Method for dosing antiscalant into a membrane-based water treatment system

A dosing pump (19) doses antiscalant into a membrane-based water treatment system (1). The dosing pump (19) includes a displacement body for pumping antiscalant into the membrane-based water treatment system (1) in doses. A motor drives the displacement body. A control module controls the motor. The control module is configured to vary the dosage of antiscalant pumped into the water treatment system (1) based on a temperature corrected system variable (SVTc) being based on a plurality of operating variables of the water treatment system (1).

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 of European Application 17 196 291.3, filed Oct. 13, 2017, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to a dosing pump for dosing antiscalant into a membrane-based water treatment system, a dosing system comprising such a pump and a method for dosing antiscalant into a membrane-based water treatment system. The membrane-based water treatment system may in particular be a reverse osmosis (RO) desalination system. However, the dosing pump, dosing system and method described herein may be used in any membrane-based desalination and water treatment application. This includes, but is not limited to municipal and industrial desalination, municipal water treatment, industrial water treatment, groundwater filtration, drinking water filtration, surface water filtration, high purity water, cooling water treatment, water reuse, brackish water filtration, seawater desalination, produced water and others.

BACKGROUND

Membrane-based water desalination and water treatment with reverse osmosis (RO) or nanofiltration (NF) is a popular technology that greatly contributes in solving the problem of water scarcity and water pollution in different countries regardless of geographical location. RO is a membrane technology that is well established and widely used in membrane-based water desalination (e.g. seawater, brackish water, treated wastewater, impaired waters, produced water and others) and water treatment (e.g. surface water, groundwater, water reuse).

A widely used performance indicator for such systems is “recovery” or “yield” being defined as the ratio between the outflow of permeate, i.e. water produced by the system, and the inflow of feed, i.e. water conveyed to the system. The water discharged by the system is referred to as concentrate or brine. Membrane-based water treatment systems may comprise one or more membranes. In a multiple-membrane system, the membranes may be arranged in series and/or in parallel, wherein the concentrate of membrane i may serve as feed to the subsequent membrane i+1.

The system recovery is always lower than 100%, and it depends on the salt content of the feed water and other factors like membrane scaling. Scaling is a type of membrane fouling that blocks the membrane and reduces its filtration and water diffusion performance. Scaling occurs when one or more scale forming salt species exceed their solubility limits, supersaturate and precipitate. The increased concentration of scale forming species occurs when the recovery (permeate water/feed water) increases, which enhances the concentration polarization in the region next to the membrane surface on the feed side. Therefore, the recovery is limited by scaling.

Furthermore, membrane fouling, particularly scaling, deteriorates membrane performance, which results in limited membrane life, higher use of pressure and thus energy, more frequent cleaning of the membrane and system downtime. The consequences are costly operating expenses and limited system performance. Scaling is a phenomenon where precipitation of minerals/salts in solution causes them to form nuclei that promote crystallization (mineral growth) and ultimately produce scaling (fouling) onto the membrane. Precipitation starts when the activity of ions reaches their saturation limit and the solution becomes supersaturated. The risk or extent of scaling can be reduced or even prevented by using a chemical additive known as antiscalant. Antiscalants are commercially available chemical products designed for being added into the feed water stream of a membrane system against scaling. Antiscalants usually come with a dosage recommendation by the antiscalant manufacturer to achieve a target recovery.

However, the most efficient dose of an antiscalant is highly application and system dependent. The antiscalant manufacturers' dosage recommendations are often overestimated to guarantee a promised target recovery in a wide range of applications and systems. Apart from an ineffective overconsumption of costly antiscalant, such overdosing of antiscalant creates operational problems, for instance a membrane's ability to be fully recovered by a cleaning-in place (CIP) can be negatively affected by an over-dosage of antiscalant. Overdosed and thus ineffective antiscalant may end up in the concentrate stream being discharged into an external water body or a sewer drain. This creates problems for water authorities and personal responsible for the wastewater treatment plant and constitutes an environmental pollution of the water receiving body (natural or wastewater treatment plant). As a result, the membrane-based water treatment system may be shut down by authorities or penalties may be imposed on operators of the system.

SUMMARY OF THE INVENTION

The dosing pump, dosing system and method disclosed herein provide a solution for dosing the least amount and/or flow of antiscalant necessary to achieve a target recovery in any membrane-based water treatment system it is used in, and thus to minimise or prevent overdosing of antiscalant.

In accordance with a first aspect of the present disclosure, a dosing pump is provided for dosing antiscalant into a membrane-based water treatment system. The dosing pump comprisesa displacement body or impeller for pumping antiscalant into a membrane-based water treatment system in a dosed manner,a motor for driving the displacement body or impeller, anda control module for controlling the motor,wherein the control module is configured to vary the dosage of antiscalant pumped into the water treatment system based on a temperature corrected system variable (SVTc) being based on a plurality of operating variables of the water treatment system.

The term “membrane-based water treatment system” shall herein encompass any form of reverse osmosis (RO), forward osmosis (FO), membrane distillation (MD), electrodialysis (ED), nanofiltration (NF), microfiltration (MF), or ultrafiltration (UF) system, in particular desalination system, using membrane technology, e.g. graphene and carbon nanotubes membrane, ion exchange membrane, electrodialysis reversal (EDR) membrane, capacitive de-ionization membrane (CDI) and other types of membrane used in desalination and water treatment. In general, any membrane-based desalination and water treatment technology (e.g. RO, NF, FO, MD, ED, EDR, CDI) involving the use of antiscalants can be areas of application of the present disclosure.

