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

Abstract

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).

Claims

1. A method for dosing antiscalant into a membrane-based water treatment system, the method comprising the steps of: determining a temperature corrected system variable based on a plurality of operating variables of the membrane-based water treatment system, wherein the temperature corrected system variable includes a temperature correction, the temperature corrected system variable being dependent on scaling of a membrane of the membrane-based water treatment system; and varying the dosage of antiscalant fed into the membrane-based water treatment system based on the temperature corrected system variable, wherein varying the dosage comprises adapting the dosage of antiscalant fed into the membrane-based water treatment system upon a change of a first time derivative of the temperature corrected system variable.

2. The method according to claim 1, wherein the plurality of operating variables of the membrane-based water treatment system comprises at least one operating variable of the group of operating variables consisting of: feed pressure, feed electrical conductivity, feed temperature, feed pH, difference between feed pressure and concentrate pressure, permeate pressure, permeate temperature and permeate electrical conductivity.

3. The method according to claim 2, wherein the step of varying the dosage comprises recursively adapting the dosage of antiscalant fed into the membrane-based water treatment system based on a previously determined value of the temperature corrected system variable as feedback value.

4. The method according to claim 1, wherein the temperature corrected system variable is a temperature corrected net driving pressure including the temperature correction, or a temperature corrected permeate flow including the temperature correction, or a temperature corrected net driving pressure including the temperature correction and a temperature corrected permeate flow including the temperature correction, or a combination of a temperature corrected net driving pressure including the temperature correction and a temperature corrected permeate flow including the temperature correction.

5. The method according to claim 4, wherein the step of varying the dosage comprises recursively reducing the dosage of antiscalant fed into the membrane-based water treatment system as long as a first time derivative of the temperature corrected net driving pressure does not increase or as long as a first time derivative of the temperature corrected permeate flow does not decrease or as long as the first time derivative of the temperature corrected net driving pressure does not increase and the first time derivative of the temperature corrected permeate flow does not decrease.

6. The method according to claim 4, wherein the step of varying the dosage comprises increasing the dosage of antiscalant fed into the membrane-based water treatment system if a first time derivative of the temperature corrected net driving pressure has increased or if a first time derivative of the temperature corrected permeate flow has decreased or if the first time derivative of the temperature corrected net driving pressure has increased and the first time derivative of the temperature corrected permeate flow has decreased.

7. The method according to claim 4, wherein the step of varying the dosage comprises recursively increasing the dosage of antiscalant pumped into the membrane-based water treatment system as long as a first time derivative of the temperature corrected net driving pressure is above an initial first time derivative or as long as a first time derivative of the temperature corrected permeate flow is below an initial first time derivative or as long as the first time derivative of the temperature corrected net driving pressure is above an initial first time derivative and the first time derivative of the temperature corrected permeate flow is below an initial first time derivative.

8. The method according to claim 4, further comprising the steps of: monitoring the temperature corrected net driving pressure against a maximum threshold value or monitoring the temperature corrected permeate flow against a minimum threshold value or monitoring the temperature corrected net driving pressure against a maximum threshold value and monitoring the temperature corrected permeate flow against a minimum threshold value; and initiating a cleaning-in-place of a membrane if the temperature corrected net driving pressure exceeds the maximum threshold value or if the temperature corrected permeate flow falls below the minimum threshold value or if the temperature corrected net driving pressure exceeds the maximum threshold value and the temperature corrected permeate flow falls below the minimum threshold value.

9. The method according to claim 1, further comprising the steps of: monitoring a feed temperature against a minimum threshold value and a maximum threshold value; monitoring a feed pH against a minimum threshold value and a maximum threshold value; and resetting the dosage of antiscalant to an initial value if the feed temperature or the feed pH exceed the respective maximum threshold value or falls below the respective minimum threshold value or if the feed temperature and the feed pH exceed the respective maximum threshold value or falls below the respective minimum threshold value.

