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JournalR. Ramesh of the Babu, University N.S. Bhadrinarayana, of Chemical Technology K.M.Meera and Sheriffa Metallurgy, Begum, 42, N.Anantharaman 2, 2007, 201-206

TREATMENT OF TANNERY WASTEWATER BY ELECTROCOAGULATION

R. Ramesh Babu, N.S. Bhadrinarayana, K.M.Meera Sheriffa Begum*, N.Anantharaman

Department of Chemical Engineering, Received 15 September 2006 National Institute of Technology, Accepted 30 April 2007 Tiruchirappalli— 620 015, India Email: [email protected], [email protected]

ABSTRACT

Treatment of tannery effluent is a challenging task as it is a biogenic matter of hides and a large variety of organic and inorganic chemicals. The objective of the present work is to study the application of electro-coagulation technique in the treatment of tannery effluent. The experiments were carried out in an electrochemical reactor using sacrificial elec- trodes, as and aluminum as in a continuous process. Operating parameters such as current density and time were studied for maximum (COD) reduction, Biochemical Oxygen Demand (BOD) reduction, reduction (TDS) and Chromium removal. Other relevant parameter such as power consumption for COD removal was also evaluated. The higher flow rate resulted in improved COD, BOD, TDS and chromium removal. The applied current density was also found to significantly influence the reduction of COD, BOD, TDS and chromium removal. Keywords: electrocoagulation, COD, BOD, Chromium, TDS, current density.

INTRODUCTION cipitation of soluble metal species as well as other in- organic species from aqueous streams, thereby permit- The tannery effluent is a mixture of biogenic ting their removal through sedimentation or . matter of hides and a large variety of organic and inor- , lime, and/or have been the chemical ganic chemicals. It usually contains high concentrations coagulants used. of chlorides, aliphatic sulfonates, sulfates, aromatic and These processes, however, tend to generate large aliphatic ethoxylates, sulfonated polyphenols, acrylic acid volumes of sludge with a high bound-water content condensates, fatty acids, dyes, , soluble carbo- which can slow down filtration operation and are also hydrates, sodium sulfide and inorganic like chromium. difficult to dewater. These treatment processes also tend The potential environmental impact of the chemicals to increase the total dissolved solids content of the ef- used in tannery operations is well known. fluent, making it unacceptable for reuse in industrial Conventional physico-chemical treatments of applications. tannery effluents consist of pretreatment, , Conventional chromium removal methods include sedimentation and sludge handling. , chemical precipitation, biological degrada- Chemical coagulation has been used for decades tion, exchange, etc. For the adsorption process, to destabilize colloidal suspensions and to effect pre- adsorbents investigated include bioadsorbent [1-3], red

