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Perspective for Emission and Control of Nitrous Gas in Biological Wastewater Treatment

Being the third most significant anthropogenic greenhouse gas, nitrous gas (N2O) has 300-fold stronger effect than carbon dioxide (CO2) and 4 ~ 30-fold stronger effect than methane (CH4). In this study, the main sources and mechanisms of N2O emission from biological wastewater treatment were reviewed, and the possible mitigation strategies were discussed. Parameters including dissolved oxygen, temperature, pH, organic carbon, and nitrite concentration have influences on the emission of N2O. The possible mitigation strategies were put forward by controlling these parameters in biological wastewater treatment and inducing modified technologies such as simultaneous nitrification-denitrification, denitrifying phosphorus removal, and aerobic granular sludge. In order to obtain a near-zero N2O emission, applying typical catalysts in the nitrification or denitrification tank to decompose N2O into harmless gas is recommended. Moreover, a method of developing photochemical processes to transform N2O could also be suggested.


Introduction
Greenhouse gas (GHG) emission has received great attention over the past decades . Typical GHG includes nitrous gas (N2O), carbon dioxide (CO2), and methane (CH4). Among them, N2O has 300-fold stronger effect than CO2 and 4 ~ 30-fold stronger effect than CH4 (Muthuraman et al., 2019). N2O featured in colorless, nonflammable, sweet-smelling is stable in the atmosphere for prolonged periods (Government of Canada, 2017). It has been identified as a significant constituent in several aspects of atmospheric chemistry (Miyahara et al., 2010). Being the third most significant anthropogenic greenhouse gas, N2O has been categorized as the greatest contributor to stratospheric ozone depletion (Portmann et al., 2012). The estimated growth of N2O emission is expected up to 14.49 Mt/y by 2020 (Muthuraman et al., 2019).
The industrial sector is regarded as the most significant emission source after agriculture Yu et al. 2019), where wastewater treatment is an important resource for N2O emission. According to data from Ednhofer et al. (2014), the emission of N2O from wastewater treatment accounted for 3.56% of the total emission load, increasing 20% compared with the past twenty years. Khalil and Rasmussen (1992) also estimated the global N2O emission, putting forward that the annual N2O emission rate from sewage was 0.3 × 10 12 ~ 3.0 × 10 12 kg/y, contributing to 3% ~ 30% of total N2O emission. Therefore, the emission of N2O during wastewater treatment cannot be ignored, and it converts water pollution to air pollution problems, which is opposite to the initial biological design intention to remove nitrogen from wastewater.
Online and continuous monitoring of N2O has been employed in recent years for accurate quantification of N2O emissions from biological wastewater treatment systems (Sabba et al., 2018). In a biological wastewater treatment plant, N2O discharged from an activated sludge pool is usually collected using a closed floating chamber. This technology was originally designed based on a solid surface emission test. During the aeration phase, dissolved N2O is stripped from the liquid phase; during the non-aeration phase, fresh air can be added from the bottom of the vessel and the sampled N2O can be extracted from the top of the vessel. After the sample of exhausted gas is extracted, the sample can be analyzed by gas chromatograph (Law et al., 2012;Chen et al., 2018).
Studies showed that substantial amount of N2O was produced during nitrogen removal process in biological nutrient removal (BNR) plants (Osada et al., 1995). It is reported that about 0.05% ~ 25% of nitrogen is converted to N2O in the process of BNR (Vasilaki et al., 2019). There are two biological processes aiming at nitrogen removal: nitrification and denitrification. The main formation of nitrous in wastewater is ammonium, which is converted into nitrite or nitrate during nitrification process. While nitrite is reduced to dinitrogen gas during denitrification process. These two nitrous removal proc-esses both lead to the emissions of N2O. N2O is an intermediate in the heterotrophic denitrification process and is often generated by ammonia-oxidizing bacteria (AOB). In order to complete nitrogen conversion in a limited time, relatively high nitrification and denitrification rates are required, contributing to the emission of N2O. In addition, bacteria communities change due to the changes of process conditions, which will also affect N2O production. The N2O emission is also associated with processes of biological wastewater treatment, during which the emission load is various depending on operation parameters (Kampschreur et al., 2009). Therefore, in this study, the main sources and mechanisms of N2O emission from biological wastewater treatment were reviewed, and the possible mitigation strategies were discussed.