The term “antiscalant” shall mean herein any form of chemical additive effective to reduce the risk or extent of membrane scaling, in particular membrane fouling. Antiscalants may for example be chelating agents, delay agents and/or dispersants. Chemical formulations used in antiscalants may be classified into solubility modifiers (e.g. polyphosphates, phosphonates, phosphates esters, polyacrylates, EDTA), crystal modifiers (e.g. polymaleic acid, sulfonated polystyrene), surfactants used for dispersion (e.g. metal sulphonates, metal phenolates, fatty acid phosphates), and others. Antiscalants may comprise combinations and/or blends of such formulations.

The term “dosage” of antiscalant shall mean herein an amount and/or flow of antiscalant. The dosing pump may pump antiscalant into the membrane-based water treatment system in a dosed manner by regulating a continuous flow of antiscalant into the membrane-based water treatment system. Alternatively, the flow of antiscalant into the membrane-based water treatment system may be discontinuous in regular or irregular intervals, wherein the dosage may be varied by the interval rate and/or the amount of antiscalant per interval. The dosing pump is preferably a displacement pump with a displacement body for displacing defined portions of water in order to be able to control the pumped dosage by controlling the number and/or frequency of displaced portions.

The term “temperature corrected system variable” (SVTc) may be defined as any system variable that is dependent on the scaling of the membrane and that includes a temperature correction. For instance, the temperature corrected system variable (SVTc) may be a temperature corrected net driving pressure (NDPTc). The term “net driving pressure” (NDP) may be defined herein as:

NDP=Pf-Δ⁢⁢Pfc2-Pp-πfc+πp,
wherein Pfdenotes the feed pressure, ΔPfcis the difference between the feed pressure and the concentrate pressure, Ppis the permeate pressure, πfcis the feed-concentrate osmotic pressure, and πpis the permeate osmotic pressure. The temperature corrected net driving pressure NDPTc may be defined as: NDPTc=NDP·TCF, wherein TCF is a temperature correction factor being a function of a membrane-specific temperature correction constant Ctand the feed temperature Tf: TCF=f(Ct, Tf).

The temperature corrected net driving pressure (NDPTc) is a preferred choice of the temperature corrected system variable (SVTc) in case the water treatment system is operated at an essentially constant permeate flow. If the throughput through the membrane drops due to scaling and fouling, a feed pump speed or power may be increased to achieve a constant permeate flow. The scaling and fouling of the membrane will then show as an increase in the temperature corrected net driving pressure (NDPTc).

An alternative choice of the temperature corrected system variable (SVTc) may be a temperature corrected permeate flow (PFTc) being defined herein as:

PFTc=QpTCF,
wherein Qpdenotes the permeate flow and TCF is the same temperature correction factor as described above. If Qfis 10.0 m3/h and TCF=0.747, then PTFc would be 13.39 m3/h.

The temperature corrected permeate flow (PFTc) is a preferred choice of the temperature corrected system variable (SVTc) in case the water treatment system is operated at an essentially constant feed pressure and/or with a constant feed pump speed/power. If the throughput through the membrane drops due to scaling and fouling, the permeate flow drops and the scaling and fouling of the membrane will then show as a decrease in the temperature corrected permeate flow (PFTc). The dependency of PFTc from the scaling and fouling is thus inversely to the dependency of NDPTc from the scaling and fouling.

There are more options for temperature corrected system variable (SVTc) than NDPTc and PFTc. For instance, a temperature corrected water permeability (KWTc) being defined as

KWTc=PFTcNDPTc·Am
may serve as SVTc, wherein Amis the total membrane area. KWTc falls with scaling like PFTc. If PTFc was 13.39 m3/h, NDPTc was 5.57 bar and Amwas 600 m2, then KWTc would be 0.004 m/(bar·h).

Another example for a suitable SVTc may be a temperature corrected membrane resistance RMTc being defined as

RMTc=NDPTc·Am·105PFTc·μrt,
wherein μrtis the water dynamic viscosity at a reference temperature. RMTc rises with scaling like NDPTc. If PTFc was 13.39 m3/h, NDPTc was 5.57 bar, Amwas 600 m2, and μrtwas 8.9·10−4N·S/m2, then RMTc would be 2.3 10−11m−1. KWTc and RMTc are examples for a combination of NDPTc and PTFc.

In the following, where SVTc is assumed to be NDPTc, the skilled reader will readily understand that PFTc, KWTc or RMTc could alternatively serve as SVTc, wherein PFTc and KWTc rise with scaling and NDPTc and RMTc fall with scaling.

A low value of NDPTc (and thus KWTc) and a high value of PTFc (and thus RMTc) are in principle desirable to achieve a high recovery. However, the value of NDPTc increases over time due to scaling, i.e. NDPTc may show a rising slope over time. During the development of the dosing pump described herein, investigations have revealed an unexpected correlation between the dosage of antiscalant and the temperature corrected net driving pressure NDPTc and/or the temperature corrected permeate flow PFTc. It was shown that the slope of NDPTc, i.e. the first time derivative of NDPTc, i.e. ΔNDPTc/Δt, not only increases when the dosage is below a minimal dosage, but also increases when the dosage is above a maximum dosage. Analogously, the slope of PFTc, i.e. the first time derivative of PFTc, i.e. ΔPFTc/Δt, was shown not only to decrease when the dosage is below a minimal dosage, but also to decrease when the dosage is above a maximum dosage. Furthermore, the investigations have shown that, within a relatively wide range of antiscalant dosage between the minimal dosage and the maximum dosage, the antiscalant dosage has almost no influence on the slope of SVTc, i.e. ΔSVTc/Δt. Thus, it is desirable to operate as close as possible to the minimal dosage in order not to waste ineffective (and in excess even harmful) antiscalant. The control module of the present dosing pump is thus configured to make use of this finding by varying the dosage of antiscalant in dependence of NDPTc and/or ΔNDPTc/Δt, and thereby optimizing the antiscalant consumption to a minimum. In case of PFTc as SVTc, the dosage of antiscalant is varied in dependence of PFTc and/or ΔPFTc/Δt, and thereby optimizing the antiscalant consumption to a minimum.

