- Email: [email protected]
Development of a tank-submerged type membrane filtration system
DESALINATION
ELSEVIER
Desalination 119 (1998) 151-158
Development of a tank-submerged type membrane filtration system K. Suda*, S. Shibuya, Y. Itoh, T. Kohno Engineering Department Municipal Water Works Division, EBARA Corporation, 1-6-27 Kohman, Minato-ku, Tokyo 108, Japan, Tel.: +81-3-5461-5501, Fax: +81-3-5461-5784, E-mail. [email protected]
Received 22 June 1998; accepted 29 June 1998
Abstract A tank-submerged type membrane filtration system was developed for filtering highly turbid influent of water purification plants. Testing of microfiltration hollow fiber modules of this system revealed that a housing-less, sheet type module was most effective for the removal of suspended matter. Stable operation was enabled by the use of this module, with no rise in the transmembrane pressure difference during the inflow of highly turbid influent water. Test results indicated that this system was capable of long-term, continuous operation, without any influence by the quality of raw water. Solids entrapped in the filter media were able to be removed by air scrubbing only, therefore making backwashing unnecessary. Keywords:
Sheet type MF module; Inter-fiber clogging; Tank-submerged type; Energy consumption
1. Introduction The use of m e m b r a n e filtration is becoming more widespread in Japan recently. It is being m a i n l y adopted in water purification facilities of small-scale water purification plants. This may be attributed to the success of MAC21 (Membrane Aqua C e n t u r y 21) P r o j e c t ( 1 9 9 1 - 1 9 9 3 ) for promoting the use of membrane filtration. This project was administered by Japan's Ministry of Health and Welfare, National Institute of Public health, and involving eighteen engineering corporations. Another factor behind the increase in m e m b r a n e filtration is that Japan is becoming aware of the hazards caused bywater-borne protozoa, namely Cryptosporidium parvum and Giardia lamblia.
The authors have started R&D on water purification systems featuring m e m b r a n e filtration since about eight years ago. Last year marked the adoption of several tanksubmerged type membrane filtration systems, d e v e l o p e d by the authors, in different locations in Japan. Development of an actualscale system was centered on: 1) Development of an energy-efficient filtration system capable of stable operation, 2) Development of an optimal membrane cleaning method, and 3) Development of a membrane filtration module. The following discusses development factors and plant-scale test results.
2. Development concept Most water purification plants in Japan draw their raw water from rivers. Rivers in
*Corresponding author. Presentedat the Conferenceon Membranesin Drinkingand IndustrialWaterProduction,Amsterdam, September21-24, 1998, International Water ServicesAssociation, EuropeanDesalination Societyand AmericanWater Works Association 0011-9164/98/$ - See froat matter © 1998 ElsevierSeieaeeB.V. All fights reserved. PII S0011-9164(98)00136-2
152
K. Suda et al. / Desalination 119 (1998) 151-158
Japan tend to have rapid flows, unlike those in the USA and Europe, due to the majority of the land being mountainous with only a few plane areas. Consequently, the turbidity of river water is subject to intense fluctuation during rainy periods. There is much rainfall in June and September during which typhoons hit the Japanese islands. Explicit maintenance is carried out in Japanese water purification plants to cope with rapid fluctuations in the turbidity of raw water, as well as with highly turbid raw water, due to such geographical and climatic factors. Our development concept was therefore centered on developing a membrane filtration system capable of energy-efficient stable operation, and of adequately coping with high turbidity. As for MF hollow fiber modules, we decided to use an open type, i.e. a housing-less type, and for the filtration, an external pressure filtration method. The open types were selected as this eliminates the problem of turbid matter clogging inside the housing (inter-fiber clogging) when there is an in-low of highly turbid influent. Removal of such matter is no easy task. In both external and internal pressure type filtration methods, using incased membrane filtration modules could necessitate pretreatment, thus complicating the system. Dead-end filtration was decided over cross-flow filtration, considering that the former was more costand energy-efficient. Thus started our development of a membrane filtration system.
after the inflow startup. When not in use, the membranes could be stored in a dry state, with no need to immerse them in preservative solution like formalin. Three types of modules, namely a cylindrical type, a sheet prototype, and a sheet final type, were used in the course of development. The following explains each. River water was used as the influent in all tests.
3. Some development details
3.2. Cylindrical module A housing-less cylindrical module (see Fig. 1) was used at the initial stage of the development. It was set horizontally in a square shaped tank. An open tank submerged filtration method was applied here. This enabled a cost-saving in that use of a pressure tank became unnecessary, also in that it was possible to immerse the membrane filtration system in an existing concrete tank at a water purification plant. Table 2 shows experimental conditions, while Fig. 2 is a flow chart of the system. Fig. 3 shows conditions of the membrane module upon completion of a test. The cylindrical membrane became subject to inter-fiber clogging within a short time. This is because neither backwashing nor airscrubbing could smoothly remove turbid matter in the center of the module. We t h e r e f o r e d e c i d e d to r e d e s i g n the configuration of the module. The next module we designed was one that was capable in having turbid matter removed only by airscrubbing, i.e. without using backwashing. This attempt was made to allow an increase in the water recovery rate.
