Why is the COD in tank O of the A/O process higher than that in tank A?

What is the BOD of the incoming water? Generally, the ratio of BOD5 to TN should be greater than 4; to be on the safe side, it must be greater than 5 in order to ensure that denitrification can take place.

If only ammonia nitrogen is to be removed, it is necessary to rely on nitrifying bacteria and aerobic conditions in order to oxidize ammonia nitrogen into nitrate nitrogen; moreover, the concentration of organic matter must not be too high, as otherwise aerobic bacteria will inhibit the activity of nitrifying bacteria. The pH value at the aerobic outlet must be greater than 7.

Nitrifying bacteria have a relatively long generation time; generally, a sludge age of over 20 days is required to ensure their growth and reproduction, so the MLSS level needs to be kept at a higher level.

Around TP1, B/C ratio of 0.4

A BOD5/TN ratio of less than 3 is unfavorable for denitrification. However, since you are only concerned about ammonia nitrogen, as long as the sludge age is sufficient and there is enough dissolved oxygen, and the BOD level in the effluent remains below 100 while the pH is above 7, then the ammonia nitrogen level in the effluent can be kept below 100. However, the presence of nitrifying bacteria is necessary; these bacteria generally have poor stress resistance and require a long time to grow and reproduce.

The BOD/TN ratio is around 0.4, and it fails to meet the standards; using coagulants after the A/O process also does not yield good results, so we plan to try MBR.