10. A method for dosing antiscalant into a membrane-based water treatment system, the method comprising the steps of: determining a temperature corrected system variable based on a plurality of operating variables of the membrane-based water treatment system, wherein: the temperature corrected system variable includes a temperature correction based on a measured temperature at the membrane-based water treatment system, the temperature corrected system variable being dependent on scaling of a membrane of the membrane-based water treatment system; and the temperature corrected system variable is a temperature corrected net driving pressure including the temperature correction, or a temperature corrected permeate flow including the temperature correction, or a temperature corrected net driving pressure including the temperature correction and a temperature corrected permeate flow including the temperature correction, or a combination of a temperature corrected net driving pressure including the temperature correction and the temperature corrected permeate flow including the temperature correction; and varying the dosage of antiscalant fed into the membrane-based water treatment system based on the temperature corrected system variable comprising recursively reducing the dosage of antiscalant fed into the membrane-based water treatment system as long as the slope a first time derivative of the temperature corrected net driving pressure does not increase or as long as the slope a first time derivative of the temperature corrected permeate flow does not decrease or as long as the slope first time derivative of the temperature corrected net driving pressure does not increase and the slope first time derivative of the temperature corrected permeate flow does not decrease.

11. The method according to claim 10, wherein the step of varying the dosage further comprises increasing the dosage of antiscalant fed into the membrane-based water treatment system if a first time derivative of the temperature corrected net driving pressure has increased or if a first time derivative of the temperature corrected permeate flow has decreased or if the first time derivative of the temperature corrected net driving pressure has increased and the first time derivative of the temperature corrected permeate flow has decreased.

12. The method according to claim 11, wherein the plurality of operating variables of the membrane-based water treatment system comprises at least one operating variable of the group of operating variables consisting of: feed pressure, feed electrical conductivity, feed temperature, feed pH, difference between feed pressure and concentrate pressure, permeate pressure, permeate temperature and permeate electrical conductivity.

13. The method according to claim 10, wherein the step of varying the dosage comprises recursively adapting the dosage of antiscalant fed into the membrane-based water treatment system based on a previously determined value of the temperature corrected system variable as feedback value.

14. The method according to claim 10, wherein the step of varying the dosage comprises recursively increasing the dosage of antiscalant pumped into the membrane-based water treatment system as long as a first time derivative of the temperature corrected net driving pressure is above an initial first time derivative or as long as a first time derivative of the temperature corrected permeate flow is below the initial first time derivative or as long as the first time derivative of the temperature corrected net driving pressure is above an initial first time derivative and the first time derivative of the temperature corrected permeate flow is below an initial first time derivative.

15. The method according to claim 10, further comprising the steps of: monitoring the temperature corrected net driving pressure against a maximum threshold value or monitoring the temperature corrected permeate flow against a minimum threshold value or monitoring the temperature corrected net driving pressure against a maximum threshold value and monitoring the temperature corrected permeate flow against a minimum threshold value; and initiating a cleaning-in-place of a membrane if the temperature corrected net driving pressure exceeds the maximum threshold value or if the temperature corrected permeate flow falls below the minimum threshold value or if the temperature corrected net driving pressure exceeds the maximum threshold value and the temperature corrected permeate flow falls below the minimum threshold value.

16. The method according to claim 10, further comprising the steps of: monitoring the feed temperature against a minimum threshold value and a maximum threshold value; monitoring the feed pH against a minimum threshold value and a maximum threshold value; and resetting the dosage of antiscalant to an initial value if the feed temperature or the feed pH exceed the respective maximum threshold value or falls below the respective minimum threshold value or if the feed temperature and the feed pH exceed the respective maximum threshold value or falls below the respective minimum threshold value.

17. A method for dosing antiscalant into a membrane-based water treatment system, the method comprising the steps of: determining a temperature corrected system variable based on a plurality of operating variables of the membrane-based water treatment system, wherein the temperature corrected system variable includes a temperature correction factor based on a measured temperature of a feed temperature and the plurality of operating variables of the membrane-based water treatment system comprises at least one operating variable of the group of operating variables consisting of: feed pressure, feed electrical conductivity, water feed temperature, feed pH, difference between feed pressure and concentrate pressure, permeate pressure, permeate temperature and permeate electrical conductivity, the temperature corrected system variable being dependent on scaling of a membrane of the membrane-based water treatment system; and varying the dosage of antiscalant fed into the membrane-based water treatment system based on the temperature corrected system variable, wherein varying the dosage comprises adapting the dosage of antiscalant fed into the membrane-based water treatment system upon a change of a first time derivative of the temperature corrected system variable.

