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Desalination for landfill leachates containing condensed inorganic salts
Desalination, 97 (1994) 415-425 Elsevier Science B.V., Amsterdam -
415 Printed in The Netherlands
Desalination for landfill leachates containing condensed inorganic salts Toshio Kawanishi, Shingo Matsumoto, Yasuo Horii and Morio Masuzaki Kubota Corporation, 2-47, Shikitsuhigashi I-chome, Naniwa-ku, Osaka 556 (Japan)
SUMMARY
Recently in Japan, leachates from general waste landfills have increased their inorganic salt concentration and have caused various problems inside and outside treatment facilities. Because there is strong demand for countermeasures to cope with these problems, the authors have undertaken studies on leachate desalination using a continuous type electrodialyzer. The results of the experiments confirmed that stable desalination can be achieved by conducting the conventional leachate treatment as pre-treatment. Furthermore, comparing desalination methods, we have come to the conclusion that electrodialysis is most appropriate for landfill leachate desalination.
INTRODUCTION
In recent years plastic refuse has been incinerated in municipal waste plants in Japan. As a result, fly ash with a high chlorine content is discharged from the exhaust gas hydrogen chloride scrubbing equipment. This ash is eventually disposed of in general waste landfills where leachates have increased their inorganic salt concentration and cause various problems inside and outside the treatment facilities. There is strong demand for countermeasures to cope with these problems. OOll-9X34/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDIOOll-9164(94)00104-V
416
An outstanding example of ecological damage caused by salt is the appearance in mountain rivers of diatom (Amphiprora alata) which grow in brackish water such as water in a river mouth. Present countermeasures against salt damage, for example damage to paddy rice plants, has been the dilution of leachate with rain water or underground water. The development and practical use of desalination technology for leachate treatment is also expected. Against this background, the authors have undertaken a series of studies on leachate desalination using a continuous type electrodialyzer.
EXPERIMENTALDESIGN
Raw water
Our sample was a leachate from a lysimeter (experimental landfill tub) filled with mixed refuse (Cl- content 2.8% by weight) containing 70% incineration residue, 20% noncombustible shredded refuse, and 10% municipal refuse compost. Equipment
1. Lysimeter - a square steel tub of 6 m width, 10 m length (2x5 m chambers) and 2.5 m height having a hopper base and a total capacity of 112.5 m3 (nominal). The five chambers are of three types: three semiaerobic, one anaerobic, and one leachate circulating semi-aerobic. Table I lists the refuse filling conditions for the lysimeter of which leachate was sampled. TABLE I Filling conditions for lysimeter Status of landfill
Completed
Landfill structure Water content, %
Semi-aerobic
Wet filled weight (t)
20.0
Filled volume (m3)
15.8
19.9
417
14’r”r mnval I .
.
Biol ical Q+tiag trea#ent se#~ti~
sandfiltration, Activated carbon
adsorption OesaliMtiMl AA
r-l-
Fig. 1. Flow diagram of test plant.
2. Leachate treatment test plant - Fig. 1 shows a flow diagram of the test plant based on the addition of sodium carbonate in combination with ferric chloride for Ca removal and a contact aeration process with saddleshaped packing for biological treatment. 3. Desalination equipment - The equipment used was a continuous type electrodialyzer (70 cm W x 120 cm L x 154 cm H) comprising an electrodialysis bath, a demineralized water circulator, a concentrated water circulator, and other elements. Table II gives outline specifications of the electrodialyzer. Fig. 2 is a flow diagram of the electrodialyzer. The multichamber electrodialysis bath employed ion-exchange membranes pretreated for selective passage of monovalent cations and anions based on remarkable differences in ion permeability (Na+ > Ca*+; Cl- b SO:-. TABLE II Outline specifications of electrodialyzer Method of water feed
Continuous type
Electrodialysis
Effective membrane surface area 2dm2 x lopairs
bath
Cation membrane (12 units) Strongly acidic Anion membrane (10 units) Strongly alkaline Rectifier
DC outout = 10 A x 55 V
418
ElectrcdialysSs bath
kctif ier
Fig. 2. Flow diagram of electrodialyzer.
