METHOD FOR DETERMINING A DOSE OF COAGULANT FOR TREATING RAW WATER

Abstract

A method for determining an optimal dose of coagulant for raw water includes a step of determining a value, for the raw water, of a first organic parameter; a step of determining a value, for the raw water, of a second mineral parameter; a step of determining a class of water for the raw water, characterized by the value of the first organic parameter and the value of the second mineral parameter; a step of determining a value, for the raw water, of a third organic parameter; a step of defining a target value, for the clarified water, of the third organic parameter; a step of selecting a function between the third organic parameter and an added dose of coagulant, said function being selected for the class of water and for the value, for the raw water, of the third organic parameter; a step of using the function to determine a first dose of coagulant in order to reach the target value.

Claims

1. A method for determining an optimal dose ([COAG].sub.OPT) of coagulant for raw water (EB), comprising the following steps: a step of determining a value (P.sub.ORG1_EB), for the raw water, of a first organic parameter (P.sub.ORG1) for providing information on the ability of a water to coagulate; a step of determining a value (P.sub.MIN2_EB), for the raw water, of a second mineral parameter (P.sub.MIN2) for providing information on the mineral load of a water; a step of determining a class of water (CL.sub.EB), for the raw water, as a function of the values, determined for the raw water, of the first organic parameter and of the second mineral parameter, a class of water being characterized by a first range of values of the first, organic parameter (P.sub.ORG1) and a second range of values of the second, mineral parameter (P.sub.MIN2); a step of determining a value (P.sub.ORG3_EB), for the raw water, of a third, organic parameter (P.sub.ORG3), said third parameter being for providing information on the quantity of organic matter in a water; a step of defining a target value (P.sub.ORG3_ED), for the clarified water, of the third organic parameter (P.sub.ORG3); a step of selecting a function (f.sub.i) for establishing a relationship between the third organic parameter (P.sub.ORG3) and a dose of coagulant ([COAG]) added to the raw water, said function being selected for the class of water (CL.sub.EB) determined for the raw water and for the value (P.sub.ORG3_EB), determined for the raw water, of the third, organic parameter (P.sub.ORG3); a step of using the function (f.sub.i) selected, so as to determine a first dose of coagulant ([COAG1]) corresponding to the target value (P.sub.ORG3_ED), defined for the clarified water, of the third organic parameter (P.sub.ORG3).

2. The method as claimed in claim 1, comprising a preliminary step of determining a plurality of classes of water (CL.sub.i), each class of water being characterized by at least one first range of values of at least one first, organic parameter (P.sub.ORG1) for providing information on the ability of a water to coagulate, and a second range of values of at least one second, mineral parameter (P.sub.MIN2) for providing information on the mineral load of a water.

3. The method as claimed in claim 1, the at least one first, organic parameter (P.sub.ORG1) being selected from the UV absorbance, preferably at 254 nm, the dissolved organic carbon (DOC), the ratio (SUVA) between the UV absorbance, preferably at 254 nm, and the dissolved organic carbon (DOC), or a combination of said parameters.

4. The method as claimed in claim 1, the at least one second, mineral parameter (P.sub.MIN2) being selected from the complete alkalimetric titer (TAC), the concentration of chloride ions, the concentration of sodium ions, the concentration of sulfate ions, the concentration of calcium ions, the concentration of magnesium ions, the concentration of silicate ions, the conductivity, or a combination of said parameters.

5. The method as claimed in claim 1, the third, organic parameter (P.sub.ORG3) being selected from the UV absorbance, preferably at 254 nm, or the dissolved organic carbon (DOC).

6. The method as claimed in claim 1, further comprising a step of determining the coagulation pH (pH.sub.C), the function (f.sub.i) being further selected for the coagulation pH (pH.sub.C).

7. The method as claimed in claim 1, the function (f.sub.i) being an exponential function for the entirety of the classes of water, such that:
P.sub.ORG3=ƒ.sub.i([COAG])=A.sub.ie.sup.−B.sup.i.sup.[COAG]+C.sub.i; the coefficients A.sub.i, B.sub.i and C.sub.i being determinable by relationships (R.sub.i1, R.sub.i2, R.sub.i3) which are given according to the class of water (CL.sub.i), for a given value (P.sub.ORG3_EB), for the raw water, of the third, organic parameter (P.sub.ORG3), and for a coagulation pH (pH.sub.C).

8. The method as claimed in claim 7, the coefficient C.sub.i being defined as being the value of noncoagulable organic matter, for the given value (P.sub.ORG3_EB), for the raw water, of the third, organic parameter and for a given coagulation pH (pH.sub.C).

