Abstract

This paper presents methodology, concept and results of the WateReuse Foundation project WFR – 09 – 06b when developing a high pressure membrane, reverse osmosis (RO) and nanofiltration (NF) online membrane integrity testing (MIT) technique. The use of pressure-driven membrane processes, particularly RO, has grown significantly over the past few decades in water treatment and reuse applications to safeguard water supplies against harmful pathogens and impurities. In principle, RO membranes should provide a complete physical barrier to the passage of nanosize pathogens (e.g., enteric viruses). However, in the presence of imperfections and/or membrane damage, membrane breaches as small as 20 to 30 nm can allow enteric viruses to pass through the membrane and contaminate the product water stream, thereby posing a potential health hazard that is of particular concern for potable water production. This project was focused on evaluating a pulsed-marker membrane integrity monitoring (PM-MIMo) approach for RO processes on the basis of the use of a fluorescent marker. The monitoring approach employs pulsed dosing (via a precision metering pump) of a marker into the RO feed stream coupled with online marker concentration monitoring in the RO permeate by an inline spectrofluorometer. Membrane integrity is then inferred on the basis of real-time analysis of the marker permeate time − profile concentration in response. The basic concept of the PM-MIMo approach for detecting membrane breaches was successfully demonstrated, by comparing intact and damaged membranes, in a series of experiments using a diagnostic plate-and-frame RO system and spiral-wound RO pilot system. Results of the developed technique are presented in the project report to allow the industry to consider adopting this technique for RO/NF online integrity monitoring.

INTRODUCTION

Monitoring and control of pathogens in water treatment processes are a daunting challenge for the water industry and governmental regulators (Lozier 2003; Mi et al. 2004; Kumar et al. 2007; Phattaranawik et al. 2008; Brehant et al. 2010; Guo et al. 2010). Of the different pathogens, waterborne viruses are especially challenging because of their small size, high mobility, and resistance to chlorination (Grabow 2001; Theng-Theng Fong 2005; Pontius et al. 2009). Waterborne enteric viruses have been linked to a variety of diseases, including poliomyelitis, heart diseases, encephalitis, aseptic meningitis, hepatitis, gastroenteritis, and even paralysis in immunocompromised individuals (Theng-Theng Fong 2005). Enteric viruses, which are nucleic acid strands surrounded by protein protective coats (capsids), are oblate intracellular parasites, requiring host cells in order to replicate. In the absence of host cells, enteric viruses are essentially inert nanoparticles (NPs), typically in the size range of approximately 30 to 100 nm (See Figure 1).

Figure 1

Depiction of surfaces of common waterborne enteric virus capsids (shaded area) and their typical surrogate (MS2) bacteriophage. Reprinted with permission from Carrillo-Tripp et al. (2009). Copyright 2009 Nucleic Acids Research.

Figure 1

Depiction of surfaces of common waterborne enteric virus capsids (shaded area) and their typical surrogate (MS2) bacteriophage. Reprinted with permission from Carrillo-Tripp et al. (2009). Copyright 2009 Nucleic Acids Research.

The application and integration (with conventional processes) of pressure-driven membrane processes have grown significantly, over the past few decades, as part of a multibarrier water treatment strategy to safeguard water supplies from harmful pathogens and impurities. Low-pressure membrane (LPM) processes such as microfiltration (MF) and ultrafiltration (UF) typically provide physical barriers for particles larger than >0.1 to 5 mc and >0.05 to 0.1 mc, respectively. High-pressure membrane (HPM) processes such as nanofiltration (NF) can reject multivalent ions and materials of >0.0005 to 0.001 mc, whereas reverse osmosis (RO) can reject materials as small as monovalent ions. Given the typical size of enteric viruses (30 − 100 nm), LPM processes such as UF can be effective in providing near-complete rejection of enteric viruses. In principle, HPM processes (RO/NF) should provide a complete physical barrier to nanosized enteric viruses. Membrane and membrane module imperfections and/or damage, however, may render both LPM and HPM membrane processes ineffective in removing viruses.