Method
Leachate was temporarily stored in the feed tank and then pumped to the test plant in a fixed volume for pretreatment. After activated carbon adsorption, the pretreated leachate was desalinated under three sets of conditions: Run 1 for a target Cl- level of 200 mg/l, Run 2 for 500 mg/l, and Run 3 for 1000 mg/l.
RESULTS AND DISCUSSION
Results of pretreatment
Leachate feed rates were in the 50-60 l/d range. Results of the pretreatment are given in Table III, in which figures for water quality parameters are mean values over the experimental desalination period. BOD, COD and TOC removal
Fig. 3 shows concentration changes by process. BOD was reduced to 8.5 mg/l (99% removal) by contact aeration, marking good performance
7.0 816
7.8
398
135
405
732
0.66
480
2.117
PH M-alkalinity, mg/l
ORP, mV
TOC, mg/l
BOD, mgh
DO, mg/l
COD&, mg/l
COD,,
8.9
221
18,400
1,450
1,363
9,377
TDS, mg/l
T-Ca, mg/l
Ca2+, mg/l
Cl-, mg/l
36
NH,-N, mgl
SS, mg/l
87
Kje-N, mg/l
NO,x-N, mg/l
96
T-N, mg/l
mg/l
23
22
Temperature, “C
9,300
20
24
18,650
28
9.7
29
38
48
1,585
447
637 -
400
134
Calcium removal
Water quality parameter
Test plant treatment results
TABLE III
8,943
27
34
16,550
12
37
0.9
19
55
507
207
2.7
8.5
209
120
788
8.3
23
9,911
30
36
16,250
22
28
1.8
15
41
433
116
-
6.5
148
133
269
7.2
22
9,411
46
56
17,550
21
27
2.4
16
43
347
110
3 -
130
145
262
6.8
22
Contact Coagulating Sand aeration sedimentation filtration
9,505
71
72
17,750
26
28
1.4
4.4
32
243
13
2 -
19
80
211
6.6
22
Activated carbon
191
0.6
0.7
860
0.4
1.1
0.4
0.9
2.0
4.5
4
0.4 -
5
52 -
8.0
22
RUN 1
479
0.7
0.9
1,130
1.8
0.8
0.1
0.8
1.6
8
4
1.3 -
6
65 -
7.4
26
RUN 2
Electrodialysis
939
0.9
1
2,450
2.6
2.3
0.3
0.5
2.8
10
6
1.5 -
8
160 -
7.1
27
RUN 3
P e
420
A=T 0
C
OfB
0 0 x -CODnn 0 =CODcr
Fig. 3. Concentration
changes by process (1).
of the biological treatment. Membrane pollution with organic substances did not occur in the electrodialysis bath, probably because of the low residual BOD of 2 mg/l in the water treated with activated carbon (hereinafter referred to as “pretreated leachate”). Considerable amounts of COD substances remained in the treated water after contact aeration at 207 mg/l for COD,, and 507 mg/l for COD,,, which are attributable to sparingly biodegradable COD substances characteristic of leachate. In the coagulating sedimentation treatment which followed, coagulation was conducted in an acid range (pH 5-6) with a relatively high COD,, removal of 76%. TOC was removed in a pattern similar to that of COD,, removal, with a value of 19 m/l for the pretreated leachate. Nitrogen removal
Fig. 4 shows concentration changes by process. the changes in NH,-N and NO,-N concentrations suggest fair nitrification in the contact aeration chamber. Calcium removal
The mean T-Ca concentration of leachate was 1450 mg/l, 94% of which was accounted for by soluble Ca2+ ions. The Ca removal process reduced the T-Ca content to 24 mg/l (98%) removal) but was followed by a slight increasing tendency after biological treatment.
421
Fig. 4. Concentration changes by process (2).
