Ultralow range fluorometer calibration

A fluorometer may be used to measure ultralow concentrations of fluorescing species, such as ultralow concentrations of fluorescent tracer passing through a reverse osmosis membrane into a permeate stream. In some examples, the fluorometer may be recalibrated by resetting some but not all of the calibration parameters used to determine the concentration of fluorescent tracer in the permeate based on the measured fluorescent response of the fluorometer. For example, an intercept of a calibration curve may be reset or recalibrated for the fluorometer in situ, potentially providing significant accuracy improvements even though the fluorometer has not undergone a full recalibration.

TECHNICAL FIELD

This disclosure relates to the calibration of fluorometers and, more particularly, to the calibration of fluorometers used to monitor low concentrations of fluorophores in membrane separation processes.

BACKGROUND

Membrane separation is a technology that selectively separates materials via pores and/or minute gaps in the molecular arrangement of a continuous membrane structure. Membrane separations can be classified by pore size and by the separation driving force. Example membrane separation techniques include microfiltration (MF), ultrafiltration (UF), ion-exchange (IE), and reverse osmosis (RO). For example, reverse osmosis is widely used in water purification processes to remove ions, bacteria, and other molecules and larger particles from the water. In a reverse osmosis process, an applied pressure is used to overcome an osmotic pressure across the membrane, allowing substantially pure solvent (e.g., water) to pass through the membrane while a residual solute is retained on the pressurized side of the membrane.

In practice, the degree of purification achieved using the membrane separation process is dictated, at least in part, by the quality and integrity of the membrane used in the process. If there are chemical and/or mechanical failures to the membrane structure, impurities can pass through the membrane breach and into the resulting “purified” product stream. In the case of water purification, harmful impurities and pathogens (e.g., waterborne enteric viruses,Cryptosporidium, Giardiacysts, nanoparticles, organic compounds, and so forth), which can be in the nanosize range, can pass through the membrane breach into the clean water stream, potentially creating health risks.

For these and other reasons, techniques have been used to monitor the performance of membrane separation processes. As one example, a fluorometric monitoring process may be used to monitor the performance of a membrane separation process by introducing a fluorescent tracer into a feed stream and then detecting the fluorescent tracer in one or more streams downstream of the separation membrane. The extent to which the fluorescent tracer passes through the membrane can provide an indication of the integrity of the membrane.

Practical challenges arise when attempting to fluorometrically monitor the performance of a membrane separation process. Since a properly-functioning membrane may separate most of the fluorescent tracer from the purified product stream (also referred to as the permeate), the fluorometer may need to detect very small concentrations of tracer. Small changes in the calibration or measurement accuracy of the fluorometer may cause large errors in the measured tracer concentration at these very small concentrations. Moreover, since many membrane separation processes operate continuously, for example supplying critical water needs, there may be little or no opportunity to recalibrate the fluorometer.

SUMMARY

In general, this disclosure is directed to systems and techniques for calibrating fluorometers, including systems and techniques for monitoring and/or controlling membrane separation processes utilizing such fluorometers. In some examples, the techniques include partial recalibration of a fluorometer in-situ based on control of a fluorescent tracer introduced into a feed stream being separated using a membrane separation process. Prior to or concurrent with startup of a membrane separation process, a fluorometer used to monitor the process may undergo a full, multi-point calibration. The full calibration process may involve fluorometrically analyzing a reference solution that is a blank or zero fluorescence solution in addition to one or more other reference solutions having known concentrations of fluorescent tracer. A single or higher order calibration curve having a slope and an intercept relating the measured fluorometric response of the fluorometer to a fluorescent tracer concentration can then be stored in a computer readable memory associated with the fluorometer.

In subsequent operation, the fluorescent tracer can be introduced into a feed stream before a membrane used in a membrane separation process. The fluorometer can monitor the feed stream and/or one more streams downstream of the membrane, such as a permeate stream. With reference to the calibration information generated during the full calibration, the concentration of fluorescent tracer in the monitored stream can be monitored based on the fluorescent response of the stream by the fluorometer. Over time, however, the fluorometer may lose calibration. The fluorometer may lose calibration for a variety of reasons, such as fouling buildup, changing electrical resistance of a circuit, changing light source strength, and/or other factors. Accordingly, the accuracy of measurements made by the fluorometer may deteriorate over time.

In accordance with some examples of the present disclosure, a zero point recalibration is performed on the fluorometer without performing a full, multipoint recalibration. When performing the zero point recalibration, the intercept of the calibration curve stored for the fluorometer may be reset, e.g., without changing other parameters defining the calibration curve. It has been found that, in some applications, recalibrating the intercept of the recalibration curve of the fluorometer, particularly when detecting very low concentrations of fluorescent tracer, can provide highly beneficial accuracy improvements even though a full recalibration is not performed. As described in greater detail below, such a partial recalibration may be performed without removing the fluorometer from its operating environment, allowing the accuracy of the fluorometer to be enhanced even though a full recalibration may not be feasible.

Without wishing to be bound by any particular theory, it is believed that, in some applications, zero point drift is a more significant contributor to measurement inaccuracies than other calibration inaccuracies. For example, for fluorometers used to measure ultralow concentrations of fluorescent tracer, such as in membrane separation permeate streams, small shifts in zero point calibration have been found to cause large inaccuracies in measured tracer concentration. For instance, a shift in the zero point of the calibration curve may cause a measurement inaccuracy equivalent to 0.01 μg/L or greater. If measuring higher concentrations of fluorescent tracer, such as 1-1,000 μg/L, the measurement inaccuracy is 1% or less. However, when measuring ultralow concentrations of fluorescent tracer, such as 0.1 μg/L or less, that same measurement inaccuracy causes a measurement error of 10% or greater.

In some examples, a fluorometer zero point recalibration process can be performed by controlling a fluorescent tracer introduced into a feed stream that is undergoing membrane separation. Typically, the fluorescent tracer will be introduced into the feed stream at a substantially constant rate and/or in an amount effective to achieve a substantially constant concentration. During zero point recalibration, the concentration of the fluorescent tracer may be adjusted to provide a fluorescent signal change that can be used to recalibrate the intercept of the calibration curve for the fluorometer. In different examples, introduction of the fluorescent tracer into the feed stream may be stopped, or increased or decreased relative to a baseline introduction rate. The fluorescent signal measured by the fluorometer can be used to adjust the intercept of the calibration curve, allowing one or more adjusted calibration curve parameters to be stored in a memory associated with the fluorometer for subject use.

In one example, a method of calibrating a fluorometer used to monitor a reverse osmosis membrane separation process is described. The method includes introducing a fluorescent tracer into a feed stream to provide a first concentration of fluorescent tracer in the feed stream and contacting a reverse osmosis membrane with the feed stream, thereby generating a permeate stream and a concentrate stream. The method also includes fluorometrically analyzing the permeate stream generated from the feed stream having the first concentration of fluorescent tracer with a fluorometer and determining therefrom a first measured concentration of the fluorescent tracer in the permeate stream based on a stored calibration curve that includes an intercept. The method further includes adjusting a concentration of the fluorescent tracer introduced into the feed stream to provide a second concentration of fluorescent tracer in the feed stream different than the first concentration. Further, the example method includes fluorometrically analyzing the permeate stream generated from the feed stream having the second concentration of fluorescent tracer with the fluorometer and determining therefrom a second measured concentration of the fluorescent tracer in the permeate stream based on the calibration curve. In addition, the method includes determining an intercept shift for the calibration curve based on comparison of the first measured concentration to the second measured concentration and determining an adjusted intercept for the calibration curve based on the intercept shift.