3.1. Membrane specifications Identical hollow fiber membranes, which constituted the hub of the system being developed, were used in the course of development. Table 1 shows membrane specifications. The membranes were made of polyethylene, a material chosen for its resistance to oxidizing agents (e.g. sodium hypochlorite), acids and alkalis. The hollow fiber itself carried flexible and durable qualities. The membranes were hydrophilic, thus minimizing membrane fouling by organic matter and enabling satisfactory filtration performance within a short time
3.3. Prototype sheet type membrane module A highly efficient prototype sheet type MF module was developed (see Fig. 4). Sheet form membranes were set in an array to increase the air-scrubbing effect, thus eliminating the inter-fiber clogging seen in the case of the cylindrical module. The volumetric efficiency was about 40% lower than that of the cylindrical module. The transmembrane pressure difference rose gradually (see Fig. 5) and there was almost no inter-fiber clogging. There was almost no attachment of turbid matter inside the module and long-term filtration was possible with
K. Suda et al. /Desalination 119 (1998) 151-158
Fig. 3. Module with inter-fiber clogging.
Fig. 1. Cylindrical module 1800 x L 1000 mm x 5.
". . . . ~ , t
q)J
!
:~
)~
Submerged ta°~
/~ ~ j ) Blower
I /
j._,
Jr*oata° J
"~-~-~"*ar t'°k I
'?"
Waste sludge
Fig. 2. Flow chart of experimental apparatus. 0.6
E
0.5
-.~ 0.4 x,,
0.3 ~3
0.2 .
& o.1 0 0
50
1O0 150 Elapsed time(days)
200,
Fig. 5. Changes in the transmembrane pressure difference of the prototype module.
Fig. 4. Prototype sheet type membrane module.
153
154
K. Suda et al. /Desalination 119 (1998) 151-158
Raw water
i ~e 35 FL A i r vent
ropemtur(~°~ra-v~~ t-e-r............ I -m-Transmernbranep~ressuredifference ~=] 1.5
~--I
I __
l
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0.9 i
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Blowor
,I ,
20
0.7
15
v
0.5
=E
_jr
' _
.
5
.
.
ta
.
---:
0 0
Waste s I udge
Fig. 6. Flow chart of gravity filtration making use of the difference in influence and treated water levels.
;
Eo
1O0
0.1
-0.1 400
200 300 Elapsed time(days)
Fig. 9. Changes in the temperature of raw water and transmembrane pressure difference (No. 1).
I - e - Raw water
50
-__/"~"Tr~_e_a_ted_w_ater3-|
~
~--/
40 ~3o r-- 20 10
0 ~ 0
~ 1O0
200 Elapsed time(days)
300
400
Fig. 10. Turbidity of raw water and treated water (No. 1). Fig. 7. Final sheet type module membrane. --~-Temperature . . . . . .of . . .raw . . . . water .. -m-Transmembrane pressure difference
35
=
l 0.9 0.8 0.7 0.6
Fig. 8. View of the experimental system,
oT
P
0
50
!
r
1O0 150 Elapsed time (days)
r 200
.... 0/ 250
Fig. 11. Changes in the temperature of raw water and transmembrane pressure difference (No. 2).
oE
155
K. Suda et al. / Desalination 119 (1998) 151-158
30[ 25-
T I - - - t .......
20
30 F-
--e--Raw water ~ ~ Treated waterj -
~
TT--
,~
_
--
-O-Temperatura of raw water -II-Transmembrane pressure difference
1
i
0.9
25
iT
°~
0.8 0.7 ~"
20
0.6 0.5 0.4
v
~
x
lO
~5 "~ ~
0.3
I---
0.2
5
0.1 --
0 0
50
100 150 Elapsed time(days)
200
50
1O0
150
25O
200
Elapsed time(days)
Fig. 12. Turbidityof raw water and treated water (No. 2). -O-Temperature of raw water .. _-'~Transn~embrane press~differ_e_nc.e
30
0
250
0
?~--] 1 / 0.9
Fig. 15. Changes in the temperature of raw water and transmembrane pressure difference (No. 4). 1.6 1.4
0.7 ~ 0.6 ~ 0.5 ~ 0.4 ~
20
g 10
0.3
5
0.2 0,1
0 0
I
•
0.4 0.2
p-
l -
L_-~_Trae~e!w-a~er]
==
0
50
100 150 Elapsed time(days)
200
250
Fig. 16. Turbidityof raw water and treated water (No. 4). months, i.e., when the t r a n s m e m b r a n e pressure difference rose at low temperature periods. Running costs were able to be kept down by use of this gravity filtration. Table 2 shows the experimental conditions.
L ...... i
. . . . I - o - R s w water
20
1
0.8 0.6
Fig. 13. Changes in the temperature of raw water and transmembrane pressure difference (No. 3). 30
l
== .¢~,, t
0 150
,~n 100 Elapsed time(days)
1.2
lO
3.4. Final sheet type m e m b r a n e module
5
o
~L~ 0
20
]
.
40
.
.
.
.
.
