi and increased the activity of carbohydrate-active enzymes and related metabolic functions, which effectively disrupted the recalcitrant structure of lignocellulose to achieve a high humification degree. Spearman correlation analysis indicated that Stappia of the Proteobacteria phylum, Ilumatobacter of the Actinomycetes phylum, and eleven genera of Ascomycota were the main HA producers. This study provides new ideas for bagasse treatment and recycling and realizing the comprehensive use of resources.

Low-intensity alternating ventilation achieves effective humification during food waste composting by enhancing the intensity of microbial interaction and carbon metabolism

Vertical static composting is an efficient and convenient technology for the treatment of food waste. Exploring the impact of oxygen concentration levels on microbial community structure and functional stability is crucial for optimizing ventilation technology. This study set three experimental groups with varying ventilation intensities based on self-made alternating ventilation composting reactor (AL2: 0.2 L kg-1 DM·min-1; AL4: 0.4 L kg-1 DM·min-1; AL6: 0.6 L kg-1 DM·min-1) to explore the optimal alternating ventilation rate. The results showed that the cumulative ammonia emission of AL2 group reduced by 25.13% and 12.59% compared to the AL4 and AL6 groups. The humification degree of the product was 1.18 times and 1.25 times higher than the other two groups. AL2 increased the relative abundance of the core species Saccharomonospora, thereby strengthening microbial interaction. Low-intensity alternating ventilation increased the carbon metabolism levels, especially aerobic_chemoheterotrophy, carbohydrate and lipid metabolism. However, it simultaneously reduced nitrogen metabolism. Structural equation model analysis demonstrated that alternating low-intensity ventilation effectively regulated both microbial diversity (0.81, p < 0.001) and metabolism (0.81, p < 0.001) by shaping the composting environment. This study optimized the intensity of alternating ventilation and revealed the regulatory mechanism of community structure and metabolism. This study provides guidance for achieving efficient and low-consumption composting.

Fungi play a crucial role in sustaining microbial networks and accelerating organic matter mineralization and humification during thermophilic phase of composting

Fungi play an important role in the mineralization and humification of refractory organic matter such as lignocellulose during composting. However, limited research on the ecological role of fungi in composting system hindered the development of efficient microbial agents. In this study, six groups of lab-scale composting experiments were conducted to reveal the role of fungal community in composting ecosystems by comparing them with bacterial community. The findings showed that the thermophilic phase was crucial for organic matter degradation and humic acid formation. The Richness index of the fungal community peaked at 1165 during this phase. PCoA analysis revealed a robust thermal stability in the fungal community. Despite temperature fluctuations, the community structure, predominantly governed by Pichia and Candida, remained largely unaltered. The stability of fungal community and the complexity of ecological networks were 1.26 times and 5.15 times higher than those observed in bacterial community, respectively. Fungi-bacteria interdomain interaction markedly enhanced network complexity, contributing to maintain microbial ecological functions. The core fungal species belonging to the family Saccharomycetaceae drove interdomain interaction during thermophilic phase. This study demonstrated the key role of fungi in the composting system, which would provide theoretical guidance for the development of high efficiency composting agents to strengthen the mineralization and humification of organic matter.

The impact of microbial inoculants on large-scale composting of straw and manure under natural low-temperature conditions

Understanding large-scale composting under natural conditions is essential for improving waste management and promoting sustainable agriculture. In this study, corn straw (400 tons) and pig manure (200 tons) were composted with microbial inoculants. The thermophilic phase of composting lasted for fourteen weeks, resulting in an alkaline final product. Microbial systems with low-temperature initiation and high-temperature fermentation played a crucial role in enhancing lignocellulose degradation and humic substances (HS) formation. Adding microbes, including Rhodanobacter, Pseudomonas, and Planococcus, showed a positive correlation with degradation rates of cellulose, hemicellulose, and lignin. Bacillus, Planococcus, and Acinetobacter were positively correlated with HS formation. Microorganisms facilitated efficient hydrolysis of lignocelluloses, providing humic precursors to accelerate composting humification through phenolic protein and Maillard pathways. This study provides significant insights into large-scale composting under natural conditions, contributing to the advancement of waste management strategies and the promotion of sustainable agriculture.

