Luận án Development of an appropriate treatment system for natural rubber processing wastewater treatment

· In this research, the energy-recovery type wastewater treatment system UASB-DHS system was applied to natural rubber processing wastewater in Vietnam. The key point for the application of the UASB reactor in natural rubber processing wastewater was combined with an efficient pretreatment process. ABR system was used in this study, however, accumulation of natural rubber particulars in the UASB column is always happed and leaded sludge washed-out. Therefore, an effective pre-treatment process is needed to research. Moreover, the production process of natural rubber should be considered like acid and ammonia addition. · Our research and present researches often exceed discharge standards of ammonia and nitrogen contents. An effective nitrogen removal process should be researched to achieve the discharged standard. Moreover, autotrophic denitrification processes such as the anammox process would be applied to this wastewater in order to reduce operational costs for wastewater treatment. · This study investigated that the current anaerobic treatment system emitted a large amount of GHG. Therefore, further study on emission principles and reduction methods should be studied.

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UASB reactor should be equipped with an ‘excess sludge and rubber particle removal system’ in addition to an ST for treating natural rubber processing wastewater that contains a large amount of residual natural rubber particles. In addition, some soluble organic removal was observed in the ST during phases 3 and 4. The DHS reactor can serve as an effective post-treatment system for residual organic particles and TSS removal. In this study, the DHS reactor removed 83.5 ± 10.0% of total COD, 82.6 ± 11.2% of total BOD, and 73.5 ± 20.0% of TSS during the entire experiment. These organic removal efficiencies were higher than the post-treatment DHS reactor treating the ABR effluent [9]. DO level of the DHS effluent was only 0.5 ± 0.3 mg·L-1 in phase 1. After the effluent was recirculated to DHS, DO increase to 0.9 ± 0.5 mg·L-1 in the DHS effluent. BOD of the DHS effluent also increased to 30 ± 16 mg·L-1 in phase 2. Okubo et al. (2016) reported that the effluent recirculation improved the DO levels in the system, however, DO was consumed very quickly to degrade high concentration organics in the upper part of the reactor [45]. During phase 3, the concentration of the organic in the DHS effluent was 140 ± 64 mg·L-1 for total COD and 31 ± 12 mg·L-1 for total BOD, indicating that most of the biodegradable organic compounds were removed from the system. Thus, the DHS reactor can be used as an effective post-treatment process for treating this wastewater. Figure 3.11 Accumulation of rubber particular in feed pipe and photo of wastewaters. Table 3.4 Summary of the process parameters of the system during entire experimental period. Figure 3.12 Time course of (A) Total COD removal efficiency and organic loading rate of UASB reactor, (B) Total BOD removal efficiency. Table 3.5 Biogas production and compositions of the UASB reactor. 3.4.2 Nitrogen removal and greenhouse gas emissions Table 3.6 lists the concentrations of TN, ammonia, nitrate, and nitrite in the treatment system. The TN in the ABR influent and effluent were 184 ± 93 mg-N·L-1 and 155 ± 72 mg-N·L-1, respectively. On the other hand, ammonia concentrations of the ABR influent and effluent were 122 ± 49 mg-N·L-1 and 151 ± 70 mg-N·L-1, indicating that ammonia could be produced from organic nitrogen by anaerobic digestion. In addition, small amounts of nitrate detected in the ABR effluent suggested the occurrence of nitrification in ABR. According to Tanikawa et al. (2016a) [24], ammonia was oxidized to nitrate at the surface of ABR, and nitrous oxide was emitted to the atmosphere. In fact, 213 ppm of nitrous oxide was detected in the biogas collected from ABR on day 190 of this study. Nitrate reduction in the UASB reactor indicated the possibility of denitrification of