Patent Publication Number: US-2022226782-A1

Title: Systems and techniques for cleaning pressure membrane systems using a water-in-air cleaning stream

Description:
RELATED MATTERS 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/138,770, filed Jan. 18, 2021, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to systems and techniques for cleaning pressure membrane systems, particularly pressure membrane systems used upstream of a water analysis device, using a water-in-air cleaning stream. 
     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. 
     During operation of a membrane separation system, clean water can pass through the membrane surface while contaminants can sit on the surface of the membrane. Contaminants can build up over time, even if there is a crossflow across the surface of the membrane, due to a phenomenon called concentration polarization. The length of time the membrane can stay in service before replacement or shutdown for cleaning can depend on how well concentration polarization and subsequent filter fouling are controlled. 
     SUMMARY 
     In general, this disclosure is directed to systems and technique for controlling fouling in closed membrane separation systems using a water-in-air stream in which the feed stream supplied to the separation system contains a greater volume of air than water. Introducing air into the feed stream can provide a scouring effect on the surface of the membrane that helps prevent deposition of foulant and/or helps remove built-up foulant. It has been found that using a feed stream where the amount of air exceeds the amount of water results in unexpectedly better fouling control and membrane performance improvements than when using a comparable feed stream where the amount of water exceeds the amount of air in the stream. 
     Although the systems and techniques of the disclosure can be used in a variety of different applications, in some implementations, the systems and techniques are utilized to control fouling in a pre-filtration system upstream of an automated water analysis system. An automated water analysis system may be used to analyze one or more chemical species in a variety of different industrial waters. This can provide information concerning the concentration of the one or more chemical species of interest in the industrial water, allowing the operator to take appropriate corrective action based on the measured concentration. For example, the operator may adjust an operating parameter of the industrial water system and/or control the introduction of a chemical additive to the industrial water system that interacts with the chemical species of interest. 
     When an automated water analysis system is implemented as an online tool, the system may receive a sample of water from the industrial water system, perform a prefiltration on the water sample, and then analyze the resulting filtered sample for the one or more chemical species of interest. In some implementations, the automated water analysis system may control addition of an optical indicator to the filtered water sample and perform a colorimetric optical analysis to determine the concentration of the one or more chemical species of interest. Performing prefiltration on the water sample can remove comparatively large size particulates and contaminants, which may otherwise interfere with the optical analysis performed on the water sample. 
     In practice, an operator may install an automated water analysis system with the intent that the water analysis system operates continuously for an extended period of time without requiring user intervention. If the prefiltration system of the water analysis system becomes prematurely fouled, this can require accelerated user intervention for maintenance on the water analysis system. 
     In accordance with some examples of the present disclosure, a membrane filtration system utilizing a feed stream that contains a greater amount of air than water is utilized as a prefiltration step for an automated water analysis system. This can generate a filtered water stream for downstream analysis, such as the determination of the concentration of one or more chemical species of interest in the water stream. By distributing the water stream to be filtered in the membrane system in an airstream of greater volume, fouling on the membrane element may be reduced or eliminated as compared to when the water stream is processed directly on the membrane and/or processed with a lesser volume of air. This can extend the service life and duration between which user maintenance is needed on the membrane filtration system. 
     While the systems and techniques of the disclosure can be beneficially used to support extended operation of an automated water analysis system, the disclosure is not limited to the specific application. As another example, the systems and techniques can be used in larger scale water purification processes in which a contaminated water stream is processed by one or more membrane filtration elements to produce a clean water stream for downstream use. 
     In one example, a method of controlling pre-filtration fouling in an automated water analysis system is described. The method includes generating a feed stream by combining a flow rate of air with a flow rate of water, where a ratio of the flow rate of air divided by the flow rate of water is greater than 1. The method also includes contacting a membrane with the feed stream inside of a housing, thereby generating a permeate stream and a concentrate stream, wherein the housing pressure isolates the membrane from an ambient environment. In addition, the method involves analyzing the permeate stream to determine a concentration of at least one chemical species in the permeate stream. 
     In another example, a method of controlling membrane fouling comprising is described that includes generating a feed stream by combining a flow rate of air with a flow rate of water, wherein a ratio of the flow rate of air divided by the flow rate of water is greater than 1. The method also includes contacting a membrane with the feed stream inside of a housing, thereby generating a permeate stream and a concentrate stream, where the housing pressure isolates the membrane from an ambient environment. 
     In an additional example, a system is described that includes a membrane, a water metering device, an air metering device, an analyzer, and a controller. The membrane is positioned in a housing that pressure isolates the membrane from an ambient environment, with the membrane being configured to separate a feed stream into a permeate stream and a concentrate stream. The water metering device is configured to provide a flow rate of water. The air metering device is configured to provide a flow rate of air that combines with the flow rate of water to generate the feed stream supplied to the housing. The analyzer is positioned to measure a concentration of at least one chemical species in the permeate stream. The controller is communicatively coupled to the water metering device, the air metering device, and the analyzer. The example specifies that the controller is configured to control the water metering device and the air metering device to generate the feed stream with a ratio of the flow rate of air divided by the flow rate of water is greater than 1, and control the analyzer to analyze the permeate stream and determine the concentration of that at least one chemical species in the permeate stream. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example membrane separation system that receives a water-in-air feed stream containing a greater volume of air than water at least periodically during operation. 
         FIG. 2  is a block diagram illustrating one example configuration of an automated water analysis system that can be used to analyze water received from the membrane separation system of  FIG. 1 . 
         FIG. 3  is plot showing normalized permeate flow rate versus time using a water-in-air feed stream compared to an air-in-water feed stream. 
     