REGULATIONS

According to the U.S. Environmental Protection Agency (EPA) Surface Water Treatment Rule (SWTR), inadequately treated water may contain disease-causing microorganisms, which include bacteria, viruses, and parasites (California Department of Public Health 2009a). In 1986, the SWTR established requirements for removal and inactivation of these pathogens from the surface water systems. These regulations are on the basis of a minimum log removal value (LRV):
formula
(1)
where Cf and Cp are the concentrations of the pathogen or microorganism in the feed and permeate streams, respectively. Under the SWTR, LRVs in water treatment processes are regulated as follows (Haymore et al. 2001):
  • 99% (2-log) removal and/or inactivation of Cryptosporidium

  • 99.9% (3-log) removal and/or inactivation of Giardia

  • 99.99% (4-log) removal and/or inactivation of viruses

Some states implement a more stringent regulation for removal and/or inactivation of microorganisms from water. In California, for example, 4-log removal and/or inactivation of Cryptosporidium and 99.999% (5-log) removal and/or inactivation of viruses are required for disinfected recycled water (Haymore et al. 2001; California Department of Public Health 2009b).

The typical efficiencies of MF and UF membranes for virus removal have been reported in the literature, with LRV ranges of 0 to 4 for MF and 5.4 to 7.9 for UF (Haymore et al. 2001). A survey conducted by the U.S. EPA in 2001 (See Figure 2) indicated that most of the 29 U.S. states with policies regarding the use of membrane filtration have given virus removal credits for both MF and UF processes. These virus removal credits can be counted toward the required regulatory LRV for virus removal from surface and recycled waters. In awarding virus removal credits, states rely on membrane integrity monitoring (MIMo) techniques that have been well established for LPM processes (California Department of Public Health 2009c). In contrast, virus removal credits are rarely given to NF and RO processes because of the difficulty of online MIMo of HPM processes.

Figure 2

Summary of state virus removal credit for MF and UF. Note: No Standard – credit is typically awarded on a case-by-case basis. Source: Haymore et al. 2001 .

Figure 2

Summary of state virus removal credit for MF and UF. Note: No Standard – credit is typically awarded on a case-by-case basis. Source: Haymore et al. 2001 .

The absence of reliable membrane integrity testing (MIT) procedures for HPM limits the evaluation and thus regulatory acceptance of the typically high removal efficiencies that are provided by membrane technologies. Overcoming the above challenges is paramount to greater acceptance and use of membrane technologies to purify impaired water sources while optimizing virus inactivation with a multiple-barrier design.

MEMBRANE INTEGRITY MONITORING METHODS

A summary of various MIT methods that are applicable to HPM systems (i.e., NF and RO) is summarized in Table 1.

Table 1

Comparison of HPM integrity testing methods for virus removal

MethodAdvantagesDisadvantages
I. Direct methods 
a) Marker-based methods 
Fluorescent-tagged bacteriophages 
  • Size, structure, morphology, chemical composition and surface properties are similar to those of enteric viruses

  • LRV obtained by both intact and compromised membranes in agreement with enteric viruses

 
  • Possibility of membrane fouling

  • Potential to undergo biological inactivation and growth during study

  • Transport behavior influenced by visible lights, temperature, pH and presence of contaminants in water

  • Lack of reliable methods to recover phages from water

  • High minimum detection limit

 
Fluorescent-tagged NPs 
  • Size similar to that of enteric viruses

  • Surface properties can be modified to mimic those of enteric viruses

  • High fluorescence intensity

  • Photostable

  • LRV obtained by both intact and compromised membranes in agreement with enteric viruses

 
  • Potential to form aggregates in aqueous suspension

  • Transport behavior influenced by pH, temperature, ionic strength, presence of contaminants in water

  • Potential to cause membrane fouling

  • Some NPs' potentially toxicity

  • Insufficient commercial availability

  • Expense

 
Fluorescent-tagged macromolecules 
  • Size similar to that of enteric viruses

  • Nontoxic

  • Stable size and transport behavior in water

  • Moderately inexpensive

 
  • Efficiency as tracers in water applications unevaluated

  • Potential of membrane fouling unexamined.