9. The method as claimed in claim 8, the coefficient Ci being connected to the value (P.sub.ORG3_EB), for the raw water, of the third, organic parameter by first, linear relationships R.sub.i1.

10. The method as claimed in claim 8, the coefficient Ci being connected to the coagulation pH (pH.sub.C) by a second, linear, polynomial or exponential, relationship R.sub.i2.

11. The method as claimed in claim 7, the coefficient A.sub.i being equal to the value (P.sub.ORG3_EB), determined for the raw water, of the third, organic parameter, minus the coefficient C.sub.i.

12. The method as claimed in claim 11, the coefficient B.sub.i being calculated on the basis of a second derivative value α.sub.i of the function (f.sub.i), of the coefficient Ai and of the value (P.sub.ORG3_EB), determined for the raw water, of the third, organic parameter (P.sub.ORG3).

13. The method as claimed in claim 12, the second derivative value α.sub.i of the function (f.sub.i) being attained for a coagulant dose ([COAG]) equal to a maximum economically allowable dose (DMEA), said maximum economically allowable dose being the coagulant dose on the basis of which the cost of treatment with the coagulant becomes greater than the cost of the treatment by an alternative reagent, and being determinable by third, linear relationships R.sub.i3 as a function of the value (P.sub.ORG3_EB), determined for the raw water, of the third, organic parameter.

14. The method as claimed in claim 1, the values, for the raw water, of the first, organic parameter (P.sub.ORG1), of the second, mineral parameter (P.sub.MIN2) and of the third, organic parameter (P.sub.ORG3) being determined by measurements of the raw water, for example in-line measurements performed by a dissolved organic carbon sensor, a UV sensor, a conductivity sensor, or sampling measurements performed by a complete alkalimetric titer (TAC) analyzer, an ion concentration analyzer, or a combination of such measurements.

15. The method as claimed in claim 1, further comprising determining a dose of a second reagent (REAC), for example a powdered activated carbon or an acid, and determining the first coagulant dose ([COAG1]) to be added to attain the target value (P.sub.ORG3_ED), defined for the clarified water, of the third, organic parameter (P.sub.ORG3) with the second reagent.

16. The method as claimed in claim 1, further comprising: a step of defining a target turbidity value (TURB_.sub.ED) for the clarified water; a step of determining a second coagulant dose ([COAG2]) to be added to the raw water (EB) for attaining the target turbidity value (TURB_.sub.ED) for the clarified water; a step of determining the optimal coagulant dose ([COAG].sub.OPT) to be added to the raw water, comprising the comparison of the first coagulant dose ([COAG1]) and the second coagulant dose ([COAG2]), said optimal dose being the greatest dose between the first coagulant dose ([COAG1]) and the second coagulant dose ([COAG2]).

17. A method implemented on a computer medium for effecting the steps of the method as claimed in claim 1.

18. A raw water treatment process comprising at least a step of coagulating the raw water, the coagulant dose added being the optimal coagulant dose ([COAG.sub.OPT]), determined by the determination method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0105] The invention will be better appreciated, and other advantages will become apparent, on a reading of the detailed description which is given by way of example and without limitation, the description being illustrated by the appended figures, in which:

[0106] FIG. 1 illustrates a first embodiment of the method of the invention for determining the optimal coagulant dose;

[0107] FIG. 2 represents a function enabling calculation of the UV absorbance of a water as a function of the injected coagulant dose;

[0108] FIG. 3 represents an example of division of waters into classes of water, defined as a function of the concentration of Cl.sup.− and Na.sup.+ ions, of the complete alkalimetric titer (TAC) and of the ratio UV.sub.254 nm/DOC (SUVA);

[0109] FIGS. 4A and 4B represent first and second relationships enabling calculation of the value of noncoagulable organic matter, as a function of the UV absorbance of the raw water and as a function of the coagulation pH, for one class of water;

[0110] FIG. 5 represents schematically the way of calculating a maximum economically allowable dose of coagulant (DMEA);

[0111] FIG. 6 represents a series of third, linear relationships enabling calculation of the DMEA as a function of the UV absorbance of the raw water, for one class of water;

[0112] FIG. 7 illustrates a water treatment process comprising a coagulation step and a downstream step;

[0113] FIGS. 8A and 8B illustrate one particular embodiment of the method for determining a first coagulant dose, enabling the integration of other reagents;

[0114] FIG. 9 illustrates a second embodiment of the method of the invention for determining the optimal coagulant dose;

[0115] FIG. 10 illustrates fourth relationships enabling determination of a second coagulant dose;

[0116] FIG. 11 illustrates the second embodiment.