This post was last edited by zhaolijun on 2015-11-12 09:44 to share the pilot test results with you; I hope it will be helpful to you. MBR Membrane Pilot Test Operation Report, Su Jing Environmental Protection New Materials Co., Ltd. Organic pigment wastewater represents a significant challenge in industrial wastewater treatment, as it features high acidity, high COD levels, high color intensity, high salt content, and organic substances that are difficult to biodegrade. Traditional A2/O processes often fail to provide satisfactory treatment results for such wastewater. The membrane bioreactor (MBR) is a new type of wastewater treatment process that combines membrane separation technology with the traditional activated sludge method; it offers advantages such as high-quality treated water, reduced space requirements, and lower sludge production. To meet the needs for upgrading and enhancing nitrogen and phosphorus removal in wastewater treatment plants in our country, integrating the MBR process with the A2/O process, and taking advantage of the high sludge load in the membrane tank as well as the separate HRT and SRT, can effectively improve the treatment efficiency of this combined process. 1 Materials and Methods 1.1 Pilot-scale apparatus The pilot-scale apparatus is located within the sewage treatment workshop; it is 1.5 m long, 0.6 m wide, and 2 m high, with a designed treatment capacity of 11.5 m3/day. The purpose of this pilot study is to gain a preliminary understanding of the treatment efficiency of MBR for relevant pollutants under actual water quality conditions, as well as the influencing factors, in order to provide design parameters and operational guidelines for the upgrade project of the wastewater treatment plant. The membrane module uses PVDF hollow fiber membranes, a total of 4, and their performance parameters are shown in Table 1. Table 1 Performance parameters of hollow fiber membrane modules: Parameter, Pore size/μm, Dimensions/mm, Membrane area/m2; Values: 0.1, Ø165*164, 515; 1.2. Test procedure: The pilot-scale process is shown in Figure 1. file:///C:/Users/zhaolj1/AppData/Local/Temp/msohtmlclip1/01/clip_image001.gif Figure 1: Process flow of the pilot plant. The water fed into the pilot plant comes from the biochemical aeration tank in the sewage treatment facility; after membrane separation, the resulting water flows into the sedimentation tank. The experiment began on January 10. In the first phase, the membrane flux was set at 10 L/(m2·h), corresponding to a flow rate of 0.6 m3/h. After 20 days of stable operation, the flow rate was increased to 0.9 m3/h in order to test the limits of the membrane’s performance; at this point, the membrane flux reached 15 L/(m2·h). By March 18, the second phase of the experiment had lasted a total of 48 days. During the test, the pilot plant operated in cycles of 10 minutes, with 8 minutes of operation followed by a 2-minute interval. 1.3 Analysis items and methods: The analysis items include pH value, temperature, MLSS, COD, turbidity, etc., all of which are determined using national standard methods; for specific details, refer to \"Methods for the Monitoring and Analysis of Water and Wastewater\" (3rd edition). 1.4 Water quality of influent and operating parameters The water quality indicators and operating parameters for the aeration tank are shown in Table 2. Table 2 Water quality and operating parameters of the aeration tank: Statistical values – T/°C, pH, DOM, LSS, COD. Maximum values: 21.2, 7.62, 4.4, 5.04, 1280; Minimum values: 16.5, 6.98, 1.1, 3.49, 498; Average values: 18.5, 7.3, 2.3, 4.5, 481.2. 2 Results and discussion: After two phases of commissioning and operation, the efficiency of the system in removing pollutants such as COD and SS was evaluated, while the operational stability of the system was also examined. 2.1 MBR treatment efficiency 2.1.1 Removal efficiency of COD The system’s efficiency in removing COD is shown in Figure 2 (data available in Appendix 1). Figure 2 shows the variations in COD concentration and removal efficiency across various components of the system. As can be seen from Figure 2, during the commissioning phase, the COD concentration in the aeration tank varied significantly, ranging from 498 to 1280 mg/L, with an average value of 812 mg/L ； In the secondary sedimentation tank, the COD level ranged from 296 to 640 mg/L, with an average concentration of 443 mg/L; the average removal rate was 45% ； Meanwhile, the COD of the water discharged from the MBR membrane ranged from 176 to 496 mg/L, with an average of 294 mg/L, and the average removal rate was 64%. The results show that, compared to the secondary sedimentation tank, the pilot-scale device can increase the COD removal rate by about 35% at most, and the average removal rate can be increased by 19%. Therefore, the efficient retention by membrane modules plays a better role in stabilizing the quality of the effluent water. 