201 Journal of the University of Chemical Technology and Metallurgy, 42, 2, 2007

mud, phosphate treated sawdust, Fe3+/Cr3+ and • Formation of coagulants by electrolytic oxida- activated carbon. The major disadvantages of the adsorp- tion of the ‘sacrificial ’; tion process are the regeneration of the spent adsorbent • Destabilization of the contaminants, particu- and subsequent treatment of the backwash water. late and breaking of ; Chemical precipitation usually employs the • Aggregation of the destabilized phases to form following four major steps. The Cr6+ is first reduced to flocs. less toxic and less soluble Cr3+ and the Cr3+ is then When a potential is applied from an external precipitated as Cr(OH) at high pH. In the third stage power source, the anode material undergoes oxidation, 3 the insoluble metal hydroxide is settled. Finally the while the cathode will be subjected to reduction or re- sludge is dewatered and disposed properly [3]. As the ductive deposition of elemental metals. The electro- particle size of sludge produced in the process is rela- chemical reactions with metal M as anode may be sum- tively small and difficult to settle completely, a filtra- marized as follows: tion process is necessary after sedimentation. Alterna- • At the anode: tive technique like foam flotation has been used in which M M n+ + ne” (s) → (aq) surfactant is added to increase the contact between the 2H O +2e- →4H+ + O + 4e” 2 (l) (aq) 2(g) flocs and the bubbles. When the concentration of Cr6+ is low, is usually adopted. The main short- • At the cathode: coming of this process is the high cost of the resins. M n+ + ne- M (aq) → (s) Of late, an electrochemical technique called 2H O + 2e- →H + 2OH- 2 (l) 2(g) electro-coagulation has attracted significant attention for chromium removal process due to its operational sim- If iron or aluminum are used, the gen- plicity. This technique uses an electrocoagulation cell erated Fe 3+ or Al 3+ ions will immediately undergo (aq) (aq) in which iron dissolve and produce Fe2+. This further spontaneous reactions to produce corresponding newly produced Fe2+ directly reduces Cr6+ to Cr3+ lead- and/or polyhydroxides. For example, Al3+ ions ing to the precipitation of Cr(OH) and Fe(OH) . This on may generate Al(H O) 3+, Al(H O) OH2+, 3 3 2 6 2 5 process does not require the addition of chemicals due Al(H O) (OH)2+ and the hydrolysis products may form 2 4 to the oxidation and reduction reactions which take many monomeric and polymeric species such as place. This influences the reduction of COD and BOD. Al(OH)2+, Al (OH) 4+, Al(OH)4", Al (OH) 3+, 2 2 6 15 Therefore, the maintenance and operation of the sys- Al (OH) 4+, A (OH) 4+, Al O (OH) 7+, 7 17 l8 20 13 4 24 tem is simple. Power consumption is also expected to Al (OH) 5+over a wide pH range. Similarly, ferric ions l3 34 be low. The present investigation is aimed at studying generated by electrochemical oxidation of iron electrode the effectiveness of electrocoagulation technique. may form monomeric ions, Fe(OH) and polymeric hy- 3 droxy complexes, namely: Fe(H O) 3+, Fe(H O) (OH)2+, 2 6 2 5 Fe(H O) (OH)2+, Fe (H O) (OH) 4+ and Fe (H O) (OH) 4+ 2 4 2 2 8 2 2 2 6 4 THEORY OF ELECTROCOAGULATION depending on the pH of the aqueous medium. These hy- droxides/polyhydroxides/polyhydroxymetallic compounds The electrocoagulation (EC) process depends on have strong affinity for dispersed particles as well as responses of water contaminants to strong electric fields counter ions to cause coagulation. The gases evolved at and electrically induced oxidation and reduction reactions. the electrodes may impinge on and cause flotation of the Electrocoagulation technique utilizes direct current to cause coagulated materials. sacrificial electrode ions to remove undesirable contami- Wastewater containing Cr6+ (CrO 2") ions can 4 nants either by chemical reaction and precipitation or by be removed by this electrocoagulation technique using causing colloidal materials to coalesce and then they are iron as the sacrificial anode. The ferrous ion (Fe2+) gen- removed by electrolytic flotation. In an electrocoagula- erated by electro oxidation of the iron anode can re- tion process the coagulating of ions are produced ‘in situ’ duce Cr6+ to Cr3+ under alkaline conditions and is it- and it involves three successive stages: self oxidized to ferric (Fe3+) ion according to:

202 R. Ramesh Babu, N.S. Bhadrinarayana, K.M.Meera Sheriffa Begum, N.Anantharaman

CrO2- +3Fe2+ +4 H O →3Fe3+ +Cr3+ + 8OH- Electrocoagulation unit 4 (aq) (aq) 2 (l) (aq) (aq) (aq) (or) An electrocoagulation cell measuring 20 cm di- CrO2- +3Fe2+ +4 H O +4OH- →3Fe (OH) + ameter and 15 cm height fabricated out of Perspex ma- 4 (aq) (aq) 2 (l) (aq) 3 ↓ terial was used in the present study. The effective vol- +Cr(OH) 3 ↓ ume of the cell is about 6.5 liters. Electrodes used are The Cr3+ ion is then precipitated as Cr(OH) (aq) 3(s) iron and aluminum plates of 100 mm in length, 100 by raising the pH of the solution. The Fe2+ ions can (aq) mm in width and 2 mm in thickness. The effective area also reduce Cr O 2 - under acidic conditions according 2 7 of electrode is 100 cm2 . Here iron is used as anode and to the following reaction: aluminum as cathode. The feed was recircu1ated by using Cr O 2-+6Fe2+ +14H+ → 2Cr3+ +6Fe3+ +7H O 2 7 (aq) (aq) (aq) (aq) 2 (l) a pump as shown schematically in Fig.1. The H produced as a result of the reaction 2 may remove dissolved organics or any suspended mate- rials by flotation. However, the Fe3+ ions may undergo hydration and depending on the pH of the solution. Fe(OH)2+, Fe(OH) + and Fe(OH) species may be present 2 3 under acidic conditions. The reactions involved are:

Fe3+ + H O →Fe(OH)2+ +2H+ (aq) 2 (l) (aq) (aq) Fe3+ +2 H O →Fe(OH)+ +2H+ (aq) 2 (l) 2 (aq) (aq) Fe3+ + 3 H O →Fe(OH) + 3 H+ (aq) 2 (l) 3 (aq)

Under alkaline conditions, Fe (OH) - and Fe 6 (OH) - ions may be present. It is therefore, quite appar- 4 ent that EC of both anionic and cationic species is pos- sible by using an iron plate/rod as a sacrificial electrode.

Fig. 1. Schematic diagram of the system. EXPERIMENTAL (1- D.C. power supply; 2- electrocoagulation tank; 3- feed tank; 4 – pump; 5 – rotameter; 6 - electrodes (anode: iron; cathode: Samples used in this study were collected from aluminum). the tank of a Common Effluent Treatment Plant (CETP). Experimentation The typical characteristics of these samples are presented The tannery effluent from the feed tank is pumped in Table 1. into the reactor tank. The anode and cathode were con- nected to the respective terminals of the DC rectifier. Table 1. Characteristics of tannery effluent. Electric power was supplied by a stabilized power source through the DC rectifier fitted with ammeter and volt-

S. No Parameters Value meter. The effluent from outlet of the system is again

1 COD (ppm) 3200 recirculated into reactor. The sludge is removed from the bottom opening of the reactor. The current (I) was 2 BOD (ppm) 1250 varied from 1 to 5 A and the electrical potential (E) was 3 Total Dissolved solids (mg/l) 6700 varied between 4 and 20 V. The efficiency of the electro-

4 Suspended solids (mg/l) 1300 chemical reactor was studied at various conditions such as different current densities and volumetric flow rates. 5 Total Solids (mg/l) 8000 The effluent was treated at three different cur- 6 Chromium (ppm) 40 2 rent densities, viz., 15, 20 and 25 mA/cm . The flow 7 pH 6.8 rates of liquid into the reactor were 2 lpm, 4 lpm, 6

8 colour dark lpm and 8 lpm. The flow rate of the effluent was mea- brown sured by using calibrated rotameter. The duration of

203 Journal of the University of Chemical Technology and Metallurgy, 42, 2, 2007 electrolysis was 4 hours. Samples were collected every 1 70 hour. The pH was maintained between 6 and 7. The pH 60 of solution was adjusted by using 1:5 (volume) sulfuric 50 acid solution. The buffer solution used in adjusting the 2 LPM pH in the coagulation unit was prepared by mixing 0.2 M 40 4 LPM NaOH and 0.1 M Na CO with distilled water. 30 6 LPM 2 3 8 LPM 20 BOD ReductionBOD (%) Analysis 10 Standard methods prescribed by American Pub- 0 lic Health Association [4] were adopted for quantitative 10 15 20 25 30 estimation of Chemical Oxygen Demand (COD), Bio- Current Density (mA/cm2) chemical Oxygen Demand (BOD), chromium, total sol- ids, suspended solids and total dissolved solids (TDS). Fig. 3. Effect of current density on BOD reduction.

RESULTS AND DISCUSSION 80 70 In the present investigation the operating pa- 60 2 LPM 50 rameters such as flow rate, current density and elec- 4 LPM 40 trolysis time were varied to explore their effect on COD, 6 LPM 30 BOD, TDS and chromium removal. The results showed 8 LPM 20 that the reduction of COD, BOD, TDS and the removal of TDS (%) Removal 10 of chromium were higher at higher charge input and 0 electrolysis time. 10 15 20 25 30

The effect of current density on the reduction Cur rent De nsi ty (mA/ cm2) of COD, BOD and TDS and the removal of chromium were studied by conducting experiments at different Fig. 4. Effect of current density on Total dissolved solids removal current densities, viz., 15, 20, 25 mA/cm2. The current density is defined as the ratio of current input to the electrolytic cell to the surface area of the electrode. The current supplied to the electrochemical reactor is usu- 80 70 ally expressed in terms of current density. The pH was 60 2 LPM 50 maintained between 6 and 7. Figs 2, 3, 4, 5 and 6 show 4 LPM 40 6 LPM 30 20 8 LPM 10 80 0 Removal of Chromium (%) 70 10 15 20 25 30