60 80 1O0 Elapsed time(days)
120
140
Fig. ]4. T u r b i d i t y o f r a w w a t e r a n d treated w a t e r (No. 3).
only air-scrubbing. For e n e r g y - s a v i n g , gravity filtration was attempted making use of the difference in the water levels of the influent and treated water tanks (see Fig. 6). A water level difference of 3-4 m was enough to carry out filtration without use of a pump, except during three to four winter
The final sheet type membrane module was basically the same as the prototype except for the configuration. It is almost the same as the one currently being used. Fig. 7 shows this module and Table 3 shows its specifications. The module shown in Fig. 7 is an external pressure type with a membrane surface area of 56 m 2. Another module, this one with a membrane surface area of 78.4 m 2 was also used. Either of these two modules were used depending on the flow rate of the influent. The white portion stretching horizontally .constitutes the hollow fiber membrane. A both end catchment method was used, with
156
K. Suda et al. /Desalination 119 (1998) 151-158
Table 1. Membrane specification Type Hollow fiber (MF) Material Hydrophilic polyethylene Nominal pore size 0.1 ~tm Internal/external diameter 270 ~tm/410 rtm hollow fiber Table 2. Experimental conditions of membrane modules Condition 1 (cylin~trical) 2 (sheet type) Filtration Suctionpump- Gravityand pumpinduced induced Flux 0.75m/d 0.5 ~ 0.75 m/d Pretreatment Sodium hypochlorite injection Physical Air-scrubbing Air-scrubbingand washing backwashing Table 3. Specifications of final sheet type membrane module Dimensions H 515 x L 800 × W 350 mm Tot. membr, surface area 56 m2 Catchm. plate material Polyurethane Catchm. locations 4 (at upper comers of module) Securing frame material SUS 304 the water catchment located at four upper comer areas. The four catchment pipes are merged into one pipe at the upper part of the module, therefore one catchment pipe for each module. The number of fibers for the membrane with face area of 56 m 2 was about 50,000. There were ten sheets per module. Compared to the prototype, the final module had less clearance between the sheets to enable efficient air-scrubbing. Its volumetric efficiency was thus improved and it was more practical. Its volumetric efficiency was thus improved and it was more practical. When c o m p a r i n g it to the e a r l i e r - m e n t i o n e d cylindrical type, the volumetric efficiency was almost identical and there was no decrease in the turbidity removal performance due to less clearance between the sheets.
4. Experimental results of sheet type module A pilot-scale experimental system, equipped with final sheet type MF modules, was built and field-tested at 4 domestic locations, each with different quality raw water. 4.1. Tests
Main
components
of the experimental
system: a tank equipped with the submerged MF modules, a suction pump, a mobile aeration system, a blower, a raw water storage tank, a treated water tank, and various valves. The mobile aeration system and the blower were equipped for carrying out periodical air-scrubbing. This set up enabled the modules to be cleaned uniformly. Aeration from the lower part of the module made the membrane sheets shake. Consequently, the resulting friction between sheets and the separation effect of the air itself worked to remove matter attaching on the membranes. The system had a drain valve which was used to discharge drainage prior to the start of the cleaning operation. Fig. 8 shows the actual experimental system. The module was installed in the center of the submerged tank. It is also possible to set 2 modules, one on top of the other. A hatch was made on the side of the submerged tank to load/unload the modules. The module itself weighed about 20 kg in a wet state, so the u n l o a d i n g c o u l d be sufficiently done by two adults. 4.2. Test conditions
Tests using this system were done at four domestic locations. Table 4 shows some experimental data. Constant operation was carried out at a flux of 0.5 m3/d. No coagulants were charged. The quality of the raw water and the operation method were different in each of the four tests. Only gravity filtration, with a water level difference of about 4 m, was used in Test 1. Table 5 shows the average quality of the raw water for each tests. Basically, each test was conducted automatically. As for checking the water quality, a continuous type, high-sensitivity turbidimeter was set to measure the turbidity and to check any breakthrough in the membrane. W a t e r a n a l y s i s was also periodically conducted. 4.3. Test results Test 1: Fig. 9 shows changes in the trans-
membrane pressure difference along the elapse of time and Fig. 10 shows the relationship between raw water and treated water turbidity. A significant drop in the t r a n s m e m b r a n e p r e s s u r e d i f f e r e n c e is indicated at the 70th day, 100th day, and
K. Suda et al. /Desalination 119 (1998) 151-158
157
28000mm
ra .
,
Tit
r" r
ZIZ
V]r
r X
r
.
.
.
.
.
