Papers - SUZUKI Yoshihiro
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Genetic diversity of dissolved free extracellular DNA compared to intracellular DNA in wastewater treatment plants Reviewed
Tamai, Soichiro Okuno, Miki Ogura, Yoshitoshi Suzuki, Yoshihiro
Science of The Total Environment 970 178989 2025.3
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)
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Detection and collection of shiga toxin-producing Escherichia coli using foam concentration without membrane filtration Reviewed
Tamai, S. Katafuchi, M. Hui, X. Suzuki, Y.
Ecotoxicology and Environmental Safety 2025.3
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)
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Optimizing the processing conditions of ballasted flocculation of actual turbid river water Reviewed
Suzuki, Yoshihiro Takahashi, Katsuya Kanai, Miyuka Tamai, Soichiro Annaka, Yuko Chuganji, Nobuaki
Urban Water Journal 22 ( 2 ) 236 - 243 2025.1
Authorship：Lead author, Corresponding author Language：English Publishing type：Research paper (scientific journal)
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Crop contamination evaluation by antimicrobial-resistant bacteria via livestock waste compost-fertilized field soil Reviewed
Suzuki, Yoshihiro Horita, Tomoyuki Nishimura, Emi Xie, Hui Tamai, Soichiro Kobayashi, Ikuo Fukuda, Akira Usui, Masaru
Journal of Hazardous Materials 480 135987 2024.9
Authorship：Lead author, Corresponding author Language：English Publishing type：Research paper (scientific journal)
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Year-round monitoring of antibiotic-resistant bacteria in pristine uppermost stream and estimation of pollution sources Reviewed
Nishimura, Emi Xie, Hui Tamai, Soichiro Nishiyama, Masateru Nukazawa, Kei Hoshiko, Yuki Ogura, Yoshitoshi Suzuki, Yoshihiro
Frontiers in Environmental Science 12 1439174 2024.9
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal)
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Shwe Sin P.P., Wai S.H., Ota Y., Nishioka K., Suzuki Y.
International Journal of Hydrogen Energy 53 86 - 92 2024.1
Publishing type：Research paper (scientific journal) Publisher：International Journal of Hydrogen Energy
Hydrogen production from renewable electricity offers an eco-friendly alternative to fossil fuels. Proton exchange membrane (PEM) electrolysis is a well-known method for this purpose. Studies have primarily focused on reducing costs of noble catalysts, improving efficiency, managing system degradation, and addressing membrane thinning caused by contaminated cations. However, techniques for PEM recovery post-degradation are still under development. This study investigated the effects of cations on PEM cells using artificial soft water, and analyzed two recovery methods to restore cell performance. Our findings indicate a significant rise in cell operating voltage and a decrease hydrogen production over 8 h of operation with soft water. After introducing both recovery methods, the initial operating value was reinstated in both cases. Only nitric acid treatment, however, achieved hydrogen production levels comparable to those of ultrapure water.
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Mineda H., Nukazawa K., Suzuki Y.
Journal of Water and Climate Change 14 ( 12 ) 4821 - 4836 2023.12
Publishing type：Research paper (scientific journal) Publisher：Journal of Water and Climate Change
Hydrological alterations by dams and climate change can reduce aquatic biodiversity by disrupting the life cycles of organisms. Here, we aimed to evaluate and compare the hydrological alterations caused by dams and climate change throughout the Omaru River catchment, Japan, using a distributed hydrological model (DHM). First, to assess the impacts of dam and climate change independently, we performed runoff analyses using either dam discharge or future climatic data (two future periods, 2031–2050 and 2081–2100 x three representative con-centration pathways). Subsequently, we derived indicators of hydrologic alterations (IHA) to quantify changes in flow alterations by comparing them to IHA under natural conditions (i.e., without dam or climate change data). We found that dams altered IHAs more than climate change. However, on a catchment-scale standpoint, climate change induced wider ranges of flow alterations, such as a further decrease in low flow metrics along the tributaries and uppermost main stem, suggesting a catchment-level shrinkage in important corridors of aquatic organisms. We also observed that the altered flow by water withdrawals was ameliorated by the confluence of tributaries and downstream hydropower outflows. Our approach using a DHM captured the various patterns of flow alterations by dams and climate change.
