Intermittent aeration reducing N2O emissions from bioreactor landfills with gas-water joint regulation

Nitrogen in landfills: Sources, environmental impacts and novel treatment approaches

Nitrogen (N) accumulation in landfills is a pressing environmental concern due to its diverse sources and significant environmental impacts. However, there is relatively limited attention and research focus on N in landfills as it is overshadowed by other more prominent pollutants. This study comprehensively examines the sources of N in landfills, including food waste contributing to 390 million tons of N annually, industrial discharges, and sewage treatment plant effluents. The environmental impacts of N in landfills are primarily manifested in N2O emissions and leachate with high N concentrations. To address these challenges, this study presents various mitigation and management strategies, including N2O reduction measures and novel NH4+ removal techniques, such as electrochemical technologies, membrane separation processes, algae-based process, and other advanced oxidation processes. However, a more in-depth understanding of the complexities of N cycling in landfills is required, due to the lack of long-term monitoring data and the presence of intricate interactions and feedback mechanisms. To ultimately achieve optimized N management and minimized adverse environmental impacts in landfill settings, future prospects should emphasize advancements in monitoring and modeling technologies, enhanced understanding of microbial ecology, implementation of circular economy principles, application of innovative treatment technologies, and comprehensive landfill design and planning.

Black soldier fly larvae mitigate greenhouse gas emissions from domestic biodegradable waste by recycling carbon and nitrogen and reconstructing microbial communities

Black soldier fly larvae have been proven to reduce greenhouse gas emissions in the treatment of organic waste. However, the microbial mechanisms involved have not been fully understood. The current study mainly examined the dynamic changes of carbon and nitrogen, greenhouse gas emissions, the succession of microbial community structure, and changes in functional gene abundance in organic waste under larvae treatment and non-aeration composting. Thirty percent carbon and 55% nitrogen in the organic waste supplied were stored in larvae biomass. Compared to the non-aeration composting, the larvae bioreactor reduced the proportion of carbon and nitrogen converted into greenhouse gases (CO2, CH4, and N2O decreased by 62%, 87%, and 95%, respectively). 16S rRNA sequencing analysis indicated that the larvae bioreactor increased the relative abundance of Methanophaga, Marinobacter, and Campylobacter during the bioprocess, enhancing the consumption of CH4 and N2O. The metagenomic data showed that the intervention of larvae reduced the ratio of (nirK + nirS + nor)/nosZ in the residues, thereby reducing the emission of N2O. Larvae also increased the functional gene abundance of nirA, nirB, nirD, and nrfA in the residues, making nitrite more inclined to be reduced to ammonia instead of N2O. The larvae bioreactor mitigated greenhouse gas emissions by redistributing carbon and nitrogen and remodeling microbiomes during waste bioconversion, giving related enterprises a relative advantage in carbon trading.

Analysis of latrine fecal odor release pattern and the deodorization with composited microbial agent

As an important source of malodor, the odor gases emitted from public toilet significantly interfered the air quality of living surroundings, resulting in environmental problem which received little attention before. Thus, this paper explored the odor release pattern of latrine feces and deodorization effect with composited microbial agent in Chengdu, China. The odor release rules were investigated in sealed installations with a working volume of 9 L for 20 days. The odor units (OU), ammonia (NH3), hydrogen sulfide (H2S) and total volatile organic compounds (TVOC) were selected to assess the release of malodorous gases under different temperature and humidity, while the highest malodor release was observed under 45℃, with OU and TVOC concentration was 643.91 ± 2.49 and 7767.33 ± 33.50 mg/m3, respectively. Microbes with deodorization ability were screened and mixed into an agent, which composited of Bacillus amyloliquefaciens, Lactobacillus plantarum, Enterococcus faecalis and Pichia fermentans. The addition of microbial deodorant could significantly suppress the release of malodor gas during a 20-day trial, and the removal efficiency of NH3, H2S, TVOC and OU was 81.50 %, 38.31 %, 64.38 %, and 76.86 %, respectively. The analysis of microbial community structure showed that temperature was the main environmental factor driving the microbial variations in latrine feces, while Firmicutes, Actinobacteria, Proteobacteria and Bacteroidetes were the main bacteria phyla involved in the formation and emission of malodorous gases. However, after adding the deodorant, the abundance of Bacteroidetes, Proteobacteria and Actinobacteria were decreased, while the abundance of Firmicutes was increased. Furthermore, P. fermentans successfully colonized in fecal substrates and became the dominant fungus after deodorization. These results expanded the understanding of the odor release from latrine feces, and the composited microbial deodorant provided a valuable basis to the management of odor pollution.

