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N2O emission and bacterial community dynamics during realization of the partial nitrification process

In this study, greenhouse gas emissions and microbial community succession during the realization of the partial nitrification (PN) process were studied. The results show that N2O emission mainly occurred in the aerobic stage and the PN reactor released about 20 mg of N2O gas each cycle. There is a positive correlation between the dissolved N2O concentration and the temperature of a typical cycle. High-throughput sequencing was used to illustrate succession in the microbial community structure. The most significant microfloral change during the PN startup process was that some aerobic bacteria were relatively enriched and some anaerobic bacteria were weeded out. The ammonia oxidizing bacteria (AOB) like Nitrosomonadaceae were enriched on account of the suitable external environment. Pseudomonas whose main function is denitrification declined and Planctomyces (anammox) showed the same tendency. This study comprehensively demonstrates the fluctuations of dissolved and emitted N2O while researching the succession of the microbial community in the culture of the PN process.


Introduction
With the development of industry, the problem of climate change has drawn attention from all circles of society. 1 The everincreasing greenhouse gases (GHGs) are really important for addressing the problem of global climate change. 2 The production of CO 2 , CH 4 and N 2 O is the leading contributor to all of the GHGs and takes the main responsibility for global warming. 3 Biological treatment is generally applied in sewage disposal plants and has always been deemed a major pathway for GHG emission. 4 The process involves the shiing of organics and nitrogen, and generates GHGs in the process of biological sewage treatment. 5 CO 2 and CH 4 are produced alongside the microbial degradation of organic carbon in the biological sewage treatment process. 6 It's reported that CH 4 production from WWTPs accounts for $5% of global CH 4 emissions. 7 In addition, the N 2 O derived from the process of nitrogen transformation especially the nitrication and deni-trication processes is also produced during this process. 8 Research indicates that N 2 O emissions from WWTPs are 0.22 TgN per year which is 1.3% of the total N 2 O production. 9 Based on the above evidence, the problem of GHGs discharged from biological sewage treatment plants can counterbalance parts of their environmental benets. 10 The partial nitrication (PN) process is a relatively efficient technique for the biotransformation of nitrogen which costs less energy and has a high nitrogen conversion ratio. 11 As shown in Fig. 1, the rst step is the oxidation of NH 4 + to NH 2 + OH by amo and the second step is the oxidation of NH 2 + OH to NO 2 À by hao. 9 There is an unstable intermediate named NOH which can produce N 2 O by chemical decomposition. Another potential Fig. 1 The partial nitrification metabolic pathway. pathway is the biological reduction process of NO produced through the oxidation of NH 2 + OH.
The PN process that oxidizes NH 4 + to NO 2 À has been reported to be technologically feasible and economically viable, especially when wastewater has high concentrations of ammonium and low levels of organic carbon for denitrication. 12 The main advantages of partial nitrication compared with complete nitrication are a lower oxygen demand during aeration, less of a requirement for organic substrates for sequential heterotrophic denitrication, low emissions of carbon dioxide (CO 2 ) and less sludge production. 13 Therefore, this technology enables the low cost transformation of nitrogen from wastewater.
The PN process has been used to provide suitable conditions for anammox (anaerobic ammonia oxidation) bacteria by transforming NH 4 + to NO 2 À in practical applications. As reported by Assis et al. (2017), the PN process has been started successfully by culturing with poultry slaughterhouse wastewater in which the NH 4 + concentration was 64 AE 10 mg L À1 . 14 The microbiological analysis demonstrated that Nitrosomonas was enriched to transform NH 4 + to NO 2 À . Some researchers have also reported that the PN strategy has been applied to the landll leachate treatment eld. The PN process is required to transfer half of the NH 4 + to NO 2 À to ensure there is the correct ratio of NH 4 + and NO 2 À because of a deciency of NO 2 À in the raw landll leachate. 15 With the achievement of the PN process, the composition of the various existing microbial communities shis to an ammonia oxidizing bacteria (AOB) dominant community. 16 The realization and control of the PN process is not very easy. Many strategies have been used to enhance the PN process and many researchers have studied the underlying causes affecting the PN process. A nitrifying granular sludge reactor has been used to accumulate nitrite and it sustained a stable PN process for more than 300 days. A 5 mT magnetic eld was used to enhance the PN process by improving the bacterial activity through regulating the expression of transshipment protein genes. 17 A salinity of 1.35% was conrmed to enhance both AOB and anammox bacteria activities during the combined PN-anammox process. 18 There are three major N 2 O production pathways during wastewater treatment, namely NH 2 OH oxidation, nitri-er (AOB) denitrication, and heterotrophic denitrication pathways, among which NH 2 OH oxidation and nitrier (AOB) denitrication exist in the PN process. 19 Although a lot of researchers have focused on the microbial community, few studies have been carried out regarding N 2 O emission and microbial community dynamic changes during the startup of the PN process. As low dissolved oxygen (DO) concentrations and high nitrite concentrations in the PN process could increase N 2 O emissions, the characteristics of N 2 O emission may differ from the complete nitrication process. 20 In this study, a lab-scale sequencing batch reactor (SBR) for the PN process was constructed and the characteristics of N 2 O emission during this process were investigated. Dynamic changes in the characteristics of microbial communities during the PN process were also evaluated using high-throughput sequencing techniques.