 
Fluorescent molecular dyes 
  • Information on their stability and transport behaviors in water is established

  • Minimum detection limit is low

  • LRV obtained by both intact and compromised membranes in correlation with enteric viruses

  • Low potential membrane fouling

  • Inexpensive

 
  • Size not comparable to enteric viruses

  • Potential sorption by membranes

 
b) Pressure-based methods 
Vacuum-hold test 
  • Sensitivity to membrane breaches

  • Ability to correlate results to marker-based challenge test results

  • Established ASTM standard method

  • Simple procedure

 
  • Lack of broad range of LRV correlation against marker-based challenge tests

  • Need for system shutdown and draining of membrane elements

 
Pressure decay test 
  • Sensitivity to membrane breaches

  • Simple procedure

 
  • Ability to damage membrane with permeate-side pressure application

  • Need for system shutdown and draining of membrane elements

 
II. Indirect methods 
Conductivity monitoring 
  • Low cost

  • Simple procedure

  • Effectiveness at detecting gross membrane failure

 
  • Insensitivity to minor membrane breaches

  • Sensitivity's dependence on feed salinity

 
Conductivity probing 
  • Low cost

  • Simple procedure

  • Ability to isolate location(s) of membrane integrity failure

  • Greater sensitivity than conductivity monitoring

 
  • Insensitivity to minor membrane breaches

  • Linkage of cost to size of the plant

  • Sensitivity's dependence on feed salinity

 
Sulfate monitoring 
  • Greater sensitivity than conductivity monitoring

 
  • Insensitivity to minor membrane breaches

  • Sensitivity's dependence on feed water salinity

 
TOC monitoring 
  • Improved detection limit of TOC monitoring devices improving resolution and sensitivity

  • Greater sensitivity than sulfate and conductivity monitoring

 
  • Sensitivity's dependence on feed water quality

  • Current lack of correlation between TOC removal and virus removal

 
Particle-counting methods 
  • Greater sensitivity than conductivity monitoring

  • Simple procedure

  • Ability to achieve high LRV in cases where feed particle loading is high

 
  • Instrument resolution's insufficiency for detection of particles smaller than 100 nm

  • Sensitivity's dependence on feed water quality and its decrease when feed is prefiltered via MF or UF

  • Greater effectiveness for LPM testing

  • Insensitivity to minor membrane breaches in RO/NF

 
MethodAdvantagesDisadvantages
I. Direct methods 
a) Marker-based methods 
Fluorescent-tagged bacteriophages 
  • Size, structure, morphology, chemical composition and surface properties are similar to those of enteric viruses

  • LRV obtained by both intact and compromised membranes in agreement with enteric viruses

 
  • Possibility of membrane fouling

  • Potential to undergo biological inactivation and growth during study

  • Transport behavior influenced by visible lights, temperature, pH and presence of contaminants in water

  • Lack of reliable methods to recover phages from water

  • High minimum detection limit

 
Fluorescent-tagged NPs 
  • Size similar to that of enteric viruses

  • Surface properties can be modified to mimic those of enteric viruses

  • High fluorescence intensity

  • Photostable

  • LRV obtained by both intact and compromised membranes in agreement with enteric viruses

 
  • Potential to form aggregates in aqueous suspension

  • Transport behavior influenced by pH, temperature, ionic strength, presence of contaminants in water

  • Potential to cause membrane fouling

  • Some NPs' potentially toxicity

  • Insufficient commercial availability

  • Expense

 
Fluorescent-tagged macromolecules 
  • Size similar to that of enteric viruses

  • Nontoxic

  • Stable size and transport behavior in water

  • Moderately inexpensive

 
  • Efficiency as tracers in water applications unevaluated

  • Potential of membrane fouling unexamined.

