Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Feb 28;5(9):4576–4587. doi: 10.1021/acsomega.9b04016

Investigating COD and Nitrate–Nitrogen Flow and Distribution Variations in the MUCT Process Using ORP as a Control Parameter

Xiaoyu Zhang †,‡, Xiaoling Wang †,‡, Weihao Feng †,‡, Xueqi Li †,‡, Hai Lu §,*
PMCID: PMC7066563  PMID: 32175504

Abstract

graphic file with name ao9b04016_0009.jpg

This study aimed to reveal the flow and distribution lows of chemical oxygen demand (COD) and nitrate–nitrogen under different main anoxic stage oxidation–reduction potential (ORPan) conditions based on the analysis of material balance in each reaction stage of the modified university of cape town (MUCT) process, combined with the biochemical reaction principles of activated sludge. The rule of the carbon source saving effect was also clarified. The study adopted the programmable logic controller automatic control system and the feedback control structure using the inner circulation flow of nitrate as the controlled variable. The ORPan setting values of control parameters were −140, −125, −110, −95, −70, and −60 mV, respectively. The results showed that when the ORPan setting value was −95 mV, COD distribution ratios of phosphorus-accumulating bacteria reached the highest in the anaerobic stage and preanoxic stage, with the values of 51.74 and 7.70%, respectively. The COD was distributed between heterotrophic bacteria and denitrifying bacteria in the main anoxic stage, and the distribution ratios were 4.40 and 7.19%, respectively, when the ORPan setting value was −95 mV. The study also showed the distribution of nitrate–nitrogen between denitrifying bacteria and denitrifying phosphorus-accumulating bacteria in the main anoxic stage, and when ORPan increased from −140 to −60 mV, the distribution ratios of denitrifying phosphate-accumulating bacteria increased from 76.46 to 86.32%. When there was no denitrification and phosphorus absorption, the acetic acid dosage increased from 20.33 g/d at −140 mV to 24.76 g/d at −95 mV, and the carbon source saving rate increased from 23.19 to 26.56% under similar conditions. Therefore, in the MUCT process, the regulation of ORPan changed the material flow direction and mass quality distribution of COD and nitrate nitrogen. When ORPan set value was −95 mV, COD and nitrate–nitrogen got the best distribution and the carbon source saving effect was the most significant.

1. Introduction

In order to alleviate water pollution, nitrogen and phosphorus removal is required in the process of municipal sewage treatment, but most of the time, the ratio of carbon–nitrogen and carbon–phosphorus ratio in municipal sewage are gradually decreasing, thereby making the removal of nitrogen and phosphorus difficult.1,2 Generally, in the synchronous biological nitrogen and phosphorus removal system of sewage, phosphorus-accumulating bacteria, nitrifying bacteria, and denitrifying bacteria coexist to complete for the process of phosphorus release, phosphorus absorption, nitrification, denitrification, and other biochemical reactions. However, the growth environment of each strain is quite different, resulting in a variety of contradictory relations.3−10 In recent years, autotrophic denitrification technology has gradually gained massive attention for its efficiency in the treatment of low carbon–nitrogen ratio sewage. However, because hydrogen production bacteria, sulfur bacteria, and iron bacteria are autotrophic bacteria, the growth and reproduction rate is relatively low, and they often need a large volume; thus, they have not been applied in urban sewage treatment plants.11,12 In the 1990s, researchers discovered that denitrifying phosphorus-accumulating bacteria exists in the anoxic stage of the sewage treatment plant. The stable metabolic process of the bacteria could alleviate the above contradiction to a certain extent.13−20 The electron acceptor of denitrifying phosphorus-accumulating bacteria is nitrate–nitrogen, and its metabolic characteristics are the same as those of common phosphorus-accumulating bacteria.21−23 However, after nearly 20 years of development, the process of denitrifying phosphorus uptake has not completely solved the problem of poor effect of simultaneous nitrogen and phosphorus removal in low carbon/nitrogen (C/N) ratio wastewater, mainly because the quality and the quantity of influent water fluctuate greatly and the operating parameters of the sewage plant cannot respond immediately, and the denitrifying phosphorus-accumulating bacteria cannot reach the optimum state. The above problems can be solved to a certain extent by constructing a denitrifying phosphorus absorption process control system and developing control parameters and optimum control strategy in the sewage treatment plant.24

In the construction of the control system, the real-time, stable, and accurate detection of control parameters are very important. Presently, the control parameters commonly used in sewage treatment plants include dissolved oxygen (DO), pH, and oxidation–reduction potential (ORP) values, among which DO is used to control the air supply of the blower in the aerobic stage, whereas the pH value is used to monitor the operation of the system, and ORP is used to monitor the REDOX environment in the nonaerated stage. The ORP online detector has the advantages of on-line detection, fast response, high control accuracy, and easy access to the computer.24−28 In the field of environmental engineering, many countries and regions in the world have taken ORP value as one of the important detection indicators for disinfection of swimming pool water, drinking water, and hot spring water, and some experts and scholars have applied ORP to industrial process control.29−31 Over 90% of wastewater treatment plants for nitrogen and phosphorus removal in China are equipped with ORP online measuring instruments in the nonaerated section, which is used to monitor the redox level of the treatment environment and avoid the high oxygen content affecting the phosphorus release and denitrification effect. However, there are few reports on its regulation as a process control parameter of continuous sewage biological treatment system. Among them, Ma and Peng studied the feasibility of realizing the optimal control of denitrification process by using ORP as the fuzzy control parameter of denitrification reaction in anoxic/aerobic activated sludge process and obtained the conclusion. They showed that, when the internal circulation flow rate was controlled separately, the ORP value at the end of denitrification zone was (−86 ± 2) mV; and when the external carbon source was controlled alone, the ORP value was (−90 ± 2) mV.32 In preliminary studies, researchers took low carbon–nitrogen ratio sewage as the treatment object and studied the feasibility of ORPan as the control parameter of the continuous flow single sludge sewage treatment system. It has been determined that regulating the parameter can change the total phosphorus (TP) and total nitrogen (TN) concentration of the system effluent and at an optimal value of −95 mv.25 On this basis, the metabolism of poly-hydroxy alkanoates (PHA) and TP under different ORPan conditions was discussed.33 In order to meet the needs of constructing denitrifying phosphorus absorption process control system and strengthen and stabilize denitrifying phosphorus absorption performance, this paper traces the flow and distribution of carbon and nitrogen under different ORPan conditions and evaluates the carbon source savings produced by ORPan as the operation control parameter. Mainly, the research is based on the biochemical reaction process of common heterotrophic bacteria, denitrifying bacteria, nitrifying bacteria, and phosphorus accumulating bacteria, combined with the principle of material balance.

