A Comparative and Optimal Experiment of a Sequential Batch Enhanced Pre-Treatment Reactor without Sludge Retention

: This article focuses on the limitations of the traditional treatment technology of leachate, based on the principles of reaction kinetics and analysis of biological treatment technology, and conducts a comparative study of the synchronization experiments of a sequential batch enhanced pre-treatment reactor without sludge retention (SBER for short), sequential batch reactor (SBR for short), and continuous stirred tank reactor (CSTR for short). It has been conﬁrmed that the SBER has higher and more stable pre-treatment efﬁciency for organic matter and nutrients, signiﬁcantly improves the biodegradability and nutrient ratio of pre-treatment, and creates favorable conditions for subsequent aerobic biological treatment. The further orthogonal test conﬁrmed that dissolved oxygen (DO for short), hydraulic retention time (HRT for short), pH, and sequencing period were signiﬁcant inﬂuencing factors on the treatment efﬁciency of the SBER, and the optimal operating parameters of the SBER were obtained: aeration time (T a for short) = 8 h/d, HRT = 4 d, pH = 8.0, and sequencing period (T s ) = 6 h.


Introduction
In order to solve thorny problems of high concentration of nitrogen, low carbonnitrogen ratio, and poor biodegradability in the treatment of landfill leachate [1][2][3][4][5], the leachate treatment project of Chongqing Comprehensive Disposal Site adopted pre-treatment and contact oxidation technology (shown in Figure 1).However, significant problems were exposed: firstly, although the continuous stirred tank reactor (CSTR for short) pretreatment process is suitable for the removal of organic matter, which is easily degradable, and ammonia nitrogen (NH 4 + -N for short), the concentration of organic matter was difficult to degrade and total nitrogen (TN for short) remains high, while the carbon-nitrogen ratio (C/N ratio for short) of the effluent is too low with poor biodegradability; secondly, nutrient imbalance of the pre-treatment effluent affects the microbial activity of the subsequent aerobic biological reaction tank, further affecting the overall treatment efficiency.After that, a sequential batch reactor (SBR for short) was adopted to improve the pre-treatment process, and the C/N ratio and biodegradability of the pre-treatment effluent were improved, but there were still shortcomings: firstly, the improvement of TN removal efficiency is limited, and C/N ratio is still low; secondly, the increased amplitude of biochemical oxygen demand (BOD 5 for short)-chemical oxygen demand (COD Cr for short) ratio is not significant, and the biodegradability is not ideal; thirdly, the treatment effect is unstable, which is not conducive to the subsequent operation of aerobic biological treatment.Considering that the pre-treatment effects of both the pre-treatment processes of the traditional continuous stirred tank reactor (CSTR for short) and conventional sequential batch reactor (SBR for short) are not ideal, and based on the principles of reaction kinetics and the technology of biological treatment, this study propose the concept of a "Sequential Batch Enhanced Pre-treatment Reactor (SBER for short)" by conducted a long-term series of experiments and carefully summarized experience.
The SBER adopts sequential batch operation, intermittent aeration, without sludge retention and controlling the pH, utilizes a micro-oxygen and aerobic alternating environment to promote hydrolysis and acidification, and degrades the large molecule refractory organic matter into soluble small molecule substances to improve the biodegradability of the effluent.It also controls the solid retention time (SRT for short) reasonably and promotes the selection of dominant bacterial species, and thereby promotes shortcut nitrification and denitrification, improves the removal efficiency of NH4 + -N and TN, optimizes the C/N ratio of pre-treatment effluent, and provides favorable conditions for subsequent aerobic treatment [6][7][8][9].