2.1.2 Removal efficiency of SS: On the 12th day of operation, measurements were taken of the SS concentration in the water exiting the MBR. The treatment efficiency for SS is shown in Figure 3 (data are provided in Appendix 2). Figure 3 shows the changes in SS concentration in various parts of the system. As can be seen from Figure 3, the SS concentration in the secondary sedimentation tank is generally higher than that in the MBR effluent. At the beginning of operation, the SS concentration in the membrane effluent fluctuated significantly; after 18 days of operation, however, this concentration became more stable. During operation, the SS in the water leaving the MBR membrane ranged from 6 to 125 mg/L, with an average of 44.1 mg/L. Meanwhile, the SS in the secondary sedimentation tank was between 46 and 260 mg/L, with an average concentration as high as 161.4 mg/L. The results show that, compared with the secondary sedimentation tank, the pilot-scale device can reduce the average concentration of SS in the effluent by about 73%, and once the reactor is operating stably, the SS concentration can be brought below 20 mg/L. 2.2 Operational stability of MBR 2.2.1 COD volumetric load The effect of the volumetric load on the COD in the effluent is shown in Figure 4 (data are provided in Appendix 3). file:///C:/Users/zhaolj1/AppData/Local/Temp/msohtmlclip1/01/clip_image007.gif Figure 4 shows the effect of volume load on the COD level in the effluent. As can be seen from Figure 4, the quality of the MBR effluent changes with the volume load; the COD volume load in the aeration tank ranges from 91.4 to 234.6 g/(m3·d). During the 20 days of the first phase of operation, the COD volumetric load in the aeration tank showed significant fluctuations, while the quality of water discharged from the MBR remained within the range of 250–320 mg/L, indicating stable water quality from the MBR. During the first 27 days of the second phase of operation, the volumetric load remained within the range of 157.8–234.6 g/(m3·d), while the MBR effluent concentration fluctuated significantly between 176.5–496 mg/L. This was likely due to a 50% increase in membrane flux, which exceeded the critical flux level and disrupted the existing filter cake layer, thereby affecting the MBR system’s ability to retain COD. As the experiment progressed, the transmembrane pressure difference increased gradually; sludge flocs and EPS accumulated on the membrane surface, gradually forming a gel layer. After 28 days, the capacity to retain pollutants in the mixture improved significantly, and the COD concentration in the effluent remained stable, at around 280 mg/L on average. The results show that the MBR system has strong adaptability. 2.2.2 During membrane negative pressure and membrane flux operation, the water temperature fluctuated between 16.5 and 21.2°C. The trends in MBR membrane negative pressure and membrane flux are shown in Figure 6 (data are provided in Appendix 4). Figure 6 shows the changes in the negative pressure across the MBR membrane and the membrane flux during operation. From the start of operation until day 20, the temperature fluctuated between 18.7 and 21.1°C. As can be seen from Figure 6, the negative pressure across the MBR membrane remained at 0.012–0.014 MPa, while the membrane flux stayed constant at 10 L/(m2·h). This indicates that the pilot-scale system operated stably and efficiently. 20 days later, the temperature dropped slightly, ranging between 16.5 and 20.8°C. The negative pressure across the membrane fluctuated, but not significantly, remaining between 0.01 and 0.015 MPa, while the membrane flux stayed stable at 15 L/(m2·h). Throughout the operation period, the membrane negative pressure was maintained below 0.015 MPa. The results show that the system can operate stably when the water temperature is between 16.5 and 21.1°C. 3 Conclusions ① The use of the MBR process for treating pigment wastewater yields effluent quality that is significantly better than that obtained using a secondary sedimentation tank: the average COD concentration is 294 mg/L, representing an improvement in removal efficiency of about 19% compared to the secondary sedimentation tank ； The average SS concentration in the effluent was 44.1 mg/L, which represents a reduction of about 73% compared to that in the secondary sedimentation tank. ② This pilot-scale MBR membrane system shows good adaptability to pigment wastewater and can operate stably. Schedule 1: Data related to COD concentration and removal rate of each part of the system Operating time (days) Middle part of aeration tank (COD) MBR (COD) Secondary sedimentation tank (COD) Secondary sedimentation tank removal rate MBR removal rate January 10th 173818735452.03 74.66 January 11th 2536.526434435.88 50.79 January 12th 363025232548.41 60.00 January 13th 4707.5318.538645.44 54.98 January 14th 5731.5316.5454.537.87 56.73 January 15th 685030644048.24 64.00 January 16th 7531.528936431.51 45.63 January 17th 8521304.534633.59 41.55 January 18th 9512319.537925.98 37.60 January 19th 10669329.538442.60 50.75 January 20th 11771316439.543.00 59.01 January 21st 12682326375.544.94 52.20 January 22nd 13622.5296375.539.68 52.45 January 23rd 14498.5276380.523.67 44.63 January 24th 15503.5266.535130.29 47.07 January 25th 16499296.542015.83 40.58 January 26th 17583276.535638.94 52.57 January 27th 18548.5296.5380.530.63 45.94 January 28th 19612.5237405.533.80 61.31 January 29th 2073027729659.45 62.05 January 30th 21100037045554.50 63.00 January 31st 22941358.551844.95 61.90 February 1st 23937.528345151.89 69.81 February 2nd 241150.5317.546659.50 72.40 February 3rd 251018.535443357.49 65.24 February 4th 2693533549247.38 64.17 February 5th 27892.5305458.548.63 65.83 February 6th 28880.5376514.541.57 57.30 February 7th 2987121243949.60 75.66 February 8th 30861176.5458.546.75 79.50 February 9th 311166.525649257.82 78.05 February 10th 321280330521.559.26 74.22 February 11th 331102.5315511.553.61 71.43 February 12th 341112423620.544.20 61.96 February 13th 35925.532556139.38 64.88 February 14th 361043.530549252.85 70.77 February 15th 3799422750249.50 77.16 February 16th 381141.523645360.32 79.33 February 17th 