Removal of bisphenol A (BPA) from biologically treated wastewater by microfiltration and nanofiltration
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Bisphenol A (BPA) is an endocrine disruptor that is difficult to completely remove from wastewater by conventional biological methods. Increased post-treatment BPA removal with ceramic membranes is worth investigating because of these membranes’ mechanical and chemical stability and lifespan. To determine the effectiveness of ceramic membranes for post-treatment of biologically treated BPA-contaminated wastewater, microfiltration (MF) and nanofiltration (NF) were conducted. Both processes removed BPA completely at an initial BPA concentration of 0.3 ± 0.14 mg/L. Increased concentration of 0.7 ± 0.29 mg/L decreased BPA removal. MF removed at least 24 % of BPA, presumably because BPA was adsorbed on particulate matter, which was retained by the membrane, or adsorbed on its surface. NF removed up to thrice more BPA. MF and NF completely removed suspended solids and 40–60 % COD. Filtration capacity decreased with time due to fouling but did not depend on initial BPA concentration. BPA concentrations in municipal wastewater are typically lower than the lowest concentration tested, where MF completely removed BPA. Hence, MF ceramic membranes appear a promising solution for post-treatment of BPA-containing wastewater. MF can be used at a much lower transmembrane pressure than NF, requiring less energy to pump wastewater through the membrane, thus reducing costs.
KeywordsSecondary effluent Endocrine disrupting compound Membrane filtration Membrane fouling
Bisphenol A (2,2-bis-4-hydroxyphenylpropane, BPA) is a widely used xeno-estrogen, mainly in the production of polycarbonate plastics and epoxy resins. It is considered an environmental pollutant with comparatively high biological activity and is classified as an endocrine disrupting compound (EDC). The most common source of BPA in natural water is wastewater. Although BPA can be degraded by microorganisms, it is hard to be completely removed from wastewater by conventional biological treatment methods. As a result, residual BPA is present in the effluents from municipal wastewater treatment plants in concentrations ranging from 0.01 μg/L (Nasu et al. 2001) to 86.0 μg/L (Kasprzyk-Hordern et al. 2009). This is a cause for concern because BPA is estrogenic at concentrations below 1 ng/L (Tanaka et al. 2000).
One of the reasons that biological treatment is not completely effective is because BPA can be sorbed on suspended solids or biofilm. BPA has a moderate affinity for the solid phase because it has an octanol–water coefficient (log Kow) of 3.32 (Stringfellow and Alvarez-Cohen 1999). Thus, BPA can sorb to suspended solids that were not completely removed in a secondary clarifier. This makes post-treatment necessary to lower the concentration of BPA in effluent and limit its adverse effects on water ecosystems.
One of the options for post-treatment is membrane filtration, which produces high-quality effluents with low concentrations of organic compounds. BPA rejection by membranes ranges from 18 % (Kimura et al. 2004) to >99.9 % (Agenson et al. 2003). This wide range of rejection rates is due to the fact that there is a strong relationship between rejection rate and membrane type; for phenolic compounds in particular, there is a linear relationship between rejection efficiency and molecular weight cut-off (Jung et al. 2007). For this reason, the rejection efficiency of EDCs, including BPA, decreases in the following order: reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF).
Among the various treatment options for EDC removal from solutions, NF and low-pressure techniques (MF and UF) are worth examining. NF requires high transmembrane pressure, which increases operating costs. However, it can separate organic micropollutants of low molecular weight and operate at pressures lower than RO. In contrast, although MF and UF are relatively less effective, they offer the advantage of operation at lower transmembrane pressures. This makes these processes worth investigating with the aim of improving their efficiency. So far, MF has been able to remove from 20 to 95 % of BPA from drinking water; efficiency dropped as the membrane became saturated with BPA (Bing-zhi et al. 2010).
The use of only membrane filtration for wastewater purification is limited by the clogging of membranes with pollutants (Sun et al. 2015), which shortens the filtration cycle and lower membrane life. So called fouling accompanies membrane filtration and is caused by the presence of organic compounds in wastewater. This may affect the removal of low-molecular mass organic micropollutants. To delay the drop in removal efficiency and to lengthen membrane life, different hybrid processes can be used to remove BPA and other EDCs. These hybrid processes combine membrane filtration with BPA degradation during Fenton’s process, among other techniques. However, significant membrane fouling was observed in the NF of the effluent even after Fenton oxidation (Escalona et al. 2014).