The Basic Characteristics of the SBER
The design purpose of the SBER is to degrade organic compounds with large molecules as much as possible, promote shortcut nitrification and denitrification, optimize the biodegradability and nutrient ratio of the effluent while ensuring treatment efficiency, and provide favorable conditions for subsequent biological treatment, through sequential batch inflow, intermittent aeration, the removal of mixed liquid, and reasonable control of pH value.Therefore, compared with the traditional SBR and CSTR pre-treatment, the SBER's operation method has the following characteristics: firstly, micro-oxygen and anoxic intermittent operation.The SBER achieves alternating changes in the state of microoxygen (0.5~1.0 mg/L) and anoxic (0.2~0.5 mg/L) in the reactor through intermittent low intensity aeration; secondly, the control of a large gradient of concentration.In order to achieve a large gradient of concentration of substrate and microorganisms in the reactor, Considering that the pre-treatment effects of both the pre-treatment processes of the traditional continuous stirred tank reactor (CSTR for short) and conventional sequential batch reactor (SBR for short) are not ideal, and based on the principles of reaction kinetics and the technology of biological treatment, this study propose the concept of a "Sequential Batch Enhanced Pre-treatment Reactor (SBER for short)" by conducted a long-term series of experiments and carefully summarized experience.
The SBER adopts sequential batch operation, intermittent aeration, without sludge retention and controlling the pH, utilizes a micro-oxygen and aerobic alternating environment to promote hydrolysis and acidification, and degrades the large molecule refractory organic matter into soluble small molecule substances to improve the biodegradability of the effluent.It also controls the solid retention time (SRT for short) reasonably and promotes the selection of dominant bacterial species, and thereby promotes shortcut nitrification and denitrification, improves the removal efficiency of NH 4 + -N and TN, optimizes the C/N ratio of pre-treatment effluent, and provides favorable conditions for subsequent aerobic treatment [6][7][8][9].

The Basic Characteristics of the SBER
The design purpose of the SBER is to degrade organic compounds with large molecules as much as possible, promote shortcut nitrification and denitrification, optimize the biodegradability and nutrient ratio of the effluent while ensuring treatment efficiency, and provide favorable conditions for subsequent biological treatment, through sequential batch inflow, intermittent aeration, the removal of mixed liquid, and reasonable control of pH value.Therefore, compared with the traditional SBR and CSTR pre-treatment, the SBER's operation method has the following characteristics: firstly, micro-oxygen and anoxic intermittent operation.The SBER achieves alternating changes in the state of micro-oxygen (0.5~1.0 mg/L) and anoxic (0.2~0.5 mg/L) in the reactor through intermittent low intensity aeration; secondly, the control of a large gradient of concentration.In order to achieve a Water 2023, 15, 4072 3 of 12 large gradient of concentration of substrate and microorganisms in the reactor, the SBER receives and discharges water immediately, with a relatively short settling time and without sludge retention; thirdly, the control of a moderate pH value.The optimal pH value for nitrite bacteria is 7.0-8.5, and the optimal pH value for nitrate bacteria is 6.5-7.5.A controlled pH between 7.5 and 8.5 is conducive to achieving shortcut nitrification and denitrification; fourthly, the control of shorter SRT.The SBER significantly improves biological activity by eliminating the mixture and, without sludge retention, controlling a shorter sludge age, promoting the selection and separation of dominant strains.Therefore, the SBER has higher removal efficiency, greater improvement in biodegradability, and a more reasonable nutrition ratio of effluent.

The Theory of the Biochemical Reaction of the SBER
Sequential batch influent has the advantages of both the continuous stirred tank reactor (CSTR) and plug-flow reactor (PFR), with a high driving force for biochemical reactions [10][11][12][13][14][15][16][17][18].According to the principles of biochemical reaction kinetics, in a fully mixed pre-treatment reactor with continuous flow, the distribution of substrate, microorganisms, and dissolved oxygen is uniform; however, due to the low gradient of substrates and microorganisms, the driving force of biochemical reactions is relatively small, and the reaction rate and removal efficiency are relatively low.In a push-flow reactor, the maximum gradient and biochemical reaction driving force of the substrate and microorganisms are maintained [19][20][21][22].During the aeration and stirring stages of the SBER, the substrate concentration and microbial concentration show a mixed state in space.During the entire run cycle, the SBER exhibits a push-flow state over time, and therefore it has both the advantages of the CSTR and PFR.Moreover, compared with the SBR pre-treatment process, the SBER has a longer aeration time and shorter SRT control time, and can present a fully mixed state during the aeration stage.The gradient of concentration is larger, presenting a more ideal mixing type in space and a more ideal push-flow type in time, and this is incomparable with SBR and CSTR pre-treatment processes.

