- Email: [email protected]
Electrochemical removal of nitrite and ammonia for aquaculture
War. Res. Vol. 30, No. 3, pp. 715-721, 1996
~ Pergamon
0043-1354(95)00208-1
Copyright© 1996ElsevierScienceLtd Printed in Great Britain,All rights reserved 0043-1354/96 $15.00+ 0.00
E L E C T R O C H E M I C A L R E M O V A L OF NITRITE A N D
AMMONIA FOR AQUACULTURE S. H. LIN *@ and C. L. WU Department of Chemical Engineering, Yuan Ze Institute of Technology, Neili, Taoyuan 320, Taiwan, Republic of China (First received March 1994; accepted in revised form August 1995)
A~traet--Electrochemical nitrite and ammonia removal from aqueous solution was investigated. Operating variables of the electrochemical method including the current input, pH, conductivity, buffer solution and initial nitrite and ammonia concentrations were considered to determine their respective effect on the efficiency of nitrite and ammonia removal. The experimental results indicated that the time for complete nitrite removal and power consumption decrease with an increase in conductivity. For nitrite removal, the electrochemical process was found to consume less power when utilizing a current input larger than 2 A. The pH effect on the nitrite removal was observed to be significantly smaller than those of conductivity and current input. While the acidic environment favored the nitrite removal, the alkaline environment appeared to be beneficial to the ammonia removal. The ammonia removal was found to be a much slower process than the nitrite removal. Key words--electrochemical method, nitrite and ammonia removal, aquacultural water
INTRODUCTION Aquaculture is an important business in many countries around the world. Traditional aquacultural activities are conducted in large ponds with low fish density. Such activities require a large amount of land and water resources. Due to increasing cost and decreasing availability of these resources, the traditional aquacultural activities are becoming relatively uneconomic. An emerging recent approach is to employ high density aquaculture which requires significantly less amount of land and water resources than the conventional method (Liao and Mayo, 1972; Otte and Rosenthal, 1979). The economic advantages of this approach are enormous. However, the high density aquaculture incurs a new problem. An increase in fish density in a limited aquatic space would lead to a more rapid degradation of the water quality. Hence, treatment of aquacultural water to maintain high water quality required for such an activity becomes increasingly crucial (Liao and Mayo, 1972; Otte and Rosenthal, 1979). It is important that the water quality problem be resolved efficiently before high density aquaculture can yield real economic benefit. A major difficulty of high density aquacultural system stems from the rapid nitrite and ammonia accumulations in the water. The problem is caused primarily by the fish excretion and decomposition of *Author to whom all correspondence should be addressed [Tel.: (886) 3 463 8910; Fax: (886) 3 455 9373].
unconsumed fish foods. Nitrite and ammonia are toxic to marine fishes and the tolerable concentration levels of those compounds for an aquacultural system are quite low, usually being much less than 1 mg-l-' (Liao and Mayo, 1972; Otte and Rosenthal, 1979; Poxton and Allhouse, 1982). The water quality of a high density and untreated aquacultural pond can deteriorate fairly rapidly and fall below the acceptable level. External water treatment units are therefore necessary if the water quality is to be sufficiently maintained for a normal aquacultural activity (Kruner and Rosenthal, 1983; Bovendeur et al., 1987). Treatment methods proposed by the previous investigators included nitrification/denitrification using fixed film or fluidized bed biological reactors, adsorption by activated carbon and suspended solids removal by sedimentation (Liao and Mayo, 1972; Otte and Rosenthal, 1979; Poxton and Allhouse, 1982; Kruner and Rosenthal, 1983; Bovendeur et al., 1987). In recent years, nitrification by ozonation has received increasing attention (Anon., 1972; Blogoslawski et al., 1975; Honn and Chavin, 1974; Otte and Rosenthal, 1979). Ozonation of the aquacultural water is capable of efficiently converting the toxic nitrite and ammonia to non-toxic nitrate at a relatively higher cost than the other methods. A method that has not been considered for nitrification is the electrochemical method. This method has been successfully employed to deal with various industrial wastewaters (Beck et al., 1974; Biwyk, 1980; Matis, 1980; Ramirez, 1981; Cenkin and Belevtsev, 1985; Lin and Peng, 1994). Hence it would be of significant academic and
715
716
S.H. Lin and C. L. Wu
practical interest to test whether the electrochemical oxidation method is a potential alternative for the nitrite and ammonia removal from aqueous solution. The purpose of this study is to conduct experimental investigations to address this issue. A bench-top electrolytic apparatus was assembled to perform the simulation tests. Several aspects regarding the effects of current, type of electrodes, conductivity, pH, buffer solution and initial nitrite and ammonia concentrations on the nitrite and ammonia removal efficiencies are explored. As will be seen later, the electrochemical method is at least as effective and promising as the other treatment methods. EXPERIMENTAL
The batch experimental apparatus is shown in Fig. 1. Two pairs of anodic and cathodic electrodes situated approx. 1.5 era apart to each other and dipped in the prepared sample solution containing nitrite and/or ammonia. The current input was controlled by an ammeter and the total power consumption was integrated and registered by a power integrator. In each run, approximately 1 1 of sample solution was placed in the electrolytic cell. The sample solution (single component or mixture) was prepared by dissolving an appropriate amount of reagent grade sodium nitrite or ammonium chloride (E. Merck, Inc., Germany) in deionized water. Single component sample solution contained either sodium nitrite or ammonium chloride only whereas mixture sample solution contained both components. The total effective surface area of electrodes was 22.6 cm 2. The initial pH of the sample solution in the electrolytic cell was adjusted to around 7 except for those samples intended to investigate the pH effect on the nitrite or ammonia removal efficiency. The conductivity of the freshly prepared sample doluyion was very low, generally no more than 70 # mho-cm -t. At such a low conductivity, the current that could be applied to the electrodes was very low indeed which was insufficient
2
to initiate and maintain normal nitrite and ammonia oxidation. To overcome this difficulty, a small amount of table salt was added to the sample solution to raise its conductivity to over 1500 # mho-cm-~ for all experimental runs. The nitrite and ammonia concentrations were determined, respectively, by the standard photometric and Nessler methods (APHA, 1992) using a Hitachi u,v. spectrophotometer (Model U3410, Hitachi, Inc., Japan). In the initial experimental tests, the cast iron electrodes were employed to investigate the efficiencies of nitrite and ammonia removal from the aqueous solution. The removal efficiencies turned out to be rather low, being no more than 20% for a 2-h test run. Moreover, the cast iron electrodes turned from light grey into black, presumably due to oxidation on the electrode surface. Search for replacement electrodes resulted in choosing the high purity graphite rod as anodes and titanium dioxide as cathodes. Such electrode pairs, which had a surface area of 22.6 cm2, performed well for the present nitrite and ammonia removal experiments. RESULTS AND DISCUSSION
The mechanism of electrochemical oxidation process in aqueous systems is inherently complex (Pletcber and Walsh, 1990). It is generally believed that there are two possible mechanisms involved in the process: electro-flotation and electro-oxidation. Oxidation and reduction o f the electrochemical process occur, respectively, at the anode and cathode of the electrodes (Pletcher and Walsh, 1990). With sufficient power supply, the nitrite or ammonia molecules are oxidized at the anode to nitrate. Electro-flotation is caused by the hydrogen gas generated during the electrochemical process. In the present study, the hydrogen gas did not generate much electro-flotation effect due primarily to that there were no suspended or dissolved solids in the present aqueous solution, unlike the ordinary industrial wastewaters.
