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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
The application of microfiltration-reverse osmosis/ nanofiltration to trace organics removal for municipal wastewater reuse a
b
a
N. Garcia , J. Moreno , E. Cartmell , I. Rodriguez-Roda a
bc
a
& S. Judd
Cranfield Water Science Institute, Cranfield University, Bedford MK43 0GA, UK
b
Laboratory of Chemical and Environmental Engineering (LEQUiA), Institute of the Environment, University of Girona, Girona, Spain c
Catalan Institute for Water Research (ICRA), University of Girona, Girona, Spain Published online: 14 Jun 2013.
To cite this article: N. Garcia, J. Moreno, E. Cartmell, I. Rodriguez-Roda & S. Judd (2013) The application of microfiltrationreverse osmosis/nanofiltration to trace organics removal for municipal wastewater reuse, Environmental Technology, 34:24, 3183-3189, DOI: 10.1080/09593330.2013.808244 To link to this article: http://dx.doi.org/10.1080/09593330.2013.808244
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 24, 3183–3189, http://dx.doi.org/10.1080/09593330.2013.808244
The application of microfiltration-reverse osmosis/nanofiltration to trace organics removal for municipal wastewater reuse N. Garciaa , J. Morenob , E. Cartmella , I. Rodriguez-Rodab,c and S. Judda∗ a Cranfield Water Science Institute, Cranfield University, Bedford MK43 0GA, UK; b Laboratory of Chemical and Environmental Engineering (LEQUiA), Institute of the Environment, University of Girona, Girona, Spain; c Catalan Institute for Water Research (ICRA), University of Girona, Girona, Spain
Downloaded by [University of Connecticut] at 06:02 07 October 2014
(Received 9 January 2013; final version received 19 May 2013 ) The fate of organic micropollutans (MPs) in a membrane system based on microfiltration (MF) and reverse osmosis/nanofiltration (RO/NF) has been investigated for the case of wastewater reuse. Both an operating full-scale water reuse plant and a pilot plant were employed, with 22 individual organic compounds at their ambient concentrations studied for the former and the latter employing two target compounds over a range of feed concentrations. Results revealed removal efficiencies higher than 75% for most compounds in the full-scale plant, though mass flow studies on all streams revealed a significant imbalance of material for some compounds. Rejection efficiencies measured for candidate commercial NF and RO membranes tested at pilot scale challenged with a pharmaceutically active compound (ibuprofen, IBU) and an endocrinedisrupting chemical (nonylphenol, NP) exceeded 99%. Permeate concentrations were 0.005–0.14 μg/L for IBU and below the limit of detection for NP. A mass balance of the MPs for the full-scale plant across the MF and RO stages revealed a significant imbalance associated with the challenge of accurate determination of low concentrations. Differences in pilot plant and full-scale data were otherwise attributed to the impact of membrane ageing (and specifically hydrolysis) on RO rejection of the MPs examined. Keywords: reverse osmosis; microfiltration; wastewater reuse; organic micropollutants; mass balance
1. Introduction The presence of organic micropollutants (MPs) in municipal wastewater is considered an important issue of environmental concern, and the low concentrations at which they arise makes their efficient removal challenging. Methods for their removal have increasingly focused on membrane technologies, from microfiltration (MF) through to reverse osmosis (RO), both for metals [1,2] and organics.[3–12] The recalcitrant nature of the organic compounds has provided opportunities for membrane bioreactors (MBRs),[2– 6] which operate under at longer sludge ages than classical activated sludge processes thus providing more intensive biotreatment. However, the increasing focus on water reuse has led to many recent studies of these residual substances in dense membrane systems, and RO and nanofiltration (NF) specifically,[4–12] with particular interest in the increasingly accepted two-stage MBR-RO/NF treatment scheme.[4–6] Within the past 2–3 years especially, attention has turned to the relative efficacy of RO and NF against organic MPs.[6–10] It has been demonstrated [8] that NF provides an effective barrier for pharmaceuticals, pesticides, endocrine disruptors and other organic contaminants in wastewater. Average removals reported for neutral com∗ Corresponding
