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The efficient and economical disposal of urea wastewater for promoting wastewater reuse, are still tough obstacles for the sustainable development of urea industry. In order to explore newly efficient and economical alternative for urea wastewater treatment, a novel one-stage autotrophic nitrogen removal reactor (submerged aerated biological filter, SABF) simultaneously combining with urea hydrolysis, partial nitritation (PN) and anammox was carried out and stably operated for 126 days. The SABF reactor not only presented excellent urea removal efficiency (˃ 99.5%) with average effluent urea concentration of 0.83 mg/L to meet reuse standard, but also achieved high nitrogen removal loading, ammonium removal efficiency and total nitrogen removal efficiency as 1.5 kg N/(m3·d), 89.46% and 77.09%, respectively. The urea removal mechanism was proven as the intracellular urea hydrolysis, which was conducive to accomplish the persistent free ammonia (FA) inhibition on nitrite-oxidizing bacteria for stable PN and guaranteed total nitrogen removal by anammox. High-throughput sequencing analysis results revealed that Nitrosomonas accounting for 15.92% should be responsible for excellent urea hydrolysis and PN performance and Candidatus_Brocadia with relative abundance of 1.63% was the main anammox bacteria for further nitrogen removal in the SABF reactor. The proposed process showed significant advantages of extremely low operational cost and high removal efficiency with recyclable effluent for urea wastewater treatment. This study provided a novel and superior alternative for the cleaner production of urea manufacturing enterprises, which can well alleviate the stress of urea wastewater treatment and further benefit the sustainable development of urea industry in the future. [Display omitted] • A novel one-stage nitrogen removal process for urea wastewater was developed. • Intracellular urea hydrolysis, partial nitritation, anammox were integrated in SABF. • A high N removal rate of 1.5 kg N/(m3·d) was achieved at 30 °C. • Effluent urea concentration was stably lower than 1 mg/L. [ABSTRACT FROM AUTHOR]

For the sake of high efficiency and saving operational cost for high-concentration urea wastewater treatment, a novel two-stage partial nitritation (PN)-anammox process containing simultaneous urea hydrolysis and PN in sequencing batch reactor (SBR) was investigated. Although the influent urea concentration increased from 500 to 1200 mg/L, the SBR simultaneously achieved urea removal efficiency higher than 98% and stable PN with effluent NO 2 −-N/NH 4 +-N ratio of 1.0–1.3 without any extra alkalinity addition. The intracellular hydrolysis was the dominant mechanism for urea removal and persistent free ammonia inhibition on nitrite-oxidizing bacteria was the main reason for nitrite accumulation of 97.92% in SBR. The subsequent anammox reactor showed efficient nitrogen removal performance with average ammonium removal efficiency, nitrogen removal efficiency and maximum nitrogen removal loading rate of 98.08%, 81.45% and 1.05 kg N·m−3·d−1 respectively. High-throughput sequencing results indicated Gemmatimonadetes became the most abundant bacterial phylum related to potential intracellular urea hydrolysis and displayed obvious ammonium-oxidizing bacteria enrichment and nitrite-oxidizing bacteria inhibition in SBR, and the dominant anammox bacteria (Candidatus_Kuenenia) in anammox reactor. The proposed process was proven to be promising for high-concentration urea wastewater treatment, facilitating the sustainable development of the urea industry in the future. [Display omitted] • A novel biological nitrogen removal process for urea wastewater was investigated. • Autotrophic N removal with no alkalinity added was achieved for urea wastewater. • Intracellular urea hydrolysis and stable partial nitritation was integrated in SBR. • 81.45% of total N was removed by anammox with N removal rate of 1.05 kg N·m−3·d−1. [ABSTRACT FROM AUTHOR]