'''~'I]
7 Fig. 17. Dimensions of 5,000 m3/d membrane filtration system.
Table 4. Experimental data No. 1 2 3 4 Water source River River River Lake Period Dec 95 - Sept 96 Mar 96 - Oct 96 Aug 96 - Dec 96 June 96 Effluent flow rate 25 m3/d 25 m3/d 50 m3/d 25 m3/d Operation Gravity filtration Pump induced Pump induced Pump induced Flux: 0.5 m/d, Filtration: external pressure type filtration/constant flow filtration, Filtration time: 20-30 min/cycle, Cleaning method: air-scrubbing, Washing time: 0.5-1.0 min/cycle, Pretreatment: sodium hypochlorite injection. Table 5. Average of raw water quality No. 1 2 (river) (river) Turbidity 10.1 4.9 Color 17 19.4 KMnO4 cons. 5.7 7.3 E260 0.138 0.08 DOC 1.4 2.2 NH4+-N 0.14 0.17 Iron 0.57 0.22 Manganese 0.05 0.04 THMFP 0.03 0.32 Gen. bact. (c/l) 15,000 1,600 Colif. group (c/l) 340 50 E260 at 5 cm cell
(mg/1) 3 (river) 2 8 2.8 0.1> 0.1 0.017 0.02 210 -
4 (lake) 1.0 5 4.5 0.11 0.01 0.033 540 -
160th day after the start of the test. This was due to chemical cleaning mainly using acids, alkalis, and sodium hypochlorite. These elements were charged into the submerged tank. The first two chemical cleaning attempts resulted in insufficient filtration recovery and there was an immediate increase in the transmembrane pressure difference. However, the transmembrane pressure difference dropped to below 0.1 kg/cm 2 after the third cleaning and stable operation was able to be continuously maintained. There
was no increase in the pressure difference along fluctuations in the turbidity of raw water. The turbidity in the submerged tank was constantly at 150 degrees. The operation performance by this MF system was stable even when there was highly turbid raw water. The turbidity of the treated water constantly indicated zero. As gravity filtration was used in this test, power consumption was able to be kept extremely low, 0.3 kwh/m 3 for the duration of the test. The suction pump was used for only 4 months during the entire test period. Test 2: Fig. 11 shows changes in the transmembrane pressure difference along the elapse of time and Fig. 12 changes in the turbidity along the same. No chemical cleaning was done in this test. although there was a tendency for the pressure difference to gently rise, the operation was stable. Tests 3 and 4: Figs. 13 and 15 show changes in the transmembrane pressure difference along the elapse of time and Figs. 14 and 16 changes in the turbidity along the same, for tests 3 and 4 respectively. The turbidity of the raw water was lower than that in the former tests. There was therefore no rapid increase in transmembrane pressure
158
K. Suda et al. / Desalination 119 (1998) 151-158
Table 6. Average of treated water quality (mg/1) No. 1
Turbidity Color KMnO4 consumption E260 DOC NH4+-N Iron Manganese THMFP General bacteria (c/I) Coliform group (c/l)
No. 2
No. 3
No. 4
(mg/1)
Removal ratio (%)
(mg/1) Removal ratio (%)
(mg/I)
Removal ratio (%)
(rag/l)
Removal ratio (%)
0 2 3.2 0.109 1.0 0.11 0 0.017 0.021 0 0
100 88.2 43.9 21.0 28.6 21.4 100 66.0 30.0 100 100
0 4.7 5.3 0.08 1.8 0.12 0 0.02 0.027 0 0
0 3 2.8
100 62.5 0.0
0 2 5.2
100 60.0 0.0
0.04 0.005
63.6 50.0
0 0
100 100
100 75.7 28.1 7.0 19.4 32.0 100 59.0 17.2 100 100
0.1> 0.03> 0.01> 0 0
100 100
Note 1:E260 at 5 cm cell; Note 2: Left column in each test = measured value; Note 3: Right column in each test = removal percentage.
difference and operation was very stable in both cases. Average water quality of filtrates: Table 6 shows the average water quality of the filtrates, the removal of turbidity, general bacteria, and coliform groups was 100% in all cases. The removal of insoluble color was also high. In contrast, the removal percentage of soluble matter was not so high. 5. Model case
The said filtration system is currently being used in 6 water purification plants. The smallest system filters 50 m3/d and the largest system, 700 m3/d. Trouble-free, stable automatic operation is maintained. Using data obtained from the previously-mentioned 4 tests conducted and those from actual operation data collected so far, we will attempt to make a model system capable of filtering 5,000 m3/d. Let us consider that the filtration capacity can be accomplished by immersing membrane modules in an existing concrete tank at a water purification plant. Fig. 17 is its dimensions of such a membrane filtration system. It will be (H) 4 m x (L) 13 m x (W) 28 m and which will be immersed in a sedimentation basin. The total number of the membrane modules (surface area of each membrane: 78.4 m 2) will be 144.
6. Conclusions
A housing-less, sheet type MF module was developed and practicalized. This energyefficient module made it possible for stable operation of highly turbid raw water, as well as coping with fluctuations in turbidity. Energy consumption was greatly minimized by the application of gravity filtration, by which filtration can be done making use of the difference in the water levels of the influent and treated water tanks, without the need of a suction pump, except in cold temperature periods. Chemical cleaning of the membranes can be minimized to no more than twice a year, depending on the quality of the raw water. Water purification plants in Japan are currently facing modifications and our membrane filtration system which uses these MF modules is advantageous in that it can be submerged in tanks, thus enabling a saving in initial costs and s p e e d y construction.
References [1]
[2]
S. Shibuya, Y. Itoh, and T. Kuono, Performance of tank-submerged type membrane filtration, International Congress on Membranes and Membrane Process, 1966, pp. 1048-1049. Y. Itoh, S. Shibuya, and K. Suda, Performance of membrane filtration for drinking water. JSME Centennial Grand Congress, 97 Symposium on Environmental Engineering, 1997, pp. 328-331.
