Effect of biochar on the mitigation of organic volatile fatty acid emission during aerobic biostabilization of biosolids and the underlying mechanism

Mechanism of biochar in alleviating the inhibition of anaerobic digestion under ciprofloxacin press

The antibiotic ciprofloxacin (CIP), detected in various aqueous environments, has broad-spectrum antimicrobial properties that can severely affect methanogenic performance in anaerobic systems. In this study, a novel strategy to alleviate the inhibition of AD performance under CIP press with the direct addition of biochar (BC) prepared from corn stover was proposed and the corresponding alleviation mechanism was investigated. When the dosage of BC was 5 and 20 g/L, the cumulative methane production in AD could reach 317.9 and 303.0 mL/g COD, and the CIP degradation efficiencies reached 94.1 % and 96.6 %, significantly higher than those of 123.0 mL/g COD and 81.2 % in the Control system. BC avoided excessive reactive oxygen species in anaerobic systems and induced severe oxidative stress response, while protecting the cell membrane and cell wall of microorganisms. Microorganisms could consume and utilize more organic extracellular polymeric substances for their growth and metabolism. When BC was involved in AD, fewer toxic intermediates were generated during CIP biodegradation, reducing acute and chronic toxicity in anaerobic systems. Microbial diversity suggested that BC could enrich functional microorganisms involved in direct interspecies electron transfer like Methanosaeta, norank_f_Bacteroidetes_vadinHA17, JGI-0000079-D21 and Syntrophomonas, thus facilitating the methanogenic process and CIP degradation. Genetic analyses showed that BC could effectively upregulate functional genes related to the conversion of butyrate-to-acetate and acetyl-to-methane under CIP stress, while functional gene abundance associated with CIP degradation enhanced partially, about encoding translocases, oxidoreductases, lyases, and ligases. Therefore, BC can be added to AD under CIP press to address its inhibited methanogenic performance.

Phosphate, magnesium containing additives and biochar regulate compost maturity and synergistically reduce odor emission in chicken manure composting: Role of physicochemical, bacterial and fungal dynamics

This study explored the odor composition and emission in chicken manure composting process, employing chemical fixatives and biochar to mitigate odors effectively. Compost maturity, ammonia, sulfur-containing odor emissions, as well as the bacterial and fungal community structure were analyzed to assess composting performance and mechanisms. The results indicated that four malodorous substances were identified as major contributors: dimethyl disulfide (Me2S2), hydrogen sulfide (H2S), methyl sulfide (Me2S), and ammonia (NH3). Biochar (BC) augmented compost maturity by enhancing the relative abundance of Thermobifida and Saccharomonospora, while reducing the emission of total malodorous sulfur-containing components by 65.3% by mitigating Halocella and Hydrogenispora, albeit without affecting NH3 emissions. Superphosphate (SP) mitigated malodorous sulfur-containing components by 51.3% through its calcium (Ca) components and associated bacteria (Pseudomonas, Halocella and Hydrogenispora). Notably, magnesium (Mg) emerged as a limiting factor for NH3 fixation by SP. The combination of SP and magnesium sulfate (MS) decreased NH3 emissions by 47.9% via the formation of struvite crystals (MgNH4PO4) and further reduced the emission of malodorous sulfur-containing components by 60.0% by enhancing functional fungi inhibiting malodorous substance production. Ultimately, the combined application of BC, SP, and MS yielded the most significant odor reduction effects, with reductions of 86.8%, 35.3%, 92.8%, and 38.8% observed in Me2S2, H2S, Me2S, and NH3, respectively.

Response of food waste anaerobic digestion to the dimensions of micron-biochar under 30 g VS/L organic loading rate: Focus on gas production and microbial community structure

Biochar modification is an effective approach to enhance its ability to promote anaerobic digestion (AD). Focusing on the physical properties of biochar, the impact of different particle sizes of biochar on AD of food waste (FW) at high organic loading rate (OLR) was investigated. Four biochar with different sizes (40-200 mesh) were prepared and used in AD systems at OLR 30 g VS/L. The research results found that biochar with a volume particle size of 102 μm (RBC-P140) had top-performance in promoting cumulative methane production, increasing by 13.20% compared to the control group. The analysis results of the variety in volatile acids and alkalinity in the system did not show a correlation with the size of biochar, but small size has the potential to improve the environmental tolerance of the system to high acidity. Microbial community analysis showed that the abundance of aceticlastic methanogen and the composition of zoogloea were optimized through relatively small-sized biochar. Through revealing the effect of biochar particle size on AD system at high OLR, this work provided theoretical guidance for regulating fermentation systems using biochar.

