DO is a key factor affecting the nitrogen treatment efficiency of the SBBCOR, which directly affect the activity time of different strains, and change the specific growth rate of different microbial species. When DO is too high, the nitrifying bacteria compete with the denitrifying bacteria, and even cause the denitrifying bacteria to be washed out, which reduces the nitrogen treatment efficient. At the same time, too low concentration of DO is not conducive to the growth of aerobic heterotrophic bacteria, and nitrite bacteria and nitrate bacteria cannot survive in the reactor, which can also decrease the treatment performance of nitrogen [36–40].
By controlling I/II-SBBCOR’s water temperature of 20 ± 5 ° C, HRT of 4 d/2 d, SBC of 8 h, aeration cycle is 60 min, aeration ratio is 50%, investigate the nitrogen treatment efficiency of SBBCOR under the conditions of DO of 1.0 ± 0.2/1.5 ± 0.2 mg/l, 1.5 ± 0.2/2.0 ± 0.2 mg/l, 2.0 ± 0.2/2.5 ± 0.2 mg/l, 2.5 ± 0.2/3.0 ± 0.2 mg/l during aeration.
When Do of I/II-SBBCOR was 1.0 ± 0.2/1.5 ± 0.2 mg/l during aeration, the removal rate of NH4+-N and TN was only 60% and 62% (that of TN was slightly higher than that of NH4+-N). As DO increased to 2.5 ± 0.2/3.0 ± 0.2 mg/l during aeration, the removal rates of NH4+-N increased greatly, while that of TN first increased and then decreased, which reached the highest as DO = 2.0 ± 0.2/2.5 ± 0.2 mg/l during aeration (shown as Fig. 9,10.).
µN—nitrate bacteria growth rate yield;
µN, max—nitrate bacteria maximum growth rate yield;
K0—nitrite bacteria oxygen saturation constant.
When DO in the reaction system is much higher than K0, then µN = µN, max. The oxygen saturation constant k0 of nitrite bacteria is much lower than that of nitrate bacteria. The decrease of DO in a stable nitrifying system may lead to the competition for oxygen between the two groups of bacteria, which may lead to the dynamic succession of bacteria, and then affect the degree of biological oxidation of ammonia.
At the same time, high DO may result in too small anaerobic zone or incomplete anaerobic environment in activated sludge floc or biofilm, and affect the growth activity of denitrifying bacteria, resulting in poor nitrogen removal efficiency. Low DO will eliminate the nitrifying bacteria and inhibit the nitrification, which will affect the denitrification efficiency of the reactor. Therefore, only by keeping the reasonable DO can make the nitrification and denitrification go on smoothly.
The important conditions for SBBCOR nitrogen removal are intermittent aeration and limited oxygen supply, which made DO change at different period and was uneven in different space, promoted the simultaneous nitrification and denitrification. Due to the limitation of oxygen diffusion, DO gradients are produced in the microbial floc or biofilm, i. e., DO on the outer surface of the microbial floc or biofilm is high, and the aerobic nitrifying bacteria and ammoniating bacteria are dominant. Due to the obstruction of oxygen transfer and the large consumption of external oxygen, the anoxic zone is formed, and denitrifying bacteria are dominant, thus forming a micro-environment conducive to simultaneous nitrification and denitrification. Different zones of anoxic and aerobic zones were formed in the SBBCOR, which provided a favorable environment for the action of denitrifying bacteria and nitrifying bacteria, Nitrification and Denitrification kinetics were formed, and Nitrification and Denitrification were synchronized in the SBBCOR, so as to improve the effect of nitrogen removal.
According to the analysis of nitrogen in the I/II-SBBCOR (shown in Fig. 11, 12), it is considered that the nitrogen in the effluent mainly exists as nitrate (NO3-_N) when DO is high, and mainly exists as ammonia nitrogen (NH4+_N) and nitrite (NO2-_N) when DO is lower. When DO is too high, the denitrification was inhibited, the NO3-_N accumulates more in the effluent, and the nitrogen treatment is inefficient; and when DO is too low, the nitrification was inhibited, the NH4+_N and NO2-_N accumulates more in the effluent, and the nitrogen removal effect is not ideal.
