Elsevier

Water Research

Volume 45, Issue 19, 1 December 2011, Pages 6461-6470
Water Research

N2O emission from a partial nitrification–anammox process and identification of a key biological process of N2O emission from anammox granules

https://doi.org/10.1016/j.watres.2011.09.040Get rights and content

Abstract

Emission of nitrous oxide (N2O) during biological wastewater treatment is of growing concern. The emission of N2O from a lab-scale two-reactor partial nitrification (PN)–anammox reactor was therefore determined in this study. The average emission of N2O from the PN and anammox process was 4.0 ± 1.5% (9.6 ± 3.2% of the removed nitrogen) and 0.1 ± 0.07% (0.14 ± 0.09% of the removed nitrogen) of the incoming nitrogen load, respectively. Thus, a larger part (97.5%) of N2O was emitted from the PN reactor. The total amount of N2O emission from the PN reactor was correlated to nitrite (NO2) concentration in the PN effluent rather than DO concentration. In addition, further studies were performed to indentify a key biological process that is responsible for N2O emission from the anammox process (i.e., granules). In order to characterize N2O emission from the anammox granules, the in situ N2O production rate was determined by using microelectrodes for the first time, which was related to the spatial organization of microbial community of the granule as determined by fluorescence in situ hybridization (FISH). Microelectrode measurement revealed that the active N2O production zone was located in the inner part of the anammox granule, whereas the active ammonium consumption zone was located above the N2O production zone. Anammox bacteria were present throughout the granule, whereas ammonium-oxidizing bacteria (AOB) were restricted to only the granule surface. In addition, addition of penicillin G that inhibits most of the heterotrophic denitrifiers and AOB completely inhibited N2O production in batch experiments. Based on these results obtained, denitrification by putative heterotrophic denitrifiers present in the inner part of the granule was considered the most probable cause of N2O emission from the anammox reactor (i.e., granules).

Highlights

► We determined N2O emission from the lab-scale PN and anammox process. ► Average emission of N2O from the PN process was 4.0 ± 1.5% of incoming nitrogen load. ► Average emission of N2O from the anammox process was 0.1 ± 0.07% of incoming nitrogen load. ► The N2O emission from anammox granules could be originated from heterotrophic denitrification.

Introduction

Nitrous oxide (N2O) has a more than 300-fold greater potential for global warming effects than carbon dioxide, even though N2O only accounts for approximately 0.03% of total greenhouse gas emissions (Bates et al., 2008). Thus, the actual impact of N2O on global warming has been estimated up to 10% of total greenhouse gas emissions. It also takes part in stratospheric ozone depletion and is toxic to humans. Wastewater treatment systems, especially, biological nitrogen removal processes, have been known to be a potential N2O emission source. It is, therefore, in urgent need of reducing the emission and of identifying the factors that control the emission of N2O from wastewater treatment plants (WWTPs).

Several measurements at lab-scale and full-scale WWTPs have indicated that N2O can be produced in substantial amounts from biological nitrogen removal processes (Foley et al., 2010, Osada et al., 1995, Tallec et al., 2006, Kampschreur et al., 2008, Kampschreur et al., 2009b). Both nitrification and denitrification processes can lead to emission of N2O. However, N2O emissions are extremely variable and depend on many operational parameters such as dissolved oxygen (DO) and nitrite (NO2) concentrations in both nitrification and denitrification stage (Beline et al., 2001, Gejlsbjerg et al., 1998, Itokawa et al., 2001, Kampschreur et al., 2008, Park et al., 2000) and carbon availability (low chemical oxygen demand (COD)/N ratio) in the denitrification stage (Itokawa et al., 2001, Park et al., 2000). A recent review by Kampschreur et al. (2009a) showed that there are large variations in the N2O emissions from full-scale WWTPs (0–14.6% of the nitrogen load) and lab-scale WWTPs (0–95% of the nitrogen load).