Optionally, the control module may be configured to receive and/or determine the temperature corrected system variable SVTc. Thus, the calculation of SVTc may be performed by the control module or by a calculation module being remotely located from the dosing pump. The decision algorithm to determine whether at all and/or by how much the dosage of antiscalant is reduced may be also performed by the control module and/or remotely by a calculation module. If SVTc is calculated remotely and the decision algorithm for dosage change is performed within the control module, the control module may receive the calculated SVTc from a remote calculation module. Alternatively or in addition, the decision algorithm may at least partially be performed remotely, wherein the control module may be configured to receive a command whether at all and/or by how much the dosage of antiscalant is to be changed.

Optionally, the plurality of operating variables of the water treatment system may comprise at least one of the group consisting of: feed electrical conductivity, feed temperature, feed pH, difference between feed pressure and concentrate pressure, permeate pressure, permeate temperature and permeate electrical conductivity. In case of a multiple-membrane system with multiple membrane vessels in series, the difference between feed pressure and concentrate pressure may be determined at the last membrane vessel in the series. Preferably, the operating variables feed electrical conductivity, feed temperature and the difference between feed pressure and concentrate pressure at the last membrane vessel are at least used to determine SVTc. A plurality of at least four sensors, i.e. a feed electrical conductivity sensor, a feed temperature sensor, a feed pressure sensor at the last membrane vessel and a concentrate pressure sensor at the last membrane vessel, may provide these operating variables for determining SVTc. An additional feed pH sensor may be used to monitor the feed for major changes in the chemical composition of the feed. A plurality of three more sensors, i.e. a permeate electrical conductivity sensor, a permeate temperature sensor and a permeate pressure sensor may provide more operating variables for determining NDPTc more accurately. A permeate flow sensor may be useful to measure the permeate flow for the determination of PFTc. SVTc may be determined in a calculation module being remotely located from the dosing pump and in signal communication with the control module and the plurality of sensors. Alternatively or in addition, SVTc may be determined by the control module itself being in signal communication with the plurality of sensors. The signal communication may be via cable connection or wireless with corresponding transmitters and receivers. The control module and/or the calculation module may be in one-way or two-way communication with a database and/or computing cloud comprising one or more processors and servers.

In case the information of one or more of the permeate sensors, e.g. the permeate electrical conductivity sensor, the permeate temperature sensor and/or the permeate pressure sensor, is not available, the permeate pressure Ppand/or the permeate osmotic pressure πpmay be approximated to be a constant C or zero. In this situation, the temperature corrected net driving pressure may be determined as

NDPTc=TCF·(Pf-Δ⁢⁢Pfc2-πfc-C)
based on the operating variables feed pressure Pf, feed electrical conductivity γf, feed temperature Tfand the difference between feed pressure and concentrate pressure at the last membrane vessel ΔPfc.

The variables feed electrical conductivity γfand feed temperature Tfare used to determine the temperature correction factor TCF and the feed-concentrate osmotic pressure πfc. The formula for the temperature correction factor TCF depends on the membrane type and may thus be provided by the membrane manufacturer. A plurality of TCF formula options may be stored in a database for a plurality of membrane types and products. The correct TCF formula may be selected from the database dependent on the membrane type used in the water treatment system. For instance, TCF for a composite membrane at a reference temperature of 25° C. may be given by:

The osmotic pressure π may be derived of the van't Hoff formula π=R·T·ϕ·Σiαici, wherein R is the universal gas constant, T is the temperature in ° K, ϕ is the osmotic coefficient, αiis the activity coefficient for ionic species i and ciis the concentration of ionic species i. The feed-concentrate osmotic pressure πfcmay be deduced therefrom to be:

πfc=2.654·Tf·cfc106-cfc,
wherein the actual feed temperature Tfis input in ° K and cfcis the concentration of salts in the feed-concentrate. The concentration of salts in the feed-concentrate cfccan be derived from the concentration of salts in the feed cfby making use of the known total recovery Rec:

cfc=-CP·cf·ln⁡(1-Rec)Rec,
wherein CP is the water-dependent concentration polarization factor typically in the range of 1 to 2, e.g. CP=1.1 for low brackish groundwater. The concentration of salts in the feed cfin units of mg/l may be determined by using the measurement of the feed electrical conductivity γfin units of μS/cm: cf=0.76·γf−3.07. For example, if γfis 1500 μS/cm, cfis 1136.93 mg/l. If the total recovery is 70% for an application in low brackish groundwater, i.e. CP=1.1, the concentration of salts in the feed-concentrate cfcis 2151 mg/l, which results in a feed-concentrate osmotic pressure πfcof 1.65 bar.

