Method of flushing desalination apparatus equipped with reverse osmotic membrane module and apparatus therefor

A desalination method and apparatus for separating permeate water (WP) from raw water (WF) pressure-fed by a pump (PM) through a reverse osmotic membrane module (U) to discharge concentrated water (WB) from an outlet valve (V2). When the membrane performance of the reverse osmotic membrane module (U) deteriorates due to formation of a gel layer (G). The number of revolutions of a pump driving motor (MO) is reduced and the opening of the outlet valve (V2) is increased, thereby increasing the flow velocity inside the reverse osmotic membrane module (U) so that the gel layer (G) is removed by the raw water (WF). Provided are a number-of-revolutions control means (1) for controlling the number of revolutions of the motor (MO), an opening control means (2) for controlling the opening of the outlet valve (V2) and an operation means (5) for operating the number-of-revolutions control means (1) and the opening control means (2) when the membrane performance of the reverse osmotic membrane module (U) is lowered.

TECHNICAL FIELD 
This invention relates to a method of flushing a desalination apparatus 
equipped with a reverse osmotic membrane module and an apparatus therefor. 
BACKGROUND ART 
As is well known, the construction of a desalination apparatus with a 
reverse osmotic membrane module is such that, as shown in FIG. 1, the 
reverse osmotic membrane module U is adapted to separate permeate water 
(pure water) WP from raw water (seawater) WF that is pressure-fed by a 
pump PM through a permeable membrane M, concentrated water (concentrated 
brine) WB being finally discharged from an outlet valve V2. In this kind 
of desalination apparatus, a gel layer G (FIG. 2) is formed on the 
permeable membrane M by contaminants, microbes or the like in the water 
over a long period of operation and thus the membrane performance or 
permeation performance deteriorates. It has thus been the practice to 
recover the permeation performance of the permeable membrane M by closing 
valve V1 at predetermined intervals of time T1 (once a month in the 
example shown), as shown by a dotted line A in FIG. 3, the gel layer G on 
the surface of the permeable membrane M then being dissolved and removed 
with a liquid chemical. Such removal operation is ordinarily performed at 
a given frequency within the range of from once every half a month to once 
every two months, depending on the specific operating conditions, such as 
the quality of raw water, the recovery rate, etc. Such being the case, 
certain problems are encountered since not only must the system be shut 
down, but also the cost of the liquid chemical and labour involved are 
considerable. These problems lead to an overall increase in the cost of 
the desalination. 
DISCLOSURE OF INVENTION 
An object of this invention is to provide a method of flushing a 
desalination apparatus having a reverse osmotic membrane and an apparatus 
therefor which can serve to reduce the number of times it is necessary to 
remove a gel layer with a liquid chemical. 
The inventors of this invention found, as a result of various studies, that 
when a large quantity of low-pressure water is fed through a reverse 
osmotic membrane over a predetermined period of time, the total quantity 
of raw water flows to the outlet valve such as to increase the flow 
velocity through the reverse osmotic membrane, whereby the gel layer is 
removed by the shearing force of the flow of raw water. The present 
invention was accomplished having taken notice of this principle. 
The present invention relates to a desalination method utilizing a reverse 
osmotic membrane module, by which means permeate water is separated from 
raw water pressure-fed by a pump through a reverse osmotic membrane, 
concentrated water being discharged from an outlet valve, wherein when the 
membrane performance of the reverse osmotic membrane module deteriorates 
due to a gel layer, the number of revolutions of a pump driving motor is 
reduced and the opening of the outlet valve is increased, thereby 
increasing the flow velocity inside the reverse osmotic membrane module so 
that the gel layer is removed by the raw water. 
Further, the present invention relates to a desalination apparatus for 
separating by a reverse osmotic membrane module permeate water from raw 
water pressure-fed by a pump through a reverse osmotic membrane, 
concentrated water being discharged from an outlet valve, comprising a 
number-of-revolutions control means for controlling the number of 
revolutions of a pump driving motor, an opening control means for 
controlling the opening of the outlet valve and an operation device 
operable to control the number-of-revolutions control means and the 
opening control means to reduce the number of revolutions of the pump 
driving motor and increase the opening of the outlet valve when the 
membrane performance of the reverse osmotic membrane module is lowered. 