    
    
     DETAILED DESCRIPTION 
     In general, this disclosure describes systems and techniques for operating a membrane separation system by utilizing a feed stream that is predominantly gas and contains a lesser amount of the liquid to be filtered. Where the liquid to be filtered is an aqueous stream and the gas source is air, the feed stream may be referred to as a water-in-air stream to indicate that the aqueous portion of the feed stream is present in a lesser amount than the gaseous portion of the feed stream. In some implementations, the water-in-air stream may be continuously supplied to the membrane system such that any liquid filtered using the membrane system is supplied as part of a feed stream that contains a greater amount of air that liquid. In other implementations, the water-in-air stream may be intermittently supplied to the membrane system such that the membrane system filters a feed stream containing only the liquid or the liquid with a lesser amount of air (e.g., such that there is more liquid than air present in the feed stream) between periods when an water-in-air stream is supplied to the membrane system. In either case, the water-in-air stream supplied to the membrane system can help provide exceptional fouling control, minimizing the amount of fouling on the membrane and increasing the performance of the membrane separation system. 
     The fouling control benefits associated with using a water-in-air stream are particularly observed in closed membrane system in which a membrane is enclosed within a housing that pressure isolates the membrane from ambient environment. These closed membrane systems, which may also be referred to as housed membrane systems, generally operate by supplying a feed stream to a pressure vessel containing one or more membrane elements. As a result, the entire volume of liquid and air entering housing containing the membrane element contacts the surface of the membrane elements inside of the housing. 
     Closed membrane systems are generally distinguishable from a second type of membrane system, referred to as a submerged membrane system. In a typical submerged membrane system, one or more membrane elements are submerged in feed water and aeration is preformed underneath the membrane elements. This creates a two-phase turbulence under the membrane elements. This configuration is commonly used in membrane bioreactor (MBR) processes for treating wastewaters. However, in a submerged membrane system, the feed liquid and membrane elements are typically at atmospheric pressure with aeration bypassing the membrane elements. As a result, much of the air supplied underneath the membrane elements passes around the membrane elements without contacting the surface of the membrane elements or having a material impact on the fouling of the membrane element. 
       FIG. 1  is a conceptual diagram illustrating an example membrane separation system  100  according to the disclosure that receives a water-in-air feed stream containing a greater volume of air than water at least periodically during operation. In the specific example of  FIG. 1 , membrane separation system  100  is implemented as a pre-filtration step for a downstream automated water analysis system  200 . In other examples, membrane separation system  100  may process a feed stream to produce a filtered stream for downstream uses other than for analysis by water analysis system  200 . 
     In the example of  FIG. 1 , system  100  includes a separation membrane  102  contained within a housing  104 . Housing  104  pressure isolates membrane  102  from an ambient environment, e.g., such that the pressure inside of the housing is higher or lower than ambient pressure surrounding the housing. System  100  in the illustrated example also includes a controller  106 , a water metering device  108 , and an air metering device  110 . Water metering device  108  is in fluid communication with a source  112  of water to be purified using membrane  102 . Air metering device  110  is in fluid communication with a source of air  115  to combine with water from source  112  to supply a feed stream  114  containing a greater volume of air than water to housing  104  and membrane  102  contained therein. In operation, feed stream  114  is supplied to membrane  102 , 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 stream  116  and a concentrate stream  118  (which may also be referred to as a reject stream). 
     At least a portion of the purified water generated by membrane system  100  is supplied to downstream automated water analysis system  200  in the example arrangement of  FIG. 1 . Operating under the control of controller  106  (or a separate controller from the controller that controls membrane separation system  100 ), water analysis system  200  can analyze water received from membrane separation system  100  to determine one or more characteristics of the water, such as the concentration of one or more chemical species in the water. This information can then be used to control one or more aspects of the water source  112 , such as the addition of one or more chemical agents selected to control the chemical species measured by the water analysis system. 
     One or more controllers  106  (which is illustrated as a single controller) can be communicatively coupled to various components within membrane separation system  100  and water analysis system  200  to manage the overall operations of the system. For example, controller  106  may be communicatively connected to water metering device  108 , air metering device  110 , water analysis system  200 , and optionally any other controllable components or sensors that may be desirably implemented in system  100  and/or system  200 . Controller  106  includes processor  120  and memory  122 . Controller  106  communicates with controllable components via connections. For example, signals generated by water analysis system  200  may be communicated to controller  106  via a wired or wireless connection, which in the example of  FIG. 1  is illustrated as wired connection. Memory  122  stores software for running controller  106  and may also store data generated or received by processor  120 . Processor  120  runs software stored in memory  122  to manage the operation of system  100  and/or system  200 . 
