Effect of low COD/N ratios on stability of single-stage partial nitritation/anammox (SPN/A) process in a long-term operation

doi:10.1016/j.biortech.2017.07.127

Bioresource Technology

Volume 244, Part 1, November 2017, Pages 192-197

https://doi.org/10.1016/j.biortech.2017.07.127 Get rights and content

Highlights

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Low COD/N could improve nitrogen removal of SPN/A.
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AMX activity was decreased without influent COD.
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The microbial distribution difference between granules/flocs was observed.
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Biodiversity of AOB and NOB increased with COD/N decreasing at low DO (0.2 mg L⁻¹).

Abstract

This study investigates the effects of varying COD/N ratios on single-stage partial nitritation/anammox (SPN/A) process in a SBR. The operational period was divided into three phases with different influent COD/N ratios (0.4, 0 and 0.5). Stable nitrogen removal was achieved in phase I with a COD/N of 0.4. In phase II COD was absent, effluent nitrite and nitrate increased and nitrogen removal performance gradually deteriorated. In phase III SPN/A failed to recover from nitrate accumulation when COD/N was increased. Microbial activity was measured and microbial community was analyzed by high-throughput sequencing. These results revealed that ordinary heterotrophic organisms (OHO) was suppressed when influent COD was absent, leading to the promotion of nitrification even at a low DO (0.2 mgL⁻¹). Therefore, nitrite oxidizing bacteria (NOB) was gradually enriched and anammox bacteria was suppressed. Besides, it was observed that flocs were sensitive to influent COD variations than granules, which requires further investigation.

Graphical abstract

Introduction

The single-stage partial nitritation/anammox (SPN/A) process is an efficient and economically viable method for the removal of ammonium from wastewater (Liang et al., 2014). In SPN/A processes, ammonium is partly oxidized to nitrite by ammonium oxidizing bacteria (AOB), then the produced nitrite and remaining ammonium are converted to nitrogen gas by anammox bacteria (AMX). The SPN/A process has several advantages to conventional nitrogen removal methods, such as requiring no organic carbon, the low output of excess sludge and low energy requirements (Gao et al., 2014, Siegrist et al., 2008). To date, SPN/A processes have been applied to high ammonium wastewaters such as landfill-leachate, digested sludge supernatant and industrial wastewater (Cydzik-Kwiatkowska et al., 2014, Lackner and Horn, 2013, Vazquez-Padin et al., 2009, Xiao et al., 2009).

For optimization of the SPN/A process, different environmental conditions have been investigated such as temperature, inorganic carbon concentration, organic carbon concentration and inhibitory substances (Dapena-Mora et al., 2007, Laureni et al., 2016, Li et al., 2012, Zhang et al., 2011, Zhang et al., 2016), with studies finding influent organic carbon to be critical for stable operation of SPN/A processes. When influent COD/N ratios increase over a certain level, ammonium removal rates reduce due to the suppression of anammox bacteria activity (Ni et al., 2012). Chamchoi et al., observed a gradual reduction of anammox activity with increased COD concentrations in the range of 100–400 mg L⁻¹ (Chamchoi et al., 2008). During long-term operation of SPN/A systems under high COD loading rates, dominant nitrogen removal processes shifts from anammox process to denitrification due to competition for living space (Tang et al., 2010) and substrate (electron acceptors) (Wang et al., 2010). However, at optimum influent COD concentrations, nitrogen removal efficiency can be promoted by the coexistence of denitrification and anammox processes (Chen et al., 2016, Wang et al., 2010). Chen et al., reported that nitrogen removal was enhanced by the coexistence of denitrification and anammox processes under COD/N ratios of 0.8 (Chen et al., 2016). Overall, in the SPN/A process, nitrogen removal performance is significantly affected by the influent organic carbon, as it is a key factor regulating the synergy of anammox and denitrification (Li et al., 2016). In addition, influent COD has also been reported to significantly alter microbial communities in SPN/A processes (Lackner et al., 2008), with microbial diversity and abundance being strongly correlated with the influent COD/N ratios (Cydzik-Kwiatkowska et al., 2014, Jenni et al., 2014, Liang et al., 2014, Zhang et al., 2015b). It has been established that the diversity of anammox bacteria is affected by the influent carbon source, with high concentration influent carbon resulting in the dominant anammox bacterial population shifting toward Candidatus ‘Brocadia’ fulgida, which is capable of organotrophic nitrate reduction (Kartal et al., 2008).

So far, the effect of COD/N ratios on nitrogen removal performance and microbial communities in SPN/A systems have been widely investigated, although most studies have focused on system performance with increased COD/N ratios, where typically maximum influent COD/N ratios were determined and an optimum COD/N ratio established. The performance of SPN/A process under decreasing COD/N ratios has not been well investigated to date, particularly its long-term stability under extremely low COD/N ratios. For digested wastewater and several industrial wastewaters, the influent carbon source can be very limited, therefore nitrogen removal performance and microbial community dynamics require further research in SPN/A systems with low COD/N ratios, for optimization and effective practical application of SPN/A systems in wastewater treatment.

In this study, a sequencing batch reactor (SBR) was used to establish the SPN/A process, with the SBR systematically fed wastewater at different COD/N ratios of 0.4, 0, and 0.5. The main goals of this study were: (I) to investigate the effect of influent COD/N ratios on nitrogen removal performance; (II) to establish the relative abundance and activity of key microorganisms involved in SPN/A processes; (III) to establish the microbial distribution of the SPN/A system using high-throughput sequencing.

Section snippets

Experimental set-up and operation

A schematic diagram of the SPN/A system is outlined in Fig. 1, using an SBR made of polymethyl methacrylate (PMMA), with a total working volume capacity of 10 L (dimension: 18.5 cm internal diameter × 44 cm height). The SBR was equipped with a pH/DO meter (Multi 3420, WTW Company, Germany), mechanical stirrer (IKA RW20, Germany) and an air pump (MUON 3500, Japan). The SBR operational temperature was maintained at 28.8 ± 2 °C via heated water bath using a temperature controller. The SBR cycle time was

Effect of the COD/N ratios on nitrogen removal performance

The SBR nitrogen removal performance under different COD/N ratios is illustrated in Fig. 2. In phase I, influent COD and ammonium average concentrations were 105.2 and 263.1 mg L⁻¹, respectively with an average COD/N ratio of 0.4. As showed in Fig. 2, the nitrogen removal rate was 0.33 kg N m⁻³ d⁻¹ at the start of phase I, followed by a rise in NRR to 0.86 kg N m⁻³ d⁻¹ when the aeration rate was increased in a step-wise manner from 0.3 to 0.6 L min⁻¹. During the steady period in phase I, the aeration rate

Conclusion

This study confirms that low influent COD/N have a significant effect on SPN/A. The decrease of COD/N from 0.4 to 0 induced SPN/A deterioration. Furthermore, it failed to recover from nitrate accumulation, despite addition of acetate. it was mainly due to the decrease of OHO activity in absence of COD, while nitrifying activity was promoted and anammox activity was suppressed at low DO (0.2 mgL⁻¹). Microbial relative abundances in flocs were found to be more sensitive to the variable COD/N

Acknowledgements

This work was financially supported by Natural Science Foundation of China (21677005 and 51608013), the 111Project. Thanks Beijing Drainage Group Co. Ltd (BDG) for providing the seeding sludge.

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Effect of low COD/N ratios on stability of single-stage partial nitritation/anammox (SPN/A) process in a long-term operation

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Experimental set-up and operation

Effect of the COD/N ratios on nitrogen removal performance

Conclusion

Acknowledgements

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