DOI： 10.2166/wcc.2023.538
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Tamai S., Suzuki Y.
Microorganisms 11 ( 12 ) 2023.12
Publishing type：Research paper (scientific journal) Publisher：Microorganisms
Enterococcus spp. are common bacteria present in the intestinal tracts of animals and are used as fecal indicators in aquatic environments. On the other hand, enterococci are also known as opportunistic pathogens. Elucidating their composition in the intestinal tracts of domestic animals can assist in estimating the sources of fecal contamination in aquatic environments. However, information on the species and composition of enterococci in animal hosts (except humans) is still lacking. In this study, enterococci were isolated from the feces of cattle, pigs, birds, and humans using selective media. Enterococcal species were identified using mass spectrometry technology, and each host was characterized by diversity and cluster analysis. The most dominant species were E. hirae in cattle, E. faecium in birds, and E. faecalis in pigs and humans. Cattle had the highest alpha diversity, with high interindividual and livestock farm diversity. The dominant enterococcal species in pigs and humans were identical, and cluster analysis showed that the majority of the two hosts’ species clustered together.
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Optimum conditions for high-speed solid–liquid separation by ballasted flocculation Reviewed
Suzuki Y., Kaku R., Takahashi K., Kanai M., Tamai S., Annaka Y., Chuganji N.
Water Science and Technology 88 ( 1 ) 35 - 46 2023.7
Publishing type：Research paper (scientific journal) Publisher：Water Science and Technology
In the ballasted flocculation, high-speed sedimentation of suspensions is achieved using a microsand as a ballast material and a polymer flocculant combined with microflocs made of polyaluminum chloride (PAC) as an inorganic coagulant. In this study, three turbid water samples containing kaolin clay (kaolin concentration: 20, 200, and 500 mg/L) were treated by coagulation–sedimentation and ballasted flocculation. The effects of pH and PAC dosage, which are the controlling parameters for coagulation, and the microsand (silica sand) and polymer dosages, which are the controlling parameters for ballasted treatment, on the treatment efficiency and floc settling velocity were examined. The floc settling velocity under the optimum conditions was 17 times higher than that of the conventional coagulation–sedimentation process using PAC. The turbidity was 0.54 turbidity unit (TU) (TU as the kaolin standard), and its removal efficiency was 99.7%. Furthermore, turbid water samples with different kaolin concentrations (20 and 500 mg/L) were treated via the ballasted flocculation. In this study, fundamental information on the optimization of each dosage condition of coagulant, ballast, and polymer and pH condition in ballasted flocculation was obtained, and the removal mechanisms under optimal, underoptimistic and overoptimistic conditions were proposed.
DOI： 10.2166/wst.2023.204
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Xie H., Yamada K., Tamai S., Shimamoto H., Nukazawa K., Suzuki Y.
Environmental Science and Pollution Research 2023.6
Publishing type：Research paper (scientific journal) Publisher：Environmental Science and Pollution Research
Antibiotic-resistant bacteria (ARBs) can now be detected not only in clinical institutions but also in wastewater treatment plants (WWTPs), extending the range of emergence to residential areas. In this study, we investigated the change of antibiotic-resistant Escherichia coli (E. coli) and other coliforms in each treatment process at WWTPs. Throughout the treatment process, the numbers of E. coli and other coliforms were significantly reduced to less than 5.7 ± 0.5 CFU/100 ml and 2.4 ± 0.0×102 CFU/100 ml, respectively. However, ESBL-producing E. coli and other coliforms were detected in each treatment process (even after chlorination) at 5.6% and 4.8%, compared to the total E. coli and other coliforms counts. Then, ESBL-producing-related genes were identified via PCR analyses, and the most predominant gene was CTX-M-9 in both E. coli (47.2%) and other coliforms (47.3%). Although actual WWTPs greatly reduced the number of bacteria, the relative prevalence of ESBL-producing bacteria was increased, suggesting that ESBL-producing bacteria remain in the effluent at minimal concentrations and could be diffusing to water bodies.
