1. Introduction
The acceleration of urbanization has prompted China’s urban sewage discharge volume in 2020 to reach 57.14 billion m3/year [1]. Wastewater treatment plants (WWTPs) have been operating at high load for a long time, seriously affecting the quality of the effluent [2]. Wastewater treatment has become a significant challenge in ecological construction. Improving the efficiency of wastewater treatment plants and reducing the effective concentration of nitrogen, phosphorus, and other pollutants in the effluent are effective ways to solve this problem [3,4]. Currently, oxidation ditches and A2/O processes are widely used in urban WWTPs both domestically and internationally due to their high efficiency of simultaneous nitrogen and phosphorus removal from wastewater, low operating costs, regional selectivity, stable operation, and good shock load resistance [2,5,6]. The oxidation ditches process consists of a groove body, aeration equipment, inlet device, flow diversion, and mixing equipment. The sewage and activated sludge show a circulating flow that connects the head to the tail in the aeration area, with a long hydraulic residence time and low organic load. While the A2/O process is an anaerobic–hypoxia–aerobic biological process. The organic combination of three different conditions and different types of microbial flora can simultaneously remove organic matter, as well as nitrogen and phosphorus [7].
The wastewater treatment efficiency of oxidation ditches and A2/O processes is significantly dependent on the nitrogen and phosphorus concentration in the wastewater. For example, the removal efficiencies of various nutrients in the effluent from oxidation ditches were consistently above 90% when treating the wastewater with high nitrogen and phosphorus concentrations, which achieved the high requirements of the discharge standard [8]. In contrast, the treatment efficiency of the A2/O process is generally poor, especially the lower efficiency of nitrogen and phosphorus removal [9,10]. In order to solve the above problems of the A2/O process, previous studies constructed an A3/O structure on the basis of the traditional A2/O process by adding a pre-denitrification zone or building the integrated multi-tank anaerobic-anoxic/oxic-A2/O process and implemented a real-time control strategy, which used the “phased isolation tank” technology to change the aeration and water flow path to achieve automatic circulation in eight stages [11,12]. Additionally, some factors, such as the C/N ratio and sludge microorganisms in the wastewater, also affect the treatment efficiency of these two processes to varying degrees [4,6]. Furthermore, the environmental background of WWTPs is also an important factor influencing the treatment efficiency of these two processes besides the characteristics of the wastewater [3,13,14]. It could indicate that the treatment efficiency of these two processes might differ significantly in treating wastewater with different pollutant characteristics. Therefore, selecting an appropriate treatment process for urban wastewater with different pollutant characteristics is crucial to improve treatment efficiency, cost reduction, and achievement of stable and compliant sewage discharge.
In order to clarify the main differences in wastewater treatment and explore a more suitable wastewater treatment process for urban wastewater, characteristics between oxidation ditches and modified high-efficiency multi-cycle A2/O processes should be analyzed. This study selected Ningyang’s sewage treatment plant as a research object, comprehensively analyzed the nutrient removal efficiency, and compared the sludge volume index (SVI) of oxidation ditches and high-efficiency multi-cycle A2/O processes under the condition of consistent influent water quality. The aims of this work are as follows: (1) elucidate the main differences in treatment efficiency of nutrients (COD, NH4+-N, TN, and TP) between the two processes; (2) provide scientific evidence for ensuring the region treatment efficiency and improving the effluent quality; and (3) comprehensively analyze the two processes costs and operating costs.
2. Materials and Methods
2.1. Wastewater Treatment Processes
The Ningyang sewage treatment plant (116°8′26.828″ E and 35°33′27.192″ N) was built in two phases. The wastewater treatment process for Phase I is as follows: coarse screen and pumping station + fine screen and sedimentation tank + anoxic tank + A/O (oxidation ditch) + secondary sedimentation tank + intermediate pumping station + grid flocculation sedimentation tank + fiber bundle filter + disinfection process. The treatment process for Phase II is as follows: coarse screen and pumping station + fine screen and cyclone sedimentation tank + high-efficiency multi-cycle A2/O tank + secondary sedimentation tank + intermediate pumping station + grid flocculation sedimentation tank + V-shaped filter + disinfection process. The effluent achieves Grade A according to the “Pollutant Discharge Standard for Urban Wastewater Treatment Plants (GB18918-2002, China)” [15].