NDP and NDPTc are now available assuming zero permeate pressure Ppand zero permeate osmotic pressure πp. If the measured permeate pressure Ppand the measured permeate electrical conductivity γpare available from respective sensors, however, the net driving pressure NDP and thus the temperature corrected net driving pressure NDPTc may be more precisely determined by adding the measured permeate pressure Ppto NDP and subtracting the permeate osmotic pressure πpfrom NDP. As the concentration of salts in the permeate is in general quite low, the osmotic pressure πpin units of bar may be derived directly from the measured permeate electrical conductivity γpin units of μS/cm by: πp=7.49·10−4·γp−0.19·10−3. For instance, if the permeate electrical conductivity γpis 13 μS/cm, πpwould be 0.01 bar.

Thus, if the feed pressure sensor shows 9.41 bar, the concentrate pressure sensor shows 9.11 bar, the permeate pressure sensor shows 0.03 bar, the feed electrical conductivity sensor shows 1500 μS/cm, the permeate electrical conductivity sensor shows 13 μS/cm and the feed temperature sensor shows 15° C., NDP would be 7.59 bar and NDPTc would be 5.57 bar for a membrane with TCF=0.734 at 15° C.

Optionally, the control module may be configured to recursively adapt the dosage of antiscalant pumped into the water treatment system based on a previously determined temperature corrected system variable SVTc as feedback value. Alternatively or in addition, the control module may be configured to adapt the dosage of antiscalant pumped into the water treatment system upon a change of the slope of the temperature corrected system variable (ΔSVTc/Δt). The control module may thus be configured to perform a closed-loop regulation of the dosage of antiscalant with SVTc and/or ΔSVTc/Δt as feedback value. The slope of SVTc, i.e. ΔSVTc/Δt, may be approximated by determining SVTc(t1) at a first point in time t1and SVTc(t2) at a second point in time t2and calculating the differential ΔSVTc=SVTc(t2)−SVTc(t1) for the time interval Δt=t2−t1.

Optionally, the control module may be configured to recursively reduce the dosage of antiscalant pumped into the water treatment system as long as the slope of the temperature corrected net driving pressure (ΔNDPTc/Δt) does not increase and/or as long as the slope of the temperature corrected permeate flow (ΔPFTc/Δt) does not decrease. A recursion loop may be repeated after the time interval Δt and ΔSVTc/Δt of the current loop may be compared with ΔSVTc/Δt of the previous loop. If ΔSVTc/Δt of the current loop k is within limits, e.g. 1%, the same as ΔSVTc/Δt of the previous loop k−1, ΔSVTc/Δt has not changed. The limits may be pre-determined and/or set by a user and/or depend on the accuracy and precision at which SVTc is determined. The amount by which the dosage of antiscalant is reduced per recursion loop may be a pre-determined amount and/or set by the user and/or dependent on the chosen time interval Δt used for determining ΔSVTc/Δt. The longer the chosen time interval Δt is, the larger can the amount of reduction per recursion loop be and vice versa. The time interval Δt may be pre-determined and/or set by a user and/or automatically adapted to the volatility of SVTc and/or ΔSVTc/Δt. The higher the volatility of SVTc and/or ΔSVTc/Δt is, the shorter the time interval Δt may be chosen and vice versa. In case of statistical fluctuations of SVTc, SVTc may be sampled to an average value of SVTc over a certain number of samples or low-pass filtered. For instance, if SVTc is sampled at a sampling rate of 1 Hz over a sliding window of 16 seconds, 16 values of SVTc may be recorded for the sliding window. The first 12 samples may be averaged to SVTc1of a first window part and the last 4 samples may be averaged to SVTc2of a second window part. ΔSVTc/Δt may then be determined as (SVTc2−SVTc1)/8 s.

Optionally, the control module may be configured to increase the dosage of antiscalant pumped into the water treatment system if the slope of the temperature corrected net driving pressure ΔNDPTc/Δt has increased and/or if the slope of the temperature corrected permeate flow ΔPFTc/Δt has decreased. As above, a recursion loop may be repeated after the time interval Δt, and ΔSVTc/Δt of the current loop with the current dosage may be compared with ΔSVTc/Δt of the previous loop with the previous dosage. If ΔNDPTc/Δt of the current loop is above a limit, e.g. 1.01·ΔNDPTc/Δt of the previous loop, ΔNDPTc/Δt has increased. Analogously, if ΔPFTc/Δt of the current loop is below a limit, e.g. 0.99·ΔPFTc/Δt of the previous loop, ΔPFTc/Δt has decreased. The control module thus increases the dosage for the subsequent recursion loop. As above, the limits may be pre-determined and/or set by a user and/or depend on the accuracy and precision at which SVTc is determined. Also, the amount ΔD by which the dosage of antiscalant is increased for the next recursion loop may be a pre-determined amount and/or set by the user and/or dependent on the chosen time interval Δt used for determining ΔSVTc/Δt. The longer the chosen time interval Δt is, the larger can the amount ΔD of reduction per recursion loop be and vice versa. The time interval Δt may be pre-determined and/or set by a user and/or automatically adapted to the volatility of SVTc and/or ΔSVTc/Δt. The higher the volatility of SVTc and/or ΔSVTc/Δt is, the shorter the time interval Δt may be chosen and vice versa. The values of Δt and/or the amount ΔD by which the dosage is adapted may be the same or different between the case when ΔNDPTc/Δt has not increased and the case when ΔNDPTc/Δt has increased.