18. The method according to claim 17, wherein the step of varying the dosage comprises recursively adapting the dosage of antiscalant fed into the membrane-based water treatment system based on a previously determined value of the temperature corrected system variable as feedback value.

19. The method according to claim 17, wherein the temperature corrected system variable is a temperature corrected net driving pressure including the temperature correction, or a temperature corrected permeate flow including the temperature correction, or a temperature corrected net driving pressure including the temperature correction and a temperature corrected permeate flow including the temperature correction, or a combination of a temperature corrected net driving pressure including the temperature correction and a temperature corrected permeate flow including the temperature correction.

20. The method according to claim 19, wherein the step of varying the dosage comprises recursively reducing the dosage of antiscalant fed into the membrane-based water treatment system as long as a first time derivative of the temperature corrected net driving pressure does not increase or as long as a first time derivative of the temperature corrected permeate flow does not decrease or as long as the first time derivative of the temperature corrected net driving pressure does not increase and the first time derivative of the temperature corrected permeate flow does not decrease.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 is a schematic view showing a membrane-based water treatment system with an example of a first embodiment of the dosing system described herein;

(3) FIG. 2 is a schematic view showing a membrane-based water treatment system with an example of a second embodiment of the dosing system described herein;

(4) FIG. 3a is a flow diagram view showing a decision algorithm with an example of an embodiment of the method for dosing antiscalant into a membrane-based water treatment system described herein;

(5) FIG. 3b is a flow diagram view showing a decision algorithm with an example of an embodiment of the method for dosing antiscalant into a membrane-based water treatment system described herein;

(6) FIG. 4 is a combined diagram of NDPTc and antiscalant dosage over time when operation is started with a new membrane or after a cleaning-in-place (CIP) of the membrane;

(7) FIG. 5 is a combined diagram of ΔNDPTc/Δt, NDPTc and antiscalant dosage over time when the antiscalant dosage is reduced;

(8) FIG. 6 is a combined diagram of ΔNDPTc/Δt, NDPTc and antiscalant dosage over time when the antiscalant dosage is increased and reduced again;

(9) FIG. 7 is a combined diagram of monitored ΔNDPTc/Δt, pH.sub.f, T.sub.f and NDPTc over time with respective thresholds; and

(10) FIG. 8 is a combined diagram of ΔNDPTc/Δt, average NDPTc and direct NDPTc over time.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(11) FIG. 1 shows a schematic illustration of a membrane-based water treatment system 1 in form of a reverse osmosis desalination system comprising a membrane 3 in a vessel 5 with an inlet port 7, an outlet port 9 and a discharge port 11. It should be noted that the membrane 3 in a vessel 5 may be a system of a plurality of membranes in vessels arranged in parallel and/or in series. The inlet port 7 is in fluid connection with a feed line 13, the outlet port is in fluid connection with a permeate line 15 and the discharge port 11 is in fluid connection with a concentrate line 17. Salt water or brackish water to be desalinated is fed by a feed pump 18 into the vessel 5 via the feed line 13 and desalinated fresh water is output into the permeate line 15. Concentrate, also referred to as brine, having a high concentration of salt is discharged through the concentrate line 17. Over time and usage of the system 1, the membrane 3 may show scaling and fouling reducing the system performance.

(12) Therefore, the membrane-based water treatment system 1 is equipped with a first embodiment of a dosing system for dosing antiscalant into the feed line 13 of the membrane-based water treatment system 1. The dosing system comprises a dosing pump 19 for dosing antiscalant from an antiscalant reservoir 21 into the feed line 13. An inlet port 23 of the dosing pump 19 is in fluid connection with an intake port 20 within the antiscalant reservoir 21 and an outlet port 25 of the dosing pump 19 is in fluid connection with the feed line 13. A check valve 27 before the feed line 13 prevents water pressure in the feed line 13 from pushing back antiscalant towards the dosing pump 19. Another check valve 28 upstream the inlet port 23 of the dosing pump 19 and downstream of the intake port 20 prevents antiscalant pressure in the reservoir 21 from pushing antiscalant towards the dosing pump 19. The filling level of antiscalant in the reservoir 21 is monitored by a fluid level sensor 29 controlling an angled valve 31 downstream of the outlet port 25 of the dosing pump 19 via a signal line 33. The angled valve 31 may be shut if the level of antiscalant in the reservoir 21 falls below a minimum threshold.

