defluoridation of drinking water in batch and continuous-flow electrocoagulation systems
TRANSCRIPT
Poll Res. 32 (4) : 727-736 (2013)Copyright © EM InternationalISSN 0257–8050
DEFLUORIDATION OF DRINKING WATER IN BATCH ANDCONTINUOUS-FLOW ELECTROCOAGULATION SYSTEMS
J. MANOJ BABU AND SUDHA GOEL*
Civil Engineering Department, IIT Kharagpur, Kharagpur 721 302, India
(Received 13 April, 2013; accepted 6 May, 2013)
ABSTRACT
The objective of this study was to evaluate fluoride removal from drinking water usingelectrocoagulation (EC) in batch or continuous-flow reactors under different operating conditions.Effects of varying applied voltage, initial concentration and initial pH on fluoride removalefficiencies were evaluated using mild steel electrodes. Results for distilled water and groundwater were compared. Maximum F removal efficiencies from distilled water were 84.9 % and 79.4%in batch and continuous mode, respectively for the highest applied voltage of 25 V. Removalefficiencies from ground water were 79.6% and 28.7% in batch and continuous mode, respectively.At low F concentrations of 10 mg/L and initial pH ranging from 5 to 10, the final pH ranged from6.35 to 7.96, highlighting the advantage of EC over conventional coagulation for drinking watertreatment. Maximum F removal efficiency of 84.9% was observed at an initial pH of 6.42.Increasing fluoride concentrations from 20 mg/L to 50 mg/L resulted in an increase in pHirrespective of the applied voltage. As expected, an increase in initial F concentrations resulted indecreased F removal efficiencies. These results demonstrate the importance of initial Fconcentrations on both, final pH of the treated water and F removal efficiencies.
KEY WORDS : Electrocoagulation, Filtration, pH, Mild steel, Concentration
Corresponding author’s email : [email protected], [email protected]
INTRODUCTION
Water and energy have been identified as the toptwo global challenges for the 21st century. Waterquality and quantity are being challenged byincreasing pollution particularly in India. Today,India is relying increasingly on groundwater forboth, irrigation and drinking water requirements,unlike in the past, when surface water was the mainsource of water. Common pollutants in groundwaterinclude fluoride and nitrate which are of mainlynatural and anthropogenic origins, respectively. Theworst-affected areas in the country are rural areaswhere basic infrastructure for water supply andtreatment is generally lacking. Several specifictechniques for the removal of these contaminants areavailable and include coagulation, chemicalprecipitation, electrocoagulation (EC), adsorption,and ion exchange. Innovative, cheap, decentralizedand effective methods to purify ground water forhuman consumption, especially in rural areas are
needed and EC is one possible treatment strategy.Electrocoagulation: The process of destabilizing
and/or oxidizing suspended, emulsified, ordissolved contaminants in an aqueous medium byintroducing an electric current into the medium isknown as EC. Due to the passage of current, theanode dissolves during EC, and is often termed the‘sacrificial’ electrode. It is generally made of metalslike Al or Fe. Dissolution of the anode results in theformation of metal cations as shown in equation 1.These metal cations form polymeric metalhydroxide species (shown in equation 3), similar tocoagulant salts like alum and ferric chloride inconventional chemical coagulation. These cationsand other charged polymeric metal hydroxidespecies can bring about neutralization of negativelycharged particles and compounds. Once they areneutralized, the particles can bind together to formaggregates or flocs resulting in contaminant removal(Kumar and Goel, 2009).
Anode: oxidation and dissolution of electrode
728 BABU AND GOEL
Ma → Man+ + ne- .. (1)
2H2O → 4H+ + O2↑ + 4e- .. (2)Cathode: deposition of metal oxide layerMa
n+ + (OH-)n→ Ma(OH)n .. (3)2H2O + 2e- → H2↑ + 2OH- .. (4)
In the EC process, no chemical coagulants orflocculants are added, thereby reducing the amountof sludge. Gas bubbles that are produced duringelectrolysis can carry the contaminant to the top ofthe solution where it can be easily concentrated,collected and removed. EC is controlled electricallyand has no moving parts, and therefore, requireslow maintenance. In summary, the EC processincludes coagulation, electroflotation and electro-oxidation in a single unit.
