Performance Evaluation of Electrocoagulation Technique for Removing Groundwater Hardness of Tikrit University

The performance of Electrocoagulation (EC) process for removal of hardness in groundwater (GW ) of Tikrit University in Salahaddin province, north of Baghdad in Iraq has been studied using aluminum (Al) electrodes with both bipolar and monopolar configurations. The effect of initial pH , applied voltage (U) , electrolysis time (t) , and electrodes configurations on the performance of EC has been investigated. It was found that the best initial pH value to remove hardness (HD) was 9.5. The results indicated that increasing U and t had a positive effect on the hardness removal efficiency( hardness %E ) to reach 90.4%andit was influenced by the electrodes configuration in which hardness %E was 83.5% in bipolar connection compared with 66.2%in monopolar connection.Also The electrical energy consumption hardness %E and the experimentally and theoreticallyelectrodes consumption ( exp W ) and ( theo W )were calculated.It was absorbed,as determined, that there is no significant difference between pseudo-first and second-order kinetic model except at 40 volt that the pseudo second-order kinetic model fits better than the first-order kinetic model with the data of the electrocoagulation process. Finally the cost at themost favorable conditions for EC process was calculatedresulting that the electrocoagulation process is successfully applied to remove the hardness rate from groundwater with high performance.


INTRODUCTION
uitable and available water for human consumption is highly limited and likewise, available drinking water has been reduced because of the pollution created naturally and artificially. Among water quality parameters, hardness has always been investigated as an important factor.Hardness which is one of the chemical characteristics of water is caused mainly by the presence of calcium and magnesium [1] .Hardness of waters varies considerably from place to place. In general, groundwater is harder than surface waters andwater gets harder if the amount of calcium and magnesium is increased. Hardness can be expected in regions where large amounts of limestone are found [2] . Hardness ions create a lot of problems for life and industry [3] ,therefore knowing the hardness of water is important in evaluating its use as a domestic or industrial water supply [2] .
There are various techniques for the removal of water hardness, such as using electromembrane processes, nano-filtration [4] chemical or natural substances [5] , ion exchange resins [6] , and electrodialysis [7] .
Recently, growing demand for high quality water has justified the development of modern and low cost technologies for hard and very hard water softening [4] .One of these techniques is electrochemical technology such as electrocoagulation process, which is being used for the removal of colloidal and suspended particles, ions [7] heavy metals [8] , dyes, [9] , organic matters [10] and oily matters [11] from water and aqueous environments.
Electrocoagulation is a complex process occurring via serial steps such as; electrolytic reactions at electrode surfaces, formation of coagulants in aqueous phase, adsorption of soluble or colloidal pollutants on coagulants which are removed by sedimentation or flotation [3] .
A simple electro-coagulating batch reactor is made up of anode and cathode with monopolar or bipolar configuration [12] . When a potential is applied from an external power source, the anode material undergoes oxidation, while the cathode will be subjected to reductive deposition of elemental metals [13] . Following equations, describe EC process in relation to Al electrode: Atanode: 3  Al(OH) + , 4 717 Al(OH) + , 4 820 Al(OH) + , 7 13424 AlO(OH) + , 5 1334 Al(OH) + , which transform finally into 3(s) Al (OH) according to complex precipitationkinetics [13,14] . " sweep flocs" have large surface areas which are beneficial for a rapid adsorption of soluble organic compounds and trapping of colloidal particles. Finally, these flocs are removed easily from aqueous medium by sedimentation or H 2 flotation [15] .EC technology, compared with other techniques, enjoys some advantages like plain equipment, easy functionality, short resistance time, no need of chemicals, low sludge production, sludge stability, suitable sedimentation of sludge, dewatering and environmental compatibility [15] . The aim of this study was to investigate EC process performance to remove HD from GW using Al electrodes as a substitute for other water softening techniques and determining the optimal pH, U,t and electrode configuration, alsocalculate the hardness %E , exp W and theo W . In addition to determinetheorder and the kinetic model ofthe obtained data of EC process and its cost.