wastewater in the UASB reactor. The concentrations of nitrous oxide in the biogas produced in UASB were 213 ppm, 72 ppm, and 614 ppm on day 42, day 190, and day 264, respectively. The nitrous oxide production ratio from 1 m3 of treated RSS wastewater was 4.737 × 10-6 m3·m-3-w.w during phase 3. The maximum nitrous oxide concentration of 614 ppm was observed on day 264. The production rate equivalent to carbon dioxide for 1 m3 of treated RSS wastewater for nitrous oxide in this UASB reactor was calculated as 2.77 × 10-5 t-CO2 eq·m-3-w.w during phase 3. The DHS reactor was installed for the nitrification of wastewater as well as residual organics removal in this system. During phase 1, the DHS reactor demonstrated low TN and ammonia removal efficiencies of 38.8 ± 16.0% and 19.3 ± 5.8% (Figure 3.13). A small amount of nitrate production was also observed in the DHS reactor. However, TN and ammonia reduction suggested that nitrification occurred in the reactor, and nitrification products were immediately utilized by denitrifying bacteria in the DHS reactor. Araki et al. (1999) and Machdar et al. (2000) reported that the inner section of the DHS sponge carrier is anaerobic [21][18]. Therefore, denitrification would be occurred to some degree in the DHS reactor. In addition, the low BOD concentration in the DHS effluent suggested that autotrophic bacteria could be in abundance in that reactor. Several studies reported the coexistence of anaerobic ammonia-oxidizing bacteria and heterotrophs in the DHS reactor. During phase 2 through phase 4, the DHS effluent was recirculated back as DHS influent to enhance TN removal efficiency. The TN removal efficiency of the DHS reactor was increased to 52.9 ± 5.1% during phase 2. Ikeda et al. (2013) demonstrated the advantage of effluent recirculation up to 2.0 in the DHS reactor treating industrial wastewater containing a high concentration of organic and ammonia for enhancing denitrification [47]. The nitrification ratio (based on ammonia oxidization) of the DHS reactor also increased to 0.42 ± 0.03 kg-N·m-3·day-1 during phase 4. This nitrification rate was greater than the same sponge-type DHS reactor treating sewage and natural rubber processing wastewater in other studies [19][48]. During phase 4, the TN removal efficiency of the DHS reactor was decreased to 35.9± 10.7% due to a high nitrogen loading rate operation of 0.68 ± 0.22 kg-N·m-3·day-1. Nitrous oxide emissions from the DHS reactor were evaluated by a small closed cylinder with a sponge carrier that retained sludge. The biogas from the DHS reactor contained 99.2 % of nitrogen, 0.8% of carbon dioxide, and 85.4 ppm of nitrous oxide on 190. The amount of biogas from the DHS reactor was under the detection limit. Kampschreur et al. (2009) reported that low DO and low COD/N ratios were the most important operational parameters leading to nitrous oxide emissions [35]. Thus, the DHS reactor could emit nitrous oxide from nitrification and/or denitrification processes. Kampschreur et al. (2008) also reported that 0.6% of the nitrogen load was emitted as nitrous oxide in full-scale nitrifying and denitrifying sewage treatment plants [38]. According to this nitrous oxide emission ratio (0.6% of the nitrogen load), nitrous oxide emissions from the DHS reactor were calculated as 0.00026 t-CO2 eq·m-3 - w.w. during phase 3 [38]. Therefore, nitrous oxide emissions from the DHS reactor are an important parameter to consider when designing full-scale treatment systems. In total, the TN removal efficiency was 33.6 ± 17.7%, 51.3 ± 34.0%, 68.3 ± 15.1%, and 57.9 ± 7.0% in phases 1 to 4, respectively. However, ammonia and TN remained in the final effluent. Therefore, a system modification such as the addition of a denitrification tank or autotrophic nitrogen removal process is required. The emission ratios for 1 m3 of RSS wastewater treatment for ABR, UASB, and DHS were calculated as 0.0129 t-CO2eq·m-3, 0.0045 t-CO2eq·m-3 and 0.00026 t-CO2eq·m-3, respectively. The UASB reactor can recover biogas as energy, thus GHGs emission ratio from the proposed system can be reduced to 0.013 t-CO2eq·m-3, corresponding to a 92% reduction of GHGs emissions compared with the existing open-type anaerobic treatment systems. However, pre-treatment ABR emitted most of the GHGs emissions in this study. Therefore, the development of effective closed-type pretreatment systems is needed for further reductions in GHGs emissions. Table 3.6 Nitrogen concentrations (mg-N·L-1) in the proposed system. Figure 3.13 (A) Total nitrogen and (B) ammonia removal efficiency of the total system and DHS reactor during phase 1 to phase 4. 