A novel mechanism of enhanced PCBs degradation associated with nitrogen in the rhizosphere of the wetland plant Myriophyllum aquaticum

Co-contamination of polychlorinated biphenyls (PCBs) and nitrogen (N) is widespread. Here, N removal and PCBs degradation were investigated in constructed wetlands populated with Myriophyllum aquaticum, and the role of N in PCBs degradation was explored as well. Nearly 97% of N was removed in the planted system, whereas less than 40% was removed in the plant-free system. Compared to the treatment with plants and no N amendment, N addition enhanced plant growth by 31.9% and PCBs removal by 9.90%. PCBs attenuation was mainly attributed to microbial degradation rather than plant uptake. Using DNA stable-isotope probing, 26 operational taxonomic units were identified across all treatments, of which 25 were linked to PCBs degradation for the first time. Some PCB-degraders were associated with nitrification/denitrification and were significantly enriched in the treatment that included both plants and N application, indicating that PCBs degradation was promoted by recruiting ammonia-oxidising and denitrifying microbes with PCBs metabolic ability. This was confirmed by the higher A13/A12 ratios for the bphC, amoA, and nirK genes and their significant positive correlations. Overall, the findings clarify the novel mechanism by which N promotes PCBs degradation in constructed wetlands and offers a theoretical basis for efficiently removing inorganic elements and persistent organic pollutants.

The key role of denitrification and dissimilatory nitrate reduction in nitrogen pollution along vertical landfill profiles from metagenomic perspective

Landfill are persistent sources of nitrogen (N) pollution even in the decades after closure. However, the biological pathways of N-pollution, particularly N₂O and NH₄⁺, at different landfill depths have received little attention. In this study, metagenomic analysis was conducted on landfill refuse from vertical reservoir profiles in two closed landfills named XT and MT. NH₄⁺ concentrations were found to be higher in deeper layers of MT, while greater potential for N₂O emissions occurred in XT and the shallow layers of MT. Furthermore, the community structure and function of N-metabolizing microbes were more strongly defined by landfill depth than landfill type. Denitrification, involving abundant nirK and norB genes, was identified as the major pathway for N₂O production in both XT and MT-shallow, while dissimilatory nitrate reduction with abundant nirBD genes was identified as the major pathway for NH₄⁺ accumulation. Microbes of norB-type and nirBD-type were positively affected by NO₃^- in XT, whereas negatively affected by contents of organic material and moisture in MT-shallow. The mechanism by which nitrogen fixation, with abundant nifH genes, contributes to NH₄⁺ accumulation in MT-deep should be further elucidated. These findings can provide a theoretical basis for governing scientific N-pollution control strategies throughout the entire landfill process.

Characteristics of denitrification activity, functional genes, and denitrifying community composition in the composting process of kitchen and garden waste

N₂O can be easily produced during the co-composting of kitchen waste (KW) and garden waste (GW). This study investigated the effects of the co-composting of KW and GW at different ratios (1:2, 1:1.5, 1:1, and 1.5:1) on the denitrifying activities, functional genes, and community composition of denitrifiers. The results showed that the denitrification activity of KW and GW at a 1:2 ratio was the lowest. The gene abundances of nirS, nirK, nosZI, and nosZII were high on days 12 and 28 under the four different ratios. Network analysis demonstrated that nosZ-type denitrifiers could construct a complex and reciprocal bacterial network to promote the reduction of N₂O to N₂. Mantel test results revealed that nirS-, nirK-, nosZI-, and nosZII-type denitrifiers were significantly positively correlated with pH, C/N, and moisture content. These findings demonstrated that composting with appropriate proportions of KW and GW could reduce N₂O emissions caused by denitrification.

Additional ratios of hydrolysates from lignocellulosic digestate at different hydrothermal temperatures influencing anaerobic digestion performance

