Method for washing inner surface of tubular permeable membrane

A method for washing an inner surface of a tubular permeable membrane which comprises: PA0 a friction piece having a hardness of from about 10.degree. to 30.degree. measured according to JIS K6301, 5-2 attached to an end of an elastic rod-like support, and PA0 operating the support to reciprocate the friction piece within the tubular permeable membrane to thereby act a rub-washing force of from about 0.1 to 1.0 kg/cm.sup.2 between the inner surface of the tubular permeable membrane and the friction piece so as to remove contaminants adhered to the inner surface of the tubular permeable membrane.

FIELD OF THE INVENTION 
The present invention relates to a method for washing an inner surface of a 
tubular permeable membrane 
BACKGROUND OF THE INVENTION 
In the case of membrane separation process using a membrane such as an 
ultrafiltration membrane or a reverse osmosis membrane, solid materials in 
a raw liquid adhere. to the inner surface of the membrane with the lapse 
of time and a so-called membrane contamination proceeds. As a result, 
deterioration in permeation performance of the membrane is unavoidable. 
Therefore, the membrane must be periodically washed to recover the 
permeation performance of the membrane. 
The efficiency of washing can be evaluated by the degree of recovery of the 
permeation performance of the membrane, viz., the ratio of the amount of 
the membrane permeated liquid after washing the membrane to the initial 
amount of the membrane permeated liquid. Any criterion has not been 
conventionally established to determine the efficiency of washing and this 
efficiency factor is appropriately determined in accordance with various 
conditions such as the manner of washing, the degree of contamination of 
the membrane, the period of washing, etc. 
The present inventors have found that there is the relationship such that 
the higher the efficiency of washing is, the lcwer the rate of membrane 
contamination becomes, between the efficiency of washing (i.e., the degree 
of the amount of contaminants adhered to the surface of the membrane after 
washing) and the rate of membrane contamination after washing (i.e., 
reduction in liquid permeation rate with the lapse of time). For example, 
referring to FIG. 2, curves A, B and C each shows the state of reductron 
in liquid permeation rate after washing when the liquid permeation rate 
decreases to 9.462 lpm (liter per minute; hereinafter the same) in an 
ultrafiltration apparatus using a tubular permeable membrane having the 
initial amount of the membrane permeated liquid of 30.28 lpm. That is, the 
curves A, B, and C show the states after washing the membrane so as to 
obtain the liquid permeation rate of 21.763 lpm (the degree of recovery: 
about 72%), after washing the membrane so as to obtain the liquid 
permeation rate of 24.60 Zpm (the degree of recovery: about 81%), and 
after washing the membrane so as to obtain the liquid permeation rate of 
28.387 lpm (the degree of recovery: about 94%), respectively. It is 
apparent from the results that slight increase in the efficiency of 
washing greatly contributes to preventing the liquid permeation rate from 
lowering. For example, on comparison between the curves B and C shown in 
FIG. 2, in the case of the curve B, the degree of recovery is 81% and the 
degree of reduction in the liquid permeation rate after washing (30 days 
after) is 42% (the ratio of the lowered liquid permeation rate after 30 
days to the liquid permeation rate immediately after washing). On the 
other hand, in the case of the curve C, the degree of recovery is 94% and 
the degree of reduction in the liquid permeation rate after washing is 
13%. Therefore, the degree of reduction in the liquid permeation rate can 
be decreased about 30% by only increasing the degree of recovery by 13%. 
As described above, in the membrane washing, the more the degree of 
recovery in the permeation performance of the membrane is increased, the 
more the degree of membrane contamination after washing can be decreased, 
so that it is possible to make the intervals of washing of the membrane 
longer to thereby minimize the frequency of washing. This is convenient in 
the maintenance of membrane. 
A method in which a chemical agent is filled in a tubular membrane to 
dissolve contaminants adhered to the membrane is conventionally known as a 
method for washing a tubular permeable membrane. In this method, however, 
it is extremely difficult to reach the above-described degree of recovery 
near 100%. Therefore, a further methcd is known in which after washing the 
membrane with the chemical agent, a washing ball such as sponge ball or 
the like is introduced into the tubular membrane to run it within the 
tubular permeable membrane by a fluid back pressure. In this method, 
contaminants adhered to the surface of the membrane are removed by 
shearing force (hereinafter referred to as "rub-washing force") caused 
between the washing ball and the membrane surface. 
The rub-washing force & is expressed as follows: 
EQU .tau.=A.sub.1 (P.sub.1 -P.sub.2)/2A.sub.2 
wherein P.sub.1 and P.sub.2 represent upstream and downstream fluid 
pressures acting onto the washing ball, respectively, and A.sub.1 and 
A.sub.2 represent contacting areas between the ball and the fluid and 
between the ball and the membrane, respectively. In order to increase the 
rub-washing force, the fluid pressure must be increased. However, a limit 
exists to increase the pressure P.sub.1 in view of the pressure resistance 
of the membrane. Further, there is an inconvenience that if the pressure 
P.sub.1 is increased, the flow rate correspondingly increases to thereby 
cause a difficulty in liquid operation. Thus, a limit exists to increase 
the rub-washing force. In this case, it is also difficult to reach the 
above-described degree of recovery near 100%. 