MATERIALS AND METHODS
The GW was collected from many wells located at Tikrit University in Salahaddin Province in the north of Baghdad in Iraq, and its principle characteristics are listed in Table 1 . Water samples were taken from the underground hadan average concentration about 2070 mg/l of total hardness rate. Fig.(1.a,b) shows an overview of EC equipments which include DC power supply (Type SDR 4010, SODILEC Model; 40V, 10A, Japan) withseparated sheets of Al electrodes of (100x35x8) mm of 35cm 2 an active area and an inter distance (d) of 5 and 2 cm in monopolar and bipolar configuration respectively were putting into a plexiglas reactor tank of 170×120×80 mm and 0.5L volume of solution. pH of the sample was adjusted using sulphuric acid and normal sodium hydroxide; the reactor was tested with water samples of different pHs (4.4, 7.2 and 9.5) under four applied voltages (10, 20,30 and 40v).
Under each testing conditions, four reaction durations were tested: 5, 15, 30 and 50 min. Samples were (25 mL) from the middle of the reactor, thenfiltered in order to remove the formed flocs.Finally, filtered samples were analyzed by atomic absorption spectrophotometer concerning and determined the total hardness removalefficiency hardness %E . At the end of each run, the electrodes were washed thoroughly with dilute acid and water to remove any solid residues on the surfaces, dried and re-weighted.pHsolution was also analyzed at the end of the experiment.In the present study, the hardness %E by EC process has been evaluated at different condition pH, U, t and different electrodes configuration. Where o HD and t HD are theconcentration of hardness rate before and after EC process respectively.Electrical energy consumption EEC is a very important economical parameter in the electrocoagulation process. The EEC was calculated using the following equation: Where; EEC is the electrical energy consumption (Wh/m 3 ), U is the applied voltage (volt), I is the current intensity (Ampere), t is the electrocoagulation time (hr)&V is the volume (m 3 ) of the treated water. Also, the amount of electrode dissolved was calculated theoretically by using Faraday's law and compared with the experimental amounts.
Where W theo (kg Al electrode/m 3 of treated water) is the theoretical amount of ion produced by current intensity I (Ampere) passed for a duration of operating time t EC (sec), N is the number of electrons involved in the oxidation/reduction reaction; for Al N Al =3, M is the atomic weight of anode material (M for Al = 0.02698 kg/mol), F is the Faraday's constant (96485C/mol) and V is the volume (m 3 ) of the GW in the EC reactor. Hence, the amount of aluminum adsorption increased with the increase in adsorbent concentration, which indicated that the adsorption depended on the availability of binding, sites for aluminum hydroxides flocs [16] .

RESULTS AND DISCUSSION Effect of pH on Hardness Removal
The pH of the solution is one of the most important parameters that govern the removal efficiency in the EC process. To examine this effect, the sample was adjusted to the desired pH for each experiment by using 0.1M NaOH solution and 0.1M H 2 SO 4 solution.As showed inFig. (2) Al(OH) were produced. Since 3 Al(OH) had higher weight and density, it settled faster and had higher efficiency. Therefore, it acts better in enmeshment in a precipitate. Hence, based on the results of the present study and previous studies electrocoagulation process can act as pH moderator [17,18] .

Effect of Different Electrode Connections
In order to improve thehardness removal efficiency from the ground water, electrode configuration can have a justified effect on the sludge formation as well as on the corrosion of the electrode. Effect of electrode configuration (monopolar and bipolar) for the hardness %E by EC is shown in Fig.(3). Investigation was performed for softening GW of 2070mg/l total HD with constant other parameters of (5cm) distance between electrodes,t of (10min) and Uwere maintainedto be (10-40volt)during the experiments. It was observedthat with the passage of time hardness concentrationinside the EC bath was decreased for both electrode connections.It is also seen from Fig.(3)that the hardness %E for actual acidity of7.2 pH and for 10 min time at 10 to 40 volt in monopolar connectionwere25.2% to 66.2%, whereas forbipolar connection, were 42 to 83.5% respectively. In bipolar connection, a higher surface area compared to thatof monopolar connection favored the adequate anodic oxidation.As a result, with the same applied voltage or potential (current applied) for both kindof connection, the intensity is higher in the bipolar connection,thereforethe hardness %E of the GW were foundmore than that was observed in monopolar electrode connection [19] .

Effect of applied potential and Time
The effect of U on the were achieved from18.9 to 54.6%after 5min, 32.4 to 74.8% for 15min, 38.7 to 88.8% for 30min and 54.2 to 90,3%after 50min of electrolysis from 10 to 40 volt respectively. As the applied voltage increased, the removal efficiency of hardnessincreased. Simultaneously, the sufficient current passingthrough the solution rise due to the increased applied voltage. Dueto sufficient current through the solution, the metal ions generatedby the dissolution of the sacrificial electrode were hydrolyzedto form a series of metallic hydroxide species. However, it took about 15 min to reach over 75% ofhardnessremoval, and30 min to reach 88.8 when the appliedvoltage was40V while under 20 and 40 volt it took 50 min to reach 72% and 90.3% hardness %E respectively. This is primarily due to an insufficient amount ofelectric power supplied at 10V for the complete destabilization ofthe suspendedmetallichydroxide species in the solution. Both 30 and 40 volts aresuitable for this electrocoagulationexperiment;also30 to 50 min was the requiring treatment times to reach over88-90% hardnessremoval. Therefore the optimal reaction time is 30-50 min for this equipment considering the treatment cost and efficiency. As time progressed and dissolved coagulants at the aluminum electrodeincreased, there was an increase in the removal efficiency, whichcould be explained by a sufficient amount of coagulant dissolvingfrom the aluminum electrode to effectively reduce the layer of thesuspended metallic hydroxide species to destabilize the metallichydroxide species. In order to investigate the optimum applied voltage,the consumption of the specific energy for an applied voltageduring electrocoagulation process was evaluated [20,21] .