3.4.3 Performance comparison of ABR-UASB-DHS system and existing treatment system The characteristics of natural rubber processing wastewater are different in the local factory and producing countries (Table 3.7). This difference composition of wastewater is related to which acids are used for the coagulation process of latex. Acetic acid, formic acid, and sulfuric acid are normally used in the coagulation process in natural rubber factories [49]. The most natural rubber-produced countries Thailand and Malaysia have used sulfuric acid because of its high oxidizing activity and low reagent costs. Therefore, wastewater in these countries did not contain a large number of residual rubber particles due to the high oxidizing power of sulfuric acid, but it still had a large impact on aquatic environments. On the other hand, acetic acid and/or formic acid have been typically used for the coagulation process in Vietnamese local natural rubber processing factories, because they have a lower impact on the environment compared to sulfuric acid. However, the natural rubber processing wastewater from the Vietnamese factories contains a large number of residual rubber particles and these particles have a negative effect on the wastewater treatment processes reactor due to the accumulation of floating particles [7][9]. From this difference in wastewater composition, the wastewater treatment process is different between Vietnam and other natural rubber processing countries. Table 3.7 Characteristics of natural rubber processing wastewater in Thailand, Malaysia, and Vietnam. Acetate and propionate are the main organic compounds in the natural rubber processing wastewater in Vietnam. This organic compound is easy to convert to methane by acetate utilizing methanogen. Therefore, the natural rubber processing wastewater in Vietnam could be easy to degrade. However, the natural rubber particulars are remained in the wastewater, and post-treatment removing this particular to introduce the wastewater treatment process is required. Nguyen (1999) firstly applied a UASB reactor for treating natural rubber processing wastewater in Vietnam [6]. Nevertheless, a large number of rubber particulars accumulated in the UASB column, and its process performance deteriorated. The high biodegradability of natural rubber processing wastewater in Vietnam after removing natural rubber particulars was reported by Watari et al. (2016) [9] and Thanh et al. (2016) [42]. The laboratory scale UASB reactor performed a high methane recovery ratio of 93.3% calculated based on the removed total COD [9]. Thus, the efficient process performance of natural rubber processing wastewater treatment process is necessary to develop effective rubber particular process. The natural rubber processing wastewater in Thailand and Malaysia contains sulfide acid. The sulfate-rich wastewater creates the onset of toxic H2S gas production in the wastewater holding ponds and wastewater treatment process. H2S gas production was carried out by the SRB, which could proliferate in wastewater containing sulfate and other sulfur compounds under aerobic composition. The activated sludge process and oxidation ditch process could be carried out, but the energy cost for air delivery systems has always discouraged the continual operation. In many cases, the systems were left an-aerated and turned to be aerobic pounds which again cause H2S emission. Therefore, a closed system such as an anaerobic digester is an attractive treatment system. Tanikawa et al. (2017) designed two-stage UASB reactors that