Results of desalination Table IV gives the operating conditions for the continuous type electrodialyzer. The results of the desalination treatment are given in Table III. Fig. 5 shows desalination characteristics for Run 2. Because reduction in permeate flux, due to density polarization in the ion-exchange membrane of the electrodialysis bath, membrane surface, organic pollution, etc., is critical in the practical application of the system, permeate flux changes were examined in each run. The results are given in Table V. Chlorine ion permeate flux (J Cl), an index of the Cl- permeability of the ion-exchange membrane, was obtained using the Nernst-Planck approximate equation as follows: JCl
= -2 Cl U Cl C Cl
l
d4ldx
(1)
TABLE IV Desalination
conditions
Item
RUN1
RUN2
RUN3
Cl- target level, mg/l Leachate feed rate, l/min Membrane surface flow velocity Stationary DC voltage, V
200 1.2-2.0 2.5-3.9 7- 10
500 1.2-3.2 2.3-6.2 8-10
1000 3.0-3.3 5.8-6.4 9-10
(cm/s)
422
Fig. 5. Desalination
characteristics.
TABLEV Chlorine ion permeate flux by run Flux (mol/cm2
where 2 Cl represents an ionic valency, C Cl the Cl concentration in the membrane, U Cl the Cl- mobility through the membrane, $J the potential, and x a variable (cm) at a right angle with respect to the membrane surface. Permeate flux Chlorine ion permeate flux values (Table V) demonstrated no significant reduction due to membrane pollution in any of the runs. The results show that the greater the target level (200 mg/l, 500 mg/l, 1000 mg/l), the greater the chlorine ion permeate flux. Desalination level We were able to desalinate a pretreated leachate containing about 10,000 mg/l chlorine ions below the target Cl- concentrations of 200,500,
423
and 1000 mg/l for Runs 1, 2 and 3, respectively. The TDS/Cl- ratio increased from about 1.9-2.0 for the leachate to 2.6-4.5 for the desalinated water, suggesting selective permeation of Cl- ions. Other water quality parameters
BOD decreased from 2 mg/l for the pretreated leachate to 0.4-1.5 mg/l, and TOC from 19 mg/l to 5-8 mg/l, indicating a slight removal of organic substances. As for nitrogen, high N removal efficiencies were obtained, irrespective of the form of N - with treated water T-N concentrations of 1.6-2.8 mg/l, or 91-95% T-N removal. Use of this system shows the potential to achieve effluent T-N levels of under 3 mg/l.
PRACTICAL
LEACHATE
DESALINATION
We also conducted an experiment with a sampling of treated leachate from an actual facility. It confirmed that the leachate can be desalinated using our experimental electrodialyzer. Table VI gives an example of the practical leachate desalination results. TABLE VI
Practical leachate desalination results Chlorine ion concentration
RUN A
RUNB
Leachate Cl-, mg/l Treated water Cl-, mg/l
5192 268
5203 573
COMPARISON
OF LEACHATE
DEiSALINATION METHODS
Currently available methods of chlorine ion removal include ion exchange, distillation, reverse osmosis, and electrodialysis. Of these the ion-exchange method is used at the lower extreme of Cl- concentrations of 500 mg/l or lower, while the distillation method is used at the higher extreme, or Cl- concentrations of a few tends of thousands of milligrams per liter or higher. Since the adaptability of these methods for landfill
424
TABLE VII Comparison of desalination methods Item
Electrodialysis
Reverse osmosis
Membrane-passing substance (s)
Cations and anions
Water
Membrane type
Ion-exchange membrane
Semi-permeable membrane
Recovery ratio
90-95 I
50-70 %I
Features
aHigher valencies facilitate separation *Inappropriate membrane surface flow rates result in concentration polarization tending to reduce the desalination efficiency
When the silica (Si02) content increases above 20 mg/l, membrane clogging is likely
TABLE VIII Si and SiO, concentration changes by process Process
Si (mg/l)
SiO, (mg/l)
Feed tank Calcium removal Contact aeration Coagulating sedimentation Sand filtration Activated carbon Electrodialysis
147 119 95 56 46 43 20
316 254 203 121 98 92 43
leachate treatment is poor, the reverse osmosis and electrodialysis methods are under investigation as practical methods. Table VII lists the characteristics of these latter two methods for comparison. Table VIII gives Si and SiO, concentration changes by process. The leachate contained a high concentration - 316 mg/l - of SK&. The SiO, concentration decreased to
425
92 mg/l after activated carbon treatment but remained above the value of 20 mg/l. Membrane clogging is therefore likely in the reverse osmosis method.
