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The physicochemical and biological characteristics of milli-microbubbles were compared to evaluate their performance on hydrogenotrophic denitrification (HD) for groundwater treatment in remote areas. The hydrogen supply was controlled at 1.14 L/d with 40 mgN/L of NO3-N. The microbial community structure in two bubble reactors was investigated by high throughput sequencing. Microbubbles enhanced biodegradation in the HD system, providing a maximum nitrogen removal efficiency of 99%. Approximately 50% of total hydrogen was utilized for biological nitrate removal with the highest hydrogen effectiveness achieved at 1.21 g N/g H2. In contrast, millibubbles achieved less than 10% efficiency and 9.9% of total hydrogen was consumed for biological nitrogen removal. Thaueraspp., Hydrogenophagaspp. and Rhodocyclaceaeof Proteobacteriaphylum were the dominant bacteria in the microbubble reactor, whereas Methyloversatilis spp.was dominant in the millibubble reactor, in which a relatively low amount of hydrogen (0.6 mg/L) was dissolved. The differences can be attributed to the higher hydrogen transfer efficiency (45×10-3s-1) and lower rising velocity (0.31 mm/s) of the microbubbles system than the millibubbles system (2×10-3s-1and 480 mm/s). The micro-hydrogen bubble technology affords increased hydrogen effectiveness, reduced energy consumption, and modified system design. Therefore, it is more appropriate for enhancing HD.

Due to the high water demand and unsustainable water resource, wastewater reclamation and wastewater treatment prior to discharge have become current important issues. Various treatment technologies, such as biological processes, have been improved as alternatives. In this study, the biological nitrogen removal system using pure-culture Bacillus licheniformis was developed and used as an internal treatment unit in an aquarium to improve the effluent quality for water reuse. The efficiencies for NH4-N and total nitrogen (TN) removal and the quality of treated water verified the occurrence of heterotrophic nitrification and aerobic denitrification; the nitrification rate was 0.84 mg/L-h and the denitrification rate was 0.62 mg/L-h. The maximal NH4-N and TN removal efficiencies were approximately 73% at the influent NH4-N of 30 mg/L. However, the other competitive heterotroph of Pseudomonas sp. was observed, which resulted in dramatically decreasing efficiencies and an enlarged ratio of carbon consumption and nitrogen removal. Although the overall performance of the B. licheniformis system was lower than the system using mixed-culture nitrifying and heterotrophic denitrifying microorganisms, the advantages of the B. licheniformis system were ease of operation and the fact that it is a land-limited treatment system. The research is ongoing to enhance performance and maintain excellent efficiency in a long-term operation.

Nitrate removal during anaerobic ammonium oxidation (anammox) treatment is a concern for optimization of the anammox process. This study demonstrated the applicability and long-term stability of the coupled anammox and hydrogenotrophic denitrification (CAHD) process as an alternative method for nitrate removal. Laboratory-scale fixed bed anammox reactors (FBR) supplied with H2 to support denitrification were operated under two types of synthetic water. The FBRs showed simultaneous NH4-N and NO3-N removal, indicating that the CAHD process can support NO3-N removal during the anammox process. Intermittent H2 supply (e.g. 5 mL/min for a 1-L reactor, 14/6-min on/off cycle) helped maintain the CAHD process without deteriorating its performance under long-term operation and resulted in a nitrogen removal rate of 0.21 kg-N/m3/d and ammonium, nitrate, and dissolved inorganic nitrogen removal efficiencies of 73.4%, 80.4%, and 77%, respectively. The microbial community structure related to the CAHD process was not influenced by changes in influent water quality, and included the anammox bacteria ‘Candidatus Jettenia’ and a Sulfuritalea hydrogenivorans-like species as the dominant bacteria even after long-term reactor operation, suggesting that these bacteria are key to the CAHD process. These results indicate that the CAHD process is a promising method for enhancing the efficiency of anammox process.

High nitrate-nitrogen (NO3−–N) content is a typical feature of groundwater, which is the primary water source in the Kathmandu Valley, Nepal. Considering the Kathmandu Valley’s current problem of water scarcity, a user-friendly system for removing NO3−–N from groundwater is promptly desired. In this study, a simplified hydrogenotrophic denitrification (HD) reactor was developed for the Kathmandu Valley, and its effectiveness was evaluated by its ability to treat raw groundwater. The reactor operated for 157 days and showed stability and robustness. It had an average nitrogen removal efficiency of 80.9 ± 16.1%, and its nitrogen loading rate and nitrogen removal rate varied from 23.8 to 92.3 g–N/(m3∙d) and from 18.3 to 73.7 g–N/(m3∙d), respectively. Compared to previous HD reactors, this simplified HD reactor is a more user-friendly option for the Kathmandu Valley, as most of the materials used for the reactor were locally available and require less maintenance. The reactor is recommended for groundwater treatment at the household level. It has a current treatment capacity of 40 L/d, which can fulfill the daily requirements for drinking and cooking water in a household with 4–5 people.

Nitrate removal during anaerobic ammonium oxidation (anammox) treatment is a concern for optimization of the anammox process. This study demonstrated the applicability and long-term stability of the coupled anammox and hydrogenotrophic denitrification (CAHD) process as an alternative method for nitrate removal. Laboratory-scale fixed bed anammox reactors (FBR) supplied with H