     As described in greater detail below, controller  106  can control water metering device  108  and air metering device  110  to generate feed stream  114  supplied to membrane  102 . Controller  106  can control water metering device  108  and air metering device  110  so that the amount of air in feed stream  114  is greater than the amount of water in the feed stream for at least a portion of the time the water is supplied to membrane  102  from source  112 . Controller  106  can control water metering device  108  and air metering device  110  to supply feed stream  114  to membrane  102  and generate permeate stream  116  in response to a request from water analysis system  200  calling for water to facilitate analysis by the water analysis system. While various components are described as being controlled by controller  106 , in other implementations, some or all of the components may be manually controlled by an operator in the course of performing a technique according to the disclosure. 
     During operation of system  100 , membrane  102  can be contacted with fluid to be purified from source  112  to remove ion, molecules, pathogens, and/or other particulate contaminants. For example, feed stream  114  can contain various solutes, such as dissolved organics, dissolved inorganics, dissolved solids, suspended solids, the like or combinations thereof. Upon separation of feed stream  114  into permeate stream  116  and concentrate stream  118 , in membrane  102 , 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 stream  118  can have a higher concentration of dissolved and/or suspended solutes as compared to the feed stream. In this regard, the permeate stream  116  represents a purified feed stream, such as a purified aqueous feed stream. 
     System  100  and membrane  102  can 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, system  100  and membrane  102  may be implemented as a reverse osmosis, ultrafiltration, microfiltration, or nanofiltration membrane separation process. 
     In reverse osmosis, feed stream  114  is typically processed under cross flow conditions. When so configured, feed stream  114  may 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. 
     System  100  can employ a variety of different types of membranes as membrane  102 . 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, membrane  102  can 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 one or more housings  104 . Within one or more housings  104 , feed stream  114  can 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 permeate streams from the different housings or stacks may be combined to form a combined permeate stream  116 . 
     Within most reverse osmosis systems, pressure vessels (e.g., housing  104 ) 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 system  100  is implemented as a reverse osmosis process, one or more membranes  102  may be configured as a multi-stage and/or multi-pass system. 
     While system  100  and membrane  102  may be implemented as cross-flow filtration process, in other configurations, the system may be arranged for conventional filtration of suspended solids by passing feed stream  114  through a filter media or membrane in a substantially perpendicular direction. This arrangement can create one exit stream (e.g., purified stream  116 ) 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, system  100  may have a feed stream  114 , a purified stream  116 , and a backwash stream  118 . 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. 
     Independent of the specific configuration of system  100  and membrane  102 , one or more membranes may be contained within housing  104  during operation to pressure isolate the membrane from the ambient environment. In general, housing  104  may be an enclosed pressure vessel that separates an interior of the housing from an exterior environment. As a result, a pressure inside of housing  104  may be different (e.g., higher or lower) than ambient environmental pressure surrounding the exterior of the housing. By positioning membrane  102  inside of housing  104 , substantially all air introduced into the housing as part of a water-in-air feed stream may contact the surface of the membrane. For example, at least 90 volume percent of the air introduced into housing  104  may contact the surface the membrane, such as at least 95 volume percent, at least 98 volume percent, at least 99 volume percent, or at least 99.5 volume percent of the air. 
     In some implementations, housing  104  is implemented as a closed chamber having a feed inlet, a permeate outlet, and a concentrate outlet. Feed stream  114  can enter the closed chamber via the feed inlet. Permeate stream  116  can discharge from the closed chamber via the permeate outlet. Further, concentrate stream  118  can discharge from the closed chamber via the concentrate outlet. 
     While housing  104  and the various inlets and/or outlets of the housing can be oriented in various ways, in some examples, the components are arranged to facilitate an upward flow of the incoming air-in-water stream against the membrane surface. For example, housing  104  may be oriented vertically with respect to gravity, with the feed inlet positioned adjacent a bottom end of the housing, and the permeate outlet positioned adjacent a top end of the housing. When so configured, feed stream  114  may enter housing  104  at or adjacent a bottom of the housing, and flow upwardly with respect to gravity through the housing before permeate stream  116  discharges at or adjacent a top of the housing. In other implementations, the housing  104  may be oriented vertically with respect to gravity, with the feed inlet positioned adjacent a top end of the housing and the permeate outlet positioned adjacent a bottom end of the housing. 
     In general, membrane separation system  100  is configured to establish a transmembrane pressure across membrane  102  that acts as a driving force for generating permeate stream  116  and concentrate stream  118  from feed stream  114 . In some implementations, the transmembrane pressure may be generated by applying a suction force downstream of membrane  102  (e.g., on permeate stream  116 ) that draws feed stream  114  through housing  104  and membrane  102 . Additionally or alternatively, the transmembrane pressure may be generated by supplying a pressurized feed stream  114  to housing  104 , where the pressure of the feed stream is greater than the downstream pressure of the permeate stream and/or concentrate stream. 