Optionally, the control module may be configured to recursively increase the dosage of antiscalant pumped into the water treatment system as long as the slope of the temperature corrected net driving pressure (ΔNDPTc/Δt) is above an initial slope (ΔNDPTc0/Δt) and/or as long as the slope of the temperature corrected permeate flow (ΔPFTc/Δt) is below an initial slope (ΔPFTc0/Δt), e.g. the initial slope determined when operating with a recommended dosage of antiscalant.

The dosing pump as described above may be applied in a water treatment system being already equipped with a plurality of sensors used for other purposes. Quite often, feed pressure, feed pH, feed temperature, feed electrical conductivity and concentrate pressure are monitored for surveillance purposes in water treatment systems anyway. Such sensors may be used to determine the operating variables on which the determination of SVTc is based. The control module may simply be in communication with the sensors to determine SVTc from the operating variables or may receive an already determined SVTc from a calculation module in order to adapt the dosage of antiscalant accordingly. Alternatively or in addition, a calculation module may also perform the decision algorithm for adapting the dosage of antiscalant and send a respective command for increase/reduction of the dosage to the control module.

However, according to a second aspect of the present disclosure, a dosing system for dosing antiscalant into a membrane-based water treatment system is provided, wherein the dosing system comprisesa dosing pump as described above, anda plurality of sensors for determining a plurality of operating variables of the water treatment system,
wherein the control module is configured to vary the dosage of antiscalant pumped into the water treatment system based on a temperature corrected system variable SVTc being based on a plurality of operating variables of the water treatment system determined by the plurality of sensors.

The dosing system thus comprises the sensors required to determine the operating variables on which the determination of SVTc is based. This is in particular useful if the water treatment system is not yet equipped with such a plurality of sensors.

Optionally, the dosing system may comprise a calculation module configured to determine the temperature corrected system variable SVTc. The calculation module may be implemented on a local or remote programmable logic controller (PLC) or programmable logic relay (PLR) for processing the sensor signals and calculating SVTc. The calculation module may be configured to receive the plurality of operating variables of the water treatment system from the plurality of sensors. Thus, the calculation module may be remotely located from the dosing pump and in signal communication with the control module and the plurality of sensors. The calculation module may be configured to calculate SVTc and to perform the decision algorithm whether at all and/or by how much the dosage of antiscalant is to be changed. The calculation module may then command the control module to increase or decrease the dosage of antiscalant accordingly.

According to a third aspect of the present disclosure, a method for dosing antiscalant into a membrane-based water treatment system is provided, the method comprising the steps of:determining a temperature corrected system variable SVTc based on a plurality of operating variables of the water treatment system,varying the dosage of antiscalant fed into the water treatment system based on the temperature corrected system variable SVTc.

Optionally, the plurality of operating variables of the water treatment system may comprise at least one of the group consisting of: feed electrical conductivity, feed temperature, feed pH, difference between feed pressure and concentrate pressure, permeate pressure, permeate temperature and permeate electrical conductivity.

Optionally, the step of varying the dosage may comprise recursively adapting the dosage of antiscalant fed into the water treatment system based on a previously determined temperature corrected net system variable SVTc as feedback value.

Optionally, the step of varying the dosage may comprise adapting the dosage of antiscalant fed into the water treatment system upon a change of the slope of the temperature corrected system variable (ΔSVTc/Δt).

Optionally, the temperature corrected system variable (SVTc) may be a temperature corrected net driving pressure (NDPTc) and/or a temperature corrected permeate flow (PFTc) and/or a combination thereof.

Optionally, the step of varying the dosage may comprise recursively reducing the dosage of antiscalant fed into the water treatment system as long as the slope of the temperature corrected net driving pressure (ΔNDPTc/Δt) does not increase and/or as long as the slope of the temperature corrected permeate flow (ΔPFTc/Δt) does not decrease.

Optionally, the step of varying the dosage may comprise increasing the dosage of antiscalant fed into the water treatment system if the slope of the temperature corrected net driving pressure (ΔNDPTc/Δt) has increased and/or if the slope of the temperature corrected permeate flow (ΔPFTc/Δt) has decreased.

Optionally, the step of varying the dosage may comprise recursively increasing the dosage of antiscalant pumped into the water treatment system as long as the slope of the temperature corrected net driving pressure (ΔNDPTc/Δt) is above an initial slope (ΔNDPTc0/Δt) and/or as long as the slope of the temperature corrected permeate flow (ΔPFTc/Δt) is below an initial slope (ΔPFTc0/Δt), e.g. the initial slope determined when operating with a recommended dosage of antiscalant.

Optionally, the method may further comprise a step of monitoring the temperature corrected net driving pressure NDPTc against a maximum threshold value (NDPTcmax) and/or monitoring the temperature corrected permeate flow PFTc against a minimum threshold value (PFTcmin), and a step of requesting a cleaning-in-place (CIP) of a membrane if the temperature corrected net driving pressure NDPTc exceeds the maximum threshold value (NDPTcmax) and/or if the temperature corrected permeate flow PFTc falls below the minimum threshold value (PFTcmin).

Optionally, the method may further comprise a step of monitoring the feed temperature against a minimum threshold value (Tf,min) and a maximum threshold value (Tf,max), a step of monitoring the feed pH against a minimum threshold value (pHf,min) and a maximum threshold value (pHf,max), and a step of resetting the dosage of antiscalant to an initial value, e.g. a recommended dosage by the antiscalant manufacturer, if the feed temperature and/or the feed pH exceeds the respective maximum threshold value or falls below the respective minimum threshold value.