The coupled process of thiosulfate-driven denitrification (NO 3 −→NO 2 −) and Anammox (TDDA) was a promising process for the treatment of wastewater containing NH 4 +-N and NO 3 −-N. However, the high concentration of SO 4 2− production limited its application, which needs to be alleviated by an economical and effective way to promote the application of TDDA process. In this study, TDDA process was started in a relatively short time by stepwise replacing nitrite with nitrate and operated continuously for 146 days. Results presented that the average total nitrogen removal efficiency of 82.18% can be acquired at a high loading rate of 1.98 kg N/(m3·d) with maximum nitrogen removal efficiency up to 87.04%. It was observed that the increase of S/N ratio improved the denitrification efficiency and slightly inhibit the Anammox process. Batch tests showed that Sulfammox process appeared in TDDA process under certain conditions, further contributing 2.59% nitrogen removal and 10.46% sulfur removal (14.42 mg/L NH 4 +-N and 37.68 mg/L SO 4 2--S were removed). This finding was mainly attributed to the reduction of sulfate in TDDA system to elemental S0 or HS−, which subsequently was used as an electron donor to realize the recycling of sulfate (SO 4 2--S) pollutants and promote the sulfur-nitrogen (S–N) cycle. High-throughput analysis displayed that Anammox bacteria (Candidatus_Kuenenia), Sulfur-oxidizing bacteria (Thiobacillus) with relatively high abundance of 5.37%, 7.74%, respectively, guaranteeing the excellent nitrogen and sulfate removal performance in the reactor. The enrichment of phyla Chloroflexi (31.79%), Proteobacteria (31.82%), class Ignavibacteriales (10.55%), genus Planctomycetes (13.57%) further verified the exitence of Sulfammox process in the TDDA reactor. This study provides a new perspective for the practical application of TDDA in terms of reducing the production of high concentration SO 4 2− and saving operational cost and strengthening deeply nitrogen removal. • The TDDA was started in a short time via stepwise replacing nitrite with nitrate. • The Highest TN removal (87.04%) was acquired at the NLR of 1.98 kg N/(m3·d) in TDDA. • Sulfammox process appeared in TDDA process which accelerated S–N cycle of system. • The contributions of various nitrogen and sulfur removal pathways were investigated. • The enrichment of Ignavibacteriales (10.55%) verified the exitence of Sulfammox. [ABSTRACT FROM AUTHOR]

The coupled process of thiosulfate-driven denitrification (NO 3 −→NO 2 −) and Anammox (TDDA) was a promising process for the treatment of wastewater containing NH 4 +-N and NO 3 −-N. However, the high concentration of SO 4 2− production limited its application, which needs to be alleviated by an economical and effective way to promote the application of TDDA process. In this study, TDDA process was started in a relatively short time by stepwise replacing nitrite with nitrate and operated continuously for 146 days. Results presented that the average total nitrogen removal efficiency of 82.18% can be acquired at a high loading rate of 1.98 kg N/(m3·d) with maximum nitrogen removal efficiency up to 87.04%. It was observed that the increase of S/N ratio improved the denitrification efficiency and slightly inhibit the Anammox process. Batch tests showed that Sulfammox process appeared in TDDA process under certain conditions, further contributing 2.59% nitrogen removal and 10.46% sulfur removal (14.42 mg/L NH 4 +-N and 37.68 mg/L SO 4 2--S were removed). This finding was mainly attributed to the reduction of sulfate in TDDA system to elemental S0 or HS−, which subsequently was used as an electron donor to realize the recycling of sulfate (SO 4 2--S) pollutants and promote the sulfur-nitrogen (S–N) cycle. High-throughput analysis displayed that Anammox bacteria (Candidatus_Kuenenia), Sulfur-oxidizing bacteria (Thiobacillus) with relatively high abundance of 5.37%, 7.74%, respectively, guaranteeing the excellent nitrogen and sulfate removal performance in the reactor. The enrichment of phyla Chloroflexi (31.79%), Proteobacteria (31.82%), class Ignavibacteriales (10.55%), genus Planctomycetes (13.57%) further verified the exitence of Sulfammox process in the TDDA reactor. This study provides a new perspective for the practical application of TDDA in terms of reducing the production of high concentration SO 4 2− and saving operational cost and strengthening deeply nitrogen removal. • The TDDA was started in a short time via stepwise replacing nitrite with nitrate. • The Highest TN removal (87.04%) was acquired at the NLR of 1.98 kg N/(m3·d) in TDDA. • Sulfammox process appeared in TDDA process which accelerated S–N cycle of system. • The contributions of various nitrogen and sulfur removal pathways were investigated. • The enrichment of Ignavibacteriales (10.55%) verified the exitence of Sulfammox. [ABSTRACT FROM AUTHOR]

In order to assess the performance of anaerobic ammonium oxidation (anammox) bioreactors, it is necessary to study the stoichiometry of the anammox reaction and pH. This study focused on the effect of the hydraulic retention time (HRT) on the effluent pH in anammox-upflow anaerobic sludge blanket (UASB) bioreactors. Anammox-UASB bioreactors with and without a recirculation system were used to investigate the effluent pH and bioreactor performance. It was concluded that under varying HRT conditions, the decrease in effluent pH did not indicate the deterioration of nitrogen removal, but did indicate that the nitrogen removal efficiency was reduced owing to a sudden increase in the nitrogen loading rate resulting from the decrease in HRT. Moreover, the results showed that the HRT directly affected the concentration of OH−, which affected the increase/decrease in effluent pH. This study demonstrated that effluent pH is a more powerful tool than previous techniques used to assess bioreactor performance. We suggest that the effluent pH could be used for preliminary assessment. • The decrease in effluent pH does not always indicate the deterioration of nitrogen removal. • The clear decrease in the effluent pH showed that the NRE decreased. • The change in the effluent pH was related to the NRE rather than the NRR. • Two UASB reactors with and without recirculation were used for effluent pH and NRE investigation. • The effect of the HRT on the effluent pH was investigated. [ABSTRACT FROM AUTHOR]