New insight into the spatio-temporal patterns of functional groups of hotspot inside the composting aggregates by synchrotron-based FTIR in hyperthermophilic composting

Hyperthermophilic composting (HTC) is a recently developed and highly promising organic fraction of municipal solid waste (OFMSW) treatment technology. Investigation of organic matter (OM) dynamics in compost particle is thus crucial for the understanding of humification of HTC process. Herein, this work aimed to study the chemical and structural changes of OM at the molecular level during HTC of OFMSW using EEM and SR-FTIR analyses. Additionally, two-dimensional correlation spectroscopy (2D-COS) was also utilized to probe and identify the changes in chemical constituents and functional groups of organic compounds on the surface of compost particles during different composting periods. Results show that SR-FTIR can detect fine-scale (~μm) changes in functional groups from the edges to the interior of compost particles during different composting periods by mapping the particles in situ. In the hyperthermophilic stage (day 9), the extracted μ-FTIR spectrum reveals a distinct boundary between anaerobic and aerobic regions within the compost particle, with a thickness of anaerobic zone (1460 cm-1) of approximately 30 μm inside the particle's core. This provides direct evidence of anaerobic trends at compost microscales level within compost particles. 2D-COS analysis indicated that organic functional groups gradually agglomerated in the order of 1330 > 2930 > 3320 > 1600 > 1030 > 895 cm-1 to the core skeleton of cellulose degradation residues, forming compost aggregates with well physicochemical properties. Overall, the first combination of SR-FTIR and EEM provides complementary explanations for the humification mechanism of HTC, potentially introducing a novel methodology for investigating the environmental behaviors and fates of various organic contaminants associated with OM during the in-situ composting biochemical process.

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.

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.

Predicting maturity and identifying key factors in organic waste composting using machine learning models

The measurement of germination index (GI) in composting is a time-consuming and laborious process. This study employed four machine learning (ML) models, namely Random Forest (RF), Artificial Neural Network (ANN), Support Vector Regression (SVR), and Decision Tree (DT), to predict GI based on key composting parameters. The prediction results showed that the coefficient of determination (R2) for RF (>0.9) and ANN (>0.9) was higher than SVR (<0.6) and DT (<0.8), suggesting that RF and ANN displayed superior predictive performance for GI. The SHapley additive exPlanations value result indicated that composting time, temperature, and pH were the important features contributing to GI. Composting time was found to have the most significant impact on GI. Overall, RF and ANN were suggested as effective tools for predicting GI in composting. This study offers the reliable approach of accurately predicting GI in composting processes, thereby enabling intelligent composting practices.

Insight into the humification and carbon balance of biogas residual biochar amended co-composting of hog slurry and wheat straw

The impact of biogas residual biochar (BRB) on the humification and carbon balance process of co-composting of hog slurry (HGS) and wheat straw (WTS) was examined. The 50-day humification process was significantly enhanced by the addition of BRB, particular of 5% BRB, as indicated by the relatively higher humic acid content (67.28 g/kg) and humification ratio (2.31) than other treatments. The carbon balance calculation indicated that although BRB addition increased 22.16-46.77% of C lost in form of CO2-C, but the 5% BRB treatment showed relatively higher C fixation and lower C loss than other treatments. In addition, the BRB addition reshaped the bacterial community structure during composting, resulting in increased abundances of Proteobacteria (25.50%) during the thermophilic phase and Bacteroidetes (33.55%) during the maturation phase. Combined these results with biological mechanism analysis, 5% of BRB was likely an optimal addition for promoting compost humification and carbon fixation in practice.

Promoting nitrogen conversion in aerobic biotransformation of swine slurry with the co-application of manganese sulfate and biochar

This study aimed to explore the co-application of MnSO4 (Mn) and biochar (BC) in nitrogen conversion during the composting process. A 70-day aerobic composting was conducted using swine slurry, supplemented with different levels of Mn (0, 0.25%, and 0.5%) and 5% BC. The results demonstrated that the treatment with 0.5MnBC had the highest levels of NH4+-N (3.07 g kg-1), TKN (29.90 g kg-1), and NO3--N (1.94 g kg-1) among all treatments. Additionally, the 0.5MnBC treatment demonstrated higher urease, protease, nitrate reductase, and nitrite reductase activities than the other treatments, with the peak values of 18.12, 6.96, 3.57, and 15.14 mg g-1 d-1, respectively. The addition of Mn2+ increased the total organic nitrogen content by 29.59%-47.82%, the acid hydrolyzed ammonia nitrogen (AN) content by 13.84%-57.86% and the amino acid nitrogen (AAN) content by 55.38%-77.83%. The richness of Chloroflexi and Ascomycota was also enhanced by the simultaneous application of BC and Mn. Structural equation modeling analysis showed that Mn2+ can promote the conversion of Hydrolyzed Unknown Nitrogen (HUN) into AAN, and there is a positive association between urease and NH4+-N according to redundancy analysis. Firmicutes, Basidiomycota, and Mortierellomycota showed significant positive correlations with ASN, AN, and NH4+-N, indicating their crucial roles in nitrogen conversion. This study sheds light on promoting nitrogen conversion in swine slurry composting through the co-application of biochar and manganese sulfate.