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Tamai S., Ogura Y., Okuno M., Nukazawa K., Suzuki Y.
Environmental Science: Water Research and Technology 9 ( 7 ) 1792 - 1803 2023.4
Publishing type：Research paper (scientific journal) Publisher：Environmental Science: Water Research and Technology
Dissolved free extracellular DNA (exDNA), DNA released outside the cell, is present in aquatic environments at very low concentrations. Additionally, exDNA passes through a 0.22 μm filter, making it difficult to capture for analysis. Thus, information on exDNA is scarce, and the composition of genes present in exDNA is unknown. In this study, we developed a process for highly concentrating exDNA adsorbed on colloids and then dissolving the colloids with an acid using colloidal adsorption and foam concentration. After applying this strategy, 66.3% ± 3.3% of the uidA gene and 39.4% ± 2.4% of the tetA gene were recovered from genomic DNA carrying the uidA gene and plasmids carrying the tetA gene (n = 3), respectively. Using the developed method, 1000 mL of effluent of a wastewater treatment plant was concentrated to 400 μL, and exDNA was rapidly recovered at a high concentration (concentration ratio: 2500×). Furthermore, metagenomic analysis revealed the function of some exDNA genes, such as those encoding antibiotic resistance proteins. The information revealed in this study contributes to the elucidation of the mechanisms of horizontal gene transfer in aquatic environments.
DOI： 10.1039/d3ew00015j
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Tamai S., Shimamoto H., Nukazawa K., Suzuki Y.
Microorganisms 11 ( 4 ) 2023.4
Publishing type：Research paper (scientific journal) Publisher：Microorganisms
High counts of bacteria are present in beach sand, and human health threats attributable to contact with sand have been reported. In this study, we investigated fecal indicator bacteria in the top surface sand of coastal beaches. Monitoring investigations were performed during a monsoon when rainfall occurs randomly, and the composition of the coliforms was analyzed. The coliform count in the top surface sand (depth < 1 cm) increased by approximately 100 fold (26–2.23 × 103 CFU/100 g) with increasing water content because of precipitation. The composition of the coliforms in the top surface sand changed within 24 h of rainfall, with Enterobacter comprising more than 40% of the coliforms. Estimation of factors that changed the bacterial counts and composition revealed that coliform counts tended to increase with increasing water content in the top surface sand. However, the abundance of Enterobacter was independent of the sand surface temperature and water content. Coliform counts in the top surface sand rapidly increased and the composition showed remarkable variations because of the supply of water to the beach following rainfall. Among them, some bacteria with suspected pathogenicity were present. Controlling bacteria in coastal beaches is important for improving public health for beachgoers.
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Yadai T., Suzuki Y.
npj Clean Water 7 2023.2
Authorship：Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：npj Clean Water
Efficient desalination through a reverse osmosis (RO) membrane requires the prior removal of blockade-causing substances from raw seawater. To achieve ultrahigh-speed processing of a pretreatment process for seawater RO desalination, we combine traditional softening with ballasted flocculation (SBF) for Ca2+ and Mg2+ removal. An alkaline mixture of Ca(OH)2 and Na2CO3 was the most suitable softening agent for Ca2+ and Mg2+ removal with a reduced amount of generated sludge. This softening treatment simultaneously removed the suspended solids and bacteria from actual seawater. The settling velocity of the suspended solids generated via seawater softening was extremely low. Under the optimum conditions for desalinating actual seawater using an anionic polymer flocculant and microsand, the settling velocity exceeded 3.5 cm/s, 833 times higher than that of softening without ballasted flocculation. The amount of sludge after standing for 3 min was 76.5% lower in SBF than in conventional softening. The silt density index of the treated seawater met the water-supply standard of RO membranes (i.e., <3.0). Furthermore, the SBF-generated sludge exhibited considerably improved dewatering property than the sludge obtained via conventional softening. SBF can efficiently and quickly remove the causative substances of RO membrane fouling from seawater, thereby improving the treatability of generated sludge. SBF provides a new pretreatment process for seawater desalination using RO membranes.