1
['~--]wh
o I
I .....
II
1. D. C. Power Supply 2. Digital Ammeter 3. Power Integrator
5 4!
4. Parallel Graphite Anodes
+i
5. Parallel Titanium Dioxide Cathodes 6. Magnetic Bar-Stirrer 7. Sampling Valve 8. Electrolytic Cell
o{9
9. Digital Magnetic Stirring Controller
I Fig. 1. Experimental setup.
717
Aquacultural nitrite and ammonia removal ,00
--¢
80
5000 --
•
100,O....o'•~O~
I I
r
d"
//
6o0
_,,o"
'E 4000
,Y
E 3000 .~ ~"~ 2000 -o
Initial concentration (mg-I-I )
/ / 20
l•/ / ] 20
I 30
I 40
I 50
f 60
Time (min) Fig. 2. Effect of initial nitrite concentration on the nitrite removal with 1.5 A current and 2000 mg-1-~ salt concentration. The effect of initial nitrite concentration on the nitrite removal is demonstrated in Fig. 2. An initial nitrite concentration of 10mg-1 -~ or less shown in this figure is about the maximum nitrite concentration level that could be found in an untreated, high density fish hatchery (Liao and Mayo, 1972; Otte and Rosenthal, 1979; Poxton and Allhouse, 1982; Kruner and Rosenthal, 1983; Bovendeur et al., 1987). It appears that at I mg-1-~ initial nitrite, complete nitrite removal is achieved in just about four minutes. As the nitrite concentration was elevated to 5 mg-l -~, the time for total nitrite removal increased rapidly to more than 30 min, as demonstrated in Fig. 3 which reveals a linear relationship between the time for total nitrite removal and the initial nitrite concentration. As mentioned earlier, the sample solution prepared by dissolving sodium nitrite or ammonium chloride in deionized water consistently had a conductivity of no more than 70 p mho-cm -~ which is too low for an effective electrochemical treatment. Addition of 1.5 g of table salt to l 1 of sample wastewater (with a salt
0
1000
I 2000
1 3000
I 4000
Salt concentration (mg-l -I) Fig. 4. Conductivity of the aqueous solution as a function of salt concentration. concentration of 1500 mg-1-1 ) raised the conductivity to slightly over 2000 # mho-cm- 1, as shown in Fig. 4. The conductivity was further elevated to over 4000 V mho-cm- l by maintaining a 3000 mg-l- ~ of salt concentration. According to Krstajic and Nakic (1987), the added chloride ion in the aqueous solution will be converted anodically to chlorine which is further converted to hypochloric acid. The reaction sequence can be represented by 2 CI----, C12 + 2 e C12 + H20----~HC1 + HOC1. The hypochloric acid (HOCI) is a strong oxidant which oxidizes nitrite to nitrate according to NO~- + HOCI --* NO~- + CI-. This oxidation reaction enhances the electrochemical nitrite removal process. This may account for the strong influence of the conductivity on the nitrite removal as seen in Fig. 5. Here the current input was
1oo80/y
80 E "d
•
1000 I 10
0
-
0
I / ~
-
?
60 --
O
E
40 e.
Z, 20
40
o
m
-I-')
• 20
v-
•
0
j
2
4
6
8
10
12
Initial NO2-N concentration (rag-1 -I) Fig. 3. Time for total nitrite removal vs initial nitrite concentration with 1.5 A current and 2000 mg-l- t salt concentration.
0
10
I 20
I 30
I 40
Time (rain) Fig. 5. Effect of salt concentration on the nitrite removal with 1.5 A current and 5 mg-l-~ initial nitrite concentration.
S. H. Lin and C. L. Wu
718 1o0
?2.0
~n'| /
50--
O,, ' ' ' I / !
E
v
80
--
35
-
25
--
IS
Time
40 0
>
60
Z
40
"~ 30 I cq
0
Current
/
O O
20
L
I
I
L
I
10
20
30
40
50
I
I
L
1000
2000
3000
10
Time (min)
5 4000
NaCl(mg-1 -I)
Fig. 6. Effect of current input on the nitrite removal with 5 mg-1- ~ initial nitrite concentration and 2000 rag-l- ~ salt concentration.