According to the present invention, by reducing the number of revolutions 
of the pump and increasing the opening of the outlet valve, the pressure 
inside the reverse osmotic membrane module is reduced and the flow rate is 
increased, so that the quantity of water permeating through the reverse 
osmotic membrane module becomes substantially zero. The increased quantity 
of water makes the flow velocity high such as to allow the gel layer 
attached to the reverse osmotic membrane to be easily peeled off. Since 
the gel layer can be effectively be peeled off and removed by the shearing 
force of such a large quantity of high-velocity raw water flow, it is 
possible to make the interval of liquid chemical flushing twice that with 
a conventional apparatus, make the life of the reverse osmotic membrane 20 
percent longer than that of a conventional apparatus, shorten the 
shut-down period of the desalination apparatus, and reduce the cost of 
desalination. 
It is preferable in embodiments of the present invention to use an inverter 
or a fluid coupling as the number-of-revolutions control means. 
It is preferable in embodiments of the present invention to use an 
electrical or pneumatic positioner as the opening control means. 
In embodiments of the present invention, the operation device is preferably 
constructed by a timer and a controller. Such an arrangement is suitable 
for a steady operation. 
It is preferable in embodiments of the invention to use as the operation 
device a control unit comprising a microcomputer which calculates the 
membrane performance from the flow rates, pressures and electric 
conductivities of any two of the raw water, permeate water and 
concentrated water, and which outputs control signals to the 
number-of-revolutions control means and the opening control means when the 
membrane performance reaches a predetermined value at which flushing is 
required. Such an arrangement is suitable for an unsteady operation.

BEST MODE FOR CARRYING OUT THE INVENTION 
Embodiments of the present invention will be described below with reference 
to the drawings. 
FIG. 4 shows an embodiment for a steady operation. In this drawing, raw 
water WF is pressurized by a turbo-type pump PM driven by a motor MO and 
fed through a pipe line L1 to a reverse osmotic membrane module U. The 
reverse osmotic membrane module U separates permeate water WP from the raw 
water WF. The separated permeate water WP is recovered through a pipe line 
L2, while concentrated water WB is discharged by way of a pipe line L3 
through an outlet valve V2. 
Connected to the motor MO is a number-of-revolutions control means for 
controlling the number of revolutions of the motor, such as an inverter 1, 
and the outlet valve V2 is provided with an opening control means for 
controlling the opening thereof, for example, a positioner 2. 
The inverter 1 and the positioner 2 are connected through wirings l1, l2, 
respectively, to a controller 3. To the controller 3 a timer 4 is 
connected, the controller 3 and the timer 4 constituting an operation 
device 5. 
The operation device 5 outputs, in operation, a control signal of, for 
example, 8 mA to the inverter 1 at a predetermined time, i.e., when the 
membrane performance of the reverse osmotic membrane M of the reverse 
osmotic membrane module U has fallen to a predetermined level due to the 
formation of a gel layer G (at the time t shown in an enlarged figure of 
the portion enclosed by a circle shown in FIG. 3), and performs control to 
reduce the number of revolutions of the motor MO, which is normally 
controlled by the inverter 1 with a control signal of 18 mA, from 100% to 
40%. The operation device 5 also outputs a control signal of, for example, 
4 V to the positioner 2 and performs control to increase the opening of 
the outlet valve V2, which is normally controlled at 15% by the positioner 
2 with a control signal of 2 V, to increase to 80%. 
The characteristics of the pump P, i.e., the relationship between the head 
H (pressure) and the flow rate Q, are as shown in FIG. 5, and the 
resistance R of the pipe line increases in proportion to the flow rate Q. 
Normally, the pump is operated at the state corresponding to an 
intersection D1 of a curve of H (n=100%) and a curve of R (o=15%). If, for 
example, the number of revolutions n of the motor decreases to 40% and the 
outlet valve opening o increases to 80%, the head H falls to a curve H 
(n=40%) and the pipe line resistance R falls to a curve R (o=80%). Thus, 
the operation of the pump R becomes coincident with the intersection D2 
between the two curves. This allows a large quantity of low-pressure raw 
water WF to flow into the reverse osmotic membrane module U. In this case, 
the power level required for operating the pump may be lower than that for 
the normal operation. 