author. Email: s.j.judd@cranfield.ac.uk
© 2013 Taylor & Francis
pounds were 82% and 85% for NF and RO, respectively, with corresponding values of 97% and 99% for ionic compounds. A study of an MBR/NF hybrid system [5] for the removal of nutrients and organic MP (pharmaceuticals and personal-care products, PPCPs) from municipal wastewaters revealed >98% for ibuprofen (IBU).[9] Notwithstanding the increased focus on the removal of organic MPs by dense membrane processes for water reuse, there have been few studies conducted either at full scale on the fate of these substances within the complete treatment scheme, or of the impact of their concentration on removal from real waters using full-scale membrane modules. A recent study on full-scale MF and RO membrane facilities suggests that removal exceeds 97%,[11] though a number of studies based on commercial technologies have indicated rejection to be related to the rating of the membrane (Table 1). Data in Table 1 indicate that there is a tendency for the expected decrease in rejection with decreasing membrane selectivity; e.g. from 99% for NF90 down to 39% for NF40 in the case of IBU for a 0.11–0.14 μg/L feed,[5] compared with >99% for an RO membrane.[6,11] Of those chemicals listed, nonylphenol (NP) appears to present the greatest challenge regarding its removal: rejection is as low as 83% for an RO membrane.[13] Against this, removal
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Table 1. Organic MPs rejection data from RO/NF. Membrane process
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NF40 NF70 NF90 NF40 NF70 NF90 RO (BW30 and ESPA2/NF (NF270 and NF90)
NF90
NF RO RO
RO/NF RO/NF RO RO
MicroRemoval Feed pollutant efficiency (%) concentration Ref. IBU DFC IBU
BPA DFC E1 EE2 IBU E1 E2 EE2 BPA DFC IBU NP BPA DFC NP BPA E1 IBU DFC IBU
39 56 98.6 86 100 100 >99/>99
>98/>97 93/92 99/99 99/99 97 92 97 94 90 100 100 >99 96 98 100 83/74 96/95 >76/>76 >95 >90 99
108 ng/L
[5]
138 ng/L 2 μg/L
[6]
5–18 μg/L
[8]
122 ng/L
[9]
410 ng/L 133 ng/L 550 ng/L 270 ng/L 0.94 μg/L 6.06 ng/L 7.46 ng/L 90 ng/L 500 μg/L 78.8–202 ng/L
[11]
[13] [14] [15] [16]
Notes: IBU, ibuprofeno; BPA, bisphenol A; DFC, diclofenac; NP, nonylphenol; E1, estrone; E2, 17β-oestradiol and EE2, 17αethynylestradiol.
data reported by the most selective of the commercial NF membranes tested (NF90) are comparable to those of RO. Comparative studies of NF and RO performance for organic MP rejection for both drinking water [9] and wastewater reuse [10] have demonstrated only marginally increased rejection by RO over NF. This is supported by a review of organic MPs rejection data from papers based on benchscale to full-scale operation (Table 1), which appears to suggest that the difference in rejection performance is negligible. It is thus questionable that, unless desalination is a requirement, the additional purification afforded by the RO is justifiable given its higher operational costs incurred from higher pressure operation. It is also the case that for any dense membrane process there remains an issue of the management of the concentrate stream where almost all of the MP content resides. The current study aimed to establish: • The fate of MPs in a complete full-scale membrane polishing process, based on a mature two-stage MFRO water reuse installation.
• The impact of membrane selection (NF vs. RO) on removal efficiency, based on a full-scale module. • The integrity of the data. Data integrity was assessed through completion of a mass balance across the two individual membrane separation stages for a range of 22 individual MPs. It is only through conducting the mass balance at full scale that any constraints regarding the accuracy of the data can be identified, since the mass flow of MPs in the concentrate and permeate streams should equal that of the feed stream in each case. Mass balances of this type are often overlooked in studies of this nature. The supplementary test of the efficacy of the NF vs. the RO membrane was assessed with reference to a pharmaceutically active compound IBU and an endocrine-disrupting chemical (NP).