Direct humification of biowaste with hydrothermal technology: A review

Hydrothermal humification of biowaste, in comparison to the traditional coal-based humic acid extraction process, better aligns with the goals of carbon neutrality and sustainability. This article provided a comprehensive review on the current advancements in hydrothermal humification of biowaste. Hydrothermal humic acid (HHA) derived from different biowaste sources was compared, exhibiting significant differences in their hydrophobicity, oxygen-containing functional group content, and structural characteristics. The influence of key parameters, including reaction temperature, residence time, pH and the action of catalysts on HHA yield was analyzed. The pathways through which biowaste and its major components transform into HHA were elucidated. Coal-like hydrochar has shown significant potential for producing HHA through hydrothermal treatment, with HHA selectivity exceeding 65 %. HHA also exhibits promising performance in agriculture and environmental remediation, offering comparable value to commercial humic acid. Future research should concentrate on establishing the correlation between hydrothermal conditions and the efficiency of biowaste humification, thereby facilitating the development of a predictive model for assessing efficiency. Additionally, exploring the application value of hydrothermal-synthesized HHA with diverse chemical characteristics will guide the optimization of hydrothermal conditions and selection of suitable feedstock.

Relative contribution of ammonia-oxidizing bacteria and denitrifying fungi to N2O production during rice straw composting with biochar and biogas residue amendments

Nitrous oxide (N2O) production is associated with ammonia-oxidizing bacteria (amoA-AOB) and denitrifying fungi (nirK-fungi) during the incorporation of biochar and biogas residue composting. This research examined the relative contribution of alterations in the abundance, diversity and structure of amoA-AOB and nirK-fungi communities on N2O emission by real-time PCR and sequence processing. Results showed that N2O emissions showed an extreme relation with the abundance of amoA-AOB (rs = 0.584) while giving credit to nirK-fungi (rs = 0.500). Nitrosomonas and Nitrosospira emerged as the dominant genera driving ammoxidation process. Biogas residue changed the community structure of AOB by altering Nitrosomonadaceae proportion and physiological capacity. The denitrification process, primarily governed by nirK-fungi, served as a crucial pathway for N2O production, unveiling the pivotal mechanism of biochar to suppress N2O emissions. C/N and NH4+-N were identified as significant parameters influencing the distribution of nirK-fungi, especially Micromonospora, Halomonas and Mesorhizobium.

Effects of oyster shells on maturity and calcium activation in organic solid waste compost

With the rapid development of aquaculture, the production of oyster shells has surged, posing a potential threat to the environment. While oyster shell powder is widely recognized for its inherent alkalinity and rich calcium carbonate content, making it a superior soil conditioner, its role in organic solid waste composting remains underexplored. To investigate the effects of varying concentrations of oyster shell powder on compost maturation and calcium activation, this study employed thermophilic co-composting with acidic sugar residue and bean pulp, incorporating 0% (control), 10% (T1), 20% (T2), 30% (T3), and 40% (T4) oyster shell powder. Findings revealed that appropriate proportions of oyster shell powder significantly enhance temperature stability during composting and elevate maturation levels, notably reducing ammonia emissions between 62.5% and 76.7%. Intriguingly, the calcium in the oyster shell powder was significantly activated during composting, with the 40% addition group achieving the highest calcium activation rate of 48.5%. In summation, the inclusion of oyster shell powder not only optimizes the composting process but also efficiently activates the calcium, resulting in an alkaline organic-inorganic composite soil conditioner with high exchangeable calcium content. This research holds significant implications for promoting the high-value utilization of oyster shells.

Dynamics of antibiotic resistance genes and bacterial community during pig manure, kitchen waste, and sewage sludge composting

Organic solid wastes (OSWs) are important reservoirs for antibiotic resistance genes (ARGs). Aerobic composting transforms OSWs into fertilizers. In this study, we investigated ARGs dynamics and their driving mechanisms in three OSW composts: pig manure (PM), kitchen waste (KC), and sewage sludge (SG). The dominant ARGs were different in each OSW, namely tetracycline, aminoglycoside, and macrolide resistance (PM); tetracyclines and aminoglycosides (KC); and sulfonamides (SG). ARGs abundance decreased in PM (71%) but increased in KC (5.9-fold) and SG (1.3-fold). Interestingly, the ARGs abundance was generally similar in all final composts, which was contributed to the similar bacterial community in final composts. In particular, sulfonamide and β-lactam resistant genes removed (100%) in PM, while sulfonamide in KC (38-fold) and tetracycline in SG (5-fold) increased the most. Additionally, ARGs abundance rebounded during the maturation period in all treatments. Firmicutes, Proteobacteria, and Actinobacteria were the main ARGs hosts. Several persistent and high-risk genes included tetW, aadA, aadE, tetX, strB, tetA, mefA, intl1, and intl2. The structural equation models showed ARGs removal was mainly affected by physicochemical parameters and bacterial communities in PM, the ARGs enrichment in KC composting correlated with increased mobile genetic elements (MGEs). In general, thermophilic aerobic composting can inhibit the vertical gene transfer (VGT) of pig manure and horizontal gene transfer (HGT) of sludge, but it increases the HGT of kitchen waste, resulting in a dramatic increase of ARGs in KC compost. More attention should be paid to the ARGs risk of kitchen waste composting.