2. Test Materials and Methods

2.1. Sewage Water Quality Characteristics

The pollutant composition and concentration in the manually configured sewage to be treated in modified university of cape town (MUCT) process system were determined by referring to the water quality of 10 sewage treatment plants in Changchun City, northern China.1,2,34−37 The dosage of each substance was 50 mg/L of whole milk powder, 0.5 mL/L of brewery wastewater, 50 mg/L of NH4Cl, 3.1 mg/L of KH2PO4, 0.4 g/L of NaHCO3, 10 mg/L of CaCl2, and 50 mg/L of MgSO4, respectively. In order to meet the growing needs of activated sludge, 0.6 mL/L of micronutrient solution was added, including 0.9 g/L of FeCl3, 0.15 g/L of H3BO3 and CoCl2·7H2O 0.15 g/L, CuSO4·5H2O 0.03 g/L, KI 0.18 g/L, CoCl2·4H2O 0.06 g/L, Na2MoO4·2H2O 0.06 g/L, and zinc ZnSO4·7H2O 0.12 g/L.

2.2. Main Operating Parameters of MUCT Device

As shown in Figure 1, the MUCT test system consisted of a 252 L sewage tank, 90 L MUCT reactor, precipitator with a diameter of 50 cm, and automatic control system. The size of the sewage tank was 80 cm × 70 cm × 45 cm (L × B × H), and the super height is 5 cm. The MUCT reactor was 75 cm × 30 cm × 45 cm (L × B × H), with a 5 cm super height. This included the anaerobic stage of 18 L, preanoxic stage of 9 L, main anoxic stage of 18 L, and aerobic stage of 45 L. The reactor was designed as a double gallery; each gallery was divided into five compartments with partition plates. The effective volume of each compartment was 9 L, and the size was 15 cm × 15 cm × 45 cm (L × B × H). The stirrer was installed in the sewage tank and nonaerated section, and the air diffuser was arranged in the aerobic section. The mixed liquid of the reactor flowed into the precipitator for solid–liquid separation. The settling device was equipped with a scraper with a rotating speed of 5 rpm. The MUCT system established an automatic control system, which consisted of ORP on-line detector, DO on-line detector, the programmable logic controller (PLC), computer, and electric control valve.

Figure 1.

Figure 1

Open in a new tab

MUCT process test device diagram. (1) Sewage tank; (2) anaerobic section; (3) preanoxic section; (4) main anoxic section; (5) aerobic section; (6) clarifier; (7) influent water; (8) effluent water; (9) excess sludge; (10) sludge reflux; (11) aerobic/main anoxic nitrifying fluid circulation; (12) preanoxic/anaerobic mixture circulation.

2.3. Test Scheme

The daily water treatment capacity of the test system was 240 L/d. Water quality parameters were assessed according to German (ATV-DVWK Specification),38 the design of Municipal Wastewater Treatment Plants (Volume 2: Liquid Treatment Processes) of United States,39 the Dutch Biological Phosphorus Removal Manual for Design and Operation, and Chinese Outdoor Drainage Design Specification (GB50014) (2016 edition).40 According to the regulations, operation parameters such as sludge load, hydraulic retention time, and sludge age were determined. “Chinese Outdoor Drainage Design Specification” (GB50014) (2016 edition) takes hydraulics, microbiology, and so forth as the scientific support and combines with the engineering design and operation experience of China’s urban drainage system, to analyze the design parameters. It is the design standard of the urban drainage system in China.

During the experiment, ORPan was controlled by the PLC automatic control system. The control system adopted a feedback control structure, the control parameters, controlled variables, and the actuators were ORPan, the internal circulation flow rate of nitrate, and electric control valve, respectively. The control process was as follows: the ORP value was detected by an ORP online measuring instrument at the end of the main anoxic stage (the output signal was 4–20 mA analogue signal), transmitted to the data acquisition card of PLC system. After being converted into a digital signal by A/D converter and compared with ORPan setting value of proportional–integral–derivative regulator, the deviation was determined. Next, after the control of proportional, integral, and differential calculation, the results were given and converted into analogue signals (4–20 mA) by a D/A converter. As the output value, the results were fed back to the internal circulation electric control valve of nitrate. The Scientific Apparatus Maker’s Association (SAMA) diagram and logic control diagram of the automatic control system are shown in Figure 2. The experiment was carried out in six stages with ORP set values of −140, −125, −110, −95, −75, and −60 mV. The other operating parameters remained unchanged except the internal circulation flow of nitrate. Three sludge retention time were operated in each stage for 250 days. See Table 1 for the test scheme. The sludge retention time (SRT) was controlled by the hydraulic method; a peristaltic pump was used for continuous discharge of excess sludge from the main anoxic section at a rate of 7.5 L/d.26

Figure 2.

Figure 2

Open in a new tab

SAMA diagram and logic control diagram of an automatic control system.

Table 1. Start Scheme and Test Scheme of MUCT Reaction Systema.

  stage timebd sludge load kg/(kg·d) MLSS mg/L HRT h sludge return ratio s predeficient—anaerobic mixture reflux ratio r aerobic—main anoxic mixture reflux ratio a SRT d ORPan (mV) temperature control value °C DO concentration setting value at the end of aerobic section mg/L surface load of precipitator m3/(m2·h)
inoculation inoculation 1 1–7 0.899c 1000 9 1 0.5 1.5 12   22 ± 1 2.0 ± 0.1 0.14
  inoculation 2 8–25 0.388 2500   0.5 1 2.5          
  inoculation 3 26–37 0.253 3500       3          
experiment experiment 1 1–30 0.253 3500 9 0.5 1 regulated by ORPan 12 –140 22 ± 1 2.0 ± 0.1 0.14
  experiment 2 31–60               –125      
  experiment 3 61–90               –110      
  experiment 4 91–120               –95      
  experiment 5 121–150               –70      
  experiment 6 151–180               –60      
Open in a new tab
a

The data listed in the table are all MUCT process design values or set values, rather than test values during operation.

b

The operation time was different from that in Figure 3, and the two should be unified. This part of the time was the total running time, and the time in Figure 3 was the running data of 20 d selected during the statistical analysis of data.

c

Calculation value of adding activated sludge to the reactor quantitatively on the first day of inoculation.

2.4. Activated Sludge Inoculation and System Startup

During the system startup, see Table 1 for operation parameters of the MUCT system, the recirculated sludge from the secondary sedimentation tank of Changchun Beijiao sewage treatment plant was added to the reactor for continuous operation after inoculation, and the activated sludge was cultured and domesticated. The concentration of water quality indicators was tested every day.

2.5. Detection Indicators and Methods

During the experiment, the samples were taken from anaerobic, preanoxic, main anoxic, and aerobic stages, respectively. After water samples were taken out and centrifuged at 4500 rpm for 7 min, the concentration of chemical oxygen demand (COD), TP, TN, NH4+–N, and NO3–N in the supernatant was determined. The content of PHA in sludge after freeze-drying was determined by gas chromatography.37,41−43 The DO concentration in the main anoxic section was measured using a WTW-pH/OXi340 portable DO detector. DO in the aerobic section was regulated by PLC automatic control system, and the value of the DO detector was read as the test result. The data used in the analysis of the test results were taken from the indexes for 20 days during the stable operation period of each section. Chemical samples used in this study are shown in Table 2.