Some or all of the steps of the method described above may be implemented in form of compiled or uncompiled software code that is stored on a computer readable medium with instructions for executing the method. Alternatively or in addition, some or all method steps may be executed by software in a cloud-based system, in particular the calculation module may be partly or in full implemented in a cloud-based system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1shows a schematic illustration of a membrane-based water treatment system1in form of a reverse osmosis desalination system comprising a membrane3in a vessel5with an inlet port7, an outlet port9and a discharge port11. It should be noted that the membrane3in a vessel5may be a system of a plurality of membranes in vessels arranged in parallel and/or in series. The inlet port7is in fluid connection with a feed line13, the outlet port is in fluid connection with a permeate line15and the discharge port11is in fluid connection with a concentrate line17. Salt water or brackish water to be desalinated is fed by a feed pump18into the vessel5via the feed line13and desalinated fresh water is output into the permeate line15. Concentrate, also referred to as brine, having a high concentration of salt is discharged through the concentrate line17. Over time and usage of the system1, the membrane3may show scaling and fouling reducing the system performance.

Therefore, the membrane-based water treatment system1is equipped with a first embodiment of a dosing system for dosing antiscalant into the feed line13of the membrane-based water treatment system1. The dosing system comprises a dosing pump19for dosing antiscalant from an antiscalant reservoir21into the feed line13. An inlet port23of the dosing pump19is in fluid connection with an intake port20within the antiscalant reservoir21and an outlet port25of the dosing pump19is in fluid connection with the feed line13. A check valve27before the feed line13prevents water pressure in the feed line13from pushing back antiscalant towards the dosing pump19. Another check valve28upstream the inlet port23of the dosing pump19and downstream of the intake port20prevents antiscalant pressure in the reservoir21from pushing antiscalant towards the dosing pump19. The filling level of antiscalant in the reservoir21is monitored by a fluid level sensor29controlling an angled valve31downstream of the outlet port25of the dosing pump19via a signal line33. The angled valve31may be shut if the level of antiscalant in the reservoir21falls below a minimum threshold.

The dosing system further comprises a plurality of sensors35, i.e. eight sensors, for determining a plurality of operating variables, i.e. eight variables, of the water treatment system1. The feed line13is equipped with a feed electrical conductivity sensor35a, a feed temperature sensor35b, a feed pH sensor35cand a pressure sensor35d. The feed line13is further equipped with a feed flow sensor37afor measuring the flow in the feed line13. The feed electrical conductivity sensor35a, the feed temperature sensor35b, the feed pH sensor35cand the feed flow sensor37aare located upstream of the feed pump18, whereas the feed pressure sensor35dis located downstream of the feed pump18and upstream of the vessel5. A concentrate pressure sensor35eis located downstream the discharge port11at the discharge line17for determining the concentrate pressure Pc. The permeate line15is equipped with a permeate pressure sensor35f, a permeate temperature sensor35gand a permeate electrical conductivity sensor35hof the dosing system. Furthermore, the permeate line15is equipped with a permeate flow sensor37b, which may be used to determine a temperature corrected permeate flow PFTc. The permeate pressure sensor35fis located closest to the vessel5downstream the outlet port9. The eight sensors35of the dosing system are in signal communication with a control module, comprising one or more processors, of the dosing pump19via wireless or cabled signal line39with associated transmitters and receivers. The control module of the dosing pump19is configured to receive and process the operating variables provided by the sensors35via signal line39. The control module of the dosing pump19determines here a temperature corrected net driving pressure NDPTc as temperature corrected system variable SVTc based on the plurality of received operating variables and varies the dosage of antiscalant fed into the feed line13based on the determined NDPTc.

The control module of the dosing pump19determines a net driving pressure (NDP) by:

NDP=Pf-Δ⁢⁢Pfc2-Pp-πfc+πp,
wherein Pfdenotes the feed pressure, ΔPfcis the difference between the feed pressure Pfand the concentrate pressure Pc, Ppis the permeate pressure, πfcis the feed-concentrate osmotic pressure, and πpis the permeate osmotic pressure. NDPTc may then be determined by: NDPTc=NDP·TCF, wherein TCF is a temperature correction factor being a function of a membrane-specific temperature correction constant Cfand the feed temperature Tf: TCF=f(Ct, Tf).

The output of the feed pressure sensor35d, i.e. feed pressure Pf, and of the concentrate pressure sensor35e, i.e. concentrate pressure Pc, are combined to the differential pressure ΔPfc=Pf−Pcas one of the operating variables for determining NDPTc. The variables feed electrical conductivity γfand feed temperature Tfare used to determine the temperature correction factor TCF and the feed-concentrate osmotic pressure πfc. The temperature correction factor TCF depends on the membrane type and is thus provided by the membrane manufacturer. A plurality of TCF formula options is stored in a database for a plurality of membrane types and products. The correct TCF formula may be selected from the database dependent on the membrane type used in the water treatment system. For instance, TCF for a composite membrane may be given as:

The osmotic pressure π may be derived of the van't Hoff formula π=R·T·ϕ·Σiαici, wherein R is the universal gas constant, T is the temperature in ° K, ϕ is the osmotic coefficient, a; is the activity coefficient for ionic species i and ciis the concentration of ionic species i. The feed-concentrate osmotic pressure πfcmay be deduced therefrom to be:

πfc=2.654·Tf·cfc106-cfc,
wherein the actual feed temperature Tfis input in ° K and cfcis the concentration of salts in the feed-concentrate. The concentration of salts in the feed-concentrate cfccan be derived from the concentration of salts in the feed cfby making use of the total recovery Rec:

cfc=-CP·cf·ln⁡(1-Rec)Rec,
wherein CP is the water-dependent concentration polarization factor typically in the range of 1 to 2, e.g. CP=1.1 for low brackish groundwater. Recovery Rec=Qp/Qfmay be assumed to be a given fixed nominal target value, typically in the range of 30% to 90%, or may be determined from the measurements of the flow sensors37a,b.

