Electrocoagulation-Electroflotation As A Surface Water Treatment For Industrial Uses
Electrocoagulation-Electroflotation As A Surface Water Treatment For Industrial Uses
Electrocoagulation-Electroflotation As A Surface Water Treatment For Industrial Uses
a r t i c l e i n f o a b s t r a c t
Article history: Water is a natural product that is needed in many industrial uses, but some processes like washing
Received 2 March 2010 or cooling do not require drinking water. In this work we investigated the efficiency of an electrolytic
Received in revised form 29 June 2010 treatment of surface waters in order to increase their quality. The waters were taken from a river and in
Accepted 30 June 2010
a pond and they were treated by electrocoagulation–electroflotation with an aluminum soluble anode.
Vital nutriments for the bacteria development were consumed during the electrolysis. This treatment led
Keywords:
to great decreases of molecular oxygen, phosphate and nitrate anions and dissolved organic compounds.
Bacteria
Each of these decreases may explain the disinfection effect that was observed for the total flora. Moreover,
Disinfection
Aluminum
the X-ray diffraction of the electro-generated solid showed the presence of nanocrystallites that could be
Nanoparticles involved in a bactericidal effect. After the electrocoagulation–electroflotation treatment, the investigated
Adenosine 5 -triphosphate waters exhibited an increased quality for a cooling use.
© 2010 Elsevier B.V. All rights reserved.
1383-5866/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.seppur.2010.06.024
C. Ricordel et al. / Separation and Purification Technology 74 (2010) 342–347 343
2.1. Water substrates All chemical analyses were carried out by following standard
methods: Alcalimetric Title and Complete Alcalimetric Title (AT and
All the experiments were performed with tap water or water CAT) NF EN ISO 9963-1 (T 90-036); total hardness (Mg2+ + Ca2+ )
samples taken in a river and a pond. These samples were stored (NF T 90-003); hardness (Ca2+ ) (NF T 90-016); chloride (NF T 90-
at 4 ◦ C and brought to room temperature before experimentation. 014); permanganate index (KMnO4 ) NF EN ISO8467 (T 90-050);
Table 1 gathers the main physicochemical data of these surface suspended matters (NF T 90-105); phosphate NF EN ISO 6878
waters. (T 90-023). Nitrate concentration was measured by the Reflecto-
quant method (Merck). The detection range was between 5 and
2.2. Equipment and electrolysis 225 mg L−1 .
The ECEF reactor was a 2 L electrolytic cell with two parallel 2.5. Electrochemical analysis
aluminum plates, each having a surface area of 38.4 cm2 . The elec-
trodes were installed vertically in the middle of the reactor with Specific electrochemical analyses were done during electrolysis
an electrode gap of 2 cm. Before electrolysis, the electrodes were performed in a 0.5 L three-neck round-bottom flask equipped with
immersed in 2 M NaOH during 5 min and then rinsed with water. aluminum electrodes, a platinum electrode and a saturated calomel
Finally, they were dried with absorbent paper and weighed. The electrode (SCE) as a sensor of the oxidation reduction potential
electrodes were connected to a DC power supply (Micronix MX300- (ORP) and with a Clark electrode for the oxygen concentration mea-
1) providing a controlled voltage or current up to 300 V or 1 A, surement. The electrolysis was stopped during the electrochemical
respectively. All the runs were performed at room temperature, measurements.
under a magnetically agitation. Current intensity was chosen in
order to avoid any heating of the solution, a phenomenon which 2.6. X-ray diffraction
would influence the disinfection action of electrolysis. The applied
tension and the water temperature were measured during the elec- The solid obtained after electrolysis carried out in 0.01 M NaCl
trolysis. The conductivity and the pH of the waters were measured with a soluble aluminum anode was collected by filtration. It was
with a WTW 315i apparatus. After each run, the aluminum elec- washed with water, dried at 105 ◦ C and then characterized by X-ray
trodes were washed with water, dried and weighed. Water samples powder diffraction (Bragg-Brentano geometry, Rigaku-Geigerflex
were taken and used for analysis after sedimentation. Neither cen- diffractometer). The diffraction pattern was scanned from 10◦ to
trifuging nor filtration was performed. Parallel blank analyses were 90◦ (2) using Cu K␣ = 1.54178 Å and a step length of 0.02◦ (2).