Table 2. Chemical Samples Used and the Suppliersa.

no. component no. component
1 potassium bichromate (guaranteed reagent) 18 sodium carbonate
2 1,10-phenanthroline 19 sodium bicarbonate
3 ferrous sulfate 20 sodium nitrate (guaranteed reagent)
4 ammonium ferrous sulfate 21 methanol (chromatographic purity)
5 concentrated sulfuric acid 22 carboxybenzene (chromatographic purity)
6 potassium persulfate 23 potassium dihydrogen phosphate
7 ascorbic acid 24 calcium chloride
8 ammonium molybdate 25 magnesium sulfate
9 potassium hydrogen phosphate (guaranteed reagent) 26 iron chloride
10 sodium hydroxide 27 boric acid
11 hydrochloric acid 28 cobalt chloride heptahydrate
12 potassium nitrate (guaranteed reagent) 29 copper sulfate pentahydrate
13 trichloromethane 30 cobalt chloride tetrahydrate
14 potassium iodide 31 sodium molybdate
15 mercury iodide 32 zinc sulfate heptahydrate
16 potassium sodium tartrate 33 a mixture of PHB and PHA (mass ratio 9:1)
17 ammonium chloride (guaranteed reagent) 34 PH2MV
Open in a new tab
a

Except for a mixture of PHB and PHA (mass ratio 9:1) and PH2MV were supplied by Sigma Corporation of America, the other Chemical samples were all supplied by Tianjin Guangfu Fine Chemical Research Institute, China.

2.6. Material Balance Analysis

Based on the principle of material balance, each reaction section was taken as the boundary of the system, and the amount of material reaction was calculated using the substance detection concentration. The results were expressed as mean ± error bar.

Assuming that the amount of material accumulated during the stable operation of the system was 0, the formula is as follows;

Anaerobic stage

2.6. 1

Preanoxic stage

2.6. 2

Main anoxic stage

2.6. 3

Aerobic stage

2.6. 4

where subscripts 0, 1, 2, 3, 4, and 5 represent inlet water, anaerobic stage, preanoxic stage, main anoxic stage, aerobic stage, and outlet water, respectively; ΔS denotes the amount of material reaction, g/d; Q represents inflow flow, L/d; V represents the effective volume of the reactor, L; S denotes the concentration of substance, mg/L; s represents sludge reflux ratio; a represents aerobic/main anoxic nitrifying fluid circulation ratio; and r denotes preanoxic/anaerobic mixture circulation ratio.

2.7. Chemometrics and Coefficient of Reaction Process

The biochemical reaction processes of MUCT process included COD aerobic degradation, anaerobic phosphorus release, denitrifying phosphorus absorption, aerobic phosphorus absorption, denitrification, and nitrification. COD in sewage is expressed by empirical molecular formula C18H19O9N.

The empirical molecular formula of cell material is C5H7NO2, and the empirical molecular formula of PHA is CH1.5O0.5. The chemical equation for the oxidation of C18H19O9N to CO2 is shown in eq 5.fd544 Formula 5 shows that the oxygen equivalent of C18H19O9N was 1.42 gCOD/gC18H19O9N.

2.7. 5

The chemical equation for the oxidation of PHA to CO2 is shown in eq 6. Formula 6 shows that the COD equivalent of PHA was 1.67 gCOD/gCH1.5O0.5.

2.7. 6

The biochemical reaction equation for aerobic degradation of COD is shown in eq 7. According to the formula, the stoichiometric coefficient of O2–C18H19O9N–COD was 2.19 gCOD/gO2.

2.7. 7

The denitrifying biochemical reaction equation using COD as a carbon source is shown in eq 8. It can be seen from the formula that the stoichiometric coefficient of NO3–C18H19O9N–COD was 0.18 gNO3/gCOD, and the stoichiometric coefficient of NO3–COD–C18H19O9N–NO3 was 5.35 gCOD/gNO3.

2.7. 8

The denitrifying biochemical reaction equation using acetic acid as a carbon source is shown in eq 9. According to the formula, the stoichiometric coefficient of C2H4O2–NO3 was 5.10 gC2H4O2/gNO3.

2.7. 9

3. Test Results

3.1. Start-Up

After 37 days of continuous operation, the nitrification effect of the system was good, the activated sludge showed a strong ability of phosphorus release and absorption, the effluent concentration of TN and TP were below 20 and 1 mg/L, respectively, and the activated sludge had been cultured and domesticated. The biological phase of the activated sludge sample was examined by a microscope, and it was found that the characteristics of the activated sludge bacterial micelles were good.

In a previous study (2015)45 based on the principle of chemometrics, the proportion of main microorganisms in the MUCT process was calculated by using material balance: 35.94% of common heterotrophic bacteria, 16.84% of denitrifying bacteria (with COD as carbon source), 41.54% of phosphorus accumulating bacteria (of which, denitrifying phosphorus accumulating bacteria accounted for 20.63%), and 5.78% of nitrifying bacteria. The operation parameters and influent water quality of the above study were similar to that of the start-up period (8–37 d), especially in the later period (25–37 d). In addition, because the two studies were carried out in the same set of MUCT system test device, the above calculation results can be used as the microbiological basis for qualitative analysis in this paper.

3.2. ORPan and System Performance

During the test, the change rule of COD, nitrogen, phosphorus and other pollution index concentration, and PHA content in sludge in each reaction section of the system are shown in Figure 3. The errors of the measured values were less than 2 mV. It can be seen from Figure 3 that when the carbon–nitrogen ratio of the influent water was low, the setting value of ORPan could be adjusted and controlled, which had little influence on COD, ammonia nitrogen, and nitrite–nitrogen. The effluent concentration changed in the range of 10.26–23.65, 1.16–4.54, and 0–0.03 mg/L, respectively. However, the effluent concentration of TN, TP and nitrate–nitrogen was greatly affected by the regulation of ORPan setting value. The effluent concentration was 12.06–24.58, 0.21–5.03, and 8.67–22.34 mg/L, respectively. When the ORPan set value was −95 mV, the effluent concentration of the three was the lowest. Therefore, it can be inferred that −95 mV was the best setting value.

Figure 3.

Figure 3

Open in a new tab

Variation of the material index in each stage of the MUCT process. (a) COD variation; (b) PHA variation; (c) TP variation; (d) TN variation; (e) NO4+–N variation; (f) NO3––N variation. Note: ☆ in (f) indicates the concentration of nitrite nitrogen of AS (effluent).

In addition, the removal rates of three pollutants were determined based on the calculation results of the material balance of COD, TN, and TP. When the ORP setting value was controlled at −140, −125, −110, −95, −75, and −60 mV, the removal rates of COD were basically stable, 238.26 ± 24.95 mg COD/(gMLSS·d); the removal rates of TN were 28.45 ± 3.52, 29.99 ± 1.23, 31.05 ± 0.94, 35.02 ± 0.83, 34.52 ± 1.08, and 34.11 ± 2.04 mg TN/(gMLSS·d), TP were 3.26 ± 0.22, 3.98 ± 0.82, 4.83 ± 1.45, 5.48 ± 0.76, 5.38 ± 0.45, and 5.20 ± 0.63 g TP/(gMLSS·d), respectively. The results showed that the COD removal rate was not affected by ORP regulation value but TN and TP changed significantly. When ORP regulation value was −95 mV, both of them reached the highest values, which also confirmed that −95 mV was the best setting value.