The Mechanism of N2O Emission during Biological Wastewater Treatment
The BNR processes in biological wastewater treatment produce a large number of GHG such as CO2, CH4, and N2O. In this section, the mechanisms of N2O production, including nitrification and denitrification are discussed.

Nitrification
In general, during the nitrification process, NH4 + or NH3is oxidized to NO3via NO2 -. Most of the microorganisms involved in the nitration reaction are obligate autotrophic microorganisms. Nitrification process can be divided into two phases. In the first phase, ammonia-oxidizing bacteria convert NH4 + into NO2 -; there are two enzymes that participate in the reaction: ammonia monooxygenase (AMO) and hydroxylamine oxidoreductase (HAO) (Scala and Kerkhof, 1998). In the second phase, NO2is converted to NO3by nitrite oxidizing bacteria; the catalytic enzyme is nitrite oxidoreductase (NOR). N2O is neither intermediate nor final product throughout the reaction; therefore, N2O is a by-product. The complete path is described in Figure 1. The process of biological nitrification is a series of enzymatic reactions. According to Law et al. (2012), the autotrophic AOB usually lack the nitrous oxide reductase gene, which is the root cause of N2O production. The enzyme participates in the reaction, but its total amount before and after the reaction does not change. The activity of the enzyme is related to the temperature and the pH of the environment.

Denitrification
Denitrification is a process in which NO3or NO2is reduced to N2O and N2 by heterotrophic facultative anaerobic microorganisms under anaerobic or hypoxic conditions. Enzymes play a key role in denitrification process, and there are four enzymes involved: nitrate reductase (Nar), nitrite reductase (Nir), nitricoxide reductase (Nor), and nitrous oxidereductase (Nos) . The complete path is shown in Figure 2. Two modes of nitrate removal can occur in biological processes, including assimilatory and dissimilatory nitrate reduction. Dissimilatory denitrification is responsible for biological denitrification for enhanced nitrogen removal and bacterial cell respiratory electron transport chain for the oxidation of a variety of organic and inorganic substrates.
Compared with the nitrification process, N2O is an intermediate substance that is produced during the denitrifycation process. At present, there are two main explanations for the production of N2O in the denitrification process. The first one is losing of Nos activity in the denitrifying bacteria. N2O generated in the third stage was unable to escape, resulting in N2O accumulation from the water (Parton et al., 1996). The second is that part of the denitrifying bacteria does not have a Nos system, and its final product is only N2O. The amount of N2O produced is directly related to the activity of Nar (Parton et al., 1996). Similar with the nitrification process, the activity of the enzyme in denitrification process is determined by factors such as dissolved oxygen, organic carbon sources, and nitrites (Song et al., 2018;.

Factors Influencing the Emission of N2O in Biological Wastewater Treatment
The main form of nitrogen in wastewater is ammonia, which can be converted into nitrite or nitrate during nitrification process and thus dinitrogen gas is reduced in denitrification stage. Both nitrification and denitrification processes can lead to the emission of nitrogen oxides (Kampschreur et al., 2009). Figure 3 summarizes the important factors which have significant effects on N2O emission. The concentrations of dissolved oxygen (DO) and nitrite play important roles in both nitrification and denitrification processes, and the ratio of chemical oxygen demand/nitrogen (COD/N) affects denitrification process. In this section, the key factors influencing N2O emission are reviewed and discussed.

Dissolved Oxygen
The N2O emission is affected by the concentration of dissolved oxygen in both nitrification and denitrification processes (Kampschreur et al., 2009). During the nitrification process, the ammonia is converted into nitrite by oxidation, in which process N2O is produced, followed by the oxidation from nitrite to nitrate. Under the condition of low oxygen, the oxidation rate of nitrite to nitrate is lower than that of ammonia to nitrite. The emission of N2O increases with low dissolved oxygen concentration resulting from insufficient aeration (Zeng et al., 2003). The reason for this is that in oxygen-limiting conditions, autotrophic ammonia oxidizers use nitrite as the terminal electron acceptor to save oxygen for the oxygenation reaction of ammonia to hydroxylamine (Kampschreur et al., 2009). As a result, more nitrite is consumed by AOB and ammonium-oxidizing archaea (AOA), leading to the rising of N2O emission.
On the other hand, The N2O emission is also enhanced in the denitrification process if overload oxygen is provided. The concentration of dissolved oxygen can influence activities of many kinds of dominant enzymes (Tallec et al., 2006). During denitrification, dissolved oxygen can induce N2O production by affecting the function of the denitrifying enzyme system, among which the nitrous oxide reductase is more sensitive to dissolved oxygen compared to others. As a result, even a small amount of dissolved oxygen presented will cause the decline of nitrous oxide reductase, leading to the emission of N2O. Garrido et al. (1997) did a research and worked out a model to investigate the influence of dissolved oxygen concentration on nitrite accumulation and N2O emission in a suspension reactor. The results showed that with the increase of dissolved oxygen concentration, the nitrite concentration increased, reach-ing a peak at about 1.5 mg/L, and then climbed down. There was almost no nitrite when the dissolved oxygen concentration was over 2.5 mg/L. The similar trend was also observed in the nitrite simulation curve. The concentration of nitrate witnessed an upward trend as dissolved oxygen inclined, with a similar trend of simulation data, staying almost stable after 2.5 mg/L oxygen concentration. In contrast, ammonia decreased as dissolved oxygen rose up. The conclusion was described as that in the low oxygen concentration, nitrite concentration was higher, which resulted in the higher emission of N2O.