effectively utilized SRB by effluent recirculation and demonstrated COD removal efficiency of 95.7±1.3% at an OLR of 0.8 kg ·m-3·day-1 [15]. Previous studies on the process performance of existing treatment systems for natural rubber processing wastewater are summarized in Table 3.8. The combined ABR (HRT=3.4 day) - UASB (HRT=1.8 day) - ST (HRT=0.6 day) - DHS (HRT=0.5 day) system removed 94.8 ± 2.1% of total COD, 98.0 ± 0.9% of total BOD, 71.8 ± 22.6% of TSS, and 68.3 ± 15.1% of TN during phase 3. A combination of anaerobic and aerobic lagoons has also been widely used in Thailand, Vietnam, and Malaysia because of its low operational cost and easy maintenance [14][7][15]. Oxidation ditches were used for the treatment of natural rubber processing wastewater due to their high efficiency in removing nitrogen [14][7]. Tanikawa et al. (2017) reported that the process performance of full-scale DAF–anaerobic lagoon–anoxic lagoon–aerated tank system achieved the removal efficiencies of 89% for TSS, 98% for total COD, 91% for TN in South Vietnam [15]. Thus, the ABR-UASB-ST-DHS system developed in this study demonstrated similar removal efficiencies to existing treatment systems. Moreover, our system could reduce approximately 80% of HRT similar to the existing systems (e.g. existing anaerobic-aerobic lagoon needs 1 month for treatment) [7]. The final effluent of our system met the required Vietnamese national technical regulation on effluent of the natural rubber processing industry-B except for the ammonia content (QCVN01: 2008/BTNMT, pH: 6-9, Total BOD: < 50 mg·L-1, Total COD: < 250 mg·L-1. TSS: < 100 mg·L-1, TN: < 60 mg-N·L-1, Ammonia: < 40 mg-N·L-1). Several current treatment systems exceed the effluent regulations in Vietnam [7]. Thus, an effective nitrogen removal process is required in both the proposed and existing systems. However, the pilot scale ABR-UASB-ST-DHS system demonstrated high potential for the treatment of natural rubber processing wastewater in Vietnam. Furthermore, the full-scale UASB-DHS system that will be developed based on the results obtained in the pilot scale system in this study and previous studies will likely achieve a high process performance and energy recovery potential in the form of methane. Table 3.8 Process performance of the existing treatment system for treating natural rubber processing wastewater. 3.5 Design for full-scale UASB-DHS system for natural rubber processing wastewater in Vietnam From this research, the UASB-DHS system could be an appropriate treatment system for natural rubber processing wastewater in Vietnam. To design a full-scale UASB – DHS system, some key factor (scale and cost) was calculated in the following section. A large-scale natural rubber processing factory surveyed (Section 3.1) is selected for designing the proposed treatment system. The factory daily produced 200 tons of latex and 30 tons of cup lumps, respectively. The factory discharged 1,000 m3·day-1. The characteristic of wastewater used for this design was showed in Table 3.9. Table 3.9 Water quality of natural rubber processing wastewater for simulation. Content Unit Concentration pH 5.6 Total COD mg-COD·L-1 6,430 Soluble COD mg-COD·L-1 6,020 TSS mg·L-1 650 VSS mg·L-1 250 TN mg·L-1 420 Acetate mg-COD·L-1 810 Propionate mg-COD·L-1 760 3.5.1 Reactor design for natural rubber processing wastewater 3.5.1.1 Pre-treatment process for UASB reactor The previous study reported that the UASB reactor operated without any pre-treatment process and accumulated a large number of natural rubber particulars in the UASB column and finally the reactor was broken [6]. In addition, in our pilot-scale experiment happened a large amount of sludge washed-out due to the wastewater containing a large number of rubber particulars. Therefore, a pre-treatment process for the UASB reactor treating this wastewater is necessary. Current local natural rubber processing factories have been widely used in the anaerobic lagoon and can be modified to the baffled reactor with easy modification. Thus, a modified anaerobic lagoon would be recommended for a