DETAILED Description OF VARIOUS EMBODIMENTS OF THE INVENTION

[0117] In the description, the invention is described with the example of raw water. However, the invention can be applied to any other liquid containing organic matter and turbidity.

[0118] FIG. 1 illustrates a first embodiment of a method for determining the optimal dose of coagulant to be added to the raw water, the method comprising the following steps, which are also described subsequently: [0119] a step 110 of determining, for the raw water, a value (P.sub.ORG1_EB) of a first, organic parameter (P.sub.ORG1) for providing information on the ability of a water to coagulate; [0120] a step 120 of determining, for the raw water, a value (P.sub.MIN2_EB) of a second, mineral parameter (P.sub.MIN2) for providing information on the mineral load of a water; [0121] a step 130 of determining, for the raw water, a class of water (CL.sub.EB) as a function of the values, determined for the raw water, of the first, organic parameter and of the second, mineral parameter, a class of water being characterized by a first range of values of the first, organic parameter (P.sub.ORG1) and a second range of values of the second, mineral parameter (P.sub.MIN2); [0122] a step 140 of determining, for the raw water, a value (P.sub.ORG3_EB) of a third, organic parameter (P.sub.ORG3), said third parameter being for providing information on the quantity of organic matter in a water; [0123] a step 150 of defining, for the clarified water, a target value (P.sub.ORG3_ED) of the third, organic parameter (P.sub.ORG3); [0124] a step 160 of selecting a function (f.sub.i) for establishing a relationship between the third, organic parameter (P.sub.ORG3) and a dose of coagulant ([COAG]) added to the raw water, said function being selected for the class of water (CL.sub.EB) determined for the raw water and for the value (P.sub.ORG3_EB), determined for the raw water, of the third, organic parameter (P.sub.ORG3); [0125] a step 170 of using the function (f.sub.i) selected, so as to determine a first dose of coagulant ([COAG1]) corresponding to the target value (P.sub.ORG3_ED), defined for the clarified water, of the third, organic parameter (P.sub.ORG3).

[0126] According to this embodiment, the optimal dose is the first coagulant dose.

[0127] According to one particular embodiment example, the third, organic parameter P.sub.ORG3 is the UV absorbance at 254 nm of the clarified water, expressed in m.sup.−1. It may be called “UV” throughout the description.

[0128] The UV absorbance (typically UV at 254 nm) is a physical measurement for evaluating the aromatic organic matter contained within the water. The UV absorbance is measured by means of the UV photometer (typically at 254 nm), where the sample is placed in a UV-transparent quartz cell with a thickness in the cm range—for example, of 1 cm, 3 cm, 5 cm or 10 cm. The measurement is simpler and in general more cost-efficient than the measurement of TOC (Total Organic Carbon) and even than DOC (Dissolved Organic Carbon). This method by photometry gives a result in m.sup.−1, corresponding to the loss of luminous intensity at the selected wavelength (typically 254 nm) through a sample of water in the cell with a thickness of 1 cm. The organic matter thus detected contains aromatic rings and double bonds, such as, especially, humic acids. These aromatic organic substances are particularly effectively removed by coagulation.

[0129] According to another embodiment example, it is possible to measure the DOC, which will also be a function of the coagulant dose.

[0130] For each class of water CL.sub.i, the UV or the DOC is a function (f.sub.i) of the coagulant dose.

[0131] To facilitate reading, the remainder of the description will use the term UV absorbance or UV, with the proviso that the measurement in question may alternatively be of DOC or of another third, organic parameter.

[0132] The various steps are described more later in the description.

[0133] There may additionally be a preliminary step 105 of determining a plurality of classes of water.

[0134] There may additionally be a step 145 of determining the coagulation pH, with the function f.sub.i being selected, moreover, for the coagulation pH.

[0135] Other steps, not shown in FIG. 1, may be added. They are described in particular in the remainder of the description.

[0136] FIG. 2 illustrates the preferred embodiment wherein the function f.sub.i selected during the selection step 160 is an exponential function:

UV or DOC=ƒ.sub.i([COAG])=A.sub.ie.sup.−B.sup.i.sup.[COAG]+C.sub.i  (1)

[0137] where COAG is the coagulant dose added, expressed in ppm.

[0138] The step 160 of selecting the function f.sub.i then comprises a step of determining the coefficients A.sub.i, B.sub.i, C.sub.i.