Nitrite Concentration
It is acknowledged that nitrite concentration has a big influence on both nitrification and denitrification processes. Shen et al. (2014) put forward that the emission of N2O increased with high nitrite concentration. Peng et al. (2015) did a research and found that AOB was very sensitive to nitrite. The denitrification of AOB, which was accelerated with increasing concentration of nitrite, caused accumulation of nitrous gas. The reason for this was that the AOB denitrification process was promoted by nitrite reductase Gene nirK and nitric oxide reductase Gene norB mRNA under the condition of high nitrite concentration .

Ammonia Concentration
The emission of N2O increased as increasing ammonia concentration (Ahn et al., 2010). Shen et al. (2014) indicated that one possible reason was that a high nitrifying activity occurred under high ammonium conditions, which might induce high N2O emission. According to the results from Shen et  Kampschreur et al. 2008 Full scale commercial wastewater treatment plant Reject water with TKN = 1265 ± 41 mg/L Dissolved oxygen, nitrite, aeration rate, pH Teiter and Mander, 2005 Municipal wastewater treated by wetlands BOD7 = 96 g/m 2 ; total N = 34 g/m 2 ; total P = 4.7 g/m 2 Operation factors of wetland Kong et al. 2002 Soil al. (2014), it could be observed that the nitrite production rate was increased from 3.30 to 4.78 mg/g/h with increasing initial ammonium concentrations from 7.7 to 52.4 mg/L, indicating that nitrite-oxidizing bacteria (NOB) activities might be lowered with increasing ammonium concentrations. The activity of NOB decreased as the concentration of nitrate showing a decreasing trend from 5.65 to 4.5 mg/g/h. This meant that with increasing of the initial ammonium concentration, the activity of AOB did not change, while that of NOB decreased, which could be due to a higher competition ability of AOB than NOB for oxygen. As a result, N2O emission changed from 0.01 to 0.09 mg/g/h, showing that high N2O emission rate occurred with increasing ammonium concentration.

Temperature
The emission of N2O is high under the condition of low water temperature (Sun et al., 2013). According to Adouani et al. (2015), the N2O emission rate and percentage of N2O rose up due to the decrease of temperature. The reason for this was that low temperature slowed down all denitrification enzyme activities and especially N2O reductase activities, inducing important emissions of N2O. In addition, N2O reductase could be inhibited by the accumulation of N2O in the biological reactor, which explained the accumulation and emission of N2O, causing a significant pollution transfer to the atmosphere.

COD/N Ratio
During denitrification process, limited organic carbon was observed to increase the emission of N2O (Vonschulthess et al., 1995). Quan et al. (2012) investigated the impact of various COD/N ratio in synthetic wastewater mixed with pig manure digestate liquid. The studies showed the proportions of N2O emission to the influent nitrogen loading rate at the aeration rates at 0.2, 0.6, and 1.0 L air/min were 8.2, 6.1, and 3.8% at a COD: N ratio of 1:0.22; 7.0, 5.1, and 3.5% at a COD: N ratio of 1:0.15; and 4.4, 2.9, and 2.2% at a COD: N ratio of 1:0.11, respectively. It could be seen that at the same aeration rate, with the decreasing of COD: N ratio, the proportions of N2O emission rose up.
In addition, organic carbon source also plays an important role in the generation of N2O. The type of microorganisms could be influenced by various carbon sources, resulting in different emission load of N2O. Studies showed that high N2O emission was caused by the low diversity of population due to single carbon source (Lemaire et al., 2006). Addition of an external organic carbon source, methanol, led to an appreciable reduction of the N2O emission from 4.5 to 0.2% of the nitrogen load (Park et al., 2006). The reason for this might be due to the activity of different bacteria which digested carbon source and emitted N2O. N2O accumulated as soon as organic carbon became limiting and bacteria started to consume internal sto-rage compounds (poly-b-hydroxybutyrate) (Kampschreur et al., 2008).