In another example, a system is described that includes a fluorescent tracer pump configured to introduce fluorescent tracer into a feed stream, a membrane configured to separate the feed stream into a permeate stream and a concentrate stream, a fluorometer configured to fluorometrically analyze the permeate stream, and a controller communicatively coupled to the fluorescent tracer pump and the fluorometer. The example specifies that the controller is configured to control the fluorescent tracer pump to introduce the fluorescent tracer into a feed stream to provide a first concentration of fluorescent tracer in the feed stream. In addition, the controller is configured to control the fluorometer to fluorometrically analyze the permeate stream generated from the feed stream having the first concentration of fluorescent tracer and determine therefrom a first measured concentration of the fluorescent tracer in the permeate stream based on a stored calibration curve that includes an intercept. In addition, the example specifies that the controller is configured to control the fluorescent tracer pump to adjust a concentration of the fluorescent tracer introduced into the feed stream to provide a second concentration of fluorescent tracer in the feed stream different than the first concentration. In addition, the controller is configured to control the fluorometer to fluorometrically analyze the permeate stream generated from the feed stream having the second concentration of fluorescent tracer and determine therefrom a second measured concentration of the fluorescent tracer in the permeate stream based on the calibration curve. The example further states that the controller is configured to determine an intercept shift for the calibration curve based on comparison of the first measured concentration to the second measured concentration and determine an adjusted intercept for the calibration curve based on the intercept shift.

In another example, a method of calibrating a fluorometer used to monitor a reverse osmosis membrane separation process is described. The method includes introducing a fluorescent tracer into a feed stream and contacting a reverse osmosis membrane with the feed stream, thereby generating a permeate stream and a concentrate stream. The method further involves terminating the introduction of fluorescent tracer introduced into the feed stream and fluorometrically analyzing the permeate stream generated from the feed stream following termination of the fluorescent tracer and determining therefrom a measured concentration of the fluorescent tracer in the permeate stream based on a stored calibration curve that includes an intercept. The method further includes determining an adjusted intercept for the calibration curve of the fluorometer by at least using the measured concentration of the fluorescent tracer as an intercept shift.

In another example, a system is described that includes a fluorescent tracer pump configured to introduce fluorescent tracer into a feed stream, a reverse osmosis membrane configured to separate the feed stream into a permeate stream and a concentrate stream, a fluorometer configured to fluorometrically analyze the permeate stream, and a controller communicatively coupled to the fluorescent tracer pump and the fluorometer. The example specifies that the controller is configured to control the fluorescent tracer pump to introduce a fluorescent tracer into the feed stream and subsequently control the fluorescent tracer pump to terminate the introduction of fluorescent tracer introduced into the feed stream. The controller is also configured to control the fluorometer to fluorometrically analyze the permeate stream generated from the feed stream following termination of the fluorescent tracer and determine therefrom a measured concentration of the fluorescent tracer in the permeate stream based on a stored calibration curve that includes an intercept. In addition, the example states that the controller is configured to determine an adjusted intercept for the calibration curve of the fluorometer by at least using the measured concentration of the fluorescent tracer as an intercept shift.

In another example, a method for calibrating a fluorometer is described. The method includes performing a multi-point calibration with a fluorometer, the multi-point calibration comprising fluorometrically analyzing a first fluid substantially devoid of a fluorescent tracer and a second fluid having a known concentration of the fluorescent tracer and determining therefrom a calibration curve for the fluorometer that includes a slope and an intercept. The method includes subsequent to performing the multi-point calibration, fluorometrically analyzing, with the fluorometer, an aqueous stream in which the fluorescent tracer is introduced at a first concentration level and determining a measured concentration of the fluorescent tracer in the aqueous stream based on the calibration curve. The method further includes adjusting the concentration of fluorescent tracer introduced into the aqueous stream by one of terminating an addition of the fluorescent tracer to the aqueous stream, increasing an amount of the fluorescent tracer added to the aqueous stream, or decreasing the amount of the fluorescent tracer added to the aqueous stream. In addition, the method involves fluorometrically analyzing, with the fluorometer, the aqueous stream following adjustment of the concentration of fluorescent tracer and determining an intercept shift for the calibration curve based on fluorometric analysis of the aqueous stream following adjustment of the concentration of fluorescent tracer without determining an adjusted slope for the calibration curve. The method also includes storing an adjusted intercept for the calibration curve compensating for the intercept shift in a non-transitory memory associated with the fluorometer.

DETAILED DESCRIPTION

In general, this disclosure describes systems and techniques for calibrating fluorometric sensors as well as membrane separation systems and processes utilizing such fluorometric sensors. In some examples, a multi-point calibration is performed with the fluorometer prior to use in the membrane separation process. The multipoint calibration process may involve fluorometrically analyzing a first fluid substantially or entirely devoid of a fluorescent tracer, a second fluid having a known concentration of the fluorescent tracer, and optionally one or more additional fluids having known concentrations of the fluorescent tracer different than the second fluid. A calibration curve for the fluorometer can be determined from the fluorometric analysis of the different fluids. The calibration curve may be in the form of a single or higher order equation having a slope and an intercept.

In subsequent use, the fluorometer can be used to fluorometrically analyze an aqueous stream in which the fluorescent tracer for which the fluorometer was previously calibrated is introduced. For example, the fluorometer may analyze a permeate stream passing through a membrane filter in a reverse osmosis process in which the fluorescent tracer is introduced into the corresponding feed stream. The fluorescent tracer may be introduced at a first concentration level and a measured concentration of the fluorescent tracer in the aqueous stream being monitored determined based on the calibration curve.

To recalibrate the fluorometer, the concentration of fluorescent tracer introduced into the aqueous stream may be adjusted. For example, introduction of the fluorescent tracer may be decreased up to and including being terminated (or, in other cases, may not be terminated) or increased relative to the first concentration level. The fluorometer can then fluorometrically analyze the aqueous stream following adjustment of the concentration of fluorescent tracer. Based on the fluorometric response measured, an intercept shift for the calibration curve can be determined, e.g., without determining an adjusted slope for the calibration curve. This intercept shift can be used to determine an adjusted intercept for the calibration curve that is then stored for subsequent use.

FIG. 1is a conceptual diagram illustrating an example membrane separation system100that can utilize one or more fluorometers that may be calibrated as described herein. System100includes a separation membrane102, at least one fluorometer104, and a controller106. System100inFIG. 1is also illustrated as including a feed stream pressurization pump108and a fluorescent tracer pump110. Feed stream pressurization pump108is in fluid communication with a source112of fluid to be purified using membrane102. Fluorescent tracer pump110is in fluid communication with a source of fluorescent tracer114to be introduced into a feed stream contacting membrane102. In operation, a feed stream114is supplied to membrane102, which is capable of treating or purifying the feed stream by dividing the feed stream into at least a first stream and a second stream, such as a permeate stream116and a concentrate stream118(which may also be referred to as a reject stream).