Fig. 8. Time for total nitrite removal and power consumption vs salt concentration 5 mg-I -~ nitrite concentration and 1.5 A current input.
kept at 1.5 A for the three test runs. The time for total nitrite removal was seen to be virtually cut in half as the salt concentration was increased from 1500 to 3000 mg-l- ~. Beyond the 3000 rag-l- ~ level, the effect of salt concentration on the time for total nitrite removal was significantly reduced. Hence 3000 mg-lof salt concentration with a corresponding conductivity around 4000/~ mho-cm-~ would be the optimal value. The present conclusion confirms that observed in our previous report for electrochemical treatment of textile wastewater (Lin and Peng, 1994). By employing an initial 2000 rag-1 ~ salt concentration in the sample solution, the curent input was varied between 1 and 2.5 A to examine its effect on the nitrite removal. As seen in Fig. 6, the effect of current input was as significant as that of conductivity. An increase in the current input considerably enhances the oxidation power of the anode, leading to an increase in the nitrite removal or a shorter time for complete nitrite removal. The time for total nitrite
removal as a function of current input is demonstrated in Fig. 7, Also shown in this figure is the power consumption pertaining to the current input. The curve dearly demonstrates a steady increase in power consumption as the current input increases from 1 to 2 A. Beyond that, the power consumption decreases with increasing current due to a rapid decrease in the time for total nitrite removal. Hence, in terms of power consumption, an applied current input larger than 2 A is strongly recommended. A similar figure for the time of total nitrite removal and the power consumption vs the salt concentration is displayed in Fig. 8. The rapid decrease in the time for complete nitrite removal is due primarily to enhanced anodic oxidation by hypochloric acid at higher salt concentration. Unlike that of Fig. 7, the power consumption in this figure is seen to decrease monotonically with increasing salt concentration. This, however, may not be construed that the salt concentration can be much further increased in order
60
--
I00 -
30
o i ° ~.,Oa 0
Time Time
~e _
s0
80 -
Powor 40
--
"~
--
10
--
.p"
/ ~ ¢,~"
20
60
'°t/;7
Initial pH
f,/
30 20
.y
10
--
L
I
l
2
L4°°z 50 /7'
0 3
Current (A) Fig. 7. Time for total nitrite removal and power consumption vs current input with 5 mg-1- ~nitrite concentration and 2000 mg-l-~ salt concentration.
oS7
0
.9
I
L
L
L
10
20
30
40
Time (min) Fig. 9. Effect of pH on the nitrite removal with 5 mg-1initial nitrite concentration, 2000 mg-1-~ salt concentration and 1.5 A current input.
719
Aquacultural nitrite and ammonia removal to achieve a greater power consumption efficiency because presence of excessive amount of chloride ions in the aqueous solution will not be ideal for the treated wastewater. The initial pH of the sample solution in the previous figures was adjusted to 7 before the experimental run started. Figure 9 shows the effect of initial pH on the nitrite removal. The pH chosen here covers the actual pH range realizable in an aquacultural effluent. Although the initial pH effect on the nitrite removal is seen to be still significant, it is not of the same magnitude as those of conductivity and current input, This figure also indicates that the acidic pH range favors the nitrite removal and seems to have a stronger effect on the nitrite removal than the alkaline pH range. The pH of the sample solution was observed to decrease from 7 to less than 5 during the electrochemical process of a typical test run. This was due to conversion of nitrite to nitrate, the latter being a much stronger acid. Hence it would be of practical interest to see whether pH control during the electrochemical process will benefit the nitrite removal. Hence, experimental runs were conducted with and without boric acid/sodium borate buffer solution. The buffer solution was capable of controlling the pH to between 7 and 6 during the electrochemical process, which is much narrower than that of the unbuffered case. The results in Fig. 10 clearly indicate that control of the pH in the aqueous solution during the process has an adverse effect on the nitrite removal. This is in line with what was observed in Fig. 9 that the more acidic environment of the unbuffered case is favored when compared to that of the buffered case. The above experiments were performed using sample solution containing nitrite only. In many practical situations, both nitrite and ammonia will be present simultaneously in the aqueous solution. Figure 11 compares the nitrite removal of a single-
100
j•j•J•J•
80
> E Q
60
i Z 40 © Z 20
• Nitrite-ammonia
I 10
0
E 20
I 30
f 40
Time (rain) Fig. 11. Comparison of nitrite removal for nitrite and nitrite-ammonia solutions with 5 rag-1 1 initial nitrite and 20 rag-l- t initial ammonia concentration. component (nitrite) system and that of a twocomponent (nitrite-ammonia) system. Presence of ammonium ion is seen to strongly retard the nitrite removal. As noted by Benefield et al. (1982), oxidation of ammonium ion in the aqueous solution is a two-step process. In the first step, ammonium ion is oxidized to nitrite which then is further oxidized to nitrite in the second step. Hence, during the electrochemical oxidation process, certain amount of nitrite was generated in the first stage ammonia oxidation for the two-component system. The actual amount of nitrite existing in the aqueous solution would be more than that derived from the initial nitrite. This results in a significant slowdown of the nitrite removal, as shown in this figure. Because of the two-step nature of its electrochemical oxidation, ammonia removal will be expected to be slower than that of nitrite. Figure 12 demonstrates the ammonia removal as a function of time for two different pHs. In comparison with that of Fig. 9 80
100
80
> Q
60
/
p
E z
40
Buffer
'C I
z~ 2o
z
20
0
10
20
30
40
I 50
Time (min) Fig. 10. Effect of buffer solution on the nitrite removal with 5 mg-1-~ initial nitrite concentration and 2000rag-l- t salt concentration.
0
J I I0
I 20
I 30
I 40
] 50
I 60
Time (rain) Fig. 12. Effect of pH on the ammonia removal with 20 mg-I-~ initial ammonia concentration, 2000mg-I-~ salt concentration and 2.5 A current input.
720
S. H. Lin and C. L. Wu lOO(~l)-l(ql~,)~,~,,
'~ 80
-
-
60-.,'~ Temperature (°(3) \~mk~ • 16 ~.~\t~\ Q 40 -20
-
o
0
28
I
I
I
I
8
9
10
11
pH Fig. 13. Percent of un-ionized ammonia (NH3) existing in the aqueous solution as a function of pH and temperature.