 
Fluorescent molecular dyes 
  • Information on their stability and transport behaviors in water is established

  • Minimum detection limit is low

  • LRV obtained by both intact and compromised membranes in correlation with enteric viruses

  • Low potential membrane fouling

  • Inexpensive

 
  • Size not comparable to enteric viruses

  • Potential sorption by membranes

 
b) Pressure-based methods 
Vacuum-hold test 
  • Sensitivity to membrane breaches

  • Ability to correlate results to marker-based challenge test results

  • Established ASTM standard method

  • Simple procedure

 
  • Lack of broad range of LRV correlation against marker-based challenge tests

  • Need for system shutdown and draining of membrane elements

 
Pressure decay test 
  • Sensitivity to membrane breaches

  • Simple procedure

 
  • Ability to damage membrane with permeate-side pressure application

  • Need for system shutdown and draining of membrane elements

 
II. Indirect methods 
Conductivity monitoring 
  • Low cost

  • Simple procedure

  • Effectiveness at detecting gross membrane failure

 
  • Insensitivity to minor membrane breaches

  • Sensitivity's dependence on feed salinity

 
Conductivity probing 
  • Low cost

  • Simple procedure

  • Ability to isolate location(s) of membrane integrity failure

  • Greater sensitivity than conductivity monitoring

 
  • Insensitivity to minor membrane breaches

  • Linkage of cost to size of the plant

  • Sensitivity's dependence on feed salinity

 
Sulfate monitoring 
  • Greater sensitivity than conductivity monitoring

 
  • Insensitivity to minor membrane breaches

  • Sensitivity's dependence on feed water salinity

 
TOC monitoring 
  • Improved detection limit of TOC monitoring devices improving resolution and sensitivity

  • Greater sensitivity than sulfate and conductivity monitoring

 
  • Sensitivity's dependence on feed water quality

  • Current lack of correlation between TOC removal and virus removal

 
Particle-counting methods 
  • Greater sensitivity than conductivity monitoring

  • Simple procedure

  • Ability to achieve high LRV in cases where feed particle loading is high

 
  • Instrument resolution's insufficiency for detection of particles smaller than 100 nm

  • Sensitivity's dependence on feed water quality and its decrease when feed is prefiltered via MF or UF

  • Greater effectiveness for LPM testing

  • Insensitivity to minor membrane breaches in RO/NF

 

JUSTIFICATION OF PM-MIMo APPROACH

Practical online membrane integrity methods for HPM systems should be as listed in Table 2.

Table 2

Desired characteristics of online membrane integrity monitoring system for virus removal in HPMs

CharacteristicsDescription
Sensitive 
  • Detect low concentrations of target species in permeate from a single or multiple element breaches within a full-scale membrane train

  • Quantify the range of target species LRV

 
Real-time 
  • System shutdown is not required

  • Normal filtration continues during integrity monitoring

 
Comprehensive 
  • Characterize the extent of breach

  • Provide useful information to identify membrane breach location for corrective actions

 
Cost-effective 
  • Do not require significant additional equipment or overhauls for existing plants

  • Expensive or rare/specialty chemicals are not required

 
CharacteristicsDescription
Sensitive 
  • Detect low concentrations of target species in permeate from a single or multiple element breaches within a full-scale membrane train

  • Quantify the range of target species LRV

 
Real-time 
  • System shutdown is not required

  • Normal filtration continues during integrity monitoring

 
Comprehensive 
  • Characterize the extent of breach

  • Provide useful information to identify membrane breach location for corrective actions

 
Cost-effective 
  • Do not require significant additional equipment or overhauls for existing plants

  • Expensive or rare/specialty chemicals are not required

 

Given these desired attributes, direct MIMo techniques based on fluorescent marker monitoring appear to be most promising. Such markers have been shown to be detectable at sufficient resolution and sensitivity (using commercial spectrofluorometers) and thus should be suitable for monitoring membrane integrity. The existing approaches, however, rely on constant marker concentration in the feed stream, which is an impractical approach in water treatment facilities because of the (a) high cost of markers and (b) temporal variations in water quality. Moreover, comparison of LRV of the marker for a constant feed marker concentration between intact and compromised membranes may be insufficient for assessing the extent and characteristics of membrane breaches.

