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As a direct carbon emission source, the amount of nitrous oxide (N 2 , which is actually caused by AOB denitrification. To control the N 2 O emission during biological N-removal, complete HND and NO 2 - accumulation for AOB denitrification should be avoided to a large extent. For this purpose, DO in aerobic tanks should be controlled at a normal level (approximately 2 mg·L -1 ), and solid retention time (SRT) should be extended, up to 20 d, which would avoid accumulating N 2 O for AOB denitrification. Additionally, external carbon should be supplemented in time to promote HDN approaching the end, N 2 . This review summarizes the mechanisms of all the mentioned N 2 O emission pathways and discusses the control strategies of N 2 O emission according to the associated mechanisms.

High value-added extracellular polymer substance (EPS) extracted from excess sludge can effectively promote resource recovery from wastewater. EPS can replace traditional alginate in the food, medicine, textile, printing and dyeing, papermaking, and household chemicals industries. Moreover, its unique performance as a flame retardant has shown attractive potential for aircraft including space shuttles. This is due to the complicated chemical structure and composition of EPS, the excellent compatibility, adhesion, and other advantages of which could yield environmental-friendly flame-retardants. Therefore, a systematic analysis and summary on the mechanisms of EPS as flame retardants is of significance for future application. On the basis of the advantages and disadvantages of other fire-resistant materials on the market, the characteristics and application potential of EPS are analyzed and summarized. Second, the possible fire-resistant mechanisms of phosphorus and alginate-like substance (ALE) in EPS are revealed, and the synergistic flame-retardant effects of extracellular-proteins are also elucidated. Based on this, the flame-retardant characteristics of EPS are comprehensively evaluated and compared with other fire-resistant materials. To further improving the performance of EPS as a flame-retardant material, some modification strategies are proposed, such as increasing their phosphorus content, purifying and enhancing the content of ALE in EPS, and optimizing the modification methods of EPS on their substrates.

Using a 150 L moving bed biofilm reactor (MBBR), with the temperature controlled at 28℃ and high NH 4 + -N concentration (average concentration 350 mg·L -1 ), inorganic wastewater was used as an influent to start the completely autotrophic nitrogen removal over nitrite (CANON) process. Meanwhile, the flocculent sludge was taken into a 5 L sequencing batch reactor, and the influent NH 4 + -N concentration was maintained at 90-200 mg·L -1 for the recovery of short-cut nitrification. The results showed that in the MBBR reactor, when the average hydraulic retention time (HRT) was 12 h, short-cut nitrification and total nitrogen (TN) removal rate were mutually constrained, the average TN removal rate was 38.2%, and the average δ NO 3 - -N/TN value was 0.274; when the HRT dropped to 6 h, the δ NO 3 - -N/TN value decreased from 0.347 to 0.146. The sequencing batch reactor (SBR) maintained aeration and anoxic time for 30 min and 20 min, respectively, by intermittent aeration, while the dissolved oxygen concentration during the aerobic process was 0.5 mg·L -1 to 0.6 mg·L -1 , the free nitrous acid concentration was higher than 0.18 mg·L -1 at the end of each cycle, NAR increased from 0 to 99.2% after 12 days, NUR decreased to 0 from an initial 24.8 mg·(g·h) -1 , and the TN removal rate decreased from 13% to 3%; the system successfully converted to short-range nitrification. High-throughput sequencing results showed that the abundance of Candidatus Kuenenia in the flocculent sludge and biofilm in the MBBR reactor were 7.91% and 17.38% respectively, Nitrosomonas accounted for 27.43% and 2.55%, respectively, while Nitrospira accounted for 0.30% and 0.28%, respectively. After the recovery of short-cut nitrification in the SBR, the abundance of ammonia-oxidizing bacteria and anaerobic ammonia oxidation decreased to 1.18% and 0.01%, respectively, and the abundance of Nitrospira increased to 1.39%.

The focus of this paper, was low temperature, high ammonia nitrogen wastewater. The operation characteristics of the biofilm CANON process during the temperature reduction process were determined, by continuously adjusting different operating conditions. The aim was to explore the methods needed for the CANON process to obtain stable shortcut nitrification and a good nitrogen removal effect, when the influent NH 4 + -N concentration is high and the temperature low. The results showed that, ① compared with the biofilm CANON reactor temperature changing from medium to low temperature directly (30℃±1℃→19℃), it was more conducive to adapt the nitrogen-removing bacteria to the low-temperature environment, while the temperature was gradually lowered. Moreover, the extent of each reduction should be minimized. Besides, the operating conditions should be adjusted to ensure the nitrogen removal effect. ② The temperature was gradually reduced to about 19℃ after 25 d, and then decreased to about 15℃ after another 18 d. The NH 4 + -N and TN removal rates could be respectively stable at 90% and 70% over a long period of time. The TN removal rate and removal load could still reach 72.52% and 0.78 kg·(m 3 ·d) -1 , respectively, even when the temperature dropped to 12℃. ③ When adapting biological CANON sludge during the temperature reduction process, shortcut nitrification should be given priority. A stable shortcut nitrification effect should be obtained by maintaining a certain concentration of residual NH 4 + -N, and by strictly controlling the DO concentration to restrain NOB activity.