In the modules for BPA rejection, different types of membranes were used, particularly ones fabricated from organic materials such as polysulfone and polyamide (Nghiem et al. 2008), polyethylene (Chen et al. 2008) or polyvinylidenefluoride (Bing-zhi et al. 2010). However, there are few papers on the use of inorganic ceramic membranes that are characterized by good thermal and chemical stability, high pressure and mechanical resistance, long life and good antifouling properties.
Although BPA in biologically treated wastewater can be both adsorbed to particulate matter and present in soluble form, previous studies with membrane bioreactors have focused on removing only adsorbed BPA with MF and UF (Zuehlke et al. 2003, Schröder 2006, Chen et al. 2008). The use of these membrane bioreactors had several drawbacks. To ensure high efficiency, these systems were operated at a high hydraulic retention time (HRT) of over 10 h, or additional physicochemical processes were performed, such as ozonation of the permeate. In contrast to the above studies, NF would allow removal of both adsorbed and soluble BPA, but it is not known whether the use of NF would substantially increase the efficiency of BPA removal because the size of this increase would largely depend on the amount of BPA adsorbed to suspended solids. If most BPA is adsorbed to these solids, then MF could be sufficient for post-treatment because the MF filter can serve as a secondary clarifier and separate BPA sorbed on suspended solids from the secondary effluent. However, the relative removal efficiency of MF versus NF has not been investigated. Thus, we compared MF and NF ceramic membranes for the removal of BPA from effluent from reactors with immobilized biomass operated at an HRT of only 1.5 h. At such a short HRT, high concentrations of suspended solids in the effluent are observed, to which BPA and other hydrophobic compounds may be sorbed. Because high concentrations of solids can foul membranes, we tested the susceptibility of the ceramic membranes to fouling. These results will help to determine the most effective combination of membrane selectivity and permeability when using ceramic membranes for BPA removal from secondary effluent.
Materials and methods
Characteristics of feed wastewater
The experiments were run with biologically treated wastewater that contained BPA. The biological treatment system consisted of an aerobic up-flow reactor with biomass immobilized on a stationary porous support. This reactor treated synthetic wastewater, imitating municipal wastewater that was spiked with BPA at concentrations of 2.5, 5.0 and 10.0 mg/L (Zielińska et al. 2014). The volumetric loading rate was 7 kg COD/(m3 d), and the organic compounds-to-nitrogen ratio (COD/N) was 7.2. Due to the very short hydraulic retention time in the bioreactor of 1.5 h, this effluent had concentrations of suspended solids of 78 ± 12 mg TSS/L and concentrations of COD of 212 ± 36 mg/L. Concentrations of BPA were 0.3 ± 0.14, 0.5 ± 0.19 and 0.7 ± 0.29 mg/L, respectively at the initial BPA concentrations in synthetic wastewater of 2.5, 5.0 and 10.0 mg/L. The overall efficiency of the bioreactor ranged from 61 to 66 % COD removal and from 87 to 92 % BPA removal.
Membrane filtration protocol
The membrane installation was operated in two variants. In the first variant, an MF membrane was placed in the membrane module. In this setup, the membrane functioned as a secondary clarifier. In the second variant, an NF membrane was used in the membrane module to remove soluble and adsorbed pollutants.
Before filtration, the membrane module was flushed by circulating distilled water for 20 min. After that, pure water permeation was measured. The average permeation flux of distilled water (JW) was 1500 L/(m2 h) for the MF membrane and 80 L/(m2 h) for the NF membrane. Filtrations were performed with a feed flow velocity of 22–29 L/min and a temperature of 21 °C. The initial transmembrane pressure (TMP) (i.e., the difference in pressure between feed and filtrate) was 0.3 MPa for MF and 0.7 MPa for NF. Turbulent flow in the membrane channels lowered concentration polarization.
