Nitrogen removal, microbial community and electron transport in an integrated nitrification and denitrification system for ammonium-rich wastewater treatment

https://doi.org/10.1016/j.ibiod.2018.07.014Get rights and content

Highlights

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    The removal efficiency of NH4-N was 53.3% in the integrated PND reactor.

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    Thauera, Nitrosomonas and Acidovorax had abundant nitrogen-functional genes.

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    High NO2-N resulted in the increased NOB activity.

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    High NO2-N contributed to more N2O emission during denitrification.

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    A modified model of denitrifying electron transport was proposed.

Abstract

Partial nitrification and denitrification (PND) is a promising technology for removing nitrogen from wastewater. This study aimed to optimize the operation for a stable partial nitrification in a PND reactor with ammonia-rich influent. Metagenomics was used to analyse the functional genes and relative microbial community for nitrogen removal. Experiments with respiratory chain inhibitors were conducted to investigate the electron transport chain of denitrification. The achieved removal efficiency of ammonium nitrogen (NH4-N) was 53.3%, and the effluent nitrite nitrogen (NO2-N) to NH4-N concentration ratio was 0.74. Nitrosomonadaceae was the dominant species of ammonia oxidizing bacteria and Nitrospira was the dominant nitrite oxidizing bacteria (NOB). Denitrifiers mainly belonged to Thauera, Phycisphaerales and Paracoccus. Thauera, Aequorivita and Rhodothermus possessing the gene nos were able to reduce N2O. High NO2-N concentrations resulted in the increasing NOB activities, which impeded the stable partial nitrification, and contributed to more N2O emission during denitrification. A modified model of denitrifying electron transport was proposed and explained that high NO2-N concentration improved the capacity of nitrite reductase for electron competition and caused an insufficient electron supply for N2O reductase, which contributed to more N2O emission.

Introduction

Safe and efficient nitrogen removal from ammonia-rich wastewater has received intensive attention. Partial nitrification and denitrification (PND) is a promising technology for nitrogen removal, which can reduce 25% oxygen demand and 40% carbon requirement, in comparison with the complete nitrification and denitrification process (Wei et al., 2017). The integrated system combines nitrification with denitrification within one reactor, which eliminates the need for separate tanks and consequently simplifies the design of wastewater treatment. However, stable partial nitrification and efficient nitrogen removal may be difficult to be achieved due to the different growth requirements of nitrifiers and denitrifiers, especially for ammonia-rich wastewater with low organic carbon to nitrogen ratios. Therefore, it is necessary to optimize the nitrogen removal performance in the integrated system.

Nitrite nitrogen (NO2-N) accumulation is achieved in partial nitrification. When it happens, high concentrations of NO2-N, and consequently high free nitrous acid (FNA) could inhibit the activity of nitrifiers (Wu et al., 2016). Ammonia oxidizing bacteria (AOB) could be inhibited by 50% at the FNA concentration of 0.42–1.72 mg/L, while nitrite oxidizing bacteria (NOB) could be inhibited by 100% at the FNA concentration of 0.026–0.22 mg/L (Laloo et al., 2018). Besides, a high NO2-N concentration was found to result in the high emission of a greenhouse gas-nitrous oxide (N2O) during nitrification (Castro-Barros et al., 2016; Hu et al., 2018). For sustainable development of wastewater treatment, further investigation is required to clarify the effect of NO2-N on nitrification in the PND system.

When PND is applied, NO2-N is the main electron acceptor rather than the nitrate nitrogen (NO3-N) during denitrification. The FNA can also inhibit denitrifier activities. The NO3-N reduction rate was completely inhibited at the FNA concentration of 0.2 mg/L (Zhou et al., 2011). Du et al. (2016) found that N2O emission during denitrification via NO2-N was higher than via NO3-N. The N2O conversion rate was improved with increasing NO2-N concentrations (Sun et al., 2018). The electron acceptors for traditional denitrification include NO3-N, NO2-N, NO and N2O, and denitrification requires the participation of nitrate reductase (NAR), nitrite reductase (NIR), NO reductase (NOR) and N2O reductase (NOS). NAR is not included during denitrification via NO2-N. Pan et al. (2013) illustrated that electron competition existed among denitrification reductases. The electron distribution to NAR under different organic concentrations was stable, while the electron distributions among NIR, NOR and NOS were greatly affected by organic concentrations (Pan et al., 2013). Chen et al. (2017) observed that the electron affinity of NAR was stronger than other denitrification reductases and NAR tended to have the priority to utilize electrons. Therefore, electron acceptors might affect the electron competition among denitrification reductases and the denitrification performance. It is required to further investigate the effect of electron acceptors on denitrification.

Electrons are usually originated from carbon metabolisms during denitrification. Denitrification reductases can obtain the electrons through the electron transport system. At present, there are two primary models for denitrification electron transport, i.e. the ASMN model and the ASM-ICE model (Pan et al., 2015). The ASMN model is based on a simplified hypothesis that electrons are supplied for the four denitrification steps sufficiently and independently. Based on the ASMN model, the ASM-ICE model makes some improvement and assumes that the produced electrons are firstly transferred to the intermediate electron carrier and then to the denitrification electron acceptors. If the reductases obtain the electrons from the same carrier, electron competition exists, and further investigation on denitrification electron transport would be beneficial to figure out the mechanism of electron competition.