Solution pH Level
The emission of N2O is also affected by the pH level of wastewater. The level of pH was shown to have a major impact on the N2O production rate. Under the condition of investigated pH range of 6.0 ~ 8.5, the specific N2O production was observed to be the lowest between pH 6.0 and 7.0 at a rate of 0.15 ± 0.01 mg N2O-N/h/g volatile suspended solids (VSS). Nevertheless, N2O emission rate increased with pH, reaching a maximum rate of 0.53 ± 0.04 mg N2O-N/h/g VSS at pH = 8.0. (Law et al., 2011). This result indicated that the tested pH caused an increase in the N2O production rate by promoting the activity of the AOB. Pan et al. (2012) did a similar study and concluded that the maximum N2O reduction rate was much more sensitive to pH variation than nitrate and nitrite reduction rates. The optimal range of pH was 7.5 to 8.0. It could be summarized that in pH range of 6.0 ~ 9.0, the maximum N2O reduction rate was strongly influenced by pH, with its value gradually increasing from pH = 6.0, reaching its highest level at about 350 mg N/(gVSS × hour) at pH = 8.0, followed by a decrease to about 200 mg N/(gVSS × hour) at pH = 9.0. A similar trend with respect to pH changing was witnessed in nitrate and nitrite reduction rates. However, the nitrite and nitrate reduction rates were lower than N2O emission rate at the same pH level. These results suggested that the electrons competition among different nitrogen oxides reductase likely played a role in N2O accumulation at low pH conditions. The affinity of N2O reductase with respect to N2O decreased as pH increasing (Pan et al., 2012).

Possible Mitigation Strategies
N2O is a powerful greenhouse gas with a very long lifetime of 150 years and a 320-fold stronger effect than CO2 (Adouani et al., 2015). It is, therefore, necessary to investigate and control the anthropogenic emissions of N2O. Anthropogenic N2O emissions come mainly from energy processes, chemical industries and agriculture and waste treatments. Wastewater treatment is responsible for 3.2% ~ 10% of the total N2O emissions (Daelman et al., 2015). Therefore, mitigating the emission of N2O in wastewater treatment process is of great need.

Control of Operating Parameters
Dissolved oxygen: As for the concentration of dissolved oxygen, low dissolved oxygen in nitrification process and high dissolved oxygen in denitrification process contribute to increased emission of N2O. This means that during the nitrification process the supply of oxygen should be sufficient in order to minimize N2O emission. Therefore, high aeration can be applied in nitrification process to support sufficient oxygen, inhibiting N2O emission (Kampschreur et al., 2008). Some of the biological wastewater treatment plants tend not to apply high aeration due to taking energy consumption into considera-tion. The emitters might consider that increased energy consumption will lead to rising level of carbon dioxide, which is also a dominant greenhouse gas. However, during nitrification process, obtaining enough oxygen is necessary because N2O is much more harmful and difficult to handle than CO2.
However, in some cases involving denitrification process such as biofilm-granular sludge systems, high aeration could be practically difficult because increased oxygen concentrations resulted in inhibition of denitrification (Kampschreur et al., 2008). Law et al. (2012) pointed out that the process characteristics of wastewater treatment plants designed and operated for near-complete denitrification contributed to lower and more stable N2O generation than the plants with partial denitrification. Therefore, in order to obtain lower N2O generation, features concerning in denitrification process employed in wastewater treatment plants should be designed to achieve nearcomplete denitrification. The balance between influent flow and load, low dissolved oxygen concentration, high recycle rate, large bioreactor volume, long solid retention time, and external carbon source dosing are the design features which need to be considered.
Nitrite concentration: Taking nitrite concentration into consideration, high nitrite concentration in both nitrification and denitrification processes will lead to the emissions of N2O. The factors influencing high nitrite accumulation in nitrification process include insufficient aeration, low solid retention time, low temperature and high ammonia concentration. To control such factors, in full-scale wastewater treatment plants, designing features should be taken into consideration. In detail, loadings are buffered and the risk of transient oxygen depletion is reduced due to the systems equipped with large bioreactor volumes and influent flow-balancing (Law et al., 2012). Similarly, Desloover et al. (2012) also concluded some strategies to reduce accumulation of NH2OH and NO2 − . The strategies included sufficiently high and constant dissolved oxygen, adapted aeration regime, low free nitrous acid. An end-of-pipe treatment was put forward, which could be supported by the implementation of a newly proposed GHG crediting system. The end of pipe treatment was employed to ensure sufficient copper availability, supporting N2O reductase synthesis.