As shown in Figure 1A, the oxidation ditch process of Phase I uses meter exposure, and the aeration equipment is a rotary disk, with a total of 8 units, with a speed of 55–65 rpm. Based on the conventional A2/O process, the tank shape was modified to form a high-efficiency multi-cycle A2/O process (Figure 1B), including the addition of guide walls and pushers in the aerobic section and the division of the aerobic section into multiple groups. The main advantages of this modified A2/O process are as follows: (1) creating a completely mixed mixture + plug flow mode throughout the biological section; (2) using gas flow meters and dissolved oxygen (DO) detectors (which measure the concentration of dissolved oxygen in water) to provide feedback and control the air supply from blowers, forming a gradient dissolved oxygen region in the aerobic section to reduce energy consumption; and (3) increasing the anoxic time and reducing the dissolved oxygen in the aerobic section to achieve efficient denitrification based on the influent water quality with the low C/N ratio (the mass ratio of carbon to nitrogen in wastewater).
The design and actual influent conditions and water quality characteristics of the wastewater treatment plant are described in Table 1. The designed influent capacity is 40,000 t/d for Phase I and 20,000 t/d for Phase II. The actual average influent values are 33,423.6 t/d and 20,685.1 t/d, respectively. The actual range of influent for Phase I and Phase II is 21,155–45,778 t/d (p < 0.001) and 8793–32,281 t/d (p = 0.0034), respectively. In terms of water quality, the designed influent values for COD, NH4+-N, TN, and TP are 450, 35, 40, and 4 mg/L, respectively, while the actual average values are 400.8, 22.73, 34.09, and 4.01 mg/L. The actual range of these nutrients is 95.3–964, 14.9–29.0, 17.4–63.2, and 1.02–16.5 mg/L, respectively. Except for TP (p = 0.95), all other water quality indicators show significant differences from the designed values (p < 0.001).
As shown in Table 2, the hydraulic retention times (HRTs) for the anaerobic, anoxic, and aerobic zones in Phase I were 3.02, 5.53, and 17.95 h, respectively. While in Phase II, the HRTs were 1.51, 3.02, and 14.92 h, respectively. The sludge return ratio was 100% in both phases. The total nitrogen load was 0.011 kgTN/(kgMLSS·h) in Phase I and 0.005 kgTN/(kgMLSS·h) in Phase II. The surface load of the secondary sedimentation tank was 1.04 m3/(m2·h) in Phase I and 0.62 m3/(m2·h) in Phase II. The nitrified liquid return ratios were 250% and 200% for Phase I and Phase II, respectively.
2.2. Water Quality and Sludge Index Monitoring Methods
The wastewater quality monitoring included five key indicators: COD, TN, NH4+-N, TP, and SVI. The water samples were taken from the sampling sites per day from 1 January 2022 to 30 December 2022, filtered using a 0.22 μm membrane, and stored at 4 °C. The sampling sites were selected at the mixing well of influent and the outlet of the secondary sedimentation tank, and sludge samples were taken from the mixed liquid at the end of the aerobic tank. The COD was measured using the dichromate method (HJ 828-2017) [16], TN was determined through alkaline potassium persulfate digestion with UV spectrophotometry (HJ 636-2012) [17], NH4+-N was analyzed using the Nessler reagent spectrophotometry method (HJ 533-2009) [18], TP was assessed through potassium persulfate digestion and molybdate antimony resistance photometry (GB11893-89) [19], and the SVI was calculated using the sedimentation method (CJ/T221-2023) [20].
2.3. Statistical Analysis
A two-tailed Student’s t-test was used to quantify the differences in five key wastewater quality indexes during the one-year operation of wastewater treatment processes in Phase I and Phase II, including COD, NH4+-N, TN, TP, and SVI. In addition, in order to quantify the wastewater treatment efficiency of Phase I and Phase II, the removal rates of five key wastewater quality indexes were calculated, respectively. The calculation formula is as follows:
where i is the sewage quality index, Ii is the influent concentration of i, Oi is the effluent concentration of i.Similarly, a two-tailed Student’s t-test was used to quantify the differences in the removal rates of five key wastewater quality indexes during the one-year operation of wastewater treatment processes in Phase I and Phase II. Statistical analysis was performed in SPSS 19.0.