Fluorometer104is optically connected to one or more of feed stream114, permeate stream116, and/or concentrate stream118and is configured to fluorometrically analyze the stream. In the illustrated configuration, a single fluorometer104is illustrated as being positioned to receive slip streams from each of the feed stream114, permeate stream116, and concentrate stream118. When so configured, valves or other flow control mechanisms may be used to selectively place the fluorometer in fluid communication with each of the respective streams at different times. In other configurations, fluorometer104may be implemented to only fluorometrically analyze a single stream (e.g., feed stream114or permeate stream116), or two of the three stream (e.g., feed stream114and permeate stream116). In these alternative configurations, system100may include more than one fluorometer, such as a separate fluorometer for each stream to be fluorometrically analyzed during operation.

Controller106is communicatively connected to fluorometer104, feed stream pressurization pump108, fluorescent tracer pump110, and optionally any other controllable components or sensors that may be desirably implemented in system100. Controller106includes processor120and memory122. Controller106communicates with controllable components in system100via connections. For example, signals generated by fluorometer104may be communicated to controller106via a wired or wireless connection, which in the example ofFIG. 1is illustrated as wired connection. Memory122stores software for running controller106and may also store data generated or received by processor120, e.g., from fluorometer104. Processor120runs software stored in memory122to manage the operation of system100.

As described in greater detail below, fluorometer104may be used to fluorometrically analyze the separation performance of membrane102. Fluorometer104can emit excitation light into a fluid stream/sample under analysis and receive fluorescent emissions generated in response to the excitation light. The amount of fluorescent emission light detected by the fluorometer can be processed with reference to calibration information stored in memory to determine a concentration of a fluorescing tracer in the fluid sample under analysis. This, in turn, can provide an indication of the separation performance of membrane102. Fluorometer104can be recalibrated as will be described to help calibration errors that may arise during operation of the fluorometer.

During operation of system100, membrane102can be contacted with fluid to be purified from source112to remove ion, molecules, pathogens, and/or other particulate contaminants. For example, feed stream114can contain various solutes, such as dissolved organics, dissolved inorganics, dissolved solids, suspended solids, the like or combinations thereof. Upon separation of feed stream114into permeate stream116and concentrate stream118, in membrane102, the permeate stream can contain a substantially lower concentration of dissolved and/or suspended solutes as compared to the feed stream. On the other hand, the concentrate stream118can have a higher concentration of dissolved and/or suspended solutes as compared to the feed stream. In this regard, the permeate stream116represents a purified feed stream, such as a purified aqueous feed stream.

System100and membrane102can be configured for any desired type of membrane separation process, including cross flow separation processes, dead-end flow separation processes, reverse osmosis, ultrafiltration, microfiltration, nanofiltration, electrodialysis, electrodeionization, pervaporation, membrane extraction, membrane distillation, membrane stripping, membrane aeration and the like or combinations thereof. Typically, however, system100and membrane102may be implemented as a reverse osmosis, ultrafiltration, microfiltration, or nanofiltration membrane separation process.

In reverse osmosis, feed stream114is typically processed under cross flow conditions. When so configured, feed stream114may flow substantially parallel to the membrane surface such that only a portion of the feed stream diffuses through the membrane as permeate. The cross flow rate is typically high in order to provide a scouring action that lessens membrane surface fouling. This can also decrease concentration polarization effects (e.g., concentration of solutes in the reduced-turbulence boundary layer at the membrane surface, which can increase the osmotic pressure at the membrane and thus can reduce permeate flow). The concentration polarization effects can inhibit the feed stream water from passing through the membrane as permeate, thus decreasing the recovery ratio, e.g., the ratio of permeate to applied feed stream. A recycle loop(s) may be employed to maintain a high flow rate across the membrane surface.

System100can employ a variety of different types of membranes as membrane102. Such commercial membrane element types include, without limitation, hollow fiber membrane elements, tubular membrane elements, spiral-wound membrane elements, plate and frame membrane elements, and the like. For example, reverse osmosis typically uses spiral wound elements or modules, which are constructed by winding layers of semi-porous membranes with feed spacers and permeate water carriers around a central perforated permeate collection tube. Typically, the modules are sealed with tape and/or fiberglass over-wrap. The resulting construction may have one channel that can receive an inlet flow. The inlet stream flows longitudinally along the membrane module and exits the other end as a concentrate stream. Within the module, water can pass through the semi-porous membrane and is trapped in a permeate channel, which flows to a central collection tube. From this tube it can flow out of a designated channel and is collected.

In different applications, membrane102can be implemented using a single membrane element or multiple membrane elements depending on the application. For example, multiple membrane elements may be used forming membrane modules that are stacked together, end to end, with inter-connectors joining the permeate tubes of the first module to the permeate tube of the second module, and so on. These membrane module stacks can be housed in pressure vessels. Within the pressure vessel, feed stream114can pass into the first module in the stack, which removes a portion of the water as permeate water. The concentrate stream from the first membrane can then become the feed stream of the second membrane and so on down the stack. The permeate streams from all of the membranes in the stack can be collected in the joined permeate tubes. In these applications, the feed stream entering the first module and/or the combined permeate stream and/or the final concentrate stream from the last module in the stack may be monitored using one or more fluorometers104.

Within most reverse osmosis systems, pressure vessels may be arranged in either “stages” or “passes.” In a staged membrane system, the combined concentrate streams from a bank of pressure vessels can be directed to a second bank of pressure vessels where they become the feed stream for the second stage. Commonly, systems have two to three stages with successively fewer pressure vessels in each stage. For example, a system may contain four pressure vessels in a first stage, the concentrate streams of which feed two pressure vessels in a second stage, the concentrate streams of which in turn feeds one pressure vessel in the third stage. This is designated as a “4:2:1” array. In a staged membrane configuration, the combined permeate streams from all pressure vessels in all stages may be collected and used without further membrane treatment. Multi-stage systems are commonly used when large volumes of purified water are required, for example for boiler feed water. The permeate streams from the membrane system may be further purified by ion exchange or other means.

In a multi-pass system, the permeate streams from each bank of pressure vessels are collected and used as the feed to the subsequent banks of pressure vessels. The concentrate streams from all pressure vessels can be combined without further membrane treatment of each individual stream. Multi-pass systems are typically used when very high purity water is required, for example in the microelectronics or pharmaceutical industries. When system100is implemented as a reverse osmosis process, one or more membranes102may be configured as a multi-stage and/or multi-pass system.

While system100and membrane102may be implemented as cross-flow filtration process, in other configurations, the system may be arranged for conventional filtration of suspended solids by passing feed stream114through a filter media or membrane in a substantially perpendicular direction. This arrangement can create one exit stream (e.g., purified stream114) during the service cycle. Periodically, the filter may be backwashed by passing a clean fluid in a direction opposite to the feed, generating a backwash effluent containing species that have been retained by the filter. In this arrangement, system100may have a feed stream114, a purified stream116, and a backwash stream118. This type of membrane separation is typically referred to as dead-end flow separation and is typically limited to the separation of suspended particles greater than about one micron in size.

To monitor the performance of membrane102, a fluorescent tracer from fluorescent tracer source113can be introduced into feed stream114. Operating under the control of controller106, fluorescent tracer pump110can inject fluorescent tracer into feed stream114upstream of membrane102. In the illustrated example, fluorescent tracer is shown as being introduced upstream of feed stream pump108, although in other configurations, may be introduced downstream of the feed stream pump. In either case, the feed stream114containing an amount of fluorescent tracer can contact membrane102to undergo a separation or purification process.