Nitrite bacteria, nitrate bacteria and denitrifying bacteria existed in the SBBCOR, which compete with each other for DO and substrates. In the process of NH4+_N being oxidized to NO2-_N and NO2-_N being oxidized to NO3-, nitrite bacteria and nitrate bacteria compete with each other for oxygen, but the affinity of nitrite bacteria to oxygen is stronger than that of nitrate bacteria, lower DO is beneficial to eliminate nitrate bacteria and inhibit the transformation of NO2-_N to NO3-_N, and will also affect the transformation of NH4+_N to NO2-_N. The process of denitrifying bacteria converting NO2- to Nitrogen(N2) and the process of nitrate bacteria oxidizing NO2-_N to NO3-_N requires a common substrate NO2-, but the former is anaerobic and the latter is aerobic, the lower DO inhibited the transformation of NO2- _N to NO3-_N, and the nitrite bacteria took NH4+_N as the substrate, if too much NH4+_N was oxidized to NO2-, it would lead to the accumulation of NO2-; If the amount of excess NH4+_N is too high, will affect the nitrogen removal performance of the SBBCOR. Therefore, DO will directly affect the number and activity of nitrite bacteria, nitrate bacteria and denitrifying bacteria in SBBCOR, and restrict the transformation relationship among NH4+_N, NO2- _N, NO3-_N and TN, is the critical factor of nitrogen removal efficiency of SBBCOR.
Therefore, because of the DO gradient from the surface to the interior of the biofilm carrier of SBBCOR, with corresponding aerobic, anoxic and facultative zones, enables the simultaneous presence of both aerobic and anaerobic environments in SBBCOR, if nitrification and denitrification kinetics were well controlled, it could be carried out in the same reactor to achieve simultaneous nitrification and denitrification.
3.3 Optimal Experiment of the SBBCOR
3.3.1. Design of the Orthogonal Test
Through the analysis of the long-term series experiments, we found that HRT, SBC and DO are the major influencing factors for the nitrogen treatment effect of the SBBCOR. Based on pre-analysis, an L9 (34) orthogonal test was designed (shown in Table 2), aiming to optimize the operating parameters of the SBBCOR. This orthogonal test was conducted at room temperature (20 ± 5 ° C), with aeration cycle of 60min and aeration rate of 50%, HRT, SBC and DO are shown in Table 2. Each scheme has a testing period of 30 days and sample-taking frequency of one time per day; to ensure the rationality of the experiment and the reliability of the data, two parallel experiments were conducted for each sampling, and the mean of the test data was taken for statistical analysis.
Table 2
Design and statistics of the orthogonal test (L9(34)).
Factor Level | HRT(d) | SBC(h) | DO(mg/L) | Empty Column | Average Removal Rate of NH4+-N (%) | Average Removal Rate of TN (%) |
A | C | B | D |
1 | 1 (3d/1.5d) | 1(6.0h) | 1 (1.5/2.0mg/L) | 1 | 62.13 | 63.89 |
2 | 1 (3d/1.5d) | 2(8.0h) | 2 (2.0/2.5mg/L) | 2 | 69.03 | 63.88 |
3 | 1 (3d/1.5d) | 3(10.0h) | 3(2.5/3.0mg/L) | 3 | 69.63 | 61.03 |
4 | 2 (4d/2d) | 1(6.0h) | 2 (2.0/2.5mg/L) | 3 | 84.65 | 77.32 |
5 | 2 (4d/2d) | 2(8.0h) | 3(2.5 /3.0mg/L) | 1 | 91.36 | 73.68 |
6 | 2 (4d/2d) | 3(10.0h) | 1 (1.5 /2.0mg/L) | 2 | 76.39 | 68.95 |
7 | 3 (6d/3d) | 1(6.0h) | 3(2.5 /3.0mg/L) | 2 | 93.65 | 83.26 |
8 | 3 (6d/3d) | 2(8.0h) | 1 (1.5 /2.0mg/L) | 3 | 86.69 | 76.36 |
9 | 3 (6d/3d) | 3(10.0h) | 2 (2.0 /2.5mg/L) | 1 | 87.69 | 79.65 |
Note: (*/*) denotes the parameters of I/II-SBBCOR. The control of HRT, SBC, DO were mainly realized by controlling the flowmeter, the automatic inlet and outlet water device, the aeration device, and the stirring device in the reactor. Attention was paid to the real-time monitoring of the flow rate, volume load, aeration time ratio and so on, so as to control various parameters reasonably. |
3.3.2. Statistical Analysis of the Orthogonal Test
The statistical analysis of the orthogonal test of the SBBCOR is shown in Table 3.