Fluoride in drinking water: Fluoride contaminationhas been a serious problem in several parts of Indiaas well as in different parts of the world. Theacceptable safe limit of fluoride recommended byIndian standards is 1 mg/L [BIS, 1993]. However,there are several places in India where peopleconsume ground water, vegetables and other foodshaving fluoride levels higher than the acceptablelimit. It has been reported that many agriculturalproducts from Andhra Pradesh in India containfluoride ranging from 0.2 to 11 mg/kg (Rao andMahajan, 1991).
In order to achieve cost-effective fluoride removalfrom drinking water, a techno-economical treatmentmethod is undoubtedly essential. Defluoridation byEC has been demonstrated in several studies(Behbahani et al., 2011; Emamjomeh et al., 2011;Ghosh et al., 2008; Ghosh et al., 2011; Hu et al., 2003;Hu et al., 2007; Mameri et al., 1998; Montero-Ocampoand Villafane, 2010; Orescanin et al., 2011; Feng et al.,2003; Xu et al., 2011; Zhu et al., 2007). Only two ofthese studies were done with continuous flowsystems and in both studies, Al electrodes wereused. No systematic comparison of F removalefficiencies in batch versus continuous flow reactorswith Fe electrodes was found in the literature.Further, only four of the twelve studies reviewedwere done with real waters and only one studyshowed a comparison between F removalefficiencies with synthetic and real water samples.
The objective of the present study was to evaluatethe extent of fluoride removal from drinking waterusing EC in batch and continuous-flow reactorsunder different operating conditions. Effects ofvarying applied voltage, initial concentration andinitial pH on fluoride removal efficiency were
evaluated. Fluoride removal efficiencies in double-distilled water solutions and ground water from adeep tubewell were compared. In all experiments,changes in pH, conductivity, turbidity, sludgegeneration and electrode dissolution weremonitored. The distribution of F in different phases(supernatant, sludge and electrode deposition) wasalso evaluated.
MATERIALS AND METHODS
Collection of water samples: Fluoride solutionswere prepared using double-distilled water orground water. Ground water samples were collectedfrom the tubewell in Dandakaranya pump house infront of the VSR Complex, IIT Kharagpur, as andwhen required. The depth of the tubewell isapproximately 250 feet.
Experimental setup and procedure in EC batchreactor
A glass beaker of 1 L volume served as the EC batchreactor (Fig. 1). Two iron electrodes of size 14.5 cm x2.5 cm x 0.1 cm with immersion depth of 9 cm andinter-electrode distance of 3 cm were used. Theseelectrodes were connected to a transformer ofcapacity 0 to 30 volts and 0 to 3 Amperes, which wasused for converting AC current to DC current.Experiments were conducted at applied voltages of10V, 15V, 20V, and 25V. Conductivity, pH andturbidity were monitored during each experiment.The supernatant at the top of the reactor wascollected at regular intervals and stored in test tubes.All supernatant samples were filtered usingcellulose nitrate paper of diameter 47 mm with anominal pore size of 0.45 micrometer [WhatmanIndia].
Fig. 1. EC batch reactor and experimental setup for theremoval of fluoride.
DEFLUORIDATION OF DRINKING WATER IN BATCH AND CONTINUOUS-FLOW 729
Sampling: Current was passed for 180 minutes withcontinuous stirring using a magnetic stirrer. Sampleswere collected from the top of the reactor at intervalsof 15, 30, 45, 60, 80, 100, 120, 150 and 180 min.Samples were analyzed for pH, turbidity,conductivity, and fluoride. For experiments withgroundwater, samples were taken additionallyevery 20 min in the 4th hour.
Experimental setup and procedure for EC conti-nuous flow reactor
The continuous-flow EC reactor was made fromacrylic sheet with inner dimensions of 36 cm x 12 cmx 11.5 cm and 4 L volume (Fig. 2). Two iron (mildsteel) electrodes of size 14.5 cm x 2.5 cm x 0.1 cmwith an immersion depth of 10 cm and 3 cm inter-electrode distance were used. These electrodes wereconnected to a transformer of capacity 0 to 30 voltsand 0 to 3 Amperes, used for converting AC currentto DC current. One peristaltic pump (Miclins India,Model No PP 30) was used to regulate the flow rateto the reactor. The EC process was carried out for aperiod of 6 hours at a flow rate of 1 L/h. Fluoridesolutions were prepared with distilled water orgroundwater spiked with 10 mg/L of fluoride.