As with other molecules or particulates being separated out of the feed stream, a majority of the fluorescent tracer may be concentrated in concentrate stream118. Only a small minority of the fluorescent tracer introduced into feed stream114may carry through to permeate stream116, e.g., when membrane102is functioning as intended. The amount of fluorescent tracer passing through membrane102from feed stream114and into permeate stream116may be indicative of the quality and/or operational efficiency of the membrane. For example, if membrane102has an integrity breach affecting the separation efficiency of the membrane, a higher concentration of fluorescent tracer introduced into feed stream114via fluorescent tracer pump110may carry through to permeate stream116than if the membrane does not have such a breach.

Operating on a periodic or continuous monitoring basis, one or more fluorometers104can monitor the concentration of fluorescent tracer in one or more corresponding streams of system100to evaluate the performance of the system. For example, fluorometer104may measure feed stream114to determine a measured concentration of the fluorescent tracer introduced into the stream by fluorescent tracer pump110. Fluorometer104may also measure permeate stream116to determine a measured concentration of the fluorescent tracer passing through membrane102and present in permeate stream116.

Various performance metrics can be determined based on the measured fluorescent properties of the monitored streams. As one example, controller106may calculate a dye rejection efficiency factor Rtbased on the following equation:

In the equation above, Rtis the dye rejection efficiency, CFis the fluorescent dye concentration of the feed stream, Cpis the fluorescent dye concentration of the permeate stream, CF,BKGis the background fluorescence of the feed stream, and CP,BKGis the background fluorescence of the permeate stream. Additional performance parameters that may be calculated by controller106with reference to information stored in memory122and data from fluorometer104are described in U.S. Pat. No. 6,838,001, the entire contents of which are incorporated herein by reference.

In normal operation, the dye rejection efficiency of membrane102may be greater than 95 percent, such as greater than 98 percent, greater than 99 percent, or greater than 99.9 percent. For example, controller106may control fluorescent tracer pump110to introduce an amount of fluorescent tracer into feed stream114effective to achieve a concentration ranging from 10 parts per billion (ppb) to 100,000 ppb, such as from 10 ppb to 1000 ppb, or from 10 ppb to 10,000 ppb. By comparison, the amount of fluorescent tracer passing through membrane102and present in permeate stream116at these feed stream concentrations may be less than 10 ppb, such as less than 5 ppb, or less than 1 ppb, or less than 100 parts per trillion (ppt). Controller106can control fluorescent tracer pump110to introduce the tracer at a substantially constant rate and/or to achieve a substantially constant concentration in feed stream114(e.g., adjusting the rate of introduction based on flow rate changes to feed stream114). Alternatively, the rate and/or concentration of the fluorescent tracer may vary over time.

In general, the fluorescent tracer introduced into feed stream114is an inert tracer. The term “inert” refers to a fluorescent tracer that is not appreciably or significantly affected by any other chemistry in the system, or by the other system parameters such as pH, temperature, ionic strength, redox potential, microbiological activity or biocide concentration. The fluorescent tracer should be transportable with the water of the membrane separation system and thus substantially, if not wholly, water-soluble therein at the concentration it is used, under the temperature and pressure conditions specific and unique to the membrane separation system. In other words, the fluorescent tracer may display properties similar to a solute of the membrane separation process in which it is used.

In some examples, the fluorescent tracer added to feed stream114is a component of a formulation, rather than as a separate component, such as a dry solid or neat liquid. For example, the fluorescent tracer may be contained in (e.g., intermixed with) treatment chemicals injected into feed stream114to enhance the membrane separation process, e.g., antiscalants that retard/prevent membrane scale deposition, antifoulants that retard/prevent membrane fouling, biodispersants, microbial-growth inhibiting agents, such as biocides and cleaning chemicals that remove membrane deposits. The composition containing the fluorescent tracer may include an aqueous or other water-soluble solution or other substantially homogeneous mixture that disperses with reasonable rapidity in the membrane separation system to which it is added. In applications where the fluorescent tracer composition (or product containing the fluorescent tracer) is in solid form, fluorescent tracer pump108may be replaced with a solid metering device.

In some examples, the fluorescent tracer is not a visible dye, e.g., such that the fluorescent tracer is a chemical species that does not have a strong absorption of electromagnetic radiation in the visible region, which extends from about 4000 Angstroms to about 7000 Angstroms (from about 400 nanometers (“nm”) to about 700 nm). For example, the fluorescent tracer may be chosen from a class of materials which are excited by absorption of light and produce fluorescent light emission, where the excitation and emission light occurs at any point within the far ultraviolet to near infrared spectral regions (e.g., wavelengths from 200-800 nm).

System100can be used to purify any desired type of fluid. Example aqueous (water-based) liquid feed sources112that may be purified using system100include raw water streams (e.g., extracted from a fresh water source), waste water and recycle water streams (e.g., from municipal and/or industrial sources), streams in food and beverage processes, streams in pharmaceutical processes, streams in electronic manufacturing, streams in utility operations, streams in pulp and paper processes, streams in mining and mineral processes, streams in transportation-related processes, streams in textile processes, streams in plating and metal working processes, streams in laundry and cleaning processes, streams in leather and tanning processes, streams in paint processes, and combinations thereof.

The one or more fluorometers104used in system100may be implemented in a number of different ways in system100. In the example shown inFIG. 1, a pipe, tube, or other conduit is connected between a main fluid pathway and a flow chamber of fluorometer104, e.g., providing a slip stream or sample stream from the bulk of flowing liquid. In such examples, the conduit can fluidly connect the flow chamber (e.g., an inlet of the flow chamber) of fluorometer104to the main fluid pathway. As fluid moves through the main fluid pathway, a portion of the fluid may enter the conduit and pass adjacent a sensor head positioned within a fluid chamber, thereby allowing fluorometer104to determine one or more characteristics of fluid flowing through the fluid pathway. After passing through the flow chamber, analyzed fluid may or may not be returned to the main fluid pathway, e.g., via another conduit connecting an outlet of the flow chamber to the fluid pathway. In alternative configurations, fluorometer104positioned in-line with a main fluid pathway, e.g., allowing the fluorometer to directly sample and/or fluorometrically analyze the primary flowing fluid stream without drawing a slip stream.

In either case, when implemented to receive fluid directly from a main fluid pathway or stream without user intervention, fluorometer104may be characterized as an online optical sensor. Controller106may control fluorometer104to continuously fluorometrically analyze a fluid stream over a period of time or intermittently fluorometrically analyze the fluid stream at periodic intervals. When fluorometer104is implemented as an online fluorometer, it may be difficult to remove the fluorometer from service for calibration if such removal may require shutting down system100or causing undesirable monitoring gaps in the performance of the system.

In other applications, fluorometer104may be used to fluorometrically analyze a stationary volume of fluid that does not flow through a flow chamber of the optical sensor. For example, in these alternative configurations, fluorometer104may be implemented as an offline monitoring tool (e.g., as a handheld sensor), that requires filling the optical sensor with a fluid sample manually extracted from system100.

FIG. 2is a block diagram illustrating an example of fluorometer104that may be used in the fluid separation system ofFIG. 1. Fluorometer104includes controller106, one or more optical emitters222(referred to herein as “optical emitter222”), and one or more optical detectors224(referred to herein as “optical detector224”). Controller106includes previously-described processor120and a memory122. Optical emitter222directs light into fluid pathway230and optical detector224receives transmitted light on the opposite side of the fluid pathway. The components of fluorometer104may be implemented on a single printed circuit board (PCB) or may be implemented using two or more PCB boards. Further, in some examples, fluorometer104communicates with an external device, such as a system controller controlling system100, remote server, cloud-computing environment, or other physically remote computing device.

