| Abstract | Laboratory scale experiments on electrochemical precipitation (ECP) treatment of
Cr(VI) wastewaters were conducted to investigate its optimum conditions and applications.
The monopolar ECP consists of a direct current (DC) power supply, iron electrode plates
and an electrolyte solution. The distance between the anode and cathode electrodes was fixed
at 5 cm. Bipolar ECP unit was modified from the monopolar ECP unit in which four steel
plates were inserted between anode and cathode. The distance between these electrodes were
fixed at I cm. The voltage (E) supplied to the ECP unit was controlled by a variable voltage
transformer.
This study was devided into three phases, as follows:
I Determination of optimum conditions for Cr removal in the ECP unit
2 Application of the ECP process for treatment of electroplating and dye wastewaters
3 Comparison of the investment and operation costs between the ECP process and a
conventional chemical precipitation process for Cr removal.
Based on the experimental results obtained from Phase-I, the parameters affecting the
ECP efficiencies were found to be: initial wastewater pH (pHi), electrical current (I), the
operation resistance (R), temperature (T), operation time (OT) and initial Cr concentration
(C0 ). The optimum conditions of the ECP process for synthetic Cr wastewater, Cr
concentrations of 570-660 mg/l, were found to be: pHi - above 3, operation pH - 10-11, R -
12-15 ohms, minimum OT (Min OT) - 15 min and I - 1.0-3.0 amp or current density (i) -
33.3-100.0 amp/m2
• At these optimum conditions, the electric power and steel electrode
plate consumptions were 12 - 23 kWh/m3 and 1.0-2.8 kg-Fe/kg Cr removed, respectively.
X-ray fluorescence and X-ray diffractometric analysis of the precipitated sludge in the ECP
unit revealed the main compounds of the precipitated sludge to be maghemite (Fe20 3) and
chromite (FeCr20 4). The percent Fe20 3 and FeCr20 4 contents of the dried ECP sludge were
-ii-
58-68% and 20-25%, respectively. From the mass balance analysis, the amount of Cr
removed by precipitation and adsorption in the ECP unit were 85.1 % and 14.8%,
respectively. Based on the Freundlich isotherm constants, the Cr adsorption capacity of the
ECP sludge was about 50 times greater than that of a powder activated carbon.
Results of the Phase-2 experiments showed the ECP process to be a feasible
alternative method in treating a Cr wastewater. The bipolar ECP unit was found to be
superior to the monopolar ECP unit for Cr ions removal from an electroplating wastewater
due to its lower power consumption and operation time requirement. The optimum
conditions of the bipolar ECP unit treating the Cr wastewater, Cr concentrations of 570-2100
mg/l, to achieve the Cr concentrations in the mixed liquor filtrate of less than 0.5 mg/l were
found to be: pHi - above 3, I - 1.0-1.5 amp (or i - 6.7-10.0 amp/m2
), Min OT - 15 min, and
T - 25-45 °C. At these optimum conditions, the electric power and steel electrode plate
consumptions were 12-45 kWh/m3 and 0.7-1.8 kg-Fe/kg Cr removed, respectively. The ECP
unit was also effective in removing color and TCOD from a dye wastewater whose color
and TCOD were 15200 ADMI and 3500 mg/l, respectively. The optimum conditions of the
ECP unit to achieve 90-96% of color removal and 60% of TCOD removal were: I - 15 amp,
OT - longer than 45 min, and T - 25-45 °C.
An economic of analysis of an ECP unit treating a Cr wastewater was conducted in
the Phase-3 study. At the Cr wastewater flow rates of 10, 20, 30, 40 and 50 m3
/d,
expenditures (including investment and operation and maintenance costs) were found to be
32550, 56700, 80700, 104800 and 128900 Baht/month (the repayment periods and interest
rate were assumed to be 10 years and 13% for soft loan, respectively). Comparing between
the operation costs (including electrode, chemical, electricity and sludge treatment costs) of
the ECP, ion exchange and conventional chemical precipitation processes, the ECP and ion
exchange processes were found slightly lower than the conventional chemical precipitation
process. |