To determine the effect of BPA concentrations on filtration by both MF and NF membranes, biologically treated wastewater with different concentrations of BPA was used. In series 1 (S1), BPA concentration in the feed solution was 0.3 ± 0.14 mg/L; in series 2 (S2), it was 0.5 ± 0.19 mg/L; in series 3 (S3), 0.7 ± 0.29 mg/L. During filtration, permeation tests were conducted. The time necessary to collect half a liter of permeate was measured. These measurements were taken until the membrane was totally clogged. At the end of each filtration experiment, the membranes were rinsed with distilled water.
The fouling intensity was determined by calculating the normalised permeate flux (α), defined as the ratio of permeate flux (JV) to the flux of distilled water (JW). An α ratio near 1 indicates that the wastewater tends to clog the membrane very little, whereas a ratio near 0 indicates that it tends to clog the membrane quickly.
The abbreviations used in the equations are as follows: A—membrane filtration area (m2), CF—BPA concentration in the feed solution (mg/L), CP—BPA concentration in the permeate (mg/L), CR—BPA concentration in the retentate (mg/L), t—time for collecting a known volume of permeate (h), TMP—transmembrane pressure (MPa), VF—initial feed volume (L), VP—total withdrawn permeate volume (L), VR—retentate volume (L).
In order to characterize the feed solution, the permeate and the retentate, their basic physicochemical descriptors were determined, including organic compounds as COD and BOD5, total suspended solids (TSS) and volatile suspended solids (VSS), according to APHA (1992).
BPA concentrations were determined in the feed solution, the permeate and the retentate. To prepare the samples for HPLC, solid-phase extraction (SPE) was performed, using Supelclean™ ENVI™-18 SPE/3 mL/500 mg columns (Supelco). Before extraction, columns were treated with 8 mL methanol and 8 mL distilled water. To remove BPA from the solid particles in the samples, retentate samples were pretreated before SPE enrichment. For this pretreatment, a solution was prepared with retentate and methanol in a 3:1 ratio (v/v), and then shaken for 2 h at 150 rpm before filtering. After filtration, the samples were added to the SPE columns. Columns have been dried under vacuum. Samples were eluted five times with 1 mL of acetonitrile, and then the acetonitrile was evaporated under a nitrogen stream to a volume of 2 mL. Finally, 1 mL of sample was analyzed by HPLC. This analysis was performed with a Supelcosil LC-PAH column from Supelco (5 µm particles, 4.6, 150 mm) using a Varian HPLC system with a UV–Vis detector at 278 nm. The mobile phase (acetonitrile/water = 70/30, v/v) was pumped at a flow rate of 1 mL/min. The temperature of the column oven was set at 35 °C.
The normality of the distribution was tested with Shapiro–Wilk’s test. The differences between the mean values derived from particular groups were examined with ANOVA and Tukey’s test. The strength of the relationships between groups of results was determined using Pearson’s correlation coefficient (R). With all statistical analyses, p ≤ 0.05 was considered significant. Statistica 9.0 PL (StatSoft) was used.
Results and discussion
BPA removal by the MF and NF
In this study, the effluent from the reactor with immobilized biomass was post-treated by membrane filtration because of high concentrations of COD, suspended solids and BPA. Two kinds of filtration were selected to differentiate the amount of BPA adsorbed on suspended solids, which would be removed by MF, and the amount of BPA in soluble forms, which would be removed by NF in addition to the BPA adsorbed on suspended solids.
For a given BPA concentration in the feed solution, characteristics of wastewater pumped into MF and NF installations were the same. In S2 and S3, BPA removal was higher with NF than with MF due to several factors. In general, the size exclusion is considered the dominant mechanism for rejection of large organic compounds, such as BPA. Electrostatic repulsion is negligible, as the other mechanism contributes to BPA removal, due to the nonionic form of this pollutant in neutral pH (Bolong et al. 2010). Size exclusion was a probable mechanism for BPA rejection with NF because the molecular mass of BPA (228.29 g) is higher than the cut-off of this membrane. Weaker removal of BPA by MF than by NF showed that the MF membrane is too open for BPA; the pore size of the membrane is several orders of magnitude larger than the size of the BPA molecule. Higher removal with NF also indicates that part of the BPA or its by-products were present in the liquid phase. In addition, the higher transmembrane pressure in NF (0.7 MPa) than in MF (0.3 MPa) increased the removal of BPA because the higher pressure increased the rate of water transport through the membrane, and in consequence, lowered BPA concentration in the permeate. Because BPA removal with MF was above 24 %, it might be expected that some of the BPA that was present in the feed solution was bound by particulate organic matter in the wastewater and allowed BPA to be separated by this membrane. Therefore, it can be stated that the presence of organic matter significantly improves EDC rejection by membranes (Jin and Hu 2015).