7, ammonia existing in its un-ionized form (NH3) also increases. This accounts for the favorable effect of the alkaline pH range on the ammonia removal shown in Fig. 12. The ammonia removal for the single-component (ammonia) and the two-component (ammonia-nitrite) systems by electrochemical oxidation is compared in Fig. 14. The current input was only 1.5 A which is lower than the 2.5 A current employed in the single component case shown in Fig. 12. The ammonia removal is seen to be considerably lower in the present case, Essentially linear ammonia removal was observed for both single-component and mixture cases. A much higher ammonia removal for the single component case than that of mixture is anticipated because part of the power input was utilized for nitrite oxidation in the latter. CONCLUSIONS
which shows total nitrite removal in the neighborhood of 30min, that of the ammonia removal in Fig. 12 is about 40% or less for the same time period and the removal is essentially linear for that period. Comparison of these two figures also reveals that while acidic pH range is beneficial to the nitrite removal, the ammonia removal shows an opposite pH trend. According to Emmerson et al. (1980), ammonia in aqueous solution can exist in either un-ionized form (NH3) and/or ionized form (NH~-). Of these two forms of ammonia, the un-ionized one is much easier to oxidize (Benefield et al., 1982). For a given pH and temperature, these two forms of ammonia establish an equilibrium following the equation (Emmerson et al., 1980) NH3 + H 2 0 ~ N H
~ +OH-.
The percentage of ammonia existed in ionized form as a function of pH and temperature is shown in Fig. 13. It is apparent that as the pH increases above 20 -Ammonia
1015_
Electrochemical method has been employed in the present study to investigate nitrite and ammonia removal from aqueous solution. Several aspects, such as the sample solution conductivity, pH, current input and type of electrodes, are explored to determine their respective effects on the electrochemical removal efficiency. The iron electrodes were found to be unsuitable for the present process due to its low removal efficiency. Pairs of graphite anode and titanium dioxide cathode considerably elevate the removal efficiency and were adopted for the present study. Among the three major operating variables examined here, conductivity and current input were found to exert much stronger influence on the nitrite and ammonia removal efficiency than the pH. While a current input larger than 2 A was observed to yield lower power consumption than that below, the power consumption decreases steadily with increasing conductivity in the aqueous solution. It is also seen in the experimental results that the nitrite and ammonia removal is enhanced, respectively, in the acidic and alkaline environments. Although the pH of the aqueous solution decreases significantly during the electrochemical process, pH control appears to exert a negative effect on the nitrite removal. Presence of a second component in the aqueous solution was found to adversely affect nitrite and ammonia removal. Acknowledgements--The authors wish to sincerely thank
/
the Yuan Ze Memorial Foundation for the financial support (under the grant DRA 83001) of this project.
Ammonia-nitrite
REFERENCES
0
10
20
30
f 40
Time (min) Fig. 14. Comparison of ammonia removal for nitrite and nitrite-ammonia solution with 5 mg-l- ~initial concentration, 20 mg-I-~ ammonia concentration and 1.5 A current input.
Anonymous (1972) Use of ozone in sea water for cleansing shellfish. Wat, Eft. Treat. J. 12, 260. APHA (1992) The Standard Methods for Water and Wastewater Examination, 17th edn, Am. Publ. Hlth Assoc., Washington, D.C. Beck E. C., Gianmini A. P. and Ramirez E. R. (1974) Electroeoagulation clarifies food wastewater. Food Teehnol. 28, 2.
Aquacultural nitrite and ammonia removal Benefield L. D., Judkins J. F. and Weand B. L. (1982) Process Chemistry for Water and Wastewater Treatment. Prentice-Hall, Englewood Cliffs, N.J. Biwyk A. (1980) Electrocoagulation of biologically treated sewage. In Proc. 35th Purdue Ind. Waste Conf., Lafayette, Ind. Blogoslawski W. J., Brown C., Rhodes E. W. and Broadhurst M. (1975) Ozone disinfection of seawater supply system. In Proc. 1st Int. Syrup. on Ozone for Water and Wastewater Treatment, Cleveland, Ohio, Bovendeur J., Eding E. H. and Henken A. M. (1987) Design and performance of a water recirculation system for high-density culture of the African catfish. Aquacult. 63, 329. Cenkin V. E. and Belevtsev A. N. (1985) Electrochemical treatment of industrial wastewater. War. Eft. Treat. J. 25, 243. Emmerson K. R., Russo R. C., Lund R. E. and Thurston R. V. (1980) Aqueous ammonia equilibrium calculation: Effect of pH and temperature. J. Fish Res. Board Can. 32, 2379. Honn K. and Chavin W. (1976) Utility of ozone treatment in the maintenance of water quality in a closed marine system. Marine Biol. 34, 201.
721
Kruner G. and Rosenthal H. (1983) Efficiency of nitrification in trickling filter using different substrate. Aquacult. Engng 2, 49. Krstajic G. and Nakic V. (1987) Hypochloric production: A model of the cationic reactions. J. appl. Electrochem. 17, 77. Liao P. B. and Mayo R. D. (1972) Intensified fish culture combining water reconditioning with pollution abatement. Aquacult. 3, 61. Lin S. H. and Peng C. F. (1994) Treatment of textile wastewater by electrochemical method. Wat. Res. 28, 277. Matis K. A. (1980) Treatment of industrial liquid wastes by electroflotation. Wat. Pollut. Control. 19, 136. Otte G. and Rosenthal H. (1979) Management of a closed brackish water system for high density fish culture by biological and chemical water treatment. Aquacult. 18, 169. Pletcher D. and Walsh F. C. (1990) Industrial Electrochemistry, 2nd edn. Chapman & Hall, London. Poxton M. G. and Allhouse S. B. (1982) Water quality criteria for marine fisheries. Aquacuh. Engng l, 153. Ramirez E. R. (198 I) Physicochemical treatment of rendering wastewater by electrocoagulation. In 36th Purdue Indust. Waste Conf., Lafayette, Ind.