Table 3
The statistics of the SBBCOR’s nitrogen removal performance of the orthogonal test.
| A | B | C | D | NH4+-N Removal Rate | TN Removal Rate |
NH4+-N Removal Rate | Kj1 | 200.79 | 240.43 | 225.21 | 241.18 | K = 721.22 P = 57795.37 Q = 58796.33 | |
Kj2 | 252.40 | 247.08 | 241.37 | 239.07 |
Kj3 | 268.03 | 233.71 | 254.64 | 240.97 |
Qj | 58620.82 | 57825.16 | 57940.18 | 57796.27 |
Sj2 | 825.46 | 29.79 | 144.82 | 0.90 | ST2 = 1000.97 | |
TN Removal Rate | Kj1 | 188.80 | 224.47 | 209.20 | 217.22 | | K = 648.02 P = 46658.88 Q = 47155.67 |
Kj2 | 219.95 | 213.92 | 220.85 | 216.09 | |
Kj3 | 239.27 | 209.63 | 217.97 | 214.71 | |
Qj | 47091.19 | 46697.76 | 46683.43 | 46659.93 | |
Sj2 | 432.31 | 38.88 | 24.55 | 1.05 | | ST2 = 496.79 |
Note: KjL = sum of 3 trial results relative to level L in column j; K = Sum of all trial results; Qj =(ΣKjl2)/3;P = K2/3; Q = Sum of squares of all trial data; Sj2 = Qj-P༛ST2 = ΣSj2. |
(1) Analysis of Influencing Factors |
Affecting factors of the nitrogen treatment efficiency of the I/II-SBBCOR were determined by using extremum difference analysis (shown in Table 4).
The analysis of variance for the significance of treatment efficiency of NH4+-N showed that Sj2A > Sj2C>Sj2B, the significance order of NH4+-N treatment efficiency from highest to lowest was: HRT, DO, SBC.
The analysis of variance for the significance of treatment efficiency of TN showed that Sj2A > Sj2B>Sj2C. The significance order of TN treatment efficiency from highest to lowest was: HRT, SBC, DO.
(2) Significance Test of Influencing Factors
Variance analysis was used to investigate whether there is a significant difference in the treatment efficiency of the I/II-SBBCOR under various factors and levels (shown in Table 4). The significance of the influencing factors is tested at the significance level of α = 0.10, 0.05( F0.90 (2,2) = 9, F0.95 (2,2) = 19). Orthogonal test of NH4+-N and TN treatment performance showed that FA, FB, FC > F0.95 (2,2) > F0.90 (2,2), HRT, SBC, DO have significant impact on the NH4+-N and TN removal efficiency.
Table 4
Square deviation analysis of I/II-SBBCOR’s removal performance of the orthogonal test.
Factor | Freedom | Variance Analysis of NH4+-N Removal Rate | Variance Analysis of TN Removal Rate |
Mean Square Sum | F | Significance | Mean Square Sum | F | Significance |
A | 2 | 825.46 | 916.47 | ** | 432.31 | 410.36 | ** |
B | 2 | 29.79 | 33.08 | ** | 38.88 | 36.91 | ** |
C | 2 | 144.82 | 160.79 | ** | 24.55 | 23.30 | ** |
error | | 0.90 | | | 1.05 | | |
Note: F = Sj2(3 − 1)/[Se2(3 − 1)]。Se2 is the sum of the squares of the within-group differences;* indicates significant under conditions of α = 0.10, ** indicates significant under conditions of α = 0.05. |
3.3.3. Determination of Optimal Parameters
(1)Determination of Single Operating Parameters
If the purpose is to remove NH4+-N, the best factor level combination is A3B1C3, the operating parameters are “HRT = 6d/3d, SBC = 6 h, DO = 2.5 /3.0mg/L”; If TN removal is the goal, the best factor level combination is A3B1C2; the optimal operating parameters are “HRT = 6d/3d, SBC = 6 h, DO = 2.0 /2.5mg/L ”.
(2)Determination of Overall Operating Parameters
In production practice, the main problem and key purpose of the I/II-SBBCOR is to enhance the total nitrogen removal efficiency; in order to achieve an ideal nitrogen treatment effect and minimize operating costs, the optimal operating parameters of the I/II-SBBCOR are: HRT = 6d/3d, SBC = 6 h, DO = 2.0 /2.5mg/L. Under this operating conditions, the NH4+-N and TN removal rates of the I/II-SBBCOR are usually more than 94% and 84% at normal temperature, the NH4+-N and TN of the effluent drops to 1/4 and 1/3 of that of prototype (the biological contact oxidation treatment project of leachate from a municipal solid waste comprehensive disposal site in Chongqing), which is more conducive to the subsequent advanced treatment processing.