Insight into effects of mature compost recycling on N2O emission and denitrification genes in sludge composting

Inhibition effect of magnetic field on nitrous oxide emission from sequencing batch reactor treating domestic wastewater at low temperature

This study aims to investigate the effect of a magnetic field on nitrous oxide (N₂O) emission from a sequencing batch reactor treating low-strength domestic wastewater at low temperature (10°C). After running for 124 days in parallel, results indicated that the conversion rate of N₂O for a magnetic field-sequencing batch reactor (MF-SBR) decreased by 34.3% compared to that of a conventional SBR (C-SBR). Meanwhile, the removal efficiencies for total nitrogen (TN) and ammonia nitrogen (NH₄-N) of the MF-SBR were 22.4% and 39.5% higher than those of the C-SBR. High-throughput sequencing revealed that the abundances of AOB (Nitrosomonas), NOB (Nitrospira) and denitrifiers (Zoogloea), which could reduce N₂O to N₂, were promoted significantly in the MF-SBR. Enzyme activities (Nir) and gene abundances (nosZ nirS and nirK) for denitrification in the MF-SBR were also notably higher compared to C-SBR. Our study shows that application of a magnetic field is a useful approach for inhibiting the generation of N₂O and promoting the nitrogen removal efficiency by affecting the microbial characteristics of sludge in an SBR treating domestic wastewater at low temperature.

Effects of macroporous adsorption resin on antibiotic resistance genes and the bacterial community during composting

Swine manure is considered a reservoir for antibiotic resistance genes (ARGs), which may enter the soil and then the food chain to endanger human health. This study investigated the effects of adding 0%, 5%, and 15% (w/w) macroporous adsorption resin (MAR) on ARGs and the bacterial community during composting. The results showed that the addition of MAR reduced the abundances of ARGs (14.14-99.44%) and mobile genetic elements (MGEs) (47.83-99.48%) after swine manure composting. Significant positive correlations were detected between ARGs and MGEs, and thus the variations in MGEs may have led to the changes in ARGs. Redundancy analysis showed that MGEs had stronger effects on ARGs than environmental factors and the bacterial community. Network analysis suggested that ARGs and MGEs co-existed in common host bacteria. In conclusion, the results showed that adding 5% MAR can reduce the risk of ARG transmission.

Influence of matured compost inoculation on sewage sludge composting: Enzyme activity, bacterial and fungal community succession

The influence of matured compost inoculation during sewage sludge with sawdust composting was assessed. Mature compost reduced the heating rate, thermophilic phase, peak temperature, and volatile solid degradation rate, with no significant effect on pH and germination index. Matured compost addition also increased the cellulase, peroxidase, arylsulfatase, and urease contents during the mesophilic phase, and increased the urease content but decreased the cellulase, peroxidase, protease, and arylsulfatase contents during the cooling phase, with no significant effect on enzyme activities at the thermophilic phase. Matured compost increased the diversity of bacteria during the mesophilic and thermophilic phases, but reduced the fungal diversity throughout composting. Matured compost significantly improved uniformity of the bacterial community and affected the structure of the bacterial and fungal communities, while changing the correlation between some functional microorganisms and enzyme activities. These results provide guidance for optimizing the composting process when matured compost as bulking agent.

Biochar combined with montmorillonite amendments increase bioavailable organic nitrogen and reduce nitrogen loss during composting

This study aimed to compare the effects of biochar, montmorillonite and their mixture on nitrogen availability and nitrogen loss during chicken manure composting. Four lab-scale composting experiments, the control (CK), 5% biochar addition (BC), 5% montmorillonite addition (M) and 2.5% biochar + 2.5% montmorillonite addition (BCM), were established. Results showed that the addition of BC, M and BCM significantly improved the contents of bioavailable organic nitrogen and NH₄⁺-N in composts. In addition, BC and BCM reduced N loss by 19.2% and 12.2%, respectively, in comparison with CK. Significant shift of key bacterial communities associated with N transformation were also found in four treatments. Redundancy analysis and structural equation models indicated different additives changed the correlation among bacterial communities, environmental factors and organic N fractions. Comparison of N availability and N loss indicated that the combination of biochar and montmorillonite are more effective than that of separate application during composting.