The Theory of Hydrolysis Acidification of the SBER
The SBER's periodic inflow, without sludge retention, and intermittent aeration promote bacterial separation, resulting in high sludge activity and hydrolysis acidification effect.
Firstly, during the SBER's sequencing batch cycle, the concentration of organic matter is high in the initial stage of influent, creating nutritional conditions for the hydrolysis acidification reaction; at the end of the reaction, the facultative hydrolysis bacteria are in a state of hunger [23][24][25][26][27].This alternating environmental condition of "feast" and "hunger", as well as the reasonable control of the SRT, can promote the growth of hydrolysis bacteria and fermentation bacteria, which is conducive to improving the biological activity of facultative hydrolysis bacteria.
Secondly, intermittent aeration and stirring provide the alternating environment of micro-oxygen and anoxic for the SBER.The micro-oxygen conditions can avoid the excessive accumulation of dissolved organic matter and provide convenient conditions for the next stage of anaerobic hydrolysis [27][28][29][30][31][32]; the anoxic state can inhibit the rapid removal of dissolved organic matter and promote hydrolysis, thereby improving the biodegradability of the effluent.At the same time, intermittent aeration with micro-oxygen and anoxic in the SBER results in periodic changes in DO concentration, pH value, and oxidation-reduction potential (ORP for short) in the reactor, especially during the non-aeration stage when DO concentration and ORP decrease and pH value slightly increases, creating more favorable reaction conditions for hydrolysis and acidification.Through the action of extracellular enzymes from the acid-producing bacteria, complex macromolecular organic matter is promoted to be converted into simple soluble small molecules [33][34][35][36][37], promoting facultative or specialized acid-producing bacteria to convert hydrolysis products into neutral compounds such as short-chain organic acids, alcohols, aldehydes, etc. Carbohydrates are degraded into fatty acids while increasing the proportion of volatile fatty acids, further optimizing the biodegradability of effluent.
Thirdly, the SBER achieves no sludge retention by removing the mixed liquid and controlling a shorter SRT.The generation time of heterotrophic bacteria and facultative hydrolytic bacteria is shorter, and controlling shorter SRT is conducive to the growth and reproduction of heterotrophic bacteria and hydrolytic bacteria.Therefore, the SBER has higher sludge activity, better pre-treatment efficiency, and a strong impact load resistance, creating good conditions for subsequent aerobic treatment.

The Theory of Nitrification and Denitrification of the SBER
By controlling the batch cycle, SRT, DO, and pH, the SBER achieves substrate sharing and acid-base balance and enhances the ability of nitrification and denitrification.
Firstly, the inhibitory concentrations of free ammonia on nitrite and nitrate bacteria are 10-150 mg/L and 0.1-10 mg/L, respectively.The SBER can increase the concentration of NH 4 + -N and form a certain concentration of free ammonia in the early stage of influent water by strengthening periodic batch influent, inhibiting the growth of nitrate bacteria, and effectively promoting shortcut nitrification and denitrification [11,12].
Secondly, the SBER utilizes the characteristic that the specific growth rate of nitrite bacteria is higher than that of nitrate bacteria when the temperature is greater than 20 • C. By controlling a shorter SRT, nitrate bacteria with a longer generation time can be washed out of the SBER, thereby promoting shortcut nitrification and denitrification.
Thirdly, the SBER utilizes the character of the oxygen affinity of nitrite bacteria to higher than that of nitrate bacteria to limit the growth of nitrate bacteria by controlling low levels of DO.Through periodic aeration, DO in the SBER at the aeration stage is controlled at a micro-oxygen level (approximately 0.8 mg/L) and NH 4 + -N is accumulated with oxidized and nitrate (nitrite) bacteria, providing a substrate for the next stage of denitrification; when stopping aeration and stirring, the SBER is in an anoxic state (approximately 0.2 mg/L).The denitrifying bacteria use carbon as electron donors to reduce nitrate (nitrite) to nitrogen, thereby avoiding the excessive accumulation of TN and facilitating the next cycle of shortcut nitrification and denitrification.Therefore, intermittent aeration and micro-anoxic periodic changes may control the nitrification reaction in the nitrite stage and utilize the influent organic carbon source for denitrification, which not only saves oxygen consumption and the organic carbon source, but also provides the possibility for achieving shortcut nitrification and denitrification and significantly improves pre-treatment efficiency.
At the same time, the optimal pH ranges for nitrite bacteria and nitrate bacteria are 7.0-8.5 and 6.5-7.5, respectively.Controlling pH between 7.5 and 8.5 is conducive to achieving shortcut nitrification and denitrification.Nitrification produces acid, denitrification produces alkali, and through nitrification and denitrification in the same reactor, the SBER achieves internal acid-base balance from a spatial perspective.It not only has better ammonia and nitrogen removal efficiency, but also has strong impact load resistance and stable effluent.