Suspended solids and COD removal by the MF and NF
Suspended solids were present in the feed solutions in concentrations of 78 ± 12 mg TSS/L; these solids were completely removed by MF and NF membranes. COD concentration in the feed solutions was 212 ± 36 mg/L. The experimental series did not differ significantly in terms of concentration of suspended solids and of COD in the feed solution. COD retention on the membranes ranged from 40 to 60 %, independently of the membrane type and the composition of the feed solution. The majority of COD retained on the membranes were in particulate form or were adsorbed on the bio-floc, hence COD removal by NF, which did not exceed 60 %, was similar to that by MF, and the differences between the two were not significant. This probably indicates that in the biological effluent, hard-to-degrade organic compounds that were present in soluble forms had molecular masses lower than the cut-off of the membranes. The presence of these hard-to degrade compounds, which probably included the by-products of BPA metabolism, was indicated by a BOD/COD ratio lower than 0.1. It is possible that the total efficiency of the removal of organic compounds could have been affected by extracellular polymeric substances being released from biological solids to the liquid because of shearing forces caused by the high-pressure pump or by particles in wastewater becoming fragmented by these forces. However, extracellular polymeric substances have a molecular mass in the range of 31.0–97.4 kDa (Zhang et al. 2007), so they would have been present only in the MF permeate. On the other hand, the fouling layer forming on the membrane surface, as an inherent phenomenon during membrane filtration, can potentially improve the separation on membranes, giving similar results for two different membranes.
Hydraulic parameters of the MF and NF
Hydraulic parameters of MF and NF at BPA initial concentrations of 0.3 ± 0.14 mg BPA/L, 0.5 ± 0.19 mg BPA/L and 0.7 ± 0.29 mg BPA/L
BPA conc. in the feed solution (mg/L)
JV (L/(m2 h))
Rm ((MPa s)/m)
0.3 ± 0.14
0.5 ± 0.19
0.7 ± 0.29
Based on the presented results, it cannot be said that membrane cut-off is the main factor that affects the length of the filtration cycle and permeate flux. Jin et al. (2010) have reported that the flux through a ceramic membrane with pores as small as 80 nm and a smooth surface had a longer filtration cycle than flux through a membrane with pore sizes of 300 nm and an irregular surface. In the current study, the structures of the membrane surfaces were not determined. The values of JV were close for MF and NF (Fig. 4; Table 1). This may have been connected with the applied TMP (in the experiments, TMP values typical for particular membrane processes were used: 0.3 MPa for MF and 0.7 MPa for NF). Generally, the increase in TMP causes an increase in flux values, based on Darcy’s law. However, an increase in TMP could also be attributed to membrane fouling that results from concentration polarization (Sun et al. 2015). As the TMP increases, more pollutants accumulate on the membrane surface, and they form a gel layer and some pores are clogged. This increases filtration resistance and decreases the flux.
In the present study, the permeate flux decreased more with wastewater containing BPA than with distilled water. The normalised permeate flux (α) was significantly higher during NF than during MF (Table 1). When α is below 1, this indicates that the membrane is being fouled by organic matter accumulating in the pores and on the surface of the membrane, which clogs the flux. This occurred with both MF and NF; however, α was close to 0 with MF, indicating that this membrane tended to become fouled more quickly than the NF membrane. The suggested reason is the ratio between pore size and size of the particles that determines membrane clogging. Particles close to or smaller than the diameter of the membrane pores foul the pores and membrane surfaces and form a filtration cake more quickly and to a greater extent than larger particles. Therefore, in MF, the flux decline is due to the pore clogging with compounds of a size similar to the pore diameter and due to cake formation with larger particles. On the other hand, dissolved compounds are more problematic in relation to fouling of NF membranes (Zahrim et al. 2011). Opinions differ about reasons for fouling. It may be caused mainly by relatively large colloids and soluble organic compounds ranging from 0.450 to 0.026 μm (Zheng et al. 2009) or by extracellular polymers excreted by bacteria (Lee et al. 2006). Hydrophobic compounds, such as BPA, tend to strongly bind to hydrophobic materials like membranes. Hence, adsorption of organic compounds could change the hydrophobicity of the membrane surface, which could cause fouling (Escalona et al. 2014). In the current study, the intensity of fouling was probably not affected by COD in the feed solution, as indicated by the fact that the permeability of the membrane to COD was similar in MF and NF. In our study the concentration of colloidal organics, which are particles smaller than suspended solids, was not measured, though they may have caused fouling. Colloids that are deposited on the membrane may additionally adsorb dissolved organic compounds, which affect the rejection of micropollutants (Andrade et al. 2014).