Samples were collected from the outlet of thereactor every 30 minutes and analyzed for pH,conductivity, turbidity and fluoride. These sampleswere filtered through cellulose nitrate paper ofdiameter 47 mm with a nominal pore size of 0.45micrometer (Whatman India).
Analytical methods
Only analytical grade reagents were used in thestudy and double-distilled water was used toprepare all stock solutions. A stock solution of 100mg/L sodium fluoride was prepared in doubledistilled water and added to the feed water asrequired.
pH, turbidity and conductivity
pH of all solutions was monitored using a digitaldesktop pH meter [Labquest, Vernier International,USA] calibrated with a buffer solution of pH 7[Merck India]. Conductivity was measured using adigital conductivity meter [Model: YK-22CT,Lutron]. Digital Turbidity meter [Model- 331,Electronics India] was used to measure turbidity. pHwas adjusted using NaOH and HCl solutions.
Sludge collection
Sludge was filtered through 0.45 micrometer filterpaper [Filter paper No. 42, Whatman India] andweighed. Mass of sludge was determined byweighing after drying the sludge at 110 oC for 24hours.
Electrode consumption
The mass of electrodes was determined before andafter each experiment. A loss in weight of anode andgain in the weight of cathode was observed duringEC. So, electrode polarity was reversed for eachexperimental run and the electrodes were sand-papered prior to each run.
Fluoride analysis
Fluoride concentration was measured using an ion-selective fluoride electrode [Orion, USA].Measurement of fluoride concentration withoutfiltration was not possible because the Orion 4 Starelectrode [Thermo Electron Corporation, USA] ishighly sensitive and the maximum concentrationthat can be measured is 15 mg/L.
RESULTS AND DISCUSSION
Fluoride removal in an EC batch reactor
The effects of three operating parameters on fluorideremoval efficiency were examined in EC batch
Fig. 2. Experimental setup of continuous flow EC process
730 BABU AND GOEL
experiments: applied voltage, initial concentrationand initial pH. Besides fluoride, pH, turbidity, andconductivity were monitored during eachexperiment. Evaluation of sludge generation andelectrode dissolution was also done for eachexperiment.
Effect of applied potential
The rate of coagulant generation and therefore,removal of contaminant is dependent mainly on thecurrent passing through the electrochemical cell.Therefore, the voltage applied to the electrodes waskept constant for each experiment. The effect ofapplied voltage on fluoride removal efficiencies wasstudied for the same initial concentration of fluoride10 mg/L and the results are shown in Fig. 3. As theapplied potential was increased from 10 V to 25 V,percent fluoride removal increased from 57% to84.9%. With increase in applied voltage, currentpassing through the reactor increases resulting ingreater dissolution of the anode in keeping withFaraday’s law. Dissolution of the metal anode resultsin the formation of cationic coagulating ions likemetal oxides and hydroxides and the amount of ironhydroxide available in solution to form complexesand precipitate the ions increases (Modirshahla et al.,2007).
Fig. 3. Effect of applied voltage on % removal of fluoridewith time. Experimental conditions: EC batchreactor with initial F concentration of 10 mg/L.
Fig. 4. Variation in conductivity over time for differentapplied potentials. Experimental conditions: ECbatch reactor with initial F concentration of 10 mg/L.
concentration in the supernatant. This would resultin lower conductivity at higher applied voltages ascompared to lower applied voltages.pH: pH was monitored in the EC reactor at differentvoltages during the course of each experimental runand the results are summarized in Table 1(experiments 1 to 4). For all four applied voltages,the initial pH varied from 6.42 to 6.70 while the finalpH varied from 6.87 to 7.26.
Turbidity: Turbidity of the reactor supernatant wasmeasured after filtration at different time intervalsduring the experimental run and was found toincrease steadily with time at all applied voltages asshown in Fig. 5. With time, coagulant productionincreases leading to increased floc formation. Hence,final turbidity increased with increase in appliedpotential from 1.4 NTU at 10 V to 4.2 NTU at 25 V.
Effect of initial concentration
Another set of experiments was conducted at themaximum voltage of 25 Volts and by varying the
Conductivity: As shown in Fig. 4, conductivity variedthroughout the experimental run. The highestconductivity (0.062 mS/m) was observed after 180mins at 10 V and the lowest (0.045 mS/m) at 25 V inthe supernatant. It is likely that the amount ofcoagulant ions produced was highest at 25 Vresulting in the highest amount of floc formationand settling of floc. Floc settling and removal fromsupernatant can lead to decrease in ion
Fig. 5. Change in turbidity over time for different appliedvoltages. Experimental conditions: EC batchreactor with initial F concentration of 10 mg/L.
