Introduction: Electrocoagulation (EC) is an emerging technology that combines the functions and advantages of conventional coagulation, flotation, and electrochemistry in oily water and wastewater treatment by providing a quantitative appreciation of the mechanism of interactions between themi. Based on the principles of electrolysis a simplified EC system may contain single or multiple metal electrodes in the form of anode-cathode pairs and may be connected in either a monopolar or a bipolar modeii. Metals released from the anode during electrolysis cause coagulation of pollutants aiding their separation from water. The optimum process conditions in an EC such as current densities, treatment times, pH etc., are established to achieve high contaminant removal efficiencies. However, one of the challenges limiting the efficiency of EC is the need for the regular cleanup of the fouled electrodes, which causes an intermittent shut-down and a re-start of the EC process.
The ECO II project funded by PTAC/NRCAN was aimed at overcoming this challenge. An ultrasonic system was tested and the process optimized by Easwara to clean the electrodes effectively during the EC treatment of frac flow-back water without their removal from the treatment device. The results of the ECO II project have been reported in Easwara’s Final draft report. This preliminary technology assessment is based on the Easwara’s Final draft report and specific clarifications provided by Easwara via email correspondence.
Objectives: The overall objective was to determine the efficacy of the Ultrasound (US) in reducing electrode fouling during the EC treatment process. The specific objectives were as follows: (i) to optimize an US system for the effective cleanup of the fouled electrode; and (ii) to test EC for the removal of target contaminants such as total suspended solids, total hardness, heavy metals and total petroleum hydrocarbons from frac flow-back water.
Description of Experiments: The experiments were conducted at the laboratories in the Department of Chemical and Petroleum Engineering, and the Centre for Environmental Engineering Research and Education, University of Calgary. The frac flow-back water used in the experiments was obtained from hydraulic fracturing operations at Progress Energy’s Pond, in North Western Alberta. The EC cell incorporating the US transducer was tested using frac flow-back water in a process that included two steps. The first step involved the experimental optimization of the US for the cleanup of the fouled electrodes and the second step tested the efficiency of electro-coagulation for the removal of target contaminants such as suspended solids, hardness, and heavy metals and emulsified oil. The testing included the treatment of the frac flow-back water at different US regimens where the optimum operational parameters for improving EC efficiency were determined.
Results: The results of the first step of the EC process incorporating the US transducer showed the visual removal of scale deposition from EC anode completely. An ultrasound exposure at 480 Watts and frequency of 25 kHz for a time duration between 1 and 3 minutes and at a distance of 5 mm between the US transducer and the EC electrode provided the most optimum result. The results of the second step of the EC process showed a removal of suspended solids, oil and hardness from frac flow-back water up to 90%. The application of carbon dioxide gas bubbled into water during the EC process increased the removal of total hardness in water from 50% to 90%. The removal of heavy metals iron and barium was not supported with comparative before and after treatment results.
Technology assessment: The assessment of the results associated with the specific objective (i) showed that optimization of the US process for the rapid clean-up of the anode during the EC treatment of the frac flow-back water tested was highly effective. The assessment of the results associated with specific objective (ii) showed that the EC treatment for removal of total suspended solids, total hardness, and total petroleum hydrocarbons from frac flow-back water was successful. The removal of heavy metals such as iron and barium could not be verified because it not supported by comparative results before and after the treatment.
An assessment of the overall results showed that when Easwara’s US process was combined with the EC treatment, a major drawback associated with electrode fouling could be overcome successfully without the extraction of electrodes as opposed to the current practice which requires the intermittent stopping of the EC treatment process for the extraction, clean-up and re-installation of the electrodes. Relevant technology details have been outlined in the Technology Assessment Summary.