RECAPITULATION
Results of the present basic experiments on leachate desalination using a continuous type electrodialyzer are summarized as follows: 1. Even when chlorine ion concentration is as high as 10,000 mg/l, continuous desalination of the leachate to a chlorine ion concentration of 200 mg/l or lower is possible solely by using the electrodialysis method. 2. Appropriate pretreatment prevents reduction in the chlorine ion permeate flux in continuous electrodialysis treatment and increases the permeate flux with a rise in the target value for chlorine ion treatment. 3. Because leachate sometimes contains a high concentration of silica (SiO,), as in the present study, and the treatment water recovery ratio of the electrodialysis method is higher than that of reverse osmosis, then electrodialysis is the most appropriate method for leachate desalination. 4. Continuous electrodialysis proved capable of stably removing nitrogen, showing the potential for reduction of the treated water nitrogen concentration to below 3 mg/l.
CONCLUSIONS
The present study was conducted within the framework of a cooperative project by the Association for the Study of High Salt Leachate Treatment Systems. The Association will continue to study leachate desalination based on continuous electrodialysis, together with the investigation of membrane service life and the development of concentrated water recycling technology. REFERENCES 1
2
M. Hanajima,, T. Shimaoka, S. Higuchi, R. Yamaguchi, Y. Horii and H. Nagaoka, Proc., First Annual Conference of the Japan Society of Waste Management Experts, pp. 337-340. M. Hawjima, T. Shimaoka, S. Higuchi, Y. Horii and N. Nagaoka, Proc., Second Annual Conference of the Japan Society of Waste Management Experts, pp. 293-296.
415 Printed in The Netherlands
Desalination for landfill leachates containing condensed inorganic salts Toshio Kawanishi, Shingo Matsumoto, Yasuo Horii and Morio Masuzaki Kubota Corporation, 2-47, Shikitsuhigashi I-chome, Naniwa-ku, Osaka 556 (Japan)
SUMMARY
Recently in Japan, leachates from general waste landfills have increased their inorganic salt concentration and have caused various problems inside and outside treatment facilities. Because there is strong demand for countermeasures to cope with these problems, the authors have undertaken studies on leachate desalination using a continuous type electrodialyzer. The results of the experiments confirmed that stable desalination can be achieved by conducting the conventional leachate treatment as pre-treatment. Furthermore, comparing desalination methods, we have come to the conclusion that electrodialysis is most appropriate for landfill leachate desalination.
INTRODUCTION
In recent years plastic refuse has been incinerated in municipal waste plants in Japan. As a result, fly ash with a high chlorine content is discharged from the exhaust gas hydrogen chloride scrubbing equipment. This ash is eventually disposed of in general waste landfills where leachates have increased their inorganic salt concentration and cause various problems inside and outside the treatment facilities. There is strong demand for countermeasures to cope with these problems. OOll-9X34/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDIOOll-9164(94)00104-V
416
An outstanding example of ecological damage caused by salt is the appearance in mountain rivers of diatom (Amphiprora alata) which grow in brackish water such as water in a river mouth. Present countermeasures against salt damage, for example damage to paddy rice plants, has been the dilution of leachate with rain water or underground water. The development and practical use of desalination technology for leachate treatment is also expected. Against this background, the authors have undertaken a series of studies on leachate desalination using a continuous type electrodialyzer.