Genetic associations as indices for assessing nitrogen transformation processes in co-composting of cattle manure and rice straw

In this study, mechanism of nitrogen transformation was investigated in terms of genetic associations (nitrogen-related gene groups) in co-composting of cattle manure and rice straw. Mutual validation among KEGG, Pearson correlation, stepwise regression, and Path analyses indicated that the functional genes synergistically affected on nitrogen transformation in composting process. NxrA/qnorB (0.9419 ± 0.0334) and (amoA + anammox)/Bacteria (0.7187 ± 0.0334) were the key functional gene groups mediating NH₄⁺-N transformation. AmoA/(narG + napA) (-0.8400 ± 0.0129), amoA/bacteria (0.8692 ± 0.0273), and (nirK + nirS)/nosZ (1.1652 ± 0.0089) determined NO₃^--N, NO₂^--N and N₂O transformation, respectively. AmoA/(napA + narG) mediated both NO₃^--N and NO₂^--N transformation. AmoA/anammox (-0.7172 ± 0.0591) and (nirK + nirS)/nosZ (-0.6626 ± 0.0825) served as predominant factors for total nitrogen removal. These results provided a molecular-level insight that nitrification, anaerobic ammonia oxidation and denitrification (SNAD) might simultaneously contribute to nitrogen transformation during composting, rather than sequentially.

Effects of clay on nitrogen cycle related functional genes abundance during chicken manure composting

The objective of this study was to identify the effect of clay additive to explore the nitrogen cycling related ammonification, denitrifying genes and their relation N₂O and ammonia emission during the chicken manure (CM) composting. The consequences indicated that the additive clay could optimize the environment and improve the NH₄⁺-N into NO₃^--N transformation. Meanwhile, the results observed that 8% clay amendment was significant correlated the abundance of nifH and nosZ genes. Compared with the other treatments, the 6% and 10% clay treatments observed low abundance of nifH. The heat map indicated that clay amendment could enhance the function genes abundance. The redundancy analysis showed that N₂O and ammonia emission among all physiochemical parameters had great relationship with functional genes. Therefore, the addition of 10% clay into CM composting system could efficiently enhance the abundance of nirK and narG genes, and improve the composting with its application.

Insights into functional microbial succession during nitrogen transformation in an ectopic fermentation system

The ectopic fermentation system (EFS) is an advanced technology for treating farm wastewater, and it reduces ammonia nitrogen emission and nitrogen loss of fermentation products. This study observed the functional bacteria succession related to nitrogen metabolism in EFS by high throughput sequencing, and evaluated their associations with environmental factors. Results revealed that with the changes of temperature, pH, moisture content, and nitrogen content during fermentation, the species richness and diversity of ammonia oxidizing bacteria (AOB) with amoA increased, but those of denitrifying bacteria carrying nirK and nosZ decreased. During the fermentation process, the dominant bacterial populations of AOB and denitrifying bacteria changed significantly, and different bacterial populations showed different positive/negative correlations with the environmental factors. This study revealed the role of functional bacteria in ammonia removal and nitrogen conservation of EFS, and provided a theoretical basis for the improvement of microbial agents and EFS application.

Assessing key microbial communities determining nitrogen transformation in composting of cow manure using illumina high-throughput sequencing

Insight to nitrogen transformation and cycling during composting is vital in developing management strategies that improve nitrogen content and quality of the end product. In this study, a positive ventilation device was constructed and used to elucidate nitrogen transformation and microbial community structures during the composting of cow manure and rice straw. Bacterial community successions were analyzed during the composting process by examining the change in their structural dynamics using high-throughput sequencing technique. The results revealed that dominant phyla, included Acidobacter, Proteobacteria, Firmicutes, Bacteroidetes, Chloroflexi, and Actinobacteria. Furthermore, a positive strong correlation was observed between the key bacterial communities and nitrogen transformation. Analyses of functional genera, Spearman correlation and Path showed that Thermomonospora_curvata_DSM_43183 followed by Luteimonas and Simiduia, Brevundimonas and Tamlana, Pseudomonas followed by Brevundimonas and Flavobacterium were the key bacterial communities affecting NH₄⁺-N, NO₃^--N, and NO₂^--N transformation, respectively. Thauera followed by Pseudomonas_putida_NBRC_14164 played a dominant role in N₂O transformation.