2. Materials and methods 2.1. Reference plants 2.1.1. Full scale The full-scale 1200 m3 /d capacity UK-based MF-RO plant (Figure 1) was the same as that on which a previous study was based, and has been described elsewhere.[1] It is fed with secondary-treated municipal wastewater from the neighbouring wastewater treatment works, and generates desalinated water for industrial reuse. The plant comprises a 150 μm screen for protecting the MF. The hollow fibre (HF) MF operates with regular backflushing and cleaning in place (CIP) with hypochlorite, acid and alkali for maintenance of permeability. The MF filtrate is held in an intermediate storage tank prior to treatment by the 2:1 array-configured RO process. Scaling in the RO is ameliorated by upstream dosing with antiscalant and acid. The plant operates at mean recoveries of 86% at the MF stage and 73% for the RO (i.e. 1910 and 1640 m3 /d MF feed and permeate, respectively, and 1200 m3 /d RO permeate if operating at full capacity). 2.1.2. Pilot-scale Performance data for specific commercial RO membrane modules were obtained from an RO pilot plant installed at the Castell-Platja d’Aro WWTP (Catalonia, Spain) and described elsewhere.[17] The 4.3 m3 /d plant (Figure 2) treats municipal wastewater, with a MBR fitted upstream to protect the RO. The MBR is assumed to provide biotreated and microfiltered municipal wastewater in a manner analogous to the full-scale reuse plant where classical activated sludge treatment precedes the MF stage. The RO process comprised a pressure vessel housing a single element, fed from an intermediate 200 L holding tank and protected by a cartridge filter. The rig permitted either discharge or recycling of the concentrate, the latter providing increased feedwater or retentate concentrations as encountered through RO staging.
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Figure 1.
Schematic of the full-scale MF-RO plant for municipal wastewater reuse.
Figure 2.
Pilot plant, schematic.
Three standard 40–40 in −4 in (or 100 mm) in diameter and 40 in (or 1 m) long – commercial membrane modules of differing rejection properties were employed for the study: two RO membranes (HR and LE) and one nanofiltration (NF) membrane (NF270), all provided by the Dow Chemical Company. The membranes (Table 2) were selected so as to provide a range of selectivity. The methodology has been described in detail elsewhere.[1] Prior to each experiment, each membrane was conditioned using permeates from the MBR permeate tank for 16–20 h at a pressure of 4 bar for the NF membrane and 9 bar for the two RO membranes. The holding tank was then spiked with 10 μg/L of the target compounds of IBU and NP as 50 mL aliquots from a 40 mg/L standard solution. Trials were undertaken by incrementally increasing the feedwater concentration by passing 50% of the
Table 2.
Properties of the selected membranes.
Membrane LC LE-4040 LC HR-4040 NF270-4040
Area Stabilized salt Permeate flow MWCO (m2 ) rejection (%)a rate (m3 /d)a (Da) 7.2 8.7 7.6
99.2 99.7 >97.0
9.5 11 9.5
∼100 ∼100 ∼200
Notes: LE, low energy; HR, high rejection and NF270, nanofiltration. a Standard test conditions: General, 25◦ C, 15% recovery, pH 8. Salt, 2000 mg/L NaCl for LE and HR; 2000 mg/L MgSO4 for NF. TMP, 15.5 bar for HR, 8.6 bar for LE and 4.8 bar for NF.
feedwater through the RO process and returning the concentrate to the holding tank. This process was repeated seven times for each of the three membranes tested, providing
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a range of concentration factor values between unity and 4.5. This enabled the overall feedwater to be increased in accordance with retentate concentration across a full-scale RO array. Permeate recoveries of 12–15% were maintained throughout.
