Bicarbonate (HCO 3-) can be used as an inorganic carbon source for hydrogen-based denitrification (HD). Since HCO3is considered to accelerate the NO2reduction rate, this study is attempted to minimize the hydraulic retention time (HRT) of HD systems using varied amounts of HCO3-: deficient, moderate, and abundant amounts. The results implied that a low NO2amount was removed at unsuitably short HRTs, resulting in poor HD efficiency despite being supplemented with abundant HCO3amounts. HCO3assisted in rapidly acclimatizing the bacteria having nirS gene, causing higher NO2reduction rate and aided in changing the bacterial communities. Thauera spp. were the most dominant bacteria in abundant HCO3conditions, achieving high HD efficiency at 8 to 24 h HRT whereas satisfactory efficiency was achieved in the deficient and moderate HCO3amountsystems through the collaboration of Rhodocyclaceae, Alcaligenaceae, and Xanthomonadaceae as predominant bacteria in the community. A strong correlation between the abundance of nirS gene and Thauera spp. was also found. The findings in this study revealed the importance of using HCO3for the enrichment of H2-oxidizing denitrifiers containing nirS gene in order to reduce NO2accumulation to enhance the HD efficiency.

NO3-N and dye colors discharged from textile wastewater pose environmental problems in Thailand. This study aimed to observe the nitrogen removal rate (NRR) with and without RB-5 color contamination via hydrogenotrophic denitrification (HD) processing, which uses H2 gas as electron donor to reduce NO3-N and NO2-N; comparing with bioreactors treatment to evaluate systems that can simultaneously remove NO3-N and dye color. Five reactors under different operation and gas supply conditions were set-up under HRT of 24 h, including an aerobic reactor using air, two anaerobic reactors using argon and H2, and a combined process using intermittent air/argon and air/H2. NRR without dye varied between 45 and 90% for H2 and air/H2 by HD processing, while it was completely removed when adding color. H2 and air/H2 reactors experienced partial decolorization of approximately 20–30%, whereas the other three reactors remained unchanged. Effluent of NO3-N were close to wastewater standards, but the color was still easy to detect, which indicated that the treatment time needs to be sufficient. In conclusion, HD and intermittent air/H2 processing can completely remove NO3-N and NO2-N when contaminated with RB-5 color. Furthermore, RB-5 did not affect the NRR, whereas some particles of dye color can also reduce in these processes.

The development of a low-cost and efficient hydrogenotrophic denitrification (HD) system for nitrate removal from groundwater is urgently required in developing countries. In the present study, a sponge-based HD reactor was developed to examine the effects of various carrier filling ratios (0%, 10%, 20%, and 30%) on the HD performance. HD reactors with sponges showed higher nitrogen removal capacities than that without sponges. There was no significant difference in the nitrogen removal efficiency at filling ratios of 10%, 20%, and 30%. The NO3-N removal rate varied based on the filling ratio, and reached 382, 470, 548, and 530 g-N/(m 3 ・ d) at filling ratios of 0%, 10%, 20%, and 30%, respectively. Furthermore, the attached biomass was considered to play an important role in the enhancement of NO3-N removal. The coexistence of hydrogenotrophic and heterotrophic denitrification activity was observed in the reactors, and there was a strong correlation between total volatile sponge-attached biomass and heterotrophic activity. However, heterotrophic activity accounted for a maximum of only 5.1% of the dissolved inorganic nitrogen removal. The high nitrogen removal rate achieved in this study shows that sponge-based HD reactors can potentially be used for NO3-N removal from groundwater.

The anaerobic ammonium oxidation (anammox) process holds great promise for treating nitrogen-contaminated water; stable nitrite-nitrogen (NO2--N) production is significant to anammox performance. In this study, partial hydrogenotrophic denitrification (PHD) was used to stably and efficiently produce NO2--N from nitrate-nitrogen (NO3--N). An investigation of the effects of initial pH on the PHD process revealed that a high NO2--N production efficiency (77.9%) could be ensured by setting an initial pH of 10.5. A combined PHD-anammox process was run for more than three months with maximal ammonium-nitrogen (NH4+-N), NO3--N, and total dissolved inorganic nitrogen removal efficiencies of 93.4, 98.0, and 86.9%, respectively. The NO2--N to NH4+-N and NO3--N to NH4+-N ratios indicated that various bioprocesses were involved in nitrogen removal during the anammox stage, and a 16S rRNA gene amplicon sequencing was performed to further clarify the composition of microbial communities and mechanisms involved in the nitrogen removal process.