Sulfur-aided aerobic biostabilization of swine manure and sawdust mixture: Humification and carbon loss

To investigate how sulfur addition affects humification and carbon loss during swine manure (SM) biostabilisation, various proportions of sulfur, i.e., 0 (CK), 0.2%-0.8% (S1-S4) were added to SM in a 70-day pilot-scale test. Compared to CK (16.07%), sulfur addition resulted in the mineralization of 17.05%-24.27% of the total organic carbon. Sulfur addition also reduced CH4 emissions, which were 3.7%-29.3% lower than that of CK. The total global warming potential values were in the range of 913.1-968.2 g CO2 eq kg-1 for all treatments. Although the sulfur-added treatments showed lower HA/FA ratios than CK after 70 days, no significant impact on the maturity of the final products was observed. Sulfur addition impacted the microbial community, CH4, CO2, N2O emissions, and affected the variation of temperature in biowaste biostabilization. These discoveries provided an important basis for understanding the function of sulfur in regulating the aerobic bio-decomposition of organic waste.

Synergetic effect and mechanism of elementary sulphur, MgSO4 and KH2PO4 progressive reinforcement on pig manure composting nitrogen retention

The potential of sulphur (S), MgSO4 (Mg), and KH2PO4 (P) in nitrogen retention, ammonia emission decrease, and microbial community succession during composting needs to be investigated. To achieve this, different levels of S (0, 0.2, 0.4, 0.6, and 0.8% in dry weight) plus Mg and P (S + Mg + P) were progressively added in 70 days pig manure aerobic composting. The results revealed that the amendment increased salinity and lowered pH and dephytotoxication of the product with the increase of S amount. However, no significant inhibition effects were observed on the evolution of the thermophilic phase and product maturity. In addition, the amendment significantly reduced the total NH3 and N2O emissions by 29.66%-58.81% and 20.6%-56.7%, increased NH4+ level by 17.22%-73.21% in thermophilic phase and NO3- content by 26.17%-57.48% in a mature phase, and elevated the total Kjeldahl nitrogen content by 34.28%-46.6% during the composting. In addition, compared to the control, the supplement markedly encouraged the formation of guanite in the compost product. The S addition stimulated the growth of Anseongella, Actinomadura, Chelativorans, Castellaniella, Luteimonas, and Steroidobacter microbial communities which functioned well in the degradation of nitrogen-containing compounds and organic matter. Evidence from Redundancy Analysis, Firmicutes, Myxococcus, Chloroflexi, Gemmatimonadota, and Deinococcota showed positive correlations with pH. These results imply that adding S-Mg-P amendment encourages the population and activity of specific functional microorganisms, and facilitated the ammonia emission reduction by lowering pH and thus reserved nitrogen through the formation of guanite during composting. The investigation of bacterial community abundance and environmental variables at the phylum and genus levels over time revealed that adding of 0.6% S in conjunction with P and Mg minerals was suitable for nitrogen loss mitigation in composting. The findings suggest using S + Mg + P supplement to conserve nitrogen in pig dung aerobic composting.

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.

The dominant role of cooperation in fungal community drives the humification process of chicken manure composting under addition of regulatory factors

This study aimed to explore the action mechanism of fungal community on the enhancement of humification during chicken manure composting by regulating the core pathway of carbon metabolism - the tricarboxylic acid cycle. Regulators adenosine triphosphate (ATP) and malonic acid were added at the beginning of composting. The analysis of changes in humification parameters showed that the humification degree and stability of compost products were improved by adding regulators. Compared with CK, the humification parameters of adding regulators group increased by 10.98% on average. Meanwhile, adding regulators not only increased key nodes, but also strengthened the positive correlation between fungi, and network relationship was closer. Moreover, core fungi associated with humification parameters were identified by constructing OTU networks, and the division and cooperation mechanism of fungi were confirmed. Ultimately, the functional role of the fungal community acting on humification was confirmed by statistical means, that was, the fungal community promoting humification was the main group of composting process. And the contribution was more obvious in ATP treatment. This study was helpful to gain insight into the mechanism of regulators addition to advance the humification process, and provided new ideas for the safe, efficient and harmless disposal of organic solid waste.