Alternating ventilation accelerates the mineralization and humification of food waste by optimizing the temperature-oxygen-moisture distribution in the static composting reactor

Traditional unidirectional ventilation often leads to the loss of heat and moisture during composting, disrupting the favorable microenvironment required for aerobic microbes. This study developed a pulse alternating ventilation composting reactor and investigated the effects of alternating ventilation on composting efficiency compared with upward ventilation and downward ventilation. The results demonstrated that alternating ventilation stabilized the moisture content at approximately 60 % while reducing the temperature and oxygen concentration range within the reactor. Moreover, it extended the duration of high-temperature (>50 °C) by 31 % and 75 % compared to other two groups. It improved the microbial cooperation intensity and stimulated the core microbe (Tepidimicrobium). Seed germination index (GI) of the compost was improved (GI = 91.27 %), and the humic acid content was 1.23 times and 1.37 times higher than other two groups. These results showed that alternating ventilation can be used for efficient resource disposal of food waste.

A bibliometric analysis of emerging contaminants (ECs) (2001-2021): Evolution of hotspots and research trends

Emerging contaminants (ECs) have attracted increasing attention in the past two decades because of their ubiquitous existence and high environmental risk. Understanding the progress of research and the evolution of hot topics is critical. This study provides a bibliometric review, along with a quantitative trend analysis of approximately 8000 publication records dated from 2001 to 2021. Wider distribution in various subjects was discovered in terms of publication numbers, indicating a strong tendency for EC research to become an interdisciplinary topic. Visualization of term co-occurrence analysis revealed that the ECs study went through three stages over time: identification and detection, traceability and risk, and process and control. Quantitative trend analysis revealed that antibiotics, microplastics, endocrine disrupting chemicals (EDCs), per/poly-fluoroalkyl substances (PFAS), pesticides, heavy metals, and nanoparticles are attracting increasing attention, whereas conventional pharmaceuticals, persistent organic pollutants, and materials such as benzotriazole, diclofenac, bisphenol A, carbamazepine, triclosan, and titanium dioxide exhibit a downward trend. PFAS and EDCs are considered potential future core hotspots for the hysteretic rise in research attention compared with conventional ECs. Furthermore, analysis of research linkage and the developing stages of ECs could be possible approach to determine the evolution of hotspots in ECs study. This study provides objective and comprehensive insights into the research landscape of ECs, which may shed light on future developmental directions for researchers interested in this field.

A change in substance and microbial community structure during the co-composting of kitchen waste anaerobic digestion effluent, sewage sludge and Chinese medicine residue

Anaerobic digestion is a resource recovery method for organic waste, gaining attention due to carbon reduction. Disposing of anaerobic digestion effluent (ADE) is crucial for developing anaerobic digestion, but conventional wastewater treatment fails to effectively recover nutrients contained in the ADE. In the present study, the ADE without solid-liquid separation was mixed with sewage sludge and Chinese medicine residue for the composting, where the ADE could be recovered at high temperature through humification. Besides, the nitrogen balance, humification process, and microbial dynamics during the composting process were studied. The results showed that the group supplemented with ADE could increase the nitrogen retention efficiency by 2.21 % compared to the control group. High ammonia nitrogen content and salinity did not negatively affect the maturity and phytotoxicity of compost products and even increase the humification degree of compost products. Moreover, additional ADE may not alter microbial community structure, which could contribute to microbial succession. This is the first time to investigate the substance transformation and shift in microbial community structure while applying composting process for ADE treatment, in which the anaerobic-aerobic collaborative disposal process provides an alternative solution for the recovery of ADE.