EXPERIMENTALDESIGN
Raw water
Our sample was a leachate from a lysimeter (experimental landfill tub) filled with mixed refuse (Cl- content 2.8% by weight) containing 70% incineration residue, 20% noncombustible shredded refuse, and 10% municipal refuse compost. Equipment
1. Lysimeter - a square steel tub of 6 m width, 10 m length (2x5 m chambers) and 2.5 m height having a hopper base and a total capacity of 112.5 m3 (nominal). The five chambers are of three types: three semiaerobic, one anaerobic, and one leachate circulating semi-aerobic. Table I lists the refuse filling conditions for the lysimeter of which leachate was sampled. TABLE I Filling conditions for lysimeter Status of landfill
Completed
Landfill structure Water content, %
Semi-aerobic
Wet filled weight (t)
20.0
Filled volume (m3)
15.8
19.9
417
14’r”r mnval I .
.
Biol ical Q+tiag trea#ent se#~ti~
sandfiltration, Activated carbon
adsorption OesaliMtiMl AA
r-l-
Fig. 1. Flow diagram of test plant.
2. Leachate treatment test plant - Fig. 1 shows a flow diagram of the test plant based on the addition of sodium carbonate in combination with ferric chloride for Ca removal and a contact aeration process with saddleshaped packing for biological treatment. 3. Desalination equipment - The equipment used was a continuous type electrodialyzer (70 cm W x 120 cm L x 154 cm H) comprising an electrodialysis bath, a demineralized water circulator, a concentrated water circulator, and other elements. Table II gives outline specifications of the electrodialyzer. Fig. 2 is a flow diagram of the electrodialyzer. The multichamber electrodialysis bath employed ion-exchange membranes pretreated for selective passage of monovalent cations and anions based on remarkable differences in ion permeability (Na+ > Ca*+; Cl- b SO:-. TABLE II Outline specifications of electrodialyzer Method of water feed
Continuous type
Electrodialysis
Effective membrane surface area 2dm2 x lopairs
bath
Cation membrane (12 units) Strongly acidic Anion membrane (10 units) Strongly alkaline Rectifier
DC outout = 10 A x 55 V
418
ElectrcdialysSs bath
kctif ier
Fig. 2. Flow diagram of electrodialyzer.
Method
Leachate was temporarily stored in the feed tank and then pumped to the test plant in a fixed volume for pretreatment. After activated carbon adsorption, the pretreated leachate was desalinated under three sets of conditions: Run 1 for a target Cl- level of 200 mg/l, Run 2 for 500 mg/l, and Run 3 for 1000 mg/l.
RESULTS AND DISCUSSION
Results of pretreatment
Leachate feed rates were in the 50-60 l/d range. Results of the pretreatment are given in Table III, in which figures for water quality parameters are mean values over the experimental desalination period. BOD, COD and TOC removal
Fig. 3 shows concentration changes by process. BOD was reduced to 8.5 mg/l (99% removal) by contact aeration, marking good performance
7.0 816
7.8
398
135
405
732
0.66
480
2.117
PH M-alkalinity, mg/l
ORP, mV
TOC, mg/l
BOD, mgh
DO, mg/l
COD&, mg/l
COD,,
8.9
221
18,400
1,450
1,363
9,377
TDS, mg/l
T-Ca, mg/l
Ca2+, mg/l
Cl-, mg/l
36
NH,-N, mgl
SS, mg/l
87
Kje-N, mg/l
NO,x-N, mg/l
96
T-N, mg/l
mg/l
23
22
Temperature, “C
9,300
20
24
18,650
28
9.7
29
38
48
1,585
447
637 -
400
134
Calcium removal
Water quality parameter
Test plant treatment results
TABLE III
8,943
27
34
16,550
12
37
0.9
19
55
507
207
2.7
8.5
209
120
788
8.3
23
9,911
30
36
16,250
22
28
1.8
15
41
433
116
-
6.5
148
133
269
7.2
22
9,411
46
56
17,550
21
27
2.4
16
43
347
110
3 -
130
145
262
6.8
22
Contact Coagulating Sand aeration sedimentation filtration
9,505
71
72
17,750
26
28
1.4
4.4
32
243
13
2 -
19
80
211
6.6
22
Activated carbon
191
0.6
0.7
860
0.4
1.1
0.4
0.9
2.0
4.5
4
0.4 -
5
52 -
8.0
22
RUN 1
479
0.7
0.9
1,130
1.8
0.8
0.1
0.8
1.6
8
4
1.3 -
6
65 -
7.4
26
RUN 2
Electrodialysis
939
0.9
1
2,450
2.6
2.3
0.3
0.5
2.8
10
6
1.5 -
8
160 -
7.1
27
RUN 3
P e
420
A=T 0
C
OfB
0 0 x -CODnn 0 =CODcr
Fig. 3. Concentration
changes by process (1).