For purposes of discussion, controller106described with respect toFIG. 1as controlling system100is also illustrated as the controller controlling fluorometer104. In practice, fluorometer104may have a separate controller from one or more system controller controlling the overall operation of system100. Accordingly, it should be appreciated that the computing functionality attributed to controller106in system100and fluorometer104may be performed on any one or more controllers associated with the system, be it physically onsite or remotely located, and the functionalities described herein are not limited to being performed on any specific hardware device.

Memory122stores software and data used or generated by controller106. For example, memory122may store data representative of one or more calibration curves232used by controller106to determine a concentration of a fluorescent tracer in fluid medium passing through fluid pathway230. Calibration curve data232may relate fluorescent emission light detected by optical detector224to a concentration of a fluorescent tracer in the fluid under analysis. In some examples, calibration curve data232is in the form of an equation that relates light measurements taken by optical detector224to fluorescent tracer concentration information. For example, the equation may be a single or higher-order equation having one or more slope coefficients and an intercept, each of which are stored in memory and referenced by controller106to convert light information measured by optical detector224to fluorescent tracer concentration information.

For ease of description, calibration curve data232is generally described below as being calibration information that is determined by fluorometer104and stored in memory122of the fluorometer. In other examples, calibration curve data232may be determined separately from fluorometer104(e.g., using a laboratory spectrophotometer and computing device) and stored in memory122and/or a separate computing device communicatively coupled to fluorometer104. Therefore, although fluorometer104is described below as being configured to determine calibration curve data232and further being configured to determine a measured concentration of fluorescent tracer, it should be appreciated that the disclosure is not limited to such an example sensor. In different examples, hardware and/or software operating outside of fluorometer104may be utilized to implement functions attributed to fluorometer104in this disclosure.

In examples in which fluorometer104determines calibration curve data232, the calibration curve data may be based on an analysis of baseline detection values produced by optical detector224and processed by controller106. The baseline detection values may be detected by optical detector224when one or more fluid solutions having a known concentration of fluorescent tracer are passed through fluid pathway230. These fluid solutions having a known concentration of fluorescent tracer may be referred to as reference solutions. For example, controller106may determine calibration curve data by fluorometrically analyzing a first fluid substantially or entirely devoid of a fluorescent tracer, a second fluid having a known concentration of the fluorescent tracer, and optionally one or more additional fluids having known concentrations of the fluorescent tracer different than the second fluid (e.g., covering the range of fluorescent tracer concentrations expected to be measured by fluorometer104in subsequent operation).

Upon receiving detection values from the reference fluids, processor120of controller106(or a processor of another computing device) can analyze the detection values to establish a relationship between the known characteristic and the detection values. For example, processor120may perform a curve fitting process such as linear regression to determine a relationship between the known concentration of fluorescent tracer and the detected fluorescent emissions. The determined relationship (or coefficients associated therewith) can then be stored as calibration curve data232.

In the example of a single order calibration curve, for example, controller106may fit a curve representing known fluorescent tracer concentration values plotted on a y-axis of a graph with corresponding measured fluorescent emissions plotted on the x-axis of the graph. A first order curve having the form y=m*x+b, where y is the fluorescent tracer concentration, x is the measured fluorescent emissions, and m is the slope of the curve, and b is the intercept of the curve, may be fit to the data to determine slope and intercept calibration values. The slope and intercept values can be stored as calibration curve data232in memory122. In the case of a higher order curve (e.g., second order, third order, or higher), additional coefficients corresponding to the higher order curve may be stored in memory (e.g., in addition to the intercept). Controller106may employ any suitable statistical software package such as, e.g., Minitab, Excel, or the like, to generate calibration curve data232.

Processor120runs software stored in memory122to perform functions attributed to fluorometer104and controller106in this disclosure. Components described as processors within controller106, controller106, or any other device described in this disclosure may each include one or more processors, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), programmable logic circuitry, or the like, either alone or in any suitable combination.

Optical emitter222includes at least one optical emitter that emits radiation having a specified wavelength or wavelength range. In different examples, optical emitter222can emit radiation continuously or intermittently. In some examples, optical emitter222emits radiation at a plurality of discrete wavelengths. For example, optical emitter222may emit at two, three, four or more discrete wavelengths.

Optical emitter222can emit light at any suitable wavelength, as described in greater detail below. In some examples, optical emitter222emits light within a spectrum ranging from 10 nm to 700 nm. Light emitted by optical emitter222propagates through fluid pathway230of fluorometer104and may be detected by optical detector224. In response to receiving the optical energy, fluorescing molecules within the fluid may excite, causing the molecules to produce fluorescent emissions. The fluorescent emissions, which may or may not be at a different frequency than the energy emitted by optical emitter222, may be generated as excited electrons within fluorescing molecules change energy states. The energy emitted by the fluorescing molecules may be detected by optical detector224. For example, optical detector224may detect fluorescent emissions emitted in a frequency range from 50 nm to 800 nm.

Optical detector224includes at least one optical detector that detects radiation within associated wavelength ranges within the UV light spectrum. Optical detector224detects radiation that is emitted by optical emitter222and that has propagated through fluid pathway230and any fluid solution in the fluid pathway. Optical detector224may be implemented using multiple detectors, one for each wavelength or wavelength range, or may be implemented using a single detector such as, e.g., a detector that is programmable to detect multiple wavelength ranges.

Controller106controls the operation of optical emitter222and receives signals concerning the amount of light detected by optical detector224. In some examples, controller106further processes signals, e.g., to develop calibration curve data232and/or to determine a concentration of fluorescent tracer in a fluid solution passing through fluid pathway230using calibration curve data232. While controller106may perform various signal processing functionalities, as described herein, controller106need not be configured to perform any or all of the described functionalities. In different examples, signal processing, e.g., to develop calibration curve data232and/or to determine a characteristic of a fluid solution based on calibration curve data232may be performed outside of fluorometer104. For example, controller106(FIG. 1) or another controller may perform one or more functions generally attributed to controller106in this disclosure. Accordingly, it should be appreciated that functions attributed to controller106herein are for ease of description, and the described functions may, in fact, be implemented within fluorometer104or within one or more separate devices, which may or may not be communicatively coupled to fluorometer104.

In one example in which controller106processes signals, controller106controls optical emitter222and optical detector224to create calibration curve data232. When fluorometer104is informed of one or more known characteristics of a reference solution flowing through fluid pathway230, controller106controls optical emitter222to emit radiation at one or more wavelengths and further controls optical detector224to detect the radiation at one or more wavelengths. Fluorometer104may be so informed, for example, via manual input from a user. Controller106processes signals concerning the amount of light detected by optical detector224to determine fluorescent emission characteristics of the reference solution. For example, controller106may process signals concerning the magnitude and/or wavelength(s) of light detected by optical detector224for different reference solutions having different known concentrations of fluorescent tracer. Controller106may store the data in memory122.

After determining the amount of light detected by optical detector224through the reference solutions, controller106can process the light detection information to develop a relationship between the known concentration of fluorescent tracer in the reference solutions and the light detection information. Controller106can store the relationship as calibration curve data232in memory122. In subsequent operation, controller106can determine a measured concentration of the fluorescent tracer based on the magnitude of fluorescent emissions detected by224from a fluid having an unknown concentration of the fluorescent tracer (e.g., feed stream114and/or permeate stream116) using calibration curve data232, which relates measured fluorescent emissions to fluorescent tracer concentrations.