Comparative Experiment of Pre-Treatment Processes
To scientifically verify the effectiveness of the SBER, synchronous comparative experiments were conducted on the CSTR, SBR, and SBER pre-treatment processes.

Testing the Water Quality
In the early stage of sludge cultivation, diluted leachate was used as the substrate, and an appropriate amount of glucose was added.BOD5/CODCr was controlled to be no less than 0.5, CODCr/NH4 + -N was controlled to be no less than 10, and the pH was controlled between 7.5 and 8.5, providing a reasonable nutrient ratio and acid-base environment for microorganisms.Afterwards, the dilution ratio and glucose input were gradually reduced until all the influent water was leachate.During the entire start-up period of the experiment, CODCr, NH4 + -N, TN, and TP of the influent were 4060~4990 mg/L, 221~1520 mg/L, 290~2050 mg/L, and 1.80~20.50mg/L, respectively.The volumetric loads of CODCr were increased from 0.80 kg/(m 3 • d) to 1.20 kg/(m 3 • d), while NH4 + -N volumetric loads were increased from 0.04 kg/(m 3 • d) to 0.36 kg/(m 3 • d).

Operation Mode
Under the same influent concentration, temperature (20 ± 5 °C), HRT (4 d), pH (7.5~8.5), and relevant conditions, three reactors operated synchronously in different ways: the SBER, sequencing batch influent, intermittent aeration (with DO controlled at 0.8 ± 0.2 mg/L during aeration), stirring, and discharging supernatant, with the sequencing batch cycle of 12 h, and the aeration cycle of 2 h, accounting for 60% of the aeration time, and controlling sludge age to improve microbial activity; the SBR, sequencing batch feeding, intermittent aeration (with DO controlled at 2.0 ± 0.2 mg/L during aeration), sedimentation, and discharging supernatant, with the sequencing batch cycle of 6 h and the aeration time accounting for 30%; the CSTR, operating with continuous influent and aeration (DO = 1.0 ± 0.2 mg/L).

Comparison of Removal Effects
The synchronous comparative experiments show that the SBER has higher removal efficiency and more stable effluent quality compared with the SBR and CSTR.After 28 days of domestication, CODCr removal rate of the SBER increased to 65.85% and tended to be stable (shown in Figure 3).After three weeks of start-up, NH4 + -N removal rates of

Testing the Water Quality
In the early stage of sludge cultivation, diluted leachate was used as the substrate, and an appropriate amount of glucose was added.BOD 5 /COD Cr was controlled to be no less than 0.5, COD Cr /NH 4 + -N was controlled to be no less than 10, and the pH was controlled between 7.5 and 8.5, providing a reasonable nutrient ratio and acid-base environment for microorganisms.Afterwards, the dilution ratio and glucose input were gradually reduced until all the influent water was leachate.During the entire start-up period of the experiment, COD Cr , NH 4 + -N, TN, and TP of the influent were 4060~4990 mg/L, 221~1520 mg/L, 290~2050 mg/L, and 1.80~20.50mg/L, respectively.The volumetric loads of COD Cr were increased from 0.80 kg/(m 3 •d) to 1.20 kg/(m 3 •d), while NH 4 + -N volumetric loads were increased from 0.04 kg/(m 3 •d) to 0.36 kg/(m 3 •d).

Operation Mode
Under the same influent concentration, temperature (20 ± 5 • C), HRT (4 d), pH (7.5~8.5), and relevant conditions, three reactors operated synchronously in different ways: the SBER, sequencing batch influent, intermittent aeration (with DO controlled at 0.8 ± 0.2 mg/L during aeration), stirring, and discharging supernatant, with the sequencing batch cycle of 12 h, and the aeration cycle of 2 h, accounting for 60% of the aeration time, and controlling sludge age to improve microbial activity; the SBR, sequencing batch feeding, intermittent aeration (with DO controlled at 2.0 ± 0.2 mg/L during aeration), sedimentation, and discharging supernatant, with the sequencing batch cycle of 6 h and the aeration time accounting for 30%; the CSTR, operating with continuous influent and aeration (DO = 1.0 ± 0.2 mg/L).