Although the permeate flux differed in the various series, this was not due to different concentrations of suspended solids in the feed solution because these concentrations did not differ significantly. Similarly, Muthukumaran et al. (2011) found that the permeate flux was not influenced by changes in the concentrations of suspended solids in biologically treated effluent, which ranged from 13 to 30 mg/L. Bendick et al. (2005) did find that the permeate flux decreased when the concentration of suspended solids in the feed solution increased from 56 to 239 mg/L, but further increase did not change this parameter. Independent of the reason, fouling decreases the permeate flux, shortens the filtration cycle and destabilizes membrane function. These problems increase the operational cost of a membrane module.
In the series with the lowest initial BPA concentration in the present study, the permeate flux was also the lowest in both MF and NF. The low permeate flux was due to fouling; however, the fouling probably explains the complete retention of BPA in this series. When the membrane is clean at the beginning of filtration, the rejection of pollutants is determined by the nominal pore size. When fouling occurs during filtration, particles present in the wastewater clog membrane pores and form filtration cake. This decreases the filtration flux rate while simultaneously improving retention of pollutants. The rejection of particles smaller than the membrane cut-off is possible because fouling lowers the nominal diameter to the so-called effective diameter. This has been confirmed by Muthukumaran et al. (2011), who showed that fouling affects permeate flux more than pore size. Also, Boonyaroj et al. (2012) found that BPA rejection by a membrane covered with a layer of pollutant was 68.89 %, whereas rejection by a clean membrane was only 23.01 %.
In this study, membrane resistance (Rm) in each series was higher during filtration through the membrane with a lower cut-off (NF) (Table 1). It is not, however, obvious that Rm will be higher with NF than MF because a membrane with a higher cut-off is clogged mainly by pollutants that penetrate into the membrane pores. When the cut-off is lower, the membrane is clogged mainly by pollutants retained on its surface, and these pollutants could be removed by shearing forces if the filtration is performed in cross-flow mode, as in the present experiment.
In the current study, the composition of the feed solution affected the filtration process, as indicated by the positive correlations between BPA removal efficiency and membrane resistance (R = 0.83), and between BPA removal efficiency and the volumetric concentration factor (VCF) (R = 0.64). Although the VCF increased when transmembrane pressure was increased from 0.3 to 0.7 MPa (R = 0.68) (Table 1), this did not affect the retention of COD. At the highest initial BPA concentration, the permeability of the NF membrane was much lower than that of the MF membrane, as shown by the recovery value (Y), which expresses the ratio of the permeate volume to the feed solution volume.
In conclusion, our results suggest that the use of ceramic MF and NF membranes is effective in post-treating biologically treated wastewater that contains BPA. At an initial BPA concentration of 0.3 ± 0.14 mg/L, BPA was completely removed. At higher BPA concentrations in the feed solution, from 61 to 75 % of BPA was removed with NF. The efficiency of removal with MF was significantly lower (from 24 to 37 %). However, even at a membrane pore size of 0.45 µm, the MF membrane can remove BPA because it is sorbed on particulate matter, which is retained. Because the retentate from the membrane process contains a large BPA loading, further studies are necessary to find a solution for decontamination of the retentate by, for example, recirculating the retentate to a biological reactor.
This work has been financed by the Polish National Science Center and statutory project. Thanks to Mark Leonard for proof-reading.
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