Ratio of COD/N:
When it refers to the COD/N, low N2O emission could be resulted from high COD/N and extra carbon resources. To achieve sufficiently high COD/N, external COD needs to be provided in the pre-settling stage, and lower COD is able to be removed from sewage. In addition, choosing a proper carbon source is necessary (Desloover et al., 2012). Park et al. (2006) found that the amount of N2O produced during wastewater treatment decreased from 4.5% to 0.2% of the total N, by introducing methanol as additional carbon source. Shen et al. (2014) used acetate as the initial carbon source, drawing conclusions that with the initial acetate concentration at 250, 500, and 1000 mg/L, the concentration of N2O emitted at 90 min was 0.75, 0.05, and 0.035 mg/L, respectively. It can be concluded that increasing variety of carbon sources, as well as increasing concentration of carbon sources, is able to achieve low N2O emission.

Modified Biological Wastewater Treatment Technology
Simultaneous nitrification-denitrification technology: Simultaneous nitrification-denitrification (SND) is a modified biological wastewater treatment technology which was developed in recent years (Iannacone et al., 2019). Microbial simultaneous nitrification-denitrification process is the conversion of the ammonium ion to nitrogen gas in a single bioreactor. The process is dependent on flow characteristics, reaction kinetics, mass loading of readily biodegradable chemical oxygen demand (rbCOD) and DO concentration . It is a new technology for denitrification of wastewater that carries out nitrification and denitrification processes under the same operating conditions in a same reactor. In the SND process, the AOB denitrification, nitrification bacteria nitrification, heterotrophic denitrification, and other processes in the same reactor are performed at the same time. Since N2O release may occur in each reaction process, the release characteristics of N2O during the SND process are more complicated.
The N2O emission in the SND process under hypoxic conditions is an essential product of microbial metabolism. Both heterotrophic denitrifying bacteria and AOB can produce N2O, and the denitrification of AOB is the main reason for N2O production . Furthermore, denitrification using intracellular polyhydroxyalkanoates (PHAs) as a carbon source increases the amount of generated N2O (Yin et al., 2018). However, under high-loading phosphorus conditions, microorganisms can synthesize more PHAs to reduce the denitrifycation enzyme electrons, thereby reducing the amount of N2O during heterotrophic denitrification process. In addition, in the SND system, the optimal control of the particle size of the sludge aggregates can achieve N2O reduction control (Di Bella and Torregrossa, 2013). By controlling the particle size of the sludge aggregates at 0.45 to 0.9 mm, high nitrification and denitrification activities can be achieved at the same time, thereby reducing N2O emissions.
Denitrifying ohosphorus removal: Denitrifying phosphorrus removal means that denitrifying phosphate-accumulating bacteria use NO3or NO2as electron acceptors under anoxic conditions, and uses internal carbon source PHAs to achieve excess phosphorus absorption in water (Zhou et al., 2010). N2O is the main denitrification by-product in this process. The conversion of PHAs plays an important role in the accumulation of N2O. The reduction of N2O is relatively low when using PHAs as a carbon source. In addition, during this process, NO2 or free nitrous acid can inhibit the activity of denitrification enzyme and the absorption of phosphorus, resulting in denitrification efficiency and a large number of N2O production.
The production of N2O in denitrifying phosphorus removal process mainly occurs in the process of anoxic phosphorus denitrifying and aeration nitrification (Mandel et al., 2019). In the anoxic denitrification stage, continuous water intake and aeration stage can be adopted to reduce N2O production by adjusting aeration to prevent nitrification bacteria from denitrifying to avoid N2O accumulation. Research found that when continuously adding NO2 and using propionic acid as a carbon source in the denitrifying phosphorus removal process, the electronic competition between denitrifying enzymes can be reduced and NO2accumulation can be reduced (Jiang et al., 2019). Choosing the right carbon source is beneficial to the reduction of N2O in the system. Wang et al. (2014) found that increasing the concentration of influent phosphorus can reduce the formation of N2O. When the concentration of phosphorus increased from 5 to 50 mg/L, the total N2O production decreased from 1.64% of total nitrogen to 0.16%.
Aerobic granular sludge: The aerobic granular sludge process has good application prospects in wastewater treatment due to its good sedimentation performance and high microbial concentration (Mandel et al., 2019). However, its spatial structure will lead to incomplete denitrification and cause N2O production, because the denitrifying and nitrifying bacteria in the particles are difficult to have pure anoxic and aerobic conditions. Temperature can affect N2O production by affecting the metabolic activities of functional microbes. Studies from Bao et al. (2018) found that as the temperature increased from 10 to 30 °C, the average N2O emission factor decreased in the range of 0.15% ~ 0.70% N2O-N/NH4 + -N oxidized in the first aeration phase and 0.14% ~ 0.15% N2O-N/NH4 + -N-oxidized in the second aeration phase. With the increase of temperature, an aerobic environment is more likely to form within the granular sludge. The accumulation of NO2in the system decreases, contributing to the decrease of N2O production.
Other novel methods: Besides controlling operational parameters, some other novel strategies have been developed by researchers. From previous studies, it was learned that microorganisms played an important role in transferring nitrite. Richardson et al. (2009) did a research on enzyme regulation relating to bacteria in denitrification stage. The denitrifying bacteria converted endogenous cytotoxic NO to N2O in order to protect themselves. They could exist in several forms possessing very different enzymatic activities towards N2O reduction, which might be caused by changes at the catalytic site [Cu4S]. In order to reduce the generation of N2O, the genetic expression of the enzyme could be changed on some catalytic sites. Miyahara et al. (2010) studied a new branch of denitrifying bacteria called Pseudomonas stutzeri TR2. It was observed that the strain TR2 produced low levels of N2O even under aerobic conditions. After all conditions tested, the strain TR2 was proved to release little levels of N2O through exhibiting strong denitrification process. Particularly, when exposed to both N2O and nitrite, the strain TR2 bacteria had an affinity of N2O as substrate. Consequently, this kind of newly found microorganism has a potential to reduce N2O emissions when applied to sewage disposal fields.