ELSEVIER
Desalination 119 (1998) 151-158
Development of a tank-submerged type membrane filtration system K. Suda*, S. Shibuya, Y. Itoh, T. Kohno Engineering Department Municipal Water Works Division, EBARA Corporation, 1-6-27 Kohman, Minato-ku, Tokyo 108, Japan, Tel.: +81-3-5461-5501, Fax: +81-3-5461-5784, E-mail. [email protected]
Received 22 June 1998; accepted 29 June 1998
Abstract A tank-submerged type membrane filtration system was developed for filtering highly turbid influent of water purification plants. Testing of microfiltration hollow fiber modules of this system revealed that a housing-less, sheet type module was most effective for the removal of suspended matter. Stable operation was enabled by the use of this module, with no rise in the transmembrane pressure difference during the inflow of highly turbid influent water. Test results indicated that this system was capable of long-term, continuous operation, without any influence by the quality of raw water. Solids entrapped in the filter media were able to be removed by air scrubbing only, therefore making backwashing unnecessary. Keywords:
Sheet type MF module; Inter-fiber clogging; Tank-submerged type; Energy consumption
1. Introduction The use of m e m b r a n e filtration is becoming more widespread in Japan recently. It is being m a i n l y adopted in water purification facilities of small-scale water purification plants. This may be attributed to the success of MAC21 (Membrane Aqua C e n t u r y 21) P r o j e c t ( 1 9 9 1 - 1 9 9 3 ) for promoting the use of membrane filtration. This project was administered by Japan's Ministry of Health and Welfare, National Institute of Public health, and involving eighteen engineering corporations. Another factor behind the increase in m e m b r a n e filtration is that Japan is becoming aware of the hazards caused bywater-borne protozoa, namely Cryptosporidium parvum and Giardia lamblia.
The authors have started R&D on water purification systems featuring m e m b r a n e filtration since about eight years ago. Last year marked the adoption of several tanksubmerged type membrane filtration systems, d e v e l o p e d by the authors, in different locations in Japan. Development of an actualscale system was centered on: 1) Development of an energy-efficient filtration system capable of stable operation, 2) Development of an optimal membrane cleaning method, and 3) Development of a membrane filtration module. The following discusses development factors and plant-scale test results.
2. Development concept Most water purification plants in Japan draw their raw water from rivers. Rivers in
*Corresponding author. Presentedat the Conferenceon Membranesin Drinkingand IndustrialWaterProduction,Amsterdam, September21-24, 1998, International Water ServicesAssociation, EuropeanDesalination Societyand AmericanWater Works Association 0011-9164/98/$ - See froat matter © 1998 ElsevierSeieaeeB.V. All fights reserved. PII S0011-9164(98)00136-2
152
K. Suda et al. / Desalination 119 (1998) 151-158
Japan tend to have rapid flows, unlike those in the USA and Europe, due to the majority of the land being mountainous with only a few plane areas. Consequently, the turbidity of river water is subject to intense fluctuation during rainy periods. There is much rainfall in June and September during which typhoons hit the Japanese islands. Explicit maintenance is carried out in Japanese water purification plants to cope with rapid fluctuations in the turbidity of raw water, as well as with highly turbid raw water, due to such geographical and climatic factors. Our development concept was therefore centered on developing a membrane filtration system capable of energy-efficient stable operation, and of adequately coping with high turbidity. As for MF hollow fiber modules, we decided to use an open type, i.e. a housing-less type, and for the filtration, an external pressure filtration method. The open types were selected as this eliminates the problem of turbid matter clogging inside the housing (inter-fiber clogging) when there is an in-low of highly turbid influent. Removal of such matter is no easy task. In both external and internal pressure type filtration methods, using incased membrane filtration modules could necessitate pretreatment, thus complicating the system. Dead-end filtration was decided over cross-flow filtration, considering that the former was more costand energy-efficient. Thus started our development of a membrane filtration system.
after the inflow startup. When not in use, the membranes could be stored in a dry state, with no need to immerse them in preservative solution like formalin. Three types of modules, namely a cylindrical type, a sheet prototype, and a sheet final type, were used in the course of development. The following explains each. River water was used as the influent in all tests.
3. Some development details
3.2. Cylindrical module A housing-less cylindrical module (see Fig. 1) was used at the initial stage of the development. It was set horizontally in a square shaped tank. An open tank submerged filtration method was applied here. This enabled a cost-saving in that use of a pressure tank became unnecessary, also in that it was possible to immerse the membrane filtration system in an existing concrete tank at a water purification plant. Table 2 shows experimental conditions, while Fig. 2 is a flow chart of the system. Fig. 3 shows conditions of the membrane module upon completion of a test. The cylindrical membrane became subject to inter-fiber clogging within a short time. This is because neither backwashing nor airscrubbing could smoothly remove turbid matter in the center of the module. We t h e r e f o r e d e c i d e d to r e d e s i g n the configuration of the module. The next module we designed was one that was capable in having turbid matter removed only by airscrubbing, i.e. without using backwashing. This attempt was made to allow an increase in the water recovery rate.