2.2. MP fate analysis 2.2.1. Chemical analysis MP removals at the full-scale installation were determined through sampling of the various streams, and specifically the feed and backwash/reject streams of both the MF and RO processes. Grab samples from this site were taken 2–3 times daily over a three-day period, and contaminants assayed by Anglian Water Laboratories (Huntingdon, UK) according to standards methods based on GC-ICP-MS. Twenty-two MP ‘priority’ compounds were assayed: EDTA, NP, estrone (E1), 17βoestradiol (E2), 17α-ethynylestradiol (EE2), tributyltin, naphthalene (NAPHT), IBU, ofloxacin (OFLX), oxytetracyc (OXTCY), erythromycin, propranolol, fluoxetine , triclosan, diclofenac (DFC), 22 44 -tetrabromodiphenyl ether (BDPE47), 22 44 5-pentabromodiphenyl ether (BDPE99), 22 44 6-pentabromodiphenyl ether (BDPE100), 22 44 55 hexabromodiphenyl ether (BDPE153), bis-(2-ethylhexyl) phthalate (DEHP), glyphosate (GLYPH) and mecoprop (MCPP). Limits of detection ranged from 0.01 μg/L for
Figure 3.
the pharmaceutical compounds to 0.07–0.19 μg/L for the BDPE compounds. At the pilot plant, removals by the NF and RO membranes’ processes were determined through sampling of permeate and concentrate streams. The sampling campaign was carried out over a three-day period, one day dedicated to each membrane, and MPs assayed by the Catalan Institute for Water Research (ICRA) (Girona, Spain) using standard methods based on ultraperformance liquid chromatography-mass spectrometer.[18] The robustness of the concentration data was appraised through a mass balance for all target compounds for both the full-scale (at both the MF and the RO stages) and pilot-scale plants. Mass flows, in milligram/day for each compound, were calculated for feed, retentate and permeate flows and the % recovery of the feed mass flow in the two membrane outlet streams determined.
3. Results and discussion 3.1. Removal of MPs, full-scale plant The fate of the organics within the process was determined through measurement of the mean concentration values for each compound. Concentrations ranged from around 0.1–1 ng/L for the estrogenic (E1, E2 and EE2) and the polybrominated diethers (PBDE) compounds to 1– 100 ng/L for most of the other compounds other than EDTA
Permeate water quality: *% of samples where the compound was detectable.
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Environmental Technology
Figure 4.
MP rejection data.
Figure 5.
Mean mass flows (mg/d), full-scale installation.
(∼500 μg/L). The overall standard deviation values for the full-scale MP concentration data ranged from 2% to 90%, with the higher values arising at concentrations close to the limit of detection. Summary data for the detectability, rejection and mass flows of those compounds for which data were acquired are depicted in Figures 3–5, respectively. It is only those compounds having a high percentage detection value combined
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with a high mean permeate concentration that can be considered to be routinely poorly rejected by the membrane, since a single anomaly can skew the mean concentration value. On this basis, all organics other than EDTA, NAPHT, E1 and BDPEs 47 and 99 can be considered to be substantially removed by the MF/RO process with less than half of the RO permeate samples analysed having detectable levels of the compound. For most compounds, removal by
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the process exceeded 75% (Figure 4); for 10 of the 22 compounds assayed the RO permeate concentration was below the limit of detection. On the other hand, 13 of the 22 compounds were detected in all of the MF permeate samples analysed, albeit at very low concentrations in many cases. A few compounds, primarily the BDPEs and DEHP, were significantly removed at the MF stage (by 47–85%) due to their strong association with the suspended solids.[19] Observed trends are similar to results reported in the literature, with some compounds such as DFC (>98% rejection) almost completely removed by RO membranes,[9,11, 15] compared with a very low level of rejection (99% throughout based on the limit of detection of 0.05 μg/L. For the IBU, the maximum permeate measured was 0.14 μg/L at feed concentrations between 10 and 29 μg/L, implying a rejection of around 99.5% for this compound. Reported data from a previous trial conducted at comparable feed concentration
Table 3.
Water quality, pilot plant.
