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TITLE OF THE INVENTION CONTROL ARRANGEMENT AND CONTROL METHOD FOR SEAWATER DESALINATION SYSTEM
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control arrangement and control method for a seawater desalination system. More particularly, the present invention is concerned with a control arrangement and control method for a seawater desalination system which is preferably adapted to a seawater desalination system that uses semipermeable membranes to obtain fresh water from seawater or brackish water. 2.Description of the Related Arts
In recent years, in relation to seawater desalination systems employing semipermeable membranes, or especially, reverse osmotic membranes, cases where fouling of the membranes (a phenomenon in which a slightly soluble component, a polyelectrolyte solute, colloid, microscopic solids, or the like, which is contained in raw water, is deposited on the membranes to deteriorate a permeation flux) poses a significant problem have been reported.
In order to remove a fouling causative component contained inseawater, flocculationtreatment, coagulating sedimentation treatment, sand filtration treatment, multimedia filter

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treatment, floatation separation treatment, microfiltration membrane filtration treatment, ultrafiltration membrane filtration treatment, or any other various pretreatment techniques have been employed. In particular, a concentration of a fouling causative component contained in seawater greatly varies depending on a season, time, or weather. Running control through pretreatment dependent on the variation is essential in terms of fouling of semipermeable membranes and minimization of an environmental load.
In case flocculation treatment or coagulating sedimentation treatment which employs a flocculant is adopted from among the pretreatment techniques, the injection rate of the flocculant is controlled through injection ratio stabilization control or turbidity proportioning control. In seawater desalination, a silt density index (SDI) that is an index for a particulate substance is widely employed. Control based on the SDI value is also implemented.
In contrast, membranous fouling is reportedly caused not only by turbid particles but also a low molecular organic substance. For example, non-patent document 1 ("Pretreatment for RO Seawater Desalination and Fouling," Bulletin of the Society of Seawater Science Japan, Vol. 63, p.367-371, 2009) describes that transparent exopolymer particles (TEP) containing an organic substance, or more particularly, gelatinous polysaccharides largely contributes to occurrence

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of fouling.
The TEPs are formed when low molecular polysaccharides in seawater are flocculated to become particulate or colloidal. After phytoplanktons or the like are sheared during pretreatment to be performed using ultrafiltration membranes or the like, when polysaccharides flow out to flocculate, the TEPs are also formed. A change in the concentration of the TEPs little correlates to a change in the turbidity of seawater. For example, when seawater temperature is high and an amount of sunlight is large, the TEP concentration in seawater increases irrespective of the turbidity of seawater.
In conventional injection ratio stabilization control, concentration proportioning control, and flocculant injection control based on an SDI value, it is impossible to implement control dependent on a change in the concentration of polysaccharides in seawater. Fouling of semipermeable membranes may make rapid progress.
In order to prevent rapid progress of fouling of semipermeable membranes, such measures can be taken that when injection ratio stabilization control is implemented, the ratio is increased, or that when turbidity proportioning control is implemented, an offset value is increased. However, in either of the cases, a flocculant is excessively injected. This poses a problem in that a chemical cost or a sludge disposal cost

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increases.
In treatment in which a f locculant is employed, a component of seawater is flocculated, and sedimented, filtered, or floated and separated. Herein, flocculation requires presence of a turbid substance serving as a nucleus.
In case seawater that is raw water contains a little turbid substance, growth of floe is insufficient. Therefore, a fouling causative component such as polysaccharides or TEPs is not sufficiently convoluted into the floe. Eventually, it becomes hard to separate the fouling causative component from seawater.
As a result, the fouling causative component cannot be satisfactorily removed through pretreatment. This poses a problem in that fouling of semipermeable membranes makes rapid progress.
The present invention addresses the foregoing problems. An object of the present invention is to provide a control arrangement and control method for a seawater desalination system which controls pretreatment according to a quantity of a fouling causative component contained in seawater or brackish water so that fouling of semipermeable membranes can be minimi zed, and which provides fresh water stably at a low running cost.
StJMMARY OF THE INVENTION
For accomplishing the foregoing object, a control

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arrangement for a seawater desalination system of the present invention includes: a semipermeable membrane treatment apparatus that uses semipermeable membranes to desalinate seawater or brackish water; a pretreatment apparatus that is disposed on a stage preceding the semipermeable membrane treatment apparatus, and pretreats seawater or brackish water to be fed to the semipermeable membrane treatment apparatus; a pretreated water polysaccharides measurement unit that measures a concentration of polysaccharides contained in treated water provided by the pretreatment apparatus; and a control unit that calculates an operation rate of the pretreatment apparatus on the basis of a pretreated-water polysaccharide-concentration measured value provided by the pretreated water polysaccharides measurement unit, and a predetermined target signal.
For accomplishing the aforesaid object, a control method for a seawater desalination system of the present invention comprises: for desalinating treated water, which is produced from seawater or brackish water by a pretreatment apparatus, using semipermeable membranes of a semipermeable membrane treatment apparatus, measuring a concentration of polysaccharides, which are contained in the treated water of the pretreatment apparatus, by using a pretreated water polysaccharides measurement unit; comparing a measured value of the concentration of polysaccharides with a predetermined

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target signal; calculating an operation rate of thepretreatment apparatus on the basis of the comparison value, and controlling the operation rate using a control unit.
According to the present invention, pretreatment is controlled according to a quantity of a fouling component contained in seawater or brackish water so that fouling of semipermeable membranes can be minimized. Eventually, fresh water can be provided stably at a low running cost.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a flowchart showing an embodiment 1 of a control arrangement for a seawater desalination system in accordance with the present invention;
Fig. 2 is a flowchart for explaining stances of an evaluation index comparison block, an evaluation index arithmetic block, and a disturbance generation block which are incorporated in the embodiment 1 of the control arrangement for a seawater desalination system in accordance with the present invention;
Fig. 3 is a characteristic diagram showing a relationship between a concentration of polysaccharides and an increasing speed of a resistance to semipermeable membrane filtration in the embodiment 1 of the control arrangement for a seawater desalination system in accordance with the present invention;
Fig. 4 is a flowchart showing an embodiment 2 of the control arrangement for a seawater desalination system in accordance

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with the present invention;
Fig. 5 is a flowchart showing an embodiment 3 of the control arrangement for a seawater desalination system in accordance with the present invention; and
Fig. 6 is a flowchart showing an embodiment 4 of the control arrangement for a seawater desalination system in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A control arrangement for a seawater desalination system of the present invention will be described below based on various embodiments. Among the embodiments, the same reference numerals denote identical entities. [Embodiment 1]
Fig. 1 shows a flow in an embodiment 1 of a control arrangement for a seawater desalination system in accordance with the present invention.
As shown in the drawing, a control arrangement for a seawater desalination system in accordance with the embodiment 1 includes : a semipermeable membrane treatment apparatus 12 that uses semipermeable membranes to desalinate seawater 1; a pretreatment apparatus 10 that is disposed on a stage preceding the semipermeable membrane treatment apparatus 12 and that pretreats the seawater 1 to be fed to the semipermeable membrane treatment apparatus 12; a pretreated water polysaccharides