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Suzuki Y., Shimizu H., Tamai S., Hoshiko Y., Maeda T., Nukazawa K., Iguchi A., Masago Y., Ishii S.
Environmental Monitoring and Assessment 195 ( 2 ) 2023.2
Authorship：Lead author, Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Environmental Monitoring and Assessment
Waterborne diseases due to pathogen contamination in water are a serious problem all over the world. Accurate and simultaneous detection of pathogens in water is important to protect public health. In this study, we developed a method to simultaneously detect various pathogenic Escherichia coli by sequencing the amplicons of multiplex PCR. Our newly designed multiplex PCR amplified five genes for pathogenic E. coli (uidA, stx1, stx2, STh gene, and LT gene). Additional two PCR assays (for aggR and eae) were also designed and included in the amplicon sequencing analysis. The same assays were also used for digital PCR (dPCR). Strong positive correlations were observed between the sequence read count and the dPCR results for most of the genes targeted, suggesting that our multiplex PCR-amplicon sequencing approach could provide quantitative information. The method was also successfully applied to monitor the level of pathogenic E. coli in river water and wastewater samples. The approach shown here could be expanded by targeting genes for other pathogens.
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Ito K., Matsunaga M., Itakiyo T., Oishi H., Nukazawa K., Irie M., Suzuki Y.
International Journal of Sediment Research 2022.12
Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：International Journal of Sediment Research
Sediment causes a serious problem in relation to dam function. A cooperative sediment sluicing operation has been under way since 2017 to prevent sediment from accumulating in dams in the Mimi River, Miyazaki, Japan. To achieve a smooth and stable operation, it is very important to determine the sediment source and a sediment transport system to maintain the dam's function. In the current study, the source and transport of sediment from the Mimi River basin have been analyzed with X-ray diffraction (XRD) to reveal the peaks of mineral species. The sediment samples were collected in the Mimi River basin from the Tsukabaru Dam to the sea in an area including 4 dams and 5 tributaries. In addition, the minerals in samples collected in 2014, before the start of the sediment sluicing operation, and from 2018 to 2020, after the start of the operation, were analyzed. An evaluation of the similarity of the sediment at each sampling point based on the X-ray diffraction peaks of mineral species showed that sediment distributed upstream was transported downstream in the year when the sediment sluicing was done. This result indicates that the sediment sluicing operation at dams ensured the continuity of the sediment distribution in the Mimi River basin. In addition, an investigation done in 2020 showed that sediment particles very similar to those of the upstream tributaries were deposited downstream because of extensive flooding caused by a large typhoon. The management of sediment transport has the greatest importance in a river basin where a dam is to be constructed. It is possible to trace the history of the sediment distribution and movement resulting from the operation of sediment sluicing by using sediment mineral analysis.
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Application of Coagulation and Foam Concentration Method to Quantify Waterborne Pathogens in River Water Samples Reviewed International coauthorship
Suzuki Y., Jikumaru A., Tamai S., Nukazawa K., Masago Y., Ishii S.