~ Pergamon
0043-1354(95)00208-1
Copyright© 1996ElsevierScienceLtd Printed in Great Britain,All rights reserved 0043-1354/96 $15.00+ 0.00
E L E C T R O C H E M I C A L R E M O V A L OF NITRITE A N D
AMMONIA FOR AQUACULTURE S. H. LIN *@ and C. L. WU Department of Chemical Engineering, Yuan Ze Institute of Technology, Neili, Taoyuan 320, Taiwan, Republic of China (First received March 1994; accepted in revised form August 1995)
A~traet--Electrochemical nitrite and ammonia removal from aqueous solution was investigated. Operating variables of the electrochemical method including the current input, pH, conductivity, buffer solution and initial nitrite and ammonia concentrations were considered to determine their respective effect on the efficiency of nitrite and ammonia removal. The experimental results indicated that the time for complete nitrite removal and power consumption decrease with an increase in conductivity. For nitrite removal, the electrochemical process was found to consume less power when utilizing a current input larger than 2 A. The pH effect on the nitrite removal was observed to be significantly smaller than those of conductivity and current input. While the acidic environment favored the nitrite removal, the alkaline environment appeared to be beneficial to the ammonia removal. The ammonia removal was found to be a much slower process than the nitrite removal. Key words--electrochemical method, nitrite and ammonia removal, aquacultural water
INTRODUCTION Aquaculture is an important business in many countries around the world. Traditional aquacultural activities are conducted in large ponds with low fish density. Such activities require a large amount of land and water resources. Due to increasing cost and decreasing availability of these resources, the traditional aquacultural activities are becoming relatively uneconomic. An emerging recent approach is to employ high density aquaculture which requires significantly less amount of land and water resources than the conventional method (Liao and Mayo, 1972; Otte and Rosenthal, 1979). The economic advantages of this approach are enormous. However, the high density aquaculture incurs a new problem. An increase in fish density in a limited aquatic space would lead to a more rapid degradation of the water quality. Hence, treatment of aquacultural water to maintain high water quality required for such an activity becomes increasingly crucial (Liao and Mayo, 1972; Otte and Rosenthal, 1979). It is important that the water quality problem be resolved efficiently before high density aquaculture can yield real economic benefit. A major difficulty of high density aquacultural system stems from the rapid nitrite and ammonia accumulations in the water. The problem is caused primarily by the fish excretion and decomposition of *Author to whom all correspondence should be addressed [Tel.: (886) 3 463 8910; Fax: (886) 3 455 9373].
unconsumed fish foods. Nitrite and ammonia are toxic to marine fishes and the tolerable concentration levels of those compounds for an aquacultural system are quite low, usually being much less than 1 mg-l-' (Liao and Mayo, 1972; Otte and Rosenthal, 1979; Poxton and Allhouse, 1982). The water quality of a high density and untreated aquacultural pond can deteriorate fairly rapidly and fall below the acceptable level. External water treatment units are therefore necessary if the water quality is to be sufficiently maintained for a normal aquacultural activity (Kruner and Rosenthal, 1983; Bovendeur et al., 1987). Treatment methods proposed by the previous investigators included nitrification/denitrification using fixed film or fluidized bed biological reactors, adsorption by activated carbon and suspended solids removal by sedimentation (Liao and Mayo, 1972; Otte and Rosenthal, 1979; Poxton and Allhouse, 1982; Kruner and Rosenthal, 1983; Bovendeur et al., 1987). In recent years, nitrification by ozonation has received increasing attention (Anon., 1972; Blogoslawski et al., 1975; Honn and Chavin, 1974; Otte and Rosenthal, 1979). Ozonation of the aquacultural water is capable of efficiently converting the toxic nitrite and ammonia to non-toxic nitrate at a relatively higher cost than the other methods. A method that has not been considered for nitrification is the electrochemical method. This method has been successfully employed to deal with various industrial wastewaters (Beck et al., 1974; Biwyk, 1980; Matis, 1980; Ramirez, 1981; Cenkin and Belevtsev, 1985; Lin and Peng, 1994). Hence it would be of significant academic and
715
716
S.H. Lin and C. L. Wu
practical interest to test whether the electrochemical oxidation method is a potential alternative for the nitrite and ammonia removal from aqueous solution. The purpose of this study is to conduct experimental investigations to address this issue. A bench-top electrolytic apparatus was assembled to perform the simulation tests. Several aspects regarding the effects of current, type of electrodes, conductivity, pH, buffer solution and initial nitrite and ammonia concentrations on the nitrite and ammonia removal efficiencies are explored. As will be seen later, the electrochemical method is at least as effective and promising as the other treatment methods. EXPERIMENTAL
The batch experimental apparatus is shown in Fig. 1. Two pairs of anodic and cathodic electrodes situated approx. 1.5 era apart to each other and dipped in the prepared sample solution containing nitrite and/or ammonia. The current input was controlled by an ammeter and the total power consumption was integrated and registered by a power integrator. In each run, approximately 1 1 of sample solution was placed in the electrolytic cell. The sample solution (single component or mixture) was prepared by dissolving an appropriate amount of reagent grade sodium nitrite or ammonium chloride (E. Merck, Inc., Germany) in deionized water. Single component sample solution contained either sodium nitrite or ammonium chloride only whereas mixture sample solution contained both components. The total effective surface area of electrodes was 22.6 cm 2. The initial pH of the sample solution in the electrolytic cell was adjusted to around 7 except for those samples intended to investigate the pH effect on the nitrite or ammonia removal efficiency. The conductivity of the freshly prepared sample doluyion was very low, generally no more than 70 # mho-cm -t. At such a low conductivity, the current that could be applied to the electrodes was very low indeed which was insufficient
2
to initiate and maintain normal nitrite and ammonia oxidation. To overcome this difficulty, a small amount of table salt was added to the sample solution to raise its conductivity to over 1500 # mho-cm-~ for all experimental runs. The nitrite and ammonia concentrations were determined, respectively, by the standard photometric and Nessler methods (APHA, 1992) using a Hitachi u,v. spectrophotometer (Model U3410, Hitachi, Inc., Japan). In the initial experimental tests, the cast iron electrodes were employed to investigate the efficiencies of nitrite and ammonia removal from the aqueous solution. The removal efficiencies turned out to be rather low, being no more than 20% for a 2-h test run. Moreover, the cast iron electrodes turned from light grey into black, presumably due to oxidation on the electrode surface. Search for replacement electrodes resulted in choosing the high purity graphite rod as anodes and titanium dioxide as cathodes. Such electrode pairs, which had a surface area of 22.6 cm2, performed well for the present nitrite and ammonia removal experiments. RESULTS AND DISCUSSION
The mechanism of electrochemical oxidation process in aqueous systems is inherently complex (Pletcber and Walsh, 1990). It is generally believed that there are two possible mechanisms involved in the process: electro-flotation and electro-oxidation. Oxidation and reduction o f the electrochemical process occur, respectively, at the anode and cathode of the electrodes (Pletcher and Walsh, 1990). With sufficient power supply, the nitrite or ammonia molecules are oxidized at the anode to nitrate. Electro-flotation is caused by the hydrogen gas generated during the electrochemical process. In the present study, the hydrogen gas did not generate much electro-flotation effect due primarily to that there were no suspended or dissolved solids in the present aqueous solution, unlike the ordinary industrial wastewaters.