The proposed MIT technique (Frenkel & Cohen 2014) was focused on advancing current fluorescent-based MIMo approaches via a PM-MIMo approach as a practical, online monitoring strategy. Pulse injection of markers into the RO feed stream has been proposed, studied and tested to allow monitoring of the dynamic change in marker concentration in the permeate stream in real time. The dynamic change in marker concentration in the permeate stream, coupled with analysis of marker membrane transport behavior, can then be utilized to both detect and assess the extent of membrane integrity breaches.

PM-MIMo SYSTEM TESTING

Five types of commercially available molecular fluorescent markers were selected after initial screening of over 20 markers as shown in Figure 3. Fluorescein (C20H12O5), uranine (C20H12Na2O5), eosin B (C20H6Br4Na2O5), and lissamine green B (C27H25N2NaO7S2) were obtained from Fisher Scientific (Pittsburgh, PA) in a powder form. Rhodamine WT (C29H29N2O5Na2Cl) was obtained from Keystone Aniline Corp. (Chicago, IL) in the form of a 20% (w/w)- aqueous solution. In addition to these molecular markers, a fluorescent-labeled macromolecule (FITC − dextran) was also tested in this study. The marker solutions were prepared by dispersing a predetermined mass or volume in ultrapure deionized water (conductivity of 0.18 μS) obtained by filtering distilled water through a Milli-Q water system (Millipore Corp., San Jose, CA).

Figure 3

Images of 2.5 ppm fluorescent marker solution (from left to right: eosin B, uranine, rhodamine WT, fluorescein, and lissamine green B).

Figure 3

Images of 2.5 ppm fluorescent marker solution (from left to right: eosin B, uranine, rhodamine WT, fluorescein, and lissamine green B).

Spectrofluorometer system

The fluorescence spectrometer (spectrofluorometer) in conjunction with a fluorescence flow cell, pulsed light source, and filters was obtained from Ocean Optics, Inc. (Dunedin, FL). The spectrometer was based on a charge-coupled device (CCD) detector suitable for the range of 175 to 1,100 nm. During the initial phase of the study, the spectrofluorometer system consisted of the pulsed xenon light source, which emitted light in the range of 300 to 750 nm, and the monochromator, which transmitted a selectable narrow band of desired wavelength selected from a wider range of wavelengths from the input light. The fluorescence flow cell was made of chemically resistant polyether ether ketone PEEK material and was designed specifically to accommodate low flow rates (up to 6 mL/min). The detected fluorescence from the samples was emitted to the spectrometer, where the light intensity was quantified in a relative unit called ‘counts’. A diagram of the spectrofluorometer setup is shown in Figure 4. The pulsed light source, coupled with the monochromator, enabled varying the wavelength of the light source entering the fluorescence flow cell. This work enabled determination of the appropriate excitation wavelength to optimize the spectrometer sensitivity for detecting the target tracer.

Figure 4

Diagram of the original spectrofluorometer setup.

Figure 4

Diagram of the original spectrofluorometer setup.

