Elemental mercury production from contaminated riparian soil suspensions under air and nitrogen bubbling conditions.

The dynamic change of redox conditions is a key factor in emission of elemental mercury (Hg0) from riparian soils. The objective of this study was to elucidate the influences of redox conditions on Hg0 emission from riparian soils. Soil suspension experiments were conducted to measure Hg0 emission from five Hg-contaminated soil samples in two redox conditions (i.e., treated with air or with N2). In four of the five samples, Hg0 emission was higher in air treatment than on N2 treatment. Remaining one soil, which has higher organic matter than other soils, showed no distinct difference in Hg0 production between air and N2 treatment. In soil suspensions subject to N2 treatment, the dissolved organic carbon (DOC) and Fe2+ concentrations were 3.38- to 1.34-fold and 1.44- to 2.28-fold higher than those in air treatment, respectively. Positive correlations were also found between the DOC and Fe2+ (r = 0.911, p < 0.01) and Hg2+ (r = 0.815, p < 0.01) concentrations in soil solutions, suggesting Fe2+ formation led to the release of DOC, which bound to Hg2+ in the soil and, in turn, limited the availability of Hg2+ for reduction to Hg0 in N2 treatment. On the other hand, for remaining one soil, more Hg2+ might be adsorbed onto the DOM in the air treatment, resulted in the inhibition of Hg0 production in air treatment. These results imply that the organic matter is important to prevent Hg0 production by changing redox condition. Further study is needed to prove the role of organic matter in the production of Hg0.

Microaerophilic Activated Sludge System for Ammonia Retention toward Recovery from High-Strength Nitrogenous Wastewater: Performance and Microbial Communities.

A transition to ammonia recovery from wastewater has started; however, a technology for sustainable nitrogen retention in the form of ammonia and organic carbon removal is still in development. This study validated a microaerophilic activated sludge (MAS) system to efficiently retain ammonia from high-strength nitrogenous wastewater. The MAS is based on conventional activated sludge (CAS) with aerobic and settling compartments. Low dissolved oxygen (DO) concentrations (<0.2 mg/L) and short solids retention times (SRTs) (<5 days) eliminated nitrifying bacteria. The two parallel MASs were successfully operated for 300 days and had ammonia retention of 101.7 ± 24.9% and organic carbon removal of 85.5 ± 8.9%. The MASs mitigated N2O emissions with an emission factor of <0.23%, much lower than the default value of CAS (1.6%). A short-term step-change test demonstrated that N2O indicated the initiation of nitrification and the completion of denitrification in the MAS. The parallel MASs had comparable microbial diversity, promoting organic carbon oxidation while inhibiting ammonia-oxidizing microorganisms (AOMs), as revealed by 16S rRNA gene amplicon sequencing, the quantitative polymerase chain reaction of functional genes, and fluorescence in situ hybridization of ß-proteobacteria AOB. The microbial analyses also uncovered that filamentous bacteria were positively correlated with effluent turbidity. Together, controlling DO and SRT achieved organic carbon removal and successful ammonia retention, mainly by suppressing AOM activity. This process represents a new nitrogen management paradigm.

Exploring the Functions of Efficient Canonical Denitrifying Bacteria as N2O Sinks: Implications from 15N Tracer and Transcriptome Analyses.