Comparison of Removal Effects
The synchronous comparative experiments show that the SBER has higher removal efficiency and more stable effluent quality compared with the SBR and CSTR.After 28 days of domestication, COD Cr removal rate of the SBER increased to 65.85% and tended to be stable (shown in Figure 3).After three weeks of start-up, NH 4 + -N removal rates of the SBER increased to 53.26%, while TN removal rates increased to 44.09%, higher than those of the SBR and significantly higher than those of the CSTR (shown in Figures 4 and 5).
the SBER increased to 53.26%, while TN removal rates increased to 44.09%, higher than those of the SBR and significantly higher than those of the CSTR (shown in Figures 4 and  5).After 50 days of cultivation, all reactors were successfully started.The average CODCr, NH4 + -N, and TN in the SBER effluent were approximately 1560 mg/L, 460 mg/L, and 700 mg/L, and the removal rates of CODCr, NH4 + -N, and TN were stable at approximately 66%, 68%, and 62%; the removal rates of CODCr, NH4 + -N, and TN in the SBR are between 60 and 68%, 60 and 69%, and 30 and 40%; the removal rates of CODCr, NH4 + -N, and TN in the CSTR range from 60% to 67%, 60% to 68%, and 25% to 30%, respectively.The SBER has a higher and more stable removal rate of organic matter and nutrients compared with the SBR and CSTR.
By regression analysis, the cell yield of mixed bacteria in the SBER was 0.4947 gVSS/(gTN•d), which was between the cell yield of the shortcut nitrification-denitrification process and that of the full nitrification-denitrification process.The results showed that there were nitrite bacteria, nitrate bacteria, and denitrifying bacteria in the mixed bacteria.the SBER increased to 53.26%, while TN removal rates increased to 44.09%, higher than those of the SBR and significantly higher than those of the CSTR (shown in Figures 4 and  5).After 50 days of cultivation, all reactors were successfully started.The average CODCr, NH4 + -N, and TN in the SBER effluent were approximately 1560 mg/L, 460 mg/L, and 700 mg/L, and the removal rates of CODCr, NH4 + -N, and TN were stable at approximately 66%, 68%, and 62%; the removal rates of CODCr, NH4 + -N, and TN in the SBR are between 60 and 68%, 60 and 69%, and 30 and 40%; the removal rates of CODCr, NH4 + -N, and TN in the CSTR range from 60% to 67%, 60% to 68%, and 25% to 30%, respectively.The SBER has a higher and more stable removal rate of organic matter and nutrients compared with the SBR and CSTR.
By regression analysis, the cell yield of mixed bacteria in the SBER was 0.4947 gVSS/(gTN•d), which was between the cell yield of the shortcut nitrification-denitrification process and that of the full nitrification-denitrification process.The results showed that there were nitrite bacteria, nitrate bacteria, and denitrifying bacteria in the mixed bacteria.After 50 days of cultivation, all reactors were successfully started.The average COD Cr , NH 4 + -N, and TN in the SBER effluent were approximately 1560 mg/L, 460 mg/L, and 700 mg/L, and the removal rates of COD Cr , NH 4 + -N, and TN were stable at approximately 66%, 68%, and 62%; the removal rates of COD Cr , NH 4 + -N, and TN in the SBR are between 60 and 68%, 60 and 69%, and 30 and 40%; the removal rates of COD Cr , NH 4 + -N, and TN in the CSTR range from 60% to 67%, 60% to 68%, and 25% to 30%, respectively.The SBER has a higher and more stable removal rate of organic matter and nutrients compared with the SBR and CSTR.
By regression analysis, the cell yield of mixed bacteria in the SBER was 0.4947 gVSS/(gTN•d), which was between the cell yield of the shortcut nitrificationdenitrification process and that of the full nitrification-denitrification process.The results showed that there were nitrite bacteria, nitrate bacteria, and denitrifying bacteria in the mixed bacteria.

Comparison of Biodegradability
Biodegradability efficiency of pre-treatment was determined by measuring BOD 5 /COD Cr .The SBER significantly increases BOD 5 /COD Cr by converting macromolecular substances into easily degradable small molecule substances.In the later stage of start-up, when the influent is completely leachate, the average BOD 5 /COD Cr of the SBER increases from the influent's 0.250 to the effluent's 0.440 (shown in Figure 6), and the biodegradability of the effluent is greatly improved.Meanwhile, the average BOD 5 /COD Cr of the effluent of the SBR and CSTR were only 0.324 and 0.275, respectively.Compared with the SBR and CSTR, the BOD 5 /COD Cr of the SBER effluent increased significantly.