carried out on untreated waters. The grain size of crystallized alumina was estimated from the full
width at half-maximum values of the X-ray diffraction using Scher-
2.3. Bacteria analysis rer’s formula.
Total bacteria and algae were counted according to the standard 3. Results and discussion
method NF EN ISO 6222(T 90401) [34]. The count of the revival
colonies was obtained at 37 ◦ C on Plate Count Agar (PCA). Two The first point of interest was to investigate the efficiency of
characteristics were determined: (i) T0 , the initial bacterial con- ECEF in improving the water quality for an industrial application in
centration in unit forming colonies (UFC mL−1 ) and (ii) Tf , the final cooling towers. The second point concerned the capacity of ECEF to
bacterial concentration (UFC mL−1 ). disinfect surface waters.
For the effluent analysis, the Quench—Gone cATP (QGA) technol-
ogy was used. The decanted solid was analyzed for Total Control 3.1. Removal of chemical species
for Microbial growth control (TCM). TCM and QGA are technolo-
gies from LuminUltra working on the measurement of adenosine The efficiency of the ECEF process on the chemical composition
5 -triphosphate (ATP). ATP is a direct and interference-free indica- of river water and pond water is given in Table 2. The experiments
tor of the total biomass. The results are first expressed in Relative were carried out at 17 ◦ C with a river water having initial pH = 7.60
344 C. Ricordel et al. / Separation and Purification Technology 74 (2010) 342–347
Table 2
Removal efficiency of ECEF treatment applied to surface water and drinking water.
Chemical parameters (conductivity) Tap water River water (0.41 mS cm−1 ) Pond water (0.55 mS cm−1 )
Ca2+ : 150 ◦ F
and a conductivity of 0.41 mS cm−1 and at 12.3 ◦ C with a pond water and hydrogen carbonate leads to the formation of calcium carbon-
having initial pH = 6.94 and a conductivity of 0.55 mS cm−1 . In order ate and hydrogen carbonate on the cathode according to reactions
to determine the influence of some water parameters on the ECEF (6) and (7) [37]:
process, we investigated the efficiency of the treatment applied
HCO3 − + OH− → CO3 2− + H2 O (6)
to tap water after NaCl addition. The calculation of the chemical
2− 2+
removal efficiency (RE%) was performed using formula (5) where CO3 + Ca → CaCO3 (7)
C0 and C are concentrations of the chemical before and after elec-
trolysis: The reduction of the permanganate index is similar to results
obtained in studies carried out on organic matter removal [38,39].
(C0 − C) × 100 The nitrate removal results confirm the experiments that were
RE (%) = (5)
C0 carried out by Emamjomeh and Sivakumar [40] by a batch ECEF
process. The nitrate removal efficiency depends on electrolysis time
The results (Table 2) show a very good efficiency for orthophos- and current values. At both low current and short electrolysis time,
phate ions and fairly good ones for the suspended solids, nitrate ions the nitrate removal efficiency was very low.
