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The partial denitrification and anammox (PDA) process has received attention for its ability to optimize treatment of wastewater containing a low -N concentration. This study investigated the suitable operational conditions for -N accumulation by hydrogenotrophic denitrification (HD) in operation of a laboratory-scale moving bed biofilm reactor, for future application in the PDA process. -N accumulation was achieved by minimizing the H2 flow rate under optimized conditions (i.e., 15 mL/min H2 flow rate, 40 mg-N/L influent -N, 7.0 h hydraulic retention time, and 2 L working volume). Hydrogenophaga comprised 39.2% of the bacterial abundance after -N accumulated, indicating its contribution to the -N accumulation. In addition, an intermittent H2 supply maintained the -N accumulation rate (NAR) and maximized the nitrite accumulation efficiency (NAE). A H2 supply ratio of 0.7 (i.e., ON: 7 min, OFF: 3 min) was optimal, which induced increases in NAR, NAE, and the -N removal efficiency that reached 0.07±0.01 kg-N/m3/d, 64.4±14.5%, and 89.2±8.9%, respectively. The ratio of H2 supply rate to the -N loading rate was calculated as 4.3 in this experiment, which may represent the optimal balance for maximization of -N accumulation by HD. HIGHLIGHTS This study demonstrated suitable operational conditions for -N accumulation through the hydrogenotrophic denitrification process.; -N accumlation after its maximization maintained for 60 days.; A suitable input ratio of H2 and -N for maximizing -N accumulation was 4.3.; Hydrogen gas oxidizing bacterial genus Hydrogenophaga was the most dominant bacteria with 39.2% of relative abundance.;

Ammonium (NH4+) contamination makes groundwater undrinkable. The dropping nitrification unit, a simple and low-cost biological unit, has been found to be effective for NH4+-removal from contaminated groundwater at a near-neutral pH. However, the pH of groundwater varies widely and is highly alkaline (pH 8.7−10) in some areas of the world, which could negatively affect the biological nitrification process. The objectives of this study were to investigate the NH4+-removal from alkaline groundwater using dropping nitrification units with sponge or biofringe material, compare their removal efficiencies, and characterize the effect of alkaline groundwater on the growth and activity of nitrifying bacteria. Synthetic alkaline groundwater (50 mg-NH4+-N L−1; pH 9.4 ± 0.1) was dropped from the top of 1-m long hanging units at 3 mL min−1 for 56 days. The NH4+-removal efficiency of sponge units (> 88%) was significantly higher than that of biofringe units (56−89%). The abundance of amoA gene of ammonia-oxidizing bacteria increased significantly over 56 days and was significantly higher in sponge units than in biofringe units, resulting in higher NH4+-removal in sponge units than that in biofringe units. This study demonstrated that dropping nitrification units can be used effectively for NH4+-removal from groundwater having a neutral to alkaline pH.

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.

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.

A dropping nitrification reactor was proposed as a low-cost and energy-saving option for the removal of NH4+-N from contaminated groundwater. The objectives of this study were to investigate NH4+-N removal performance and the nitrogen removal pathway and to characterize the microbial communities in the reactor. Polyolefin sponge cubes (10 mm × 10 mm × 10 mm) were connected diagonally in a nylon thread to produce 1 m long dropping nitrification units. Synthetic groundwater containing 50 mg L−1 NH4+-N was added from the top of the hanging units at a flow rate of 4.32 L day−1 for 56 days. Nitrogen-oxidizing microorganisms in the reactor removed 50.8−68.7% of the NH4+-N in the groundwater, which was aerated with atmospheric oxygen as it flowed downwards through the sponge units. Nitrogen transformation and the functional bacteria contributing to it were stratified in the sponge units. Nitrosomonadales-like AOB predominated and transformed NH4+-N to NO2−-N in the upper part of the reactor. Nitrospirales-like NOB predominated and transformed NO2−-N to NO3−-N in the lower part of the reactor. The dropping nitrification reactor could be a promising technology for oxidizing NH4+-N in groundwater and other similar contaminated wastewaters.

The increasing concentration of nitrogen compounds in the groundwater is of a growing concern in terms of human health and groundwater quality. Although an excess of nitrogen compounds in the groundwater of the Kathmandu Valley has been reported, the seasonal variations of the fate of the nitrogen compounds and their relationships to the subsurface sediments are unknown. In this study, spatially distributed shallow dug well samples were collected during both the dry and wet seasons of 2016, and the nitrogen compound, chloride (Cl−), and iron (Fe2+) concentrations were analyzed. Two shallow dug wells and one deep tube well were monitored monthly for 2 years. Although NH4-N concentrations were similar in the clay-dominated areas during both seasons (1 and 0.9 mg-N/L), they were lower in the gravel-dominated areas during wet season (1.8 > 0.6 mg-N/L). The NO3-N concentration differed depending upon the soil type which increased during the wet season (clay 4.9 < 13.6 mg-N/L and gravel 2.5 < 6.8 mg-N/L). The Fe2+ concentration, however, was low during the wet season (clay 2.7 > 0.4 mg/L and gravel 2.8 > 0.3 mg/L). Long-term analysis showed higher fluctuation of nitrogen compounds in the gravel-bearing areas than in the clay-bearing areas.

Groundwater available in the Kathmandu Valley is not suitable for drinking due to chemical and microbial contamination. We installed a treatment system, which was made with locally available materials and was low-cost, and supplied drinking water to the intervention site where groundwater contains high amounts of ammonia, iron, and turbidity. This research aims to evaluate the socioeconomic impact of treated water distribution. One hundred households were randomly selected and asked to use treated water for drinking, and another 100 households in the nearby community were taken randomly as a control. We conducted questionnaire surveys with the enrolled households before and five months after starting water distribution to assess the water use patterns and quality perceptions. The socioeconomic impact of the intervention was evaluated by a pre-post comparison and by the difference-in-difference method. The intervention significantly enhanced most of the parameters of water quality perception, reduced the in-house water treatment, and improved the perceived water stress and quality of life. For the control site, these parameters generally became worse in the post-survey, which suggests that the survey might have affected people’s mindset regarding water security. The system is an option for sustainable management of drinking water in the water-scarce, hard-hit areas in the developing countries.