Hydrothermal treatment (HT) is envisaged as a promising technology to treat the lignocellulosic biomass. HT temperature is an important parameter influencing the hydrolysate compositions such as organic compounds and potential inhibitors, and therefore affect the subsequential anaerobic digestion (AD) process. Herein, HT-AD was employed to treat the wheat straw-derived digestate. The HT temperature of 190 °C was proved to be the best performance with a higehst reducing sugar yield (45.05 mg g^-1) in the hydrolysate and a highest methane yield (120.8 mL g_TS^-1) from the AD of the hydrolysate, which was 42.5% higher than the methane yield in the control without the hydrolysate addition (84.8 mL g_TS^-1). 3-Furaldehyde was the dominant organic in the hydrolysates. The HT temperature of 210 °C led to the presence of AD inhibitory moieties (e.g., phenols and furans) in the hydrolysate, resulting in a low methane yield. Although the treatments with the addition of 100% hydrolysate outperformed those of 50% hydrolysate in the methane yields in the late stage, the latter had higher methane yields in the first stage, suggesting that the additional ratios of hydrolysates should be carefully considered in AD, especially the detrimental effects of inhibitors and adaptability issues of AD consortia. The MiSeq sequencing showed that the hydrolysis/acidogenesis was dominant in the first stage, while methanogenesis became dominant in the late stage with the acetoclastic/hydrogenotrophic methanogens (Methanosarcina and Methanobacterium) enriched in the hydrolysate-feeding reactors. These findings demonstrated that a integration of HT-AD was a promising approach for the digestate valorization and to reduce the potential carbon emission from waste treatments.

Insights into leachate reduction in landfill with different ventilation Rates: Balance of Water, waste physicochemical Properties, and microbial community

Ventilation is an efficient approach employed for accelerating stabilization and reducing aftercare of landfill, but its effect on leachate reduction is still elusive. To fill this knowledge gap, five lab-scale landfill reactors with different ventilation rates were established in this study. Suitable ventilation (e.g. 0.25-0.5 L·min^-1·kg^-1 dry solid of waste (DS)) was beneficial to promoting the stabilization of landfill, which effectively accelerated the degradation of organic matter and reduced water content of landfilled waste. Based on the mass balance of water, the dominant input water was initial water of landfilled waste (more than 94 %), which was partially converted to leachate and evaporated water. Ventilation enhanced the intensity of biochemical reactions heat to increase evaporated water content from 0 to 0.29 t/t DS while reducing the leachate generation significantly from 0.69 to 0.49 t/t DS with the increase of ventilation rate. Besides, the hydrophilic substances, such as humic acid-like substances, in landfilled waste increased, and the surface of the landfilled waste converted from smooth to rough. The reduction of the bound water content has a significant correlation with the degradation of organic matter content (p less than 0.05), which reduced the water-holding capacity of waste. Actinobacteriota and Firmicutes were the key bacterial phyla in the degradation of organic matter to promote bio-heat and evaporation of water, thus reducing leachate production under suitable ventilation conditions. Carbohydrates and amino acids were the main energy metabolism sources of bacteria during the landfill process. This study deepens our understanding of the leachate reduction mechanism in the micro-aerobic landfill.

Analysis of Bacterial Diversity in Different Types of Daqu and Fermented Grains From Danquan Distillery

Bacterial communities in high-temperature Daqu and fermented grains are important for brewing Jiang-flavor Baijiu such as Danquan Baijiu. Daqu is a saccharifying and fermenting agent, which has a significant impact on the flavor of Baijiu. However, bacterial communities in three different types of samples from the Danquan distillery (dqjq_ck, dqjqcp, and dqjp3) were still unclear, which limited further development of Danquan Baijiu. “dqjq_ck” and “dqjqcp” indicate high-temperature Daqu at days 45 and 135, respectively. “dqjp3” indicates fermented grains. In this study, the bacterial communities of three samples were analyzed by Illumina Miseq high-throughput sequencing. The bacterial communities of three samples primarily composed of thermophilic bacteria and bacteria with stress resistance. The most abundant species in dqjq_ck, dqjqcp, and dqjp3 were Comamonas, Bacillus, and unclassified Lactobacillales, respectively. The main bacteria included Bacillus, Comamonas, Myroides, Paenibacillus, Acetobacter, Kroppenstedtia, Staphylococcus, Saccharopolyspora, Planifilum, Lactobacillus, Acinetobacter, Oceanobacillus, Enterococcus, Thermoactinomyces, Lactococcus, Streptomyces, Saccharomonospora, Tepidimicrobium, Anaerosalibacter, unclassified_Lactobacillales, unclassified_Thermoactinomycetaceae_1, unclassified_Bacillaceae_2, unclassified_Bacillales, unclassified_Microbacteriaceae, unclassified_Rhodobacteraceae, unclassified_Actinopolysporineae, and unclassified_Flavobacteriaceae in three samples (percentage was more than 1% in one of three samples). In our study, the succession of microbiota in three samples representing three important stages of Danquan Baijiu brewing was revealed. This article lays a good foundation for understanding the fermentation mechanism and screening some excellent indigenous bacteria to improve the quality of Danquan Baijiu in future.