     The transmembrane pressure can be measured as the pressure difference between the pressure of feed stream  114  in the pressure of permeate stream  116 . The target transmembrane operating pressure for membrane separation system  100  may vary, e.g., depending on the characteristics of feed stream  114  and the configuration of the membrane system, such as the number and type of membrane elements  102  in the system. That being said, in some examples, membrane separation system  100  is configured to operate at a transmembrane pressure of at least 1 kPa, such as a transmembrane pressure ranging from 1 kPa to 600 kPa. 
     When membrane separation system  100  operates at a positive pressure, feed stream  114  may be a pressurized stream the enters housing  104 . Water metering device  108  and/or air metering device  110  may pressurized water from water source  112  and air from air source  115 , respectively, to generate the pressurized feed stream. Feed stream  114  may be pressurized to a pressure of at least 1 kPa, such that the feed stream contacts membrane  102  inside of housing  104  at a pressure of at least 1 kPa. For example, feed stream  114  may be pressurized to a pressure of at least 2 kPa, such as at least 5 kPa, at least 10 kPa, at least 50 kPa, at least 100 kPa, or at least 250 kPa. 
     In other examples when membrane separation system  100  operates in a vacuum pressure, one or more suction pumps may be implemented downstream of housing  104  to create a suction pressure that draws feed stream  114  through housing  104  and membrane  102 . For example, a suction pump may be connected to permeate stream  116  that creates a vacuum pressure inside of housing  104 , drawing feed stream  114  through the housing. The suction pump may generate a suction pressure on the permeate stream of at least 1 kPa, such as a suction pressure of at least 2 kPa or of at least 5 kPa, such as at least 10 kPa, at least 50 kPa, at least 100 kPa, or at least 250 kPa. In these implementations, feed stream  114  entering housing  104  may or may not be pressurized at a positive pressure (e.g., by water metering device  108  and/or air metering device  110 ). 
     System  100  can be used to purify any desired type of fluid. Example aqueous (water-based) liquid feed sources  112  that may be purified using system  100  include 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. For example, a membrane separate process may commonly be deployed for water treatment and the preparation of water such as drinking water, pure water, ultra-pure grade water, process water for electricity, electronic and/or semiconductor industries, process water for the medical field, water for agents, water for injection, aseptic pyrogen-free pure water, process water of food and beverage uses, water for a boiler, and/or water for washing and cooling. A membrane separation processes can also be applied to fields such as the desalination of seawater or brackish water. 
     In some examples, water source  112  is a boiler water stream. In other examples, water source  112  is a cooling water stream. A cooling water stream may be a stream obtained from a water system that includes a cooling tower that reduces a temperature of a cooling water stream through evaporative cooling. The water system in these examples may also include one or more heat exchanges in which the cooling stream passes on a cold side of the heat exchanger and picks up thermal energy from a comparatively hot process stream passing on a hot side of the heat exchanger. 
     Independent of the source  112  of water used to generate the water-in-air feed stream  114  supplied to membrane  102 , the water may contain various organic and/or inorganic species. In this regard, it should be appreciated that reference to “water” and a “water stream” (e.g., water-in-air stream) in the present disclosure is not intended to exclude the presence of one or more additional chemical species in the water, unless otherwise indicated. Example chemical species that may be present in the water include salts (e.g., calcium, sodium, magnesium), metal component (e.g., iron, aluminum, and/or zinc), phosphates, and/or biological organism. When source  112  includes water containing organic and/or inorganic species, the water molecules (H 2 O) may constitute greater than 75 weight percent of the water stream, such as greater than 90 weight percent, greater than 95 weight percent, or greater than 98 weight percent. 
     In some example, a membrane cleaning agent (e.g., biofouling control agent) may be added to water obtained from source  112  prior to contacting membrane  102  with the water. Example cleaning agents that may be used include chlorine, chlorine dioxide, chloramine, bromine (e.g., DBNPA), at the like. In other examples, a membrane cleaning agent is not added to the water obtained from source  112  prior to introduction into housing  104 . 
     As mentioned, controller  106  can control membrane system  100  can generate a water-in-air feed stream  114  containing a greater volume of air than water at least periodically during operation. Controller  106  can generate the feed stream  114  having a greater volume of air than water by supplying a greater volume of air from air source  115  than the volume of water supplied from water source  112  to housing  104  containing membrane  102 . Controller  106  may control water metering device  108  and air metering device  110  to control the volumes of water and air, respectively, supplied to housing  104 . 
     In general, controller  106  can generate a water-in-air feed stream by supplying a greater volumetric flow rate of air to housing  104  than the volumetric flow rate of water supplied to the housing. As a result, the ratio of the flow rate of air divided by the flow rate of water may be greater than 1. In some implementations, the volume of air supplied to housing  104  may be significantly greater than the volume of water supplied to the housing, when supplying a water-in-air stream. For example, the ratio of the volumetric flow rate of air delivered to housing  104  divided by the volumetric flow rate of water delivered to the housing may be at least two, such as at least three, at least five, at least 10, at least 20, or at least 25. In some specific implementations, for instance, the ratio may range from about two to about 50, such as about three to about 25, from about three to about 20, or from about four to about 10. 
