Liquid purification or separation – Casing divided by membrane into sections having inlet – Each section having inlet
Reexamination Certificate
1999-03-05
2001-05-01
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Casing divided by membrane into sections having inlet
Each section having inlet
C210S321840, C210S231000, C210S232000, C210S641000, C210S321810, C210S321900
Reexamination Certificate
active
06224766
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a membrane treatment method and apparatus for filtering a raw liquid such as sludge in an aeration tank for biological treatment, sludge discharged from the aeration tank, concentrated sludge obtained therefrom, waste water containing human excrement before being subjected to biological treatment, etc. More particularly, the present invention relates to a membrane treatment method and apparatus which can reduce costs, increase flux (flow volume per unit area of membrane), and decrease installation space.
2. Description of the Related Art
Waste water containing organic substances, nitrogen, phosphorus, etc., which would contaminate oceans, rivers and the like is generally subjected to biological treatment for conversion to clean water and is then discharged into a river, for example.
As means for separating the solid and liquid components of a reaction mixture resulting from biological treatment, a gravity-type settling tank has conventionally been used.
However, in recent years, a membrane separation technique has been used so as to reduce installation space and facilitate maintenance.
In such a membrane separation technique, steady production of filtrate over a prolonged period of time is very important. However, the volume of filtrate unavoidably decreases with passage of time. This problem is considered to be partly attributed to separated concentrated substances which deposit on the surface of a membrane and form a gel layer, which grows and hinders the passage of liquid to be filtered. The thickness of the gel layer increases as the concentration of contaminants in sludge increases and as the volume of filtrate increases. Accordingly, in the membrane separation technique, reduction in the thickness of the gel layer and removal of the generated gel layer are quite important.
Conventionally, a membrane treatment apparatus as shown in
FIG. 5
is known. In
FIG. 5
, numeral
10
denotes a membrane apparatus, numeral
11
denotes a raw liquid tank for storing a raw liquid such as sludge, and numeral
12
denotes a pressurization pump. Numeral
13
denotes a frame which can be disassembled after removal of unillustrated packing seals. A plurality of membrane plates
14
are removably disposed within the frame
13
. Each membrane plate
14
consists of a membrane support member
17
and membranes
18
.
The membranes
18
are attached to both faces of the membrane support member
17
with a clearance
17
a
on each side.
Openings
15
and
16
for forming fluid passages are formed at upper and lower ends of each membrane plate
14
, respectively.
Numeral
19
denotes discharge ports through which filtrate is discharged. Numeral
20
denotes a raw liquid inlet formed in the frame, and numeral
21
denotes a concentrated liquid outlet. Numeral
22
denotes inter-membrane passages through which raw liquid and/or concentrated liquid flows.
The raw liquid in the raw liquid tank
11
is led to the raw liquid inlet
20
by the pressurization pump
12
. The raw liquid led to the membrane apparatus
10
flows into the inter-membrane passages
22
directly or via the opening(s)
16
, so that the raw liquid is separated into concentrated liquid and filtrate that passes through the membranes
18
.
The filtrate is led to the outside of the membrane apparatus
10
through the discharge ports
19
. The concentrated liquid is returned to the raw liquid tank
11
via the concentrated liquid outlet
21
and is mixed with the raw liquid within the raw liquid tank
11
. The above-described circulation is repeated by the action of the pressurization pump
12
.
In general, the volume of liquid circulating within the inter-membrane passages
22
is determined on the basis of the flow rate of the liquid flowing through the inter-membrane passages
22
. But, it is more important that the circulation volume is restricted depending on the diameter of the openings
15
and
16
formed at the upper and lower ends of the membrane plate
14
. The openings
15
and
16
are designed to have a relatively large diameter such that a high circulation volume is secured in order to obtain a desired volume of filtrate; e.g., to have a diameter of about 65 mm.
Therefore, in order to conform to the relatively large openings, feed piping from the raw liquid tank
11
to the raw liquid inlet
20
and return piping from the concentrated liquid outlet
21
to the raw liquid tank
11
are designed to have a large diameter, thus increasing facility cost. Further, in addition to the piping, various types of accessories provided in the piping become larger, resulting in further increased facility costs.
In the conventional pressurized-type membrane processing apparatus using the pressurization pump
12
, the horsepower (electrical power) of the pressurization pump
12
must be increased, since the volume of raw liquid fed from the raw liquid tank
11
to the inter-membrane passages
22
is large, and the raw liquid must be pressurized within the inter-membrane passages
22
. Therefore, the conventional apparatus involves a problem of increased operating cost. Further, a pump of a large horsepower requires a large installation area.
When the membrane processing apparatus is operated in a state in which a pressure is applied to the raw liquid on the side of the membrane facing the inter-membrane passages
22
(on the side where sludge is circulated), the flow volume of filtrate increases temporarily. However, due to the increase in the flow volume of filtrate, growth of the gel layer on the membrane surface accelerates, with the result that the volume of filtrate decreases. In order to maintain a large flow volume of filtrate, higher power cost becomes necessary.
In order to solve the problems involved in the conventional pressurized-type membrane processing apparatus, a bubble-circulation-type membrane processing apparatus has been proposed.
As shown in
FIG. 6
, the proposed bubble-circulation-type membrane processing apparatus differs greatly from the conventional pressurized-type membrane processing apparatus in that no pressurization pump is used.
In
FIG. 6
, reference numeral
30
denotes a circulation tank disposed parallel to a membrane apparatus
10
. The circulation tank
30
and the membrane apparatus
10
are connected with each other via an inlet pipe
32
for leading concentrated raw liquid to the membrane apparatus
10
and a discharge pipe
33
for discharging the concentrated raw liquid from the membrane apparatus
10
. Thus, a circulation system is formed.
Reference numeral
40
denotes an aeration pipe inserted into the lower openings
16
and adapted to discharge fine air bubbles, reference numeral
41
denotes bubble discharge holes formed in the aeration pipe
40
, and reference numeral
50
denotes a suction pump for suctioning filtrate.
The circulation tank
30
is constructed such that raw liquid is fed from an unillustrated raw liquid tank to a raw-liquid receiving port
31
a
, while excess concentrated liquid is allowed to overflow via a concentrated liquid discharge port
31
b
to thereby return to the raw liquid tank.
When air is supplied to the aeration pipe
40
to discharge fine bubbles from the discharge holes
41
, within in the inter-membrane passages
22
there arises a difference in density between the raw liquid containing bubbles and the raw liquid newly supplied from the circulation tank
30
. Due to this difference in density, a circulation flow is created between the membrane apparatus
10
and the circulation tank
30
.
Meanwhile, filtrate is taken out to the outside via the discharge ports
19
by the action of the suction pump
50
.
This apparatus offers the following advantage. Growth of gel layers on membrane surfaces is prevented, so that blocking due to sludge can be avoided while a large flow volume of filtrate is maintained. Further, sludge blocking can be prevented uniformly over the entire surface of membranes. Moreover, disassembly of the frame and cleaning of the membranes can be perform
Hashimoto Tadaaki
Yamato Kimio
Carter, Ledyard & Milburn
Fortuna Ana
Mitsui Chemicals Inc.
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