During the test, with the main anoxic section as the system boundary, the TP concentration detection value was used to calculate the phosphorus absorption, and the results are shown in Figure 4. It can be seen from Figure 4 that with the increase of ORPan, the phosphorus absorption rate in the main anoxic stage increased from 2.57 ± 1.41 g TP/(gMLSS·d) to 13.07 ± 0.52 mg TP/(gMLSS·d). This indicated that the regulation of ORPan could enhance the function of denitrifying phosphorus-accumulating bacteria and obtained better denitrifying phosphorus absorbing effect.

Figure 4.

Figure 4

Open in a new tab

Absorption amount of TP in the main anoxic stage.

3.3. ORPan and the Quality Distribution of COD and Nitrate–Nitrogen

3.3.1. ORPan and COD Mass Distribution

Based on material balance analysis and stoichiometry, the mass distribution of COD in each stage was tracked. When ORPan was controlled as −140 mV, the mass distribution of carbon and nitrogen in each reaction section was calculated using the average value of the calculated results of material balance.

Anaerobic stage:

The amount of COD stored in the synthesized PHA is CODANS,PHA is as follows

3.3.1. 10

COD consumption in the denitrification process is CODANS,DE is

3.3.1. 11

Preanoxic stage:

COD consumption in the denitrification process is CODPAnS,DE is

3.3.1. 12

The amount of COD stored in synthetic PHA is CODANS,PHA is

3.3.1. 13

Main anoxic stage:

The amount of COD degraded by the aerobic method is CODMAnS,O is

3.3.1. 14

COD consumption in the denitrification process is CODMAnS,DE is

3.3.1. 15

The amount of nitrate–nitrogen (NO3) MAnS,COD using COD as a carbon source is

3.3.1. 16
3.3.1. 17

Using the same method and procedure, the material distribution of COD in a nonaerated section can be calculated when ORPan was controlled to be −125, −110, −95, −70, and −60, respectively. The results are shown in Figure 5. Because of the need to regulate ORPan in each test stage, the circulation ratio of nitrate was a variable, and its value was the average value of the output of the automatic control system, which was 1.23 ± 0.13, 1.51 ± 0.25, 2.34 ± 0.09, 3.23 ± 0.06, 3.37 ± 0.17, and 3.59 ± 0.10, respectively. It can be seen from Figure 5a that when ORPan was increased from—140 to −95 mV, the amount of COD in anaerobic section synthesized into PHA and stored in phosphorus-accumulating bacteria was increased from 27.01 to 35.57 gCOD/d, and the distribution rate was increased from 39.63 to 51.74%. When denitrifying bacteria were used to transform nitrate nitrogen, the amount of COD decreased to 0.75 g COD/d at −140 mV, and the flow rate was 1.10%. When the control value was −95 mV, the denitrifying bacteria did not obtain COD.

Figure 5.

Figure 5

Open in a new tab

Material distribution of carbon and nitrogen in different stages [average values with confidence bands (n = 20, P = 0.683]. (a) Distribution of COD in the anaerobic stage; (b) distribution of COD in anaerobic preanoxic stage; (c) distribution of COD in the main anoxic stage; (d) distribution of nitrate–nitrogen in main anoxic stage. Note: figures in (a) showed the percentage of COD distributed to phosphate accumulating bacteria and denitrifying bacteria in the anaerobic section of the process; figures in (b) showed the percentage of COD distributed to phosphate accumulating bacteria and denitrifying bacteria in the preanoxic stage of the process; figures in (c) showed the percentage of COD distributed to phosphate accumulating bacteria and denitrifying bacteria in the main anoxic stage of the process; figures in (d) showed the percentage of NO3––N distributed to phosphate accumulating bacteria and denitrifying bacteria in the main anoxic stage of the process.

Then again, when ORPan was set at −140 and −125 mV, the COD allocated to phosphorus-accumulating bacteria was 0 (Figure 5b). Thereafter, with the increase in ORPan value, the amount of COD allocated to phosphorus-accumulating bacteria increased. When the set value was −95 mV, the storage capacity of PHA and the allocation rate of COD reached the highest values of 1.39 g/d and 2.51%. Correspondingly, the amount of COD allocated to denitrifying bacteria decreased with the increase of ORPan. When the set value was −95 mV, the consumption and distribution rate of denitrifying bacteria reached the lowest values of 5.29 g/d and 7.70%.

From Figure 5c, it can be deduced that the distribution ratio of COD between denitrifying bacteria and heterotrophic bacteria changes regularly with the increase of ORPan. When the control value increased from −140 to −60 mV, the distribution ratio of COD to common heterotrophic bacteria increased, and the distribution ratio was 1.52, 2.10, 2.15, 4.40, 4.13, and 4.60%. Correspondingly, the COD to denitrifying bacteria decreased. The distribution rates were 10.13, 9.65, 9.28, 7.93, 7.19, and 6.15%, respectively.

To sum up, the flow and distribution of COD in MUCT process mainly occur in anaerobic, preanoxic, and main anoxic stages, where the distribution of COD in anaerobic and preanoxic stages was between phosphorus-accumulating bacteria and denitrifying bacteria. The distribution of COD in the main anoxic stage was between the common heterotrophic bacteria and denitrifying bacteria. The distribution proportion varied regularly with the difference of ORPan.

3.3.2. ORPan and Nitrate–Nitrogen Mass Distribution

When ORPan was set at −140 mV, the flow direction of nitrate–nitrogen in the main anoxic stage was calculated as Section 3.3.1. Using the same method and procedure, the distribution of nitrogen in the main anoxic stage was calculated when ORPan was controlled as −125, −105, −95, −70, and −60 mV, respectively. The results are shown in Figure 5d. With ORPan increasing from −140 to −60 mV, the amount of nitrate–nitrogen transformed by denitrifying bacteria gradually decreased, at distribution rates of 23.54, 20.86, 19.89, 16.76, 15.10, and 13.68%, respectively. Correspondingly, nitrate–nitrogen transformed by denitrifying phosphorus-accumulating bacteria was increased, at distribution rates of 76.46, 79.14, 80.11, 83.24, 84.90, and 86.32%, respectively. That is to say, when ORPan control was −95 mV, most of nitrate–nitrogen flowed to denitrifying phosphorus accumulating bacteria, and the system experienced the best effect of nitrogen and phosphorus removal.

3.4. ORPan and Carbon Source Savings

With the assumption of no denitrifying phosphorus uptake, when ORPan was controlled at −140 mV, the required amount of acetic acid (C2H4O2)ECS could be calculated according to formula 9.

3.4. 18

Under similar method and procedure as previous, we can calculate the amount of acetic acid as external carbon sources required when ORPan was controlled as −125, −105, −95, −70, and −60 mV, respectively, without anoxic phosphorus absorption. The results are shown in Figure 6. From the results, the amount of acetic acid added increased from 20.33 to 24.76 g/d when ORPan was increased from −140 to −95 mV. In other words, carbon sources can be saved during the process of phosphorus absorption by denitrification, and the carbon source saving rate increased from 23.19 to 26.56%. Subsequently, the dosage of acetic acid remained stable or decreased slightly with the increase of ORPan.

Figure 6.