Recommendations for Near-Zero Emissions of N2O from Biological Wastewater Treatment
In this study, the parameters influencing N2O emission from biological wastewater treatment and the possible mitigation strategies were comprehensively reviewed. During nitrogen removal process, N2O was produced due to the nitrifycation and denitrification. Dissolved oxygen, temperature, pH, organic carbon, and other factors affect the release of N2O. The emission of N2O can be reduced by controlling influencing parameters in biological wastewater treatment and inducing modified technologies. However, no matter how to adjust the operating parameters and water quality conditions, N2O will always be released more or less. Up to now, N2O removal technologies in biological wastewater treatment plants were not widely proposed and developed. The reason might be the little contribution to the anthropogenic N2O emission budget and the consideration of economic or legislative incentives.
Although the technologies of N2O removal in biological wastewater treatment plant are relatively few, some methods concerning N2O removal from industrial gases were investigated. The catalytic reduction of N2O by CO molecule over the surface of Si-embedded MoS2 was investigated by Esrafili (2019). The results indicated that the adsorbed N2O can be easily decomposed to N2 without activation energy. Fe-ZSM-5 catalysts were used to remove N2O from gas by Richards et al. (2018). Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N2O decomposition when propane is present. The characteristics of catalytic were influenced by Fe loading and Si:Al ratio. In addition, a photochemical process was put forward as an efficient method to remove N2O, which could be obtained under the condition of atmospheric pressure and room temperature without any catalysts. N2O was diluted and decomposed into N2, O2, and NO using a 193 nm ArF excimer laser (Tsuji et al., 2004).
Consequently, a recommendation that applying typical catalysts in the nitrification or denitrification tank to decompose N2O into harmless gas is reasonable. Moreover, methods of developing photochemical processes to transform N2O can also be considered. Nevertheless, these technologies require a relevantly high cost but the emission of N2O accounts for only a little part. Therefore, in the future work, research on N2O removal from gas emissions in biological wastewater treatment plant has great potential to mitigate GHG emission.