Electrodes Metal and Specific Energy Consumption
The synthetic GW with high concentrations HDwere softened in terms of hardness %E and specific EEC by aluminum electrocoagulation. Table 2 shows that the energy consumption in EC cell of 7.2 pH solution increased from 0.0167kWh/m 3 at 10 volt and 5min to 4.667kWh/m 3 at 40volt and 50min with increasing the removal efficiency from18.4 to 90.3% in monopolar connection,in bipolar connection increased from 0.033 to 0.220 kWh/ m 3 at rate of removal efficiency from 42 to 89.7% through 10 min. Table 2also represents a comparison between the theoretical and experimental aluminum amount that released to solution whena voltage from10 to 40 volts was applied in EC cell at pH 7.2 for various duration timefrom (5-50min).Theoretical aluminum amount was calculated by Faraday's law,equation (7) [22][23][24] .

Kinetic Study of Hardness Removal
The overall apparent kinetics of electrocoagulation process of hardness removal is described by a macro-kinetics model in which the rate constant depends on the applied voltage (or current density). This model provides preliminary data for evaluating the reaction rate constants. The kinetic rate equation for representing the removal rate of hardness concentration from the GWis described by the following m th order reaction kinetics: …. (11) whereC represents the hardness concentration, m is the order of reaction, k is the reaction rate constant, and t is the time. For a first-order reaction: The slope of the plot of lmg -1 min -1 . The values of k with first-order and second-order models for hardness removal at various applied voltages were determined graphically and are shown in Table 3, respectively for monopolar and bipolar configuration. The conformity between experimental data and the model values was evaluated by the correlation values (r 2 ). As can be seen in Table 3, regardless of the higher applied voltage, the

Eng. & Tech. Journal , Vol.30, No.18 ,2012 Performance Evaluation of Electrocoagulation Technique for Removing Groundwater Hardness of Tikrit University
3274 r 2 value for the second-order model was slightly higher than that for the first-order model.Whenfixing the other experimental conditions, but increasing the applied voltage from 10 to 20volt, the first-order rate constant K 1 increased from 1.1074 to 2.4212 E-2 min -1 while at 30 to 40volt, the second-order rate constantK 2 increased from 5.6579 to 9.5354E-2(L/g.min). It is very important to note that fast hardness removal took place at a short electrolysis time; this is considered a great advantage of using the electrocoagulation process [25,26] .

Cost of the EC process
Preliminary estimate of the cost of the removalof groundwater hardness by electrochemical process has been done considering the energy cost and the cost of electrode. From the kinetic constants obtained in the study, amount of electric power required for desired hardness reduction can be computed. The cost of electrical energy is variable in different parts of the world. Considering, as in Iraq, the cost of electrical energy about 0.02 US$ per KWhand the cost of electrode is 0.003 US$ per gram of aluminum metal.The cost of the process is determined by the sacrificial electrode loss and the electrical energy consumption. In the most favorable conditions 40 volt potential applied, 5cm distance between the electrodes7.2 pH and 50 min time, the maximum possible hardness reduction was reached90.3%. Then, the power consumption expected from equation (6) would be 2.5 kWh/m 3 for an average final value of 200mg hardness/L, and the sacrificial electrode loss is 27.9 g/m 3 of aluminum as calculated in equation (7) [27,28] .
Operating cost = aC energy + bC electrode … (14) whereC energy (kWh/m 3 ) and C electrode (kgAl/m 3 ) are consumption quantities for the Al removal, which are obtained experimentally. "a" electricalenergy price US$/kWh; "b" electrode material priceUS$/kg Al. Cost due to electrical energy (kWh/m 3 ) is calculated using above values. Operating Cost: 0.1337 US$/m 3 of solution According to this value of cost, the electrocoagulation process is successfully applied to remove the hardness rate from groundwater with high efficiency.

CONCLUSIONS
Batch EC studies with various experimental parameters such as initial pH, applied voltage, operating time and electrodes configuration were performed to evaluate the influence of electrode connection modes on the removal of hardness from groundwater. The EC process at the optimum conditions was able to softening the groundwater to acceptable rangeof hardness rate byAl electrodes.Thehighestremovalefficienciesof hardness rate is 90.3%atpH 7.2, 40 volt and 50 min.Values of kinetic rate constants for hardness rate removal at various applied voltages were calculated. The kinetic results show that there is nosignificant difference between pseudo-first and second-order kinetic model except at 40 volt that the pseudo second-order kinetic model matched satisfactorily with the experimental observations. The lowest operating costs for electricity consumption and Al electrodes consumption were also obtained. So, the