As briefly discussed above, fluorometer104may be calibrated prior to being implemented in system100or initially being installed in the system, e.g., prior to use in ongoing operation and/or during periodic full recalibration. During full calibration, calibration curve data232that includes a calibration curve intercept value may be determined and stored in memory122. As fluorometer is used during subsequent operation of system100to determine measured concentrations of fluorescent tracer (e.g., in feed stream114and/or permeate stream116), the accuracy of the measured concentrations may decrease. That is, the difference between the actual or real value of the fluorescent tracer in the stream being measured and the measured concentration determined by fluorometer104may increase over time, increasing the inaccuracy of the measured concentration determined by the fluorometer. The fluorometer may lose calibration for a variety of reasons, such as fouling buildup, changing electrical resistance of a circuit, changing light source strength, and/or other factors.

To help increase the accuracy of the measured fluorescent concentrations determined by fluorometer104, the fluorometer may undergo a partial recalibration. The partial calibration may recalibrate the intercept for the calibration curve data232, e.g., without recalibrating the slope. For example, calibration curve data232stored in memory122and used by controller106to convert measured fluorescent emission data to measured fluorescent tracer concentration data may include a stored calibration curve intercept and stored calibration curve slope coefficients. During partial recalibration, the stored calibration curve intercept may be changed without changing the stored calibration curve slope coefficients. While full recalibration may lead to even further accuracy improvements over a partial recalibration, full recalibration may require removing fluorometer104from service or otherwise exposing the fluorometer to multiple reference solutions. Partial recalibration as described in some applications herein may be achieved by controlling the introduction of fluorescent tracer into feed stream114. The partial recalibration may provide meaningful accuracy improvements for fluorometer104and, optionally, may be performed while the fluorometer remains online (e.g., without disconnecting or removing the fluorometer from a housing or fluid conduit to which it is connected or otherwise removing the fluorometer from its operating environment).

FIG. 3is an example technique for recalibrating fluorometer104. The technique ofFIG. 3is described with reference to system100inFIG. 1and fluorometer104inFIG. 2, although can be performed in other systems and with other fluorometer configurations, e.g., as described herein.

With reference toFIG. 3, the technique includes introducing a fluorescent tracer into feed stream114to provide a first concentration of fluorescent tracer in the feed stream. (300) Operating under the control of controller106in system100, fluorescent tracer pump110may be controlled to introduce fluorescent tracer into feed stream114from fluorescent tracer source113. The amount of fluorescent tracer introduced into feed stream114may be effective to provide a concentration of fluorescent tracer in feed stream114within any of the concentration ranges discussed herein, or even concentration ranges outside of those discussed herein in suitable applications.

Feed stream114contacts membrane102, separating the feed stream into permeate stream116and concentrate stream118. (302) Membrane102may be configured as a cross flow membrane, dead-end flow membrane, or have yet other configuration. In one example, system100is a reverse osmosis system. Feed stream pressure pump108can pressurize a source of liquid to be purified, overcoming the osmotic pressure of membrane102to drive the reverse osmosis process and generate the permeate stream116.

During operation, fluorometer104can fluorometrically analyze permeate stream116generated from the feed stream114in which the fluorescent tracer was introduced, e.g., in an amount effective to achieve the first concentration of fluorescent tracer in the feed stream. (304) For example, fluorometer104may be installed online in system100to measure permeate116flowing from membrane102(e.g., either in the main fluid conveyance line carrying the bulk of the permeate stream or via a slipstream connecting fluorometer104to the main conveyance line). Permeate stream116or a sample thereof can flow through fluid pathway230for measurement by fluorometer104. Controller106executing instructions stored in memory122can control optical emitter222to direct light selected to be at an excitation wavelength for the fluorescent tracer introduced into feed stream114into the permeate fluid under analysis. Fluorescent tracer molecules within permeate under analysis may fluoresce at a wavelength different than the excitation wavelength in response to the energy emitted at the excitation wavelength. Optical detector224can detect the fluorescent emissions emitted by the tracer molecules present within the permeate. The magnitude of the fluorescent emissions detected by optical detector224may vary based on the concentration of the fluorescent tracer molecules present in the permeate under analysis.

Processor120of controller106can determine a first measured concentration of the fluorescent tracer in permeate stream116based on the fluorescent emissions detected by optical detector224and calibration curve data232stored in memory122. For example, processor120of controller106may use one or more stored slope coefficients and a stored intercept of a calibration curve relating to a magnitude of light detected by optical detector224(e.g., within one or more wavelengths corresponding to fluorescent emissions of the fluorescent tracer) to a measured concentration of the fluorescent tracer. The first measured concentration of fluorescent tracer determined by processor120can be stored in memory122for subsequent use during the calibration procedure.

The example technique ofFIG. 3also includes adjusting the concentration of fluorescent tracer introduced into feed stream114to provide a second concentration of fluorescent tracer in the feed stream different than the first concentration. (306) Under the control of controller106in system100, the rate at which fluorescent tracer pump110introduces fluorescent tracer into feed stream114from fluorescent tracer source113and/or the rate at which feed stream114is delivered to membrane102may be adjusted to adjust the concentration of fluorescent tracer in the feed stream contacting member102. In one example, the concentration of fluorescent tracer is increased such that the second concentration of fluorescent tracer in feed stream114is greater than the first concentration of fluorescent tracer. In another example, the concentration of fluorescent tracer is decreased such that the second concentration of fluorescent tracer in feed stream114is less than the first concentration of fluorescent tracer. For example, as discussed with respect toFIG. 6, fluorescent tracer pump110may be stopped, terminating the introduction of fluorescent tracer into feed stream114.

In general, the concentration of fluorescent tracer in feed stream114may be adjusted an amount effective to cause the second concentration of fluorescent tracer to be measurably different than the first concentration of fluorescent tracer, e.g., taking into account noise and other system fluctuations that may typically be present. For example, the second concentration may be at least 10% different (positive or negative) than the first concentration, such as at least 25% different, at least 50% different, or at least 100% different. When the concentration of fluorescent tracer is adjusted to reduce the concentration of fluorescent tracer relative to the first concentration, the concentration may be reduced until the second concentration ranges from 0.05 to 0.95 times the first concentration, such as from 0.1 to 0.9 times the first concentration, or from 0.1 to 0.5 times the first concentration. When the concentration of fluorescent tracer is adjusted to increase the concentration of fluorescent tracer relative to the first concentration, the concentration may be increased until the second concentration ranges from 1.1 to 10 times the first concentration, such as from 1.5 to 5 times the first concentration. Accordingly, in some applications, the concentration of fluorescent tracer may be adjusted until the second concentration of fluorescent tracer is at least an order of magnitude greater than the first concentration of fluorescent tracer, or less than an order of magnitude difference. The ratio by which controller106has increased or decreased the concentration of fluorescent tracer in feed stream114(e.g., the rate at which the fluorescent tracer is introduced into the feed stream) may be stored in memory122for subsequent use during a calibration procedure. It should be appreciated that the foregoing ranges of adjusted concentrations are examples and the disclosure is not necessarily limited in this effect.

