Electrocoagulation applications for industrial wastewaters_a critical review-Kabdaşlı et al 2012_Env

Current efficiency of metal dissolution the energy

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current efficiency of metal dissolution, the energy consump- tion for the treatment of 1000 mg / L COD was estimated at 1.75 kWh / m 3 with iron electrodes and 1.53 kWh / m 3 with aluminium electrodes. These values could be reduced by 30% with a two-fold increase in the effluent conductiv- ity; the addition of NaCl at 1000 mg / L would allow such a reduction in the electric energy consumption. When electrocoagulation is considered as a treatment application, the costs of aluminium and iron sludge manage- ment should be taken into account. Hence, sludge volume and settlability are critical parameters that need to be min- imized to reduce sludge management costs. Zodi et al. [74] dealt with the treatment of textile wastewater (COD: 3260 mg / L; turbidity: 310 NTU; conductivity: 1.9 mS / cm; TSS 1700 mg / L; pH: 7) by electrocoagulation with empha- sis on the subsequent sludge settling process. The influence of electrode material, current density, pH, treatment period on sludge settling characteristics was investigated in detail. The textile wastewater under study had a high suspended solids and turbidity together with a fair COD. The initial pH and conductivity (NaCl concentration) of the reaction solutions were adjusted before electrocoagulation. The cur- rent density was supplied in the range of 5–20 mA / cm 2 . The electric current was kept constant for each run. The reaction solution was stirred at 200 rpm and continuously circulated in the flow circuit by means of a peristaltic pump at 50 mL / min. The total duration of the electroly- sis was 60 min unless stated otherwise. Iron or aluminium electrodes were used for electrocoagulation, with various conditions of initial pH ranging from 3–9, current den- sity and electrolysis time. The proposed treatment process consisted of a sequential electrocoagulation + settling sys- tem. Sludge settling velocity after electrocoagulation was measured depending on the operating conditions and used to compare the efficiency of various empirical models for estimation of sludge settling velocity. Finally, the sludge aptitude to settling was studied in terms of SVI to determine the most suitable operating conditions. For electrocoagula- tion with iron electrodes, more rapid settling was observed than with aluminium electrodes. The interface velocity was determined as 4.00, 3.75, 3.15 and 3.85 cm / min at 5, 10, 15 and 20 mA / cm 2 , respectively. For the effluent treated with aluminium electrodes at an initial pH of 9, the solid/liquid interface descended with an initial veloc- ity of 2.15 cm / min, whereas for the initial pH values of 3, 5, 6 and 7, the initial velocity was determined as 0.53, 0.55,
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12 I. Kabda¸slı et al. 0.74 and 2.05 cm / min, respectively. In addition, the sedi- ment height for electrocoagulation conducted at an initial pH of 9 was lower. The settling velocity after electroco- agulation with iron electrodes was found as 1.90, 2.80, 3.20, 3.75 and 3.13 cm / min for the pH values of 3, 5, 6, 7 and 9, respectively. With aluminium electrodes, SVI var- ied with the applied current density and the lowest value of 125 mL / g was obtained at 10 mA / cm 2 , whereas the highest
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