[0139] The coefficient C.sub.i corresponds to the residual organic matter expressed in terms of UV absorbance (also called “residual UV” in the present description or “noncoagulable UV”) when the coagulant dose has reached a maximum efficiency threshold, typically when the coagulant dose is greater than 200 ppm of solution, expressed as ppm of commercial solution.

[0140] A.sub.i corresponds to the organic matter, expressed for example in terms of UV absorbance, which is removed by coagulation when the coagulant dose has reached a maximum efficiency threshold, typically when the coagulant dose is greater than 200 ppm of solution, expressed as ppm of commercial solution.

[0141] Moreover, A.sub.i, C.sub.i and UV.sub.EB are connected by the following equation:

UV.sub.EB=A.sub.i+C.sub.i  (2)

[0142] where UV.sub.EB is the organic matter in the raw water, expressed in terms of UV absorbance.

[0143] B.sub.i is a coefficient which gives the nature of the exponential function.

[0144] A.sub.i, B.sub.i and C.sub.i are obtained by relationships given for each class of water CL.sub.i, and these relationships R.sub.i1, R.sub.i2 and R.sub.i3 enable A.sub.i, B.sub.i and C.sub.i to be deduced from the coagulation pH (pH.sub.C) and from the organic matter in the raw water (UV.sub.EB), expressed in terms of UV absorbance. These relationships are preferably available in databases associated with the classes of water.

[0145] To select the function f.sub.i, and especially to determine the coefficients in the case of an exponential function of formula (1), it is necessary to determine (determination step 130) the class of water to which the raw water belongs.

[0146] According to one preferred embodiment example, a raw water is identified in a class of water by the analysis of its following organic and mineral matrices: [0147] the organic matrix is defined by the following parameters: SUVA (which is the ratio between the UV absorbance at 254 nm, expressed in m.sup.−1, and the DOC, expressed in mg/l) and, optionally, the measurement of the DOC by liquid chromatography (LC-OCD for Liquid Chromatography-Organic Carbon Detection); [0148] the mineral matrix is defined by the following parameters: the complete alkalimetric titer (TAC), the concentration of chloride ions and/or of sodium ions, and optionally the conductivity, the concentration of silicate ions, of calcium ions, of magnesium ions, of sulfate ions, and the ionic balance.

[0149] The values of the parameters of the organic and mineral matrices (determination steps 110 and 120) may be determined by in-line analysis or by sampling, or may comprise recovery of data already available for the raw water to be treated.

[0150] According to the preferred embodiment example, the first, organic parameter (P.sub.ORG1) therefore comprises at least the SUVA, and the second, mineral parameter (P.sub.MIN2) therefore comprises at least the TAC, and also the concentration of chloride ions and/or of sodium ions.

[0151] FIG. 3 or table 1 below represents an example of mineral and organic matrices of classes of water, defined as a function of the concentration of Cl.sup.− and/or Na.sup.+ ions, the TAC and the ratio UV.sub.254nm/DOC (SUVA). The classes of water are defined as a function of the following thresholds:

TABLE-US-00001 TABLE 1 examples of mineral and organic matrices of classes of water Threshold 1 Threshold 2 Cl.sup.− and/or Na.sup.+ 60 mg/l (and/or 30 mg/l) TAC 6° f. 12° f. SUVA 2 4

[0152] For the class of water CL.sub.EB determined for the raw water EB, relationships R.sub.EB1, R.sub.EB2 and R.sub.EB3 are obtained, which are given for said class of water, and said relationships make it possible to deduce the coefficients A.sub.EB, B.sub.EB and C.sub.EB from the coagulation pH (pH.sub.C) and from the organic matter in the raw water (UV.sub.EB), expressed in terms of UV absorbance.

[0153] FIGS. 4A and 4B represent the first and second relationships REB, and R.sub.EB2, from which it is possible to calculate the value of noncoagulable organic matter, this being a function of two variables: the UV absorbance of the raw water (relationships in the form of curves are given for a given coagulation pH) and the coagulation pH (relationships in the form of curves are given for a given value of the UV absorbance of the raw water), for the class of water CL.sub.EB determined for the raw water EB. This allows the coefficient C.sub.EB to be determined.

[0154] To calculate the noncoagulable organic matter as a function of the UV absorbance (or the DOC) of the raw water, there are one or more first, linear relationships R.sub.EB1 available (three in the example illustrated), with coefficients a.sub.4, a.sub.5, a.sub.6, b.sub.4, b.sub.5, b.sub.6 which vary discretely as a function of thresholds (S.sub.4, S.sub.5) of the UV absorbance of the raw water, UV.sub.EB.

[0155] FIG. 4A shows three first linear relationships:

[0156] y=a.sub.4x+b.sub.4 up to the threshold S4;

[0157] y=a.sub.5x+b.sub.5 between the thresholds S4 and S5;

[0158] y=a.sub.6x+b.sub.5 after the threshold S5.

[0159] Depending on the classes of water, there may be a single first linear relationship or at least two first linear relationships.

[0160] To calculate the noncoagulable organic matter as a function of the coagulation pH pH.sub.C, there are also second, linear, exponential or polynomial relationships R.sub.EB2 available, according to the class of water. Said second relationships have coefficients a.sub.7, b.sub.7, c.sub.7, which are also given according to the class of water.

[0161] FIG. 4B shows a second, linear relationship of type y=a.sub.7x+b.sub.7.

[0162] Depending on the classes of water, the second relationship may alternatively be exponential, y=a.sub.7exp(b.sub.7x)+c.sub.7, or polynomial, as for example second-degree polynomial, y=a.sub.7x.sup.2+b.sub.7x+c.sub.7.

[0163] Therefore, for the class of water CL.sub.EB determined, the determination of the UV of the raw water, UV.sub.EB (or of the DOC) and of the coagulation pH, pH.sub.C, makes it possible to determine the coefficients a.sub.4, a.sub.5, a.sub.6, b.sub.4, b.sub.5, b.sub.6, a.sub.7, b.sub.7, c.sub.7, and then the noncoagulable organic matter, so giving C.sub.EB.

[0164] The coefficients a.sub.4, a.sub.5, a.sub.6, b.sub.4, b.sub.5, b.sub.6 are a function of the coagulation pH.

[0165] It would be possible to determine only the coefficients a.sub.4, a.sub.5, a.sub.6, b.sub.4, b.sub.5, b.sub.6 which are given for a fixed, unadjusted coagulation pH.

[0166] A.sub.EB is obtained via equation (2): UV.sub.EB=A.sub.EB+C.sub.EB.

[0167] The determination of the UV of the raw water, UV.sub.EB (or of the DOC.sub.EB) (determination step 140) and also the optional determination of the coagulation pH (pH.sub.C) (determination step 145) may be carried out by in-line measurement or by sampling, and/or may comprise recovery of data already available for the raw water to be treated.

[0168] Also determined is the DMEA (Maximum Economically Allowable Dose) of coagulant.

[0169] The DMEA is defined in the present description as being the coagulant dose at which the cost of treatment with the coagulant becomes greater than the cost of treatment with an alternative, generally more expensive, reagent (as an example, powdered activated carbon, CAP) for the same reduction, by coagulation, in organic matter, expressed in terms of UV absorbance, as is illustrated in FIG. 5, which provides a cost (COST) in euros per cubic meter of raw water and per unit of the organic matter removed, expressed in terms of UV, as a function of the CAP or coagulant (COAG) dose. The dotted line corresponds to the CAP and the solid curve corresponds to the coagulant. The intersection of the two gives the DMEA.

[0170] Furthermore, the inventors have found that the DMEA is independent of the coagulation pH (pH.sub.C), but that it is a function of the UV absorbance (or the DOC) of the raw water, as shown in FIG. 6.

[0171] FIG. 6 shows a series of third, linear relationships R.sub.EB3 enabling calculation of the DMEA (given in ppm of commercial solution) as a function of the UV absorbance of the raw water, UV.sub.EB (or of the DOC of the raw water, DOC.sub.EB), for the class of water CL.sub.EB determined. There are a number of third, linear relationships, whose coefficients a.sub.1, a.sub.2, a.sub.3, b.sub.1, b.sub.2, b.sub.3 vary discretely as a function of thresholds (S.sub.1, S.sub.2) of the UV absorbance of the raw water.

[0172] Determining the UV of the raw water, UV.sub.EB (or of the DOC.sub.EB) enables determination of the coefficients a.sub.1, a.sub.2, a.sub.3, b.sub.1, b.sub.2, b.sub.3, and then of the DMEA of the raw water.

[0173] According to the preferred embodiment of the invention, the DMEA is the coagulant dose corresponding to the point of inflection of the function f.sub.i (1):

UV=ƒ.sub.i([COAG])=A.sub.ie.sup.−B.sup.i.sup.[COAG]+C.sub.i  (1)

[0174] The DMEA is defined mathematically by an absolute value α.sub.i of the second-order derivative of said function, for example of between 0.0001 and 0.0009, i.e.:

ƒ″.sub.i([DMEA])=A.sub.iB.sub.i.sup.2e.sup.−B.sup.i.sup.[DMEA]=α.sub.i  (3)

[0175] Fora given class of water CL.sub.EB, the value α.sub.EB is determined as a function of the value of the UV of the raw water, UV.sub.EB (or of the DOC.sub.EB), as for example as a function of thresholds σ of UV.sub.EB as set out in table 2 below:

TABLE-US-00002 TABLE 2 examples of second derivative values as a function of the UV of the raw water UV.sub.EB [0-σ.sub.1] [σ.sub.1-σ.sub.2] >σ.sub.2 Second α.sub.EB1 α.sub.EB2 α.sub.EB3 derivative value

[0176] For the class of water CL.sub.EB, and the UV of the raw water UV.sub.EB (or the DOC.sub.EB), the coefficient B.sub.EB is determined by the relationship (3), expressed for the raw water.

ƒ″.sub.EB([DMEA])=A.sub.EBB.sub.EB.sup.2e.sup.−B.sup.EB.sup.[DMEA]=α.sub.EB

[0177] Accordingly, for the class of water CL.sub.EB determined for the raw water, and the UV.sub.EB and the coagulation pH pHc, an exponential function is obtained:

UV=ƒ.sub.EB([COAG])=A.sub.EBe.sup.−B.sup.EB.sup.[COAG]+C.sub.EB

[0178] in which the coefficients A.sub.EB, B.sub.EB and C.sub.EB are known.

[0179] With this function ƒ.sub.EB it becomes possible to calculate: [0180] the residual UV in clarified water ED (“noncoagulable UV”), irrespective of the coagulant dose applied; [0181] the first coagulant dose to be applied in order to reach a target value for the UV absorbance of the clarified water (UV.sub.ED).

[0182] A definition is made (definition step 150) of the target value of the UV absorbance of the clarified water (UV.sub.ED) or of the DOC value of the clarified water (DOC.sub.ED), which correspond to the maximum of residual organic matter in the clarified water (ED) desired.

[0183] Accordingly, the first coagulant dose is deduced from the function ƒ.sub.EB (use step 170).

[0184] Moreover, with the first, linear relationships R.sub.EB1, it is possible to calculate the UV when the coagulant dose is equal to the DMEA (referred to as “UV.sub.DMEA”) so as to decide: [0185] if UV.sub.DMEA≥UV.sub.ED, then the first coagulant dose=the DMEA; [0186] if UV.sub.DMEA<UV.sub.ED, then the function ƒ.sub.EB is used in order to calculate the first coagulant dose.

[0187] The coefficients A.sub.i, B.sub.i and C.sub.i are given for each type of coagulant.

[0188] The coagulant may be a solution based on salts of aluminum or iron, and preferably comprises the following compounds: an aluminum sulfate; an aluminum (poly)chloride; an aluminate; a ferric chloride; a ferric sulfate; a sodium or potassium ferrate ion, or a combination of said compounds. A commercial coagulant solution is, for example, an aluminum sulfate containing 8.2% of alumina Al.sub.2O.sub.5, or a ferric chloride containing 41% of FeCl.sub.3.

[0189] The target value for residual organic matter in the clarified water may be defined, furthermore, as a function of the steps downstream of the coagulation process; for example, it may be defined, in a step 300, as a function of a target value for residual organic matter in the treated water (ET). The treated water is defined as being the water obtained at the outlet of a water treatment plant.

[0190] Therefore, as illustrated in FIG. 7, if the residual organic matter in the treated water is expressed by UV absorbance, and starting from an objective to be met at the plant outlet (UV.sub.ET) and from the performance in removal of organic matter in the post-coagulation steps (%.sub.POST-COAG), the quality objective to be met in clarified water is calculated:

UV.sub.ED=UV.sub.ET/(1−%.sub.POST-COAG)

[0191] The post-coagulation performance may be calculated on the basis of in-line sensors or local measurements on the clarified water and the treated water.

[0192] According to a step 200, the data harvested from the UV sensors of the clarified water ED and the treated water ET enable calculation of the percentage removal of UV in the post-coagulation treatment steps. With knowledge of this percentage it is possible to calculate the target clarified UV, in the step 300, to enable the objective at the plant outlet (UV of treated water) to be met.

[0193] With knowledge of the level of organic matter in the raw water and of the UV objective to be met at the end of coagulation, the method makes it possible to calculate the optimal coagulant dose to be applied in order to achieve the set objectives.

[0194] It is possible to implement the method, by using, for example, a tool to install the method (for example, a tool in the form of a computer, tablet, smartphone, Cloud, etc.), without this tool being connected to in-line sensors for measuring the parameters of the raw water. In this case, the values of the parameters of the water are brought into the tool manually, and the dose is, in general, not added automatically in the coagulation/settling treatment process. The tool then contains primarily only the method steps for calculating the optimal coagulant dose. Furthermore, this calculation may take place well before the implementation of the coagulation process. It may be used to calculate a better compromise between different types of coagulants and/or the addition of acid in order to lower the coagulation pH, as is explained more below.

[0195] Conversely, it is possible to implement the method by connecting it to in-line sensors or local measurements for measuring the parameters of the raw water, for determining the class of water of the raw water and/or for determining the quantity of organic matter contained in the water during the process and at least for the raw water. Such sensors or local measurements may advantageously monitor the quality of the water in-line, all along the water treatment process. In this case, the calculated optimal dose may be added automatically in the coagulation/settlement treatment process.

[0196] To characterize the mineral matrix of the water under study, a TAC analyzer and/or an ion concentration analyzer and/or a conductivity sensor may be used.

[0197] The organic matter may be quantified by analyses of UV absorbance at 254 nm and/or DOC (dissolved organic carbon). An in-line sensor selected is preferably a UV sensor, which is easier to operate than an in-line DOC sensor.

[0198] At least three in-line sensors, preferably, are installed for determining the quantity of organic matter contained in the water during the process: a first sensor for measuring the raw water, a second sensor for measuring the water after coagulation and settling, and a third sensor for measuring the treated water either before or after the final disinfection.

[0199] The data from these sensors may be collated on a Cloud system, which may accommodate the computer tool implementing the method.

[0200] The time spacing in the acquisition of the data may be parameterized.

[0201] Alert thresholds may be defined in order to manage and warn of sensor drift. These thresholds can likewise be parameterized on a site-by-site basis. They enable especially the creation of alerts which can be notified to the user. This allows the user, in particular, to know whether it is necessary to modify the dosage of the coagulant (or of a second reagent).

[0202] The method may further comprise a step of determining a dose of another reagent, as for example powdered activated carbon (CAP), in order to improve the performance of the coagulation/settling process and/or to reach the objective for removal of the organic matter in the clarified water.

[0203] The method may furthermore comprise a step of calculating the dose of acid required to obtain a target coagulation pH. This is because it is possible to improve the removal of the organic matter by lowering the coagulation pH, typically by adding acid to the coagulation basin or reactor. The method makes it possible in particular to recover the new coefficients of the function ƒ.sub.i that correspond to this new coagulation pH, and so to recalculate the quantity of coagulant to be added in order to attain the objective for removal of the organic matter in the clarified water.

[0204] As illustrated in FIGS. 8A and 8B, the method also makes it possible to show the economic benefit of the selection of the combinations of a coagulant, another reagent and/or added acid.

[0205] FIG. 8A illustrates the UV of the water as a function of the added dose:

[0206] of coagulant at a pH of 7 (dashed curve A);

[0207] of coagulant at a pH of 6.2 (continuous curve B);

[0208] of coagulant and of CAP to be added (arrow C) when the coagulation pH is 6.2, to attain the target UV of the clarified water.

[0209] FIG. 8B indicates the comparative cost of each dosage illustrated in FIG. 8A:

[0210] a histogram corresponding to the curve A, in which the dotted line corresponds to the limit of the DMEA; above the limit of DMEA is the cost of coagulant to be added in order to attain the UV objective of the clarified water;

[0211] a histogram corresponding to the curve B, in which the dotted line corresponds to the limit of the DMEA; beyond the limit of the DMEA is the cost of coagulant to be added in order to attain the UV objective of the clarified water, with addition of the cost of the product to attain the pH of 6.2 (in black);

[0212] a histogram corresponding to the curve C, with the addition of the cost of the product to attain the pH of 6.2 (in black) and the cost of CAP to be added in order to attain the UV objective of the clarified water.

[0213] The total cost for attaining the UV objective of the clarified water, including CAP and acid, is in this case lower for attaining the UV objective of the clarified water.

[0214] Where acid is added in order to reduce the coagulation pH, the method makes it possible, by recalculating the quantity of coagulant to be added in order to attain the objective for removal of the organic matter in the clarified water, to calculate the drop in quantity of coagulant to be added, the benefit of this difference, and to make a comparison with the cost of added acid. In this way it becomes possible to know whether it is more advantageous to add acid, or to apply more coagulant, or to calculate the best compromise between the two.

[0215] Furthermore, where powered activated carbon is added, the method makes it possible, by recalculating the quantity of coagulant to be added in order to attain the objective for removal of the organic matter in the clarified water, to calculate the drop in quantity of coagulant to be added, the benefit of this difference, and to make a comparison with the cost of added CAP. In this way it becomes possible to know whether it is more advantageous to add CAP, or to apply more coagulant, or to calculate the best compromise between the two.

[0216] It is possible, moreover, to combine the addition of CAP and the addition of acid, and to calculate the calculation of the economic benefit (or loss) when CAP and acid are added.

[0217] FIG. 9 illustrates another embodiment of the method. In the embodiment illustrated, the method further comprises the following steps: [0218] a step 180 of defining a target turbidity value (TURB_.sub.ED) for the clarified water;

[0219] a step 190 of determining a second coagulant dose ([COAG2]) to be added to the raw water (EB) for attaining the target turbidity value (TURB_.sub.ED) for the clarified water; [0220] a step 200 of determining the optimal coagulant dose ([COAG].sub.OPT) to be added to the raw water, comprising the comparison of the first coagulant dose ([COAG1]) and the second coagulant dose ([COAG2]), said optimal dose being the greatest dose between the first coagulant dose ([COAG1]) and the second coagulant dose ([COAG2]).

[0221] According to this embodiment, the 3.sup.rd, organic parameter is preferably the UV.

[0222] FIG. 10 illustrates fourth relationships enabling a second coagulant dose to be determined. The fourth relationships give the coagulant dose to be added in order to attain a turbidity value in the clarified water of at least less than 5 NTU and preferably less than 3 NTU.

[0223] The fourth relationships are dependent on the turbidity of the raw water (TURB.sub.EB) and on the temperature of the raw water (T.sub.EB).

[0224] Accordingly, FIG. 10 illustrates two fourth polynomial relationships, for example of the fourth degree, connecting the coagulant dose required in order to obtain a clarified water turbidity of at least less than 5 NTU and preferably less than 3 NTU, to the turbidity of the raw water (TURB.sub.EB), the coefficients of which vary as a function of the temperature of the raw water (T.sub.EB): [0225] an equation y=a.sub.8x.sup.4+b.sub.8x.sup.3+c.sub.8x.sup.2+d.sub.8x+e.sub.8, when the temperature of the raw water is less than a threshold θ (dashed curve); [0226] an equation y=a.sub.9x.sup.4+b.sub.9x3+c.sub.9x.sup.2+d.sub.9x+e.sub.9, when the temperature of the raw water is greater than a threshold θ (continuous curve).

[0227] The second coagulant dose is obtained by measuring the turbidity of the raw water by means of an in-line turbidity sensor or of manual measurements, and by measuring the temperature by means of a temperature sensor or of manual measurements, and by using the functions defined above in order to define the coagulant dose required in order to remove the turbidity.

[0228] FIG. 11 illustrates the second embodiment, for the case where the first coagulant dose [COAG1] calculated for attaining the objective of organic matter in the clarified water (expressed in terms of UV) is greater than the second coagulant dose [COAG2] calculated for lowering the turbidity objective of the clarified water. The optimal dose in this case is the first coagulant dose which enables a reduction both in the organic matter and in the turbidity in accordance with the objectives set.

[0229] The method of the invention therefore enables calculation of the appropriate dose of coagulant, and even of one or more other reagents, to be applied in order to meet the objective of quality at the plant outlet and/or at the end of coagulation/settlement.

[0230] It enables a more accurate and more correct quantity of coagulant to be obtained, for use in a raw water, which is not specific to one site, but which is established as a function of the characteristics of the water to be treated.

[0231] It enables account to be taken of the steps downstream of the coagulation, by defining, for example, a quality objective at the plant outlet. This allows the most correct dosing.

[0232] It further enables definition of the best balance between coagulant dose to be added and/or powdered activated carbon dose to be added and/or acid dose to be added, in order to reduce the coagulation pH, with the objective of quality at the plant outlet and/or at the end of coagulation/settlement.

[0233] It enables, moreover, the operator to be notified of any changes and/or problems arising in the raw water or in the plant. This enables said operator, in particular, to know whether it is necessary to modify the dosage of the coagulant (or of a second reagent).

[0234] Lastly, the entirety of the results of the calculations, and also the in-line monitoring of the quality of the water, may be rendered on a dashboard which can be adapted to any type of medium (computer, tablet, smartphone, Cloud).

[0235] A supervision system may make it possible, moreover, to carry out remote activation of means (for example, at least one metering pump) for adjusting the dose of coagulant (or of a second reagent).