and the permanganate index. The nitrate removal efficiency was The important removal of phosphate ions is the most interest-
better for the pond water than for the river water probably because ing result. During the dissolution of aluminum anode, micro-flocs
nitrate concentration was lower in the pond water. For tap water, are formed rapidly. After the electrocoagulation, the solutions were
and Ca2+ supplemented tap water, the results given in Table 2 show maintained unstirred for a few minutes in order to allow the
that higher conductivity and Ca2+ concentration have a negative agglomeration of micro-flocs into larger flocs. During this floccu-
effect on the phosphate ions removal. But, the high concentration lation process all kinds of micro-particles and negatively charged
of Ca2+ decreased the CAT. ions are attached to the flocs by electrostatic bonding. Phosphate
According to the results of Table 2, ECEF can be successfully ions are also adsorbed onto coagulated flocs. When aluminum ions
used to remove suspended matters, orthophosphate and nitrate are present in the water, AlPO4 forms in the low pH range (<6.5)
ions and organic matter involved in the permanganate index. After and at a higher pH range (>6.5) aluminum increasingly converts
30 min of electrolysis, about 50% of the suspended matters were to oxides and hydroxides [41]. The removal of orthophosphate and
precipitated. It is well known that the similar charge of colloidal nitrate ions is very important for the success of the process. Indeed,
particles prevent their aggregation through electrostatic repul- these ions are vital nutriments for a good bacterial development.
sion. The ECEF efficiency is based on the fact that the instability Their removal inhibits biofilm formation.
of colloids, suspensions and emulsions is determined by electric In order to understand the action of electrocoagulation against
charges. Therefore, when additional electrical charges are supplied bacteria, we measured the oxygen concentration and the ORP of
to colloidal particles via appropriate electrodes, the surface charges solution during some experiments. The results are presented in
are neutralized and several particles coalesce and lead to larger Figs. 1–3.
agglomerates which may be separated by flotation or sedimenta- When the electrocoagulation experiments were performed in
tion [21,35]. the presence of chloride salts, the oxygen concentration showed a
Holt et al. [23] proved that the electrolysis current is not the sole
parameter which controls the coagulation process. Bubble produc-
tion rate and fluid regime within the reactor are also key parameters
of the process. The collision between particles, the floc growth and
the potential for material removal by flotation are controlled by
the current. Low electrolysis current produces low hydrogen bub-
ble density, leading to a low upward momentum flux, and thus a
poor mixing within the reactor. Under these conditions the sedi-
mentation is more efficient than the flotation. When the current
increases, the bubble density and the amount of mixing increase
and favor flotation over sedimentation. The operational current has
a strong influence on the dominant pollutant removal path, that is
flotation or settling, and consequently on the floc production. High
current means a small electrocoagulation cell but the process works
with a wasting electrical energy in heating up the water [30].
The decrease of the total hardness can be attributed to an elec-
trochemical generation of a softener to limit the scaling [36]. We Fig. 1. Variations of O2 concentration during electrocoagulation carried out in the
should mention that the simultaneous presence of calcium ions presence of different electrolytes (electrolyte concentration 0.1 M; I = 0.5 A).
C. Ricordel et al. / Separation and Purification Technology 74 (2010) 342–347 345
Table 4
ATP (TCM) results for isolated solids after ECEF treatment of river water and pond
water.
Ref. ATP total ATP extra ATP intra Dead cellular (%)
(pg mL−1 ) (pg mL−1 ) (pg mL−1 )
River water
[1] 33,993 11,540 22,453 34
[2] 42,790 9879 32,911 23
[3] 10,591 5598 4993 53
Pond water
[1] 31,777 17,968 3809 57
[2] 26,349 26,310 39 100
[3] 18,909 17,404 1505 92
Fig. 2. ORP variation during an electrocoagulation carried out in KCl solution (0.1 M;
I = 0.5 A).
troflotation. The bacteriological results after 30 min of electrolysis
are listed in Table 3. They show a total elimination of flora and good
disinfection efficiency. Ghernaout et al. [21] founded similar results
with surface water using steel electrodes at the same voltage but
with a current of 10 A. In our experiments, the current value was set
at 0.22 A. This low current was taken in order to avoid a temperature
increase due to a Joule effect.