After adjusting the concentration of fluorescent tracer, fluorometer104can fluorometrically analyze permeate stream116generated from the feed stream114in which the fluorescent tracer at the adjusted concentration level, e.g., in an amount effective to achieve the second concentration of fluorescent tracer in the feed stream. (308) As discussed with respect to step304in the example technique ofFIG. 3, controller106executing instructions stored in memory122can control optical emitter222to direct light selected to be at an excitation wavelength for the fluorescent tracer introduced into feed stream114into the permeate fluid under analysis. Fluorescent tracer molecules within permeate under analysis may fluoresce at a wavelength different than the excitation wavelength in response to the energy emitted at the excitation wavelength. Optical detector224can detect the fluorescent emissions emitted by the tracer molecules present within the permeate. The magnitude of the fluorescent emissions detected by optical detector224may vary based on the concentration of the fluorescent tracer molecules present in the permeate under analysis.

Processor120of controller106can determine a second measured concentration of the fluorescent tracer in permeate stream116based on the fluorescent emissions detected by optical detector224and calibration curve data232stored in memory122. For example, processor120of controller106may use one or more stored slope coefficients and a stored intercept of a calibration curve relating a magnitude of light detected by optical detector224(e.g., within one or more wavelengths corresponding to fluorescent emissions of the fluorescent tracer) to a measured concentration of the fluorescent tracer. The second measured concentration of fluorescent tracer determined by processor120can be stored in memory122for subsequent use during the calibration procedure.

In practice, changes to the concentration of fluorescent tracer introduced into feed stream114may take a period of time to show up in permeate stream116. For example, when controller106controls fluorescent tracer pump110to change a dosing rate of fluorescent tracer into feed stream114, an equilibrating period of time may need to elapse before a new equilibrium concentration is achieved in both the feed stream and permeate stream. Accordingly, controller106may control fluorometer104to determine the second measured concentration of tracer in the permeate stream a period of time after adjusting the concentration of the fluorescent tracer introduced into the feed stream effective to achieve equilibrium concentrations of the fluorescent tracer in the feed stream and in the permeate stream. Depending on the operational parameters of system100, the period of time needed to achieve equilibrium may be greater than 15 minutes, such as greater than 30 minutes, or greater than 60 minutes. For example, the period of time needed to achieve equilibrium may range from 30 minutes to 120 minutes, such as from 60 minutes to 90 minutes.

Controller106may control fluorometer104to continuously fluorometrically analyze permeate stream116, e.g., as frequently as the sampling frequency of the fluorometer will allow. When so configured, controller106may receive fluorometric data corresponding to feed stream114having the first concentration of fluorescent tracer, the second concentration of fluorescent tracer, and intermediate concentrations of fluorescent tracer as the system is undergoing equilibration. Controller106may or may not omit use of the fluorometric data generated from permeate stream116when the concentration of fluorescent tracer in the stream is not at an equilibrium level during the recalibration process. In other configurations, controller106may control fluorometer104to intermittently fluorometrically analyze permeate stream116rather than continuously analyze the stream.

To recalibrate fluorometer104, controller106may determine an intercept shift for the calibration curve stored as calibration curve data232based on first and second measured concentrations of fluorescent tracer made by fluorometer104and, e.g., the ratio by which the concentration of fluorescent tracer was adjusted. (310) The intercept shift may be an error that has developed in the calibration information, e.g., such that the measured fluorescent tracer concentration determined by controller106is offset or deviated from the true concentration of fluorescent tracer actually present in the permeate stream.FIG. 4is a graph illustrating an example intercept shift γ for an example calibration curve showing how the calibration curve may drift over time from its original position due to changing conditions. In this example, the intercept shift is shown as a negative, or downward shift, although the intercept shift may be positive or an upward shift.

To determine the extent to which the intercept of the calibration curve stored in calibration curve data232has shifted, controller106may compare the first measured concentration of fluorescent tracer in permeate stream116to the second measured concentration of fluorescent tracer in the permeate stream. (310)FIG. 5is a graph illustrating an example intercept shift γ for an example calibration curve showing how measured fluorescent tracer concentration data can be used along with information concerning the extent to which the concentration of fluorescent tracer dye has been adjusted. In this example, the concentration of fluorescent tracer in feed stream114is adjusted (increased in the illustrated example) from a first concentration D to a second concentration xD. In other words, the concentration is adjusted by a factor “x.” This is expected to cause the concentration of fluorescent tracer in permeate stream116to change from a first concentration “a” to a second concentration “b”. Since the rejection efficiency of the fluorescent tracer by membrane102is expected to be constant over the range of fluorescent tracer used and during the time span of recalibration (e.g., less than 1 ppm difference), the ratio of the second concentration of fluorescent tracer in the permeate stream “b” divided by the first concentration of the fluorescent tracer in the permeate stream “a” should be “x” if there is no intercept shift. However, where the ratio of “b” divided by “a” is different than “x,” the difference can be considered an intercept shift.

Accordingly, in some examples, controller106may determine an intercept shift with reference to memory122using the following equation:

In Equation 2 above, γ is the intercept shift, a is the first measured concentration of the fluorescent tracer, b is the second measured concentration of the fluorescent tracer, and x is the second concentration of fluorescent tracer divided by the first concentration of fluorescent tracer. Controller106may store the determined intercept shift in memory122and/or use the determined intercept shift to establish a new intercept for a recalibrated calibration curve stored as calibration curve data232.

In the example ofFIG. 3, controller106determines an adjusted intercept for the calibration curve based on the determined intercept shift (312). In some examples, controller106references calibration curve data232stored in memory122and increments or decrements the stored calibration curve intercept by an amount corresponding to the intercept shift. For example, controller106may add the intercept shift determined in step310to the stored calibration curve intercept, thereby establishing an adjusted intercept. Controller106may store the adjusted intercept as calibration curve data232for use in subsequent fluorometric measurements, e.g., replacing the intercept previously stored.

As previously noted, the calibration curve stored as calibration curve data232may include one or more slope constants in addition to an intercept value. In performing the recalibration technique ofFIG. 3, the intercept value for the calibration curve may be adjusted without changing the one or more slope constants stored. That is, the one or more slope constants generated during a full, multipoint calibration process may remain in calibration curve data232and be used during subsequent fluorometric measurements with only the intercept parameter changing due to recalibration. Accordingly, during partial recalibration, and adjusted intercept parameter for calibration curve data232may be determined without determining an adjusted slope parameter.

FIG. 6is another example technique for recalibrating fluorometer104. As with the technique ofFIG. 3, the technique ofFIG. 6is described with reference to system100inFIG. 1and fluorometer104inFIG. 2, although can be performed in other systems and with other fluorometer configurations, e.g., as described herein.

With reference toFIG. 6, the technique includes introducing a fluorescent tracer into feed stream114to provide a first concentration of fluorescent tracer in the feed stream. (600) Operating under the control of controller106in system100, fluorescent tracer pump110may be controlled to introduce fluorescent tracer into feed stream114from fluorescent tracer source113. The amount of fluorescent tracer introduced into feed stream114may be effective to provide a concentration of fluorescent tracer in feed stream114within any of the concentration ranges discussed herein, or even concentration ranges outside of those discussed herein in suitable applications. The fluorescent tracer may be introduced into feed stream114as part of an ongoing monitoring process to evaluate the rejection efficiency of membrane102.

Feed stream114contacts membrane102, separating the feed stream into permeate stream116and concentrate stream118. (602) Membrane102may be configured as a cross flow membrane, dead-end flow membrane, or have yet other configuration. In one example, system100is a reverse osmosis system. Feed stream pressure pump108can pressurize a source of liquid to be purified, overcoming the osmotic pressure of membrane102to drive the reverse osmosis process and generate the permeate stream116.