3.1. Membrane specifications Identical hollow fiber membranes, which constituted the hub of the system being developed, were used in the course of development. Table 1 shows membrane specifications. The membranes were made of polyethylene, a material chosen for its resistance to oxidizing agents (e.g. sodium hypochlorite), acids and alkalis. The hollow fiber itself carried flexible and durable qualities. The membranes were hydrophilic, thus minimizing membrane fouling by organic matter and enabling satisfactory filtration performance within a short time
3.3. Prototype sheet type membrane module A highly efficient prototype sheet type MF module was developed (see Fig. 4). Sheet form membranes were set in an array to increase the air-scrubbing effect, thus eliminating the inter-fiber clogging seen in the case of the cylindrical module. The volumetric efficiency was about 40% lower than that of the cylindrical module. The transmembrane pressure difference rose gradually (see Fig. 5) and there was almost no inter-fiber clogging. There was almost no attachment of turbid matter inside the module and long-term filtration was possible with
K. Suda et al. /Desalination 119 (1998) 151-158
Fig. 3. Module with inter-fiber clogging.
Fig. 1. Cylindrical module 1800 x L 1000 mm x 5.
". . . . ~ , t
q)J
!
:~
)~
Submerged ta°~
/~ ~ j ) Blower
I /
j._,
Jr*oata° J
"~-~-~"*ar t'°k I
'?"
Waste sludge
Fig. 2. Flow chart of experimental apparatus. 0.6
E
0.5
-.~ 0.4 x,,
0.3 ~3
0.2 .
& o.1 0 0
50
1O0 150 Elapsed time(days)
200,
Fig. 5. Changes in the transmembrane pressure difference of the prototype module.
Fig. 4. Prototype sheet type membrane module.
153
154
K. Suda et al. /Desalination 119 (1998) 151-158
Raw water
i ~e 35 FL A i r vent
ropemtur(~°~ra-v~~ t-e-r............ I -m-Transmernbranep~ressuredifference ~=] 1.5
~--I
I __
l
1.1
0.9 i
Submerged tank
Blowor
,I ,
20
0.7
15
v
0.5
=E
_jr
' _
.
5
.
.
ta
.
---:
0 0
Waste s I udge
Fig. 6. Flow chart of gravity filtration making use of the difference in influence and treated water levels.
;
Eo
1O0
0.1
-0.1 400
200 300 Elapsed time(days)
Fig. 9. Changes in the temperature of raw water and transmembrane pressure difference (No. 1).
I - e - Raw water
50
-__/"~"Tr~_e_a_ted_w_ater3-|
~
~--/
40 ~3o r-- 20 10
0 ~ 0
~ 1O0
200 Elapsed time(days)
300
400
Fig. 10. Turbidity of raw water and treated water (No. 1). Fig. 7. Final sheet type module membrane. --~-Temperature . . . . . .of . . .raw . . . . water .. -m-Transmembrane pressure difference
35
=
l 0.9 0.8 0.7 0.6
Fig. 8. View of the experimental system,
oT
P
0
50
!
r
1O0 150 Elapsed time (days)
r 200
.... 0/ 250
Fig. 11. Changes in the temperature of raw water and transmembrane pressure difference (No. 2).
oE
155
K. Suda et al. / Desalination 119 (1998) 151-158
30[ 25-
T I - - - t .......
20
30 F-
--e--Raw water ~ ~ Treated waterj -
~
TT--
,~
_
--
-O-Temperatura of raw water -II-Transmembrane pressure difference
1
i
0.9
25
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20
0.6 0.5 0.4
v
~
x
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~5 "~ ~
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0.2
5
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0 0
50
100 150 Elapsed time(days)
200
50
1O0
150
25O
200
Elapsed time(days)
Fig. 12. Turbidityof raw water and treated water (No. 2). -O-Temperature of raw water .. _-'~Transn~embrane press~differ_e_nc.e
30
0
250
0
?~--] 1 / 0.9
Fig. 15. Changes in the temperature of raw water and transmembrane pressure difference (No. 4). 1.6 1.4
0.7 ~ 0.6 ~ 0.5 ~ 0.4 ~
20
g 10
0.3
5
0.2 0,1
0 0
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•
0.4 0.2
p-
l -
L_-~_Trae~e!w-a~er]
==
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100 150 Elapsed time(days)
200
250
Fig. 16. Turbidityof raw water and treated water (No. 4). months, i.e., when the t r a n s m e m b r a n e pressure difference rose at low temperature periods. Running costs were able to be kept down by use of this gravity filtration. Table 2 shows the experimental conditions.
L ...... i
. . . . I - o - R s w water
20
1
0.8 0.6
Fig. 13. Changes in the temperature of raw water and transmembrane pressure difference (No. 3). 30
l
== .¢~,, t
0 150
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3.4. Final sheet type m e m b r a n e module
5
o
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.
40
.
.
.
.
.