DEFLUORIDATION OF DRINKING WATER IN BATCH AND CONTINUOUS-FLOW 731
initial fluoride concentration from 10 to 50 mg/L.These results are also summarized in Table 1(experiment 4 to 8). Maximum removal efficiency of84.9 % and minimum removal efficiency of 63.59 %was observed at initial concentrations of 10 mg/Land 50 mg/L, respectively.
Conductivity: Conductivity of the reactorsupernatant was measured at different timeintervals during the run and it was found that therewas some fluctuation initially and later conductivityincreased with time. In an EC process, currentincreases in proportion to conductivity because ofthe dissolution of the anode while resistancedecreases even as applied voltage is kept constant.Conductivity increased over time during theexperimental run due to generation of ions insolution. The final conductivity observed was 0.045mS/cm with an initial concentration of 10 mg/L and0.271 mS/cm with an initial concentration of 50 mg/L at 25 V.
pH: Change in pH with different initialconcentrations was also evaluated and the resultsare summarized in Table 1. The initial pH variedfrom 6.31 to 6.92 for initial concentrations varyingfrom 10 to 50 mg/L. The final pH was higher in allcases and varied between 7.26 for an initialconcentration of 10 mg/L to 9.39 for an initialconcentration of 50 mg/L. Water is converted tohydrogen gas and hydroxyl ions during the ECprocess making the solution alkaline. A majordifference between EC and conventionalcoagulation is that pH increases or remains neutralin the former and decreases in the latter.
Turbidity: Initial and final turbidity increased in allexperiments with increase in initial concentrations.The main reason for the increase in turbidity is theproduction of floc in solution during EC.
Effect of initial pH
It has been established that initial pH is animportant parameter influencing fluoride removalefficiencies and the quality of the final treated water.Experiments were conducted with double-distilledwater and an initial fluoride concentration of 10mg/L, at an applied voltage of 25 V and by varyinginitial pH from 5 to 10. Table 2 is a summary of theseresults and highlights one of the major advantagesof EC versus conventional coagulation (CC). In CC,the pH of the solution decreases after addition ofcoagulant and neutralization is generally required to
Tab
le 1
. Sum
mar
y of
res
ults
for
fluo
rid
e re
mov
al in
an
EC
bat
ch r
eact
or
Exp
erim
ent
App
lied
Co
Cf
F R
emov
alIn
itia
lFi
nal
Cha
nge
inIn
itia
lFi
nal
Ele
ctro
de
Iron
Slud
geN
umbe
rV
olta
ge(m
g/L
)(m
g/L
)ef
fici
ency
%p
Hp
Hp
HTu
rbid
ity
Turb
idit
yC
onsu
mpt
ion
dis
solv
edPr
oduc
tion
(V)
(NT
U)
(NT
U)
(mg
Fe/L
)(m
g Fe
/mg
(kg/
m3 )
F r
emov
ed)
Dis
tille
d w
ater
110
104.
357
.00
6.53
7.1
0.57
01.
470
12.2
80.
132
1510
3.21
67.9
06.
77.
110.
410
2.3
7010
.31
0.06
320
102.
7272
.80
6.46
6.87
0.41
03.
760
8.24
0.24
425
101.
5184
.90
6.42
7.26
0.84
04.
212
014
.13
0.34
525
204.
8375
.85
6.31
8.43
2.12
0.4
4.2
190
11.9
60.
446
2530
8.98
70.0
76.
518.
982.
470.
65
260
11.5
20.
647
2540
13.9
165
.23
6.92
9.28
2.36
1.2
5.2
360
11.6
90.
588
2550
18.2
63.6
06.
69.
392.
791.
16.
167
017
.29
0.70
Gro
und
wat
er9
2510
2.04
79.6
07.
6910
.97
3.28
4.3
9.1
870
109.