pre-treatment system for the UASB reactor in this wastewater. In addition, the pre-treatment anaerobic lagoon should be covered and collected biogas to reduce GHG emissions. If the factory didn't have any wastewater treatment facility, a DAF system would be recommended. The design parameter of a pre-treatment anaerobic lagoon is Flow rate: 1,000 m3·day-1 Influent COD: 6,500 mg-COD·L-1 According to the survey of OAS, a large amount of solid COD was removed until compartment 5. The volume of one compartment of OAS: 1.4 m × 1.5 m × 3.0 m = 6.3 m3 Volume of 5 compartment: 6.3 m3 × 5 = 31.5 m3 The flow rate of OAS: 110 m3·day-1 HRT of OAS until compartment 5: 31.5 m3 / 110 m3·day-1 = 8.65 hours From this calculation, a number of compartments and HRT of pre-treatment ABR are 5 compartments and 8.65 hours, respectively. The pre-treatment ABR can be designed as follows Volume of ABR : 1,000 m3·day-1 / 8.65 hours = 2,780 m3 Number of compartments: 5 The volume of each compartment: 560 m3 The calculation of ABR is w × l × h = 400 m3 (single compartment) w: wide (m) l: length (m) h: height (m) The upflow speed can be calculated by 83 m3/hour (flow rate) / l × w = 0.5 m·hour-1 The bottom area is l × w = 166 m2 The size of the bottom area can be calculated by l= w = √116 = 13 m The height of ABR is h = 560 / 166 = 3.4 m In summary, the design of pre-treatment ABR was Wide: 13m, length: 13 m, depth: 3.4 m Number of compartments: 5 The pre-treatment ABR recommended to cover to collect biogas. 3.5.1.2 UASB reactor The UASB reactor for treating natural rubber processing wastewater could be recommended to operate with OLR of 1.5 ~3.0 kg-COD·m-3·day-1 from this pilot scale experiment. A previous study mentioned the optimal upflow speed in a UASB reactor was 0.5 m·hour-1. Using this parameter, the UASB reactor for full-scale treatment might be. Influent of UASB reactor: 5,000 mg-COD·L-1 The flow rate of the UASB reactor: 1,000 m3·day-1 HRT of UASB reactor: 24 hours Upflow speed in UASB reactor: 0.5 m·hour-1 The calculation of the UASB reactor is W × L × H = 1,000 m3 W: wide (m) L: Length (m) H: Hight (m) The bottom area can be calculated by W × L = 83 m3·hour-1/ 0.5 m·hour-1 = 166 m2 W and L can be calculated by W = L = √ 166 m2 = 13 m Hight of the UASB reactor was H = 1,000 m3 / 166 m2 = 6 m In summary, the design of the UASB reactor was Wide = Length = 13 m, Hight = 6 m The UASB reactor should be equipped with GSS. 3.5.1.3 DHS reactor According to the laboratory scale and pilot scale experiment, the HRT of the DHS reactor was designed for 4 hours together with effluent recirculation. The sponge volume of the DHS reactor can be calculated by the following equation. V = Q/HRT = 250 m3 V: sponge volume (m3) Q: Flow rate (m3/day) HRT: hydridic retention time (hours) The sponge volume of 250 m3 DHS reactor will be more than 500 m3 of total volume. The expected height of the DHS rector is more than 5 m, thus it could be considered to install several DHS reactors. 3.5.2 Calculation of Energy consumption and generation for the operation of the UASB-DHS system 3.5.2.1 Energy consumption of UASB-DHS system Tandukar et al. (2007) demonstrated that the UASB-DHS system was cost-effective compared with the activated sludge process [22]. In addition, Tanikawa et al. (2016) reported two-stage UASB- DHS system treating natural rubber latex wastewater in Thailand can be reduced 95% of energy consumption [24]. The strong point of the UASB-DHS system is electricity is only required for pumping wastewater to the UASB reactor. Therefore, electricity consumption can be calculated as below. The speciation of a centrifugal pump is Flow rate: 1,000 m3·day-1 Lifting height: 6 m The centrifugal pump (GE-4M, Kawamoto pump) was selected for this calculation. The electricity consumption of this pump was estimated at 7.5 kWh. In addition, a pump for DHS recirculation is used. Therefore, the electricity consumption of this UASB-DHS system is 15.0 kWh. 3.5.2.2 Energy production of UASB-DHS system The UASB reactor could be recovered energy in the form of methane. The energy recovery from the UASB