In denitrifying reactors, canonical complete denitrifying bacteria reduce nitrate (NO3-) to nitrogen via N2O. However, they can also produce N2O under certain conditions. We used a 15N tracer method, in which 15N-labeled NO3-/nitrite (NO2-) and nonlabeled N2O were simultaneously supplied with organic electron donors to five canonical complete denitrifying bacteria affiliated with either Clade I or Clade II nosZ. We calculated their NO3-, NO2-, and N2O consumption rates. The Clade II nosZ bacterium Azospira sp. strain I13 had the highest N2O consumption rate (3.47 ± 0.07 fmol/cell/h) and the second lowest NO3- consumption rate (0.20 ± 0.03 fmol/cell/h); hence, it is a N2O sink. A change from peptone- to acetate/citrate-based organic electron donors increased the NO3- consumption rate by 4.8 fold but barely affected the N2O consumption rate. Electron flow was directed to N2O rather than NO3- in Azospira sp. strain I13 and Az. oryzae strain PS only exerting a N2O sink but to NO3- in the Clade I nosZ N2O-reducing bacteria Pseudomonas stutzeri strain JCM 5965 and Alicycliphilus denitrificans strain I51. Transcriptome analyses revealed that the genotype could not fully describe the phenotype. The results show that N2O production and consumption differ among canonical denitrifying bacteria and will be useful for developing N2O mitigation strategies.

Evaluation of Pollution Level, Spatial Distribution, and Ecological Effects of Antimony in Soils of Mining Areas: A Review.

The first global-scale assessment of Sb contamination in soil that was related to mining/smelting activities was conducted based on 91 articles that were published between 1989 and 2021. The geographical variation, the pollution level, the speciation, the influencing factors, and the environmental effects of Sb that were associated with mining/smelting-affected soils were analyzed. The high Sb values mainly occurred in developed (Poland, Italy, Spain, Portugal, New Zealand, Australia) and developing (China, Algeria, Slovakia) countries. Sb concentrations of polluted soil from mining areas that were reported in most countries significantly exceeded the maximum permissible limit that is recommended by WHO, except in Turkey and Macedonia. The soil Sb concentrations decreased in the order of Oceania (29,151 mg/kg) > North Africa (13,022 mg/kg) > Asia (1527 mg/kg) > Europe (858 mg/kg) > South America (37.4 mg/kg). The existing extraction methods for Sb speciation have been classified according to the extractant, however, further research is needed in the standardization of these extraction methods. Modern analytical and characterization technologies, e.g., X-ray absorption spectroscopy, are effective at characterizing chemical speciation. Conditional inference tree (CIT) analysis has shown that the clay content was the major factor that influenced the soil Sb concentration. Non-carcinogenic risks to the public from soil Sb pollution were within the acceptable levels in most regions. An Sb smelter site at the Endeavour Inlet in New Zealand, an abandoned open-pit Sb mine in Djebel Hamimat, Algeria, an old Sb-mining area in Tuscany, Italy, and Hillgrove mine in Australia were selected as the priority control areas. Cynodon dactylon, Boehmeria, Pteris vittata, and Amaranthus paniculatus were found to be potential Sb accumulators. All of the values of bioaccumulation factors for the crops were less than one. However, ingestion of Sb through crop consumption posed potential non-carcinogenic health risks, which should not be neglected. The soil variables (pH, Eh, total sulfur, carbon nitrogen ratio, total organic carbon, and sulfate), the total Sb and the bioavailable Sb, and heavy metal(loid)s (As, Pb, and Fe) were the major parameters affecting the microbial community compositions.

Nonlinear pattern and algal dual-impact in N2O emission with increasing trophic levels in shallow lakes.