ATP was measured before and after electrocoagulation. ATP is a
direct and interference-free indicator of total biomass. Any living
cell produces and consumes ATP. This molecule is thus specific to a
living cell and we can consider that any trace of ATP is the witness
of cells which are died or live.
Fig. 3. ORP variation during an electrocoagulation carried out in KNO3 solution
Table 4 gathers ATP results obtained on the river and pond flocs.
(0.1 M; I = 0.5 A).
Table 5 contains results of solution analysis. We observed a sig-
nificant fluctuation in results concerning the percentage of dead
90% decrease within 10 min (Fig. 1). These oxygen removals were bacteria in the flocs. These differences can result from 10% of error
due to a deoxygenating action of the hydrogen evolved at the cath- expressed by LuminUltra technologies. ATP is not completely elimi-
ode. When the electrocoagulation was realized in the presence of nated from died cells. However, we can suppose that all the bacteria
nitrate salts, the oxygen removal was less efficient. A 90% decrease did not die in the flocs. Concerning the measures of ATP QGA, results
of oxygen concentration needed 60 min. During these electrolyses of Table 5 confirm the results obtained with a standard method. The
less hydrogen was evolved at the cathode since a part of the current solution was bacteria-free. Bacteria were trapped into the flocs and
was consumed by the nitrate reduction. Oxygen removal during for most part of them they were living.
electrocoagulation was more efficient than a deoxygenating pro- The experiments reported indicate only that the bacteria are
cess performed by a nitrogen bubbling into the solution. The oxygen not detectable by any of classical cultivation or ATP methods
decrease leads to an unfavorable environment for aerobic bacteria. (Tables 3 and 5). But a significant fraction of the initial popula-
Figs. 2 and 3 present the variations of ORP during electrocoag- tion remains alive in the flocs (Table 4). To explain the dead and
ulations. As expected for a solution with decreasing concentration the concentration of bacteria in the flocs we used some working
of oxygen removal and increasing content of hydrogen, the ORP hypothesis.
decreased until −0.15 V/SCE. In agreement with a lower concen-
tration of hydrogen, the potential decreased more slowly in nitrate 3.3. Disinfection hypothesis by ECEF
solutions (Fig. 3).
According to Oss [42] bacterial adhesion to surfaces results
3.2. Bacterial removal by ECEF from the Lifshitz–Van der Waals free energy interaction and the
Lewis acid–base free energy interaction. Bacteria either donate or
The effect of ECEF on bacteria and algae development was inves- accept electrons to the surface of the substrate (in this case the
tigated on treated waters. After 10 min of electrolysis, a foam layer gas bubbles). Adhering bacteria may decrease electrostatic repul-
appeared at the surface and increased in time as the result of elec- sion allowing floc formation. The charge transfer, however, takes
place over a range shorter than 0.5 nm, so close contact is needed.
Table 3
Efficiency of ECEF treatment on the bacteriological parameters for river water and Table 5
pond water (measures by standard methods). ATP results (QGA) for water obtained after ECEF treatment and decantation.
Experiment reference ECEF 1 ECEF 2 ECEF 3 Ref. ATP (pg mL−1 ) Equivalent microorganismes % Removal
River water River water
To (UFC mL−1 ) 1.12E+02 1.12E+02 1.12E+02 Blank 335.4 3.3E+05
Tf (UFC mL−1 ) 0 0 0 Test 1 31.6 3.1E+04 91
Percentage decrease (%) 100 100 100 Test 2 14.3 1.4 E+04 96
Test 3 15.9 1.6 E+04 95
Experiment reference ECEF 1 ECEF 2
Pond water
Pond water Blank 933.1 9.3E+05
To (UFC mL−1 ) 4.15 E+04 6.05 E+05 Test 1 36.1 3.6E+04 96
Tf (UFC mL−1 ) 2.50 E+02 2.20 E+02 Test 2 101.1 1.0E+05 89
Percentage decrease (%) 99.4 99.6 Test 3 18.2 1.8E+04 98
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