Comparison of Biodegradability
Biodegradability efficiency of pre-treatment was determined by measuring BOD5/CODCr.The SBER significantly increases BOD5/CODCr by converting macromolecular substances into easily degradable small molecule substances.In the later stage of startup, when the influent is completely leachate, the average BOD5/CODCr of the SBER increases from the influent's 0.250 to the effluent's 0.440 (shown in Figure 6), and the biodegradability of the effluent is greatly improved.Meanwhile, the average BOD5/CODCr of the effluent of the SBR and CSTR were only 0.324 and 0.275, respectively.Compared with the SBR and CSTR, the BOD5/CODCr of the SBER effluent increased significantly.

Comparison of Nutrient Proportions
By comparing CODCr/NH4 + -N of the effluent from the SBER, SBR, and CSTR, the superiority and inferiority of nutrient ratios of the effluent of the three pre-treatment processes are determined (shown in Figure 7).When CODCr/NH4 + -N of the influent was 3.24~3.51,CODCr/NH4 + -N of the effluent from the SBER, CSTR, and SBR were 3.39~4.03,2.41~2.56,and 2.54~2.78,respectively.The SBER is more capable of providing excellent nutrient ratios for subsequent aerobic biological treatment than the SBR and CSTR.

Comparison of Nutrient Proportions
By comparing COD Cr /NH 4 + -N of the effluent from the SBER, SBR, and CSTR, the superiority and inferiority of nutrient ratios of the effluent of the three pre-treatment processes are determined (shown in Figure 7).When COD Cr /NH 4 + -N of the influent was 3.24~3.51,COD Cr /NH 4 + -N of the effluent from the SBER, CSTR, and SBR were 3.39~4.03,2.41~2.56,and 2.54~2.78,respectively.The SBER is more capable of providing excellent nutrient ratios for subsequent aerobic biological treatment than the SBR and CSTR.
In summary, compared with the SBR and CSTR, the SBER achieved higher hydrolysis acidification and shortcut nitrification and denitrification, significantly improving the biodegradability of effluent, and the nutrition ratio is more reasonable, providing good conditions for subsequent aerobic biological treatment.In this study, the SBER was also compared with other domestic pretreatment processes such as oil-water separation, air flotation separation, chemical softening, and UASB (up-flow anaerobic sludge blanket), and the SBER also has obvious advantages in treatment efficiency, biochemical enhancement, and the optimization of the nutrition ratio [11][12][13].In summary, compared with the SBR and CSTR, the SBER achieved higher hydrolysis acidification and shortcut nitrification and denitrification, significantly improving the biodegradability of effluent, and the nutrition ratio is more reasonable, providing good conditions for subsequent aerobic biological treatment.In this study, the SBER was also compared with other domestic pretreatment processes such as oil-water separation, air flotation separation, chemical softening, and UASB (up-flow anaerobic sludge blanket), and the SBER also has obvious advantages in treatment efficiency, biochemical enhancement, and the optimization of the nutrition ratio [11][12][13].Note: The control of aeration time, HRT, and sequence batch cycle 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, dissolved oxygen, and so on, so as to control various parameters reasonably.The pH value of influent was controlled in the range of 7.4~8.0by adding a buffer solution (Na 2 CO 3 and H 2 SO 4 of 1:5) reasonably.

Statistical Analysis of the Orthogonal Test
The statistical analysis of the orthogonal test of the SBER is shown in Table 2. Affecting factors of the treatment efficiency of the SBER were determined by using extremum difference analysis.The analysis of variance for the significance of COD Cr treatment efficiency showed that S j  2).The significance order from highest to lowest was: DO, HRT, pH, and sequence batch cycle.By analogy, the significance of NH 4 + -N treatment efficiency is ranked in descending order: DO, HRT, sequence batch cycle, and pH; the significance ranking of TN treatment efficiency from highest to lowest is: HRT, pH, sequence batch cycle, and DO.