The sensitivity of the initially assembled spectrofluorometer system was sufficient for screening of the range of suitable excitation wavelengths. However, the intensity of the filtered light (at the selected wavelength) was low, resulting in lower-than-expected excitation light intensity. As a consequence, the sensor output integration time had to be increased from 500 ms to 3 s and with increased light intensity, thereby resulting in a spectrometer temperature increase of 2 to 4 °C. The spectrofluorometer system was improved (see Figure 5) by using an LED light source to replace the previous wide-band white light source. The selected LED (based on the optimum excitation wavelength of the selected fluorescent markers) emitted light between 460 and 520 nm, which was suitable for detection of the selected markers. In addition, the monochromator was replaced with optical transmission filters (on both the excitation and emission sides) in order to further sharpen the excitation and emission spectrum. The optical filters on the excitation and emission sides were at wavelengths of 490 ± 20 nm and 530 ± 20 nm, respectively. Finally, the spectrofluorometer system was mounted inside a temperature-controlled enclosure.

Figure 5

Schematic representation of the upgraded UCLA spectrofluorometer system.

Figure 5

Schematic representation of the upgraded UCLA spectrofluorometer system.

PM-MIMo SYSTEM OPERATION

Continuous real-time PM-MIMo monitoring using PFRO (plate-and-frame RO) and SPRO (spiral wound RO) systems was conducted as single-pass RO desalting runs. The RO systems were set to operate at a desired cross-flow velocity and permeate flux, and the fluorescence backgrounds of the permeate stream were determined once the RO system reached a steady-state condition (no significant fluctuation in the permeate flux). Once a stable fluorescence background signal was attained, the marker solution was injected into the feed stream by a metering pump at a predetermined frequency and a prescribed concentration–time profile. Marker injection dose profiles were at concentrations of up to 40 ppm. The marker permeate concentration was monitored as a function of time for the duration of each marker injection event. At least 10 min was allowed between individual marker runs until the fluorescence signal returned to background level.

Two basic types of marker injection experiments were carried out. The first consisted of continuous injection of a tracer solution for a prescribed period of sufficient length to attain steady-state tracing of permeate concentration response. These runs were utilized to determine the mass transfer coefficient (kf), the reflection coefficient, and solute B values for the intact and membranes with induced integrity breaches. The second type of marker injection experiments were injection pulses (duration of 60 s) in which the permeate marker concentration was monitored to determine the fraction of tracer passage through breached membranes. Data from these experiments served to evaluate the marker permeation time distribution (MPTD) as well as the marker rejection and thus the LRV for both the intact and compromised membranes. During the experiments membranes were impacted mechanically and chemically, impacts were measured and detection limits of the proposed technique were studied.

Final selection of candidate markers

Uranine was selected as the marker for developing the PM-MIMo approach. Among the candidate markers, uranine demonstrated relatively stable fluorescence intensity at a typical RO process pH operating range (pH 6–8) along with a high level of chlorine tolerance. The detection limit for uranine if using the initial spectrofluorometer setup was 10 ppb with a detection limit of <1 ppb, attained with the use of an LED light source (at the optimal wavelength).

Uranine detection sensitivity

Following the selection of uranine as a final candidate marker, the spectrofluorometer system was upgraded with an LED light source and the optical filters designed specifically for the detection of uranine. Because the optimum uranine excitation and emission wavelengths are 485 and 520 nm, respectively, the optical filters were set at 490 nm on the excitation side and 530 ± 20 nm on the emission side. Such a combination of an LED light source and excitation–emission filters allowed sufficient separation of fluorescence excitation and emission lights for detecting uranine down to 2.5 ppb (compared to 13.0 ppb detected with the initial spectrometer setup). The higher sensitivity of the improved fluorescence detection system facilitated the use of a lower marker dose to the RO feed. The calibration curve correlating uranine concentration with fluorescence intensity was generated as shown in Figure 6, which compares the uranine calibration curve generated by the original spectrometer setup and the upgraded spectrometer setup with the LED light source. These reported limits of detection were based on the 99% confidence interval limits of the background fluorescence intensities, which were determined by using the intercept values and the linear regression statistics of the calibration lines shown in Figure 7(a) and 7(b).