In the technique ofFIG. 6, controller106controls fluorescent tracer pump110to terminate the introduction of fluorescent tracer into feed stream114. (604) After terminating the introduction of fluorescent tracer, fluorometer104can fluorometrically analyze permeate stream116generated from the feed stream114following termination of the introduction of fluorescent tracer. (606) As discussed with respect toFIG. 3, controller106executing instructions stored in memory122can control optical emitter222to direct light selected to be at an excitation wavelength for the fluorescent tracer introduced into feed stream114into the permeate fluid under analysis. Fluorescent tracer molecules within permeate under analysis may fluoresce at a wavelength different than the excitation wavelength in response to the energy emitted at the excitation wavelength. Optical detector224can detect the fluorescent emissions emitted by the tracer molecules present within the permeate. The magnitude of the fluorescent emissions detected by optical detector224may vary based on the concentration of the fluorescent tracer molecules present in the permeate under analysis.

Processor120of controller106can determine a measured concentration of the fluorescent tracer in permeate stream116based on the fluorescent emissions detected by optical detector224and calibration curve data232stored in memory122. As discussed above with respect toFIG. 3, controller106may control fluorometer104to determine the measured concentration of tracer in the permeate stream a period of time after terminating the introduction of fluorescent tracer into the feed stream effective to achieve equilibrium conditions in the feed stream and in the permeate stream. Equilibrium conditions may occur when the feed stream and permeate stream are devoid of any added fluorescent tracer molecules. Accordingly, when controller106determines a “measured concentration” of fluorescent tracer in permeate stream116—and, in fact, no fluorescent tracer is present—the “measured concentration” may represent a calibration error or offset for calibration curve data232in memory122.

To recalibrate fluorometer104, controller106may use the “measured concentration” of fluorescent tracer that is measured in the absence of any added fluorescent tracer being present in permeate stream as an intercept shift. For example, controller106may store and/or use the magnitude of fluorescent tracer measured in the absence of any added fluorescent tracer being present in permeate stream as an intercept shift.

Controller106can determine an adjusted intercept using the intercept shift. (608). For example, controller106may reference calibration curve data232stored in memory122and increment or decrement the stored calibration curve intercept by an amount corresponding to the intercept shift. For example, if the magnitude of fluorescent tracer measured in the absence of any added fluorescent tracer being present in permeate stream is negative, controller may add to the stored calibration curve intercept an amount effective to adjust the measured concentration to zero, thereby establishing an adjusted intercept. Likewise, if the magnitude of fluorescent tracer measured in the absence of any added fluorescent tracer being present in permeate stream is positive, controller may subtract from the stored calibration curve intercept an amount effective to adjust the measured concentration to zero, thereby establishing an adjusted intercept. Thus, controller106may determine a difference between the measured or reported concentration of the fluorescent tracer by fluorometer104in the permeate stream and the intercept to determine an amount by which the stored intercept should be shifted. In either case, controller106may store the adjusted intercept as calibration curve data232for use in subsequent fluorometric measurements, e.g., replacing the intercept previously stored.

The calibration curve stored as calibration curve data232may include one or more slope constants in addition to an intercept value. In performing the recalibration technique ofFIG. 6, the intercept value for the calibration curve may be adjusted without changing the one or more slope constants stored. That is, the one or more slope constants generated during a full, multipoint calibration process may remain in calibration curve data232and be used during subsequent fluorometric measurements with only the intercept parameter changing due to recalibration. Accordingly, during partial recalibration, and adjusted intercept parameter for calibration curve data232may be determined without determining an adjusted slope parameter.

The following examples may provide additional details about membrane separation systems and fluorometer calibration techniques according to the disclosure.

Example 1: Effect of Fluorescent Tracer Concentration on Membrane Rejection Efficiency

An experiment was conducted to determine the effect, if any, changing the concentration of fluorescent tracer introduced into a membrane separation system has on the measured rejection efficiency of the membrane. One 2.5″ spiral wound membrane element (DOW FILMTEC BW30-2540) was used to filter NaCl solution (1,500 mg/L) with various fluorescent tracer concentrations. All concentrate from the membrane system was recycled back to a feed tank. Water temperature was controlled at 25 degrees Celsius by circulating cooling water through a coil immersed in the feed tank. Feed pressure was set at 12 bar or 174 psi. Feed and permeate fluorescence were monitored continuously by directing feed and permeate sample through feed and fluorometers, respectively. After adding a small amount of fluorescent tracer into the feed tank, rejection efficiency was estimated based on feed and permeate fluorescence. Then, additional fluorescent tracer was added and rejection efficiency was measured after fluorescence readings became stable, and so on.

The results of the experiment are shown inFIG. 7, which is a plot of fluorescent tracer concentration in the feed stream versus fluorescent tracer concentration in the permeate and membrane rejection efficiency. As shown, the membrane rejection efficiency was nearly constant regardless of fluorescent tracer concentration in the feed stream.

Example 2: Calibration Curve Intercept Adjustment

Using the same experimental system described in Example 1 above, an experiment was conducted to evaluate an example calibration curve intercept adjustment according to the present disclosure. A feed fluorometer was calibrated for a fluorescent tracer range between 0 and 400 μg/L and an ultralow range permeate fluorometer was calibrated for a fluorescent tracer range between 0 and 1 μg/L. The fluorometers were calibrated using multi-point calibration solutions, including a zero point calibration solution devoid of fluorescent tracer and at least one calibration solution having a known concentration of fluorescent tracer. The intercept or zero point of the ultralow range fluorometer was deliberately moved by −0.02 μg/L (or −20 ng/L), thereby read −20 ng/L with DI water, artificially create a calibration error.

While filtering 1,500 mg/L NaCl solution using a DOW FILMTEC BW30 membrane, fluorescent tracer was added to the feed tank. After reaching a steady state, fluorescent tracer concentrations in the feed stream and permeate stream were measured at 167 μg/L and 31 ng/L, respectively, as shown inFIG. 8. If the intercept or zero point of the ultralow range permeate fluorometer was not moved by −20 ng/L, the fluorescent tracer concentration in the permeate would have measured at 51 ng/L. Additional fluorescent tracer was added to the feed tank to raise the tracer concentration by approximately 100%. After reaching a steady state, the fluorescent tracer concentrations in the feed stream and permeate stream were stabilized at approximately 339 μg/L and approximately 83 ng/L, respectively.

Using Equation 2 above, the intercept shift (γ) for the calibration curve data stored for the ultralow range fluorometer was calculated. The multiplication factor (x) of fluorescent tracer concentration in the feed was calculated at 2.03 (=339/167). The average fluorescent tracer concentrations in the permeate before and after raising the fluorescent tracer concentration in feed were 31 ng/L (a) and 83 ng/L (b), respectively. The intercept shift (γ) was calculated at +19.6 ng/L, which was sufficient (within +/−5%) to compensate most of the artificial zero point movement (−20 ng/L) made earlier in the study.

FIG. 8is a plot illustrating the fluorescent tracer concentrations observed during the experiment in the feed stream and the permeate stream.FIG. 8also illustrates a corrected fluorescent tracer concentration in the permeate, which was obtained by adding the 19.6 ng/L calculated correction factor to the intercept shift γ.