Figure 6

Open in a new tab

Acetic acid dosage and carbon savings under different ORPan conditions without denitrifying phosphorus uptake [average values with confidence bands (n = 20, P = 0.683)].

4. Discussion

4.1. Mass Distribution of ORP, COD, and Nitrate Nitrogen

4.1.1. ORPan and the Mass Distribution of COD

From the perspective of microbiology and metabolism, in the sewage biological treatment system, the dynamic changes of activated sludge microbiome structure and dominant microbiome were closely related to the ORP of their living environment,37 and the dominant reaction process was also quite different,41 as shown in Figure 7. In addition, the regulation of ORP altered the content of NADH/NAD+ and NADPH/NADP+ in bacterial metabolism and activated some key enzymes, thereby changing the metabolic network of substances.37 Combined with the experimental results, when ORPan was set at −95 mV, the nitrate–nitrogen reaction and phosphorus absorption in the main anoxic stage were the highest, as shown in Figures 6 and 4. This indicated that the ORPan value was the appropriate ORP range for denitrifying phosphorous accumulating bacteria, activated nitrate reductase, and PHA degrading enzyme in the bacteria to the greatest extent, which enhanced the function of denitrifying phosphorous-accumulating bacteria.

Figure 7.

Figure 7

Open in a new tab

Microbial metabolic processes and ORP ranges. (1) Organic matter oxidation; (2) phosphorus accumulation; (3) nitrification; (4) denitrification; (5) phosphorus release; (6) hydrogen sulfide production; (7) acid production; (8) methane production.

There are many kinds of redox potentials in the main anoxic section of MUCT system, which is a complex redox system. ORP is the comprehensive result of the redox reaction of many kinds of redox substances and redox substances. From the macroenvironment, the main oxidizing substances are oxygen and nitrate nitrogen, and the reducing substances are COD. Therefore, ignoring other redox processes, the ORP in the main anoxic section can be calculated by the Nernst equation, as shown in formulas 19 and 20

4.1.1. 19
4.1.1. 20

where E is the ORP value in the main anoxic section; Eθ is the standard electrode potential; R is the general gas constant; n is the number of transferred electrons; T is the temperature; F is the Faraday constant; and C [NO3], C [O2], and C [COD] are the relative values of the three concentrations to the standard concentrations. Generally, the concentration is taken for calculation; 4.54, 0.61, 8, and 1 are the stoichiometric coefficients of the reaction, respectively, as shown in formulas 7 and 8. It can be seen from the formula that ORPan in the main anoxic section is closely related to DO concentration, nitrate concentration, COD concentration, and pH value in this section.46 During the test, the DO concentration in this section was stable between 0.08 and 0.09 mg/L, and the COD concentration was stable between 18.19 and 22.30 mg/L, with little change; the pH value was stable between 7.17 and 7.71, and the variation pattern is shown in Table 3. Therefore, it can be inferred that the change of ORPan value was mainly caused by the concentration of nitrate–nitrogen, reflecting its concentration level. Based on the activated sludge reaction kinetic model 2D (ASM2d), Wang et al.47 established the MUCT process reaction kinetic model and checked and verified the kinetic parameters and stoichiometric coefficients. They also determined that the values of kinetic parameters qPHA, KA, KPP, YPO3, μAUT, and ηNO were 2.90 g·(g·d)−1, 3.85 g·m–3, 1.35 g·(g·d)−1, 0.35, 1.6, and 0.8, respectively. Other kinetic parameters and stoichiometric coefficients adopt the default values recommended by IWA. According to the model, the denitrification phosphorus-absorption performance was determined by the nitrate–nitrogen concentration in this section, which was also proved by the research of Wang and Musvoto et al.24,25

Table 3. Variation Rule of pH Value in Each Section during the Testa.
  experiment 1 experiment 2 experiment 3 experiment 4 experiment 5 experiment 6
influent 7.41 ± 0.12 7.05 ± 0.03 7.14 ± 0.07 6.88 ± 0.11 7.07 ± 0.12 7.21 ± 0.05
ANA 7.05 ± 0.11 6.97 ± 0.09 7.02 ± 0.05 7.08 ± 0.04 6.87 ± 0.08 7.01 ± 0.07
PAnS 7.21 ± 0.09 7.19 ± 0.10 7.10 ± 0.06 7.24 ± 0.06 7.14 ± 0.14 7.11 ± 0.05
MAnS 7.33 ± 0.16 7.42 ± 0.03 7.54 ± 0.12 7.62 ± 0.09 7.55 ± 0.03 7.48 ± 0.04
AS 6.95 ± 0.10 7.07 ± 0.04 6.82 ± 0.15 7.01 ± 0.05 7.17 ± 0.04 7.07 ± 0.09
Open in a new tab
a

The data in the table is the average value ± standard deviation of pH value in each stage (n = 20, P = 68.3%)

In order to make the most effective use of the denitrifying phosphorus uptake potential in the main anoxic stage, it was necessary to ensure that the nitrate–nitrogen concentration in the main anoxic stage is not too low or zero and not be too high. Explicitly, there was an optimum concentration range of nitrate–nitrogen and a corresponding optimum range in ORPan. When the concentration of nitrate–nitrogen in the main anoxic stage varied from 0.5 to 2.5 mg/L (ORPan was between −140 and −95 mV), the process of denitrifying phosphorus uptake became more complete with the increase of the set value. Nevertheless, when the concentration of nitrate–nitrogen was higher, thus ORPan was greater than −95 mV; the internal circulation flow rate of nitrate solution needed to control the value increased. Therefore, the amount of DO entering this section increased (even when DO concentration changed slightly), also the actual hydraulic retention time was shortened, resulting in the inadequate realization of denitrification and phosphorus uptake process.

In the main anoxic stage, COD was distributed between heterotrophic bacteria and denitrifying bacteria and the distribution proportion was mainly determined by the DO consumption amount. According to formula 3, when the ORPan control value increased from −140 to −60 mV, the DO reactions were 0.47, 0.65, 0.67, 1.38, 1.27, and 1.44 gO2/d, respectively. Therefore, with the increase of ORPan control value, the COD flow to heterotrophic bacteria increased, whereas the COD flow to denitrifying bacteria decreased (see Figure 5). Figure 8 shows the flow direction of COD and nitrate nitrogen in the nonaerated reaction section when ORPan was controlled at −140 and −95 mV, respectively, showing the distribution ratio of COD and nitrate nitrogen as well. The distribution ratio of COD was the ratio between the distribution amount and total COD reaction amount, and the proportion of nitrate nitrogen distribution was the ratio between the distribution amount and the reaction amount of nitrate nitrogen in the main anoxic stage.

Figure 8.

Figure 8

Open in a new tab

Flow direction of COD and nitrate nitrogen in nonaerated reaction section when ORPan was controlled at −140 and −95 mV.