Shallow lakes are considered important contributors to emissions of nitrous oxide (N2O), a powerful greenhouse gas, in aquatic ecosystems. There is a large degree of uncertainty regarding the relationship between N2O emissions and the progress of lake eutrophication, and the mechanisms underlying N2O emissions are poorly understood. Here, N2O emission fluxes and environmental variables in different lakes along a trophic state gradient in the Yangtze River basin were studied. N2O emission fluxes were -1.0-53.0 µg m-2 h-1 and 0.4-102.9 µg m-2 h-1 in summer and winter, respectively, indicating that there was marked variation in N2O emissions among lakes of different trophic state. The non-linear exponential model explained differences in N2O emission fluxes by the degree of eutrophication (p < 0.01). TN and chl-a both predicted 86% of the N2O emission fluxes in shallow lakes. The predicted N2O emission fluxes based on the IPCC EF5r overestimated the observed fluxes, particularly those in hyper-eutrophic lakes. These findings demonstrated that nutrient-rich conditions and algal accumulation are key factors determining N2O emission fluxes in shallow lakes. Furthermore, this study also revealed that temperature and algae accumulation-decomposition determine an N2O emission flux in an intricate manner. A low temperature, i.e., winter, limits algae growth and low oxygen consumption for algae decomposition. The environment leaves a high dissolved oxygen concentration, slowing down N2O consumption as the final step of denitrification. In summer, with the oxygen consumed by excess algal decomposition, the N2O production is limited by the complete denitrification as well as the limited substrate supply of nitrate by nitrification in hypoxic or anoxic conditions. Such cascading events explained the higher N2O emission fluxes from shallow lakes in winter compared with summer. This trend was amplified in hyper-eutrophic shallow lakes after algal disappearance. Collectively, algal accumulation played a dual role in stimulating and impeding N2O emissions, especially in hyper-eutrophic lakes. This study expands our knowledge of N2O emissions from shallow lakes in which eutrophication is underway.

Combination of 15N Tracer and Microbial Analyses Discloses N2O Sink Potential of the Anammox Community.

Although nitrogen removal by partial nitritation and anammox is more cost-effective than conventional nitrification and denitrification, one downside is the production and accumulation of nitrous oxide (N2O). The potential exploitation of N2O-reducing bacteria, which are resident members of anammox microbial communities, for N2O mitigation would require more knowledge of their ecophysiology. This study investigated the phylogeny of resident N2O-reducing bacteria in an anammox microbial community and quantified individually the processes of N2O production and N2O consumption. An up-flow column-bed anammox reactor, fed with NH4+ and NO2- and devoid of oxygen, emitted N2O at an average conversion ratio (produced N2O: influent nitrogen) of 0.284%. Transcriptionally active and highly abundant nosZ genes in the reactor biomass belonged to the Burkholderiaceae (clade I type) and Chloroflexus genera (clade II type). Meanwhile, less abundant but actively transcribing nosZ strains were detected in the genera Rhodoferax, Azospirillum, Lautropia, and Bdellovibrio and likely act as an N2O sink. A novel 15N tracer method was adapted to individually quantify N2O production and N2O consumption rates. The estimated true N2O production rate and true N2O consumption rate were 3.98 ± 0.15 and 3.03 ± 0.18 mgN·gVSS-1·day-1, respectively. The N2O consumption rate could be increased by 51% (4.57 ± 0.51 mgN·gVSS-1·day-1) with elevated N2O concentrations but kept comparable irrespective of the presence or absence of NO2-. Collectively, the approach allowed the quantification of N2O-reducing activity and the identification of transcriptionally active N2O reducers that may constitute as an N2O sink in anammox-based processes.

Temperature and oxygen level determine N2 O respiration activities of heterotrophic N2 O-reducing bacteria: Biokinetic study.

Nitrous oxide (N2 O), a potent greenhouse gas, is reduced to N2 gas by N2 O-reducing bacteria (N2 ORB), a process which represents an N2 O sink in natural and engineered ecosystems. The N2 O sink activity by N2 ORB depends on temperature and O2 exposure, yet the specifics are not yet understood. This study explores the effects of temperature and oxygen exposure on biokinetics of pure culture N2 ORB. Four N2 ORB, representing either clade I type nosZ (Pseudomonas stutzeri JCM5965 and Paracoccus denitrificans NBRC102528) or clade II type nosZ (Azospira sp. strains I09 and I13), were individually tested. The higher activation energy for N2 O by Azospira sp. strain I13 (114.0 ± 22.6 kJ mol-1 ) compared with the other tested N2 ORB (38.3-60.1 kJ mol-1 ) indicates that N2 ORB can adapt to different temperatures. The O2 inhibition constants (KI ) of Azospira sp. strain I09 and Ps. stutzeri JCM5965 increased from 0.06 ± 0.05 and 0.05 ± 0.02 µmol L-1 to 0.92 ± 0.24 and 0.84 ± 0.31 µmol L-1 , respectively, as the temperature increased from 15°C to 35°C, while that of Azospira sp. strain I13 was temperature-independent (p = 0.106). Within the range of temperatures examined, Azospira sp. strain I13 had a faster recovery after O2 exposure compared with Azospira sp. strain I09 and Ps. stutzeri JCM5965 (p < 0.05). These results suggest that temperature and O2 exposure result in the growth of ecophysiologically distinct N2 ORB as N2 O sinks. This knowledge can help develop a suitable N2 O mitigation strategy according to the physiologies of the predominant N2 ORB.