Electric field induces electron flow to simultaneously enhance the maturity of aerobic composting and mitigate greenhouse gas emissions

The long maturation period and greenhouse gas (GHG) emission are two major problems that arise during aerobic composting, mainly due to the low efficiency of O₂ transmission and utilization. In this study, a novel electric-field-assisted aerobic composting (EAC) process was tested by simply applying a direct-current voltage of 2 V to a conventional aerobic composting (CAC) process. Compared with the CAC process, the maturation time and the total GHG for the EAC process were reduced by 33% and 70%, respectively. Furthermore, the analyses of O₂ consumption and microbial communities demonstrated that the electric field had enhanced O₂ utilization by 30 ± 9% and increased the relative abundance of electroactive bacteria by about 3.4-fold compared to CAC. This work has represented a proof of principle for EAC and suggests that the electric field is an effective and environmentally friendly strategy for enhancing compost maturity and mitigating GHG emissions during aerobic composting.

Effect of phosphate amendments on improving the fertilizer efficiency and reducing the mobility of heavy metals during sewage sludge composting

Composting has been globally applied as an effective and cost-efficient process to manage and reuse sewage sludge. In the present study, four different phosphates as well as a mixture of ferrous sulfate and monopotassium phosphate were used in sewage sludge composting. The results showed that these phosphate amendments promoted an increase in temperature and the degradation of organic matter as well as reduction on nitrogen loss during 18 days of composting. In addition, ferrous sulfate and phosphate had a synergistic effect on reducing nitrogen loss. The contents of total phosphorus and available phosphorus in the compost with addition of 1% phosphate were 40.9% and 66.1% higher than the compost with control treatment. Using the BCR (Community Bureau of Reference) sequential extraction procedure, the addition of calcium magnesium phosphate significantly reduced the mobility factor of Cd, Zn and Cu by 24.2%, 1.7% and 18.8%, respectively. The mobility factors of Pb were increased in all samples, but the monopotassium phosphate treated sample exhibited the greatest Pb passivation ability with the lowest mobility factor increase (1.8%) among all treatments. The X-ray diffraction patterns of compost samples indicated that the passivation mechanism of Cu and Zn may be the forming CuFeS₂ and ZnCu(P₂O₇) crystals during sewage sludge composting. The germination index showed that the compost of all treatments was safe for agricultural application; the germination index of the calcium magnesium phosphate treatment was 99.9 ± 11.8%, which was the highest among all treatments.

Performance of mature compost to control gaseous emissions in kitchen waste composting

This study investigated the performance of mature compost to mitigate gaseous emissions during kitchen waste composting. Cornstalk was mixed with kitchen waste at a ratio of 3:17 (wet weight) as the bulking agent. Mature compost (10% of raw composting materials on the wet weight basis) was mixed into or covered on the composting pile. A control treatment without any addition of mature compost was conducted for comparison. Results show that mature compost did not significantly affect the composting process. Nevertheless, gaseous emissions during kitchen waste composting were considerably reduced with the addition of mature compost. In particular, mixing mature compost with raw composting materials reduced ammonia, methane, and nitrous oxide emissions by 58.0%, 44.8%, and 73.6%, respectively. As a result, nitrogen could be conserved to increase nutrient contents and germination index of the compost product. Furthermore, the total greenhouse gas emissions during kitchen waste composting were reduced by 69.2% with the mixture of mature composting. By contrast, a lower reduction in gaseous emissions was observed when the same amount of mature compost was covered on the composting pile.

Application of ceramsite and activated alumina balls as recyclable bulking agents for sludge composting

Composting is a major sludge-treatment method and bulking agents are very important in sludge composting. In this study, ceramsite and activated alumina balls were chosen as recyclable bulking agents for sludge composting. Variations in the temperature, pH, electrical conductivity, organic matter, dissolved organic carbon, moisture content, and heavy metals were detected during composting with different bulking-agent treatments as well as differences in the germination index values. The results showed that both bulking agents could ensure the maturity of the compost; further, ceramsite treatment resulted in the best water removal efficiency. According to the sequential extraction procedure, both ceramsite and activated alumina balls could stabilize Cd but they also increased the mobility of Zn. After comparing the effects of different particle sizes of ceramsite on composting, 20 mm was determined to be the most optimal value. Additionally, the recovery rates of ceramsite and activated alumina balls were 96.9% and 99.9%, respectively.

Hyperthermophilic composting significantly decreases N2O emissions by regulating N2O-related functional genes

This study reported for the first time that hyperthermophilic composting (HTC) could mitigate 90% of the cumulative amount of N₂O emissions compared to traditional composting (TC) in a full-scale experiment. The concentrations of NO₂^--N and NO₃^--N in HTC were significantly lower than those in TC, which may be the main reason for the reduced N₂O emissions. Furthermore, this study found that the decrease in N₂O emissions in HTC compared to TC was mainly due to the inhibition of the abundance of the bacterial amoA and norB genes, which could decrease the nitrification rate and control N₂O formation, respectively. Partial least squares path modeling revealed that a high temperature was the key factor in lowering N₂O emissions in HTC, while physicochemical properties were the dominant factor in enhancing N₂O emissions in TC. These results suggested that HTC is a promising technique for reducing N₂O emissions in manure composting.

