Shohei Riya

• PhD
• Professor (Assistant) at Tokyo University of Agriculture and Technology

The occurrence of the cavitation phenomena in a high-pressure jet device was verified. When water was injected at 4 MPa and 3 L min⁻¹ into the device made of a transparent material, small bubbles were observed, which would be produced by lowered pressure inside. Hydrodynamic simulation revealed that the pressure fell below saturated vapor pressure...

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...

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 bas...

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/smel...

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 denit...

A transition to ammonia recovery from wastewater has started; however, a technology for sustainable nitrogen retention in the form of ammonia 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 activate...

Soil moisture is a key factor for mercury (Hg) emission from soil. Despite its significance for Hg emissions, the effect of soil moisture on Hg flux and fractions has not been thoroughly investigated. The objective of this study was to elucidate the influences of soil moisture and temperature on Hg fluxes from soils and Hg fractions. A kinetic stud...

Nitrous oxide (N2O) is a highly potent greenhouse gas and ozone-depleting substance, produced and consumed during denitrification. Evaluation of the N2O production and consumption activities of complete denitrifying bacteria is essential for understanding their capacity to act as N2O sinks in engineered systems for cost-effective nitrogen removal v...

Ethanol fermentation with a hybrid system containing free and immobilized yeast cells on a macroporous monolithic hydrogel scaffold was investigated. Cell immobilization was achieved using N-isopropylacrylamide (NIPA) and methoxy triethyleneglycol acrylate (MTGA) hydrogels having macroporous monolithic structures containing interconnected micromete...

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 emiss...

Nitrous oxide (N2O), a potent greenhouse gas, is reduced to N2 gas by N2O‐reducing bacteria (N2ORB), a process which represents an N2O sink in natural and engineered ecosystems. The N2O sink activity by N2ORB depends on temperature and O2 exposure, yet the specifics are not yet understood. This study explores the effects of temperature and oxygen e...

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 m...

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 m...

Sacred lotus (Nelumbo nucifera) is known to vent pressurized air in the leaf blade through stomata on the central plate of the leaf via the rhizome. This study measured diurnal CH4 emissions from lotus-cultivated in tanks during July and August 2016. CH4 emissions from the central plate of leaves of different age were also evaluated. Average daily...

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 s...

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 tre...

Water-saving irrigation techniques can effectively mitigate methane emissions from rice fields but typically enhance nitrous oxide (N2O) emissions due to changes in soil moisture content. However, few studies have examined the effect of different water regimes on N2O production using different species of exogenous nitrogen (N) fertilizer applicatio...

For sustainable agriculture, it is important to manage agricultural wastes, such as crop residues and livestock wastes. Anaerobic digestion has been gathering the attention to recycle these wastes into renewable energy (biogas) and fertilizer (soil amendment) (digestate). Dry anaerobic digestion is defined as digestion at higher than 20% of total s...

The recent discovery of nitrous oxide (N2O)-reducing bacteria suggests a potential biological sink for the potent greenhouse gas N2O. While some N2O-reducing bacteria have been described, characterization of more isolates will be required for an application towards N2O mitigation. Here, we describe the successful enrichment and isolation of high-af...

This study investigated the effects of biogas digestate application to soil with rice straw on nitrate leaching potential and nematicidal activity toward root-knot nematodes Meloidogyne incognita. The following seven treatments were set up: (i) control (CONT); (ii) chemical fertilizer (CF); (iii) wet biogas digestate derived from pig manure (WBD);...

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 ci...

The aims of this study were to investigate the feasibility of partial nitrification (PN) in a sequencing batch reactor (SBR) fed with real wastewater discharged from the washing process at a semiconductor factory, to evaluate the efficacy of online control for nitrite buildup, and to examine the microbial community composition. To this end, online...

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...

Heavy metals contaminated soils are a potential exposure hazard to the public. This study demonstrated the application of chlorination and thermal treatment for remediation of heavy metals-contaminated soils on a full-industrial scale. Five soil samples were treated at a soil remediation factory equipped with an internally-heated rotary kiln (lengt...

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, w...

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 t...

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...

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 am...

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-sol...

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...

Purpose In Japan, the excavated soils produced from constructions projects, which contain relatively low levels of arsenic (As), are considered as a potential concern if they could release significant amount of As to the environment. The aim of this study was to investigate the As leaching from excavated alkaline soils and, in particular, the influ...