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measurement unit 14 that measures a concentration of polysaccharides contained in the treated water provided by the pretreatment apparatus 10; and a control unit 20 that calculates an operation rate of the pretreatment apparatus 10 on the basis of polysaccharide-concentration information 18, which is a measured value of the concentration of polysaccharides in the pretreated water provided by the pretreated water polysaccharides measurement unit 14, and a predetermined target signal 22, and outputs a pretreatment apparatus control signal 24.
The pretreatment apparatus 10 includes a flocculant injection device 27 that injects a flocculant, and a solids separation device 29 that separates solids,
Pretreated water 16 that flows out of the pretreatment apparatus 10 enters the semipermeable membrane treatment apparatus 12. The concentration of polysaccharides contained in the pretreated water is measured by the pretreated water polysaccharides measurement unit 14. The pretreated water polysaccharides measurement unit 14 can preferably measure the concentration of polysaccharides on-line in real-time. In the present embodiment, the measurement unit may be held off-line.
Polysaccharide concentration information 18 on the pretreated water 16 measured by the pretreated water polysaccharides measurement unit 14 is given to the control unit 20 to which the target signal 22 is predetermined. The

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control unit 20 calculates an operation rate of the pretreatment apparatus 10, and outputs the pretreatment apparatus control signal 24.
Herein, the target signal 22 may represent, in the form of a numerical value, a target concentration of polysaccharides contained in the treated water 16 or a stable running period of the semipermeable membrane treatment apparatus 12. Otherwise, as an item of a running condition search index to be represented by the target signal 22, a running cost of an entire seawater desalination system or an environmental load on the entire seawater desalination system may be selected.
In the semipermeable membrane treatment apparatus 12, in addition to semipermeable membranes provided as plural stages of plural semipermeable membrane units, pumps, a power recovery device, and piping for connecting the pumps and power recovery device are included. The semipermeable membrane falls into a reverse osmotic membrane and forward osmotic membrane according to a usage and material. The semipermeable membranes in the present embodiment may be either of reverse osmotic membranes and forward osmotic membranes.
The pretreatment apparatus 10 may perform any of flocculation treatment, coagulating sedimentation treatment, sand filtration treatment, multimedia filter treatment, floatation separation treatment, microfiltration membrane filtration treatment, and ultrafiltration membrane filtration

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treatment, or a combination thereof. However, for suppressing inflow of solids such as particulates, the pretreatment apparatus 10 needs the solids separation device 29 that performs any of sand filtration treatment, multimedia filter treatment, microfiltration membrane filtration treatment, and ultraf iltrationmembrane filtration treatment so as to separate solids.
In the control unit 20, as shown in Fig. 2, an evaluation index comparison block 40, an evaluation index arithmetic block 38, and a perturbation generation block 36 are included. These blocks are used for different purposes according to the target signal 22. The usages for different purposes will be described below.
For example, when the target signal 22 represents a concentration of polysaccharides contained in the pretreated water 16, the evaluation index comparison block 40 of the control unit 20 compares the target signal 22 with the polysaccharide-concentration information 18. If the polysaccharide-concentration information 18 is larger than the target signal 22, the pretreatment apparatus control signal 24 is outputted so that the pretreatment apparatus 10 can remove a larger quantity of polysaccharides. In contrast, if the polysaccharide-concentration information 18 is smaller than target signal 22, the pretreatment apparatus control signal 24 is outputted so that removal of polysaccharides by the

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pretreatment apparatus 10 can be alleviated.
When the pretreatment apparatus 10 performs flocculation treatment or coagulating sedimentation treatment, the pretreatment apparatus control signal 24 represents an injection rate of a flocculant set in the flocculant injection device 27. If the polysaccharide-concentration information 18 is larger than the target signal 22, the injection rate of the flocculant set in the flocculant injection device 27 is increased.
If the pretreatment apparatus 10 performs any of sand filtration treatment, multimedia filter treatment, microfiltration membrane filtration treatment, and ultrafiltration membrane filtration treatment, the pretreatment apparatus control signal 24 becomes a cleaning initiation signal. If the polysaccharide-concentration information 18 is larger than the target signal 22, a trigger signal that triggers physical backwashing which employs air or water is generated.
If the pretreatment apparatus 10 performs floatation separation treatment, the pretreatment apparatus control signal 24 represents an applied pressure or dissolved air capacity. If the polysaccharide-concentration information 18 is larger than the target signal 22, the applied pressure or dissolved air capacity is increased.
For example, when the target signal 22 represents a stable

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running period of the semipermeable membrane treatment apparatus 12, the evaluation index arithmetic block 38 of the control unit 20 estimates as an evaluation index a period, which elapses by chemical cleaning of membranes or replacement thereof, on the basis of the polysaccharide-concentration information 18.
To begin with, a membranous differential pressure increasing speed of semipermeable membranes due to fouling or a filtration resistance increasing speed is calculated based on the polysaccharide-concentration information 18. As shown in Fig. 3, the higher the concentration of polysaccharides is, the higher the increasing speed of a membranous differential pressure or a resistance to filtration is. The relationship is, as shown in Fig. 3, plotted non-linearly (as a curve) , and the shape of the curve varies depending on the membrane or any other condition.
Since the relationship is non-linear, a mathematical expression to be employed in the calculation should preferably be an expression of a quadratic function, but may be an approximate expression of a linear function or any other function. Alternatively, a table concerning the increasing speed of a membranous differential pressure or a resistance to filtration which is attributable to fouling may be produced in advance, and may be associated with the polysaccharide-concentration information 18. A value within a range of values that is not

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listed in the table may be obtained through interpolation or extrapolation.
A thresholdmembranous differential pressure or resistance to filtration at which membranes are chemically cleaned or replaced with new ones is obtained in advance. A difference from a currently observed membranous differential pressure of semipermeable membranes or from a currently observed resistance to filtration is dividedby the fouling increasing speed, whereby an estimated value of a stable running period of the semipermeable membrane treatment apparatus 12 can be calculated. The estimated value of the stable running period is equivalent to a value obtained when currently observed water quality (the concentration of polysaccharides given by the polysaccharide-concentration information 18) is held intact.
Further, the evaluation index comparison block 40 of the control unit 20 compares an estimated value of a stable running period, which is obtained according to the foregoing procedure, with a stable running period represented by the target signal 22. If the estimated value of the stable running period is smaller than the target signal 22, the pretreatment apparatus control signal 24 is outputted so that the pretreatment apparatus 10 can remove a larger quantity of polysaccharides.
In contrast, if the estimated value of the stable running period is larger than the target signal 22, the pretreatment apparatus control signal 24 is outputted so that removal of

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polysaccharides by the pretreatment apparatus 10 can be alleviated.
When the pretreatment apparatus 10 perforins flocculation treatment or coagulating sedimentation treatment, the pretreatment apparatus control signal 24 represents an injection rate of a flocculant. If the estimated value of a stable running period is smaller than the target signal 22, the injection rate of the flocculant set in the flocculant injection device 27 is increased.
When the pretreatment apparatus 10 performs any of sand filtration treatment, multimedia filter treatment, microfiltration membrane filtration treatment, and ultrafiltration membrane filtration treatment, the pretreatment apparatus control signal 24 becomes a cleaning initiation signal. If the estimated value of the stable running period is smaller than the target signal 22, a trigger signal that triggers physical backwashing which employs air or water is generated.
When the pretreatment apparatus 10 performs floatation separation treatment, the pretreatment apparatus control signal 24 represents an applied pressure or a dissolved air capacity. If the estimated value of the stable running period is smaller than the target signal 22, the applied pressure or dissolved air capacity is increased.
For example, when the target signal 22 represents a running