2 60 Current Density (mA/cm ) 2 LPM 50 Fig. 5. Effect of current density on Chromium removal (raw 4 LPM 40 effluent). 6 LPM . 30 8 LPM 20 the effect of current density on the removal of COD, COD Redu cti on (%) 10 BOD, TDS and chromium. The results showed that the

0 current density influences the COD reduction and in- 10 15 20 25 30 creased current density increases the reduction of COD

Cu rrent De nsi ty (mA/ cm 2) and BOD. This is due to the oxidation and reduction reactions which take place in the reactor. Fig. 2. Effect of current density on COD reduction.

204 R. Ramesh Babu, N.S. Bhadrinarayana, K.M.Meera Sheriffa Begum, N.Anantharaman

tion of oxidants and coagulants. During the process at 80 higher flow rates, the dissolution of iron metal into ions 70 increases. This in turn influences the increase of coagu- 60 lants and hence an increase in the oxidation process. 50 2 LPM 4 LPM The removal of chromium in synthetic effluent is more 40 6 LPM than that in raw effluent for the same operating condi- 30 8 LPM tions like current density, flow rate and electrolysis time. 20 This is due to the non interference of other compounds 10 Re mo va l of Ch romi um(%) like organic salts which are present in the raw effluent. 0 10 15 20 25 30 Hence, the amount of coagulants is more while pro-

2 Curre nt De nsity (mA/cm ) cessing synthetic effluent. The energy consumption in the process can be Fig. 6. Effect of current density on Chromium removal (synthetic written as effluent). E = (U * I ) / (1000 * Q) t t where, U is the total electrolysis voltage (V), t I the total electrolysis current (A), 90 t 80 Q volumetric flow rate (m3/h). 70 2 LPM ) 60 3 Fig. 7 shows that the power consumption for 50 4 LPM 40 6 LPM COD removal increases with increase in the current

(KWh/m 30 8 LPM 20 density. As the power consumption increases oxidation Power Consumption Consumption Power 10 rate increases resulting in more coagulant formation and 0 10 15 20 25 30 hence better percentage of COD reduction.

Current Density (mA/cm2) The results also showed that the flow rate of ef- fluent into the reactor also significantly influenced the Fig. 7. Effect of current density on power Consumption. power consumption. The higher power consumption leads to higher operating cost, and lower current den- The gas evolved at the electrodes namely oxygen sity may increase the electrolysis time. Therefore, the and hydrogen also influence the reduction of COD and operation of the electrochemical reactor has to be opti- BOD. The hydrogen gas liberated at cathode also helps mized for current density and flow rate to reduce the to float the contaminants. This influences the removal operating cost of the process. From the experimental of TDS. The removal of chromium is influenced by the parameters studied, the optimum current density and formation of oxy-hydroxy species in aqueous solutions. flow rate were found to be 20 mA/cm2 and 6 lpm. The hydroxy species thus formed have a pronounced tendency to undergo polymerization due to interaction CONCLUSIONS between hydroxyl groups of adjacent molecules. The polymerized complex molecules act as coagulants which From the results obtained it can be concluded that help in the removal of chromium. the electrocoagulation can be effectively used for the treat- To study the effect of the flow rate on the reduc- ment of tannery wastewater. The electrocoagulation of tion of COD, BOD and TDS and the removal of chro- tannery wastewater by generation of polymerized com- mium experiments were conducted at four different flow plex molecules and liberation of oxygen and hydrogen rates viz., 2 lpm, 4 lpm, 6 lpm, and 8 lpm. Figures 2, 3, gas at the electrodes significantly reduce the COD, BOD, 4, 5 and 6 show that the flow rate of effluent into the TDS and chromium. The optimum operating conditions reactor remarkably affected the reduction of COD, BOD, arrived at by considering maximum COD reduction and TDS and chromium. Higher flow rates showed signifi- less power consumption were 6 lpm and 20 mA/cm2 for cant reduction and this is due to the increased produc- the flow rate and current density, respectively. The in-

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