Succession of bacterial community function in cow manure composing

Succession of bacterial community, metabolism function and substrate utilization capacity in 60 days composting of cow manure were analyzed by 16S rRNA pyrosequencing, PICRUSt and Biolog method, respectively. The results showed that the number of bacterial OTUs increased from 176 in raw cow manure to 203 on Day-10, 220 on Day-30 and 313 on Day-60 of the composting, respectively. The PICRUSt analysis showed that the relative abundances of genes involved in lipid and carbohydrate metabolism increased by 28.5% and 22.4% during the incubation, respectively, but the abundances of the genes involved in nucleotide and amino acid metabolism decreased by 21.6% and 2.1%. Furthermore, the average well color development (AWCD) of carboxylic acids (0.99-0.48) and amino acids (1.61-0.89) in Biolog Eco-microplate displayed a steady downtrend through the composting process. Redundancy analysis showed that ORP, moisture and temperature could explain 68.1%, 17.6% and 14.2% of the variation in bacterial genera, respectively.

Effects of bioleaching pretreatment on nitrous oxide emission related functional genes in sludge composting process

The effect of bioleaching pretreatment on N₂O generation in sludge composting process was firstly investigated in this study. The relationships among physicochemical factors, N₂O and NH₃ emission and related functional genes were analyzed in 60 days composting of bioleaching dewatering sludge (BDS) and filter press dewatering sludge (FDS), respectively. The results showed the cumulative amounts of NH₃ and N₂O emission from the BDS composting system were reduced by 83.52% and 54.76% after bioleaching pretreatment, respectively. The lower moisture and pH, and the higher ORP and the concentrations of NH₄⁺-N, NO₃^--N and NO₂^--N were observed in BDS during the composting compared to FDS. Furthermore, bioleaching pretreatment improved the relative abundance of hao but reduced amoA, nirK and norB in the BDS during the composting. The low pH level and the reduction of nirK and norB in BDS were the main reasons mitigating NH₃ and N₂O emissions, respectively.

Insight into effects of electro-dewatering pretreatment on nitrous oxide emission involved in related functional genes in sewage sludge composting

Electro-dewatering (ED) pretreatment could improve sludge dewatering performance and remove heavy metal, but the effect of ED pretreatment on nitrous oxide (N₂O) emission and related functional genes in sludge composting process is still unknown, which was firstly investigated in this study. The results revealed that ED pretreatment changed the physicochemical characteristics of sludge and impacted N₂O related functional genes, resulting in the reduction of cumulative N₂O emission by 77.04% during 60 days composting. The higher pH and NH₄⁺-N, but lower moisture, ORP and NO₂^--N emerged in the composting of ED sludge compared to mechanical dewatering (MD) sludge. Furthermore, ED pretreatment reduced amoA, hao, narG, nirK and nosZ in ED sludge on Day-10 and Day-60 of composting. It was found that nirK reduction was the major factor impacting N₂O generation in the initial composting of ED sludge, and the decline of amoA restricted N₂O production in the curing period.

New insight of tertiary-amine modified bentonite amendment on the nitrogen transformation and volatile fatty acids during the chicken manure composting

In this study, the main objective was to investigate the potential effect of tertiary-amine modified bentonite (TAMB) on the nitrogen transformation and the volatile fatty acids (VFAs) degradation during the chicken manure (CM) composting. Six dosages of TAMB (0%, 2%, 4%, 6%, 8% and 10%) were amended into the mixture of CM and wheat straw and then composted for 50 days. The results revealed the TAMB amendment could prolong the thermophilic phase and enhance the organic matter (OM) degradation. With the increasing dosage of TAMB, the ammonia volatilization was reduced by 15.41%-65.35%. Meanwhile, the TAMB addition had a positive effect on VFAs degradation and reducing odor unit_MAX (OU_MAX) by 17.61%-59.24%. Moreover, CH₄ was reduced by 12.15%-32.78% in TAMB applied treatments compared to control. Finally, combined with all results, it indicated that TAMB amendment could reduce VFAs, CH₄ emission and nitrogen loss to improve the compost quality.