3. Results and Discussion
3.1. The Effect of Two Processes on COD Removal
In urban wastewater, the concentration of COD reflects the pollution status of organic pollutants in the wastewater. After one year of continuous monitoring, the COD concentration of the wastewater in this area was relatively high and had significant annual fluctuations, with the annual average value approaching the design value. The results indicated that the wastewater in this area was characterized by severe organic pollution. The treatment of organic pollutants was key to wastewater treatment in this area. The high variation in the concentration of COD in this WWTP might be related to the influent water quantity, water quality fluctuation (118–997 mg/L), temperature change (high treatment efficiency in summer, low efficiency in winter), sludge concentration control, process operation, and other factors [21]. As shown in Figure 2, compared with the COD concentration and removal rate of the effluent from Phase I and Phase II, it was found that the COD concentration of the effluent from Phase II was 49.9% lower than that of Phase I and the COD removal rate was 27.4% higher than that of Phase I. Although the COD removal rate of the effluent from Phase I was higher than that of previous related studies, its annual average concentration was significantly higher (p < 0.001) than the designed effluent concentration, which failed to reach Grade A standard according to the “Pollutant Discharge Standard for Urban Wastewater Treatment Plants (GB 18918-2002, China)” [5,15,22]. Whereas the annual average COD concentration of the effluent from Phase II was far below the designed effluent concentration and had achieved a Grade A discharge standard (Table 3). Therefore, the COD removal effect of the Phase II process was superior to that of the Phase I process. This is related to the use of rotary disk aeration in Phase I, and the use of bottom aeration in Phase II. The bottom aeration efficiency is higher and Phase II adopts a push flow + mixed flow state, which makes the pollutants and microorganisms more sufficient [23].
3.2. The Effect of Two Processes on NH4+-N Removal
Nitrogen is an important nutrient in wastewater and a major substance causing eutrophication of water bodies [24]. The annual average concentration of NH4+-N in the wastewater in this area was significantly lower (p < 0.001) than the design value. Additionally, compared to other areas, the NH4+-N concentration in this area was lower, indicating that the wastewater in this area does not have a serious NH4+-N pollution problem [7,25,26]. As shown in Figure 3, compared with the NH4+-N concentration and removal rate of the effluent from Phase I and Phase II, it was found that there was no significant difference (p > 0.05) between the effluent from Phase I and Phase II, and both basically had reached a Grade A discharge standard. Similarly, previous studies have found that whether it was an oxidation ditch or a highly efficient multi-cycle A2/O process, the removal rate of NH4+-N in wastewater was over 90% [3,4,5,6,27]. Therefore, there was no significant difference in the treatment of NH4+-N in wastewater between the two processes (Table 3).
3.3. The Effect of Two Processes on TN Removal
According to the TN removal efficiency requirements, based on the A2/O process biological nitrogen and phosphorus removal specifications in the “Standard for design of outdoor wastewater engineering” (GB 50014-2021, China) [28], the HRT of the anoxic zone needs to be 3 h. The HRT of the anoxic zone for Phase II was 3.03 h in actual operation, while the HRT was 5.53 h for Phase I. The TN concentrations of the effluent from both Phase I and Phase II were significantly lower than the design concentration (p < 0.001), indicating that the effluent from both phases achieved the discharge standard. As shown in Figure 4, compared with the TN concentrations and removal rates of the effluent from Phase I and Phase II, the TN concentration of the effluent from Phase I mainly ranged from 3.88 to 18.3 mg/L, with an average annual effluent TN concentration of 9.36 mg/L and an average annual removal rate of 71.01%. And the TN concentration of the effluent from Phase II mainly ranged from 4.99 to 24.0 mg/L with an average annual effluent concentration of 12.30 mg/L and an average annual removal rate of 61.34% (Table 3). The TN concentration of the effluent from Phase II was higher than that of Phase I by 40.29%, and the removal rate was lower than that of Phase I by 13.09%. The results showed that the TN concentration of the effluent from Phase II was higher than that of Phase I (p < 0.001), and the removal rate was lower than that of Phase I. This was related to the longer HRT of the anoxic zone in Phase I, which was approximately 1.83 times that of Phase II. However, compared to other studies, the TN removal rate for both Phase I and Phase II processes was not high [4,5,6,27]. This might be due to the COD/TN ratio in the wastewater, which could affect the microbial community structure and metabolic activity related to the nitrogen cycle [29,30,31]. Previous studies have found that the COD/TN ratio of 25 had a better TN removal rate in wastewater, while the average COD/TN ratio in this study was only 11.76 [30]. Additionally, abundant organic matter and microorganisms related to the nitrogen cycle in sludge might also be an important reason for affecting TN removal [32,33,34].