The concentration of salts in the feed cfin units of mg/l may be determined by using the measurement of the feed electrical conductivity γfin units of μS/cm: cf=0.76·γf·3.07. For example, if γfis 1500 μS/cm, cfis 1136.93 mg/l. If the total recovery is 70% for an application in low brackish groundwater, i.e. CP=1.1, the concentration of salts in the feed-concentrate cfcis 2151 mg/l, which results in a feed-concentrate osmotic pressure πfcof 1.65 bar.

NDP and NDPTc may now already be available if the permeate pressure Ppand zero permeate osmotic pressure πpcan be assumed to be constants or zero in case no permeate sensors35f-hwere available. However, the measured permeate pressure Ppand the measured permeate electrical conductivity γpare available from respective sensors35f-hin the shown example. The net driving pressure NDP and thus the temperature corrected net driving pressure NDPTc may be more precisely determined by adding the measured permeate pressure Ppto NDP and subtracting the permeate osmotic pressure πpfrom NDP. As the concentration of salts in the permeate is generally quite low, the osmotic pressure πpin units of bar may be derived directly from the measured permeate electrical conductivity γpin units of μS/cm by: πp=7.49·10−4·γp−0.19·10−3. For instance, if the permeate electrical conductivity γpis 13 μS/cm, πpwould be 0.01 bar.

Thus, if the feed pressure sensor shows 9.41 bar, the concentrate pressure sensor shows 9.11 bar, the permeate pressure sensor shows 0.03 bar, the feed electrical conductivity sensor shows 1500 μS/cm, the permeate electrical conductivity sensor shows 13 μS/cm and the feed temperature sensor shows 15° C., NDP would be 7.59 bar and NDPTc would be 5.57 bar for a membrane with TCF=0.734 at 15° C.

An alternative choice of the temperature corrected system variable (SVTc) may be a temperature corrected permeate flow (PFTc) being defined herein as:

PFTc=QpTCF,
wherein Qpdenotes the permeate flow measured by permeate flow sensor37band TCF is the same temperature correction factor as described above. If Qfis 10.0 m3/h and TCF=0.747, then PTFc would be 13.39 m3/h.

FIG. 2shows the membrane-based water treatment system1being equipped with a second embodiment of a dosing system for dosing antiscalant into the feed line13of the membrane-based water treatment system1. The dosing system comprises here a calculation module41being remotely installed in a PLC or a cloud-based system. The calculation module41is here in signal communication with the sensors35via the wireless or cabled signal line39in order to receive the operational parameters and to calculate NDPTc. The calculation module41is also in signal communication with the control module of the dosing pump19via a wireless or cabled signal line43. The calculation module41may send the determined NDPTc to the control module for processing within in a decision algorithm whether to increase or decrease the dosage of antiscalant. Alternatively, the calculation module41may at least partially process the decision algorithm whether to increase or decrease the dosage of antiscalant and send the according command to the control module of the dosing pump19.

FIG. 3ashows schematically a decision algorithm processed by the control module of the dosing pump19and/or the calculation module41. In a first step300, the system is operated with a new or just cleaned-in-place (CIP) membrane or multiple-membrane system. In the next step301, the dosing pump19is operated with the recommended dosage. Furthermore, the initial slope ΔNDPTc0/Δt is determined in step301.

FIG. 4shows how the initial slope ΔNDPTc0/Δt is determined. After a cleaning-in-place (CIP) of the membrane or after a new membrane is installed, a blanking time B is pre-defined for initial fluctuations of NDPTc to settle before it steadily reduces towards a stable level of NDPTc at point S. Once NDPTc starts to steadily increase at point S due to expected fouling of the membrane, ΔNDPTc0is determined over a time interval Δt to determine the slope. The dosage is kept constant to the recommended level.

In step301inFIG. 3, the determined initial slope ΔNDPTc0/Δt is recorded and a loop counter k is set to 1. In the next step303, the dosage of antiscalant is reduced by ΔD. In the following step305, the slope ΔNDPTck=1/Δt for loop k=1 is determined. The slope for loop1is then compared in step307with the initial slope. If the slope for loop1is not larger than the initial slope, the slope for loop1is recorded and the loop counter k increased to 2 in step309. The loop is then repeated to restart at step303again with reducing the dosage of antiscalant by ΔD again. The loop exits when the slope of the current loop is larger than the previous slope. Then, the dosage of antiscalant is increased by ΔD in step311. After step311, a minimal dosage of antiscalant is found without having increased the slope ΔNDPTc/Δt. The dosing pump19is thus operated with the current minimal dosage in step313. During operation of the dosing pump19with the minimal dosage in step313one closed-loop control circle315and two monitoring circles316,317are conducted by the decision algorithm. The closed-loop control circle315monitors the slope ΔNDPTc/Δt against the initial slope ΔNDPTc0/Δt. As long as the slope does not exceed the initial slope, the operation continues with the current dosage. If the slope exceeds the initial slope ΔNDPTc0/Δt, the dosage is increased by ΔD by jumping back to step311. The first monitoring circle316monitors the feed temperature Tfagainst a maximal threshold value and a minimal threshold value Tf,min, and the feed pH against a minimum threshold value pHf,minand a maximum threshold value pHf,max. The first monitoring circle thus monitors if the feed temperature and feed pH are within ranges between the respective threshold values. If the feed temperature and/or feed pH are outside their range, a significant change in the feed conditions can be assumed and the algorithm jumps back to step301to re-determine the initial slope with the recommended dosage. The threshold values may be absolute or relative values. For instance, they can be pre-determined and/or user-set parameters. Alternatively, they can be relative deviations from an averaged or low-pass filtered value. The second monitoring circle317monitors the absolute value of NDPTc against a maximal threshold value NDPTcmaxand the overall time that has lapsed since the last cleaning-in-place (CIP) of the membrane. For this purpose, the control module and/or the calculation module41may comprise a timer that can be reset by a user or automatically when the membrane is being cleaned. If any one of these two thresholds is exceeded, a CIP may be requested in step319and operation continues as before with step313. The decision algorithm restarts with the first step300after each CIP that has actually been performed upon the request.