(13) The dosing system further comprises a plurality of sensors 35, i.e. eight sensors, for determining a plurality of operating variables, i.e. eight variables, of the water treatment system 1. The feed line 13 is equipped with a feed electrical conductivity sensor 35a, a feed temperature sensor 35b, a feed pH sensor 35c and a pressure sensor 35d. The feed line 13 is further equipped with a feed flow sensor 37a for measuring the flow in the feed line 13. The feed electrical conductivity sensor 35a, the feed temperature sensor 35b, the feed pH sensor 35c and the feed flow sensor 37a are located upstream of the feed pump 18, whereas the feed pressure sensor 35d is located downstream of the feed pump 18 and upstream of the vessel 5. A concentrate pressure sensor 35e is located downstream the discharge port 11 at the discharge line 17 for determining the concentrate pressure P.sub.c. The permeate line 15 is equipped with a permeate pressure sensor 35f, a permeate temperature sensor 35g and a permeate electrical conductivity sensor 35h of the dosing system. Furthermore, the permeate line 15 is equipped with a permeate flow sensor 37b, which may be used to determine a temperature corrected permeate flow PFTc. The permeate pressure sensor 35f is located closest to the vessel 5 downstream the outlet port 9. The eight sensors 35 of the dosing system are in signal communication with a control module, comprising one or more processors, of the dosing pump 19 via wireless or cabled signal line 39 with associated transmitters and receivers. The control module of the dosing pump 19 is configured to receive and process the operating variables provided by the sensors 35 via signal line 39. The control module of the dosing pump 19 determines 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 line 13 based on the determined NDPTc.

(14) The control module of the dosing pump 19 determines a net driving pressure (NDP) by:

(15) $\mathrm{NDP}=P_f-\frac{\Delta P_{\mathrm{fc}}}{2}-P_p-{\pi }_{\mathrm{fc}}+{\pi }_p,$
wherein P.sub.f denotes the feed pressure, ΔP.sub.fc is the difference between the feed pressure P.sub.f and the concentrate pressure P.sub.c, P.sub.p is the permeate pressure, π.sub.fc is the feed-concentrate osmotic pressure, and π.sub.p is the permeate osmotic pressure. NDPTc may then be determined by: NDPTc=NDP.Math.TCF, wherein TCF is a temperature correction factor being a function of a membrane-specific temperature correction constant C.sub.f and the feed temperature T.sub.f: TCF=f(C.sub.t, T.sub.f).

(16) The output of the feed pressure sensor 35d, i.e. feed pressure P.sub.f, and of the concentrate pressure sensor 35e, i.e. concentrate pressure P.sub.c, are combined to the differential pressure ΔP.sub.fc=P.sub.f−P.sub.c as one of the operating variables for determining NDPTc. The variables feed electrical conductivity γ.sub.f and feed temperature T.sub.f are used to determine the temperature correction factor TCF and the feed-concentrate osmotic pressure π.sub.fc. 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:

(17) 0$\mathrm{TCF}=e^{2700.Math.\left(\frac{1}{298}-\frac{1}{T_f}\right)},$
wherein the actual feed temperature T.sub.f is input in ° K. For instance, if T.sub.f is 15° C., i.e. 288.15° K, TCF may be 0.734. If there is no TCF formula given or available for the used membrane type, a default TCF formula may be assumed to be: TCF=1.03.sup.(T.sup.f.sup.-298), wherein the actual feed temperature T.sub.f is input in ° K. For example, if T.sub.f is 15° C., i.e. 288.15° K, default TCF may be 0.747. The TCF formula may be a user-defined configuration parameter of the dosing pump 19 when it is installed to a certain water treatment system 1. A user may select the correct TCF formula when the dosing pump 19 is initialised or re-configured.