Bacillus licheniformis inoculation promoted humification process for kitchen waste composting: Organic components transformation and bacterial metabolic mechanism

Kitchen waste (KW) composting always has trouble with slow humification process and low humification degree. The objective of this study was to develop potentially efficient solutions to improve the humification of KW composting, accelerate the humus synthesis and produce HS with a high polymerization degree. The impact of Bacillus licheniformis inoculation on the transformation of organic components, humus synthesis, and bacterial metabolic pathways in kitchen waste composting, was investigated. Results revealed that microbial inoculation promoted the degradation of organic constituents, especially readily degradable carbohydrates during the heating phase and lignocellulose fractions during the cooling phase. Inoculation facilitated the production and conversion of polyphenol, reducing sugar, and amino acids, leading to an increase of 20% in the content of humic acid compared to the control. High-throughput sequencing and network analysis indicated inoculation enriched the presence of Bacillus, Lactobacillus, and Streptomyces during the heating phase, while suppressing the abundance of Pseudomonas and Oceanobacillus, enhancing positive microbial interactions. PICRUSt2 analysis suggested inoculation enhanced the metabolism of carbohydrates and amino acids, promoting the polyphenol humification pathway and facilitating the formation of humus. These findings provide insights for optimizing the humification process of kitchen waste composting by microbial inoculation.

Identifying the role of array electrodes in improving the compost quality of food waste during electric field-assisted aerobic composting

Low composting temperature and long maturation periods are two major problems during food waste composting. In this study, a novel array-based electric field-assisted aerobic composting (Pin-EAC) process was tested on food waste compost. Pin-EAC increase the composting temperature to 69.3 °C, and improved the germination index by 15%. The Pin-EAC took at least 40% less time to reach the standard compost maturity. The fluorescent spectroscopy results showed that Pin-EAC could increase humic acid and fulvic acid by 33% and 37%, respectively. Pin-EAC could increase the diversity of thermophilic bacteria during composting. The co-occurrence network shown that Pin-EAC are more closely related to oxygen and temperature. This work has initially shown that the use of an electric field could improve food waste composting quality, suggesting that the Pin-EAC process is an effective strategy for high-water and high-oil organic solid waste aerobic composting.

Microbiome data analysis via machine learning models: Exploring vital players to optimize kitchen waste composting system

Composting, reliant on microorganisms, effectively treats kitchen waste. However, it is difficult to precisely understand the specific role of key microorganisms in the composting process by relying solely on experimental research. This study aims to employ machine learning models to explore key microbial genera and to optimize composting systems. After introducing a novel microbiome preprocessing approach, Stacking models were constructed (R2 is about 0.8). The SHAP method (SHapley Additive exPlanations) identified Bacillus, Acinetobacter, Thermobacillus, Pseudomonas, Psychrobacter, and Thermobifida as prominent microbial genera (Shapley values ranging from 3.84 to 1.24). Additionally, microbial agents were prepared to target the identified key genera, and experiments demonstrated that the composting quality score was 76.06 for the treatment and 70.96 for the control. The exogenous agents enhanced decomposition and improved compost quality in later stages. In summary, this study opens up a new avenue to identifying key microorganisms and optimizing the biological treatment process.

Adding siderophores: A new strategy to reduce greenhouse gas emissions in composting

Microbial community is the primary driver causing the greenhouse gas emissions in composting. Thus, regulating the microbial communities is a strategy to reduce them. Here, two different siderophores (enterobactin and putrebactin) were added, which could bind and translocate iron by specific microbes, to regulate the composting communities. The results showed that adding enterobactin enriched Acinetobacter and Bacillus with specific receptors by 6.84-fold and 6.78-fold. It promoted carbohydrate degradation and amino acid metabolism. This resulted in a 1.28-fold increase in humic acid content, as well as a 14.02% and 18.27% decrease in CO₂ and CH₄ emissions, respectively. Meanwhile, adding putrebactin boosted the microbial diversity by 1.21-fold and enhanced potential microbial interactions by 1.76-fold. The attenuated denitrification process led to a 1.51-fold increase in the total nitrogen content and a 27.47% reduction in N₂O emissions. Overall, adding siderophores is an efficient strategy to reduce greenhouse gas emissions and promote the compost quality.