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cost of an entire seawater desalination system, the control unit 20 acts to shift a running condition to a running condition under which it is possible to reduce the running cost. To begin with, the evaluation index arithmetic block 38 of the control unit 20 estimates the prospective running cost of the entire seawater desalination system.
First, an estimated value of a membranous differential pressure increasing speed concerning semipermeable membranes derived from fouling, or a filtration resistance increasing speed is calculated based on the polysaccharide-concentration information 18. The higher the concentration of polysaccharides is, the higher the increasing speed of a membranous differential pressure or a resistance to filtration is. The relationship is generally non-linear (plotted as a curve) . Because of the non-linear relationship, amathematical expression employed in the calculation should preferably be an expression of a quadratic function but may be an expression of a linear function or any other function.
In addition, a table concerning the increasing speed of the membranous differential pressure or filtration resistance derived from fouling may be produced in advance, and associated with the polysaccharide-concentration information 18 . A value within a range of values that are not listed in the table may be obtained through interpolation or extrapolation.
Athresholdmembranous differential pressure or filtration

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resistance at which membranes should be chemically cleaned or replaced with new ones may be given in advance. A difference from a currently observed membranous differential pressure of semipermeable membranes or a currently observed filtration resistance may be divided by the fouling increasing speed, whereby an estimated value of a stable running period of the semipermeable membrane treatment apparatus 12 can be calculated. The estimated value of the stable running period is equivalent to a value obtained when currently observed water quality (a concentration of polysaccharides represented by the polysaccharide-concentration information 18) is held intact.
When reverse osmotic membranes are adopted as semipermeable membranes, a power used by a high-pressure pump that pressurizes the semipermeable membranes occupies a majority of a power required by an entire seawater desalination system. The cost of the power of the high-pressure pump can be calculated from the values of a membranous differential pressure and filtration flow rate. Themembranous differential pressure can be calculated from an estimated value of a membranous differential pressure increasing speed or filtration resistance increasing speed and a currently observed value of the membranous differential pressure . Using the value of the filtration flow rate and a unit price of a power (yen per kilo-watt-hour (kWh) ) , an estimated value of the power cost required over the whole of a stable running period can be

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calculated.
Further, a current f locculant injection rate is integrated over the whole of a stable running period, and multiplied by a unit price of a chemical (yen/ton). Thus, an estimated value of a chemical cost can be calculated. After the stable running period elapses, chemical cleaning of membranes or replacement thereof becomes necessary. Therefore, a chemical cleaning cost or a membrane replacement cost arises.
The sum of the power cost estimated value, chemical cost estimated value, and chemical cleaning cost or membrane replacement cost is obtained and divided by a total amount of fresh water 2 obtained over the whole of a stable running period, whereby a running cost per 1 m^ of the fresh water 2 can be obtained.
Incidentally, for calculating the running cost, a disposal cost of sludge stemming from pretreatment such as injection of a flocculant may be included. If the number of chemical cleaning enabling times to be counted until membranes are replaced with new ones is known or can be hypothesized, a value obtained by dividing the membrane replacement cost by the number of chemical cleaning enabling times may be added to the chemical cleaning cost.
As a block for calculating the running cost, the evaluation index arithmetic block 38 is included. The running cost is outputted as a calculated value of an evaluation index 44 from

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the evaluation index arithmetic block 38.
Further, the perturbation generation block 36 of the control unit 20 applies a unidirectional perturbation to a currently observed running condition that is outputted as the pretreatment apparatus control signal 24. As for a frequency at which the perturbation is applied, the perturbation may be applied regularly or irregularly.
For example, when an operation rate is expressed with a flocculant injection rate, a perturbation is regarded as an increase or decrease in the flocculant injection rate. The width of the perturbation may be regarded as a change whose effect on the polysaccharide-concentration information 18 on the treated water 16 can be recognized. As a result of the application of the perturbation, the concentration of polysaccharides given as the polysaccharide-concentration information 18 to the control unit 20 varies. The evaluation index arithmetic block 38 can calculate a running cost.
The evaluation index comparison block 40 of the control unit 20 compares the value with a running cost obtained before the application of the perturbation, and decides whichever of the running costs is larger. If the running cost obtained after the application of the perturbation is lower than the running cost obtained before the application of the perturbation, the perturbation generation block 36 applies another perturbation on the same side as a side on which the perturbation has previously

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been applied, and determines an operation rate 42 for the next time. The perturbation is kept applied until the running cost is turned to increase.
When the running cost is turned to increase, a running condition is set to an immediately previous running condition. The running is continued until the timing of the next application of the perturbation. As a result of applying an initial perturbation, if the running cost obtained after the application of the perturbation is higher than the running cost obtained before the application of the perturbation, another perturbation is applied on a side opposite to the side on which the perturbation is applied previously. If the running cost is still high, the running condition is returned to the original one. The running is then continued until the timing of the next application of the perturbation. By implementing this procedure, a seawater desalination system can be run under the condition permitting minimization of the running cost.
When the pretreatment apparatus 10 performs any of sand filtration treatment, multimedia filter treatment, microfiltration membrane filtration treatment, and ultrafiltration membrane filtration treatment, the pretreatment apparatus control signal 24 becomes a cleaning initiation signal. When the pretreatment apparatus 10 performs floatation separation treatment, the pretreatment apparatus control signal 24 represents an applied pressure or dissolved

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air capacity.
For example, when the target signal 22 represents an environmental load on an entire seawater desalination system, the control unit 20 acts to shift a running condition to a running condition that permits minimization of the environmental load. In this case, the evaluation index arithmetic block 38 of the control unit 20 outputs a calculated value of the environmental load as an evaluation index 44 by
substituting a basic unit (kg-C02/kWh) of an environmental-load power for the unit price of a power (yen/kWh), substituting a basic unit (kg-C02/ton) of an environmental-load chemical for the unit price of a chemical (yen/ton), substituting a basic unit of environmental-load chemical cleaning (kg-C02 per number of times) for the chemical cleaning cost, and
substituting a basic unit of environmental-load membrane replacement (kg-C02 per number of times) for the membrane replacement cost.
According to the same procedure as the procedure for obtaining the running cost, an operation rate 42 for the next time is obtained by repeating application of a perturbation. As a result, a seawater desalination system can be run under a condition that permits minimization of an environmental load.
As mentioned above, when the configuration of the embodiment 1 is adopted, fouling of semipermeable membranes

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employed in a seawater desalination system or performance degradation can be suppressed. Therefore, fresh water can be stably fed to a customer. In addition, a chemical cleaning frequency or replacement frequency for the semipermeable membranes can be minimized, and a quantity of a chemical for cleaning or the number of membrane modules to be abandoned can be decreased. As a result, an emission rate of an environmental load and a running cost can be minimized. Further, since pretreatment that is neither overdone nor underdone can be implemented according to a variation in seawater quality, a chemical cost or a sludge disposal cost can be minimized. [Embodiment 2]
Fig. 4 shows a flow in an einbodiment 2 of a control arrangement for a seawater desalination system of the present invention.
As shown in Fig. 4, a control arrangement for a seawater desalination system in accordance with an embodiment 2 has substantially the same configuration as the one of the embodiment 1 shown in Fig. 1 and Fig. 2. The pretreatment apparatus 10 in the embodiment 2 shall perform treatment of injecting a flocculant, that is, flocculation treatment, or coagulating sedimentation treatment.
In the embodiment 2, in addition to the configuration of the embodiment 1, a solid particulates injection apparatus 26 that injects solid particulates to seawater 1 prior to injection

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of a flocculant is included. As the solid particulates, powdered activated carbon, zeolite powder, alumina powder, or magnetic particles will prove effective because of the expectation of adsorbing polysaccharides onto the surface or surfaces thereof.
The solid particulates injection apparatus 26 includes a solid particulates tank and a solid particulates injection pump (not shown). When solid particulates are injected as slurry, the solid particulates injection apparatus further includes a liquid tank for water or the like, a mixing tank, and a mixer which are used for mixing. Tot he solid particulates injection apparatus 26, a solid particulates injection apparatus control signal 28 is sent from the control unit 20 in order to adjust an injection rate of solid particulates.
In pretreatment for injecting a flocculant, floe is produced with a turbid substance as a nucleus . Polysaccharides that are a fouling component are convoluted in the floe, and the floe is sedimented, filtered, or floated and separated. Thus, the floe can be removed from the seawater 1.
However, when the turbidity of the seawater 1 is extremely low, production of floe fails, and a quantity of polysaccharides taken in the floe decreases. As a result, the polysaccharides are left in the semipermeable membrane treatment apparatus 12. This raises the possibility that fouling occurs.
In this case, if solid particulates are injected into the

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seawater 1 on a stage preceding flocculation treatment, a quantity of a substance that becomes a nucleus of floe increases . This makes it possible to overcome a failure in production of floe. As a result, a quantity of polysaccharides to be taken into the floe increases. Eventually, fouling in the semipermeable membrane treatment apparatus 12 can be minimized. However, when solid particulates are injected, a sludge generation rate increases. Therefore, unless an injection rate of the solid particulates is appropriately controlled, not only a chemical cost of the solid particulates increases but also a sludge disposal cost increases.
It is necessary to separate a turbid substance from solid particulates injected water 3, which is obtained after solid particulates are injected, before the solid particulates injected water reaches the semipermeable membrane treatment apparatus 12. Therefore, when an injection rate of the solid particulates is excessive, the running cost of a turbid substance separation apparatus increases. Depending on the nature of powder, the powder may be reused by having an organic substance removed therefrom through warm water cleaning, vapor cleaning, heating, or any other treatment. Even in this case, a reproduction cost arises. Therefore, the injection rate of the solid particulates has to be limited to a minimum necessary rate.
The control unit 20 calculates, in addition to an operation

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rate of the pretreatment apparatus 10, an operation rate of the solid particulates injection apparatus 26, and outputs a solid particulates injection apparatus control signal 28 together with the pretreatment apparatus control signal 24. Herein, the target signal 22 may represent, in the form of a numerical value, a target concentration of polysaccharides contained in the pretreated water 16, or a stable running period of the semipermeable membrane treatment apparatus 12. Otherwise, as an item of a running condition search index to be represented by the target signal 22, a running cost of an entire seawater desalination system or an environmental load on the entire seawater desalination system may be selected.
For example, when the target signal 22 represents the concentration of polysaccharides contained in the pretreated water 16, the evaluation index comparison block 40 of the control unit 20 compares the target signal 22 with the polysaccharide-concentration information 18.
If the polysaccharide-concentration information 18 is larger than the target signal 22, the pretreatment apparatus control signal 24 and solid particulates injection apparatus control signal 28 are outputted so that the pretreatment apparatus 10 can remove a larger quantity of polysaccharides. In contrast, if the polysaccharide-concentration information 18 is smaller than the target signal 22, the pretreatment apparatus control signal 24 and solid particulates injection

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apparatus control signal 28 are outputted so that removal of polysaccharides by the pretreatment apparatus 10 can be alleviated. When the pretreatment apparatus 10 performs flocculat ion treatment or coagulating sedimentation treatment, the pretreatment apparatus control signal 24 represents an injection rate of a flocculant.
If the polysaccharide-concentration information 18 is larger than the target signal 22, a larger quantity of polysaccharides has to be removed. Therefore, increasing a flocculant injection rate and increasing a solid particulates injection rate are carried out. In contrast, if the polysaccharide-concentration information 18 is smaller than the target signal 22, polysaccharides are excessively removed. Therefore, decreasing the flocculant injection rate and decreasing the solid particulates injection rate are carried out.
Plural combinations of a flocculant injection rate and a solidparticulates injection rate for decreasing or increasing a quantity of polysaccharides are conceivable. An effect of the flocculant injection rate and solid particulates injection rate on removal of polysaccharides is predefined as a numerical model. According to the model, the most appropriate running condition may be obtained through calculation. In order to determine a unique running condition, the numerical model should specify a chemical unit price (yen/ton) of a flocculant and

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a chemical unit price (yen/ton) of solid particulates. An arithmetic operation that minimizes the sum of the chemical costs should be carried out.
Preferably, a unit price (yen/ton) of sludge disposal expense should be taken into consideration so that a disposal cost of sludge resulting from injection of a f locculant or solid particulates can be involved in the arithmetic operation. Otherwise, in order to determine a unique running condition, a numerical model is defined to specify a basic unit (k;g-C02/ton) of an environmental-load chemical of a flocculant and a basic unit (kg-C02/ton) of an environmental-load chemical of solid particulates. An arithmetic operation that minimizes the sum of the environmental loads is carried out.
Further, preferably, a basic unit (kg-C02/ton) of environmental-load sludge disposal should be taken into consideration so that an environmental load stemming from disposal of sludge that results from injection of a flocculant or sol id particulates can be involved in an arithmetic operation.
When a numerical model is not employed, the perturbation generation block 36 is used to obtain as a result of feedback an appropriate combination of a flocculant injection rate and solid particulates injection rate according to a procedure to be described below.
If the polysaccharide-concentration information 18 is larger than the target signal 22, the perturbation generation

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block 36 applies a flocculant injection rate, which is represented by the pretreatment apparatus control signal 24, as a perturbation in an increasing direction. A cycle in which the perturbation is applied may be a regular or irregular cycle. The width of the perturbation may be regarded as a change whose effect on the polysaccharide-concentration information 18 on the pretreated water 16 can be recognized.
The evaluation index arithmetic block 38 divides a product of an increment (ton) in the flocculant injection rate and a chemical unit price (yen/ton) of a flocculant by a decrement (mg/L) in a concentration of polysaccharides, and thus obtains as a calculated value of an evaluation index 44 a polysaccharide concentration decreasing cost (yen/(mg/L)) due to the flocculant.
Herein, preferably, a product of the increment (ton) in the flocculant injection rate and the sludge disposal expense unit price (yen per ton) derived from the flocculant is added to the product of the increment (ton) in the flocculant injection rate and the chemical unit price (yen/ton) of the flocculant, and the result is divided by the decrement (mg/L) in the concentration of polysaccharides.
Thereafter, the perturbation generation block 36 applies a perturbation in an increasing direction to the solid particulates injection rate represented by the solid particulates injection apparatus control signal 28. A cycle

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in which a perturbation is applied may be a regular or irregular cycle. The width of the perturbation may be regarded as a change whose effect on the polysaccharide-concentration information 18 on the pretreated water 16 can be recognized.
The evaluation index arithmetic block 38 obtains as the calculated value of an evaluation index 44 a polysaccharide-concentration minimization cost (yen/(mg/L)) due to solid particulates by dividing the product of the increment (ton) in the solid particulates injection rate and the chemical unit price (yen/ton) of the solid particulates by the decrement in the concentration of polysaccharides (mg/L) .
Herein, preferably, the product of the increment (ton) in the solidparticulates injection rate and the sludge-disposal expense unit price (yen/ton) derived fromthe solidparticulates is added to the product of the increment (ton) in the solid particulates injection rate and the chemical unit price (yen/ton) of the solid particulates, and the result is divided by the decrement (mg/L) in the concentration of polysaccharides.
The evaluation index comparison block 40 compares the polysaccharide-concentration minimization cost due to a flocculant, which is obtained according to the foregoing procedure, with the polysaccharide-concentration minimization cost due to solid particulates. If the
polysaccharide-concentration minimization cost due to a flocculant is smaller than the polysaccharide-concentration

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minimization cost due to solid particulates, the solid particulates injection rate is returned to a value obtained before application of a perturbation. A value of a flocculant injection rate obtained after the application of the perturbation is used as the operation rate 42 for the next time, and running is continued. In contrast, if the polysaccharide-concentration minimization cost due to the flocculant is larger than the polysaccharide-concentration minimization cost due to the solid particulates, the flocculant injection rate is returned to a value obtained before the application of the perturbation. A value of the solid particulates injection rate obtained after the application of the perturbation is used as the operation rate 42 for the next time, and the running is continued.
The procedure is executed until the polysaccharide-concentration information 18 becomes equal to or smaller than the target signal 22, whereby running that satisfies the target signal 22 representing the concentration of polysaccharides contained in the pretreated water 16 can be realized at a low cost. Otherwise, when emphasis is put on an environmental load rather than a running cost, the evaluation index arithmetic block 38 performs an arithmetic operation by
reading an environmental-load chemical basic unit (kg-C02/ton) of a flocculant for the chemical unit price (yen/ton) of the

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flocculant,
reading a polysaccharide-concentration decreasing
environmental load due to the flocculant (kg-C02/(mg/L)) for
the polysaccharide-concentration minimization cost due to the
flocculant (yen/(mg/L)),
reading a sludge-disposal environmental load basic unit
(kg-C02/ton) derived from the flocculant for the
sludge-disposal expense unit price (yen/ton) derived from the
flocculant,
reading an environmental-load chemical basic unit of solid
particulates (k;g-C02/ton) for the chemical unit price of the
solid particulates (yen/ton),
reading a polysaccharide-concentration decreasing
environmental load due to the solid particulates
(kg-COa/(mg/L)) for the polysaccharide-concentration minimization cost due to the solid particulates (yen/(mg/L) ) , and reading a sludge-disposal environmental load basic unit
(kg-COa/ton) derived from the solid particulates for the sludge-disposal expense unit price (yen/ton) derived from the solid particulates. Thus, running that satisfies the target signal 22 which represents the concentration of polysaccharides contained in the pretreated water 16 can be realized with a lower environmental load.
For example, assuming that the target signal 22 represents

31
a running cost of an entire seawater desalination system, the control unit 20 acts to shift a running condition to a running condition which permits minimization of the running cost. According to the procedure that is, as described in relation to the embodiment 1, followed by the evaluation index arithmetic block 38, a running cost based on the currently observed polysaccharide-concentration information 18 can be calculated as the evaluation-index calculated value 44. When an attempt is made to minimize the running cost, adjustable factors include two factors of a flocculant injection rate and a solid particulates inj ection rate. In order to obtain an appropriate combination of the flocculant injection rate and solid particulates injection rate which provides the operation rate 42 for the next time, a procedure to be described below is executed.
To begin with, the perturbation generation block 36 applies a perturbation in one direction of an increase or decrease to a flocculant injection rate represented by the pretreatment apparatus control signal 24 . A cycle in which the perturbation is applied may be a regular or irregular cycle. The width of the perturbation may be regarded as a change whose effect on the polysaccharide-concentration information 18 on the pretreated water 16 can be recognized.
As a result of the application of a perturbation, the concentration of polysaccharides to be given as the

32
polysaccharide-concentration information 18 to the control unit 20 varies. When the evaluation index arithmetic block 38 follows the foregoing procedure, a running cost can be calculated as the evaluation-index calculated value 44.
Thereafter, the flocculant injection rate is returned to a value obtained before the application of a perturbation, and the perturbation generation block 36 applies a perturbation in one direction of an increase or decrease to a solid particulates injection rate represented by the solid particulates injection apparatus control signal 28. A cycle in which the perturbation is appliedmay be a regular or irregular cycle. The width of the perturbation may be regarded as a change whose effect on the polysaccharide-concentration information 18 on the pretreated water 16 can be recognized.
As a result of the application of a perturbation, the concentration of polysaccharides to be given as the polysaccharide-concentration information 18 to the control unit 20 varies. When the evaluation index arithmetic block 38 follows the aforesaid procedure, a running cost can be calculated as the evaluation-index calculated value 44.
The evaluation index comparison block 40 compares the running cost, which is obtained after application of a perturbation as mentioned above, with the running cost obtained before the application of the perturbation, and decides whichever of the running costs is lower. If the running cost

33
obtained after the application of the perturbation is lower than the running cost obtained before the application of the perturbation, another perturbation is applied to either of a flocculant injection rate and a solid particulates injection rate on the same side as the side on which the perturbation has been applied. The perturbation is applied until the running cost is turned to increase. Once the running cost comes to increase, a running condition is set to the immediately previous condition, and then outputted as an operation rate 42 for the next time. The running is continued until the timing of the next application of a perturbation.
As a result of applying the first perturbation, if the running cost required after the application of the perturbation is higher than the running cost required before the application of the perturbation, another perturbation is applied on a side opposite to the side on which the perturbation has been applied. If the running cost is still high, the running condition is returned to the original running condition, and the running is continued until the timing of the next application of a perturbation. By executing the foregoingprocedure, aseawater desalination system can be run under a condition under which the running cost can be minimized.
For example, when the target signal 22 represents an environmental load on an entire seawater desalination system, the control unit 20 acts to shift a running condition to a running

34
condition under which an environmental load can be minimized.
In this case, the evaluation index arithmetic block 38 of the
control unit 20
substitutes an environmental-load power basicunit (kg-C02/kWh)
for the power unit price (yen/kWh),
substitutes an environmental-load chemical basic unit
(kg-COa/ton) for a chemical unit price (yen/ton),
substitutes an environmental-load chemical cleaning basic unit
(kg- C02/number of times) for a chemical cleaning cost, and
substitutes an environmental-load membrane replacement basic
unit (kg-C02/number of times) for a membrane replacement cost.
The evaluation index arithmetic block 38 then repeatedly implements an arithmetic operation and application of a perturbation according to the same procedure as the procedure concerning the running cost. As a result, the seawater desalination system can be run under a condition under which an environmental load can be minimized.
As mentioned above, when the configuration of the embodiment 2 is adopted, the same advantage as the one of the embodiment 1 can be provided. [Embodiment 3]
Fig. 5 shows a flow in an embodiment 3 of a control arrangement for a seawater desalination system of the present invention.
As shown in the drawing, the control arrangement for a

35
seawater desalination system in accordance with the embodiment 3 has substantially the same configuration as the control arrangement in accordance with the embodiment 1 shown in Fig. 1 and Fig. 2 . As the pretreatment apparatus 10 in the embodiment 3, an apparatus that performs any of sand filtration treatment, multimedia filter treatment, and floatation separation treatment is adopted.
The pretreated water 16 that flows out of the pretreatment apparatus 10 enters the semipermeable membrane treatment apparatus 12. The concentration of polysaccharides contained in the pretreated water is measured by the pretreated water polysaccharides measurement unit 14. At the same time, the turbidity of the pretreated water is measured by a turbidity measurement apparatus 30, and given as turbidity information 32 to the control unit 20. The target signal 22 and turbidity target value 34 are predetermined to the control unit 20. The control unit 20 calculates an operation rate of the pretreatment apparatus 10, and outputs the pretreatment apparatus control signal 24.
Herein, the target signal 22 may represent, in the form of a numerical value, a target concentration of polysaccharides contained in the pretreated water 16, or a stable running period of the semipermeable membrane treatment apparatus 12. Otherwise, as an item of a running condition search index to be represented by the target signal 22, a running cost of an

36
entire seawater desalination system or an environmental load on the entire seawater desalination system may be selected. The turbidity target value 34 is a target value of the turbidity of the pretreated water 16 that flows into the semipermeable membrane treatment apparatus 12.
In the embodiment 3, unlike the embodiment 1 and embodiment 2, the turbidity of the pretreated water 16 that flows into the semipermeable membrane treatment apparatus 12 is taken into consideration.
Specifically, in reality, hollow fiber membranes or spiral membranes are often employed in the semipermeable membrane treatment apparatus 12. The pretreated water 16 flows to become parallel streams in narrow channels on the membranous surfaces . At this time, if a turbid substance is present, the narrow channels are blocked. This degrades the performances of semipermeable membranes. If not only the channels are blocked but also the turbid substance adheres to or sediments on the surfaces of the semipermeable membranes, the performances of the semipermeable membranes are further degraded. Therefore, preferably, not only polysaccharides but also the turbid substance should be fully removed from the pretreated water 16 that flows into the semipermeable membrane treatment apparatus 12.
When a device that performs microfiltration membrane filtration treatment or ultrafiltration membrane filtration

37
treatment is adopted as the solids separation device 29 on the final stage within the pretreatment apparatus 10, unless the membranes are damaged, the turbid substance is nearly fully removed. However, when a device that performs sedimentation treatment, sand filtration treatment, multimedia filter treatment, or floatation separation treatment is adopted as the solids separation device 29, there arises a possibility that a turbid substance may be left intact in the pretreated water 16 depending on a running condition, and may enter the semipermeable membrane treatment apparatus 12.
When the pretreatment apparatus 10 performs coagulating sedimentation treatment and the solids separation device 29 performs only sedimentation treatment, the evaluation index comparison block 40 of the control unit 20 compares a currently observed turbidity, which is provided as turbid information 32, with the turbid target value 34.
If the currently observed turbidity is higher than the turbidity target value 34, flocculation treatment is insufficiently performed. Therefore, an injection rate of a flocculant that is outputted as the pretreatment apparatus control signal 24 is increased. If the currently observed turbidity is lower than the turbidity target value 34, flocculation treatment is performed excessively. Therefore, the injection rate of the flocculant that is outputted as the pretreatment apparatus control signal 24 is decreased. The

38
flocculant injection rate obtained according to this procedure shall be called a counter-turbidity proper flocculant injection rate.
By injecting a flocculant at a rate higher than the counter-turbidity proper flocculant injection rate, the turbidity of the pretreated water 16 that enters the semipermeable membrane treatment apparatus 12 can be suppressed to a value smaller than the turbidity target value 34. Concurrently, not only the turbidity but also fouling due to polysaccharides has to be suppressed through flocculant treatment.
For example, when the target signal 22 represents the concentration of polysaccharides contained in the pretreated water 16, the evaluation index comparison block 40 of the control unit 20 compares the target signal 22 with the polysaccharide-concentration information 18. If the polysaccharide-concentration information 18 is larger than the target signal 22, the pretreatment apparatus control signal 24 is outputted so that the pretreatment apparatus 10 can remove a larger quantity of polysaccharides.
In the case of the embodiment 3, the pretreatment apparatus 10 performs flocculation treatment or coagulating sedimentation treatment. Therefore, if the
polysaccharide-concentration information 18 is larger than the target signal 22, an injection rate of a flocculant represented

39
by the pretreatment apparatus control signal 24 is increased. In contrast, if the polysaccharide-concentration information 18 is smaller than the target signal 22, removal of polysaccharides by the pretreatment apparatus 10 is excessive. However, if an injection rate of the flocculant is decreased to fall below the counter-turbidity proper flocculant injection rate, removal of turbidity becomes insufficient. Therefore, the counter-turbidity proper flocculant injection rate is regarded as a lower limit, and the flocculant injection rate is controlled to be equal to or larger than the counter-turbidity proper flocculant injection rate.
When the target signal 22 does not represent the concentration of polysaccharides contained in the pretreated water 16, but represents the running cost of an entire seawater desalination system or an environmental load on the entire seawater desalination system, if the lower limit of the flocculant injection rate is set to the counter-turbidity proper flocculant injection rate, blocking of channels by a turbid substance and degradation of performances of semipermeable membranes can be suppressed.
As mentioned above, when the configuration of the embodiment 3 is adopted, the same advantage as the one of the embodiment 1 or embodiment 2 can be provided. [Embodiment 4]
Fig. 6 shows a flow in an embodiment 4 of a control

40
arrangement for a seawater desalination system of the present invention.
As shown in the drawing, the control arrangement for a seawater desalination system in accordance with the embodiment 4 has substantially the same configuration as the control arrangement in accordance with the embodiment 3 shown in Fig. 5. The pretreatment apparatus 10 in the embodiment 4 performs treatment that requires injection of a flocculant, that is, f locculation treatment or coagulating sedimentation treatment. A solid particulates injection apparatus 26 that injects solid particulates to seawater 1 to which a flocculant has not been injected yet is included.
As the solid particulates, powdered activated carbon, zeolite powder, alumina powder, ormagnetic particles will prove effective because of the expectation of adsorbing polysaccharides onto the surface or surfaces thereof.
The solid particulates injection apparatus 26 includes a solid particulates tank and a solid particulates injection pump (not shown). When solid particulates are injected as slurry, the solid particulates injection apparatus 26 further includes a liquid tank for water, a mixing tank, and a mixer which are used for mixing. To the solid particulates injection apparatus 26, a solid particulates injection apparatus control signal 28 is given by the control unit 20. The injection rate of solid particulates is adjusted.

41
In pretreatment in which a flocculant is injected, floe is produced with a turbid substance as a nucleus. Polysaccharides that are a fouling component are convoluted into the floe. The floe is sedimented, filtered, or floated and separated, whereby the floe can be removed from the seawater 1. However, if the turbidity of the seawater 1 is extremely low, production of the floe fails, and a quantity of polysaccharides to be taken into the floe diminishes. As a result, the possibility that fouling may occur because the polysaccharides are left intact in the semipermeable membrane treatment apparatus 12 gets risen.
In this case, when solid particulates are injected into the seawater 1 on a stage preceding the stage of flocculation treatment, a quantity of a substance that becomes a nucleus of floe can be increased, and a failure in production of floe can be overcome. As a result, a quantity of polysaccharides to be taken into the floe increases. Eventually, fouling of the semipermeable membrane treatment apparatus 12 can be minimized. However, when the solid particulates are injected, a sludge generation rate increases. Therefore, unless an injection rate of the solid particulates is appropriately controlled, not only a chemical cost of the solid particulates includes but also a sludge disposal cost increases.
Before the solid particulates injected water 3 that has solid particulates injected thereinto reaches the

42
semipermeable membrane treatment apparatus 12, a turbid substance has to be separated from the solid particulates injected water 3. If the injection rate of the solid particulates is excessively high, the running cost of a turbid substance separation apparatus increases. Depending on the nature of powder, the powder may be reused by removing an organic substance through warm water cleaning, vapor cleaning, heating, or any other treatment. Even in this case, a reproduction cost is required. Therefore, the injection rate of the solid particulates has to be suppressed to a minimum necessary rate.
The pretreated water 16 that flows out of the pretreatment apparatus 10 enters the semipermeable membrane treatment apparatus 12. The concentration of polysaccharides contained in the pretreated water is measured by the pretreated water polysaccharides measurement unit 14, and given as the polysaccharide-concentration information 18 to the control unit 20. In addition, the turbidity is measuredby the turbidity measurement apparatus 30 and given as turbidity information 32 to the control unit 20. Herein, measurement of polysaccharides and measurement of turbidity may be performed concurrently. Further, the measurement of turbidity may be performed earlier than the measurement of polysaccharides or vice versa.
The target signal 22 and turbidity target value 34 are predetermined to the control unit 20. The control unit 20

43
calculates operation rates of the pretreatment apparatus 10 and solid particulates injection apparatus 2 6 alike, and outputs the pretreatment apparatus control signal 24 and solid particulates injection apparatus control signal 28.
Herein, the target signal 22 may represent, in the form of a numerical value, a target concentration of polysaccharides contained in the pretreated water 16 or a stable running period of the semipermeable membrane treatment apparatus 12. Otherwise, as an item of a running condition search index to be represented by the target signal 22, a running cost of an entire seawater desalination system or an environmental load on the entire seawater desalination system may be selected. The turbidity target value 34 is a target value of the turbidity of the pretreated water 16 that enters the semipermeable membrane treatment apparatus 12.
In the embodiment 4, unlike the embodiment 2, the turbidity of the pretreated water 16 that enters the semipermeable membrane treatment apparatus 12 is taken into consideration. In reality, the semipermeable membrane treatment apparatus 12 often employs hollow fiber membranes or spiral membranes. The pretreated water 16 flows to become parallel streams in narrow channels on the surfaces of the membranes. At this time, if a turbid substance is present, the narrow channels are blocked, and the performances of the semipermeable membranes are degraded. If not only the channels are blocked but also the turbid substance

44
adheres or sediments onto or on the surfaces of the semipermeable membranes, the performances of the semipermeable membranes are further degraded. Therefore, not only polysaccharides but also the turbid substance should preferably be fully removed from the pretreated water 16 that enters the semipermeable membrane treatment apparatus 12.
If a device that performs microfiltration membrane filtration treatment or ultrafiltration membrane filtration treatment is adopted as the solids separation device 29 on the final stage within the pretreatment apparatus 10, unless membranes are damaged, a turbid substance is nearly perfectly removed. However, if a device that performs sedimentation treatment, sand filtration treatment, multimedia filter treatment, or floatation separation treatment is adopted as the solids separation device, there is a possibility that the turbid substance may be left intact in the pretreated water 16 and enter the semipermeable membrane treatment apparatus 12. Therefore, the injection rate of a flocculant has to be controlled so that the remaining turbid substance can be appropriately removed.
Therefore, the evaluation index comparison block 40 of the control unit 20 compares a currently observed turbidity, which is given as turbid information 32, with the turbid target value 34. If the currently observed turbidity is higher than the turbidity target value 34, flocculation treatment is

45
insufficiently performed. Therefore, the injection rate of a flocculant to be outputted as the pretreatment apparatus control signal 24 is increased. In contrast, if the currently-observed turbidity is lower than the turbidity target value 34, flocculation treatment is performed excessively. Therefore, the injection rate of the flocculant to be outputted as the pretreatment apparatus control signal 24 is decreased. The flocculant injection rate obtained according to this procedure shall be called a counter-turbidity proper flocculant injection rate.
By injecting a flocculant at a rate equal to or higher than the counter-turbidity proper flocculant injection rate, the turbidity of the pretreated water 16 that enters the semipermeable membrane treatment apparatus 12 can be suppressed to a value lower than the turbidity target value 34. Concurrently, not only the turbidity but also fouling due to polysaccharides has to be suppressed through flocculation treatment. For example, when the target signal 22 represents the concentration of polysaccharides contained in the pretreated water 16, the evaluation index comparison block 40 of the control unit 20 compares the target signal 22 with the polysaccharide-concentration information 18.
If the polysaccharide-concentration information 18 is larger than the target signal 22, the pretreatment apparatus control signal 24 and solid particulates injection apparatus

46
control signal 28 are outputted so that the pretreatment apparatus 10 can remove a larger quantity of polysaccharides. In contrast, if the polysaccharide-concentration information 18 is smaller than the target signal 22, the pretreatment apparatus control signal 24 and solid particulates injection apparatus control signal 28 are outputted so that removal of polysaccharides by the pretreatment apparatus 10 can be alleviated.
If the polysaccharide-concentration information 18 is larger than the target signal 22, a larger quantity of polysaccharides has to be removed. Therefore, both increasing a f locculant injection rate and increasing a solid particulates injection rate are carried out. In contrast, if the polysaccharide-concentration information 18 is smaller than the target signal 22, polysaccharides are excessively removed. Therefore, both decreasing the flocculant injection rate and decreasing the solid particulates injection rate are carried out. However, when the injection rate of the flocculant is decreased to fall below the counter-turbidity proper flocculant injection rate, removal of turbidity becomes insufficient. Therefore, the counter-turbidity proper flocculant injection rate is regarded as a lower limit. The flocculant injection rate is controlled to be equal to or larger than the counter-turbidity proper flocculant injection rate.
Plural combinations are conceivable as a combination of

47
a flocculant injection rate and solid particulates injection rate for bringing polysaccharides to a desired concentration. Therefore, an effect of the flocculant injection rate or solid particulates injection rate on removal of polysaccharides is predefined as a numerical model. According to the numerical model, the most appropriate running condition may be obtained through calculation. In order to obtain a unique running condition, a chemical unit price (yen/ton) of a flocculant and a chemical unit price (yen/ton) of solid particulates are specified in the numerical model, and an arithmetic operation is performed so that the sum of the chemical costs can be minimized.
Further, preferably, a sludge disposal expense unit price
(yen/ton) should be taken into consideration so that a disposal cost of sludge resulting from injection of a flocculant or solid particulates can be involved in an arithmetic operation. Otherwise, in order to obtain a unique running condition, an environmental-load chemical basic unit (kg-COa/ton) of the flocculant and an environmental-load chemical basic unit
(kg-COa/ton) of the solid particulates are specified in the numerical model. An arithmetic operation should be performed so that the sum of the environmental loads can be minimized. Further, an environmental load sludge disposal basic unit
(kg-COa/ton) should preferably be taken into consideration so that an environmental load attributable to disposal of sludge

48
resulting from injection of the f locculant or solidparticulates can be involved in the arithmetic operation.
As mentioned above, when the configuration of the embodiment 4 is adopted, the same advantage as the advantage of any of the aforesaid embodiments can be provided.

49
WHAT IS CLAIMED IS:
1. A control arrangement for a seawater desalination
system, comprising:
a semipermeable membrane treatment apparatus that uses semipermeable membranes to desalinate seawater or brackish water;
a pretreatment apparatus that is disposed on a stage preceding the semipermeable membrane treatment apparatus, and pretreats the seawater or brackish water to be fed to the semipermeable membrane treatment apparatus;
a pretreated water polysaccharides measurement unit that measures the concentration of polysaccharides contained in the treated water provided by the pretreatment apparatus; and
a control unit that calculates an operation rate of the pretreatment apparatus on the basis of a pretreated-water polysaccharide-concentration measured value obtained by the pretreated water polysaccharides measurement unit and a predetermined target signal, and outputs a control signal to the pretreatment apparatus.
2. The control arrangement for a seawater desalination
system according to Claim 1,
wherein the pretreatment apparatus includes a flocculant injection device that injects at least a flocculant and a solids separation device that separates solids from the seawater or

50 brackish water.
3. The control arrangement for a seawater desalination
system according to Claim 2,
wherein the solids separation device performs any of sand filtration treatment, multimedia filter treatment, microfiltration membrane filtration treatment, and ultrafiltration membrane filtration treatment.
4. The control arrangement for a seawater desalination
system according to Claim 1,
wherein the control unit includes
a perturbation generation block that regularly or irregularly applies a perturbation to an operation rate of the pretreatment apparatus,
an evaluation index arithmetic block that calculates a running cost or environmental load as an evaluation index, and
an evaluation index comparison block that compares the evaluation index, which is obtained before the application of the perturbation, with the evaluation index obtained after the application of the perturbation, and outputs an operation rate, which causes the evaluation index to diminish, as an operation rate for the next time.
5. The control arrangement for a seawater desalination

51
system according to Claim 1, wherein:
an apparatus that performs any of sand filtration treatment, multimedia filter treatment, and floatation separation treatment is adopted as the pretreatment apparatus;
a turbidity measurement apparatus that measures a turbidity of treated water of the pretreatment apparatus is included; and
the control unit is included for calculating an operation rate of the pretreatment apparatus on the basis of a turbidity measured value providedby the turbiditymeasurement apparatus, the pretreated-water polysaccharide-concentration measured value provided by the pretreated water polysaccharides measurement unit, the predetermined target signal, and a predetermined turbidity target value, and outputting a control signal to the pretreatment apparatus.
6. The control arrangement for a seawater desalination system according to Claim 1, wherein:
a solid particulates injection apparatus is disposed on a stage preceding the pretreatment apparatus; and
the control unit is included for calculating operation rates of the pretreatment apparatus and solid particulates injection apparatus respectively on the basis of the pretreated-water polysaccharide-concentration measured value provided by the pretreated water polysaccharides measurement

52
unit and the predetermined target signal, and output ting control signals to the pretreatment apparatus and solid particulates injection apparatus respectively.
7. The control arrangement for a seawater desalination
system according to Claim 5, wherein:
a solid particulates injection apparatus is disposed on a stage preceding the pretreatment apparatus; and
the control unit is included for calculating operation rates of the pretreatment apparatus and solid particulates injection apparatus respectively on the basis of the turbidity measured value provided by the turbidity measurement apparatus, the pretreated-water polysaccharide-concentration measured value provided by the pretreated water polysaccharides measurement unit, the predetermined target signal, and the predetermined turbidity target value, and outputting control signals to the pretreatment apparatus and solid particulates injection apparatus respectively.
8. The control arrangement for a seawater desalination
system according to Claim 6 or 7,
wherein at least one of powdered activated carbon, zeolite powder, alumina powder, and magnetic particles is used as the solid particulates of the solid particulates injection apparatus.

53
9. A control method for a seawater desalination system, comprising the steps of:
for desalinating treated water, which is produced from seawater or brackish water by a pretreatment apparatus, using semipermeable membranes of a semipermeable membrane treatment apparatus, measuring the concentration of polysaccharides, which are contained in the treated water of the pretreatment apparatus, by using a pretreated water polysaccharides measurement unit;
comparing the measured value of the concentration of polysaccharides with a predetermined target signal;
calculating an operation rate of the pretreatment apparatus on the basis of the comparison value; and
controlling the operation rate using a control unit.
10 . The control method for a seawater desalination system according to Claim 9, wherein:
the control unit includes
a perturbation generation block that regularly or irregularly applies a perturbation to an operation rate of the pretreatment apparatus,
an evaluation index arithmetic block that calculates a running cost or environmental load as an evaluation index, and
an evaluation index comparison block that compares the

54
evaluation index, which is obtained before the application of the perturbation, with the evaluation index obtained after the application of the perturbation, and outputs an operation rate, which causes the evaluation index to diminish, as an operation rate for the next time;
the target signal represents the concentration of polysaccharides contained in the treated water;
the evaluation index comparison block compares the target signal with polysaccharide-concentration information provided by the pretreated water polysaccharides measurement unit;
if the polysaccharide-concentration information is larger than the target signal, the control unit outputs a pretreatment apparatus control signal so that the pretreatment apparatus can remove polysaccharides; and
if the polysaccharide-concentration information is smaller than the target signal, the control unit outputs the pretreatment apparatus control signal so that removal of polysaccharides by the pretreatment apparatus can be alleviated.
11. The control method for a seawater desalination system according to Claim 10, wherein:
when the pretreatment apparatus performs flocculation treatment or coagulating sedimentation treatment, the pretreatment apparatus control signal represents an injection

55
rate of a flocculant; and if the polysaccharide-concentration information is larger than the target signal, the injection rate of the flocculant is increased; or
when the pretreatment apparatus performs any of sand filtration treatment, multimedia filter treatment, microfiltration membrane filtration treatment, and ultrafiltration membrane filtration treatment, the pretreatment apparatus control signal is a cleaning initiation signal; and if the polysaccharide-concentration information is larger than the target signal, a trigger signal that triggers backwashing which employs air or water is generated; or
when the pretreatment apparatus performs floatation separation treatment, the pretreatment apparatus control signal represents an applied pressure or dissolved air capacity; and if the polysaccharide-concentration information is larger than the target signal, the applied pressure or dissolved air capacity is increased.
12 . The control method for a seawater desalination system according to Claim 9, wherein:
the control unit includes
a perturbation generation block that regularly or irregularly applies a perturbation to an operation rate of the pretreatment apparatus,
an evaluation index arithmetic block that calculates a

56
running cost or environmental load as an evaluation index, and
an evaluation index comparison block that compares the evaluation index, which is obtained before the application of the perturbation, with the evaluation index obtained after the application of the perturbation, and outputs an operation rate, which causes the evaluation index to diminish, as an operation rate for the next time;
the target signal represents a stable running period of the semipermeable membrane treatment apparatus;
the evaluation index arithmetic block estimates as the evaluation index the stable running period, which elapses until the semipermeable membranes are chemically cleaned or replaced with new ones, on the basis of polysaccharide-concentration information provided by the pretreated water polysaccharides measurement unit;
the evaluation index comparison block compares the estimated value of the stable running period with a stable running period represented by the target signal;
if the estimated value of the stable running period is smaller than the target signal, the control unit outputs a pretreatment apparatus control signal so that the pretreatment apparatus can remove polysaccharides; and
if the estimated value of the stable running period is larger than the target signal, the control unit outputs the pretreatment apparatus control signal so that removal of

57
polysaccharides by the pretreatment apparatus can be alleviated.
13 . The control method for a seawater desalination system according to Claim 12, wherein:
when the pretreatment apparatus performs flocculation treatment or coagulating sedimentation treatment, the pretreatment apparatus control signal represents an injection rate of a flocculant; and if the estimated value of the stable running period is smaller than the target signal, the injection rate of the flocculant is increased; or
when the pretreatment apparatus performs any of sand filtration treatment, multimedia filter treatment, microfiltration membrane filtration treatment, and ultrafiltration membrane filtration treatment, the pretreatment apparatus control signal is a cleaning initiation signal; and if the estimated value of the stable running period is smaller than the target signal, a trigger signal that triggers backwashing which employs air or water is generated; or
when the pretreatment apparatus performs floatation separation treatment, the pretreatment apparatus control signal represents an applied pressure or dissolved air capacity; and if the estimated value of the stable running period is smaller than the target signal, the applied pressure or dissolved air capacity is increased.

5a
14. A control arrangement for a seawater desalination system, substantially as herein described with reference to accompanying drawings.
15. A control method for a seawater desalination system, substantially as herein described with reference to accompanying drawings.
Dated this IS** day of April 2012
#^'
of Anand & Anand Advocates Agents for the applicants