Recently, sustainable wastewater treatment systems that can minimize energy consumption, emission of greenhouse gases, and sludge production have been attracting the attention. A nitrogen removal process via anaerobic ammonium oxidation (anammox) has been recognized as a promising cost-effective and low energy alternative to the conventional nitrification–denitrification processes due to a significant reduction of aeration and external carbon source (van Dongen et al., 2001, Kartal et al., 2010). In nitrogen removal via anammox process, ammonium in wastewater is partly pre-oxidized to nitrite (i.e., partial nitrification) by ammonium-oxidizing bacteria (AOB) before feeding into the anammox process. The produced nitrite together with remaining ammonium is then converted to dinitrogen gas (N2) in the anammox process. In the two-reactor partial nitrification–anammox process, significant N2O production could be expected during the partial nitrification due to accumulation of high NO2 and DO-limited conditions. In addition, N2O emission can also be expected from the anammox process since the anammox processes have been generally operated at high volumetric nitrogen removal load as described by Tsushima et al. (2007) and Tang et al. (2011) and at low COD/N ratio, even though anammox bacteria have not been shown to produce N2O under physiological conditions. Emission of N2O from a full-scale two-reactor partial nitrification–anammox process treating reject water was determined to be 2.3% of the total nitrogen load (1.7% in the partial nitrification process and 0.6% in the anammox process) (Kampschreur et al., 2008). Emission of N2O from a full-scale single-stage partial nitrification–anammox reactor treating wastewater from a potato processing factory and reject water of a municipal sludge dewatering plant was 1.2% of the total nitrogen load (Kampschreur et al., 2009b), which is higher than the emission from a lab-scale single reactor partial nitrification–anammox system on artificial wastewater (less than 0.1% of the nitrogen load) (Sliekers et al., 2002).

The magnitude and source of N2O emission in the combined partial nitrification and anammox process are, however, relatively unknown, especially, the potential and mechanism of N2O emission from anammox reactors or granules (or biofilms) is also unknown. Emission of N2O from an energy-saving and cost-effective partial nitrification–anammox process would hamper the practical application and should therefore be avoided.

In this study, a lab-scale partial nitrification–anammox process was developed in two separate reactors to investigate N2O emission from both processes. In addition, further studies were performed to indentify a key biological process that is responsible for N2O emission from the anammox process (i.e., granules). In order to characterize N2O emission from the anammox granules, microelectrodes were used to determine in situ N2O production rate, which was related to spatial organization of microbial community of the granule analyzed by fluorescence in situ hybridization (FISH).

Section snippets

Lab-scale partial nitrification reactor

An up-flow biofilm partial nitrification (PN) reactor with a working volume of 800 cm3 and nonwoven fabric sheets (4.0 × 4.0 × 0.8 cm × 18 sheets; Japan Vilene Co., Ltd., Tokyo, Japan) as support material for biofilms was used. The PN reactor was established and operated for 680 days as described previously (Cho et al., 2011, Okabe et al., 2011). Synthetic nutrient medium (Okabe et al., 2011) and air was supplied continuously from the bottom of the reactor. Although the dissolved oxygen concentration

Performance of the partial nitrification (PN) and anammox reactor

The partial nitrification (PN) reactor has been operated more than 680 days in advance and only the PN reactor performance during N2O measurement (after 680 days) is shown in Fig. 1. The PN reactor was operated at a high ammonium loading rate (ALR) of 3.5–4.1 kg-N m−3 d−1 (a constant influent ammonium concentration of 300 mg-N L−1). Thereafter (after 33 days), the ALR was decreased to about 2.5 kg-N m−3 d−1 by reducing the influent flow rate (corresponding to HRT of 3.4 h) in order to achieve the stable

Conclusions

A lab-scale two-reactor partial nitrification–anammox process was developed to investigate N2O emission from both processes.

  • The average emission of N2O from the lab-scale partial nitrification and anammox process was 4.0 ± 1.5% (9.6 ± 3.2% of the removed nitrogen) and 0.1 ± 0.07% (0.14 ± 0.09% of the removed nitrogen) of the incoming nitrogen load, respectively.

  • The total amount of N2O emission from the PN reactor was correlated to nitrite (NO2) concentration in the PN effluent rather than DO

Acknowledgments

This research was financially supported by Grant-in-Aid for the “Development of High-efficiency Biological Wastewater Treatment Technology Using Artificially Designed Microbial Communities” Project from the New Energy and Industrial Technology Development Organization (NEDO), Japan and by Core Research of Evolutional Science & Technology (CREST) for “Innovative Technology and System for Sustainable Water Use” from Japan Science and Technology Agency (JST).

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