RO feed/ RO permeate Stream: Membrane concentrate (μg/L) (μg/L) %SD IBU NP
LE HR NF270 LE HR NF270
10.3–16.3 11.1–23.1 10.3–28.5 7.1–14.2 8.4–13.0 9.3–14.5
bld (
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
The application of microfiltration-reverse osmosis/ nanofiltration to trace organics removal for municipal wastewater reuse a
b
a
N. Garcia , J. Moreno , E. Cartmell , I. Rodriguez-Roda a
bc
a
& S. Judd
Cranfield Water Science Institute, Cranfield University, Bedford MK43 0GA, UK
b
Laboratory of Chemical and Environmental Engineering (LEQUiA), Institute of the Environment, University of Girona, Girona, Spain c
Catalan Institute for Water Research (ICRA), University of Girona, Girona, Spain Published online: 14 Jun 2013.
To cite this article: N. Garcia, J. Moreno, E. Cartmell, I. Rodriguez-Roda & S. Judd (2013) The application of microfiltrationreverse osmosis/nanofiltration to trace organics removal for municipal wastewater reuse, Environmental Technology, 34:24, 3183-3189, DOI: 10.1080/09593330.2013.808244 To link to this article: http://dx.doi.org/10.1080/09593330.2013.808244
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 24, 3183–3189, http://dx.doi.org/10.1080/09593330.2013.808244
The application of microfiltration-reverse osmosis/nanofiltration to trace organics removal for municipal wastewater reuse N. Garciaa , J. Morenob , E. Cartmella , I. Rodriguez-Rodab,c and S. Judda∗ a Cranfield Water Science Institute, Cranfield University, Bedford MK43 0GA, UK; b Laboratory of Chemical and Environmental Engineering (LEQUiA), Institute of the Environment, University of Girona, Girona, Spain; c Catalan Institute for Water Research (ICRA), University of Girona, Girona, Spain
Downloaded by [University of Connecticut] at 06:02 07 October 2014
(Received 9 January 2013; final version received 19 May 2013 ) The fate of organic micropollutans (MPs) in a membrane system based on microfiltration (MF) and reverse osmosis/nanofiltration (RO/NF) has been investigated for the case of wastewater reuse. Both an operating full-scale water reuse plant and a pilot plant were employed, with 22 individual organic compounds at their ambient concentrations studied for the former and the latter employing two target compounds over a range of feed concentrations. Results revealed removal efficiencies higher than 75% for most compounds in the full-scale plant, though mass flow studies on all streams revealed a significant imbalance of material for some compounds. Rejection efficiencies measured for candidate commercial NF and RO membranes tested at pilot scale challenged with a pharmaceutically active compound (ibuprofen, IBU) and an endocrinedisrupting chemical (nonylphenol, NP) exceeded 99%. Permeate concentrations were 0.005–0.14 μg/L for IBU and below the limit of detection for NP. A mass balance of the MPs for the full-scale plant across the MF and RO stages revealed a significant imbalance associated with the challenge of accurate determination of low concentrations. Differences in pilot plant and full-scale data were otherwise attributed to the impact of membrane ageing (and specifically hydrolysis) on RO rejection of the MPs examined. Keywords: reverse osmosis; microfiltration; wastewater reuse; organic micropollutants; mass balance
1. Introduction The presence of organic micropollutants (MPs) in municipal wastewater is considered an important issue of environmental concern, and the low concentrations at which they arise makes their efficient removal challenging. Methods for their removal have increasingly focused on membrane technologies, from microfiltration (MF) through to reverse osmosis (RO), both for metals [1,2] and organics.[3–12] The recalcitrant nature of the organic compounds has provided opportunities for membrane bioreactors (MBRs),[2– 6] which operate under at longer sludge ages than classical activated sludge processes thus providing more intensive biotreatment. However, the increasing focus on water reuse has led to many recent studies of these residual substances in dense membrane systems, and RO and nanofiltration (NF) specifically,[4–12] with particular interest in the increasingly accepted two-stage MBR-RO/NF treatment scheme.[4–6] Within the past 2–3 years especially, attention has turned to the relative efficacy of RO and NF against organic MPs.[6–10] It has been demonstrated [8] that NF provides an effective barrier for pharmaceuticals, pesticides, endocrine disruptors and other organic contaminants in wastewater. Average removals reported for neutral com∗ Corresponding