reactor was calculated flow as; Influent COD: 6,000 mg-COD·L-1 Influent flow rate: 1,000 m3·day-1 Estimated methane recovery ratio (based on influent total COD): 60% Estimated methane production (L-CH4·day-1) = 6,000 (mg-COD·L-1) × 1,000 (m3·day-1) × 60% / 2.857 (g-COD·L-CH4-1) = 1,260 (m3-CH4·day-1). The manual for installation of biomass plant published by the Ministry of Environment, Japan mentioned gas power generation unit is 1.8 kWh· m3-CH4. The power generation from the UASB reactor can be calculated flow as; The power generation from the UASB reactor (kWh) = Methane gas production (1,260 m3-CH4·day-1) × (1.8 kWh·m3-CH4-1)= 2,268 (KWh·day-1). Compared with the electricity consumption and electricity generation, the UASB reactor could be generated approximately 2,000 KWh·day-1. This generated electricity is enough for operating a factory. 4. Conclusions 1. The water quality and GHG emission from the existing treatment system treating natural rubber processing wastewater in Vietnam were surveyed. The effluent from the existing treatment exceeded the discharge standard. In addition, OAS emitted not only methane but also nitrous oxide and had high GWP. - The final effluent of the existing process was Total COD of 730 mg·L-1, TSS of 200 mg·L-1, and TN of 60 mg-N·L-1, respectively. - The emission rates (flux) from 1 m3 of treated RSS wastewater for methane, nitrous oxide, and total GHGs by OAS were calculated as 0.054 t- CO2eq·m-3, 0.099 t- CO2eq·m-3, and 0.153 t-CO2eq·m-3, respectively. 2. Laboratory scale UASB-DHS system and ABR system demonstrated treatment of natural rubber processing wastewater. Both systems performed good process performance and were capable of treating natural rubber processing wastewater. - The laboratory scale UASB reactor performed high-level total COD removal at 92.7 ± 2.3% with an OLR of 12.2 ± 6.2 kg-COD m−3 day−1 and 93.3 ± 19.3% methane recovery. - The laboratory scale ABR performed good process performance of 92.3 ± 0.3% COD removal efficiency with OLR of 1.4 ± 0.3 kg-COD·m-3·day-1 without pretreatment. 3. Pilot scale UASB-DHS system was operated in an actual natural rubber processing factory. - The system generated the same effluent quality compared with the current treatment system. - Approximately 80% of HRT can be reduced. - The system could be significantly reduced GHG emissions. 4. The proposed system could be an appropriate treatment system for treating natural rubber processing wastewater in Vietnam. - The system achieved high organic removal efficiency together with energy recovery from methane. - The existing treatment system and proposed system need a more effective nitrogen process to achieve the discharge standard. Recommendation for future study · In this research, the energy-recovery type wastewater treatment system UASB-DHS system was applied to natural rubber processing wastewater in Vietnam. The key point for the application of the UASB reactor in natural rubber processing wastewater was combined with an efficient pretreatment process. ABR system was used in this study, however, accumulation of natural rubber particulars in the UASB column is always happed and leaded sludge washed-out. Therefore, an effective pre-treatment process is needed to research. Moreover, the production process of natural rubber should be considered like acid and ammonia addition. · Our research and present researches often exceed discharge standards of ammonia and nitrogen contents. An effective nitrogen removal process should be researched to achieve the discharged standard. Moreover, autotrophic denitrification processes such as the anammox process would be applied to this wastewater in order to reduce operational costs for wastewater treatment. · This study investigated that the current anaerobic treatment system emitted a large amount of GHG. Therefore, further study on emission principles and reduction methods should be studied. 