Research On The Process Control Of Partial Nitrification System And Its N₂O Emission

Partial nitrification via nitrite (100% NH4+-N conversion to NO2--N) has been developed as a novel biological nitrogen removal process in recent years. Compared with conventional nitrification-denitrification process, partial nitrification theoretically saves oxygen consumption and carbon source. Therefore, it is necessary to in-depth study the principle, control and mechanism of this process, which is helpful to promote the engineering application. Although partial nitrification has expressed great advantages in application, nitrite accumulation is commonly considered as an important factor causing nitrous oxide (N2O) production. To date, limited research has been conducted to characterize N2O emission of partial nitrification process treating high ammonia wastewater. It is significant to study the N2O release characteristics, which has theoretical significance for better understand this wastewater treatment process.Based on above discussion, the objective of this paper was to evaluate on the process control and N2O emission of partial nitrification. The factors affecting the performance of partial nitrification and the mechanism of nitrite accumulation was studied. Two kinds of partial nitrification reactors were set-up and stable operated by using activated sludge and granular sludge. The phychemical properties of extracellular polymeric substances (EPS) in partial nitrification operational mode were evaluated. Especially, N2O emission of partial nitrification process was researched during stable operation stage. The obtained results could provide more useful information to determine nitrogen removal by considering N2O emission. The main conclusions are as follows:(1) The set-up and contiversion of partial nitrification to sharon processes by altering the influent ammonium concentration. After 150 days’operation, partial nitrification and sharon processes were successfully achieved when the influent NH4+ -N concentrations up to 400 and 720 mg/L, respectively. Meanwhile, sludge volumetric index (SVI) gradually decreased from 127.4 to 63.4 mL/g, while the average size of sludge improved from 29.5 to 195.6 μm by the strategy of reducing settling time. Ammonium-oxidizing bacteria (AOB) were the dominant nitrifying bacteria according to the fluorescence in-situ hybridization (FISH) analysis. Free ammonia (FA) and free nitrous acid (FNA) were the potential compounds for inhibiting the activity of nitrite-oxidizing bacteria (NOB).(2) The factors affecting the performance of partial nitrification were analyzed during long-term operation, including influent pH, dissolved oxygen (DO) and chemical oxygen demand/nitrogen (COD/N) ratio. Results showed that high DO concentration (2-4 mg/L) could damage nitrite accumulation immediately (Stage I). However, nitrite accumulation ratio (NAR) could be increased from 1.68±1.51 to 35.46±7.86% when increasing the pH values from 7.5 to 8.3 (Stage II and III). Afterwards, stable partial nitrification and high NAR could be recovered when the reactor operated under low DO concentration (0.5-1.0 mg/L) (Stage IV). However, it required a long-time to recover the partial nitrification of the reactor when altered the influent COD/N ratios (Stage V and VI). FISH analysis implied that AOB were completely recovered to the dominant nitrifying bacteria in the system. Meanwhile, SVI of the reactor gradually decreased from 115.6 to 56.6 mL/g, while the mean diameter of sludge improved from74.57 to 428.8 μm by using the strategy of reducing settling time.(3) The composition of extracellular polymeric substances (EPS) during the achievement of partial nitrification was characterized, and subsequent nitrous oxide (N2O) emission was evaluated. Accodring to low DO condition, the effluent NH4+-N concentration was less than 1 mg/L, while NO2 -N and NO3 -N were 139.82±11.19 mg/L and 26.81±6.76 mg/L, respectively. The average size of sludge flocs in the reactor increased from 102.6 to 258.5μm. The main compositions of EPS, including protein (PN) and polysaccharide (PS), increased to 65.46±3.27 and 21.63±1.08 mg/g VSS, respectively. Three-dimensional excitation-emission matrix (3D-EEM) spectroscopy implied that EPS transferred to tryptophan PN-like and humic acid-like substrates. Moreover, N2O emission accounts for 11.67% of removed nitrogen during the steady state of partial nitrification reactor.(4) N2O emissions in partial nitrifying and full nitrifying granular sludge reactors treating ammonium-rich wastewater was evaluated. During stable operation, the NH4+-N removal efficiencies of two granular reactors were 98.73±0.75% and 99.15±0.44%. Nitrate and nitrite were the main effluent nitrogen species of the two reactors, and NAR of partial nitrifying reactor was high of 87.79±2.03%. However, partial nitrifying granular-reactor had better total nitrogen removal efficiency (41.84±3.35%) than that of full nitrifying granular-reactor (19.91±2.12%). According to typical cycles, the N2O emission amount per cycle of partial nitrifying reactor account for 11.48% of the incoming nitrogen load, which was 1.5 times higher than that of full nitrifying reactor (7.47%).(5) The characterization of EPS between partial nitrifying and full nitrifying processes was evaluated. The compositions of EPS samples, including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), were measured and assessed by using 3D-EEM, fluorescence regional integration (FRI), molecular weight distribution and Fourier-transform infrared spectroscopy (FTIR). During stable operation, the NH4+-N removal efficiencies of two column-type bioreactors were high of 99.79 ±0.08% and 99.68±0.20%, respectively. NAR of partial nitrifying reactor was achieved to 88.35±0.96%. However, total nitrogen (TN) removal efficiency of partial nitrifying reactor (40.00±1.44%) was much higher than full nitrifying reactor (17.49±0.18%). Results implied that the amount of LB-EPS and TB-EPS in partial nitrifying reactor were much higher than full nitrifying reactor.3D-EEM implied that the identified fluorescence peaks showed different peak intensities and positions in both LB-EPS and TB-EPS of the two reactors. The combined analysis of FRI showed that the main fraction of TB-EPS and LB-EPS were soluble microbial products (Region IV). Based on the molecular weight distribution and FITR, the information of LB-EPS and TB-EPS could be further analyzed.