author. Email: s.j.judd@cranfield.ac.uk
© 2013 Taylor & Francis
pounds were 82% and 85% for NF and RO, respectively, with corresponding values of 97% and 99% for ionic compounds. A study of an MBR/NF hybrid system [5] for the removal of nutrients and organic MP (pharmaceuticals and personal-care products, PPCPs) from municipal wastewaters revealed >98% for ibuprofen (IBU).[9] Notwithstanding the increased focus on the removal of organic MPs by dense membrane processes for water reuse, there have been few studies conducted either at full scale on the fate of these substances within the complete treatment scheme, or of the impact of their concentration on removal from real waters using full-scale membrane modules. A recent study on full-scale MF and RO membrane facilities suggests that removal exceeds 97%,[11] though a number of studies based on commercial technologies have indicated rejection to be related to the rating of the membrane (Table 1). Data in Table 1 indicate that there is a tendency for the expected decrease in rejection with decreasing membrane selectivity; e.g. from 99% for NF90 down to 39% for NF40 in the case of IBU for a 0.11–0.14 μg/L feed,[5] compared with >99% for an RO membrane.[6,11] Of those chemicals listed, nonylphenol (NP) appears to present the greatest challenge regarding its removal: rejection is as low as 83% for an RO membrane.[13] Against this, removal
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Table 1. Organic MPs rejection data from RO/NF. Membrane process
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NF40 NF70 NF90 NF40 NF70 NF90 RO (BW30 and ESPA2/NF (NF270 and NF90)
NF90
NF RO RO
RO/NF RO/NF RO RO
MicroRemoval Feed pollutant efficiency (%) concentration Ref. IBU DFC IBU
BPA DFC E1 EE2 IBU E1 E2 EE2 BPA DFC IBU NP BPA DFC NP BPA E1 IBU DFC IBU
39 56 98.6 86 100 100 >99/>99
>98/>97 93/92 99/99 99/99 97 92 97 94 90 100 100 >99 96 98 100 83/74 96/95 >76/>76 >95 >90 99
108 ng/L
[5]
138 ng/L 2 μg/L
[6]
5–18 μg/L
[8]
122 ng/L
[9]
410 ng/L 133 ng/L 550 ng/L 270 ng/L 0.94 μg/L 6.06 ng/L 7.46 ng/L 90 ng/L 500 μg/L 78.8–202 ng/L
[11]
[13] [14] [15] [16]
Notes: IBU, ibuprofeno; BPA, bisphenol A; DFC, diclofenac; NP, nonylphenol; E1, estrone; E2, 17β-oestradiol and EE2, 17αethynylestradiol.
data reported by the most selective of the commercial NF membranes tested (NF90) are comparable to those of RO. Comparative studies of NF and RO performance for organic MP rejection for both drinking water [9] and wastewater reuse [10] have demonstrated only marginally increased rejection by RO over NF. This is supported by a review of organic MPs rejection data from papers based on benchscale to full-scale operation (Table 1), which appears to suggest that the difference in rejection performance is negligible. It is thus questionable that, unless desalination is a requirement, the additional purification afforded by the RO is justifiable given its higher operational costs incurred from higher pressure operation. It is also the case that for any dense membrane process there remains an issue of the management of the concentrate stream where almost all of the MP content resides. The current study aimed to establish: • The fate of MPs in a complete full-scale membrane polishing process, based on a mature two-stage MFRO water reuse installation.
• The impact of membrane selection (NF vs. RO) on removal efficiency, based on a full-scale module. • The integrity of the data. Data integrity was assessed through completion of a mass balance across the two individual membrane separation stages for a range of 22 individual MPs. It is only through conducting the mass balance at full scale that any constraints regarding the accuracy of the data can be identified, since the mass flow of MPs in the concentrate and permeate streams should equal that of the feed stream in each case. Mass balances of this type are often overlooked in studies of this nature. The supplementary test of the efficacy of the NF vs. the RO membrane was assessed with reference to a pharmaceutically active compound IBU and an endocrine-disrupting chemical (NP).