1
['~--]wh
o I
I .....
II
1. D. C. Power Supply 2. Digital Ammeter 3. Power Integrator
5 4!
4. Parallel Graphite Anodes
+i
5. Parallel Titanium Dioxide Cathodes 6. Magnetic Bar-Stirrer 7. Sampling Valve 8. Electrolytic Cell
o{9
9. Digital Magnetic Stirring Controller
I Fig. 1. Experimental setup.
717
Aquacultural nitrite and ammonia removal ,00
--¢
80
5000 --
•
100,O....o'•~O~
I I
r
d"
//
6o0
_,,o"
'E 4000
,Y
E 3000 .~ ~"~ 2000 -o
Initial concentration (mg-I-I )
/ / 20
l•/ / ] 20
I 30
I 40
I 50
f 60
Time (min) Fig. 2. Effect of initial nitrite concentration on the nitrite removal with 1.5 A current and 2000 mg-1-~ salt concentration. The effect of initial nitrite concentration on the nitrite removal is demonstrated in Fig. 2. An initial nitrite concentration of 10mg-1 -~ or less shown in this figure is about the maximum nitrite concentration level that could be found in an untreated, high density fish hatchery (Liao and Mayo, 1972; Otte and Rosenthal, 1979; Poxton and Allhouse, 1982; Kruner and Rosenthal, 1983; Bovendeur et al., 1987). It appears that at I mg-1-~ initial nitrite, complete nitrite removal is achieved in just about four minutes. As the nitrite concentration was elevated to 5 mg-l -~, the time for total nitrite removal increased rapidly to more than 30 min, as demonstrated in Fig. 3 which reveals a linear relationship between the time for total nitrite removal and the initial nitrite concentration. As mentioned earlier, the sample solution prepared by dissolving sodium nitrite or ammonium chloride in deionized water consistently had a conductivity of no more than 70 p mho-cm -~ which is too low for an effective electrochemical treatment. Addition of 1.5 g of table salt to l 1 of sample wastewater (with a salt
0
1000
I 2000
1 3000
I 4000
Salt concentration (mg-l -I) Fig. 4. Conductivity of the aqueous solution as a function of salt concentration. concentration of 1500 mg-1-1 ) raised the conductivity to slightly over 2000 # mho-cm- 1, as shown in Fig. 4. The conductivity was further elevated to over 4000 V mho-cm- l by maintaining a 3000 mg-l- ~ of salt concentration. According to Krstajic and Nakic (1987), the added chloride ion in the aqueous solution will be converted anodically to chlorine which is further converted to hypochloric acid. The reaction sequence can be represented by 2 CI----, C12 + 2 e C12 + H20----~HC1 + HOC1. The hypochloric acid (HOCI) is a strong oxidant which oxidizes nitrite to nitrate according to NO~- + HOCI --* NO~- + CI-. This oxidation reaction enhances the electrochemical nitrite removal process. This may account for the strong influence of the conductivity on the nitrite removal as seen in Fig. 5. Here the current input was
1oo80/y
80 E "d
•
1000 I 10
0
-
0
I / ~
-
?
60 --
O
E
40 e.
Z, 20
40
o
m
-I-')
• 20
v-
•
0
j
2
4
6
8
10
12
Initial NO2-N concentration (rag-1 -I) Fig. 3. Time for total nitrite removal vs initial nitrite concentration with 1.5 A current and 2000 mg-l- t salt concentration.
0
10
I 20
I 30
I 40
Time (rain) Fig. 5. Effect of salt concentration on the nitrite removal with 1.5 A current and 5 mg-l-~ initial nitrite concentration.
S. H. Lin and C. L. Wu
718 1o0
?2.0
~n'| /
50--
O,, ' ' ' I / !
E
v
80
--
35
-
25
--
IS
Time
40 0
>
60
Z
40
"~ 30 I cq
0
Current
/
O O
20
L
I
I
L
I
10
20
30
40
50
I
I
L
1000
2000
3000
10
Time (min)
5 4000
NaCl(mg-1 -I)
Fig. 6. Effect of current input on the nitrite removal with 5 mg-1- ~ initial nitrite concentration and 2000 rag-l- ~ salt concentration.