     In the example of  FIG. 1 , water from source  112  and air from source  115  are shown combining together upstream of housing  104  to form a combined water-in-air feed stream  114  that is supplied to housing  104 . The water and air streams may be combined together in a number of different ways. For example, the water stream may be injected into the air stream or, alternatively, the air may be injected into the water stream. In some examples, the combined water in air stream may pass through one or more mixing devices (e.g., a static mixer) upstream of housing  104  before entering the housing. In still other examples, the water and air streams may be separately introduced into housing  104  to generate the water-in-air feed stream  114  inside of the housing rather than generating the feed stream outside of the housing and supplying the combined water in air streams to the housing. 
     In some implementations, a water-in-air stream may be continuously supplied to the membrane system such that any liquid filtered using the membrane system is supplied as part of a feed stream that contains a greater amount of air that liquid. In these implementations, feed stream  114  supplied to membrane  102  may always have a greater amount of air than water. In other implementations, a water-in-air stream may be intermittently supplied to the membrane system. In this implementations, the feed stream supplied to membrane  102  may vary between being a water-in-air stream (containing a greater amount of air than water) and a water stream (e.g., containing only water from source  112 , or containing a lesser amount of air than water). 
     When membrane separation system  100  operates to intermittently supply the water-in-air stream to membrane  102 , controller  106  may intermittently terminate the supply of air from source  115  (e.g., by controlling air metering device  110 ). Membrane separation system  100  may be operated to intermittently supply the water-in-air stream to membrane  102 , e.g., with a liquid only or predominately liquid stream being supplied to the membrane between when the water-in-air stream is supplied to membrane. This can reduce air demands and associated energy operating costs for the membrane separation system. 
     In implementations when membrane separation system  100  operates to intermittently supply the water-in-air stream to membrane  102 , a minimum amount of water supplied from source  112  and contacted the membrane may be supplied as part of the water-in-air stream. For example, at least 5 volume percent of the water supplied from source  112  and processed by membrane  102  may be supplied as part of the water-in-air stream, such as at least 10 vol %, at least 30 vol %, at least 50 vol %, or at least 75 vol %. The remaining volume of water from source  112  that is introduced into housing  104  to contact membrane  102  may be supplied as part of feed stream  114  that is devoid of added air and/or that is part of the feed stream that contains a greater volume of water than air. 
     In the example of  FIG. 1 , at least a portion of permeate stream  116  generated by membrane separation system  100  is applied to automated water analysis system  200 . Automated water analysis system  200  can analyze the water to determine one or more characteristics of the water, such as temperature, pH, conductivity, and/or a concentration of one or more chemical species of interest in the water. In some examples, the information generated by automated water analysis system  200  concerning the concentration of one or more chemical species of interest can be used as a process control variable for controlling the source of water  112 . For example, the information may be used to control addition of one or more chemical agents selected to control the chemical species of interest (e.g., via precipitation or by binding to the chemically species). 
       FIG. 2  is a block diagram illustrating one example configuration of an automated water analysis system  200  that can be used to analyze water received from membrane separation system  100 . In particular,  FIG. 2  illustrates water analysis system  200  implemented as an optical sensor that can be used to measure a colorimetric response between a chemical species of interest in the water and an indicator, thereby providing an indication of the concentration of the chemical species of interest. As illustrated, the analysis system includes an optical emitter and an optical detector as well as hardware for extracting a water sample and mixing one or more reagents with the sample prior to optical analysis. In other configurations, water analysis system  200  may be implemented without the reagent and mixing features of  FIG. 2  or may have other configurations than the specific example illustrated. 
     Moreover, water analysis system  200  in  FIG. 2  is illustrated as being controlled by controller  106  discussed above with respect to  FIG. 1 . Controller  106  can be communicatively connected to the controllable components within water analysis system  200  to send and/or receive data and control signals to/from the components. However, the functions described as being performed by controller  106  in  FIG. 2  may be performed by a different system controller, one or more remote computing devices, and/or other controller hardware and/or software. 
     In the example of  FIG. 2 , water analysis system  200  is illustrated as including a sample pump  150  that can operate to provide a sample containing an unknown concentration of a chemical species of interest in permeate stream  116 . Sample pump  150  can extract a sample of water from a fluid pathway through which permeate stream  116  flows for analysis. In other examples, permeate stream  116  may be supplied under pressure to water analysis system  200 , e.g., without requiring sample pump  150 . 