2. Materials and methods 2.1. Reference plants 2.1.1. Full scale The full-scale 1200 m3 /d capacity UK-based MF-RO plant (Figure 1) was the same as that on which a previous study was based, and has been described elsewhere.[1] It is fed with secondary-treated municipal wastewater from the neighbouring wastewater treatment works, and generates desalinated water for industrial reuse. The plant comprises a 150 μm screen for protecting the MF. The hollow fibre (HF) MF operates with regular backflushing and cleaning in place (CIP) with hypochlorite, acid and alkali for maintenance of permeability. The MF filtrate is held in an intermediate storage tank prior to treatment by the 2:1 array-configured RO process. Scaling in the RO is ameliorated by upstream dosing with antiscalant and acid. The plant operates at mean recoveries of 86% at the MF stage and 73% for the RO (i.e. 1910 and 1640 m3 /d MF feed and permeate, respectively, and 1200 m3 /d RO permeate if operating at full capacity). 2.1.2. Pilot-scale Performance data for specific commercial RO membrane modules were obtained from an RO pilot plant installed at the Castell-Platja d’Aro WWTP (Catalonia, Spain) and described elsewhere.[17] The 4.3 m3 /d plant (Figure 2) treats municipal wastewater, with a MBR fitted upstream to protect the RO. The MBR is assumed to provide biotreated and microfiltered municipal wastewater in a manner analogous to the full-scale reuse plant where classical activated sludge treatment precedes the MF stage. The RO process comprised a pressure vessel housing a single element, fed from an intermediate 200 L holding tank and protected by a cartridge filter. The rig permitted either discharge or recycling of the concentrate, the latter providing increased feedwater or retentate concentrations as encountered through RO staging.
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Environmental Technology
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Figure 1.
Schematic of the full-scale MF-RO plant for municipal wastewater reuse.
Figure 2.
Pilot plant, schematic.
Three standard 40–40 in −4 in (or 100 mm) in diameter and 40 in (or 1 m) long – commercial membrane modules of differing rejection properties were employed for the study: two RO membranes (HR and LE) and one nanofiltration (NF) membrane (NF270), all provided by the Dow Chemical Company. The membranes (Table 2) were selected so as to provide a range of selectivity. The methodology has been described in detail elsewhere.[1] Prior to each experiment, each membrane was conditioned using permeates from the MBR permeate tank for 16–20 h at a pressure of 4 bar for the NF membrane and 9 bar for the two RO membranes. The holding tank was then spiked with 10 μg/L of the target compounds of IBU and NP as 50 mL aliquots from a 40 mg/L standard solution. Trials were undertaken by incrementally increasing the feedwater concentration by passing 50% of the
Table 2.
Properties of the selected membranes.
Membrane LC LE-4040 LC HR-4040 NF270-4040
Area Stabilized salt Permeate flow MWCO (m2 ) rejection (%)a rate (m3 /d)a (Da) 7.2 8.7 7.6
99.2 99.7 >97.0
9.5 11 9.5
∼100 ∼100 ∼200
Notes: LE, low energy; HR, high rejection and NF270, nanofiltration. a Standard test conditions: General, 25◦ C, 15% recovery, pH 8. Salt, 2000 mg/L NaCl for LE and HR; 2000 mg/L MgSO4 for NF. TMP, 15.5 bar for HR, 8.6 bar for LE and 4.8 bar for NF.
feedwater through the RO process and returning the concentrate to the holding tank. This process was repeated seven times for each of the three membranes tested, providing
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a range of concentration factor values between unity and 4.5. This enabled the overall feedwater to be increased in accordance with retentate concentration across a full-scale RO array. Permeate recoveries of 12–15% were maintained throughout.