Water (Switzerland) 14 ( 22 ) 3642 2022.11
Authorship：Lead author, Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：Water (Switzerland)
One of the major challenges in detecting waterborne pathogens is the low concentration of the target bacteria in water. In this study, we applied the coagulation and foam concentration method to obtain DNA from water samples collected from upstream, near an estuary. The DNA samples were analyzed using 16S rRNA gene sequencing to clarify the microbial community shifts and to identify potentially pathogenic bacteria. Bacterial communities changed as the river flowed downstream, most likely influenced by land use and human activities such as the discharge of wastewater-treatment plant effluent. Based on the 16S rRNA gene amplicon sequencing, potentially pathogenic bacteria were detected with greater than 0.1% of their relative abundances. Among these, Yersinia ruckeri and Pseudomonas alcaligenes were widely detected in the river water. In addition, digital PCR (dPCR) was used to quantify major waterborne pathogens. Shiga toxin-producing Escherichia coli (STEC), Shigella spp., and Campylobacter jejuni were all below the limit of detection. In contrast, general E. coli, which has the beta-D-glucuronidase gene (uidA) were detected by dPCR (copies/100 mL) at similar levels to those measured using the culture-based method (as colony forming units/100 mL). These results suggest that the coagulation and foam concentration method is useful for concentrating microbes and obtaining DNA from river water samples for environmental monitoring.
DOI： 10.3390/w14223642
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Effects of the Japanese Nitrate Directive Plan (NDP) to curb groundwater nitrate-nitrogen content in the Miyakonojo River basin Reviewed
Yu, Z.-Q., K. Nakagawa, R. Berndtsson, T. Hiraoka, and Y. Suzuki
Journal of Hydrology 615 128563 2022.10
Authorship：Last author Language：English Publishing type：Research paper (scientific journal)
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瀬-淵の水理学的変動を考慮した機械学習による底生動物分布モデルの構築 Reviewed
田中凌央, 糠澤桂, 宇都宮将, 鈴木祥広
土木学会論文集G（環境）地球環境研究論文集 第30巻 78 ( 5 ) I_7 - I_16 2022.9
Authorship：Last author Language：Japanese Publishing type：Research paper (scientific journal)
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Mori M., Gonzalez Flores R., Suzuki Y., Nukazawa K., Hiraoka T., Nonaka H.
Harmful Algae 117 102273 2022.8
Authorship：Last author Language：English Publishing type：Research paper (scientific journal) Publisher：Harmful Algae
Machine learning, Deep learning, and water quality data have been used in recent years to predict the outbreak of harmful algae, especially Microcystis, and analyze outbreak causes. However, for various reasons, water quality data are often High-Dimension, Low-Sample- Size (HDLSS), meaning the sample size is lower than the number of dimensions. Moreover, imbalance problems may arise due to bias in the occurrence frequency of Microcystis. These problems make predicting the occurrence of Microcystis and analyzing its causes with machine learning difficult. In this study, a machine learning model that applies Feature Engineering (FE) and Feature Selection (FS) algorithms are used to predict outbreaks of Microcystis and analyze the outbreak factors from imbalanced HDLSS water quality data. The prediction performance was verified with binary classification to determine whether Microcystis would occur in the future by applying three machine learning models to four data patterns. The cause analysis of Microcystis occurrence was performed by visualizing the results of applying FE and FS. For the test data, the predictive performance of FE and FS methods was significantly better than that of the conventional method, with an accuracy of .108 points and an F-value of .691 points higher than the conventional method. A prediction performance increase was observed with a smaller model capacity. Data-driven analysis suggested that total nitrogen, chemical oxygen demand, chlorophyll-a, dissolved oxygen saturation, and water temperature are associated with Microcystis occurrences. The results also indicated that basic statistics of the water quality distribution (especially mean, standard deviation, and skewness) over a year, not the concentrations of water components, are related to the occurrence of Microcystis. These are new findings not found in previous studies and are expected to contribute significantly to future studies of algae. This study provides a method for analyzing water quality data with high-dimensionality and small samples, imbalance problems, or both.
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Effects of Artificial River Water on PEM Water Electrolysis Performance Reviewed
Yoshimura, R., Wai, S., Ota, Y., Nishioka, K., Suzuki, Y.
Catalysts 12 ( 9 ) 934 2022.8
Language：English Publishing type：Research paper (scientific journal)