Figure 6

Emission spectra of DI water and 1- and 5-ppb aqueous uranine solutions at half the maximum LED intensity with 500 ms spectrometer integration time, 490 nm excitation filter, and 510 to 550 nm emission filter.

Figure 6

Emission spectra of DI water and 1- and 5-ppb aqueous uranine solutions at half the maximum LED intensity with 500 ms spectrometer integration time, 490 nm excitation filter, and 510 to 550 nm emission filter.

Figure 7

Concentration calibration curves for uranine in water for: (a) low concentration of uranine (measurements were carried out by using the spectrometer setup with the integration time of 3 s), (b) low concentration of uranine (measurements were carried out by using the upgraded spectrometer setup with the integration time of 500 ms), and (c) high concentration of uranine (measurements were carried out by using the upgraded spectrometer setup with integration time of 100 ms).

Figure 7

Concentration calibration curves for uranine in water for: (a) low concentration of uranine (measurements were carried out by using the spectrometer setup with the integration time of 3 s), (b) low concentration of uranine (measurements were carried out by using the upgraded spectrometer setup with the integration time of 500 ms), and (c) high concentration of uranine (measurements were carried out by using the upgraded spectrometer setup with integration time of 100 ms).

The upgraded detection system, at 50% LED intensity setting and 500 ms sensor output integration time, enabled sufficient separation of fluorescence signals of the low-concentration uranine solutions (1 ppb and 5 ppb) relative to DI water (see Figure 6). Indeed, on the basis of the background fluorescence intensities (i.e., intercepts of the calibration lines in Figure 7(a) and 7(b)), the upgraded spectrometer setup was about four times more sensitive than the initial setup. The results suggest that the upgraded spectrometer setup requires a shorter sensor output integration time (500 ms) and a lower LED intensity than the significantly longer (3 s) integration time with the initial detection system. This improvement enabled more-rapid and more-sensitive measurement of tracer concentrations.

Evaluation of marker reflection coefficient and rejection in the presence of membrane integrity breaches

In order to further quantify the extent of increased marker passage across the membranes and to identify the mechanism of marker passage due to various types of integrity breaches, the marker overall LRV (LRVoverall) and LRVs due to solution–diffusion (LRVdiff) and to convection (LRVconv), as well as the reflection coefficient (σ), were extracted from the data obtained from marker injection runs. Both the LRVdiff and LRVconv were determined through the analysis, and σ was determined via the rearranged Kedem–Katchalsky model.

Analysis of the experimental marker data (see Table 3) demonstrated that, with the presence of a membrane integrity breach, there was an increased level of convective marker transport across the membranes. This finding was demonstrated by the decrease in both the reflection coefficient and the marker LRV. The reflection coefficient decreased with the increasing extent of the membrane integrity breach. For example, σ decreased from 0.988 to 0.985 when the number of pinholes increased from one to two in the first (i.e., lead) membrane module. A decrease in σ typically indicates an increasing level of convective solute transport across the membrane. In addition, it is also apparent that, in the presence of the membrane integrity breach, the increased marker passage was controlled by convection because LRVtotal was nearly identical to LRVconv, whereas LRVdiff was in the range expected for an intact membrane.

Table 3

Impact of membrane breaches on reflection coefficient and marker LRV determined on the basis of a 2-min pulse dosing of uranine to achieve 20 ppm uranine concentration in the SPRO feed