Fig. 8. Time for total nitrite removal and power consumption vs salt concentration 5 mg-I -~ nitrite concentration and 1.5 A current input.
kept at 1.5 A for the three test runs. The time for total nitrite removal was seen to be virtually cut in half as the salt concentration was increased from 1500 to 3000 mg-l- ~. Beyond the 3000 rag-l- ~ level, the effect of salt concentration on the time for total nitrite removal was significantly reduced. Hence 3000 mg-lof salt concentration with a corresponding conductivity around 4000/~ mho-cm-~ would be the optimal value. The present conclusion confirms that observed in our previous report for electrochemical treatment of textile wastewater (Lin and Peng, 1994). By employing an initial 2000 rag-1 ~ salt concentration in the sample solution, the curent input was varied between 1 and 2.5 A to examine its effect on the nitrite removal. As seen in Fig. 6, the effect of current input was as significant as that of conductivity. An increase in the current input considerably enhances the oxidation power of the anode, leading to an increase in the nitrite removal or a shorter time for complete nitrite removal. The time for total nitrite
removal as a function of current input is demonstrated in Fig. 7, Also shown in this figure is the power consumption pertaining to the current input. The curve dearly demonstrates a steady increase in power consumption as the current input increases from 1 to 2 A. Beyond that, the power consumption decreases with increasing current due to a rapid decrease in the time for total nitrite removal. Hence, in terms of power consumption, an applied current input larger than 2 A is strongly recommended. A similar figure for the time of total nitrite removal and the power consumption vs the salt concentration is displayed in Fig. 8. The rapid decrease in the time for complete nitrite removal is due primarily to enhanced anodic oxidation by hypochloric acid at higher salt concentration. Unlike that of Fig. 7, the power consumption in this figure is seen to decrease monotonically with increasing salt concentration. This, however, may not be construed that the salt concentration can be much further increased in order
60
--
I00 -
30
o i ° ~.,Oa 0
Time Time
~e _
s0
80 -
Powor 40
--
"~
--
10
--
.p"
/ ~ ¢,~"
20
60
'°t/;7
Initial pH
f,/
30 20
.y
10
--
L
I
l
2
L4°°z 50 /7'
0 3
Current (A) Fig. 7. Time for total nitrite removal and power consumption vs current input with 5 mg-1- ~nitrite concentration and 2000 mg-l-~ salt concentration.
oS7
0
.9
I
L
L
L
10
20
30
40
Time (min) Fig. 9. Effect of pH on the nitrite removal with 5 mg-1initial nitrite concentration, 2000 mg-1-~ salt concentration and 1.5 A current input.
719
Aquacultural nitrite and ammonia removal to achieve a greater power consumption efficiency because presence of excessive amount of chloride ions in the aqueous solution will not be ideal for the treated wastewater. The initial pH of the sample solution in the previous figures was adjusted to 7 before the experimental run started. Figure 9 shows the effect of initial pH on the nitrite removal. The pH chosen here covers the actual pH range realizable in an aquacultural effluent. Although the initial pH effect on the nitrite removal is seen to be still significant, it is not of the same magnitude as those of conductivity and current input, This figure also indicates that the acidic pH range favors the nitrite removal and seems to have a stronger effect on the nitrite removal than the alkaline pH range. The pH of the sample solution was observed to decrease from 7 to less than 5 during the electrochemical process of a typical test run. This was due to conversion of nitrite to nitrate, the latter being a much stronger acid. Hence it would be of practical interest to see whether pH control during the electrochemical process will benefit the nitrite removal. Hence, experimental runs were conducted with and without boric acid/sodium borate buffer solution. The buffer solution was capable of controlling the pH to between 7 and 6 during the electrochemical process, which is much narrower than that of the unbuffered case. The results in Fig. 10 clearly indicate that control of the pH in the aqueous solution during the process has an adverse effect on the nitrite removal. This is in line with what was observed in Fig. 9 that the more acidic environment of the unbuffered case is favored when compared to that of the buffered case. The above experiments were performed using sample solution containing nitrite only. In many practical situations, both nitrite and ammonia will be present simultaneously in the aqueous solution. Figure 11 compares the nitrite removal of a single-
100
j•j•J•J•
80
> E Q
60
i Z 40 © Z 20
• Nitrite-ammonia
I 10
0
E 20
I 30
f 40
Time (rain) Fig. 11. Comparison of nitrite removal for nitrite and nitrite-ammonia solutions with 5 rag-1 1 initial nitrite and 20 rag-l- t initial ammonia concentration. component (nitrite) system and that of a twocomponent (nitrite-ammonia) system. Presence of ammonium ion is seen to strongly retard the nitrite removal. As noted by Benefield et al. (1982), oxidation of ammonium ion in the aqueous solution is a two-step process. In the first step, ammonium ion is oxidized to nitrite which then is further oxidized to nitrite in the second step. Hence, during the electrochemical oxidation process, certain amount of nitrite was generated in the first stage ammonia oxidation for the two-component system. The actual amount of nitrite existing in the aqueous solution would be more than that derived from the initial nitrite. This results in a significant slowdown of the nitrite removal, as shown in this figure. Because of the two-step nature of its electrochemical oxidation, ammonia removal will be expected to be slower than that of nitrite. Figure 12 demonstrates the ammonia removal as a function of time for two different pHs. In comparison with that of Fig. 9 80
100
80
> Q
60
/
p
E z
40
Buffer
'C I
z~ 2o
z
20
0
10
20
30
40
I 50
Time (min) Fig. 10. Effect of buffer solution on the nitrite removal with 5 mg-1-~ initial nitrite concentration and 2000rag-l- t salt concentration.
0
J I I0
I 20
I 30
I 40
] 50
I 60
Time (rain) Fig. 12. Effect of pH on the ammonia removal with 20 mg-I-~ initial ammonia concentration, 2000mg-I-~ salt concentration and 2.5 A current input.
720
S. H. Lin and C. L. Wu lOO(~l)-l(ql~,)~,~,,
'~ 80
-
-
60-.,'~ Temperature (°(3) \~mk~ • 16 ~.~\t~\ Q 40 -20
-
o
0
28
I
I
I
I
8
9
10
11
pH Fig. 13. Percent of un-ionized ammonia (NH3) existing in the aqueous solution as a function of pH and temperature.