391186236452.561.85 80.10 February 18th 40116133553154.26 71.15 February 19th 41 February 20th 42123026534472.03 78.46 February 21st 43103330551250.44 70.47 February 22nd 44109149658546.38 54.54 February 23rd 45113035762444.78 68.41 February 24th 46107137761043.04 64.80 February 25th 471269.528863549.98 77.31 February 26th 481041.527864038.55 73.31 February 27th 4997923657141.68 75.89 February 28th 501033.5344.555146.69 66.67 March 1st 5196527157640.31 71.92 March 2nd 52802285.5457.542.96 64.40 March 3rd 53797276541.532.06 65.37 March 4th 54684.5267.544634.84 60.92 March 5th 55600.527143827.06 54.87 March 6th 56600.5280.5437.527.14 53.29 March 7th 57585.5280.538933.56 52.09 March 8th 5862025138438.06 59.52 March 9th 59630280.537440.63 55.48 March 10th 60625334.537939.36 46.48 March 11th 61610241.5354.541.89 60.41 March 12th 62541.5280.535933.70 48.20 March 13th 63541.5300334.538.23 44.60 March 14th 64541290.536931.79 46.30 March 15th 65640231.535444.69 63.83 March 16th 66546315344.536.90 42.31 March 17th 67580.5260.5324.544.10 55.12 March 18th 6855123135934.85 58.08 Schedule 2: Running time of data related to SS concentration of each part of the system/d Small test (SS) Secondary sedimentation tank (SS) January 21st 12100250 January 22nd 1340 January 23rd 14100215 January 24th 1584 January 25th 1693260 January 26th 17125220 January 27th 1846 January 28th 192846 January 29th 2022 January 30th 2122115 January 31st 2220 February 1st 2314 February 2nd 24 90 February 3rd 25 February 4th 26650 February 5th 27 February 6th 28 80 February 7th 29 February 8th 30 85 February 9th 31 120 February 10th 32 February 11th 3320125 February 12th 34 February 13th 35 135 February 14th 36 February 15th 37 115 February 16th 38 140 February 17th 396 February 18th 40 160 February 19th 41 February 20th 42 February 21st 43 195 February 22nd 44 225 February 23rd 45 250 February 24th 46 February 25th 4718260 February 26th 48 February 27th 49 180 February 28th 50 170 March 1st 51 250 March 2nd 52 210 March 3 53 135 March 4 546115 Schedule 3: Data related to volumetric load and outlet water COD Operating time volumetric load MBR (COD) January 10th 1135.3 187 January 11th 298.3 264 January 12th 3115.5 252 January 13th 4129.7 318.5 January 14th 5134.1 316.5 January 15th 6155.8 306 January 16th 797.4 289 January 17th 895.5 304.5 January 18th 993.9 319.5 January 19th 10122.6 329.5 January 20th 11141.3 316 January 21st 12125.0 326 January 22nd 13114.1 296 January 23rd 1491.4 276 January 24th 1592.3 266.5 January 25th 1691.5 296.5 January 26th 17106.9 276.5 January 27th 18100.5 296.5 January 28th 19112.3 237 January 29th 20133.8 277 January 30th 21183.3 370 January 31st 22172.5 358.5 February 1st 23171.9 283 February 2nd 24210.9 317.5 February 3rd 25186.7 354 February 4th 26171.4 335 February 5th 27163.6 305 February 6th 28161.4 376 February 7th 29159.7 212 February 8th 30157.8 176.5 February 9th 31213.8 256 February 10th 32234.6 330 February 11th 33202.1 315 February 12th 34203.8 423 February 13th 35169.7 325 February 14th 36191.3 305 February 15th 37182.2 227 February 16th 38209.2 236 February 17th 39217.4 236 February 18th 40212.8 335 February 19th 41 February 20th 42225.5 265 February 21st 43189.4 305 February 22nd 44200.0 496 February 23rd 45207.1 357 February 24th 46196.3 377 February 25th 47232.7 288 February 26th 48190.9 278 February 27th 49179.5 236 February 28th 50189.5 344.5 March 1st 51176.9 271 March 2nd 52147.0 285.5 March 3 53146.1 276 March 4 54125.5 267.5 March 5 55110.1 271 March 6 56110.1 280.5 March 7 57107.3 280.5 March 8 58113.7 251 March 9th 59115.5 280.5 March 10th 60114.6 334.5 March 11th 61111.8 241.5 March 12th 6299.3 280.5 March 13th 6399.3 300 March 14th 6499.2 290.5 March 15th 65117.3 231.5 March 16th 66100.1 315 March 17th 67106.4 260.5 March 18th 68101.0 231 Appendix Table 4: MBR membrane negative pressure and flux related data operating time/d negative pressure/MPa flux/L/(m2·h) January 10th 10.0133 10 January 11th 20.0124 10 January 12th 30.0120 10 January 13th 40.0122 10 January 14th 50.0122 10 January 15th 60.0120 10 January 16th 70.0128 10 January 17th 80.0124 10 January 18th 90.0126 10 January 19th 100.0120 10 January 20th 110.0126 10 January 21st 120.0122 10 January 22nd 130.0120 10 January 23rd 140.0124 10 January 24th 150.0122 10 January 25th 160.0124 10 January 26th 170.0124 10 January 27th 180.0122 10 January 28th 190.0126 10 January 29th 200.0120 10 January 30th 210.0122 15 January 31st 220.0106 15 February 1st 230.0126 15 February 2nd 240.0130 15 February 3rd 250.0100 15 February 4th 260.0108 15 February 5th 270.0110 15 February 6th 280.0106 15 February 7th 290.0121 15 February 8th 300.0115 15 February 9th 310.0128 15 February 10th 320.0134 15 February 11th 330.0138 15 February 12th 340.0134 15 February 13th 350.0140 15 February 14th 360.0138 15 February 15th 370.0140 15 February 16th 380.0140 15 February 17th 390.0140 15 February 18th 400.0114 15 February 19th 410.0108 15 February 20th 420.0124 15 February 21st 430.0120 15 February 22nd 440.0120 15 February 23rd 450.0136 15 February 24th 460.0116 15 February 25th 470.0148 15 February 26th 480.0140 15 February 27th 490.0128 15 February 28th 500.0120 15 March 1st 510.0140 15 March 2nd 520.0140 15 March 3rd 530.0150 15 March 4th 540.0108 15 March 5th 550.0114 15 March 6th 560.0130 15 March 7th 570.0128 15 March 8th 580.0140 15 March 9th 590.0128 15 March 10th 600.0116 15 March 11 610.0120 15 March 12 620.0130 15 March 13 630.0124 15 March 14 640.0118 15 March 15 650.0122 15 March 16 660.0126 15 March 17 670.0134 15 March 18 680.0122 15