60 80 1O0 Elapsed time(days)
120
140
Fig. ]4. T u r b i d i t y o f r a w w a t e r a n d treated w a t e r (No. 3).
only air-scrubbing. For e n e r g y - s a v i n g , gravity filtration was attempted making use of the difference in the water levels of the influent and treated water tanks (see Fig. 6). A water level difference of 3-4 m was enough to carry out filtration without use of a pump, except during three to four winter
The final sheet type membrane module was basically the same as the prototype except for the configuration. It is almost the same as the one currently being used. Fig. 7 shows this module and Table 3 shows its specifications. The module shown in Fig. 7 is an external pressure type with a membrane surface area of 56 m 2. Another module, this one with a membrane surface area of 78.4 m 2 was also used. Either of these two modules were used depending on the flow rate of the influent. The white portion stretching horizontally .constitutes the hollow fiber membrane. A both end catchment method was used, with
156
K. Suda et al. /Desalination 119 (1998) 151-158
Table 1. Membrane specification Type Hollow fiber (MF) Material Hydrophilic polyethylene Nominal pore size 0.1 ~tm Internal/external diameter 270 ~tm/410 rtm hollow fiber Table 2. Experimental conditions of membrane modules Condition 1 (cylin~trical) 2 (sheet type) Filtration Suctionpump- Gravityand pumpinduced induced Flux 0.75m/d 0.5 ~ 0.75 m/d Pretreatment Sodium hypochlorite injection Physical Air-scrubbing Air-scrubbingand washing backwashing Table 3. Specifications of final sheet type membrane module Dimensions H 515 x L 800 × W 350 mm Tot. membr, surface area 56 m2 Catchm. plate material Polyurethane Catchm. locations 4 (at upper comers of module) Securing frame material SUS 304 the water catchment located at four upper comer areas. The four catchment pipes are merged into one pipe at the upper part of the module, therefore one catchment pipe for each module. The number of fibers for the membrane with face area of 56 m 2 was about 50,000. There were ten sheets per module. Compared to the prototype, the final module had less clearance between the sheets to enable efficient air-scrubbing. Its volumetric efficiency was thus improved and it was more practical. Its volumetric efficiency was thus improved and it was more practical. When c o m p a r i n g it to the e a r l i e r - m e n t i o n e d cylindrical type, the volumetric efficiency was almost identical and there was no decrease in the turbidity removal performance due to less clearance between the sheets.
4. Experimental results of sheet type module A pilot-scale experimental system, equipped with final sheet type MF modules, was built and field-tested at 4 domestic locations, each with different quality raw water. 4.1. Tests
Main
components
of the experimental
system: a tank equipped with the submerged MF modules, a suction pump, a mobile aeration system, a blower, a raw water storage tank, a treated water tank, and various valves. The mobile aeration system and the blower were equipped for carrying out periodical air-scrubbing. This set up enabled the modules to be cleaned uniformly. Aeration from the lower part of the module made the membrane sheets shake. Consequently, the resulting friction between sheets and the separation effect of the air itself worked to remove matter attaching on the membranes. The system had a drain valve which was used to discharge drainage prior to the start of the cleaning operation. Fig. 8 shows the actual experimental system. The module was installed in the center of the submerged tank. It is also possible to set 2 modules, one on top of the other. A hatch was made on the side of the submerged tank to load/unload the modules. The module itself weighed about 20 kg in a wet state, so the u n l o a d i n g c o u l d be sufficiently done by two adults. 4.2. Test conditions
Tests using this system were done at four domestic locations. Table 4 shows some experimental data. Constant operation was carried out at a flux of 0.5 m3/d. No coagulants were charged. The quality of the raw water and the operation method were different in each of the four tests. Only gravity filtration, with a water level difference of about 4 m, was used in Test 1. Table 5 shows the average quality of the raw water for each tests. Basically, each test was conducted automatically. As for checking the water quality, a continuous type, high-sensitivity turbidimeter was set to measure the turbidity and to check any breakthrough in the membrane. W a t e r a n a l y s i s was also periodically conducted. 4.3. Test results Test 1: Fig. 9 shows changes in the trans-
membrane pressure difference along the elapse of time and Fig. 10 shows the relationship between raw water and treated water turbidity. A significant drop in the t r a n s m e m b r a n e p r e s s u r e d i f f e r e n c e is indicated at the 70th day, 100th day, and
K. Suda et al. /Desalination 119 (1998) 151-158
157
28000mm
ra .
,
Tit
r" r
ZIZ
V]r
r X
r
.
.
.
.
.
'''~'I]