The completely autotrophic ammonium removal over nitrite(CANON)biofilm reactor acclimated by high-strength ammonia wastewater was used to treat low-strength ammonia wastewater. The treatment can be divided into three stages:① the nitrogen removal efficiency of anaerobic ammonia oxidation was low during the continuous aeration stage with inorganic wastewater as raw water (0-59 d) and with an aeration amount of 30 mL·min -1 and ammonia concentration of 80 mg·L -1 (until day 56), the TN removal load was only 0.13 kg·(m 3 ·d) -1 ; ② during the continuous aeration stage with domestic wastewater as raw water (60-110 d), the addition of organic carbon improved the TN removal load to 0.22 kg·(m 3 ·d) -1 on day 79; the removal rate of NH 4 + -N then reached 100% when the aeration volume improved to 100 mL·min -1 on day 103; however, the TN removal efficiency and TN removal load decreased to 42.36% and 0.14 kg·(m 3 ·d) -1 , respectively. ③ To increase both the NH 4 + -N and TN removal efficiency during the intermittent aeration stage with domestic wastewater as raw water (110-160 d), the aeration amount was increased to 50 mL·min -1 , while aeration was continued for 30 min and was stopped for the next 30 min; on day 131, the NH 4 + -N removal efficiency increased to 86.34%, the TN removal efficiency and removal load reached 85.87% and 0.3 kg·(m 3 ·d) -1 respectively; on day 141, the aeration was increased to 100 mL·min -1 and the removal efficiency of NH 4 + -N reached 100%, while the removal efficiency and removal load of TN were 64.28% and 0.22 kg·(m 3 ·d) -1 , respectively, indicating that the intermittent aeration strategy effectively improves the nitrogen removal performance of the CANON reactor. To analyze the variation of the microbial community during different stages, the samples of three stages (0, 56, and 152 d) were analyzed using high-throughput sequencing technology. The results show that:① Candidatus Brocadia is less affected than Candidatus Kuenenia during the low-strength ammonia stages with inorganic and domestic wastewater as raw water; ② Nitrosominas and Nitrospira were the dominant bacteria of AOB(ammonia oxidizing bacteria) and NOB (nitrite oxidizing bacteria), respectively. Domestic wastewater had a greater impact on Nitrosomonas than on Nitrospira ; ③ Denitrifying bacteria were present during the whole stage; Pseudomonas and Paracoccus were the most adaptable, even though their relative abundances during each stage were below 0.5%.

To Explore a suitable C/N ratio for efficient nitrogen removal and simultaneously achieving N 2 O release reduction, ammonia-rich wastewater with sodium acetate as an organic carbon source in a granular sludge completely autotrophic nitrogen removal over nitrite (CANON) reactor under different C/N water conditions were studied to determine the reactor's nitrogen removal performance and N 2 O release. The results showed that the total nitrogen (TN) removal rate and the removal load tended to increase gradually with the increase of C/N, ranging from 0 to 2.0. When C/N=0, the TN removal rate was 56.50 mg·L -1 in 7 h; the highest TN removal efficiency was 49%. When C/N=2.0, the highest TN removal rate was 71.42 mg·L -1 in 7 h; the highest TN removal efficiency was 59.52%, and the contribution of CANON to nitrogen removal gradually decreased, whereas the denitrification contribution gradually increased. When △NO 3 - -N/△TN=0.086, the contribution of CANON nitrogen removal was only 51.48% and that of denitrification was 48.52%. The N 2 O release volume and release ratio decreased with increasing C/N. When C/N=0, the N 2 O release volume and rate were the highest, namely 3.60 mg and 2.13%, respectively. The lowest N 2 O release volume and rate were 1.61 mg and 0.75%, respectively, when C/N=2.0.

To optimize the performance of completely autotrophic nitrogen removal over nitrite (CANON), a CANON process with modified polyethylene as carriers was operated in a moving-bed biofilm reactor (MBBR), using synthetic inorganic ammonia-rich wastewater (NH 4 + -N about 400 mg ·L -1 ) as influent at 30℃±1℃. With an HRT of 6 h, pH at 7.8, and filling rate of 35%, the average removal rate of NH 4 + -N and TN reached 74.28% and 87.93%, respectively, and the highest removals reached 84.68% and 98.82%, respectively, while the value of ΔNO 3 - /ΔTN was 0.12, which was close to the theoretical value of 0.127. This suggested that CANON sludge gradually adapted to the environment in the MBBR and began to enter the stable stage. Compared with a fixed-bed biofilm reactor (FBBR) under the same influent and operating conditions, the mean square error of MBBR and FBBR in terms of NH 4 + -N removal rate, TN removal rate, and TN removal load were 8.31% and 14.06%, 7.09% and 1.79%, 0.17 kg ·(m 3 ·d) -1 and 0.27 kg ·(m 3 ·d) -1 , respectively, the former are lower than the latter. Moreover, while DO concentrations of MBBR and FBBR were 1.96 mg·L - 1 and 3.09 mg ·L -1 , respectively, their TN removals of per liter carriers were 0.53 kg ·(m 3 ·d) -1 and 0.37 kg ·(m 3 ·d) -1 . Therefore, it was concluded that:① MBBR had a more stable nitrogen removal performance than did of FBBR, and ② MBBR had a higher TN removals of per liter carriers than did FBBR in addition to the higher utilization rate of oxygen.