| Traditional biological nitrogen removal techniques have many shortcomings when dealing with high ammonia wastewater, such as complex process, aeration and adding extra carbon source. Partial nitrification(PN) combined with anaerobic ammonium oxidation(anammox) technology provided an ideal choice for high ammonia wastewater treatment. Ammonia-oxidizing bacteria(AOB) were used to oxidizing half ammonia of the high ammonia wastewater to nitrite at aerobic environment, and then, mixed water of ammonia and nitrite(the ratio was about 1) was used to provide influent water for the anammox reactor to achieve the purpose of denitrification ultimately. Biological sewage treatment is an important anthropogenic source of N2 O, which amount up to 25% of the global total N2 O release. Nitrite accumulation and low oxygen environments in PN process provide the generate conditions of nitrous oxide greenhouse gas in large amounts. As a leading process of anammox reaction, N2 O generation and release of PN has an important significance in guiding the high ammonia wastewater treatment.Sequencing Batch Reactor(SBR) was used to startup PN process in this study. Batch tests were operated to explore the effect of magnetic field on PN process based on the principles of bio-magnetic effect. The promoting principle of magnetic field to PN process was explored at microorganic and genetic levels through high-throughput sequencing method. The PN reactor was studied systematically under different physical and chemical conditions(period, salinity, temperature, and magnetic field strength) after the PN process started up successfully. Periodic changes of ammonia, nitrite, nitrate, dissolved oxygen(DO), pH, N2 O and other water quality indicators were detected. Factors related to N2 O generation were clarified, which provide a theoretical basis to the N2 O reduction. The main findings of this article are as follows:(1) A weak magnetic field strength of 5 mT has significantly enhanced the PN effect, PN startup within 25 days at 5 mT magnetic field strength while the time is 35 days without external magnetic field. However, PN reaction was inhibited if the magnetic field strength higher than 15 mT. For example, ratio of nitrite to ammonium in effluent water was between 0.3 and 0.5 under 25 mT magnetic field, which cannot be increased by increasing reaction time. PN cannot be achieved.(2) N2 O release of PN reaction was systematically studied. PN reactor converted 50% ammonia to nitrite within 4 hours. Accumulation of N2 O emission was 15.05 mg and conversion rate of N2 O was 1.915% at the end of 4 hour. Accumulation of N2 O will increase 28.88 percent if extend the reaction time to 6 hours. While promoting the PN reaction rate, magnetic field also increases the N2 O production. N2 O conversion rate of PN process at 5 mT magnetic field was 2.80%, 40% higher than that with no external magnetic field addition. A certain range of salinity could promote PN process, but N2 O production was also facilitated. Accumulation of N2 O was up to 84.49 mg after 8 mg / L NaCl addition, more than four-fold compared to the control group. Between 35 oC to 25 oC, PN process decline with temperature decrease, and correspondingly, the N2 O emissions also fell off. N2 O conversion ratio of PN process at 25 oC was 1.31%, 31.41% decreased compared with that in 35 oC.(3) High-throughput sequencing method was used to explore microbial mechanism related to PN process. Magnetic fields enhanced the activity of AOB rather than increasing the proportion of it. Ratio of AOB in 5mT magnetic field even declined, High-throughput sequencing analysis showed that, ratio of Nitrosomonas is 13.9% in the absence of external magnetic field while the proportion of Nitrosomonas was dropped to 12.9% at 5 mT magnetic field strength. OUR(oxygen uptake rate) of AOB at 5 mT magnetic field strength is 0.936 mg(O2) / g(MLSS).min, which is 2.39 times of that no external magnetic field addition. The magnetic field cannot change the end of PN rather that shorten the time to the end of PN if no pH controller provided. This is because magnetic field cannot change the forms of ammonium nitrogen in the solution. Functional genes related to membrane transport, signal transduction and cell migrationin in 5 mT magnetic field were increased by 12.3%, 9.3% and 11.1% respectively compared with no extra magnetic addition. Functional gene abundance prediction indicates that the magnetic field promoting the activity of AOB may be by facilitating free ammonia went into AOB cell. N2 O was mainly produced by Nitrosomonadaceae. Changes of the physical and chemical factors result in increase or decrease of N2 O by affecting Nitrosomonadaceae related metabolic activity. |