3.4. The Effect of Two Processes on TP Removal
Other than nitrogen, phosphorus is an important nutrient in wastewater. Excessive phosphorus discharge can be toxic to aquatic organisms [13]. Therefore, the treatment of phosphorus is more important than that of nitrogen. The annual average concentration of TP in the studied area’s wastewater did not show a significant difference from the design value (p = 0.95), but it had a large fluctuation (1.02–16.5 mg/L). As shown in Figure 5, compared to the TP concentrations and removal rates of the effluent from Phase I and Phase II, the TP concentration of the effluent from Phase II was significantly lower than that of Phase I (p < 0.001), and the removal rate was significantly higher than that of Phase I (Table 3). The TP concentration of the effluent from Phase II was 51.73% lower than that of Phase I, and the TP removal rate was 17.06% higher than that of Phase I. Therefore, the TP removal rate of the Phase II process was better than that of the Phase I process. Previous studies have also found that the TP removal rate of the oxidation ditch process is not ideal, while the high-efficiency multi-cycle A2/O process has a higher removal efficiency [4,6,35,36,37]. Additionally, by comparing the average annual effluent TP concentration of Phase I with the design effluent concentration, there was no significant difference (p = 0.67), and the effluent from Phase I basically met the requirements of the discharge standard.
3.5. The Effect of Two Processes on Sludge Properties
Sludge contains abundant organic matter and microorganisms related to the nitrogen and phosphorus cycles, which plays an important role in the effectiveness of wastewater treatment [3,4,25,38]. In this WWTP, the mixed liquor suspended solids (MLSS) of Phase I and Phase II were similar ranging from 4000 to 6000 mg/L. As shown in Figure 6, the SVI of Phase I mainly ranged from 54 to 94.8 mL/g with an average annual value of 78.5 ± 11.2 mL/g, and the SVI of Phase II mainly ranged from 60 to 113 mL/g with an average annual value of 87.9 ± 12.4 mL/g. By comparing the sludge properties of Phase I and Phase II, we found that the SVI of Phase II was significantly higher than that of Phase I (p < 0.01). The sludge settling performance was an important factor affecting the effectiveness of wastewater treatment [3,26]. In this study, the SVI of the two processes was below 120 mL/g, indicating both processes had good sludge settling performance.
3.6. Analysis of the Two Processes Costs and Operating Costs
Under the same conditions of influent quality and the achievement of effluent discharge standards in both processes, the total retention time for the oxidation ditch process was 26.5 h, while the total retention time for the high-efficiency multi-cycle A2/O process was 19.45 h. Therefore, the total effective volume of the oxidation ditch process was approximately 1.36 times that of the modified A2/O process. Due to the increase in tank volume, the project cost of the oxidation ditch process also increased, resulting in a higher engineering cost per ton of wastewater.
The design capacity of Phase I of the Ningyang sewage treatment plant was 40,000 t/d, but the actual influent was approximately 33,400 t/d. Meanwhile, the design capacity of Phase II was 20,000 t/d, but the actual influent was approximately 20,700 t/d. The main energy consumption of the biochemical processes in both phases came from the power consumption of the blowers, flow propellers, and internal recycle pumps. Based on the design and equipment power calculation, the electricity consumption was 0.307 kW·h per ton of wastewater. Due to the deviation between the actual influent and the design capacity, the actual electricity consumptions for Phase I and Phase II were approximately 0.363 kW·h and 0.295 kW·h per ton of wastewater, respectively, which were lower than the average electricity consumption of China (about 0.42 kW·h/t). The electricity consumption per ton of wastewater in the high-efficiency multi-cycle A2/O process of Phase II was about 81.27% of that in the oxidation ditch process of Phase I, resulting in lower operational costs of Phase II.
4. Conclusions
This paper systematically studied the performance of the oxidation ditch process and the high-efficiency multi-cycle A2/O process in treating municipal wastewater. Through continuous monitoring over a year, we found that both processes performed well at the Ningyang sewage treatment plant. The COD concentration of the high-efficiency multi-cycle A2/O process in Phase II was 49.9% more than that of the oxidation ditch process in Phase I, and its removal efficiency increased by an average of 27.4% than Phase I. The concentration and removal efficiency of NH4+-N in Phase II were not significantly different from that in Phase I. While the TN concentration of Phase II increased by 40.29% compared with Phase I, and the removal efficiency was 13.09% compared with Phase I. The TP concentration of Phase II was 51.73% higher than that of Phase I, and the removal efficiency increased by 17.06% compared to Phase I. The above data showed that the highly efficient and multi-cycle A2/O process of Phase II was better than the oxidation groove process of the first phase and had lower construction and operating costs. The conclusion of this study provides scientific guidance for wastewater treatment processes in the region and offers a scientific basis for improving wastewater treatment efficiency.
Conceptualization, J.L. and K.C.; methodology, J.L.; software, J.L.; validation, J.L., Z.Y. and H.Y.; formal analysis, Z.Y.; investigation, J.L.; resources, H.Y.; data curation, J.L.; writing—original draft preparation, Z.Y.; writing—review and editing, K.C.; visualization, Z.Y.; supervision, K.C.; project administration, H.Y.; funding acquisition, K.C. All authors have read and agreed to the published version of the manuscript.