     Water analysis system  200  can also include an indicator pump  152  that pumps indicator from a source of indicator  154  (e.g., reservoir or container containing indicator) for mixing with the sample of water for subsequent optical analysis. The indicator can complex or otherwise react with one or more chemical species of interest in the water sample to produce a measurable optical response, the extent of which varies in response to the amount of the chemical species present in the sample under analysis. The number and type(s) of indicator combined with the water sample may be selected by the number and type(s) of chemical species of interest for measurement in the water sample. Example chemical species that may be measured include organic and/or inorganic species, such as calcium, sodium phosphate, iron, and/or other species of interest. 
     To measure the optical response of the sample containing the chemical species-indicator reaction product (e.g., a calcium-indicator complex or phosphate-indicator complex), water analysis system  200  may include one or more optical emitters  160  and one or more optical detectors  162  optically connected to a sample receiving space  164  for optical analysis. Sample receiving space  164  may be an optical cell that receives and holds a static portion of fluid that undergo optical analysis, for example, in a stop flow configuration with the sample subsequently being discharged. As another example, sample receiving space  164  may be or include a fluid conduit through which a flowing stream of fluid passes with optical analysis being performed on the flowing stream of fluid. 
     Reaction between the one or more indicators introduced into the sample and one or more of the chemical species of unknown concentration in the sample can produce an optically detectable change. The concentration of chemical species of interest can be proportional to the measured optical response of the sample. For example, the optical response may be a colorimetric change that occurs through when a complex is formed between chemical species of interest in the sample and the indicator. The reaction between the indicator and chemical species of interest may occur in or upstream of sample receiving space  164  and be detected by measuring an absorbance of the sample. 
     In some examples, water analysis system  200  is configured to introduce one or more additional reagents to a sample undergoing optical analysis. The one or more additional reagents may be present with indicator and introduced simultaneously with the indicator or may be introduced separately from the indicator. In the example of  FIG. 2 , water analysis system  200  is illustrated as including one or more additional reagent pumps  166  fluidly connected to one or more additional sources of reagent. In other examples, the indicator  154  and other desired reagents may be mixed and/or stored together and delivered through single pump  152  instead of being separately introduced. 
     Example chemical reagents that may be added to the fluid sample in addition to the indicator include, but are not limited to, a pH adjuster and/or buffer, a reaction catalyst, a sequestrant, a surfactant, a range extender or a combination thereof. For example, controller  106  may control the addition of a pH adjustor to the sample undergoing analysis so the sample is within a pH range where the calcium-indicator complex forms. The specific types of indicators and/or reagents added to the sample will vary depending on the specific application of the system. 
     Controller  106  can control the operation of optical emitter  160  and receives signals concerning the amount of light and/or frequency or wavelength(s) of light detected by optical detector  162 . In some examples, controller  106  processes signals received from optical detector  162  during analysis of a water sample containing an unknown concentration of a chemical species of interest and determines a concentration of chemical species in the sample based on calibration data stored in memory. The calibration curve data may relate light detected by optical detector  162  to a concentration of the chemical species in the fluid under analysis. 
     As discussed above with respect to  FIG. 1 , information concerning the concentration of the chemical species of interest can be used as control information for modifying the source of water  112  from which the permeate stream  116  is obtained. Controller  106  (or another controller) may be configured to take a variety of control actions based on the measured concentration of one or more chemical species of interest. 
     In some examples, controller  106  may control addition of a chemical agent to water source  112  selected to counteract the chemical species of interest and/or fouling attributable to the chemical species of interest. For example, controller  106  may control addition of a scale inhibitor and/or pH control agent to inhibit scale fouling, control addition of a corrosion inhibitor and/or pH control agent to inhibit corrosion fouling, and/or may control addition of a biocide and/or biodispersant to inhibit biofouling. Example chemical agents that may be added to water source  112  include, but are not limited to, polymers (dispersants and scale inhibitors), organophosphorus compounds such as phosphinosuccinic oligomer (PSO, scale and corrosion inhibitor), zinc (corrosion inhibitor), orthophosphate (corrosion inhibitor), polyphospahtes (scale and corrosion inhibitors), biocides, and combinations thereof. Where the chemical agent is selected to counteract a chemical species of interest (e.g. calcium, phosphate), the chemical agent may be referred to as a control agent for the chemical species (e.g., a calcium control agent, a phosphate control agent). Additionally or alternatively, one or more chemical agents may be added into the water to adjust the pH of the water. Examples of pH adjusting control agents include mineral acids, organic acids, and inorganic bases. 
     Controller  106  may control the addition of one or more chemical agents to source  112  based on the measured concentration of the chemical species of interest by starting a dosing pump or increasing an operating rate of the pump based on the measured concentration of the chemical species of interest (e.g., the measured concentration equaling or exceeding one or more concentration thresholds). Additionally or alternatively, controller  106  may stop the dosing pump or decrease an operating rate of the pump based on the measured concentration of the chemical species of interest (e.g., the measured concentration falling below one or more concentration thresholds). 
