Limits of the anammox process in granular systems to remove nitrogen at low temperature and nitrogen concentration

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Highlights

  • Effluent quality of an anammox system operated at mainstream conditions was studied.

  • Oxygen transfer from atmosphere to bulk liquid promotes nitrite oxidation.

  • Granular biomass use increases the presence of remaining substrates in the effluent.

  • Tank coverage and operation at high SRT are desired to obtain a good effluent quality.

Abstract

When partial nitritation-anammox (PN-AMX) processes are applied to treat the mainstream in wastewater treatment plants (WWTPs), it is difficult to fulfil the total nitrogen (TN) quality requirements established by the European Union (<10 g TN/m3). The operation of the anammox process was evaluated here in a continuous stirred tank reactor operated at 15 °C and fed with concentrations of 50 g TN/m3 (1.30 ± 0.23 g NO2-N/g NH4+-N). Two different aspects were identified as crucial, limiting nitrogen removal efficiency. On the one hand, the oxygen transferred from the air in contact with the mixed liquor surface favoured the nitrite oxidation to nitrate (up to 75 %) and this nitrate, in addition to the amount produced from the anammox reaction itself, worsened the effluent quality. On the other hand, the mass transfer of ammonium and nitrite to be converted inside the anammox granules involves relatively large values of apparent affinity constants (kNH4+app: 0.50 g NH4+-N/m3; kNO2-app: 0.17 g NO2-N/m3) that favour the presence of these nitrogen compounds in the produced effluent. The careful isolation of the reactor from air seeping and the fixation of right hydraulic and solids retention times are expected to help the maintenance of stability and effluent quality.

Introduction

The implementation of partial nitritation-anammox (PN-AMX) processes to remove nitrogen from the mainstream of the wastewater treatment plants (WWTPs) arises as one of the most promising options to increase the energy efficiency of these facilities (Morales et al., 2015). So far, several studies have been carried out operating PN-AMX systems at mainstream conditions, low ammonia concentration and low temperature using a one-stage configuration (Jiang et al., 2018; Lotti et al., 2014). These research works revealed two main drawbacks. On the one hand, the achieved volumetric nitrogen removal rates were below 10–30 g N/(m3·d) at 15 °C (Akaboci et al., 2018; Laureni et al., 2016; Pedrouso et al., 2018) mainly due to the oxygen limiting conditions imposed in order to maintain the balance between the activities of both ammonia-oxidizing (AOB) and anammox bacteria. On the other hand, the operational stability lost as a consequence of the development of the nitrite-oxidizing bacteria (NOB) activity (Han et al., 2016; Li et al., 2019). Both factors are responsible for the deterioration of the effluent quality, which does not meet the European discharge requirements, in sensitive areas, of 10 g/m3 of total nitrogen (TN) (Council Directive, 1991).

Since the bottleneck to apply the PN-AMX process at mainstream conditions is the stability of the PN process, efforts have been recently focused on the evaluation of the two-stage configuration systems to carry out the PN and anammox processes separately (Jin et al., 2019; Kowalski et al., 2019). Using this approach, stable long-term PN was achieved by inhibiting NOB activity due to the presence of free nitrous acid (HNO2) (Cui et al., 2019; Pedrouso et al., 2017. At this point, the performance of the subsequent anammox process needs to be further optimized to produce an effluent suitable for discharge.

The anammox process stability at mainstream conditions was demonstrated using enriched anammox biomass as inoculum and starting-up the reactors at temperatures close to 30 °C that were, then, stepwise decreased to 15 °C in a long-term acclimation period (De Cocker et al., 2018; Dosta et al., 2008; Hoekstra et al., 2018; Li et al., 2018). Another used alternative is the enrichment of anammox bacteria from activated sludge directly at low temperature (Hendrickx et al., 2014). However, although both strategies were successful, they required long start-up periods that are not useful for their implementation at full-scale WWTPs.

In the present study, a completely stirred tank reactor (CSTR), inoculated with anammox-enriched granular biomass coming from a reactor operated at 30 °C, was started-up directly at mainstream conditions (50 g TN/m3 and 15 °C). The objective was to evaluate the process stability and produced effluent quality in order to asses the anammox process feasibility at the operational conditions. Biomass mass transfer limitations and oxygen transfer from air were also considered to identify the operational considerations regarding those aspects limiting its full-scale application.

Section snippets

Set-up

The anammox process was carried out in a CSTR jacketed cylinder reactor with a useful volume of 1 L and a height to diameter (H/D) ratio of 1. The reactor temperature was maintained at 15 °C using a cryostat bath. A mechanical stirrer (58 rpm) provided a complete mixture inside the system. pH was not controlled and remained at 7.8 ± 0.2. A continuous peristaltic pump supplied the feeding while the effluent discharge took place by overflow. The reactor was inoculated with 2.8 kg VSS/m3 of anammox

Reactor performance

During the start-up period (Stage I), the applied NLR was approximately 33 g N/(m3⋅d) (Table 1). On day 7, a sudden increase in the effluent nitrate concentration was measured (Fig. 1). However, this nitrate production was not justified by the anammox process activity since ammonium concentration did not decrease proportionally. Thus, this nitrate rise was due to nitrite oxidation. Although this effect was associated with air entry into the reactor, dissolved oxygen concentration measured in the

Conclusions

When anammox biomass grows as biofilm, nitrogen mass transfer limitations reduce the reactor efficiency, which hinders compliance with discharge limits. Thus, on the one hand, the composition of the produced effluent in a PN-AMX system mainly depends on the treated wastewater nitrogen content and on the other hand, the removal efficiency depends on the NOB activity.

To reach the required low TN concentrations in the produced effluent, it is crucial to minimize the presence of dissolved oxygen in

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by the Chilean Government through the projects FONDECYT 1150285 and ANID/FONDAP/15130015. Anuska Mosquera-Corral wants to thank the MEC-CONICYT project 80180081 for its financial support in the short stay in the UAI, UdeC and UPLA (Chile) to collaborate in the elaboration of this paper. The authors from the USC belong to CRETUS Strategic Partnership (ED431E 2018/01) and to the Galician Competitive Research Group (GRC-ED431C 2017/29). All these programs are co-funded by

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