Membrane conditionReflection coefficient (σ)Marker LRVtotalMarker LRVdiffMarker LRVconv
Intact flat-sheet XLE membrane* 0.999989 3.15 3.15 5.19 
SPRO system 0.9997 3.05 3.15 3.74 
SPRO system with 1 pinhole in the 1st membrane module 0.9882 2.10 3.15 2.14 
SPRO system with 2 pinholes in the 1st membrane module 0.9848 2.00 3.15 2.03 
SPRO system with 1 pinhole in the 2nd membrane module 0.9797 1.88 3.15 1.90 
SPRO system with 2 pinholes in the 2nd membrane module 0.9780 1.84 3.15 1.86 
SPRO system after exposure to 80-ppm NaOCl solution for 8 h 0.9867 2.05 3.15 2.08 
Membrane conditionReflection coefficient (σ)Marker LRVtotalMarker LRVdiffMarker LRVconv
Intact flat-sheet XLE membrane* 0.999989 3.15 3.15 5.19 
SPRO system 0.9997 3.05 3.15 3.74 
SPRO system with 1 pinhole in the 1st membrane module 0.9882 2.10 3.15 2.14 
SPRO system with 2 pinholes in the 1st membrane module 0.9848 2.00 3.15 2.03 
SPRO system with 1 pinhole in the 2nd membrane module 0.9797 1.88 3.15 1.90 
SPRO system with 2 pinholes in the 2nd membrane module 0.9780 1.84 3.15 1.86 
SPRO system after exposure to 80-ppm NaOCl solution for 8 h 0.9867 2.05 3.15 2.08 

*The SPRO system was operated at 160 psi and feed flow rate of 6.8 L/min (average cross-flow velocity of 12.12 cm/s); The marker LRVdiff and LRVconv were estimated under the SPRO operating conditions using B and values determined from the PFRO experiment. Experimental conditions: Cf = 20 ppm, feed cross-flow velocity = 12.12 cm/s.

It is important that, even though the marker LRV for the tested intact SPRO membranes was below 4, it was possible to establish an LRV of greater than 4 for the intact SPRO system by using the PM-MIMo approach with the current spectroflurometer setup. This task can be accomplished by utilizing the membrane modules that have higher solute rejection than do the membrane modules that were utilized in the present study. Measurement of a higher LRV for a high-rejection membrane was possible, given the low detection limit of the present high-sensitivity online spectroflurometer setup (i.e., detection limit of 2.5 ppb). For example, Figure 8 shows that, given the present intact XLE membrane properties (i.e., B), the current SPRO flow conditions, and 20 ppm uranine dosing in the feed stream, a 4 to 6 LRVconv of marker (i.e., which would represent the lower LRV level for viruses) could be attained when the marker concentration in the permeate stream was approximately between 14 and 16 ppb. The above concentration level could be easily detected, given the current spectrometer setup detection limit of 2.5 ppb.

Figure 8

Total marker concentration in the permeate stream in response to marker LRV due to convection of the SPRO membrane system. Note: The example is for SPRO system operation at 160-psi feed pressure and cross-flow velocity of 12.12 cm/s with uranine RO feed concentration of 20 ppm in the SPRO feed for a pulse period of 2 min. Total permeate concentration for a given LRV due to convective transport was calculated by using Equation and for the flow conditions in the SPRO system, given Table 10 and the B value from the PFRO experiment.

Figure 8

Total marker concentration in the permeate stream in response to marker LRV due to convection of the SPRO membrane system. Note: The example is for SPRO system operation at 160-psi feed pressure and cross-flow velocity of 12.12 cm/s with uranine RO feed concentration of 20 ppm in the SPRO feed for a pulse period of 2 min. Total permeate concentration for a given LRV due to convective transport was calculated by using Equation and for the flow conditions in the SPRO system, given Table 10 and the B value from the PFRO experiment.

SUMMARY

In summary, MIMo, the newly proposed MIT process, demonstrated that the PM-MIMo approach can be utilized to detect and provide information on the characteristics of various types of membrane integrity breaches in the SPRO membrane system via real-time monitoring. The PM-MIMo approach is sensitive to minor breaches and should be able to demonstrate greater than 4 LRVs of the marker through the intact membranes of the SPRO system. The PM-MIMo approach clearly has potential for use as a real-time integrity monitoring technique for full-scale applications. In this regard, long-term pilot studies would be beneficial to demonstrate the accuracy, versatility, and robustness of this novel method of MIMo.

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