30-
732 BABU AND GOEL
bring the pH of treated water to an acceptable level.However in EC, two different phenomena wereobserved:
(a) pH increases in all experiments irrespective ofthe initial pH as shown in Table 1. Increase in initialconcentration resulted in greater pH change thanincrease in applied voltage. The treated water wasalkaline at all initial concentrations ≥ 10 mg/L.These results are similar to those for nitrate (ManojBabu, 2012; Kumar and Goel, 2010). A limitednumber of studies with EC and fluoride havereported results for final pH when evaluating theeffect of initial pH. Most of these studies showed anincrease in final pH after EC treatment (Emamjomehet al., 2011; Ghosh et al., 2008; Ghosh et al., 2011;Mameri et al., 1998; Feng et al., 2003).
(b) When the initial pH is acidic, the treatedsolution pH (final pH) value rises to a value close to7, and when the initial pH is alkaline, the final pHdrops to a value close to 7 as shown in Table 2. Theseresults are similar to those found by Guohua et al,(2000) when treating restaurant wastewater with ECand by Adapureddy and Goel (2012) when treatingdistilled water spiked with clay (kaolinite).
These results demonstrate that final pH aftertreatment is a function of initial contaminantconcentration, and nature of contaminant. Theimplications of these results for defluoridation ofdrinking water are significant. Fluorideconcentrations in Indian ground waters have beendetected up to 15 mg/L in West Bengal (Sharma,2003). Therefore, it is unlikely that pH change willbe significant for the F concentrations likely to befound in Indian ground waters and consequently,for the treatment of such waters. For F removal fromwastewaters with higher concentrations, initial pHmay well be a very important parameter.
Comparative study of distilled water with groundwater in a batch reactor
Two different source waters: double distilled waterand ground water were used for studying theremoval of fluoride and two experiments wereperformed at the same operating conditions of 25 Vapplied voltage, and initial fluoride concentration of10 mg/L. Groundwater had an average backgroundconcentration of 0.493 mg/L of fluoride and anadditional 10 mg/L of fluoride was added to theground water for the EC experiments. The groundwater examined had an initial pH of 9.17, 122.63mg/L of total dissolved solids, and 7.7 NTU ofturbidity.
No significant difference in the removal efficiencyof fluoride was observed in double distilled waterand ground water as shown in Fig. 6. The resultsdemonstrate that the removal of fluoride in distilledwater is slightly higher at 84.9% than in GW. Inground water, removal efficiency of 79.7% wasachieved with an initial concentration of 10 mg/Land final concentration of 2.03 mg/L. The smalldecrease in removal efficiency in groundwatercompared to double distilled water can be attributedto the presence of other anions apart from fluoride.
Analysis of electrode deposits and sludge using
Table 2. Variation in removal efficiency of fluoride with varying initial pH in an EC batch reactor
Initial Final Change Removal Initial Final Electrode Iron SludgepH pH in pH efficiency concentration concentration consumption, dissolved production
(%) (mg/L) (mg/L) (mg Fe/L) (mg g/LFe/mg F)
5 7.03 2.03 65.9 10 3.41 150.000 22.762 0.2136 6.35 0.35 78.8 10 2.12 140.000 17.766 0.2756.42 7.26 0.84 84.9 10 1.51 140.000 16.490 -7 7.4 0.4 81.8 10 1.82 - - -8 7.61 -0.39 79.6 10 2.04 - - -9 7.96 -1.04 73.9 10 2.61 370.000 50.068 0.26310 7.4 -2.6 69.1 10 3.09 150.000 15.353 -
Fig. 6. Comparative study regarding removal of fluoridein a batch reactor
DEFLUORIDATION OF DRINKING WATER IN BATCH AND CONTINUOUS-FLOW 733
EDX
Energy-dispersive X-ray spectroscopy (EDS or EDX)is an analytical technique used for determining theelemental composition of a sample. Electrodedeposits and sludge were analysed by EDX after ECtreatment.
EDX of electrodes: It is known that EC leads todissolution of the anode and deposition of metaloxides on the cathode. EDX analyses of the electrodedeposits obtained by sand-papering the twoelectrodes were done after a batch experiment at 25V with double-distilled water to determine theirelemental composition. Some reports in theliterature suggest that significant adsorption of Foccurs on both electrodes during EC [Zhu et al., 2007;Hu et al., 2003]. The spectral scans in our study showthe presence of Fe, O, C and Si in the deposits fromboth electrodes. A F peak was detected for thecathode deposits with a reported concentration of0%. These results show that the concentrations of Fin the electrode deposits were below the detectionlimit of 0.1% by weight for both electrodes.