7 Fig. 17. Dimensions of 5,000 m3/d membrane filtration system.
Table 4. Experimental data No. 1 2 3 4 Water source River River River Lake Period Dec 95 - Sept 96 Mar 96 - Oct 96 Aug 96 - Dec 96 June 96 Effluent flow rate 25 m3/d 25 m3/d 50 m3/d 25 m3/d Operation Gravity filtration Pump induced Pump induced Pump induced Flux: 0.5 m/d, Filtration: external pressure type filtration/constant flow filtration, Filtration time: 20-30 min/cycle, Cleaning method: air-scrubbing, Washing time: 0.5-1.0 min/cycle, Pretreatment: sodium hypochlorite injection. Table 5. Average of raw water quality No. 1 2 (river) (river) Turbidity 10.1 4.9 Color 17 19.4 KMnO4 cons. 5.7 7.3 E260 0.138 0.08 DOC 1.4 2.2 NH4+-N 0.14 0.17 Iron 0.57 0.22 Manganese 0.05 0.04 THMFP 0.03 0.32 Gen. bact. (c/l) 15,000 1,600 Colif. group (c/l) 340 50 E260 at 5 cm cell
(mg/1) 3 (river) 2 8 2.8 0.1> 0.1 0.017 0.02 210 -
4 (lake) 1.0 5 4.5 0.11 0.01 0.033 540 -
160th day after the start of the test. This was due to chemical cleaning mainly using acids, alkalis, and sodium hypochlorite. These elements were charged into the submerged tank. The first two chemical cleaning attempts resulted in insufficient filtration recovery and there was an immediate increase in the transmembrane pressure difference. However, the transmembrane pressure difference dropped to below 0.1 kg/cm 2 after the third cleaning and stable operation was able to be continuously maintained. There
was no increase in the pressure difference along fluctuations in the turbidity of raw water. The turbidity in the submerged tank was constantly at 150 degrees. The operation performance by this MF system was stable even when there was highly turbid raw water. The turbidity of the treated water constantly indicated zero. As gravity filtration was used in this test, power consumption was able to be kept extremely low, 0.3 kwh/m 3 for the duration of the test. The suction pump was used for only 4 months during the entire test period. Test 2: Fig. 11 shows changes in the transmembrane pressure difference along the elapse of time and Fig. 12 changes in the turbidity along the same. No chemical cleaning was done in this test. although there was a tendency for the pressure difference to gently rise, the operation was stable. Tests 3 and 4: Figs. 13 and 15 show changes in the transmembrane pressure difference along the elapse of time and Figs. 14 and 16 changes in the turbidity along the same, for tests 3 and 4 respectively. The turbidity of the raw water was lower than that in the former tests. There was therefore no rapid increase in transmembrane pressure
158
K. Suda et al. / Desalination 119 (1998) 151-158
Table 6. Average of treated water quality (mg/1) No. 1
Turbidity Color KMnO4 consumption E260 DOC NH4+-N Iron Manganese THMFP General bacteria (c/I) Coliform group (c/l)
No. 2
No. 3
No. 4
(mg/1)
Removal ratio (%)
(mg/1) Removal ratio (%)
(mg/I)
Removal ratio (%)
(rag/l)
Removal ratio (%)
0 2 3.2 0.109 1.0 0.11 0 0.017 0.021 0 0
100 88.2 43.9 21.0 28.6 21.4 100 66.0 30.0 100 100
0 4.7 5.3 0.08 1.8 0.12 0 0.02 0.027 0 0
0 3 2.8
100 62.5 0.0
0 2 5.2
100 60.0 0.0
0.04 0.005
63.6 50.0
0 0
100 100
100 75.7 28.1 7.0 19.4 32.0 100 59.0 17.2 100 100
0.1> 0.03> 0.01> 0 0
100 100
Note 1:E260 at 5 cm cell; Note 2: Left column in each test = measured value; Note 3: Right column in each test = removal percentage.
difference and operation was very stable in both cases. Average water quality of filtrates: Table 6 shows the average water quality of the filtrates, the removal of turbidity, general bacteria, and coliform groups was 100% in all cases. The removal of insoluble color was also high. In contrast, the removal percentage of soluble matter was not so high. 5. Model case
The said filtration system is currently being used in 6 water purification plants. The smallest system filters 50 m3/d and the largest system, 700 m3/d. Trouble-free, stable automatic operation is maintained. Using data obtained from the previously-mentioned 4 tests conducted and those from actual operation data collected so far, we will attempt to make a model system capable of filtering 5,000 m3/d. Let us consider that the filtration capacity can be accomplished by immersing membrane modules in an existing concrete tank at a water purification plant. Fig. 17 is its dimensions of such a membrane filtration system. It will be (H) 4 m x (L) 13 m x (W) 28 m and which will be immersed in a sedimentation basin. The total number of the membrane modules (surface area of each membrane: 78.4 m 2) will be 144.
6. Conclusions
A housing-less, sheet type MF module was developed and practicalized. This energyefficient module made it possible for stable operation of highly turbid raw water, as well as coping with fluctuations in turbidity. Energy consumption was greatly minimized by the application of gravity filtration, by which filtration can be done making use of the difference in the water levels of the influent and treated water tanks, without the need of a suction pump, except in cold temperature periods. Chemical cleaning of the membranes can be minimized to no more than twice a year, depending on the quality of the raw water. Water purification plants in Japan are currently facing modifications and our membrane filtration system which uses these MF modules is advantageous in that it can be submerged in tanks, thus enabling a saving in initial costs and s p e e d y construction.
References [1]
[2]
S. Shibuya, Y. Itoh, and T. Kuono, Performance of tank-submerged type membrane filtration, International Congress on Membranes and Membrane Process, 1966, pp. 1048-1049. Y. Itoh, S. Shibuya, and K. Suda, Performance of membrane filtration for drinking water. JSME Centennial Grand Congress, 97 Symposium on Environmental Engineering, 1997, pp. 328-331.