The data presented in this study are available on request from the corresponding author.
Zhen Yan was employed by Anhui Zhonghuan Environmental Protection Technology Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Footnotes
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Enlarge this image.Figure 1. The scheme diagrams for both processes: oxidation ditch (A) and modified A2/O (B) (the arrows: flow direction).
Enlarge this image.Figure 2. The effluent concentration (A) and removal rate (B) of COD (Phase I: oxidation ditch, Phase II: modified A2/O; dotted line: the COD concentration of design outflow).
Enlarge this image.Figure 3. The effluent concentration (A) and removal rate (B) of NH4+-N (Phase I: oxidation ditch, Phase II: modified A2/O; dotted line: the NH4+-N concentration of design outflow).
Enlarge this image.Figure 4. The effluent concentration (A) and removal rate (B) of TN (Phase I: oxidation ditch, Phase II: modified A2/O; dotted line: the TN concentration of design outflow).
Enlarge this image.Figure 5. The effluent concentration (A) and removal rate (B) of TP (Phase I: oxidation ditch, Phase II: modified A2/O; dotted line: the TP concentration of design outflow).
Enlarge this image.Figure 6. The average sludge SVI of the two processes (Phase I: oxidation ditch, Phase II: modified A2/O).
Design and actual influent and effluent characteristics of WWTP.
| Phase I Inflow (t/d) | Phase II Inflow | COD | NH4+-N (mg/L) | TN | TP | Suspended Solids (mg/L) | Temperature (°C) | |
|---|---|---|---|---|---|---|---|---|
| Design Inflow | 40,000 | 20,000 | 450 | 35 | 40 | 4 | 200 | - |
| Design Outflow | - | - | 50 | 5 | 15 | 0.5 | 10 | - |
| Actual Inflow (Mean) | 33,423.6 | 20,685.1 | 400.8 | 22.73 | 34.09 | 4.01 | 106 | 20 |
| Range (95%) | 21155–45778 | 8793–32281 | 95.3–964 | 14.9–29.0 | 17.4–63.2 | 1.02–16.5 | 99.6–113.3 | 15–25 |
| p-value | <0.001 | =0.034 | <0.001 | <0.001 | <0.001 | =0.95 | <0.001 | <0.001 |
Main operating parameters of two sewage treatment processes.
| Parameter | Retention Time (h) | Dissolved Oxygen (mg/L) | Sludge Return Ratio % | Total Nitrogen Load (kgTN/(kgMLSS·h)) | Surface Load of Secondary Sedimentation Tank(m3/(m2·h)) | Nitrified Liquid Return Ratio% | |||
|---|---|---|---|---|---|---|---|---|---|
| Anaerobic Zone | Anoxic Zone | Aerobic Zone | Anaerobic Zone | Aerobic Zone | |||||
| Phase I | 3.02 | 5.53 | 17.95 | 0.1–0.2 | 2.5–3.0 | 100% | 0.011 | 1.04 | 250% |
| Phase II | 1.51 | 3.02 | 14.92 | 0.1–0.2 | 2.0–3.0 | 100% | 0.005 | 0.62 | 200% |
The effluent nutrients concentration, removal rate, and results of statistical tests.
| Phase I | Phase II | df | t | p | |
|---|---|---|---|---|---|
| COD | 31.6~90.5 (56.7) | 13.5~47.0 (27.3) | 589.23 | 31.02 | <0.001 |
| COD removal rate (%) | 79.8% | 90.9% | 439.48 | 12.33 | <0.001 |
| NH4+-N | 0.73~6.98 (1.43) | 0.59~9.14 (1.45) | 713.68 | 0.15 | =0.880 |
| NH4+-N removal rate | 93.50% | 93.78% | 691.3 | 0.48 | =0.630 |
| TN | 3.88~18.3 (9.36) | 4.99~24.0 (12.30) | 645.48 | 8.81 | <0.001 |
| TN removal rate | 71.01% | 61.34% | 579.42 | 8.31 | <0.001 |
| TP | 0.15~1.65 (0.51) | 0.02~0.45 (0.17) | 418.86 | 15.035 | <0.001 |
| TP removal rate | 81.40% | 93.24% | 501.74 | 14.09 | <0.001 |
| SVI | - | - | 225.82 | 5.1734 | <0.001 |
Notes: Water quality index concentration range (mean value). Average removal rate.
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