Effect of temperature on N₂O emissions from a highly enriched nitrifying granular sludge performing partial nitritation of a low-strength wastewater

Highlights

• The partial nitritation process at mainstream conditions produced low N₂O emissions.

• A temperature effect was shown on N₂O gas emissions in a partial nitritation process.

• Temperatures higher than 15 °C increased N₂O emissions in the partial nitritation.

Abstract

In the race to achieve a sustainable urban wastewater treatment plant, not only the energy requirements have to be considered but also the environmental impact of the facility. Thus, nitrous oxide (N₂O) emissions are a key-factor to pay attention to, since they can dominate the total greenhouse gases emissions from biological wastewater treatment. In this study, N₂O production factors were calculated during the operation of a granular sludge airlift reactor performing partial nitritation treating a low-strength synthetic influent, and furthermore, the effect of temperature on N₂O production was assessed. Average gas emission relative to conversion of ammonium was 1.5 ± 0.3% and 3.7 ± 0.5% while the effluent contained 0.5 ± 0.1% and 0.7 ± 0.1% (% N-oxidized) at 10 and 20 °C, respectively. Hence, temperature increase resulted in higher N₂O production. The reasons why high temperature favoured N₂O production remained unclear, but different theoretical hypotheses were suggested.

Introduction

The implementation of the autotrophic biological nitrogen removal (BNR) in the mainstream has been proposed as the most promising solution for achieving energy-neutral or even energy-positive urban wastewater treatment plants (WWTPs) (Kartal et al., 2010, Siegrist et al., 2008). Significant efforts have been made to implement such a treatment as a one-stage system, where partial nitritation and anammox process (PN/A) are integrated in one single reactor (De Clippeleir et al., 2013, Gilbert et al., 2014, Lotti et al., 2014a, Wang et al., 2016a, Wett et al., 2013). This is based on the practise of implementing this process for sidestream treatment (Lackner et al., 2014). However the different conditions of low required effluent concentrations, lower temperature and much larger hydraulic loading relative to nitrogen loading might make a different process design more feasible. Two-stage systems have been reported as a successful alternative to face the challenges of efficient autotrophic BNR at mainstream conditions (Isanta et al., 2015, Ma et al., 2011, Pérez et al., 2015).

In the race to achieve a sustainable urban WWTP not only the energy requirements have to be considered but also the environmental impact of the facility (Morales et al., 2015). Thus, greenhouse gases emissions are a key-factor to pay attention to (Kampschreur et al., 2009). Nitrous oxide (N₂O) is produced in conventional urban WWTPs during the autotrophic nitrification and heterotrophic denitrification and, actually, N₂O emissions can dominate the total greenhouse gases emissions from biological wastewater treatment (Wunderlin et al., 2012). N₂O is an important greenhouse gas with a global warming potential of about 300 times higher than CO₂ on a 100 year time horizon (IPCC, 2013) and a substantial ozone-depleting compound in the stratosphere. Hence, mitigation strategies and control of emissions are essential issues to consider in the implementation of the autotrophic BNR in the mainstream of urban WWTPs.

It is well known that N₂O production in WWTPs is associated with nitrification by ammonia oxidizing bacteria (AOB) and to denitrification by heterotrophic bacteria (Kampschreur et al., 2009, Wunderlin et al., 2012). Furthermore, N₂O emissions can be also produced by abiotic chemical reactions (Harper et al., 2015, Kampschreur et al., 2011, Soler-Jofra et al., 2016). AOB produce N₂O by two different pathways: (i) from intermediates of the biological oxidation of hydroxylamine (NH₂OH), which is an intermediate during the ammonia oxidation to nitrite and (ii) the nitrifier denitrification pathway, which is the reduction of nitrite to N₂O with ammonia, hydrogen or pyruvate as possible electron donors (Wunderlin et al., 2012). Heterotrophic denitrifiers produce N₂O as intermediate in the denitrification so it can be released due to an imbalanced metabolic activity, a nitrite accumulation or a limited availability of biodegradable organic compounds and incomplete denitrification (Kampschreur et al., 2009, Wunderlin et al., 2012).

In the autotrophic BNR process, N₂O emissions will mainly occur in the partial nitritation step since anammox bacteria are not supposed to produce N₂O as it is not involved in their metabolism (Kartal et al., 2011). Actually, very low N₂O emissions were reported in anammox reactors and they were associated with side reactions independent of anammox bacteria (Lotti et al., 2014b), or to abiotic reactions (Kampschreur et al., 2011). In recent years, N₂O gas emissions were widely studied for PN/A systems (either in one-stage systems or in a single partial nitritation reactor) treating high-strength nitrogen wastewaters, mainly reject water (Castro-Barros et al., 2015, Desloover et al., 2011, Kampschreur et al., 2008b, Mampaey et al., 2016, Okabe et al., 2011, Pijuan et al., 2014). There was a huge variability on N₂O emissions values reported in literature, ranging from 1.5% (Rathnayake et al., 2013) to 11% (Desloover et al., 2011) of the ammonium oxidized emitted as N₂O. This variability was due to differences in reactor configurations, type of influent, conditions applied and even the methodology used for quantifying emissions (Bollon et al., 2016). In the case of PN/A systems at mainstream conditions, to the best of the authors’ knowledge, only Wang et al. (2016b) and Reino et al. (2016) reported N₂O gas emissions of a nitritation reactor treating a low-strength synthetic influent. Reino et al. (2016) reported very low values (0.36 ± 0.07% of the ammonium oxidized) in a granular sludge reactor performing partial nitritation at 10 °C, compared to N₂O gas emissions reported by Pijuan et al. (2014) (6% of the ammonium oxidized) which used the same control strategy but treating a reject water at 30 °C, and it was suggested that temperature could be an important factor affecting N₂O emissions.

The effect of temperature on N₂O emissions was never deeply studied since most studies were performed for systems treating reject water, which is characterized by high temperatures (30–35 °C). However, wastewater temperature is a key parameter in the nitrification process which affects to mass transfer, chemical equilibrium and growth rate (Van Hulle et al., 2010), so it could be also an important parameter affecting N₂O emissions. Furthermore, N₂O solubility decreases when temperature increases, which affects N₂O stripping from wastewater to gas phase resulting in the enhancement of N₂O gas emissions.

Hence, the objective of the present study was to investigate the effect of temperature on the N₂O gas emissions from a granular sludge airlift reactor performing partial nitritation of a low-strength synthetic influent. Hereto, the reactor was operated at three different temperatures: 10, 15 and 20 °C.