2.2. MP fate analysis 2.2.1. Chemical analysis MP removals at the full-scale installation were determined through sampling of the various streams, and specifically the feed and backwash/reject streams of both the MF and RO processes. Grab samples from this site were taken 2–3 times daily over a three-day period, and contaminants assayed by Anglian Water Laboratories (Huntingdon, UK) according to standards methods based on GC-ICP-MS. Twenty-two MP ‘priority’ compounds were assayed: EDTA, NP, estrone (E1), 17βoestradiol (E2), 17α-ethynylestradiol (EE2), tributyltin, naphthalene (NAPHT), IBU, ofloxacin (OFLX), oxytetracyc (OXTCY), erythromycin, propranolol, fluoxetine , triclosan, diclofenac (DFC), 22 44 -tetrabromodiphenyl ether (BDPE47), 22 44 5-pentabromodiphenyl ether (BDPE99), 22 44 6-pentabromodiphenyl ether (BDPE100), 22 44 55 hexabromodiphenyl ether (BDPE153), bis-(2-ethylhexyl) phthalate (DEHP), glyphosate (GLYPH) and mecoprop (MCPP). Limits of detection ranged from 0.01 μg/L for
Figure 3.
the pharmaceutical compounds to 0.07–0.19 μg/L for the BDPE compounds. At the pilot plant, removals by the NF and RO membranes’ processes were determined through sampling of permeate and concentrate streams. The sampling campaign was carried out over a three-day period, one day dedicated to each membrane, and MPs assayed by the Catalan Institute for Water Research (ICRA) (Girona, Spain) using standard methods based on ultraperformance liquid chromatography-mass spectrometer.[18] The robustness of the concentration data was appraised through a mass balance for all target compounds for both the full-scale (at both the MF and the RO stages) and pilot-scale plants. Mass flows, in milligram/day for each compound, were calculated for feed, retentate and permeate flows and the % recovery of the feed mass flow in the two membrane outlet streams determined.
3. Results and discussion 3.1. Removal of MPs, full-scale plant The fate of the organics within the process was determined through measurement of the mean concentration values for each compound. Concentrations ranged from around 0.1–1 ng/L for the estrogenic (E1, E2 and EE2) and the polybrominated diethers (PBDE) compounds to 1– 100 ng/L for most of the other compounds other than EDTA
Permeate water quality: *% of samples where the compound was detectable.
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Environmental Technology
Figure 4.
MP rejection data.
Figure 5.
Mean mass flows (mg/d), full-scale installation.
(∼500 μg/L). The overall standard deviation values for the full-scale MP concentration data ranged from 2% to 90%, with the higher values arising at concentrations close to the limit of detection. Summary data for the detectability, rejection and mass flows of those compounds for which data were acquired are depicted in Figures 3–5, respectively. It is only those compounds having a high percentage detection value combined
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with a high mean permeate concentration that can be considered to be routinely poorly rejected by the membrane, since a single anomaly can skew the mean concentration value. On this basis, all organics other than EDTA, NAPHT, E1 and BDPEs 47 and 99 can be considered to be substantially removed by the MF/RO process with less than half of the RO permeate samples analysed having detectable levels of the compound. For most compounds, removal by
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the process exceeded 75% (Figure 4); for 10 of the 22 compounds assayed the RO permeate concentration was below the limit of detection. On the other hand, 13 of the 22 compounds were detected in all of the MF permeate samples analysed, albeit at very low concentrations in many cases. A few compounds, primarily the BDPEs and DEHP, were significantly removed at the MF stage (by 47–85%) due to their strong association with the suspended solids.[19] Observed trends are similar to results reported in the literature, with some compounds such as DFC (>98% rejection) almost completely removed by RO membranes,[9,11, 15] compared with a very low level of rejection (99% throughout based on the limit of detection of 0.05 μg/L. For the IBU, the maximum permeate measured was 0.14 μg/L at feed concentrations between 10 and 29 μg/L, implying a rejection of around 99.5% for this compound. Reported data from a previous trial conducted at comparable feed concentration
Table 3.
Water quality, pilot plant.
RO feed/ RO permeate Stream: Membrane concentrate (μg/L) (μg/L) %SD IBU NP
LE HR NF270 LE HR NF270
10.3–16.3 11.1–23.1 10.3–28.5 7.1–14.2 8.4–13.0 9.3–14.5
bld (