     Features described as fluid metering devices in membrane separation system  100  (e.g., water metering device  108 , air metering device  110 ), may be any device that controls delivery of fluid to generate feed stream  114  for supply to housing  104 . For example, each metering device may be a pumping mechanism that receives fluid on a draw side, pressurizes, the fluid, and discharges the pressurized fluid at an increased pressure. Example pumps may comprise a peristaltic pump or other form of continuous pump, a positive-displacement pump, an air compressor, or any other type of pump appropriate for the particular application. Additionally or alternatively, one or more metering devices may be implemented as a valve or other fluid control device (e.g., when fluid from water source  112  and/or air source  115  is already pressurized or is drawn through housing  104  by a suction pump positioned downstream of the housing). 
     The source of air  115  may be ambient air that is drawn into a compressor or other pumping device for supply to housing  104 . Alternatively, the source of air  115  may be a reservoir (e.g., tank) that stores pressurized gas for delivery to housing  104 . While membrane separation system  100  is generally described as using air (e.g. approximately 78% nitrogen, approximately 21% oxygen, and approximately 1% argon), the system may operate with other gases without departing from the scope of the disclosure. Example gases that may be used include nitrogen, carbon dioxide, oxygen, and the like. 
     In the example of  FIG. 1 , permeate stream  116  generated by membrane separation system  100  is supplied to automated water analysis device  200 . This configuration may be useful to implement an online automated water analysis system that can operate without user intervention for an extended period of time (e.g., at least two weeks, at least one month, at least two months, at least six months, at least one year). By operating membrane separation system  100  with a water-in-air feed stream as described herein, the water-in-air feed stream may reduce or eliminate fouling on membrane  102  during extended service. This can allow the system to operate for an extended period of time without necessitating user intervention. In these applications, membrane separation system  100  may be a comparatively small-sized system generating a comparatively small permeate stream for downstream analysis, such as a system generating less than 100 Liters/min, such as less than 10 L/min, less than 5 L/min, less than 2 L/min, less than 1 L/min, or less than 0.5 L/min. 
     In other implementations, the systems and techniques of the present disclosure may be used to generate a permeate stream  116  intended for downstream applications other than analysis by an automated water analysis device. For example, permeate stream  116  may be used personal consumption, industrial processes, and/or any other desired application. In these implementations, the volume of water processed by membrane separation system  100  may be greater than in a comparatively smaller system feeding automated water analysis device  200 . As a result, the amount of air needed to generate the water-in-air feed stream may be particularly large. The increased energy costs associated with supplying air to these larger scale applications may limit deployment to select applications, but this does not impact the technical feasibility of the applications. 
     The techniques described in this disclosure may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the described techniques may be implemented within one or more processors, including one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. The term “processor” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. A control unit comprising hardware may also perform one or more of the techniques of this disclosure. 
     Such hardware, software, and firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     The techniques described in this disclosure may also be embodied or encoded in a computer-readable medium, such as a non-transitory computer-readable storage medium, containing instructions. Instructions embedded or encoded in a computer-readable storage medium may cause a programmable processor, or other processor, to perform the method, e.g., when the instructions are executed. Non-transitory computer readable storage media may include volatile and/or non-volatile memory forms including, e.g., random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, a hard disk, a CD-ROM, a floppy disk, a cassette, magnetic media, optical media, or other computer readable media. 
     The following examples may provide additional details about membrane separation systems and the anti-fouling efficacy of a water-in-air feed stream according to the disclosure. 
     Example: Comparison Between Water-In-Air and Air-In-Water Feed Streams 
     An example was performed to study the effect of operating a membrane separation system using a water-in-air feed stream (containing a greater volume of air than water) compared to operating using an air-in-water feed stream (containing a greater volume of water than air). The feed streams were prepared by obtaining water from an operating cooling water tower. The feed water had an unspecified quantity of organic and inorganic colloids. 
     Two different membrane modules were used to study the water-in-air and air-in-water feed streams. The first module used to study the water-in-air feed stream was a comparatively small membrane module having an inner diameter of 12 mm. The second module used to study the air-in-water feed stream was a comparatively large membrane module having an inner diameter of 25 mm. Both modules were operated under constant pressure mode by maintaining a constant transmembrane pressure of approximately 5 kPa. Table 1 shows the operating conditions for the two membrane modules supplied with water-in-air and air-in-water feed streams, respectively. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Operating conditions for experimental membrane modules. 
               