(18) The osmotic pressure π may be derived of the van't Hoff formula π=R.Math.T.Math.ϕ.Math.Σ.sub.iα.sub.ic.sub.i, 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 c.sub.i is the concentration of ionic species i. The feed-concentrate osmotic pressure π.sub.fc may be deduced therefrom to be:

(19) ${\pi }_{\mathrm{fc}}=2.654.Math.T_f.Math.\frac{c_{\mathrm{fc}}}{{10}^6-c_{\mathrm{fc}}},$
wherein the actual feed temperature T.sub.f is input in ° K and c.sub.fc is the concentration of salts in the feed-concentrate. The concentration of salts in the feed-concentrate c.sub.fc can be derived from the concentration of salts in the feed c.sub.f by making use of the total recovery Rec:

(20) $c_{\mathrm{fc}}=-\mathrm{CP}.Math.c_f.Math.\frac{\ln \left(1-\mathrm{Rec}\right)}{\mathrm{Rec}},$
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=Q.sub.p/Q.sub.f may 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 sensors 37a,b.

(21) The concentration of salts in the feed c.sub.f in units of mg/l may be determined by using the measurement of the feed electrical conductivity γ.sub.f in units of μS/cm: c.sub.f=0.76.Math.γ.sub.f.Math.3.07. For example, if γ.sub.f is 1500 μS/cm, c.sub.f is 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 c.sub.fc is 2151 mg/l, which results in a feed-concentrate osmotic pressure π.sub.fc of 1.65 bar.

(22) NDP and NDPTc may now already be available if the permeate pressure P.sub.p and zero permeate osmotic pressure π.sub.p can be assumed to be constants or zero in case no permeate sensors 35f-h were available. However, the measured permeate pressure P.sub.p and the measured permeate electrical conductivity γ.sub.p are available from respective sensors 35f-h in 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 P.sub.p to NDP and subtracting the permeate osmotic pressure π.sub.p from NDP. As the concentration of salts in the permeate is generally quite low, the osmotic pressure π.sub.p in units of bar may be derived directly from the measured permeate electrical conductivity γ.sub.p in units of μS/cm by: π.sub.p=7.49.Math.10.sup.−4.Math.γ.sub.p−0.19.Math.10.sup.−3. For instance, if the permeate electrical conductivity γ.sub.p is 13 μS/cm, π.sub.p would be 0.01 bar.

(23) 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.

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

(25) $\mathrm{PFTc}=\frac{Q_p}{\mathrm{TCF}},$
wherein Q.sub.p denotes the permeate flow measured by permeate flow sensor 37b and TCF is the same temperature correction factor as described above. If Q.sub.f is 10.0 m.sup.3/h and TCF=0.747, then PTFc would be 13.39 m.sup.3/h.

(26) FIG. 2 shows the membrane-based water treatment system 1 being equipped with a second embodiment of a dosing system for dosing antiscalant into the feed line 13 of the membrane-based water treatment system 1. The dosing system comprises here a calculation module 41 being remotely installed in a PLC or a cloud-based system. The calculation module 41 is here in signal communication with the sensors 35 via the wireless or cabled signal line 39 in order to receive the operational parameters and to calculate NDPTc. The calculation module 41 is also in signal communication with the control module of the dosing pump 19 via a wireless or cabled signal line 43. The calculation module 41 may 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 module 41 may 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 pump 19.

(27) FIG. 3a shows schematically a decision algorithm processed by the control module of the dosing pump 19 and/or the calculation module 41. In a first step 300, the system is operated with a new or just cleaned-in-place (CIP) membrane or multiple-membrane system. In the next step 301, the dosing pump 19 is operated with the recommended dosage. Furthermore, the initial slope ΔNDPTc.sub.0/Δt is determined in step 301.

(28) FIG. 4 shows how the initial slope ΔNDPTc.sub.0/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, ΔNDPTc.sub.0 is determined over a time interval Δt to determine the slope. The dosage is kept constant to the recommended level.