Experiment operation and reactor parameters
The working volume of the polymethacrylate SBR is 6 L. The SBR operating procedure is divided into four parts: 5 min feeding, 340 min aeration, 10 min settling and 5 min discharging. The exchange ratio of fresh inuent is 50%. 21 The hydraulic retention time of this system was 12 h and the temperature was around 31 AE 2 C to provide a suitable environment for the PN process. 22 A magnetic stirring apparatus was used to sustain uniform mixing conditions for the activated sludge and water to ensure all the sludge was involved in the reaction processes. A certain amount of suspended sludge was removed from the reactor every day to maintain a 3000 mg L À1 sludge concentration. 23

Acclimated sludge and synthesized wastewater
The seed sludge acclimated in the SBR with mixed liquor suspended solids (MLSS) at a concentration of 3160 mg L À1 was collected from the First Guangda Sewage Treatment Plant, in Jinan, China. An appropriate DO concentration of about 0.35-0.8 mg L À1 was maintained during the experiment for the startup of the PN process.
To enrich AOB and eliminate nitrite oxidizing bacteria (NOB), a high NH 4 + -N, and slightly alkaline medium was prepared. Water intake was 3 L per cycle. The composition of the synthetic medium was as follows: NH 4 HCO 3 (2.82 g L À1 ), KH 2 PO 4 (0.81g L À1 ), CaCl 2 (0.2g L À1 ), and MgSO 4 $7H 2 O (0.03g L À1 ). A trace element solution was prepared and added as described by . 17 The nal pH of the medium was 8.0 AE 0.1, and no pH control was applied during the experimental period. Sludge samples were taken on day 1, day 20 and day 30 for molecular testing.

Analytical methods
Water samples were ltered through syringe nylon membrane lters (0.45 mm) to remove biomass. The NH 4 + , NO 2 À and NO 3 À concentrations in the water were measured every three days. The MLSS concentration of the activated sludge was also determined. DO and pH were determined using a Hach HQ40d multimeter (Hach, America) and a Leici PHS-3C apparatus (Leici, China), respectively. Gas from the SBR was collected using a portable gas collection pump every 30 min. An AGC-ECD (Agilent 7890B, USA) equipped with a chromatographic detector was used to do the gas analysis of N 2 O. Sample analysis was nished in 24 h aer collection on account of the reference method. 24 The temperatures of the chromatographic detector for the injection port and the column were set at 50 C and 390 C. 25 2.4. Sample collection, DNA extraction, PCR amplication, sequencing and data analysis Sludge taken from the SBR was centrifuged at 6500g (Sig-ma3k15, Germany) for 15 min to remove the supernatant. Genomic DNA was isolated from 0.5 g wet sludge with the help of a power soil DNA isolation kit (MO BIO Laboratories, USA) following the protocol provided by the manufacturer. Aer the detection of the DNA content, V4 of 16S rDNA was PCR ampli-ed from microbial genomic DNA using bar-coded fusion primers (forward primer: AYTGGGYDTAAAGNG, and reverse primer: TACNVGGGTATCTAATCC). Bar-coded V4 amplicons were sequenced using the pair-end method by Illumina Miseq sequencing (Personal Biotechnology Co., Ltd., Shanghai, China). The database referenced was Greengenes. Biostatistical analyses were performed in this research using an algorithm supported by the SPSS13.0 (IBM, USA) soware. 26