            
           
           
               
               
               
            
               
                   
                 Water-in-Air 
                 Air-in-Water 
               
               
                   
                 feed stream 
                 feed stream 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Membrane Surface Area (cm 2 ) 
                 335 
                 1,000 
               
               
                   
                 Air Flow (L/min) 
                 1-2 
                 ~0.5 
               
               
                   
                 Water flow (L/min) 
                 ~0.18 
                 0.5-1.0 
               
               
                   
                 Trans-membrane pressure (kPa) 
                  5-10 
                  5-10 
               
               
                   
                 Membrane module ID (mm) 
                 12 
                 25 
               
               
                   
                   
               
            
           
         
       
     
     Because the membrane module supplied with the water-in-air feed stream has one third the surface area of the membrane module suppled with the air-in-water feed stream, it was expected that the module supplied with the air-in-water feed stream would produce more permeate. In fact, the membrane module supplied with the water-in-air feed stream produced much more permeate. 
     The permeate flow rates produced from the two membrane modules were normalized against membrane surface areas to allow more accurate comparison.  FIG. 3  is plot showing normalized permeate flow rate versus time using a water-in-air feed stream compared to an air-in-water feed stream. The data show that the water-in-air feed stream performs an average of 8 times better than the air-in-water feed stream under the same operating conditions.