Exploration and enrichment of methane-oxidizing bacteria derived from a rice paddy field emitting highly concentrated methane.

Methane-oxidizing bacteria (MOB) possess the metabolic potential to assimilate the highly potent greenhouse gas, CH4, and can also synthesize valuable products. Depending on their distinct and fastidious metabolic pathways, MOB are mainly divided into Type I and Type II; the latter are known as producers of polyhydroxyalkanoate (PHA). Despite the metabolic potential of MOB to synthesize PHA, the ecophysiology of MOB, especially under high CH4 flux conditions, is yet to be understood. Therefore, in this study, a rice paddy soil receiving a high CH4 flux from underground was used as an inoculum to enrich MOB using fed-batch operation, then the enriched Type II MOB were characterized. The transitions in the microbial community composition and CH4 oxidation rates were monitored by 16S rRNA gene amplicon sequencing and degree of CH4 consumption. With increasing incubation time, the initially dominant Methylomonas sp., affiliated with Type I MOB, was gradually replaced with Methylocystis sp., Type II MOB, resulting in a maximum CH4 oxidation rate of 1.40 g-CH4/g-biomass/day. The quantification of functional genes encoding methane monooxygenase, pmoA and PHA synthase, phaC, by quantitative PCR revealed concomitant increases in accordance with the Type II MOB enrichment. These increases in the functional genes underscore the significance of Type II MOB to mitigate greenhouse gas emission and produce PHA.

Nonlinear response of methane release to increased trophic state levels coupled with microbial processes in shallow lakes.

Shallow lakes are a crucial source of methane (CH4), a potent greenhouse gas, to the atmosphere. However, large uncertainties still exist regarding the response of CH4 emissions to the increasing trophic levels of lakes as well as the underlying mechanisms. Here, we investigate the CH4 emission flux from lakes with different trophic states in the middle and lower reaches of the Yangtze River basin, China to evaluate the effect of the trophic lake index (TLI) on CH4 emissions. The mean CH4 emission fluxes from mesotrophic, eutrophic, middle-eutrophic, and hyper-eutrophic lakes were 0.1, 4.4, 12.0, and 130.4 mg m-2 h-1, respectively. Thus, the CH4 emission flux ranged widely and was positively correlated with the degree of eutrophication. The relative abundance of methanogens with respect to the total population for the mesotrophic, eutrophic, mid-eutrophic, and hyper-eutrophic states was 0.03%, 0.35%, 0.94%, and 1.17%, respectively. The biogeographic-scale pattern of lakes classified as each of these four trophic states indicated that CH4 emissions could be well-predicted by the NH4+-N concentration in the water column, as both NH4+-N and CH4 were produced during mineralisation of labile organic matter in lake sediment. In addition, the shift from clear to turbid water, which is an unhealthy evolution for lakes, was associated with a nonlinear increase in the CH4 emissions from the studied lakes. In particular, the hypereutrophic lakes functioned as CH4 emission hotspots. Our findings highlight that nutrient levels, as a potential facilitator of CH4 emissions, should be considered in future research to accurately evaluate the greenhouse gas emissions from shallow lakes.

Eco-compatible biochar mitigates volatile fatty acids stress in high load thermophilic solid-state anaerobic reactors treating agricultural waste.

A high concentration of accumulated volatile fatty acids (VFAs) is one of the most important factors resulting in reactor failure during solid-state anaerobic digestion. In this study, the feedstock-to-inoculum (F/I) ratio (0.5, 2, 3, 4 and 6) and the recovery method after failure (biochar addition or inoculum addition) were investigated in batch solid-state anaerobic digestion fed with rice straw and pig urine. An F/I ratio of 3 was the threshold for stable operation, while the reactors failed at F/I ratios of 4 and 6 because of high accumulated VFAs concentrations (above 30 g HAc/kg). Biochar addition (10% or 20% (wet weight) of the mixture) was as effective as inoculum addition (by adjusting the F/I ratio to 2 or 3) in promoting VFAs degradation in failed reactors within a short period (<1 day). The buffering capacity of biochar was important in promoting VFAs degradation.

Reducing geogenic arsenic leaching from excavated sedimentary soil using zero-valent iron amendment followed by dry magnetic separation: A case study.

Although the deep-layer sedimentary soils excavated from construction sites contain low level of geogenic arsenic (As), remediation is necessary when the As leachability exceeds the environmental standard (10 µg/L) in Japan. In this study, the zero-valent iron (ZVI) amendment followed by dry magnetic separation (ZVI-DMS) was implemented for the treatment of a geogenic As-contaminated alkaline sedimentary soil (pH 8.9; 7.5 mg/kg of total As; 0.33 mg/kg of water-extractable As). This technology involves pH adjustment (adding H2SO4), ZVI addition, water content reduction (adding water adsorbent CaSO4·0.5H2O), and dry magnetic separation. The short-term and long-term As leachability before and after treatment was compared using sequential water leaching tests (SWLT). The results illustrated that As could be removed from the bulk soil through the magnetic separation of As-ZVI complexes, although the amount was limited (about 2% of total As). Moreover, immobilization played a dominant role in suppressing As leaching. The H2SO4 addition decreased pH to a circumneutral range and thereby suppress As release. The CaSO4·0.5H2O addition also contributed to the pH decrease and reduced As leachability. Besides, CaSO4·0.5H2O-dissolution released Ca2+ that favored As adsorption, and enhanced dissolved organic carbon (DOC) coagulation that decelerated As dissolution. SWLT results indicated that As leachability from remediated soil satisfied the environmental standard (10 µg/L) in both short-term and long-term perspective. However, the secular stability of treated soil deserves more attention due to the easy re-release of As caused by As-bearing framboidal pyrite oxidation. Additionally, during ZVI-DMS process, there is a need to scientifically decide the dosage of ZVI to avoid excessive addition. Our results demonstrated that ZVI-DMS technology could be a promising remediation strategy for geogenic As contaminated sedimentary soils/rocks.

Enrichment, Isolation, and Characterization of High-Affinity N2O-Reducing Bacteria in a Gas-Permeable Membrane Reactor.

The recent discovery of nitrous oxide (N2O)-reducing bacteria suggests a potential biological sink for the potent greenhouse gas N2O. For an application toward N2O mitigation, characterization of more isolates will be required. Here, we describe the successful enrichment and isolation of high-affinity N2O-reducing bacteria using a N2O-fed reactor (N2OFR). Two N2OFRs, where N2O was continuously and directly supplied as the sole electron acceptor to a biofilm grown on a gas-permeable membrane, were operated with acetate or a mixture of peptone-based organic substrates as an electron donor. In parallel, a NO3- -fed reactor (NO3FR), filled with a nonwoven sheet substratum, was operated using the same inoculum. We hypothesized that supplying N2O vs NO3- would enhance the dominance of distinct N2O-reducing bacteria. Clade II type nosZ bacteria became rapidly enriched over clade I type nosZ bacteria in the N2OFRs, whereas the opposite held in the NO3FR. High-throughput sequencing of 16S rRNA gene amplicons revealed the dominance of Rhodocyclaceae in the N2OFRs. Strains of the Azospira and Dechloromonas genera, canonical denitrifiers harboring clade II type nosZ, were isolated with high frequency from the N2OFRs (132 out of 152 isolates). The isolates from the N2OFR demonstrated higher N2O uptake rates (Vmax: 4.23 × 10-3-1.80 × 10-2 pmol/h/cell) and lower N2O half-saturation coefficients (Km,N2O: 1.55-2.10 µM) than a clade I type nosZ isolate from the NO3FR. Furthermore, the clade II type nosZ isolates had higher specific growth rates on N2O than nitrite as an electron acceptor. Hence, continuously and exclusively supplying N2O in an N2OFR allows the enrichment and isolation of high-affinity N2O-reducing strains, which may be used as N2O sinks in bioaugmentation efforts.

Impact of turning waste on performance and energy balance in thermophilic solid-state anaerobic digestion of agricultural waste.

Mixing is an important operation in solid-state anaerobic digestion (SS-AD) to improve the mass transfer of the solid phase. This study proposed simple turning by loader in common garage-type digester without commonly used mixer or percolation system (simplified SS-AD). In simplified-SS-AD, turning is conducted in open condition. Thus, oxidation of anaerobic sludge during turning would influence digestion performance. Therefore, in this study, the effect of turning wastes by mixing during digestion on a simplified SS-AD fed with rice straw and pig urine was investigated. Four different mixing frequency levels-no mixing (M0) and mixing once a day (M-1/1), once every 3 days (M-1/3) and once a week (M-1/7)-were conducted. Methane yields of M0, M-1/3 and M-1/7 were comparable with each other. Methane yield and lag period of M-1/1 were approximately 61% and 155% of M0 (351.2  mL/g VS and 4.7 days), respectively. Furthermore, the chemical oxygen demand (COD) of acetate accumulated in the digestate of M-1/1 was comparable to the difference in the COD of methane production between M-1/1 and the other treatments. Mixing every day also resulted in a higher oxidation-reduction potential and carbon dioxide content. These findings suggest that methanogenesis was inhibited in M-1/1 by frequent mixing in the atmosphere. Net energy analysis of SS-AD plant operation showed that M0 can obtain the highest net energy gain, whereas net energy production of M-1/7 was reduced by rewarming after mixing. Therefore, no mixing is the most effective approach for the proposed simplified process.

Comparison of leachate percolation and immersion using different inoculation strategies in thermophilic solid-state anaerobic digestion of pig urine and rice straw.

Heterogeneous distribution of substrate and microorganisms and low mass transfer limit methane production dramatically in solid-state anaerobic digestion (SS-AD). To overcome this challenge, this study determined the optimal inoculation strategy (complete premix/slurry application) for reusing solid digestate as inoculum and the optimal leachate circulation method (percolation/immersion) using batch digestion. Initially, percolation and immersion (1 h per 3 days) were compared and the result shows that immersing rice straw into leachate was superior to leachate percolation in methane production. Effect of the immersion period (24, 48 and 72 h) in each circulation cycle on methane production was then evaluated for each inoculation strategy. Methane production increased until the immersion period up to 24 h and then decreased, while the average cumulative methane yield with an immersion period of 24 h was (180 mL/g volatile solids). Slurry application with an immersion period 24 h is recommended as the optimum operating condition.

Immobilization of Azospira sp. strain I13 by gel entrapment for mitigation of N2O from biological wastewater treatment plants: Biokinetic characterization and modeling.

Development of a strategy to mitigate nitrous oxide (N2O) emitted from biological sources is important in the nexus of wastewater treatment and greenhouse gas emission. To this end, immobilization of N2O-reducing bacteria as a biofilm has the potential to ameliorate oxygen (O2) inhibition of the metabolic activity of the bacteria. We demonstrated the effectiveness of calcium alginate gel entrapment of the nosZ clade II type N2O-reducing bacterium, Azospira sp. strain I13, in reducing levels of N2O, irrespective of the presence of O2. Azospira sp. strain I13 cells in the gel exhibited N2O reduction up to a maximum dissolved oxygen concentration of 100 µM in the bulk liquid. The maximum apparent N2O uptake rate, [Formula: see text] , by gel immobilization did not appreciably decrease, retaining 72% of the N2O reduction rate of the cell suspension of Azospira sp. strain I13. Whereas gel immobilization increased the apparent half-saturation constant for N2O, [Formula: see text] , and the apparent O2 inhibition constant, [Formula: see text] , representing the degree of O2 resistance, correspondingly increased. A mechanistic model introducing diffusion and the reactions of N2O consumption was used to describe the experimental observations. Incorporating Thieles modulus into the model determined an appropriate gel size to achieve N2O reduction even under aerobic conditions.

Biokinetic Characterization and Activities of N2O-Reducing Bacteria in Response to Various Oxygen Levels.

Nitrous oxide (N2O)-reducing bacteria, which reduce N2O to nitrogen in the absence of oxygen, are phylogenetically spread throughout various taxa and have a potential role as N2O sinks in the environment. However, research on their physiological traits has been limited. In particular, their activities under microaerophilic and aerobic conditions, which severely inhibit N2O reduction, remain poorly understood. We used an O2 and N2O micro-respirometric system to compare the N2O reduction kinetics of four strains, i.e., two strains of an Azospira sp., harboring clade II type nosZ, and Pseudomonas stutzeri and Paracoccus denitrificans, harboring clade I type nosZ, in the presence and absence of oxygen. In the absence of oxygen, the highest N2O-reducing activity, Vm,N2O, was 5.80 ± 1.78 × 10-3 pmol/h/cell of Azospira sp. I13, and the highest and lowest half-saturation constants were 34.8 ± 10.2 µM for Pa. denitirificans and 0.866 ± 0.29 µM for Azospira sp. I09. Only Azospira sp. I09 showed N2O-reducing activity under microaerophilic conditions at oxygen concentrations below 110 µM, although the activity was low (10% of Vm,N2O). This trait is represented by the higher O2 inhibition coefficient than those of the other strains. The activation rates of N2O reductase, which describe the resilience of the N2O reduction activity after O2 exposure, differ for the two strains of Azospira sp. (0.319 ± 0.028 h-1 for strain I09 and 0.397 ± 0.064 h-1 for strain I13) and Ps. stutzeri (0.200 ± 0.013 h-1), suggesting that Azospira sp. has a potential for rapid recovery of N2O reduction and tolerance against O2 inhibition. These physiological characteristics of Azospira sp. can be of promise for mitigation of N2O emission in industrial applications.

Investigations of water-extractability of As in excavated urban soils using sequential leaching tests: Effect of testing parameters.

Excavated soils with low-level As contamination obtained from construction projects during city development have been of great concern in Japan. Water-extractable As represents the most easily mobilized and ecotoxicologically relevant fraction in the soil environment. In the present study, the water-extractability of As in excavated alkaline urban soils was assessed using sequential leaching tests (SLTs) with a focus on the effects of test parameters. In addition, the potentially water-leachable As over an extremely long period was assessed using the pollution potential leaching index (PPLI), from which one can estimate the number of extractions required to reduce the As in the cumulative leachates to below the Japanese environmental standard (10 µg L-1). Total As concentrations varied from 6.75 to 79.4 mg kg-1, and As was continuously detectable among replicate SLT experiments. The water-extractable As obtained in the first step of the SLT accounted for 0.41%-7.60% of total As (average: 2.36%), while the cumulative released As in the SLTs corresponded to 1.30%-21.6% of the total (average: 10.6%). The variability of the water-soluble fractions was sensitive to the test conditions. The shaking time at each SLT step had the largest effect on the As water-extractability; followed by sample storage, shaking speed and shaking interruption. A longer shaking time in the standard leaching test of excavated soils is suggested for regulatory purposes in Japan. The use of the PPLI concept for quick estimation of the potential As leachability from excavated soils was supported by the good reproducibility of PPLI results obtained from SLTs under different test parameters.

The influence of the total solid content on the stability of dry-thermophilic anaerobic digestion of rice straw and pig manure.

Dry anaerobic digestion is a promising technology for the recycling of agricultural waste to produce energy and fertilizer. Adding water to the substrate enables better handling and avoid inhibition caused by high total solid (TS) content in the reactor; however, it also increases leachate and operational costs. To assess the extent to which the amount of water added can be reduced, it was examined how the TS content in the reactor influenced the production of biogas. A semi-batch dry thermophilic anaerobic digester was fed with substrate (rice straw and pig manure) at a constant organic loading rate, and varied the TS contents (27%, 32%, 37%, and 42%) of the substrate by adding different amounts of water (representing 0-36% of the total substrate). During incubation, the TS content in the reactor gradually increased from 18% to 31%. Biogas production was stable and high (564 ±â€¯13-580 ±â€¯36 N m3 t-1 VS), and there was no accumulation of volatile fatty acids when the TS content of the reactor was between 18% and 27%. However, the rate decreased sharply and propionate and acetate were also produced when the TS content of the reactor exceeded 28%. By applying a simple TS balance model, it was found that stable biogas production could be achieved at a substrate TS content of 32%, at which reactor TS content reached 23% at steady-state condition.

Influence of feedstock-to-inoculum ratio on performance and microbial community succession during solid-state thermophilic anaerobic co-digestion of pig urine and rice straw.

This study investigated the effect of the feedstock-to-inoculum (F/I) ratio on performance of the solid-state anaerobic co-digestion of pig urine and rice straw inoculated with a solid digestate, and clarified the microbial community succession. A 44-day biochemical methane potential test at F/I ratios of 0.5, 1, 2 and 3 at 55 °C and a 35-day large-scale batch test at F/I ratios of 0.5 and 3 at 55 °C were conducted to investigate the effects of F/I ratio on anaerobic digestibility and analyze microbial community succession, respectively. The highest cumulative methane yield was 353.7 m3/t VS in the large-scale batch test. Volatile fatty acids did not accumulate at any F/I ratios. The volatile solids reduction rate was highest at a F/I ratio of 0.5. Microbial community structures were similar between F/I ratios of 3 and 0.5, despite differences in digestion performance, suggesting that stable operation can be achieved at these ratios.

Pollution potential leaching index as a tool to assess water leaching risk of arsenic in excavated urban soils.

Leaching of hazardous trace elements from excavated urban soils during construction of cities has received considerable attention in recent years in Japan. A new concept, the pollution potential leaching index (PPLI), was applied to assess the risk of arsenic (As) leaching from excavated soils. Sequential leaching tests (SLT) with two liquid-to-solid (L/S) ratios (10 and 20Lkg-1) were conducted to determine the PPLI values, which represent the critical cumulative L/S ratios at which the average As concentrations in the cumulative leachates are reduced to critical values (10 or 5µgL-1). Two models (a logarithmic function model and an empirical two-site first-order leaching model) were compared to estimate the PPLI values. The fractionations of As before and after SLT were extracted according to a five-step sequential extraction procedure. Ten alkaline excavated soils were obtained from different construction projects in Japan. Although their total As contents were low (from 6.75 to 79.4mgkg-1), the As leaching was not negligible. Different L/S ratios at each step of the SLT had little influence on the cumulative As release or PPLI values. Experimentally determined PPLI values were in agreement with those from model estimations. A five-step SLT with an L/S of 10Lkg-1 at each step, combined with a logarithmic function fitting was suggested for the easy estimation of PPLI. Results of the sequential extraction procedure showed that large portions of more labile As fractions (non-specifically and specifically sorbed fractions) were removed during long-term leaching and so were small, but non-negligible, portions of strongly bound As fractions.