In addition, microbial structure and functional genes are also affected by high NO2-N concentration in the PND system. Nitrosomonas and Nitrosospira are commonly found in the nitrification process (Liu et al., 2018). Nitrosomonas had a better tolerance for NO2-N than Nitrosospira, making them dominant at high NO2-N concentrations (Liu et al., 2018). With increasing NO2-N concentrations from 0 to 65 mg/L, the expression of gene nirK was enhanced, while similar NO3-N concentrations had no influence on nirK expression (Beaumont et al., 2004). Moreover, NO2-N concentrations led to higher mRNA concentrations of nirK and nirS and accordingly, the reduction rate of NO2-N was accelerated (Yu and Chandran, 2010). Hence, the microbial community and nitrogen removal functional genes are worthy of investigation in the integrated PND reactor.

In this study, an integrated system for PND was conducted to treat ammonia-rich wastewater. For stable partial nitrification and efficient nitrogen removal, high-throughput sequencing and metagenomics were applied to analyse the microbial structure and nitrogen removal functional genes. In addition, the effect of NO2-N on nitrification and denitrification performance was investigated. Finally, a modified model for denitrification electron transport was proposed to analyse the electron competition during denitrification via NO2-N.

Section snippets

Experimental setup and operation

A sequencing batch reactor (SBR) was operated at 25 ± 1 °C with an effective volume of 6 L. The operation cycle was 8 h, including 7 h reaction time (including 10 min feeding), 45 min settlement and 15 min drain. Feeding and drain were achieved using two peristaltic pumps controlled by timers. The hydraulic retention time was 16 h and the sludge retention time was 8.5 d. Seed sludge was taken from Nanshan wastewater treatment plant in Shenzhen, China.

There were six phases during the long-term

System performance

Long-term dynamics of nitrogen concentrations during 142 days operation are shown in Fig. 1. Under steady state conditions, the nitrogen concentrations at the end of the denitrification stage (3 h) were 0 ± 0.002 mg/L NO2-N and 0.5 ± 0.050 mg/L NO3-N. At the end of nitrification (7 h), concentrations were 280 ± 20.0 mg/L for NH4-N, 180 ± 10.0 mg/L for NO2-N and 30 ± 5.0 mg/L for NO3-N. The total ammonia removal efficiency was 53.3% and the partial nitrification ratio (NO2-N to oxidized

Conclusions

In the integrated PND system, the ammonia removal efficiency was 53.3% with the partial nitrification ratio of 85.7%. Thauera, Nitrosomonas and Acidovorax possessing functional genes for nitrification and denitrification simultaneously, could efficiently remove nitrogen in the integrated system. Nitrosomonas and Thauera with gene nirK and nor could produce N2O by AOB denitrification or heterotrophic denitrification. Thauera, Aequorivita and Rhodothermus contained gene nosZ, contributing to the

Acknowledgements

This research was supported by the Major Science and Technology Program for Water Pollution Control and Treatment of China (2018ZX07604-001) and the Development and Reform Commission of Shenzhen Municipality (urban water recycling and environment safety program).

References (50)

  • P. Manickam et al.

    Recent advances in cytochrome c biosensing technologies

    Biosens. Bioelectron.

    (2017)
  • J. Miao et al.

    Effect of organic carbons on microbial activity and structure in denitrifying systems acclimated to nitrite as the electron acceptor

    Int. Biodeterior. Biodegrad.

    (2017)
  • P.J. Murray et al.

    Denitrification potential in a grassland subsoil: effect of carbon substrates

    Soil Biol. Biochem.

    (2004)
  • Y. Pan et al.

    Electron competition among nitrogen oxides reduction during methanol-utilizing denitrification in wastewater treatment

    Water Res.

    (2013)
  • Y. Pan et al.

    Evaluating two concepts for the modelling of intermediates accumulation during biological denitrification in wastewater treatment

    Water Res.

    (2015)
  • L. Peng et al.

    The combined effect of dissolved oxygen and nitrite on N2O production by ammonia oxidizing bacteria in an enriched nitrifying sludge

    Water Res.

    (2015)
  • N. Remmas et al.

    Effects of high organic load on amoA and nirS gene diversity of an intermittently aerated and fed membrane bioreactor treating landfill leachate

    Bioresour. Technol.

    (2016)
  • D. Richardson et al.

    Mitigating release of the potent greenhouse gas N2O from the nitrogen cycle–could enzymic regulation hold the key

    Trends Biotechnol.

    (2009)
  • S. Schalk-Otte et al.

    Nitrous oxide (N2O) production by Alcaligenes faecalis during feast and famine regimes

    Water Res.

    (2000)
  • C.S. Srinandan et al.

    Assessment of denitrifying bacterial composition in activated sludge

    Bioresour. Technol.

    (2011)
  • G. Tallec et al.

    Nitrous oxide emissions from secondary activated sludge in nitrifying conditions of urban wastewater treatment plants: effect of oxygenation level

    Water Res.

    (2006)
  • B. Wang et al.

    Illumina MiSeq sequencing reveals the key microorganisms involved in partial nitritation followed by simultaneous sludge fermentation, denitrification and anammox process

    Bioresour. Technol.

    (2016)
  • D. Wei et al.

    Nitrogen removal via nitrite in a partial nitrification sequencing batch biofilm reactor treating high strength ammonia wastewater and its greenhouse gas emission

    Bioresour. Technol.

    (2017)
  • J. Wu et al.

    Differentiating two partial nitrification mechanisms: inhibiting nitrite oxidizing bacteria activity or promoting ammonium oxidizing bacteria activity

    J. Environ. Chem. Eng

    (2016)
  • D. Yang et al.

    A quasi-homogeneous catalysis and electron transfer chain for biodecolorization of azo dye by immobilized phenazine redox mediator

    Int. Biodeterior. Biodegrad.

    (2018)

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