of the biological treatment. Membrane pollution with organic substances did not occur in the electrodialysis bath, probably because of the low residual BOD of 2 mg/l in the water treated with activated carbon (hereinafter referred to as “pretreated leachate”). Considerable amounts of COD substances remained in the treated water after contact aeration at 207 mg/l for COD,, and 507 mg/l for COD,,, which are attributable to sparingly biodegradable COD substances characteristic of leachate. In the coagulating sedimentation treatment which followed, coagulation was conducted in an acid range (pH 5-6) with a relatively high COD,, removal of 76%. TOC was removed in a pattern similar to that of COD,, removal, with a value of 19 m/l for the pretreated leachate. Nitrogen removal
Fig. 4 shows concentration changes by process. the changes in NH,-N and NO,-N concentrations suggest fair nitrification in the contact aeration chamber. Calcium removal
The mean T-Ca concentration of leachate was 1450 mg/l, 94% of which was accounted for by soluble Ca2+ ions. The Ca removal process reduced the T-Ca content to 24 mg/l (98%) removal) but was followed by a slight increasing tendency after biological treatment.
421
Fig. 4. Concentration changes by process (2).
Results of desalination Table IV gives the operating conditions for the continuous type electrodialyzer. The results of the desalination treatment are given in Table III. Fig. 5 shows desalination characteristics for Run 2. Because reduction in permeate flux, due to density polarization in the ion-exchange membrane of the electrodialysis bath, membrane surface, organic pollution, etc., is critical in the practical application of the system, permeate flux changes were examined in each run. The results are given in Table V. Chlorine ion permeate flux (J Cl), an index of the Cl- permeability of the ion-exchange membrane, was obtained using the Nernst-Planck approximate equation as follows: JCl
= -2 Cl U Cl C Cl
l
d4ldx
(1)
TABLE IV Desalination
conditions
Item
RUN1
RUN2
RUN3
Cl- target level, mg/l Leachate feed rate, l/min Membrane surface flow velocity Stationary DC voltage, V
200 1.2-2.0 2.5-3.9 7- 10
500 1.2-3.2 2.3-6.2 8-10
1000 3.0-3.3 5.8-6.4 9-10
(cm/s)
422
Fig. 5. Desalination
characteristics.
TABLEV Chlorine ion permeate flux by run Flux (mol/cm2
where 2 Cl represents an ionic valency, C Cl the Cl concentration in the membrane, U Cl the Cl- mobility through the membrane, $J the potential, and x a variable (cm) at a right angle with respect to the membrane surface. Permeate flux Chlorine ion permeate flux values (Table V) demonstrated no significant reduction due to membrane pollution in any of the runs. The results show that the greater the target level (200 mg/l, 500 mg/l, 1000 mg/l), the greater the chlorine ion permeate flux. Desalination level We were able to desalinate a pretreated leachate containing about 10,000 mg/l chlorine ions below the target Cl- concentrations of 200,500,
423
and 1000 mg/l for Runs 1, 2 and 3, respectively. The TDS/Cl- ratio increased from about 1.9-2.0 for the leachate to 2.6-4.5 for the desalinated water, suggesting selective permeation of Cl- ions. Other water quality parameters
BOD decreased from 2 mg/l for the pretreated leachate to 0.4-1.5 mg/l, and TOC from 19 mg/l to 5-8 mg/l, indicating a slight removal of organic substances. As for nitrogen, high N removal efficiencies were obtained, irrespective of the form of N - with treated water T-N concentrations of 1.6-2.8 mg/l, or 91-95% T-N removal. Use of this system shows the potential to achieve effluent T-N levels of under 3 mg/l.