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Efficiency of high rate treatment of low-strength municipality sewage by a pilot-scale combination system of a sedimentation tank and a down-flow hanging sponge reactor. Environmental Technology, 1-10. Watari, T., Nakamura, Y., Kotcharoen, W., Hirakata, Y., Satanwat, P., Pungrasmi, W., Powtongsook, S., Takeuchi, Y., Hatamoto, M., & Yamaguchi, T. (2021). Application of down-flow hanging sponge–Upflow sludge blanket system for nitrogen removal in Epinephelus bruneus closed recirculating aquaculture system. Aquaculture, 532, 735997. Miwa, T., Takimoto, Y., Hatamoto, M., Kuratate, D., Watari, T., & Yamaguchi, T. (2021). Role of live cell colonization in the biofilm formation process in membrane bioreactors treating actual sewage under low organic loading rate conditions. Applied Microbiology and Biotechnology, 105(4), 1721-1729. Watari, T., Wakisaka, O., Sakai, Y., Hirakata, Y., Tanikawa, D., Hatamoto, M., Yoneyama, F. & Yamaguchi, T. (2021). Anaerobic biological treatment of EG/PG water-soluble copolymer coupled with down-flow hanging sponge reactor. Environmental Technology & Innovation, 21, 101325. Watari, T., Hata, Y., Hirakata, Y., Nguyet, P. N., Nguyen, T. H., Maki, S., Hatamoto, M., Sutani, D., Tjandra, S., & Yamaguchi, T. (2021). Performance evaluation of down-flow hanging sponge reactor for direct treatment of actual textile wastewater; Effect of effluent recirculation to performance and microbial community. Journal of Water Process Engineering, 39, 101724. Sitthi, S., Hatamoto, M., Watari, T., & Yamaguchi, T. (2020). Enhancing anaerobic syntrophic propionate degradation using modified polyvinyl alcohol gel beads. Heliyon, 6(12), e05665. Putra, A. A., Watari, T., Hatamoto, M., Konda, T., Matsuzaki, K., Kurniawan, T. H., & Yamaguchi, T. (2020). Performance of real-scale anaerobic baffled reactor-swim bed tank system in treating fishmeal wastewater. Journal of Environmental Science and Health, Part A, 55(12), 1415-1423. Fuchigami, S., Hatamoto, M., Takagi, R., Watari, T., & Yamaguchi, T. (2020). Performance evaluation and microbial community structure of mesh rotating biological reactor treating sewage. Journal of Water Process Engineering, 37, 101456. Tran, P.T., Hatamoto, M., Tsuba, D., Watari, T., & Yamaguchi, T. (2020). Positive impact of a reducing agent on autotrophic nitrogen removal process and nexus of nitrous oxide emission in an anaerobic downflow hanging sponge reactor. Chemosphere, 256, 126952. Aoki, M., Kowada, T., Hirakata, Y., Watari, T., & Yamaguchi, T. (2020). Enrichment of microbial communities for hexavalent chromium removal using a biofilm reactor. Journal of Environmental Science and Health, Part A, 55(14), 1589-1595. Nguyen, T. H., Watari, T., Hatamoto, M., Sutani, D., Setiadi, T., & Yamaguchi, T. (2020). Evaluation of a combined anaerobic baffled reactor–downflow hanging sponge biosystem for treatment of synthetic dyeing wastewater. Environmental Technology & Innovation, 19, 100913. Watari, T., Kotcharoen, W., Omine, T., Hatamoto, M., Araki, N., Oshiki, M., Mimura, K., Nagano, A., & Yamaguchi, T. (2020). Formation of denitrifying granules in an upflow sludge blanket reactor with municipal sewage and sodium nitrate feeding. Environmental Technology & Innovation, 19, 100861. Adlin, N., Hatamoto, M., Yamazaki, S., Watari, T., & Yamaguchi, T. (2020). A potential zero water exchange system for recirculating aquarium using a DHS-USB system coupled with ozone. Environmental technology, 1-11. Hirakata, Y., Hatamoto, M., Oshiki, M., Watari, T., Araki, N., & Yamaguchi, T. (2020). Food selectivity of anaerobic protists and direct evidence for methane production using carbon from prey bacteria by endosymbiotic methanogen. The ISME journal, 14(7), 1873-1885. Nguyet, P. N., Hata, Y., Maharjan, N., Watari, T., Hatamoto, M., & Yamaguchi, T. (2020). Adsorption of colour from dye wastewater effluent of a down-flow hanging sponge reactor on purified coconut fibre. Environmental technology, 41(10), 1337-1346. Satanwat, P., Tran, T. P., Hirakata, Y., Watari, T., Hatamoto, M., Yamaguchi, T., Pungrasmi, W., & Powtongsook, S. (2020). Use of an internal fibrous biofilter for intermittent nitrification and denitrification treatments in a zero-discharge shrimp culture tank. Aquacultural Engineering, 88, 102041. Putra, A. A., Watari, T., Maki, S., Hatamoto, M., & Yamaguchi, T. (2020). Anaerobic baffled reactor to treat fishmeal wastewater with high organic content. Environmental Technology & Innovation, 17, 100586. Nguyen, Q. H., Watari, T., Yamaguchi, T., Kawamura, Y., Suematsu, H., Wiff, J. P., Niihara, K. & Nakayama, T. (2020). Comparison between Nanosecond Pulse and Direct Current Electrocoagulation for Textile Wastewater Treatment. Journal of Water and Environment Technology, 18(3), 147-156. Nguyen, Q., Watari, T., Yamaguchi, T., Takimoto, Y., Niihara, K., Wiff, J.