In anaerobic stage and preanoxic stage, with nitrate–nitrogen present, COD would be distributed to denitrifying bacteria and phosphorus accumulating bacteria, respectively, and the distribution proportion was affected by the competition between denitrifying bacteria and phosphorus accumulating bacteria. It has been reported that denitrifying bacteria are dominant in this competition.43 When ORPan was controlled at a low value, the denitrification and phosphorus absorption potential in the main anoxic section had not been fully developed, and the nitrate–nitrogen concentration in the effluent was high. This caused the nitrate–nitrogen amount in the pre anoxic section to increase, while more COD flowed to denitrifying bacteria and few or almost no phosphorus accumulating bacteria. For example, when ORPan control was −140, −125, −110 mV, there was no PHA synthesis and storage in the pre hypoxia section, and the distribution proportion of phosphorus accumulating bacteria was 0; but when increased to −95, −70, and −60 mV, some COD flowed to phosphorus accumulating bacteria to synthesize PHA (see Figure 5). It can be seen from Figure 8 that when ORP was −140 mV, the COD allocation rate of phosphorus accumulating bacteria in the preanoxic stage was 0%, and denitrifying bacteria was 11.37%; when ORP was −95 mV, the COD allocation rate of phosphorus accumulating bacteria in this stage was increased to 2.52%, and denitrifying bacteria was reduced to 7.70%.

4.1.2. ORPan and Mass Distribution of Nitrate–Nitrogen

According to the principle of stoichiometric reaction, the COD flowing to denitrifying bacteria decreased and the consumption of nitrate–nitrogen decreased under conditions of increased ORPan control value. In addition, the performance of denitrifying phosphorus uptake in the main anoxic zone was enhanced with the increase in ORPan control value. Hence, the amount of nitrate–nitrogen transformed by denitrifying phosphorus-accumulating bacteria increased as well. The distribution and proportion (distribution ratio) increased similarly, as shown in Figure 5. The nitrate–nitrogen distribution ratio of denitrifying bacteria was 23.54% and that of denitrifying phosphorus-accumulating bacteria was 76.46% when the ORPan control value was −140 mV. Similarly, when the ORPan control value was −95 mV, the denitrifying bacteria distribution ratio decreased to 16.76% and that of denitrifying phosphorus-accumulating bacteria increased to 83.24%, as shown in Figure 8.

On the other hand, the competition between denitrifying bacteria and phosphorus-accumulating bacteria was enriched when nitrate–nitrogen was circulated to the anaerobic stage with mixed liquor. Compared with the situation without nitrate, the amount of COD consumed by denitrifying bacteria and the amount of phosphorus-accumulating bacteria absorbed were both transformed as shown in Figure 4. Therefore, the COD distribution ratio was also changed, as shown in Figure 8.

4.2. ORPan and Carbon saving

Assuming that the absence of denitrifying phosphorus uptake in the main anoxic stage of MUCT process, the nitrate–nitrogen consumed by the anaerobic growth of phosphorus-accumulating bacteria needed to be transformed by denitrifying bacteria, which required the addition of external carbon sources to achieve the same treatment effect (denitrifying phosphorus uptake took place). Specifically, there was a denitrifying phosphorus absorption process, which reduced the number of external carbon sources. In the wastewater treatment process, the common external carbon sources are methanol and acetic acid, which are simple, economical, and practical organic substances. The dosage of acetic acid as a carbon source was determined by stoichiometry (see Figure 7). In the MUCT process, with the increase of ORPan control value, the denitrification and phosphorus absorption performance could be enhanced, and more nitrate–nitrogen would be removed in this process. Therefore, more external carbon sources were saved. When ORPan was controlled to −95 mV, the highest carbon saving rate was 26.56%. This result was lower than the 50% saving rate obtained by Kuba et al.,48 but close to 24.29% of Wang when the internal circulation ratio of nitration liquid was 3.0 and the internal circulation ratio of mixed liquid was 1.5.49 Kuba et al. run the double sludge A2NSBR system in the laboratory: one was A/ASBR operated by anaerobic/anoxic mode to achieve denitrification and phosphorus removal; the other was NSBR operated by aerobic mode to carry out nitrification. At the end of nitrification, the effluent entered into anoxic SBR. In the A2NSBR system, denitrifying phosphorus-accumulating bacteria and nitrifying bacteria were completely separated, which had the best denitrifying phosphorus absorption performance. Therefore, the carbon source saving rate was also very high. In the MUCT process operated by Wang et al., the internal circulation ratio of nitration liquid was constant at 3.0, which was basically close to the value controlled in this paper when ORP was −95 mV, so the carbon source saving effect of the two processes was also similar.

5. Conclusions

In summary, this study assessed the distribution rate of COD between denitrifying bacteria and phosphorus-accumulating bacteria in anaerobic and preanoxic stages. The results proved that when the ORPan set value was −95 mV, the distribution rate reached the highest, 51.74 and 7.70%, respectively. COD in the main anoxic segment was distributed between ordinary heterotrophic bacteria and denitrifying bacteria. When ORPan set the value of −95 mV, the distribution proportion was 4.40 and 7.19%, respectively. In addition, the distribution of nitrate–nitrogen between denitrifying bacteria and denitrifying phosphorus-accumulating bacteria in the main anoxic stage decreased from 1.24 to 0.98 g/d with the increase of ORPan from −140 to −95 mV. Meanwhile, the amount of nitrate–nitrogen transformed by denitrifying phosphorus-accumulating bacteria increased from 4.03 to 5.18 g/d under similar conditions. Again, with the increase in ORPan control value, the acetic acid dosage increased from 20.33 g/d at −140 mV to 24.76 g/d at −95 mV, and the carbon source saving rate increased from 23.19 to 26.56%, and then remained stable.

From this study, the MUCT process can achieve good operation status and better carbon saving effect by regulating ORPan. Future studies should consider building a control system of the denitrification phosphorus absorption process to examine and realize the regulation function of this parameter.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (no. 51808254), and the Education Department of Jilin Province (no. JJKH20190856KJ).

The authors declare no competing financial interest.

