Proof of principle study of continuous flow iron(0)-electrocoagulation for removal of anthropogenic contaminants from secondary effluent

Abstract

With the rapid population growth and industrialization, water bodies are being infiltrated by a rising number of contaminants like metals, pharmaceuticals, pesticides and detergents, which requires the need for novel treatment technologies. Electrocoagulation (EC) is a water treatment technology where the coagulant is dosed electrochemically from a pure metal electrode. The performance of Fe-EC in a full water column using a continuous flow setup has had little to no attention yet. The aim of this proof of principle study is to determine the feasibility and the practical potential of Fe-EC during a continuous flow implementation, and to give insight to the general performance of the system in terms of water quality improvement.

Experiments were conducted in a continuous flow Fe-EC unit with descending flow. Two pairs of electrodes were used, evenly distributed along the length of the unit, and at the bottom an air diffuser was used to provide aeration. Operational configuration of the unit was needed as no prior experiments had been performed with this unit and the exact behavior of the electrodes, flow and aeration within the continuous flow system were unknown. Furthermore, experiments were conducted to evaluate the removal efficiency of Fe-EC on nutrients, organic micropollutants (OMPs), microbes and (heavy) metals, operating at a pH of 8 and 7.

The operational parameters were set to deliver an Fe dosage of 50 mg/L, flow rate of 7.5 L/h, and a current density of 15 mA/cm2. It was found that the system was able to achieve high removal of phosphorus up to 99% during all experiment sets. Highest removal of E. coli and coliphages was found when operating at a pH of 7, reaching 1.56 and 0.65 log removal respectively. Removal of OMPs was found to vary greatly among the compounds and measurements. More stable results were found when using increased OMP concentrations (around 50 µg/L), reaching removals varying from 7% to 25% at a pH of 8, and slightly lower removals from 1% to 15% at a pH of 7. No correlation was found between the observed removal and the acid dissociation constant (pKa) or the η–octanol–water partition ratio (Kow) of measured OMPs. Lastly, various heavy metals show affinity for removal during Fe-EC, such as arsenic, copper, zinc, manganese, chromium and vanadium. Using increased influent concentrations of the heavy metals (around 80 µg/L) resulted in removals above 85% for arsenic, zinc, chromium and vanadium.

The system has shown to have high removal potential on secondary effluent, even when contaminant concentrations reach high levels. Nevertheless, uncertainties and were found for the practical implementation of Fe-EC, and design and operational parameters first have to be optimized in order to fully utilize the potential that Fe-EC has.