On the other hand, assume that the flow rate Q, pressure P and electric 
conductivity (representing the salt concentration) C of each of the raw 
water WF, permeate water WP and concentrated water WB are indicated by 
adding thereto subscripts F, P and B, respectively, and that the mean 
pressure is indicated by PM, the mean electric conductivity being 
indicated by CM, the membrane performance by A, and the osmotic pressure 
by .pi.M. Then the following expressions are obtained: 
EQU PM=(P.sub.F +P.sub.B)/2 (1) 
EQU CM=(C.sub.F =C.sub.B)/2 (2) 
EQU .pi.M=f(CM) (3) 
EQU Q.sub.P .apprxeq.A.multidot.(PM-.pi.M) (4) 
Now, as shown in FIG. 6, for example, in the steady operation of Q.sub.F 
=100 m.sup.3 /H and P.sub.F =25 kg/cm.sup.2, the mean flow rate 
QM=116/2=58 (m.sup.3 /H) in the case of Q.sub.P =84 m.sup.3 /H and Q.sub.B 
=16 m.sup.3 /H. 
If this condition is changed to another condition of, for example, Q.sub.F 
=150 m.sup.3 /H and P.sub.F =2 kg/cm.sup.2, as shown in FIG. 7, PM of the 
expression (1) becomes low because P.sub.F is low and thus PM=.pi.M. 
Accordingly, from the expression (4), Q.sub.P =0 (m.sup.3 /H). That is, 
the whole quantity of raw water WF flows inside the reverse osmotic 
membrane module U and is discharged from the outlet valve V2. As a result, 
the flow velocity of the raw water WF inside the reverse osmotic membrane 
module is significantly increased from V1 (FIG. 2) to V2 which is more 
than 2.5 times V.sub.1, as shown in FIG. 8. By virtue of the shearing 
force generated by the high-speed flow of the large quantity of raw water, 
the gel layer G is effectively peeled off and removed. This is done at the 
predetermined time t, as shown in FIG. 3, thus making the membrane life 
20% longer than that of a conventional apparatus, and the interval T2 of 
the liquid chemical flushing about 2 times the conventional interval 
T.sub.1. 
FIG. 9 shows an embodiment for an unsteady operation, in which the pipe 
lines L1, L2 and L3 are provided with sensors 7F, 7P, 7B; 8F, 8P, 8B and 
9F, 9P, 9B for detecting the flow rate Q.sub.F, Q.sub.P, Q.sub.B, the 
pressure P.sub.F, P.sub.P, P.sub.B, and the electric conductivity C.sub.F, 
C.sub.P, C.sub.B of the raw water WF, permeate water WP and concentrated 
water WB flowing through the respective pipe lines. These sensors, the 
inverter 1 and the positioner 2 are connected to a control unit 10, the 
operation device comprising, for example, a microcomputer. In operation, 
this control unit 10 computes the membrane performance A based on the 
signals from the respective sensors in accordance with the above-described 
expressions (1), (2), (3) and (4), and, when detecting that the membrane 
performance A has been lowered to a predetermined value at which flushing 
is required, outputs control signals to the inverter 1 and the positioner 
2 to reduce the number of revolutions of the motor MO and increase the 
opening of the outlet valve V2. Then flushing similar to that in the 
above-described embodiment is effected. 
Moreover, in a case where a plurality of reverse osmotic membrane modules 
are juxtaposed, a valve may be provided in each pipe line for supplying 
the raw water to the respective reverse osmotic membrane modules. The 
flushing operation can be performed by opening one of the valves coupled 
to a corresponding reverse osmotic membrane module to be subjected to the 
flushing and by closing the remaining valves. For example, in the case 
where two reverse osmotic membrane modules are juxtaposed, as shown in 
FIG. 10, the reverse osmotic membrane modules U1 and U2 are connected in 
parallel through pipe lines La1 and La2 to the pipe line L1, and the pipe 
lines La1 and La2 are provided with change-over valves Va1 and Va2, 
respectively. Thus, by closing the change-over valve Va2 and opening the 
change-over valve Va1, for example, the flushing of the reverse osmotic 
membrane module U1 can be performed. Further, the reverse osmotic membrane 
modules U1 and U2 may be replaced with module blocks each comprising a 
plurality of modules. PG,9 
INDUSTRIAL APPLICABILITY 
As described above, the present invention produces the advantages that, 
because a gel layer attached to a reverse osmotic membrane can readily be 
removed, the intervals at which liquid chemical flushing is performed can 
be extended, the life of the reverse osmotic membrane becomes longer and 
the shut-down period of the apparatus is shortened. Therefore, the present 
invention is effective in the flushing of a desalination apparatus for 
desalinating raw water, such as seawater, sewerage water, etc., through a 
reverse osmotic membrane module.