EDX of sludge: The reactor contents at the end ofthe EC experiment were filtered using cellulose filterpaper [No. 42, Whatman India] and dried at atemperature of 110ºC for one day. The dry solids onthe filter were collected and analyzed using EDX.Four major compounds: iron, oxygen, fluorine andsodium were present in the sludge. Iron was presentdue to dissolution of the electrodes, presence ofoxygen was due to precipitation of oxides andhydroxides of iron, sodium and fluoride werepresent due to addition of the salt sodium fluorideto the initial solution. The relative distribution of theelements is summarized in Table 3.
Mass balance
A simple mass balance was done around the EC
mass adsorbed on both electrodes.Input: Initially, 10 mg/L of fluoride solution was
prepared in double distilled water.Output: Final concentration of fluoride left in the
supernatant was 1.51 mg/L.Accumulation: Amount of sludge formed was
0.1461 g/L. Based on the EDX results, fluorideconstituted 5.79% of the weight of sludge. Therefore,fluoride present in the sludge was estimated to be:0.1461 x 0.0579 g = 0.008459 g = 8.459 mg.
Based on the mass balance equation, the amountof fluoride accounted for in the output is 9.969 mgout of 10 mg. The difference in fluoride input andoutputs is insignificant at this point and the massbalance is considered complete. As shown by themass balance, no significant or measurableadsorption of F on the electrodes was observed inour study.
EC in continuous flow reactor for fluoride removal
Effect of applied voltage
Experiments were conducted at different voltages of10V, 15V, 20V and 25V for 6 hours with doubledistilled water solutions of fluoride in a continuous-flow reactor. Results are summarized in Fig. 7.Minimum fluoride removal efficiency of 55% wasobserved at 10 V leaving an exit fluorideconcentration of 4.5 mg/L (initial concentration 10mg/L) after 6 hours. The maximum removalefficiency of 79.4% of fluoride was observed at 25 Vwith an exit concentration of 2.06 mg/L (initial Fconcentration of 10 mg/L).
pH: Samples were collected at regular intervalsfor pH. The initial pH after adding sodium fluoride
Table 3. EDX of sludge
Element Weight % Atomic %
O 32.1 56.85F 5.79 8.64Na 4.15 5.11Fe 57.95 29.4Total 100 100
batch reactor with a 1 L solution. Assuming thatthere was no generation or consumption of fluoridein the reactor, the mass balance equation for F is:
Fluoride mass accumulated in sludge = Fluoridemass in - Fluoride mass out (supernatant) – Fluoride
Fig. 7. Variation in fluoride removal efficiency at differentvoltages in a continuous-flow EC reactor
ranged from 6.14 to 6.18. The final pH increased tosome extent and varied from 6.62 to 6.8. Thishighlights one of the major advantages of EC where
734 BABU AND GOEL
neutralization is not required after treatment unlikein conventional coagulation.
Conductivity: Samples were collected at regularintervals and analyzed for conductivity. It wasobserved that initial conductivity after addingsodium fluoride was 0.032 mS/cm. The maximumconductivity recorded was 0.051 mS/cm at the endof the experiment at 25 V. As electrode dissolutionand current increased with increase in appliedvoltage, the ionic strength of the solution increasedgradually leading to the small observed increase inconductivity.
Turbidity: Samples were collected at regularintervals and analyzed for turbidity. It was observedthat initial turbidity after adding sodium fluoridewas 0.3 NTU. Turbidity increased with time for allapplied voltages. The acceptable limit of turbidityfor drinking water is 5 NTU, and it was observedthat in all cases, the final turbidity was below theacceptable limit after EC and filtration.