7, ammonia existing in its un-ionized form (NH3) also increases. This accounts for the favorable effect of the alkaline pH range on the ammonia removal shown in Fig. 12. The ammonia removal for the single-component (ammonia) and the two-component (ammonia-nitrite) systems by electrochemical oxidation is compared in Fig. 14. The current input was only 1.5 A which is lower than the 2.5 A current employed in the single component case shown in Fig. 12. The ammonia removal is seen to be considerably lower in the present case, Essentially linear ammonia removal was observed for both single-component and mixture cases. A much higher ammonia removal for the single component case than that of mixture is anticipated because part of the power input was utilized for nitrite oxidation in the latter. CONCLUSIONS
which shows total nitrite removal in the neighborhood of 30min, that of the ammonia removal in Fig. 12 is about 40% or less for the same time period and the removal is essentially linear for that period. Comparison of these two figures also reveals that while acidic pH range is beneficial to the nitrite removal, the ammonia removal shows an opposite pH trend. According to Emmerson et al. (1980), ammonia in aqueous solution can exist in either un-ionized form (NH3) and/or ionized form (NH~-). Of these two forms of ammonia, the un-ionized one is much easier to oxidize (Benefield et al., 1982). For a given pH and temperature, these two forms of ammonia establish an equilibrium following the equation (Emmerson et al., 1980) NH3 + H 2 0 ~ N H
~ +OH-.
The percentage of ammonia existed in ionized form as a function of pH and temperature is shown in Fig. 13. It is apparent that as the pH increases above 20 -Ammonia
1015_
Electrochemical method has been employed in the present study to investigate nitrite and ammonia removal from aqueous solution. Several aspects, such as the sample solution conductivity, pH, current input and type of electrodes, are explored to determine their respective effects on the electrochemical removal efficiency. The iron electrodes were found to be unsuitable for the present process due to its low removal efficiency. Pairs of graphite anode and titanium dioxide cathode considerably elevate the removal efficiency and were adopted for the present study. Among the three major operating variables examined here, conductivity and current input were found to exert much stronger influence on the nitrite and ammonia removal efficiency than the pH. While a current input larger than 2 A was observed to yield lower power consumption than that below, the power consumption decreases steadily with increasing conductivity in the aqueous solution. It is also seen in the experimental results that the nitrite and ammonia removal is enhanced, respectively, in the acidic and alkaline environments. Although the pH of the aqueous solution decreases significantly during the electrochemical process, pH control appears to exert a negative effect on the nitrite removal. Presence of a second component in the aqueous solution was found to adversely affect nitrite and ammonia removal. Acknowledgements--The authors wish to sincerely thank
/
the Yuan Ze Memorial Foundation for the financial support (under the grant DRA 83001) of this project.
Ammonia-nitrite
REFERENCES
0
10
20
30
f 40
Time (min) Fig. 14. Comparison of ammonia removal for nitrite and nitrite-ammonia solution with 5 mg-l- ~initial concentration, 20 mg-I-~ ammonia concentration and 1.5 A current input.
Anonymous (1972) Use of ozone in sea water for cleansing shellfish. Wat, Eft. Treat. J. 12, 260. APHA (1992) The Standard Methods for Water and Wastewater Examination, 17th edn, Am. Publ. Hlth Assoc., Washington, D.C. Beck E. C., Gianmini A. P. and Ramirez E. R. (1974) Electroeoagulation clarifies food wastewater. Food Teehnol. 28, 2.
Aquacultural nitrite and ammonia removal Benefield L. D., Judkins J. F. and Weand B. L. (1982) Process Chemistry for Water and Wastewater Treatment. Prentice-Hall, Englewood Cliffs, N.J. Biwyk A. (1980) Electrocoagulation of biologically treated sewage. In Proc. 35th Purdue Ind. Waste Conf., Lafayette, Ind. Blogoslawski W. J., Brown C., Rhodes E. W. and Broadhurst M. (1975) Ozone disinfection of seawater supply system. In Proc. 1st Int. Syrup. on Ozone for Water and Wastewater Treatment, Cleveland, Ohio, Bovendeur J., Eding E. H. and Henken A. M. (1987) Design and performance of a water recirculation system for high-density culture of the African catfish. Aquacult. 63, 329. Cenkin V. E. and Belevtsev A. N. (1985) Electrochemical treatment of industrial wastewater. War. Eft. Treat. J. 25, 243. Emmerson K. R., Russo R. C., Lund R. E. and Thurston R. V. (1980) Aqueous ammonia equilibrium calculation: Effect of pH and temperature. J. Fish Res. Board Can. 32, 2379. Honn K. and Chavin W. (1976) Utility of ozone treatment in the maintenance of water quality in a closed marine system. Marine Biol. 34, 201.
721
Kruner G. and Rosenthal H. (1983) Efficiency of nitrification in trickling filter using different substrate. Aquacult. Engng 2, 49. Krstajic G. and Nakic V. (1987) Hypochloric production: A model of the cationic reactions. J. appl. Electrochem. 17, 77. Liao P. B. and Mayo R. D. (1972) Intensified fish culture combining water reconditioning with pollution abatement. Aquacult. 3, 61. Lin S. H. and Peng C. F. (1994) Treatment of textile wastewater by electrochemical method. Wat. Res. 28, 277. Matis K. A. (1980) Treatment of industrial liquid wastes by electroflotation. Wat. Pollut. Control. 19, 136. Otte G. and Rosenthal H. (1979) Management of a closed brackish water system for high density fish culture by biological and chemical water treatment. Aquacult. 18, 169. Pletcher D. and Walsh F. C. (1990) Industrial Electrochemistry, 2nd edn. Chapman & Hall, London. Poxton M. G. and Allhouse S. B. (1982) Water quality criteria for marine fisheries. Aquacuh. Engng l, 153. Ramirez E. R. (198 I) Physicochemical treatment of rendering wastewater by electrocoagulation. In 36th Purdue Indust. Waste Conf., Lafayette, Ind.