& Nakayama, T. (2020). COD removal from artificial wastewater by electrocoagulation using aluminum electrodes. Int. J. Electrochem. Sci, 15, 39-51. Nguyen, Q. H., Kawamura, Y., Watari, T., Niihara, K., Yamaguchi, T., & Nakayama, T. (2020). Electrocoagulation with a nanosecond pulse power supply to remove COD from municipal wastewater using iron electrodes. Int. J. Electrochem. Sci, 15, 493-504. Hirakata, Y., Hatamoto, M., Oshiki, M., Watari, T., Kuroda, K., Araki, N., & Yamaguchi, T. (2019). Temporal variation of eukaryotic community structures in UASB reactor treating domestic sewage as revealed by 18S rRNA gene sequencing. Scientific reports, 9(1), 1-11. Nguyet, P. N., Watari, T., Hirakata, Y., Hatamoto, M., & Yamaguchi, T. (2019). Adsorption and biodegradation removal of methylene blue in a down-flow hanging filter reactor incorporating natural adsorbent. Environmental technology. Ikeda, S., Watari, T., Yamauchi, M., Hatamoto, M., Hara, H., Maki, S., Yamada, M. & Yamaguchi, T. (2019). Evaluation of pretreatment effect for spent mushroom substrate on methane production. Journal of Water and Environment Technology, 17(3), 174-179. Takimoto, Y., Hatamoto, M., Ishida, T., Watari, T., & Yamaguchi, T. (2018). Fouling development in A/O-MBR under low organic loading condition and identification of key bacteria for biofilm formations. Scientific reports, 8(1), 1-9. Tanikawa, D., Watari, T., Mai, T. C., Fukuda, M., Syutsubo, K., Nguyen, N. B., & Yamaguchi, T. (2018). Characteristics of greenhouse gas emissions from an anaerobic wastewater treatment system in a natural rubber processing factory. Environmental technology. Watari, T., Mai, T. C., Tanikawa, D., Hirakata, Y., Hatamoto, M., Syutsubo, K., Fukuda, M., Nguyen, N.B., & Yamaguchi, T. (2017). Performance evaluation of the pilot scale upflow anaerobic sludge blanket–Downflow hanging sponge system for natural rubber processing wastewater treatment in South Vietnam. Bioresource technology, 237, 204-212. Watari, T., Cuong Mai, T., Tanikawa, D., Hirakata, Y., Hatamoto, M., Syutsubo, K., Fukuda, M., Nguyen, N.B., & Yamaguchi, T. (2017). Development of downflow hanging sponge (DHS) reactor as post treatment of existing combined anaerobic tank treating natural rubber processing wastewater. Water Science and Technology, 75(1), 57-68. Tran, P. T., Watari, T., Hirakata, Y., Nguyen, T. T., Hatamoto, M., Tanikawa, D., Syutsubo, K., Nguyen, M.T., Fukuda, M., Nguyen, L.H. & Yamaguchi, T. (2017). Anaerobic baffled reactor in treatment of natural rubber processing wastewater: reactor performance and analysis of microbial community. Journal of Water and Environment Technology, 15(6), 241-251. Thanh, N. T., Watari, T., Thao, T. P., Hatamoto, M., Tanikawa, D., Syutsubo, K., Fukuda, M., Nguyen, M.T., Kim, A.T, Yamaguchi, T., & Huong, N. L. (2016). Impact of aluminum chloride on process performance and microbial community structure of granular sludge in an upflow anaerobic sludge blanket reactor for natural rubber processing wastewater treatment. Water Science and Technology, 74(2), 500-507. Tanikawa, D., Syutsubo, K., Watari, T., Miyaoka, Y., Hatamoto, M., Iijima, S., Fukuda, M., Nguyen, B.N., & Yamaguchi, T. (2016). Greenhouse gas emissions from open-type anaerobic wastewater treatment system in natural rubber processing factory. Journal of Cleaner Production, 119, 32-37. Watari, T., Thanh, N. T., Tsuruoka, N., Tanikawa, D., Kuroda, K., Huong, N. L., Nguyen, M. 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