Thus, it is difficult in the conventional method for washing a surface of a 
tubular permeable membrane to reach the degree of recovery of the 
permeation performance of the membrane near 100% and the practical upper 
limit is at most 80%. 
If the degree of recovery can be increased even several %, the lowering of 
the liquid permeation rate with the lapse of time after washing or the 
progress of membrane contamination can be effectively prevented, as 
described above. In the prior art methods, however, such a technical 
advantage has not been recognized. 
The present inventors have recognized such a technical advantage and made 
various investigations on a washing method which is capable of reaching 
the degree of recovery of the permeation performance of a membrane at 
least 90%, preferably near 100%. 
A method in which in the above-described washing ball system, a pushing rod 
to directly transmit external force to a washing ball is used instead of 
fluid pressure as an external force which is a washing force source is 
known as such a method, and this method is known as a washing means in the 
field of elongated or deep vessels. 
If this washing means can be utilized to wash a tubular permeable membrane, 
it will be possible to perform washing in which the permeation performance 
of the membrane can be substantially recovered completely. As a result, 
the progress of membrane contamination after washing can be remarkably 
improved, and the total effect obtained by the combinaticn with the 
tubular permeable membranes can be unexpectedly raised. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a method for washing 
an inner surface of a tubular permeable membrane which can remove 
dontaminants adhered to the membrane safely without injuring the membrane 
by using a specific washing means 
The method for washing an inner surface of a tubular permeable membrane 
according to this invention comprises: 
preparing a friction piece having a hardness of from about 10.degree. to 
30.degree. measured using a spring type hardness tester according to JIS 
K6301, 5-2 (hereinafter the same) attached to an end of an elastic 
rod-like support; and 
operating the support to reciprocate the friction piece within the tubular 
permeable membrane to thereby act a rub-washing force of from about 0.1 to 
1.0 kg/cm.sup.2 between the inner surface of the tubular permeable 
membrane and the friction piece so as to remove contaminants adhered to 
the inner surface of the tubular permeable membrane.

DETAILED DESCRIPTION OF THE INVENTION 
This invention is explained by reference to the accompanying drawings. 
In FIG. 1, A is a membrane module and comprises an outer cylinder 1; a 
plurality of tubular permeable membrane 2 which are inserted to the outer 
cylinder and are sealed at both ends to the outer cylinder with a sealing 
material 3 (e.g., a thermosetting resin); an inlet 11 for a permeating 
liquid provided on the outer cylinder; and a screw 12 for joint. 
B is a washing device and comprises an elastic rod-like support 41 and a 
friction piece 42 attached to an end of the support. 
The washing of the tubular permeable membrane according to this invention 
is conducted in the following manners. 
A chemical agent is filled in a tubular permeable membrane in the state 
that the membrane module is connected to a pipe arrangement to thereby 
perform washing with the chemical agent, the membrane module is separated 
from the pipe arrangement, the friction piece of the above-described 
washing device is inserted into each of the tubular permeable membranes, 
and the friction piece is reciprocated within the membrane by operating an 
elastic rod-like support. In this case, the rub-washing force acts between 
the surface of the membrane and the friction piece, so that contaminants 
adhered to the inner surface of the membrane are removed as the friction 
piece moves by the rub-washing force, because the contaminants have 
swollen due to the washing with the chemical agent. Depending on the kind 
of contaminants adhered, the washing with the chemical agent may be 
omitted. 
The rub-washing force must be within a range from about 0.1 to 1.0 
kg/cm.sup.2, preferably from 0.3 to 0.7 kg/cm.sup.2. If the force is 
larger than 1.0 kg/cm.sup.2, the membranes tend to be damaged, and if the 
force is smaller than 0.1 kg/cm.sup.2, it is difficult to remove the 
contaminants. 
The moving speed of the friction piece is generally from about 0.2 to 2.0 
m/sec, preferably from 0.5 to 1.5 m/sec. 
It is desirable for the friction piece to have the hardness of from about 
10.degree. to 30.degree., preferably from 12.degree. to 18.degree., so as 
to prevent the membrane damage. Materials which can be used as the 
friction piece are sponge, foam, etc. made of polyvinyl formal rubber, 
natural rubber, butyl rubber, urethane rubber, acrylic rubber, etc. The 
shape of the friction piece is usually a spherical foam. 
In order to obtain the rub-washing force as described above by using such a 
friction piece, the outer diameter of the friction piece is from about 1.1 
to 1.4 times the inner diameter of the tubular membrane. 