(29) In step 301 in FIG. 3, the determined initial slope ΔNDPTc.sub.0/Δt is recorded and a loop counter k is set to 1. In the next step 303, the dosage of antiscalant is reduced by ΔD. In the following step 305, the slope ΔNDPTc.sub.k=1/Δt for loop k=1 is determined. The slope for loop 1 is then compared in step 307 with the initial slope. If the slope for loop 1 is not larger than the initial slope, the slope for loop 1 is recorded and the loop counter k increased to 2 in step 309. The loop is then repeated to restart at step 303 again 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 step 311. After step 311, a minimal dosage of antiscalant is found without having increased the slope ΔNDPTc/Δt. The dosing pump 19 is thus operated with the current minimal dosage in step 313. During operation of the dosing pump 19 with the minimal dosage in step 313 one closed-loop control circle 315 and two monitoring circles 316, 317 are conducted by the decision algorithm. The closed-loop control circle 315 monitors the slope ΔNDPTc/Δt against the initial slope ΔNDPTc.sub.0/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 ΔNDPTc.sub.0/Δt, the dosage is increased by ΔD by jumping back to step 311. The first monitoring circle 316 monitors the feed temperature T.sub.f against a maximal threshold value and a minimal threshold value T.sub.f,min, and the feed pH against a minimum threshold value pH.sub.f,min and a maximum threshold value pH.sub.f,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 step 301 to 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 circle 317 monitors the absolute value of NDPTc against a maximal threshold value NDPTc.sub.max and 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 module 41 may 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 step 319 and operation continues as before with step 313. The decision algorithm restarts with the first step 300 after each CIP that has actually been performed upon the request.

(30) FIG. 3b shows schematically an analogous decision algorithm processed by the control module of the dosing pump 19 and/or the calculation module 41 in case the temperature corrected permeate flow PFTc is used as temperature corrected system variable SVTc. The algorithm differs at comparison steps 307, 315 and 317 to 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 to FIG. 3b would be adequate. If a temperature corrected membrane resistance (RMTc) was used as SVTc, an algorithm according to FIG. 3a would be adequate.

(31) FIG. 5 shows in a diagram how the values of ΔNDPTc/Δt, the average NDPTc and dosage develop during the loops 1 to 4 of steps 303, 305, 307 and 309 as described above. The dosage is reduced by ΔD between the loops until an increase of the slope ΔNDPTc/Δt is detected after loop 4. The dosage is increased again in step 311 to the value of loop 3, which is the minimal dosage without increasing the slope ΔNDPTc/Δt. This is the minimal dosage the dosing pump 19 is operated with in step 313.

(32) FIG. 6 shows 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 step 311 and jumps back to step 303 to reduce the dosage of antiscalant again by ΔD. The result is visible in FIG. 6. The amount of dosage increase or decrease ΔD in the decision algorithm may be constant or variable, preferably decreasing between loops.

(33) FIG. 7 illustrates the variables ΔNDPTc/Δt, pH.sub.f, T.sub.f and NDPTc steadily increasing over time and being monitored against their respective maximum threshold values in circles 315, 316, 317. The feed temperature T.sub.f and the feed pH (pH.sub.f) are also monitored against respective minimum threshold values (T.sub.f,min, pH.sub.f,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 ΔNDPTc.sub.0/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 T.sub.f may be compared with a range of ±10% of a low-pass filtered or averaged T.sub.f value, i.e. T.sub.f,min=0.9.Math.T.sub.f and T.sub.f,max=1,1.Math.T.sub.f. Analogously, the direct measured value of pH.sub.f may be compared with a range of ±10% of a low-pass filtered or averaged pH.sub.f value, i.e. pH.sub.f,min=0.9.Math.pH.sub.f and pH.sub.f,max=1,1.Math.pH.sub.f.

(34) FIG. 8 shows 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 NDPTc.sub.1 of a first window part and the last 4 samples may be averaged to NDPTc.sub.2 of a second window part. ΔNDPTc/Δt may then be determined as (NDPTc.sub.2−NDPTc.sub.1)/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 (pH.sub.f) or feed temperature T.sub.f may 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.

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

(36) 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.

(37) 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.

(38) 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.

(39) While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.