PRACTICAL
LEACHATE
DESALINATION
We also conducted an experiment with a sampling of treated leachate from an actual facility. It confirmed that the leachate can be desalinated using our experimental electrodialyzer. Table VI gives an example of the practical leachate desalination results. TABLE VI
Practical leachate desalination results Chlorine ion concentration
RUN A
RUNB
Leachate Cl-, mg/l Treated water Cl-, mg/l
5192 268
5203 573
COMPARISON
OF LEACHATE
DEiSALINATION METHODS
Currently available methods of chlorine ion removal include ion exchange, distillation, reverse osmosis, and electrodialysis. Of these the ion-exchange method is used at the lower extreme of Cl- concentrations of 500 mg/l or lower, while the distillation method is used at the higher extreme, or Cl- concentrations of a few tends of thousands of milligrams per liter or higher. Since the adaptability of these methods for landfill
424
TABLE VII Comparison of desalination methods Item
Electrodialysis
Reverse osmosis
Membrane-passing substance (s)
Cations and anions
Water
Membrane type
Ion-exchange membrane
Semi-permeable membrane
Recovery ratio
90-95 I
50-70 %I
Features
aHigher valencies facilitate separation *Inappropriate membrane surface flow rates result in concentration polarization tending to reduce the desalination efficiency
When the silica (Si02) content increases above 20 mg/l, membrane clogging is likely
TABLE VIII Si and SiO, concentration changes by process Process
Si (mg/l)
SiO, (mg/l)
Feed tank Calcium removal Contact aeration Coagulating sedimentation Sand filtration Activated carbon Electrodialysis
147 119 95 56 46 43 20
316 254 203 121 98 92 43
leachate treatment is poor, the reverse osmosis and electrodialysis methods are under investigation as practical methods. Table VII lists the characteristics of these latter two methods for comparison. Table VIII gives Si and SiO, concentration changes by process. The leachate contained a high concentration - 316 mg/l - of SK&. The SiO, concentration decreased to
425
92 mg/l after activated carbon treatment but remained above the value of 20 mg/l. Membrane clogging is therefore likely in the reverse osmosis method.
RECAPITULATION
Results of the present basic experiments on leachate desalination using a continuous type electrodialyzer are summarized as follows: 1. Even when chlorine ion concentration is as high as 10,000 mg/l, continuous desalination of the leachate to a chlorine ion concentration of 200 mg/l or lower is possible solely by using the electrodialysis method. 2. Appropriate pretreatment prevents reduction in the chlorine ion permeate flux in continuous electrodialysis treatment and increases the permeate flux with a rise in the target value for chlorine ion treatment. 3. Because leachate sometimes contains a high concentration of silica (SiO,), as in the present study, and the treatment water recovery ratio of the electrodialysis method is higher than that of reverse osmosis, then electrodialysis is the most appropriate method for leachate desalination. 4. Continuous electrodialysis proved capable of stably removing nitrogen, showing the potential for reduction of the treated water nitrogen concentration to below 3 mg/l.
CONCLUSIONS
The present study was conducted within the framework of a cooperative project by the Association for the Study of High Salt Leachate Treatment Systems. The Association will continue to study leachate desalination based on continuous electrodialysis, together with the investigation of membrane service life and the development of concentrated water recycling technology. REFERENCES 1
2
M. Hanajima,, T. Shimaoka, S. Higuchi, R. Yamaguchi, Y. Horii and H. Nagaoka, Proc., First Annual Conference of the Japan Society of Waste Management Experts, pp. 337-340. M. Hawjima, T. Shimaoka, S. Higuchi, Y. Horii and N. Nagaoka, Proc., Second Annual Conference of the Japan Society of Waste Management Experts, pp. 293-296.