Comparative study of fluoride removal in distilledand ground water
Another experiment was done to compare fluorideremoval efficiency with double distilled waterversus ground water. While no significant differencein removal efficiency was observed for doubledistilled water in the batch and continuous flowsystems, there was a large difference in removalefficiency for groundwater in the two differentsystems (Fig. 8). In contrast to the batch process, theremoval efficiency of fluoride in groundwaterdrastically dropped down to 28.7% in thecontinuous flow reactor (Table 4). There are twopossible reasons for this observation:
1. The presence of high concentrations of totaldissolved solids (122.63 mg/L), turbidity (7.7 NTU),
Table 4. Summary of results for fluoride removal in a continuous flow EC reactor
Applied Co Cf % removal Initial Final Initial Final Electrode Sludge IronVoltage efficiency pH pH Turbidity Turbidity Consump Production dissolved(V) (mg/L) (mg/L) (NTU) (NTU) tion (mg (kg/m3) (mg Fe
Fe/L) dissolved/ mg
fluorideremoved)
Distilled water10 10 4.5 55 6.15 6.62 0.2 2.7 15 0.057 27.2715 10 3.7 63 6.18 6.8 0.3 3.2 17 0.079 26.9820 10 2.95 70.5 6.15 6.66 0.3 3.2 19 0.113 26.9525 10 2.06 79.4 6.14 6.7 0.3 3.7 29 0.129 36.52
Ground water25 10 7.13 28.7 9.17 9.27 7.7 1.7 78 0.256 271.77
high pH (9.17) and the presence of various otherions apart from fluoride in GW. Hu et al. (2003)found that if fluoride was not the dominant anion insolution, most of the fluoride ions attracted to theanode were replaced by other ions resulting in lowerfluoride removal efficiency. Anion analyses of theground water samples used in this study showsaverage concentrations of fluoride, chloride, nitrite,bromide, nitrate and sulfate as 0.493, 2.03, 0.175,0.96, 0.6 and 2.3 mg/L, respectively. Feng et al. (2003)found that anions like bromide, phosphate andsulfate resulted in a decrease in F removal efficiencywhile chloride enhanced F removal.
2. Another difference between batch andcontinuous flow studies is the lower effectivecoagulant dose that the two water samples wereexposed to in the continuous flow reactor (78 mgFe/L) as compared to the batch reactor (870 mg Fe/L). In the batch reactor, only 1 L of water wasexposed to an applied potential of 25 V over aperiod of 3 hours followed by 1 hour for settling. Inthe continuous flow reactor, 10 L of water wereexposed to the same applied voltage for an
Fig. 8. Comparative study regarding removal of fluoridein a continuous-flow EC reactor.
DEFLUORIDATION OF DRINKING WATER IN BATCH AND CONTINUOUS-FLOW 735
experimental run time of 6 hours and a hydraulicresidence time of 4 h. This results in a much lowereffective coagulant dose in the continuous flowreactor even though the residence times for the batchand continuous flow reactor are the same. The lowereffective coagulant dose had an impact only on Fremoval from GW, not distilled water implying thatthe dose was sufficient for distilled water but notGW.
CONCLUSIONS
Fluoride is one of the most common ground watercontaminants of natural origin in India. Fluorideremoval was achieved in this study throughelectrocoagulation and filtration. Filtration wasnecessary after EC to achieve acceptable waterquality in terms of turbidity. In the batch studies,three operating parameters were evaluated: appliedpotential, initial concentrations and initial pH. Effectof varying applied voltage was evaluated incontinuous flow made. Comparative studies of Fremoval from distilled water and groundwater weredone in batch and continuous-flow mode.
Applied potential: As applied potential wasincreased, percentage F removal also increased inboth, batch and continuous-flow systems. In batchelectrocoagulation studies, maximum F removalefficiency was 84.9% with distilled water, and 79.7%with ground water.
In continuous flow studies, 79.4% and 28.1% offluoride were removed from distilled water andground water, respectively. The lower F removalefficiency in continuous mode for ground water isattributed to competition from other ions and alower effective coagulant dose compared to thebatch studies.
pH: Two phenomena were observed regardingpH: (1) At low initial fluoride concentration of 10mg/L and initial pH varying from 5 to 10, nosignificant change in final pH was observed.Maximum removal of fluoride was obtained at anoptimum pH of 6.42 in batch systems. (2) The effecton final pH was found to be dependent on the initialF concentration. With increase in initial Fconcentration (≥ 10 mg/L), change in pH alsoincreased, resulting in alkaline treated water. Sincethe highest F concentrations in ground waters inIndia have been detected only up to 15 mg/L, asignificant increase in pH is unlikely after ECtreatment for F removal, i.e., neutralization will notbe required.
Sludge generation: EDX analyses of sludge andelectrode deposit samples was done and based on amass balance for fluoride around the batch reactor,it is clear that the bulk of the fluoride added (85%)was entrapped in the sludge. Oxides of iron werethe major components present in the sludge.
ACKNOWLEDGEMENTS
The authors are grateful for the financial supportprovided by Department of Science and Technology– Water Technology Initiative for this research.Chandan Bera and Iti Sharma are gratefullyacknowledged for their help with IC and EDXanalyses, respectively.
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