References

  1. Sun Y.; Chen Z.; Wu G.; Wu Q.; Zhang F.; Niu Z.; Hu H.-Y. Characteristics of water quality of municipal wastewater treatment plants in China: implications for resources utilization and management. J. Cleaner Prod. 2016, 131, 1–9. 10.1016/j.jclepro.2016.05.068. [DOI] [Google Scholar]
  2. Yang Y.; Ok Y. S.; Kim K.-H.; Kwon E. E.; Tsang Y. F. Occurrences and removal of pharmaceuticals and personal care products (PPCPs) in drinking water and water/sewage treatment plants: A review. Sci. Total Environ. 2017, 596–597, 303–320. 10.1016/j.scitotenv.2017.04.102. [DOI] [PubMed] [Google Scholar]
  3. Guerrero J.; Guisasola A.; Baeza J. A. The nature of the carbon source rules the competition between PAO and denitrifiers in systems for simultaneous biological nitrogen and phosphorus removal. Water Res. 2011, 45, 4793–4802. 10.1016/j.watres.2011.06.019. [DOI] [PubMed] [Google Scholar]
  4. Zhu Y.; Tu X.; Chai X.-S.; Wei Q.; Guo L. Biological activities and nitrogen and phosphorus removal during the anabaena flos-aquae biofilm growth using different nutrient form. Bioresour. Technol. 2018, 251, 7–12. 10.1016/j.biortech.2017.12.003. [DOI] [PubMed] [Google Scholar]
  5. Rikmann E.; Zekker I.; Tomingas M.; Vabamäe P.; Kroon K.; Saluste A.; Tenno T.; Menert A.; Loorits L.; dC Rubin S. S. C.; Tenno T. Comparison of sulfate-reducing and conventional Anammox upflow anaerobic sludge blanket reactors. J. Biosci. Bioeng. 2014, 118, 426–433. 10.1016/j.jbiosc.2014.03.012. [DOI] [PubMed] [Google Scholar]
  6. Mandel A.; Zekker I.; Jaagura M.; Tenno T. Enhancement of anoxic phosphorus uptake of denitrifying phosphorus removal process by biomass adaption. Int. J. Environ. Sci. Technol. 2019, 16, 5965–5978. 10.1007/s13762-018-02194-2. [DOI] [Google Scholar]
  7. dC Rubin S. S.; Irma M.; Manuel J. G.; Eduardo A. M.; Ivar Z.; Patxi S. M.; Nuria R.; Ricardo A. Prokaryotic diversity and community composition in the Salar de Uyuni, a large scale, chaotropic salt flat. Environ. Microbiol. 2017, 28, 3745–3754. 10.1111/1462-2920.13876. [DOI] [PubMed] [Google Scholar]
  8. Klein K.; Kattel E.; Goi A.; Kivi A.; Dulova N.; Saluste A.; Zekker I.; Trapido M.; Tenno T. Combined treatment of pyrogenic wastewater from oil shale retorting. Oil Shale 2017, 34, 82–96. 10.3176/oil.2017.1.06. [DOI] [Google Scholar]
  9. Klein K.; Kivi A.; Dulova N.; Zekker I.; Mölder E.; Tenno T.; Trapido M.; Tenno T. A pilot study of three-stage biological-chemical treatment of landfill leachate applying continuous ferric sludge reuse in Fenton-like process. Clean Technol. Environ. 2017, 19, 541–551. 10.1007/s10098-016-1245-5. [DOI] [Google Scholar]
  10. Daija L.; Selberg A.; Rikmann E.; Zekker I.; Tenno T.; Tenno T. The influence of lower temperature, influent fluctuations and long retention time on the performance of an upflow mode laboratory-scale septic tank. Desalin. Water Treat. 2016, 57, 18679. 10.1080/19443994.2015.1094421. [DOI] [Google Scholar]
  11. Zekker A.; Rikmann E.; Tenno T.; Vabamäe P.; Tomingas M.; Menert A.; Loorits L.; Tenno T. Anammox bacteria enrichment and phylogenic analysis in moving bed biofilm reactors. Environ. Eng. Sci. 2012, 29, 946–950. 10.1089/ees.2011.0146. [DOI] [Google Scholar]
  12. Zekker I.; Rikmann E.; Tenno T.; Menert A.; Lemmiksoo V.; Saluste A.; Tenno T.; Tomingas M. Modification of nitrifying biofilm into nitritating one by combination of increased free ammonia concentrations, lowered HRT and dissolved oxygen concentration. J. Environ. Sci. 2011, 23, 1113–1121. 10.1016/s1001-0742(10)60523-2. [DOI] [PubMed] [Google Scholar]
  13. Zeng W.; Li L.; Yang Y.-y.; Wang X.-d.; Peng Y.-z. Denitrifying phosphorus removal and impact of nitrite accumulation on phosphorus removal in a continuous anaerobic-anoxic-aerobic (A2O) process treating domestic wastewater. Enzyme Microb. Technol. 2011, 48, 134–142. 10.1016/j.enzmictec.2010.10.010. [DOI] [PubMed] [Google Scholar]
  14. Yuan Q.; Oleszkiewicz J. Selection and enrichment of denitrifying phosphorus accumulating organisms in activated sludge. Desalin. Water Treat. 2010, 22, 72–77. 10.5004/dwt.2010.1591. [DOI] [Google Scholar]
  15. Kapagiannidis A. G.; Zafiriadis I.; Aivasidis A. Comparison between aerobic and anoxic metabolism of denitrifying-EBPR sludge: effect of biomass poly-hydroxyalkanoates content. New Biotechnol. 2013, 30, 227–237. 10.1016/j.nbt.2012.05.022. [DOI] [PubMed] [Google Scholar]
  16. Zou H.; Wang Y. Phosphorus removal and recovery from domestic wastewater in a novel process of enhanced biological phosphorus removal coupled with crystallization. Bioresour. Technol. 2016, 211, 87–92. 10.1016/j.biortech.2016.03.073. [DOI] [PubMed] [Google Scholar]
  17. Souza S. M.; Araújo O. Q. F.; Coelho M. A. Z. Model-based optimization of a sequencing batch reactor for biological nitrogen removal. Bioresour. Technol. 2008, 99, 3213–3223. 10.1016/j.biortech.2007.05.066. [DOI] [PubMed] [Google Scholar]
  18. Vaiopoulou E.; Aivasidis A. A modified UCT method for biological nutrient removal: Configuration and performance. Chemosphere 2008, 72, 1062–1068. 10.1016/j.chemosphere.2008.04.044. [DOI] [PubMed] [Google Scholar]
  19. Duan J.; Li W.; Zhao K.; Krampe J. Variation of organics, nitrogen and phosphorus within a cycle of a Bio-Denipho system. Desalin. Water Treat. 2012, 40, 24–32. 10.1080/19443994.2012.671137. [DOI] [Google Scholar]
  20. Henze M.; Gujer W.; Mino T.; Matsuo T.; Wentzel M. C.; Marais G. v. R.; Van Loosdrecht M. C. M. Activated Sludge Model No.2d, ASM2D. Water Sci. Technol. 1999, 39, 165–182. 10.2166/wst.1999.0036. [DOI] [Google Scholar]
  21. Bortone G.; Libelli S. M.; Tilche A.; Wanner J. Anoxic phosphate uptake in the dephanox process. Water Sci. Technol. 1999, 40, 177–185. 10.2166/wst.1999.0590. [DOI] [Google Scholar]
  22. Wang J.; Wang L.; Cui E.; Lu H. Bioactivity kinetics of organic matter biodegradation and nitrification. Korean J. Chem. Eng. 2018, 35, 1274–1280. 10.1007/s11814-018-0040-x. [DOI] [Google Scholar]
  23. Nancharaiah Y. V.; Venkata Mohan S.; Lens P. N. L. Recent advances in nutrient removal and recovery in biological and bioelectrochemical systems. Bioresour. Technol. 2016, 215, 173–185. 10.1016/j.biortech.2016.03.129. [DOI] [PubMed] [Google Scholar]