FIG. 3bshows schematically an analogous decision algorithm processed by the control module of the dosing pump19and/or the calculation module41in case the temperature corrected permeate flow PFTc is used as temperature corrected system variable SVTc. The algorithm differs at comparison steps307,315and317to take into account the inverse behaviour of PFTc with respect to scaling. If a temperature corrected water permeability (KWTc) was used as SVTc, an algorithm according toFIG. 3bwould be adequate. If a temperature corrected membrane resistance (RMTc) was used as SVTc, an algorithm according toFIG. 3awould be adequate.

FIG. 5shows in a diagram how the values of ΔNDPTc/Δt, the average NDPTc and dosage develop during the loops1to4of steps303,305,307and309as described above. The dosage is reduced by ΔD between the loops until an increase of the slope ΔNDPTc/Δt is detected after loop4. The dosage is increased again in step311to the value of loop3, which is the minimal dosage without increasing the slope ΔNDPTc/Δt. This is the minimal dosage the dosing pump19is operated with in step313.

FIG. 6shows in a diagram how the values of ΔNDPTc/Δt, the average NDPTc and dosage may develop when the dosage is increased and a decrease in the slope ΔNDPTc/Δt is detected. The decision algorithm may comprise an additional step of monitoring if the slope ΔNDPTc/Δt has decreased after step311and jumps back to step303to reduce the dosage of antiscalant again by ΔD. The result is visible inFIG. 6. The amount of dosage increase or decrease ΔD in the decision algorithm may be constant or variable, preferably decreasing between loops.

FIG. 7illustrates the variables ΔNDPTc/Δt, pHf, Tfand NDPTc steadily increasing over time and being monitored against their respective maximum threshold values in circles315,316,317. The feed temperature Tfand the feed pH (pHf) are also monitored against respective minimum threshold values (Tf,min, pHf,min). It is thus monitored whether the feed temperature and feed pH are within ranges between the respective threshold values. If the feed temperature and/or feed pH are outside their range, a significant change in the feed conditions can be assumed and the initial slope ΔNDPTc0/Δt is re-determined by operating with the recommended dosage. The threshold values may be absolute or relative values. For instance, they can be pre-determined and/or user-set parameters. Alternatively, they can be relative deviations from an averaged or low-pass filtered value, e.g. the direct measured value of Tfmay be compared with a range of ±10% of a low-pass filtered or averagedTfvalue, i.e. Tf,min=0.9·Tfand Tf,max=1,1·Tf. Analogously, the direct measured value of pHfmay be compared with a range of ±10% of a low-pass filtered or averagedpHfvalue, i.e. pHf,min=0.9·pHfand pHf,max=1,1·pHf.

FIG. 8shows how the slope ΔNDPTc/Δt may be determined from average values within a sliding time window. The direct NDPTc fluctuates statistically and may be sampled at a certain sampling rate. For instance, the direct NDPTc may be sampled at a sampling rate of 1 Hz over a sliding window of 16 seconds. Thus, 16 values of direct NDPTc may be recorded for the sliding window. The first 12 samples may be averaged to NDPTc1of a first window part and the last 4 samples may be averaged to NDPTc2of a second window part. ΔNDPTc/Δt may then be determined as (NDPTc2−NDPTc1)/8 s. The window slides with time and thus yields a slope ΔNDPTc/Δt every second. Analogously, average values for other monitored variables like feed pH (pHf) or feed temperature Tfmay be averaged over a sliding time window. Alternatively or in addition, a low-pass filter may be used to reduce noise on the received signals.

The skilled reader will readily understand thatFIGS. 4 to 8could be drawn up analogously for PFTc and ΔPFTc/Δt with inverse behavior compared to NDPTc and ΔNDPTc/Δt.

Where, in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the disclosure that are described as optional, preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims.

The above embodiments are to be understood as illustrative examples of the disclosure. It is to be understood that any feature described in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. While at least one exemplary embodiment has been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art and may be changed without departing from the scope of the subject matter described herein, and this application is intended to cover any adaptations or variations of the specific embodiments discussed herein.

In addition, “comprising” does not exclude other elements or steps, and “a” or “one” does not exclude a plural number. Furthermore, characteristics or steps which have been described with reference to one of the above exemplary embodiments may also be used in combination with other characteristics or steps of other exemplary embodiments described above. Method steps may be applied in any order or in parallel or may constitute a part or a more detailed version of another method step. It should be understood that there should be embodied within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of the contribution to the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the disclosure, which should be determined from the appended claims and their legal equivalents.