Significance Test of Influencing Factors
Variance analysis was used to investigate whether there is a significant difference in the pre-treatment efficiency of the SBER under various factors and levels (shown in Table 3).Firstly, the significance of the influencing factors on the efficiency of COD Cr treatment is tested at the significance level of α = 0.05, then F 0 .95 (3,3) = 9.28.F A , F B , F C > F 0 .95 (3,3), F D < F 0 .95 (3,3); therefore, aeration time, HRT, and pH have a significant impact on the COD Cr removal efficiency, while the sequence batch cycle is not significant.DO, HRT, pH, and sequence batch cycle have a significant impact on the treatment efficiency of NH 4 + -N and TN.If TN removal is the goal, the best factor level combination is A 2 B 3 C 4 D 2 ; the optimal operating parameters are aeration time Ta = 8 h/d, HRT = 4 d, pH = 8.0, and sequencing cycle Ts = 6 h.

Determination of Overall Operating Parameters
In production practice, the purpose of the SBER is to improve the biodegradability of leachate, enhance denitrification efficiency, and provide favorable conditions for subsequent aerobic biological treatment.To achieve an ideal treatment effect and minimize operating costs, the optimal operating parameters of the SBER are: aeration time Ta = 8 h/d, HRT = 4 d, pH = 8.0, and sequencing cycle Ts = 6 h.Under these operating conditions, the COD Cr , NH 4 + -N, and TN removal rates of the SBER are usually maintained at 68%, 68%, and 62%, with BOD 5 /COD Cr increasing from an average of influent 0.25 to effluent 0.48, and COD Cr /NH 4 + -N increasing from an average of influent 3.25 to effluent 3.75.The biodegradability and carbon-nitrogen ratio have been further optimized, providing good nutrient ratios and reaction conditions for subsequent aerobic biological treatment.

Conclusions
Through the analysis of the synchronous comparative experiment, the conclusion can be drawn that the SBER adopts sequential batch influent, intermittent aeration, mixed liquid removal, and pH control to promote hydrolysis reaction and shortcut nitrification and denitrification as much as possible, improves the biodegradability and nutrient ratio of pre-treatment effluent, and creates favorable conditions for subsequent aerobic biological treatment.The orthogonal test further confirmed that DO, HRT, pH, and sequencing cycle have significant effects on the treatment efficiency of the SBER; the optimal operating

Figure 1 .
Figure 1.Leachate treatment process of Chongqing Comprehensive Disposal Site.

Figure 1 .
Figure 1.Leachate treatment process of Chongqing Comprehensive Disposal Site.

3. 1 .
Test Method 3.1.1.Test Model Device Based on the leachate treatment project of Chongqing Comprehensive Disposal Site, the experimental model (shown in Figure 2) was designed by using similar conditions that maintain the Froude number constant: similarity ratio of the prototype and model: λ l = 30, effective volume of the model: 0.016 m 3 , and size of the model: 0.2 m × 0.2 m × 0.4 m. maintain the Froude number constant: similarity ratio of the prototype and model: λl = 30, effective volume of the model: 0.016 m 3 , and size of the model: 0.2 m × 0.2 m × 0.4 m.

Figure 2 .
Figure 2. Model of pre-treatment processes and the biology contact oxidation reactor.

Figure 2 .
Figure 2. Model of pre-treatment processes and the biology contact oxidation reactor.

Figure 3 .
Figure 3.Comparison of COD Cr treatment efficiency.

Figure 5 .
Figure 5.Comparison of TN treatment efficiency.

Figure 5 .
Figure 5.Comparison of TN treatment efficiency.

Figure 7 .
Figure 7.Comparison of carbon-nitrogen ratios in effluent.

2 A > S j 2 B > S j 2 C > S j 2 D
(shown in Table

4. 3 .
Determination of Optimal Parameters 4.3.1.Determination of Single Operating Parameters If the purpose is to remove COD Cr , the best factor level combination is A 4 B 4 C 4 D 4 ; the operating parameters of the SBER can be controlled to aeration time Ta = 12 h/d, HRT = 5 d, pH = 8.0, and sequencing cycle Ts = 10 h.If the purpose is to remove NH 4 + -N, the best factor level combination is A 4 B 3 C 4 D 2 ; the operating parameters are aeration time Ta = 12 h/d, HRT = 4 d, pH = 8.0, and sequencing cycle Ts = 6 h.

Table 2 .
The statistics of the SBER's removal performance of the orthogonal test.

Table 3 .
Square deviation analysis of the SBER's removal performance of test.