  24. Musvoto E. V.; Casey T. G.; Ekama G. A.; Wentzel M. C.; Marais G. V. R.. The effect of a large anoxic mass fraction and concentrations of nitrate and nitrite. in the primary anoxic zone on low F/M filament bulking in nutrient removal activated sludge systems. B.Sc. Thesis, University of Cape Town, Rondebosch, 2000; pp 26–59. [Google Scholar]
  25. Wang X. L.; Yin J.; Gao S. Utilizing nitrate concentration in the second anoxic stage as a control parameter for MUCT process operation. Environ. Sci. 2012, 33, 175–180. [PubMed] [Google Scholar]
  26. Zhu G.; Peng Y.; Wang S.; Wu S.; Ma B. Effect of influent flow rate distribution on the performance of step-feed biological nitrogen removal process. Chem. Eng. J. 2007, 131, 319–328. 10.1016/j.cej.2006.12.023. [DOI] [Google Scholar]
  27. Soares A.; Kampas P.; Maillard S.; Wood E.; Brigg J.; Tillotson M.; Parsons S. A.; Cartmell E. Comparison between disintegrated and fermented sewage sludge for production of a carbon source suitable for biological nutrient removal. J. Hazard. Mater. 2010, 175, 733–739. 10.1016/j.jhazmat.2009.10.070. [DOI] [PubMed] [Google Scholar]
  28. Zhang W. T.; Xue X. F.; Pang H. T.; Zhang J.; Li D.; Peng Y. Z. Effect of C/N on the performance of AAO-BAF process. CIESC J. 2015, 66, 1925–1930. 10.11949/j.issn.0438-1157.20141719. [DOI] [Google Scholar]
  29. Huang M.-h.; Li Y.-m.; Gu G.-w. Chemical composition of organic matters in domestic wastewater. Desalination 2010, 262, 36–42. 10.1016/j.desal.2010.05.037. [DOI] [Google Scholar]
  30. Chuang S.; Ouyang C. F. The biomass fractions of heterotrophs and phosphate-accumulating organisms in a nitrogen and phosphorus removal system. Water Res. 2000, 34, 2283–2290. 10.1016/s0043-1354(99)00380-2. [DOI] [Google Scholar]
  31. Oehmen A.; Keller-Lehmann B.; Zeng R. J.; Yuan Z.; Keller J. Optimisation of poly-β-hydroxyalkanoate analysis using gas chromatography for enhanced biological phosphorus removal systems. J. Chromatogr. A 2005, 1070, 131–136. 10.1016/j.chroma.2005.02.020. [DOI] [PubMed] [Google Scholar]
  32. Ma Y.; Peng Y. Z.; Wang S. Y. Establishment and application of nitrification and denitrification expert system in A/O process. China Environ. Sci. 2005, 25, 252–256. [Google Scholar]
  33. Wang X.; Lu H.; Song T.; Zhao K. The effects of main anoxic section oxidation-reduction potential on the metabolism of PHA and TP in continuous-flow single-sludge treatment system. Korean J. Chem. Eng. 2019, 36, 411–422. 10.1007/s11814-018-0213-7. [DOI] [Google Scholar]
  34. Kim H.-T.; Kim G.-S.; Shin S.-W.; Oh S.-H.; Kim K.-H. Application of ORP and pH as controlling factors in sequencing batch reactor. KSCE J. Civ. Eng. 2005, 9, 73–79. 10.1007/bf02829061. [DOI] [Google Scholar]
  35. Liu X.; Chen Q.; Zhu L. Improving biodegradation potential of domestic wastewater by manipulating the size distribution of organic matter. J. Environ. Sci. 2016, 47, 174–182. 10.1016/j.jes.2016.02.004. [DOI] [PubMed] [Google Scholar]
  36. Chuang S.; Ouyang C. F. The biomass fractions of heterotrophs and phosphate-accumulating organisms in a nitrogen and phosphorus removal system. Water Res. 2000, 34, 2283–2290. 10.1016/s0043-1354(99)00380-2. [DOI] [Google Scholar]
  37. Wang X. L.; Song T. H.; Yin B. Y.; Li J. W.; Li Z. Q.; Yu Y. ORP in the main anoxic stage as the control parameter for nitrogen and phosphorus removal in the single sludge system with a continuous flow. Environ. Sci. 2015, 36, 2617–2625. [PubMed] [Google Scholar]
  38. München I. G.; Braunschweig I. K.. Design of Single Stage Activated Sludge Wastewater Treatment Plant; GFA Publishing Company: Hennef, 2000; pp 145–186. [Google Scholar]
  39. Water Environment Federation . Design of municipal wastewater treatment plants. In liquid treatment processes; McGraw-Hill, Inc.: New York, 2010; Vol. 2, pp 72–108. [Google Scholar]
  40. Shanghai Municipal Engineering Design Institute (Group) Co., LTD. Code for the Design of Outdoor Wastewater Engineering (in Chinese); China Planning Press: Beijing, 2016; pp 233–275. [Google Scholar]
  41. Caulet P.; Bujon B.; Philippe J. P.; Lefevre F.; Audic J. M. Upgrading of wastewater treatment plants for nitrogen removal: industrial application of an automated aeration management based on ORP evolution analysis. Water Sci. Technol. 1998, 37, 41–47. 10.2166/wst.1998.0338. [DOI] [Google Scholar]
  42. Kuba T.; Smolders G.; van Loosdrecht M. C. M.; Heijnen J. J. Biological Phosphorus Removal from Wastewater by Anaerobic-Anoxic Sequencing Batch Reactor. Water Sci. Technol. 1993, 27, 241–252. 10.2166/wst.1993.0504. [DOI] [Google Scholar]
  43. Henze M.; van Loosdrecht M. C. M.; Ekama G. A.; Brdjanovic D.. Biological Wastewater Treatment: Principles, Modelling and Design; IWA Publishing: London; 2010; pp 38–66. [Google Scholar]
  44. Henze M.; Harremoes P.; Jansens J. la C.; Arvin E.. Wastewater Treatment Biological and Chemical Processes; Springer-Verlag: Berlin; 2001; pp 59–72. [Google Scholar]
  45. Wang X.-L.; Song T.-H.; Yu X.-D. The biomass fraction of phosphate-accumulating organisms grown in anoxic environment in an enhanced biological phosphorus removal (EBPR) system. Desalin. Water Treat. 2015, 56, 1877–1887. 10.1080/19443994.2014.961557. [DOI] [Google Scholar]
  46. Zekker I.; Kivirüüt A.; Rikmann E.; Mandel A.; Jaagura M.; Tenno T.; Artemchuk O.; Rubin S. d.; Tenno T. Enhanced Efficiency of Nitritating-Anammox Sequencing Batch Reactor Achieved at Low Decrease Rates of Oxidation-Reduction Potential. Environ. Eng. Sci. 2019, 36, 350–360. 10.1089/ees.2018.0225. [DOI] [Google Scholar]
  47. Wang X. L.; Yin J.; Li S. K.; Wei X. D.; Gao S. Methodological study on calibrating and validating model parameters of MUCT processes at low C/N ratio. Environ. Sci. 2011, 32, 3412–3418. [PubMed] [Google Scholar]
  48. Kuba T.; van Loosdrecht M. C. M.; Heijnen J. J. Phosphorus and nitrogen removal with minimal COD requirement by integration of denitrifying dephosphatation and nitrification in a two-sludge system. Water Res. 1996, 30, 1702–1710. 10.1016/0043-1354(96)00050-4. [DOI] [Google Scholar]
  49. Wang X. L.Anoxic phosphorus uptake performance strengthening strategy in MUCT process, Ph.D. Thesis, Harbin Institute of Technology; (in Chinese), Harbin, 2010, pp 49–58. [Google Scholar]

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES