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P-nitrophenol (PNP) is highly toxic and difficult to degrade, causing great harm to the ecological environment and human health. A two-stage bench-scale membrane biofilm reactor (MBfR) was constructed to treat wastewater containing high concentration of PNP and the generated nitrogen without external organic carbon sources. The two reactors were supplied with oxygen and methane, respectively. O2-MBfR was used for the degradation of PNP and the improvement of wastewater biodegradability. CH4-MBfR was used for the total nitrogen (TN) removal treatment from O2-MBfR effluent. In this experiment, the performance of the two-stage MBfR process was evaluated and optimized by adjusting operational parameters (aeration pressure, HRT, and pH). Under the optimal operation parameters, the removal efficiencies of PNP (100 mg/L) and TN attained 89.70% and 69.24%, respectively, and the removal loads were 0.930 g·m−2·d−1 and 241.42 mg·m−2·d−1, respectively. The reactor was able to accommodate the concentrations of PNP up to 200–400 mg/L, and the reactor reached maximum efficiency throughout the process when the concentration of PNP in the wastewater was 250 mg/L. The removal rates of PNP and TN reached 95.0% and 69.48%, respectively, and the removal loads were 2.37 g·m−2·d−1 and 96.22 mg·m−2·d−1, respectively. This research provides a better solution for multi-MBfR to treat toxic industrial wastewater containing phenol, nitrophenol, and further TN removal, which would not release any air pollutants into the atmosphere.

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  • An original two-stage MBfR was successfully constructed with a stable operation effect.

  • The wastewater containing high concentration p-nitrophenol and nitrogen could be treated efficiently without external organic carbon sources.

  • The optimal operating parameters and the best threshold concentration of the two-stage MBfR were determined.

biodegradation, multi-MBfR, nitrogen removal, p-nitrophenol wastewater
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P-nitrophenol (PNP) is a sort of fine organic chemical intermediate, which is widely used in pesticide, medicine, petrochemical, plastics, paint, dye, and anti-corrosion industries (Yang et al. 2014; Dong et al. 2015). PNP is highly toxic and difficult to be degraded in the ecological environment, which can be transferred and enriched in aquatic organisms (Wu et al. 2016). It has been listed as a priority pollutant by environmental protection departments in many countries. In recent years, it has become one of the research hotspots to explore the treatment technology of PNP and other phenolic wastewater.

At present, the main methods of treating wastewater containing PNP or other refractory organic are physical (Marília et al. 2021; Muhammad et al. 2021), chemical (Chen et al. 2021a; Zahra et al. 2021), and biological (Ananya & Apurba 2020; Priyanka & Apurba 2020; Ke et al. 2021; Wang et al. 2021). Bioreactors are effective for the removal of organic contaminants at a comparatively low cost and become one of the hot topics of recent researches, as shown in Table 1. Multistage or coupled reactors are more suitable for treating high concentration PNP (>100 mg/L) wastewater. Low concentrations of PNP can be treated efficiently by single-stage bioreactors with high removal efficiencies of nitrogen. However, few studies have focused on the removal effect of nitrogen under high PNP loading. As shown in Table 2, the nitrogen removal efficiencies of refractory phenolic wastewater by bioreactors have been explored widely.

Table 1

PNP removal efficiencies and nitrogen removal efficiencies of various reactors observed in the previous publication

Reactor typesHighest influent PNPPNP removal efficiencyNitrogen removal efficiencyReferences
Nitrifying sludge bioreactor 10 mg/L 99.9% 99.5% Li et al. (2020)  
Sequencing batch reactor 40 mg/L 98.7% 94.9% Liu (2020)  
Anaerobic migrating blanket reactor/aerobic completely stirred tank reactor 100 mg/L 96%  Sponza & Kuscu (2005)  
Sieve plate tower biofilm reactor 100 mg/L 100% 87% Yuan (2020)  
Slurry bubble column 128 mg/L 100%  Salehi et al. (2010)  
Sequencing batch reactor 180 mg/L 99%  Liu et al. (2007)  
Upflow anaerobic sludge blanket 350 mg/L 97%  Xu et al. (2013)  
Upflow anaerobic sludge blanket reactor/bioelectrochemical system 350 mg/L 100%  Shen et al. (2014)  
Two integrated membrane aerated bioreactor 500 mg/L 95.86% 78.21% Mei et al. (2019a)  
Anaerobic semi-fixed bed biofilms reactor 540 mg/L 98%  Chen et al. (2021b)  
Reactor typesHighest influent PNPPNP removal efficiencyNitrogen removal efficiencyReferences
Nitrifying sludge bioreactor 10 mg/L 99.9% 99.5% Li et al. (2020)  
Sequencing batch reactor 40 mg/L 98.7% 94.9% Liu (2020)  
Anaerobic migrating blanket reactor/aerobic completely stirred tank reactor 100 mg/L 96%  Sponza & Kuscu (2005)  
Sieve plate tower biofilm reactor 100 mg/L 100% 87% Yuan (2020)  
Slurry bubble column 128 mg/L 100%  Salehi et al. (2010)  
Sequencing batch reactor 180 mg/L 99%  Liu et al. (2007)  
Upflow anaerobic sludge blanket 350 mg/L 97%  Xu et al. (2013)  
Upflow anaerobic sludge blanket reactor/bioelectrochemical system 350 mg/L 100%  Shen et al. (2014)  
Two integrated membrane aerated bioreactor 500 mg/L 95.86% 78.21% Mei et al. (2019a)  
Anaerobic semi-fixed bed biofilms reactor 540 mg/L 98%  Chen et al. (2021b)  
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Table 2

Nitrogen removal efficiencies of refractory phenolic wastewater by bioreactors

Wastewater typesReactor typesHighest influent nitrogenNitrogen removal efficiencyReferences
Phenol wastewater Sequencing batch reactor NO3−-N 240 mg/L 98% Ma et al. (2007)  
Phenol wastewater Anoxic suspension reactor NO3−-N 2 g/L 65% Bajaj et al. (2010)  
ABS resin wastewater (containing organic nitrile and aromatic toxic compounds) Cycling integrated bioreactor with gas-lift and microporrus-aeration TN 89 mg/L 60% Zhu (2014)  
Formaldehyde and phenol wastewater Anoxic upflow sludge blanket reactor NO3−-N 400 mg/L 98.4% Eiroa et al. (2005)  
Coal gasification wastewater Biological contactoxidation-Anaerobic hydrolysis acidification-anoxia-Membrane Bioreactor NH3-N 79–94 mg/L 93% − 95% Zhao et al. (2016)  
Coking wastewater (containing refractory polycyclic aromatic hydrocarbons and heterocyclic compounds) Sequential batch membrane bioreactor-Reverse osmosis TN 114 mg/L 96% Wang et al. (2014)  
Wastewater typesReactor typesHighest influent nitrogenNitrogen removal efficiencyReferences
Phenol wastewater Sequencing batch reactor NO3−-N 240 mg/L 98% Ma et al. (2007)  
Phenol wastewater Anoxic suspension reactor NO3−-N 2 g/L 65% Bajaj et al. (2010)  
ABS resin wastewater (containing organic nitrile and aromatic toxic compounds) Cycling integrated bioreactor with gas-lift and microporrus-aeration TN 89 mg/L 60% Zhu (2014)  
Formaldehyde and phenol wastewater Anoxic upflow sludge blanket reactor NO3−-N 400 mg/L 98.4% Eiroa et al. (2005)  
Coal gasification wastewater Biological contactoxidation-Anaerobic hydrolysis acidification-anoxia-Membrane Bioreactor NH3-N 79–94 mg/L 93% − 95% Zhao et al. (2016)  
Coking wastewater (containing refractory polycyclic aromatic hydrocarbons and heterocyclic compounds) Sequential batch membrane bioreactor-Reverse osmosis TN 114 mg/L 96% Wang et al. (2014)  
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Membrane biofilm reactor (MBfR) merges gas-transfer membranes with biofilms to remove water contaminants, which has the characteristics of bubble-free aeration and anisotropic mass transfer. In the study of MBfR, the types of aeration are divided into oxygen, hydrogen, and methane. The oxygen-based MBfR (O2-MBfR), also called MABR, has a unique layered structure, which presents aerobic, anoxic, and anaerobic layers. Organic matter can be degraded efficiently in the unique layered structures. Currently, O2-MBfR has been utilized by many researchers on wastewater with poor biodegradability (Grimberg et al. 2000; Lan et al. 2018; Liu et al. 2020). Lai et al. (2017) developed a O2-MBfR to treat Cetyltrimethyl Ammonium Bromide (CTAB) wastewater (400 mg/L) and the removal rate could achieve 98%. Additionally, CH4-MBfR not only has the advantage of foamless aeration and counter-diffusional biofilm but also can considerably improve the mass transfer performance of methane. CH4-MBfR has been widely used in the enrichment and application researches of denitrifying methane anaerobic oxidation (DAMO) microorganisms (Xie et al. 2017). DAMO bacteria can use methane as a carbon source to convert nitrates produced by anammox into nitrites without additional organic carbon sources. Xie et al. (2018) integrated anammox and DAMO in MBfR, and the nitrogen removal efficiency was as high as 0.2 kgN·m−3·d−1, and the total nitrogen in the effluent was only 3 mg/L. Thus, CH4-MBfR is suitable for anaerobic total nitrogen removal of wastewater containing ammonium, nitrite, and nitrate (Chai et al. 2018; Wu et al. 2019). Nowadays, many researchers use multi-stage MBfR to degrade pollutants with high concentrations (>100 mg/L) of phenolic compounds (Mei et al. 2019a; Tian et al. 2019). Biodegradation involving both anaerobic and aerobic processes is more efficient at phenolic pollutants. Anaerobic acidification can enhance the biodegradability of phenol and reduce the toxicity to bacteria. Aerobic oxidation can lead to further mineralization of phenol (Huang et al. 2016; Mei et al. 2019b). Multi-stage MBfR also solves the serious bubbling problem in the aerobic biodegradation of phenolic pollutants by traditional methods, which impact the treatment efficiency due to uncontrolled loss of biomass. Moreover, since nitrifying bacteria and denitrifying bacteria are easily poisoned by phenolic compounds (Wang et al. 2018), a multi-stage reactor can obtain simultaneous high concentrations of toxic pollutants and nitrogen removal. Tian et al. (2019) developed a two-stage bench-scale O2-MBfR (specific surface: 66.2 m2/m3 in each reactor) to treat wastewater containing the o-aminophenol (OAP) concentration of 1,179 mg/L. The results showed that reactor-1 could achieve a removal rate of 17.6 g OAP/m2d and reactor-2 could achieve more than 90% TN removal with external organic carbon sources. The integrated system removed OAP and nitrogen compounds effectively. The reason might that the additional organic carbon sources were essential for degradation, according to the previous studies (Lan et al. 2018; Liu et al. 2020). Mei et al. (2019a) constructed two integrated O2-MBfR systems with anoxic and aerated zones treating wastewater containing high levels of PNP. The biological carriers were polydimethylsiloxane (PDMS) membrane (specific surface: 201.14 m2/m3) and ceramsite with the input of organic matter. When the inlet concentration of PNP was 500 mg/L, the average removal efficiencies of PNP and TN were 95.86% and 78.21%, respectively. The results suggested that the two integrated O2-MBfR systems had the ability to degrade PNP effectively, but the denitrification ability required further research, and the addition of biological carriers led to high operation costs.

There are few researches on the advanced treatment for high PNP concentrations (>100 mg/L) without additional organic sources. Studies on the tolerance of MBfR under high impact load are limited. To efficiently achieve the simultaneous high concentrations of PNP removal and TN removal without external organic carbon sources, an original two-stage MBfR system was constructed. The two reactors were supplied with oxygen and methane, respectively. O2-MBfR was used for the degradation of PNP, the improvement of biodegradability of wastewater, and the reduction of the burden on the subsequent reactor. CH4-MBfR was used for TN removal treatment of O2-MBfR effluent. The objectives of this work were to (i) evaluate the performance of O2-MBfR and CH4-MBfR combined process; (ii) explore the effects of the aeration pressure, HRT, pH and influent concentrations of PNP; (iii) determine the acceptable concentration and the threshold concentration of PNP which could be efficiently treated by the two-stage MBfR without additional organic carbon sources. This research provided a better solution for multi-MBfR to treat toxic industrial wastewater containing phenol, nitrophenol, and further TN removal, which would not release any air pollutants into the atmosphere.

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Reactor configurations and experimental design

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The device in the experiment consisted of two bench-scale bioreactors: O2-MBfR and CH4-MBfR (Figure 1). The size of O2-MBfR was φ120 mm × 300 mm, and the effective volume was about 3 L. The size of CH4-MBfR was φ120 mm × 100 mm. To expand the volume of the reactor, CH4-MBfR was composed of two plexiglass cylinders in parallel. The total effective volume of CH4-MBfR is about 2 L. The hollow fiber membranes of the two reactors were fabric reinforced PVDF curtain membrane modules (model BP-11) launched by Tianjin Membrane Technology Co., Ltd in 2013, with average separation pore sizes of 0.03 μm. The membrane modules in the two-stage MBfR were closed-end types. In O2-MBfR, the membrane module consisted of 124 membrane filaments, the total effective membrane area was 0.202 m2 and the membrane packing density was 67.3 m2/m3. In CH4-MBfR, each reactor contained 50 hollow fiber membranes, the total effective membrane area was 0.0543 m2 and the membrane packing density was 27.15 m2/m3. The gas supply was provided by air compressors and methane gas cylinders. The gas pressure was controlled and monitored by the valves and pressure gauges, respectively.
Figure 1
Schematic diagram of O2-MBfR and CH4-MBfR combined process device.
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Schematic diagram of O2-MBfR and CH4-MBfR combined process device.

Figure 1
Schematic diagram of O2-MBfR and CH4-MBfR combined process device.
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Schematic diagram of O2-MBfR and CH4-MBfR combined process device.

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Experimental wastewater and seed sludge

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The synthetic PNP wastewater was prepared from distilled water added with PNP (Table 3), 0.41 g NaHCO3, 0.32 g KH2PO4, 0.41 g K2SO4, 0.5 g MgSO4, 0.28 g Na2CO3, 0.3 g Na2HPO4, 0.08 g ZnSO4, 0.55 g MnCl2, 0.25 g CuSO4, 0.12 g FeCl3-6H2O, 0.01 g NiCl2, 0.04 g CoCl2 and 0.04 g NaNO3 per liter. In order to ensure efficient TN removal of the subsequent system, the additional nutrients were added into the CH4-MBfR for DAMO microorganism: 1 g/L MgSO4·7H2O, 0.27 g/L CaCl2·2H2O, 0.0091 g/L FeSO4·7H2O, 2 mL/L phosphate buffer solution, and 1 mL/L trace element solution. Phosphate buffer solution: 24.4 g/L KH2PO4 and 10.2 g Na2HPO4. Trace element solution: 2.486 g/L FeSO4·7H2O, 0.5 g/L MnCl2·4H2O, 0.05 g/L ZnCl2, 0.101 g/L NiSO4·6H2O, 0.05 g/L CoCl2·6H2O, 0.31 g/L CuSO4·5H2O and 0.026 g/LNa2MoO4. The reagents (PNP, C6H12O6, NaHCO3, KH2PO4, MgSO4, Na2CO3, Na2HPO4, NiCl2, CoCl2, CuSO4, MnCl2·4H2O, ZnCl2, CuSO4·5H2O) were supplied by Tianjin Damao Chemical Reagent Factory. The reagents (FeCl3-6H2O, MgSO4·7H2O, CaCl2·2H2O, FeSO4·7H2O, NiSO4·6H2O, CoCl2·6H2O, Na2MoO4) were provided by Sinopharm Chemical Reagent Co., Ltd. The reagents (K2SO4, ZnSO4, MnCl2) were supplied by Shenyang Dongxing Reagent Factory. CH3OH, CH3COOH and CH3COONH4 of high-performance liquid chromatography grade were supplied by Tianjin Yirenda Chemical Co., Ltd.

Table 3

Operating conditions and inlet quality (on average) during the performance optimization stage

O2-MBfR
StageTime (d)PNP (mg/L)COD (mg/L)NH4+-N (mg/L)Oxygen pressure (MPa)pHHRT (h)Temperature (°C)
1-S1(I) 1–32 0–80 500–600 20–30 0.014 7.5 24 25 ± 2 
1-S1(II) 33–72 100 0 0 0.018 7.5 24 27 ± 2 
1-S2 73–96 100 0 0 0.010–0.024 7.5 24 27 ± 2 
1-S3 97–119 100 0 0 0.020 7.5 24–48 27 ± 2 
1-S4 120–139 100 0 0 0.020 4.0–9.5 36 27 ± 2 
CH4-MBfR
StageTime (d)PNP (mg/L)TN (mg/L)Methane pressure (MPa)pHHRT (h)Temperature (°C)
2-S1(I) 140–171 0 30–35 0.06 7.5 24 30 ± 1  
2-S1(II) 172–211 10–15 5–20 0.06 7.5 24 30 ± 1  
2-S2 212–299 10–15 10–16 0.01–0.11 7.5 24 30 ± 1  
2-S3 300–333 10–15 10–16 0.08 7.5 24–60 30 ± 1  
O2-MBfR
StageTime (d)PNP (mg/L)COD (mg/L)NH4+-N (mg/L)Oxygen pressure (MPa)pHHRT (h)Temperature (°C)
1-S1(I) 1–32 0–80 500–600 20–30 0.014 7.5 24 25 ± 2 
1-S1(II) 33–72 100 0 0 0.018 7.5 24 27 ± 2 
1-S2 73–96 100 0 0 0.010–0.024 7.5 24 27 ± 2 
1-S3 97–119 100 0 0 0.020 7.5 24–48 27 ± 2 
1-S4 120–139 100 0 0 0.020 4.0–9.5 36 27 ± 2 
CH4-MBfR
StageTime (d)PNP (mg/L)TN (mg/L)Methane pressure (MPa)pHHRT (h)Temperature (°C)
2-S1(I) 140–171 0 30–35 0.06 7.5 24 30 ± 1  
2-S1(II) 172–211 10–15 5–20 0.06 7.5 24 30 ± 1  
2-S2 212–299 10–15 10–16 0.01–0.11 7.5 24 30 ± 1  
2-S3 300–333 10–15 10–16 0.08 7.5 24–60 30 ± 1  
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In this experiment, the inoculated sludge in O2-MBfR was taken from the activated sludge in the secondary sedimentation tank of Shenyang South Wastewater Treatment Plant, which had the advantages of good sedimentation and a high microbial concentration. The sludge volume index (SVI) and mixed liquid suspended solids (MLSS) were 65 mL/g and 3,550 mg/L, respectively. CH4-MBfR was inoculated with sludge containing functional DAMO bacteria. The special sludge was cultivated from the sediment at the bottom of the lake in Shenyang section of Hunhe River Basin, and had been cultured by the SBR reactor for 240 days in our laboratory.

Experimental method

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This experiment adopted an intermittent operation mode. Firstly, during the performance optimization stage of the two-stage MBfR, the operating conditions and inlet quality are shown in Table 3. To make the microorganisms adapt to PNP, additional COD 500–600 mg/L and ammonium 20–30 mg/L were added in 1-S1 (I), and the concentrations of PNP gradually increased from 0 to 80 mg/L. In 1-S1 (II), stop adding organic carbon sources and nitrogen sources to make PNP the only organic carbon source, the reactor was operated stably for 40 days at the PNP concentration of 100 mg/L according to the previous study (Mei et al. 2019a). The performance of O2-MBfR was optimized by adjusting the oxygen pressure (0.010, 0.016, 0.020, 0.024 MPa) in 1-S2, HRT (24, 36, 48 h) in 1-S3 and pH (4.0, 6.0, 7.5, 8.5, 9.5) in 1-S4. Under the optimal operation conditions of O2-MBfR, the CH4-MBfR membrane hanging was operated with methane supplying. CH4-MBfR gradually stabilized after a long time in 2-S1. CH4-MBfR operation parameters were adjusted by changing the methane pressure (0.01, 0.02, 0.05, 0.08, 0.09, 0.11 MPa) in 2-S2 and HRT (24, 36, 48, 60 h) in 2-S3. After the optimal operating parameters of the two-stage MBfR were determined, the high concentrations operation experiment was conducted as shown in Table 4 to investigate the maximum concentration limit of PNP which could be treated efficiently by the two-stage MBfR.

Table 4

Operating conditions and inlet quality (on average) during the high concentrations operation stage

ReactorTime (d)PNP (mg/L)TN (mg/L)Gas pressure (MPa)pHHRT (h)Temperature (°C)
O2-MBfR 334–452 200–400 0 0.02 7.5 36 27 ± 2 
CH4-MBfR 379–452 0–110 5–35 0.08 7.5 36 30 ± 1 
ReactorTime (d)PNP (mg/L)TN (mg/L)Gas pressure (MPa)pHHRT (h)Temperature (°C)
O2-MBfR 334–452 200–400 0 0.02 7.5 36 27 ± 2 
CH4-MBfR 379–452 0–110 5–35 0.08 7.5 36 30 ± 1 
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During the experiment, digestion device (DR 200, Hashing of America Inc.) TN was determined by the potassium persulfate method (N/C3100, Analytik Jena AG Inc.); DO was determined by the DO probe of a multi-parameter water quality analyzer (HANNA HI98193, Hana Instruments Inc.); pH value was determined by the pH probe (FEP20, Mettler Toleadedo Inc.); electric thermostatic water bath (HHS, Shanghai Boxun Industrial Co., Ltd); centrifuge (H1850, Xiangyi Centrifuge Instrument Co. Ltd); PNP was determined by high-performance liquid chromatography (Agilent 1200 Infinity Series, Agilent Technology Co., Ltd). The chromatographic conditions were as follows: column: ZORBAX SB-C18 (4.6 × 150 mm, 5 μm); column temperature: 30 °C; ultraviolet detector injection volume: 20 μL. The mobile phase consisted of (40%) methanol and (60%) acetic acid-ammonium acetate (0.1 mol/L). The flow rate was 0.8 mL/min, and the detection wavelength was 254 nm.

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O2-MBfR process

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Biofilm formation and acclimation

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During the O2-MBfR system membrane hanging stage, on day 7, the biofilms could be observed to be uniform in thickness, indicating that the membrane was successfully attached. In the biofilm domestication stage of O2-MBfR, the removal efficiency of PNP was shown in Figure 2. The initial concentrations of PNP gradually increased from 0 to 80 mg/L, with the removal rate ranging from 31.35% to 98.36%, suggesting that the degradation effect of biofilm on pollutants tended to be stable. Glucose as a co-substrate contributed to the establishment of the O2-MBfR system, which was consistent with previous works (Chen et al. 2021b). However, when the initial concentration of PNP was adjusted to 100 mg/L, a large number of biofilms were ablated since the toxicity of PNP, and the removal rate of PNP dropped to 26.55%. Then the PNP removal rate increased sharply, on the 65th day, the effluent PNP concentration decreased to 2.88 mg/L, the removal rate reached a peak of 98.43% and the removal load was 1.482 g·m−2·d−1, slightly higher than AMBR/CSTR (Sponza & Kuscu 2005). O2-MBfR had a stable and efficient removal effect on 100 mg/L PNP, indicating that O2-MBfR biofilm domestication was successful. Results demonstrated that high concentrations of toxic substances had a considerable effect on microbes. Nonetheless, the PNP-degrading functional microorganisms cultured by O2-MBfR could still recover to high microbial activity and proliferate continuously to gain high treatment efficiency. The tolerance of microorganisms to PNP could be enhanced by exposing the membrane to higher acclimation concentrations, a similar result was observed by Peng et al. (2018). Since the transformation of substrate transport across the cell membrane (Bajaj et al. 2009), the substrate inhibitory effect was diminished.
Figure 2
Changes in PNP concentrations and removal efficiency during O2-MBfR hanging film domestication stage.
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Changes in PNP concentrations and removal efficiency during O2-MBfR hanging film domestication stage.

Figure 2
Changes in PNP concentrations and removal efficiency during O2-MBfR hanging film domestication stage.
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Changes in PNP concentrations and removal efficiency during O2-MBfR hanging film domestication stage.

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Optimization of O2-MBfR system

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The O2-MBfR optimum operating parameters for PNP degradation were investigated. Firstly, the PNP removal rates in O2-MBfR under different oxygen pressure conditions were considered in 1-S2, as shown in Figure 3. When the oxygen pressure increased from 0.010 MPa to 0.020 MPa, the removal rates of PNP by O2-MBfR rose rapidly, reaching the highest point of 82.60% and the removal load was 1.267 g·m−2·d−1. Since the main PNP-degrading functional microorganisms were aerobic heterotrophic bacteria (Li et al. 2020). With the increase of oxygen pressure, the oxygen environment switched from anaerobic to aerobic, which altered the layered structure of the biofilm. The metabolic rate of the bacteria in the aerobic layer was accelerated and the activity was strengthened, which led to the improvement of the degradation rate of organic matter (Liu et al. 2020). Nevertheless, when the oxygen pressure increased to 0.024 MPa, the excessive aeration pressure led to biofilms shedding, which affected the removal rate. The PNP removal rate declined to 79.13%, slightly lower than that at 0.020 MPa. Anaerobic acidification could enhance the biodegradability of PNP and reduce its toxicity to microorganisms (Tian et al. 2019), which was virtually eliminated completely under excess oxygen conditions. Another possible reason was that the excess oxygen pressure would induce tiny bubbles, which corresponded to a decrease in the oxygen mass transfer ability. The activity of aerobic heterotrophic bacteria was diminished, and the removal rate of PNP reduced accordingly (Tian et al. 2019). In the follow-up study, the oxygen pressure in O2-MBfR was selected at 0.020 MPa, which could not only ensure economic rationality but also improve the PNP degradation efficiency.
Figure 3
Changes of PNP concentrations and removal efficiency in O2-MBfR under different oxygen pressures.
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Changes of PNP concentrations and removal efficiency in O2-MBfR under different oxygen pressures.

Figure 3
Changes of PNP concentrations and removal efficiency in O2-MBfR under different oxygen pressures.
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Changes of PNP concentrations and removal efficiency in O2-MBfR under different oxygen pressures.

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Appropriate HRT can optimize the system performance, shorten the degradation time of PNP, and improve the efficiency of TN removal. The PNP removal effect of O2-MBfR altered significantly with the increase of HRT in 1-S3, as shown in Figure 4. When the HRT was 24 h, the removal rate was 66.20%. The removal rate and removal load of PNP reached the peak of 86.78% and 0.915 g·m−2·d−1 when the HRT was adjusted to 36 h. Too short HRT led to the organic loading increase, which could provide more organic carbon sources for the functional microorganisms in the aerobic layer. However, the proliferation of heterotrophic bacteria would decline the competition ability of autotrophic bacteria for oxygen, which might lead to a decrease in PNP removal efficiency (Liu et al. 2020). After the HRT increased to 48 h, the removal rate of PNP diminished to 76.50%, much lower than that at 36 h. It indicated that too long HRT led to the available organic carbon sources decreasing continuously, and the oxygen consumption of aerobic microorganisms would decrease accordingly. The anaerobic layer environment in the biofilm was tough to sustain and anaerobic bacteria were inhibited (Zheng et al. 2020). By comprehensive comparison of effluent concentrations and removal loads, the HRT of O2-MBfR was maintained at 36 h in the follow-up study.
Figure 4
Changes of PNP concentrations and removal efficiency in O2-MBfR under different HRT conditions.
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Changes of PNP concentrations and removal efficiency in O2-MBfR under different HRT conditions.

Figure 4
Changes of PNP concentrations and removal efficiency in O2-MBfR under different HRT conditions.
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Changes of PNP concentrations and removal efficiency in O2-MBfR under different HRT conditions.

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As shown in Figure 5, the pH was in the range of 4.0 to 9.5 in 1-S4. When the pH was 4.0, the degradation efficiency of PNP was only 25.76%. With the pH adjusted to 6.0, the efficiency increased rapidly. With the pH raised to 7.5, the removal rate of PNP elevated to the peak of 89.70% and the removal load was 0.930 g·m−2·d−1, suggesting that this pH value was more suitable for the growth environment of biofilms, and PNP could be metabolized effectively. Nonetheless, when the pH elevated to 9.5, the removal rate was less than 40%, demonstrating that the degradation process of PNP by the biofilms would likewise be adversely affected in the excessively alkaline environment. Moreover, the PNP removal rate at the pH of 9.5 was higher than that at 4.0, confirming that O2-MBfR had a relatively better tolerance to the excessively alkaline environment. Previous studies had also found that the over-acidic wastewater would cause the formation of sludge bulking, and make the degradation efficiency of PNP decrease sharply (Li et al. 2020), which was consistent with this study. In addition, the over-acid pH environment would cause some functional flora to lose their normal physiological activity. Some researchers also discovered that alkaline conditions not only affected the activity of microbial enzymes but also induced the ionization of PNP (Wang & Wang 2016). Consequently, the optimum pH of O2-MBfR should be preserved at about 7.5, which had the best degradation efficiency for PNP.
Figure 5
Changes of PNP concentrations and removal efficiency in O2-MBfR under different pH conditions.
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Changes of PNP concentrations and removal efficiency in O2-MBfR under different pH conditions.

Figure 5
Changes of PNP concentrations and removal efficiency in O2-MBfR under different pH conditions.
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Changes of PNP concentrations and removal efficiency in O2-MBfR under different pH conditions.

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CH4-MBfR process

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Biofilm formation and acclimation

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According to the previous experiment, the aeration pressure, HRT, and pH in O2-MBfR were 0.020 MPa, 36 h, and 7.5 respectively. During the membrane hanging stage of CH4-MBfR in 2-S1, simulated domestic sewage was introduced, and the sludge enriched with DAMO bacteria was inoculated in CH4-MBfR. As shown in Figure 6, on days 140–167, during 28 days of operation, the removal efficiency of TN increased from 80.17% to 94.68% and the removal load reached the highest of 1,101.86 mg·m−2·d−1, suggesting that CH4-MBfR had a stable operation. When the CH4-MBfR inlet was replaced by the O2-MBfR effluent, the removal efficiency of TN dropped to 54.53% sharply and did not recover for a long period of time. Since the residual PNP in O2-MBfR effluent had a strong inhibitory effect on the activity of functional flora. After 31 days of acclimation, the TN removal rate recovered to 72.39% and continued to elevate steadily in the following 10 days. On day 209, the CH4-MBfR effluent concentration of TN was as low as 1.8 mg/L, the removal rate and removal load reached 82.57% and 315.18 mg·m−2·d−1, respectively, analogous to the previous study in which the total nitrogen removal rate fluctuated around 84.3% (Xie et al. 2018), establishing that DAMO bacteria in the reactor adapted to environmental changes. Consequently, CH4-MBfR inoculated with functional microorganisms could effectively remove the nitrogen compounds from O2-MBfR effluent and exhibited a good degradation performance. Although CH4-MBfR was affected by toxic pollutants, the reactor had a strong tolerance. The successful connection of the two MBfRs proved that this method was feasible to treat the refractory organic matter.
Figure 6
Changes in TN concentrations and removal efficiency during CH4-MBfR hanging film domestication stage.
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Changes in TN concentrations and removal efficiency during CH4-MBfR hanging film domestication stage.

Figure 6
Changes in TN concentrations and removal efficiency during CH4-MBfR hanging film domestication stage.
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Changes in TN concentrations and removal efficiency during CH4-MBfR hanging film domestication stage.

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Optimization of CH4-MBfR system

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Under the optimal operating parameters of the O2-MBfR, the CH4-MBfR optimal methane pressure in 2-S3 and HRT in 2-S4 were investigated. As for pH in CH4-MBfR, DAMO bacteria have severe environmental requirements, resulting in difficulties in enrichment (Kampman et al. 2012; Bhattacharjee et al. 2016). Since the DAMO reaction process and its microflora were first reported in 2006, many experimental investigations have been carried out under the environmental conditions of pH 7.0–8.0 (Zhao et al. 2017). Some researchers studied the factors affecting the activity of DAMO flora and found that the optimal pH was about 7.6. Thus, in order to enrich DAMO bacteria and achieve a better operation effect of CH4-MBfR, the pH in CH4-MBfR was maintained at about 7.5.

As shown in Figure 7, methane partial pressure was an important factor affecting the CH4-MBfR removal of TN. The growth and reproduction of the microbial membranes in CH4-MBfR required an adequate methane supply. When the methane partial pressure raised from 0.01 MPa to 0.08 MPa, the TN removal efficiency increased significantly. On day 224, the TN removal efficiency was only 21.52% and the removal load was just 113.62 mg·m−2·d−1. On day 269, the TN removal rate elevated significantly to 42.49% and the removal load was 184.04 mg·m−2·d−1, establishing that the methane partial pressure in this range had a strong effect on the removal of TN. However, when the methane partial pressure raised from 0.09 MPa to 0.11 MPa, the growth of TN removal efficiency tended to be flat. With the elevation of methane partial pressure, the TN removal efficiency still increased continuously, reaching a peak of 48.67%. The possible reason was that when methane partial pressure was lower than 0.08 MPa, methane was in an unsaturated state. The results demonstrated that when methane partial pressure exceeded 0.08 MPa, it was no longer a limiting factor for DAMO function and was sufficient to meet the requirements of functional microorganisms. This trend was consistent with previous research by Cui et al. (2021) which found that with the methane partial pressure increasing from 10 kPa to 80 kPa, the activity of DAMO increased from 0. 020 to 0. 071 mgN·h−1· g−1 VSS; when methane partial pressure was lower than 35 kPa, DAMO activity was linearly correlated with methane partial pressure, which was in line with the methane mass transfer model of DAMO process studied by scholars (Duan et al. 2010; Cai et al. 2018). The study of Zhao et al. (2017) demonstrated that methane partial pressure above 0.049 MPa can satisfy the enrichment culture of DAMO bacteria. He et al. (2012) established a methane mass transfer model and showed that when the partial pressure of methane was 0.025 MPa, it was no longer a limiting factor for DAMO activity. However, too much methane aeration might lead to biofilm shedding and affect the growth of TN removal rate. Consequently, the appropriate methane partial pressure for the CH4-MBfR was about 0.08 MPa, which could avoid unnecessary methane waste and save energy. This methane pressure was maintained in the subsequent investigations.
Figure 7
Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT methane partial pressures.
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Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT methane partial pressures.

Figure 7
Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT methane partial pressures.
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Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT methane partial pressures.

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With the increase of HRT, the removal rates of TN by CH4-MBfR were significantly increased. As shown in Figure 8, when HRT was 36 h, the TN removal efficiency rose the fastest. The TN removal rate attained its highest point of 69.24% and the removal load was 241.42 mg·m−2·d−1. Nevertheless, the TN removal performance of the system declined when HRT exceeded 36 h, which was consistent with previous work (Xie et al. 2018). When the HRT was 48 h, the TN removal efficiency increased slightly to 70.84% and the removal load declined to 184.20 mg·m−2·d−1. Then extending the HRT to 60 h, the highest TN removal efficiency of the system reached 73.23%, the growth rate of the removal efficiency slowed down obviously and the removal load was only 155.36 mg·m−2·d−1. Since no additional organic carbon source was added in this experiment, the selection of HRT should not be too long. Long HRT led to the continuous reduction of organic carbon sources in the reactor. Due to the lack of nutrients, the proliferation of functional microorganisms was slow and the biological activity reduced accordingly. Considering the removal effect of TN and the shortening of the operation period to decrease operating costs, the optimum HRT of the reactor was 36 h.
Figure 8
Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT conditions.
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Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT conditions.

Figure 8
Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT conditions.
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Changes of TN concentrations and removal efficiency in CH4-MBfR under different HRT conditions.

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O2-MBfR and CH4-MBfR performance in long-term

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After determining the optimal operating conditions of the two-stage MBfR, the removal efficiencies of PNP (100 mg/L) and TN attained 89.70% and 69.24%, slightly lower than the previous study by AMBR/CSTR system with glucose giving 3,000 mg/L COD as co-substrate (Sponza & Kuscu 2005). The experiment entered into the high concentrations operation state (Table 4). Without additional organic carbon sources, different high concentrations of PNP were altered to determine the threshold concentration of PNP which could be effectively treated by the two-stage MBfR, as shown in Figure 9.
Figure 9
Changes of PNP, TN concentrations and removal efficiencies in two-stage MBfR under high concentrations operation conditions.
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Changes of PNP, TN concentrations and removal efficiencies in two-stage MBfR under high concentrations operation conditions.

Figure 9
Changes of PNP, TN concentrations and removal efficiencies in two-stage MBfR under high concentrations operation conditions.
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Changes of PNP, TN concentrations and removal efficiencies in two-stage MBfR under high concentrations operation conditions.

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On days 334–354, due to the sudden increase of PNP concentrations from 100 to 200 mg/L, the efficiency of O2-MBfR was severely affected. In O2-MBfR, the average removal rate of PNP within 21 days was only 38.18%. Through a 14-day adaptation period (days 356–380), the removal rate of PNP in O2-MBfR recovered up to 93.69% and the effluent concentration of PNP was only 12.9 mg/L. After O2-MBfR attained a steady state (the residual concentrations of PNP less than 30 mg/L), the effluent was used for the CH4-MBfR inlet. After O2-MBfR was connected with CH4-MBfR, the TN removal efficiency by CH4-MBfR was 53.38% and the removal load was 77.96 mg·m−2·d−1. The results demonstrated that the two-stage MBfR could adapt to the high concentration of PNP (200 mg/L). The microorganisms cultured in two-stage MBfR had strong tolerance and could achieve considerable degradation efficiency.

Subsequently, the concentrations of the PNP inlet increased by a gradient of 50 mg/L. When the concentration of PNP was 250 mg/L, the two-stage MBfR achieved the best degradation effect in the entire process. The O2-MBfR effluent concentrations of PNP and TN were about 10–40 mg/L and 5–8 mg/L, respectively. The TN removal rate increased linearly after CH4-MBfR was stabilized. The two-stage MBfR was stable under this concentration with the removal efficiencies of PNP and TN being 95.00% and 69.48%, respectively and the removal load were up to 2.37 g·m−2·d−1 and 96.22 mg·m−2·d−1, respectively. The reactor realized synchronous and efficient removal of PNP and TN. The results established that the degradation of PNP exhibited close dependence on the TN removal. The appropriate concentration of PNP could be conducive to the TN removal process. With the increasing rate of PNP degradation in O2-MBfR, the nitrogen compounds produced increased as well, establishing that O2-MBfR could not only efficiently degrade PNP but also reduce the TN load of CH4-MBfR to a certain extent. Bacteria in CH4-MBfR could utilize the residual PNP as nitrogen sources and organic carbon sources to achieve efficient removal of TN. It was consistent with the report by Tian et al. (2019), in which OAP removal load achieved 7 g·m−2·d−1 and TN removal efficiency was above 90% by two-stage O2-MBfR. Compared with our experiment, the better effect might be due to the lower toxicity of OAP than PNP, resulting in less impact on microorganisms. Moreover, the additional carbon sources increased the biodegradability of wastewater.

However, when the concentration of PNP in the inlet abruptly increased to 300 mg/L, O2-MBfR was just affected slightly, and the removal rate of PNP decreased from 95.00% to 86.76%. While CH4-MBfR was greatly deteriorative, and the removal efficiency of TN dropped from 69.48% to 59.64%. O2-MBfR showed a strong shockproof ability. Nonetheless, DAMO bacteria in CH4-MBfR were sensitive to the environmental changes and susceptible to the toxic effects of PNP. On day 407, the removal rates of PNP and TN continuously decreased to 80.27% and 49.23%, respectively. After 9 days of adaptation, the removal rates of PNP and TN increased steadily up to 89.67% and 63.35%, respectively, suggesting that the two-stage MBfR had a tolerance to the high concentration of PNP (300 mg/L) to some extent. When the concentrations of PNP were adjusted to 350 and 400 mg/L, the performance of the two-stage MBfR gradually deteriorated, and the processing effect was not stable. O2-MBfR had a relatively slight impact, with the removal rates of PNP stabilizing in ranges of 70-88%, a similar result was observed by Mei et al. (2019a). But the removal rates were hard to recover. The effluent concentrations of PNP fluctuated at 40–100 mg/L. In this case, the TN removal efficiencies of CH4-MBfR were not ideal than the previous study (Mei et al. 2019a), with only 43.33% (350 mg/L PNP) and 27.04% (400 mg/L PNP) on average. Although the degradation efficiency of PNP in O2-MBfR was stable, the residual concentrations of PNP in effluent still increased noticeably. Resulting in the increase of TN concentration in O2-MBfR effluent which might increase the burden for CH4-MBfR. Moreover, the residual high concentrations of PNP would generate toxic effects on the DAMO bacteria. The system was not able to efficiently cope with wastewater at concentrations of (350 and 400 mg/L PNP). The previous study (Mei et al. 2019a) had a better threshold concentration (500 mg/L), and the removal rates of TN were about 20% higher than this experiment, which might be due to the addition of organic matter and ceramsite and the larger specific surface area of biofilm.

In conclusion, the performance of the two-stage MBfR treating high concentrations of PNP was relatively stable. The microorganisms cultured in two-stage MBfR had a strong tolerance and could utilize PNP as nitrogen sources and organic carbon sources to achieve efficient removals. The operational parameters (aeration pressure, HRT, and pH) were significant to the removal efficiency by two-stage MBfR. And the tolerance of microorganisms to PNP could be enhanced by exposing the membrane to higher acclimation concentrations. The results demonstrated that the tolerance of the two-stage MBfR could be improved by subjecting the reactor to the proper inlet PNP concentration at about 250 mg/L. At the concentrations above the threshold concentration (300–400 mg/L), the enhancement was insignificant due to the lower tolerance toward the high toxicity of PNP. The suboptimal growth below the threshold concentration was due to the lack of organics, whereas the decreased growth rate above the threshold concentration was attributed to the increasing substrate inhibition. Compared with previous studies, the addition of glucose indeed improved the degradation and denitrification efficiency of high concentrations of PNP. During the entire operation period, O2-MBfR could achieve an efficient removal effect while CH4-MBfR was very limited by high concentrations of PNP since the microbial activity of the system was difficult to recover. The lack of organic carbon source in the reactor also led to the limitation of the system. In this experiment, the PNP and TN removal efficiencies could be further improved through (1) elevating MLVSS or exposing the activated sludge to higher acclimated concentrations; (2) integrating O2-MBfR with other pretreatment links or other subsequent nitrogen removal processes, also other fillers or carriers can be added into O2-MBfR to improve the nitrogen removal performance of the reactor; (3) adding additional carbon sources in order to improve microbial activity. Further investigations should be carried out: (1) the process of nitrification and denitrification in the two-stage MBfR to evaluate the mechanism of nitrogen conversion; (2) high-throughput sequencing to analyze the biofilm community structure and explore the PNP degradation pathway in MBfR.

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In this experiment, a two-stage MBfR process was successfully constructed to treat wastewater containing high concentrations of PNP and nitrogen without external organic carbon sources. O2-MBfR achieved high PNP removal rates and effective organic mineralization, while CH4-MBfR performed advanced nitrogen removal from the O2-MBfR effluent. The optimal operating parameters of the reactor were determined. The aeration pressure, HRT, and pH in O2-MBfR were 0.020 MPa, 36 h, and 7.5, respectively, and in CH4-MBfR were 0.080 MPa, 36 h, and 7.5, respectively. Under the optimal operation parameters, the removal efficiencies of PNP (100 mg/L) and TN attained 89.70% and 69.24%, respectively, and the removal loads were 0.930 g·m−2·d−1 and 241.42 mg·m−2·d−1, respectively. Without additional organic carbon sources, the acceptable concentration (200–400 mg/L) and the threshold concentration (250 mg/L) of the two-stage MBfR were confirmed. Under the threshold concentration, the removal rates of PNP and TN reached 95.0% and 69.48%, respectively, and the removal loads were 2.37 g·m−2·d−1 and 96.22 mg·m−2·d−1, respectively. This study lays the groundwork for further research about the practical application of the integrated system treating toxic industrial wastewater containing phenol, nitrophenol, and further TN removal.

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Jiayi Tong: Methodology, formal analysis, writing – original draft, software. Li Cui: Funding acquisition, methodology, supervision, validation. Danqi Wang: Resources, data curation, conceptualization. Xin Wang: Visualization, writing – review & editing. Zhaokun Liu: Data curation, software.

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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This research was financially supported by Funding of Department of Science and Technology, Liaoning Province (No. 2017308002) and Funding of Science and Technology Bureau, Shenyang City (No. RC180110). The authors gratefully acknowledge the reviewers for valuable insights and suggestions.

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All relevant data are included in the paper or its Supplementary Information.

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The authors declare there is no conflict.

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