A membrane-aerated biofilm reactor (MABR) provides a counter-current substrate diffusion geometry in which oxygen is supplied from a gas-permeable membrane on which a biofilm is grown. This study hypothesized that an MABR would mitigate NO and N2O emissions compared with those from a conventional biofilm reactor (CBR). Two laboratory-scale reactors...

The goal of this study was to investigate the effectiveness of a membrane-aerated biofilm reactor (MABR), a representative of counter-current substrate diffusion geometry, in mitigating nitrous oxide (N2O) emission. Two laboratory-scale reactors with the same dimensions but distinct biofilm geometries, i.e., a MABR and a conventional biofilm reacto...

The dry-thermophilic anaerobic digestibility of alternative biomasses to rice straw for pig manure treatment was evaluated. Seven grass and three woody biomasses were subjected to batch dry-thermophilic anaerobic digestion for methane (CH4) production. The CH4 production potential ranged from 109±8.1 to 347±62 m³/t-VS, with woody biomasses showing...

A pulse of nitrous oxide (N2O) emission has been observed following the disappearance of floodwater by drainage. However, its mechanism is not well understood. We conducted a column study to clarify the mechanism for N2O production during floodwater disappearance by using a microsensor and determining the bacterial gene expression. An increase in N...

The goal of this study was to elucidate the mechanisms of N2O production from a bioreactor for partial nitrification (PN). Ammonia-oxidizing bacteria (AOB) enriched from a sequencing batch reactor (SBR) were subjected to N2O production pathway tests. The N2O pathway test was initiated by supplying an inorganic medium to ensure an initial NH4⁺-N con...

We investigated the effects of free ammonia (FA) and free nitrous acid (FNA) concentrations on the predominant ammonia-oxidizing bacteria (AOB) and the emission of nitrous oxide (N2O) in a lab-scale sequencing batch reactor for partial nitrification. The reactor was operated with stepwise increases in the NH4(+) loading rate, which resulted in a ma...

Environmental applications of graphene (GN) are limited by the occurrence of aggregation. Herein, graphene oxide (GO) was synthesized, reduced to GN by ascorbic acid, and intercalated with cetyltrimethylammonium bromide (CTAB). GN-CTAB was characterized by Boehm’s titration, N2 adsorption/desorption, Fourier transform infrared spectroscopy, Raman s...

Herein, graphene (GN) was synthesized, exfoliated by anchoring dodecyl sulfate chain through hydrophobic interaction over its surface (GN-SDS), and was tested to sequester Cu(II) and Mn(II) ions in single-metal system from aqueous phase. Acid-base titrations and elemental analysis results verified successful dodecyl sulfate chain anchoring over GN-...

The effect of pig manure mixed with rice straw on methane yield and the microbial community involved in a thermophilic (55°C) anaerobic digestion process was investigated. Three substrates composed of mixed pig manure and rice straw at different ratios (95:5; 78:22 and 65:35 w/w, which resulted in C/N ratios of 10:1, 20:1 and 30:1) were used for th...

This research augmented the detailed Activated Sludge Model (ASM) to include abiotic hybrid nitrous oxide (N2O) production due to reactions between hydroxylamine (NH2OH) and nitrite. Model parameters were extracted from a series of abiotic and biological batch tests that showed that abiotic N2O production rates are 1-3 orders of magnitude greater t...

To evaluate the feasibility of swine manure treatment by a proposed Dry Thermophilic Anaerobic Digestion (DT-AD) system, we evaluated the methane yield of swine manure treated using a DT-AD method with rice straw under different C/N ratios and solid retention time (SRT) and calculated the mass and energy balances when the DT-AD system is used for s...

The use of liquid cattle waste (LCW) as a fertilizer for forage rice is important for material recycling because it can promote biomass production, and reduce the use of chemical fertilizer. Meanwhile, increase in emission of greenhouse gases (GHGs), especially CH4 and N2O would be concerned. We conducted a field study to determine the optimum load...

Nitrous oxide (N2O) is emitted from a modified Ludzak–Ettinger (MLE) process, as a primary activated sludge system, which requires mitigation. The effects of aeration rates and internal recycle flow (IRF) ratios on N2O emission were investigated in an MLE process fed with glycerol. Reducing the aeration rate from 1.5 to 0.5 L/min increased gaseous...

Effect of methanol and glycerol on nitrous oxide (N2O) emission in two laboratory-scale modified Ludzak Ettinger (MLE) processes was investigated during three distinct periods: dissolved oxygen (DO) control by intermittent aeration with a DO controller, and high and low aeration rates. N2O consumption rate in an anoxic tank and aeration mode influe...

The carbon source used for denitrification is a key factor in the reduction of nitrous oxide (N2O) produced from wastewater treatment plants because it affects denitrification activity. In this study, two laboratory-scale Modified Ludzak Ettinger (MLE) processes were operated with methanol and sodium acetate as the sole carbon sources. Removal effi...

The carbon source used for denitrification is a key factor in the reduction of nitrous oxide (N2O) produced from wastewater treatment plants because it affects denitrification activity. In this study, two laboratory-scale Modified Ludzak Ettinger (MLE) processes were operated with methanol and sodium acetate as the sole carbon sources. Removal effi...

Fertilization of aerated liquid fraction of cattle slurry (ALCS) to forage rice fields is a method to utilize surplus amount of cattle slurry and produce feed. However, ALCS application causes high emissions of CH4 and N2O from rice fields. In this study, we tested the abilities of several different field management strategies to mitigate CH4 and N...

Livestock wastewater disposal onto rice paddy fields is a cost- and labor-effective way to treat wastewater and cultivate rice crops. We evaluated the influence of nitrogen loading rates on nitrogen assimilation by rice plants and on nitrogen losses (leaching and N2O emission) in forage rice fields receiving liquid cattle waste (LCW). Four forage r...

Dry-thermophilic anaerobic co-digestion (DTAD) can be used to treat forage rice straw and pig manure and generate biogas as an energy source. Solid residue produced from DTAD process can be used as a fertilizer in forage rice fields, while addition of the residue could increase methane (CH4) and nitrous oxide (N2O) emissions from the soil. We evalu...

Nitrous oxide (N2O) production and expression of genes capable of its reduction were investigated in two full-scale parallel plug-flow activated sludge systems. These two systems continuously received wastewater with the same constituents, but operated under distinct nitrification efficiencies due to mixed liquor suspended solid (MLSS) concentratio...

In vegetable cultivation, the majority of N2O emissions occur after fertilization; it is therefore important to understand any factors contributing to this process. An experiment was conducted to investigate short-term N2O dynamics following topdressing in a greenhouse vegetable field in South China. During two topdressing processes, three differen...

Most studies on dissimilatory nitrate reduction to ammonium (DNRA) in paddy soils were conducted in the laboratory and in situ studies are in need for better understanding of the DNRA process. In this study, in situ incubations of soil DNRA using 15N tracer were carried out in paddy fields under conventional water (CW) and low water (LW) management...

Using livestock wastewater for rice production in paddy fields can remove nitrogen and supplement the use of chemical fertilizers. However, paddy fields have complicated water dynamics owing to varying characteristics and would influence nitrogen removal through nitrification followed by denitrification. Quantification of nitrification and denitrif...

To evaluate global warming potential (GWP) on livestock waste treatment and biomass production in rice field, methane (CH(4)) and nitrous oxide (N(2)O) fluxes from forage rice fields planted with 4 different cultivars (Oryza sativa L. cv. Hamasari, Leafstar, Kusahonami and Takanari) were measured. Each of the cultivars were subjected either to basa...

Behaviors of nitrogen compounds were investigated to elucidate the mechanisms of nitrogen (N) removal in a forage rice field applied with liquid cattle waste (LCW). In total, 567 kg-N· ha -1 of NH4-N was applied to the rice field by basal fertilization and three applications of LCW topdressing as supplemental fertilizer during the cultivation perio...

Three varieties of forage rice (Oryza sativa L. cv. Hamasari, Kusahonami, and Leafstar) were transplanted in plots established in a paddy field. Each of these varieties of rice was then subjected either to basal treatment alone (control plots), or to basal treatment plus subsequent treatment with liquid cattle waste (treatment plots). The nitrogen...

Flooded paddy fields perform many ecological and conservation functions and are also reported to facilitate livestock waste disposal. Paddy field infiltration rates are important for nitrogen dynamics. A laboratory study was conducted to compare the effects of infiltration rate on nitrogen dynamics including nitrogen leaching, soil adsorption, micr...

We evaluated direct and indirect greenhouse gas (methane (CH(4)) and nitrous oxide (N(2)O)) emissions from a vertical flow constructed wetland (CW) planted with forage rice. The gas fluxes from the CW surface water to the atmosphere (direct emission) and the dissolved gas concentrations in filtered water (indirect emission) from the CW were monitor...