In the above-described case, if the contact area between the friction piece 
and the inner surface of the membrane is represented by S, the external 
force in equilibrium with the maximum rub-washing force .tau..sub.max is 
S.multidot..tau..sub.max. Therefore, if the rod-like support can transmit 
an external force without bending even if the external force is larger 
than S.multidot..tau..sub.max, excessive rub-washing force may act to 
arise a risk of damage in the membrane. Therefore, it is necessary to use 
a rod-like support which has proper stiffness such that the rod may 
elastically bend by external force substantially equal to the 
above-described external force S.multidot..tau..sub.max. In general, a 
plastic rod which is made of nylon, polypropylene, polyethylene, or the 
like, or a spring wire, having an outer diameter of from about 3.0 to 6.0 
mm is used as the support. The elastic modulus of the rod-like support is 
usually from about 3.times.10.sup.3 to 10.times.10.sup.3 kg/cm.sup.3. 
It is necessary for the rod-like support to have its length slightly larger 
than that of the membrane module. The length of the module is generally 
from about 2.5 to 3.0 m and it is desirable for the length of the support 
to be about 3.5 m. 
This invention will now be explained in detail by reference to the 
following Examples and Comparative Examples. 
EXAMPLE 1 
A membrane module used was an ultrafiltration apparatus having tubular 
permeable membranes each having an inner diameter of 11.5 mm and having a 
total membrane area of 65.5 m.sup.2. A washing device used was a device 
comprising a nylon rod having an outer diameter of 4.5 mm and a length of 
3.5 m and a polyvinyl formal rubber sponge ball having an outer diameter 
of 14.5 mm and a hardness (JIS K6301, 5-2) of 15.degree. attached to an 
end thereof. 
The membrane module was used to control a paint in an electrodeposition 
coating line. The initial liquid permeation amount was 30.28 lpm. After 
0.5 year, the liquid permeation amount was decreased to 9.462 lpm and the 
weight of contaminants adhered per unit area of membrane reached 11.4 
g/m.sup.2. The tubular membranes were washed with a chemical agent (a 
mixture of organic acid, cellosolve, and a nonionic surface active agent) 
and then a polyvinyl formal rubber sponge ball was reciprocated against 
each of the tubular permeable membranes at an average speed of 1 m/sec. In 
this case, the rub-washing force was 0.4 kg/m.sup.2. 
Immediately after washing with the chemical agent, the weight of 
contaminants was 6.9 g/m.sup.2, the liquid permeation amount was 21,763 
lpm, and the degree of recovery was about 72%. Further, immediately after 
the reciprocation of the polyvinyl formal rubber sponge ball, the amount 
of contaminants adhered was very slight and it was impossible to measure 
the weight thereof. The liquid permeation amount was 28.387 lpm, and the 
degree of recovery was about 94%. 
COMATIVE EXAMPLE 1 
After washing with the chemical agent as in Example 1 above, several 
polyvinyl formal rubber sponge balls having an outer diameter of 14.5 mm 
were introduced inside the rubular ultrafiltration module and were 
reciprocated by reversing the direction of fluid stream forward and 
backward as the well-known feed flaw-type sponge ball cleaning. 
Immediately after reciprocating the ball, the weight of contaminants on 
the membrane was 3.0 g/m.sup.2, the liquid permeation amount was 24.60 
lpm, and the degree of recovery was about 81%. 
COMATIVE EXAMPLE 2 
Only washing with the chemical agent was performed in Example 1. 
Lowering of the liquid permeation amount with the lapse of time after 
washing was measured in Example 1 and Comparative Examples 1 and 2. The 
results obtained are shown in FIG. 2, wherein Curve A shows the results 
obtained in Example 1, and Curves B and C show the results obtained in 
Comparative Examples 1 and 2, respectively. 
As is apparent from those results, progress of membrane contamination after 
washing can be extremely suppressed by the method for washing tubular 
permeable membranes according to this invention, as compared with the 
conventional membrane washing method. Therefore, the number of washing can 
be reduced and maintenance of the membrane module is easy. 
EXAMPLE 2 
A membrane module used was an ultrafiltration apparatus having tubular 
permeable membranes each having an internal diameter of 11.5 mm and having 
a total membrane area of 60.0 m.sup.2. A washing device used was the same 
as used in Example 1. 
The membrane module was used to treat waste water of water soluble cutting 
oil. The initial liquid permeation amount was 43 lpm. After use for 3 
months, the permeation amount was decreased to 24 lpm. A polyvinyl formal 
rubber sponge ball was reciprocated by operating the nylon rod at a speed 
of 1 m/sec without conducting washing with a chemical agent. The liquid 
permeation amount immediately after reciprocating the ball was 39 lpm, and 
the degree of recovery was about 91%. When the permeation amount was 
measured after use for 30 days, the liquid permeation rate was 33 lpm and 
the degree of reduction in the liquid permeation amount was only 16% which 
was comparable to the value of about 13% in Example 1 (24.60% lpm with the 
lapse of 30 days after recovering to 28.387 lpm) and remarkably superior 
to the value of about 42% in Comparative Example 1 (14.19 lpm with the 
lapse of 30 days after recovering to 24.60 lpm). 
Thus, in the washing method according to the present invention, the effect 
of remarkable suppression of membrane contaminants after washing can be 
expected even if the washing with a chemical agent is not conducted in 
advance, depending on the kind of contaminants. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modification can be made therein without 
departing from the spirit and scope thereof.