Realization of partial nitrication and nitrogen transformation
Aer 35 days operation, the PN process was realized as shown in Fig. 2  During the rst 5 min, the SBR was lled with wastewater. In this stage, N 2 O was produced by bacteria dissolved in the liquid without aeration. This explains why the concentration of dissolved N 2 O was up to nearly 750 mg L À1 in the rst 5 min. Then, the anoxic stage started. During this phase, as dissolved N 2 O was emitted to the surroundings along with the bubbles created, its concentration decreased sharply during the 5-60 min timeframe. Aerwards, the concentration of dissolved N 2 O was relatively stable (60-225 min). Aer 225 min, the concentration of dissolved N 2 O became relatively lower compared with previous concentrations. Another phenomenon (Fig. 3) was that dissolved N 2 O increased along with the increase in temperature during a temperature cycle. Although the microbes related to N 2 O emission were Nitrosomonadaceae, 28 the structure of a microbial community cannot alter during a cycle. However, the activities of the enzymes in bacteria can vary with temperature. Fig. 4 shows that the discharge of N 2 O uctuated widely in a typical cycle period. The N 2 O emission rose at the beginning of

Microbial community succession during the realization of the PN process
The variation of the effluent quality during different periods implies that the cultured activated sludge has different microbial communities. Second generation DNA sequencing technology for 16S rDNA amplicons was used to study the succession of the microbial community. Sequences obtained from the second generation DNA sequencing technology of variable 16S rDNA genes were analyzed using the OTUs (Operational Taxonomic Units)-based approach. Remarkable changes in the microbial community composition and diversity appeared during the startup of the PN process. A total of 17 685, 15 562 and 15 165 reads were obtained for PN1 (sludge sampled on day 1), PN20 (sludge sampled on day 20) and PN30 (sludge sampled on day 30), respectively. The number of OTUs for PN1 was 1056 while the number of OTUs for PN20 was 394 and for PN30 the value was 279. Overall, the three samples share 81 of the same OTUs (Fig. 5). The phenomenon of the decrease in OTUs during the process of sludge acclimatization is very common. 29 In a study reported by Zhang et al., 30 the OTUs of two samples before acclimatization were 1986 and 1648 while aer acclimatization the numbers were 494 and 381.
The community richness decreased during the startup period of the PN process. As shown in Table 1, the Chao1 estimators were 1218.51, 603.72 and 396.86 on day 0, day 20 and day 30, respectively. The ACE (abundance-based coverage estimator), another index to illustrate bacterial richness, during the PN process also decreased from 1278.79 on day 1 to 720.71 on day 20 and further to 480.98 on day 30. The community diversity also decreased during the PN startup process. Community diversity was revealed by the Simpson index and the Shannon index. The Shannon estimators were 5.367, 3.867 and 3.234 on day 0, day 20 and day 30, respectively. The Simpson estimators, negatively associated with bacterial diversity, during the PN process increased from 0.012 on day 1 to 0.044 on day 20 and then to 0.082 on day 30. A similar phenomenon was also found by Liang Y. et al. 31 They reported that the Shannon indexes at the beginning of two PN processes were 5.817 and 5.860, and at the end they were 5.460 and 5.385. 31 The decreases in the biodiversity and the richness were mostly caused by the culture environment. Some controlled factors like relatively high NH 4 + and DO concentrations provided suitable environmental conditions for AOB but depressed NOB. The changes in the community structure will be discussed in detail later. Fig. 6 shows the succession of the microbial community during the PN startup process at three different taxonomic taxa through analyzing the bacterial genomic sequences from samples taken on days 1, 20 and 30. The results demonstrate that the microbial community composition experienced a remarkable change from day 1 to day 30. At the level of the phylum classication (Fig. 6A), the results show that the amount of Bacteroidetes increased from 16.8% on day 1 to 36.9% on day 20 and 45.7% on day 30. In contrast, the amount of Proteobacteria decreased from 50.5% on day 1 to 40.7% on day 20 and 37.3% on day 30. Proteobacteria (50.5%) accounted for the largest proportion at the phylum level on day 1 and includes many bacteria responsible for nitrogen xation. There were also many important AOB such as Betaproteobacteria and Gammaproteobacteria. According to the class assignment results (Fig. 6B), the relative abundance of Saprospirae belonging to Bacteroidetes increased from 11.2% on day 1 to 17.5% on day 30 while Betaproteobacteria belonging to  Proteobacteria decreased from 32.32% on day 1 to 27.02% on day 30. Based on the family level (Fig. 6C), the relative abundance of Comamonadaceae belonging to Proteobacteria increased from 5.61% on day 1 to 9.11% on day 20 and 13.98% on day 30. The family Comamonadaceae is demonstrated to be a type of nitrate reducing bacteria under a low oxygen supply. 32,33 In this research, the DO concentration was always under 0.8 mg L À1 which is a suitable condition for nitrate reducing bacteria. A total of 17 685, 15 562 and 15 165 reads were acquired on day 0, day 20 and day 30, respectively. The abundance variations of ve representative functional microbes (denitrifying bacteria (DNB), AOB, NOB, anammox bacteria and phosphorusaccumulating bacteria (PAO)) were selected to reveal the functional changes in the microbial community. Three genera of DNB (Pseudomonas, Paracoccus, and Thiobacillus) were detected and their abundances changed signicantly before and aer the startup process. From Table 2, it can be seen that the relative abundances of Thiobacillus and Pseudomonas decreased highly signicantly (p < 0.01) while the relative abundance of Paracoccus increased dramatically (p < 0.01) during the startup period. The richness of Nitrosomonadaceae (AOB) increased from 2.59% to 3.79% and reached 12.40% nally (p < 0.01) at the family level of bacteria. The relative abundances of Pseudomonas and Planctomyces (anammox) exhibited a decreasing trend during the PN process at the genus level of bacteria. Generally, PAO bacteria include Pseudomonas, Arthrobacter, Nocardia, Beyerinkia, Ozotobacter, Aeromonas, Microlunatus, and Rhodocyclus (Rittmann et al., 2012). 34 In this study, only two genera of PAO were detected, namely Pseudomonas and Arthrobacter.
Nitrosomonadaceae (AOB) have the ability to oxidize NH 4 + to NO 2 À . The increased amount of Nitrosomonadaceae indicates the successful startup of the PN process. 35 As shown by Fig. 2 and Table 2   comparisons between the three samples (Fig. 7). The results show that the microbes at the level of the genus were quite diverse among the samples. Dechloromonas (4.1%) was the most abundant genus in PN1, followed by Thiobacillus (2.0%). Klebsiella was the most abundant genus in PN20. The most abundant genus in PN30 was Thermomonas (2.6%). During the succession process, the dominant genus changed from anaerobic bacteria to facultative aerobes and facultative anaerobes. There are also other technologies applied for the treatment of wastewater containing ammonia nitrogen such as the biotreatment technology of membrane processes. The dominant bacteria changed from Ectothiorhodospira to Caenispirillum and Pannonibacter during the stable operation of a photosynthetic bacteria-membrane bioreactor. 37 The bacterial community shi along with changes in the operational conditions in a membrane-aerated biolm reactor has also been researched and the results showed that Betaproteobacteria, Gammaproteobacteria and Alphaproteobacteria were the dominant bacteria in the original sludge similar to the PN process. 38 But the abundance of Actinobacteria increased while Deltaproteobacteria decreased during the operation of this membraneaerated biolm reactor. 38

Conclusion
The PN process was acquired successfully in 35 days by controlling the DO. The N 2 O emission in a typical cycle was associated with uctuations of external factors other than the changes in microbes. The abundance of species and the community diversity of bacteria decreased when the PN startup process was completed as shown by high-throughput sequencing analysis. The relative abundance of Nitrosomonadaceae, bacteria related to N 2 O emission, increased highly signicantly. The heat map showed that the dominant genus changed from anaerobic bacteria (Dechloromonas) to facultative aerobes (Streptococcus) and facultative anaerobes (Klebsiella). Finally Thermomonas was the most abundant genus.

Conflicts of interest
There are no conicts to declare.