Horizontally embedded Fe(0) electrocoagulation to enhance As(III) removal in biologically active rapid sand filters for drinking water treatment

Abstract

The presence of high levels of arsenic in groundwater, an otherwise preferred drinking water source, urges the need for innovative and highly efficient (in terms of chemical use and energy consumption) technologies to remove arsenic during drinking water treatment. This research focussed on embedding iron electrocoagulation (Fe-EC) within a biologically active rapid sand filter (RSF) by submerging horizontally perforated (Fe(0)) electrodes. These electrodes were designed in such a way that they could easily be submerged in the filter bed during backwashing. Based on As(V)/As(III) adsorption characteristics and iron removal mechanisms in a RSF it was hypothesized that the arsenic removal could be enhanced by placing the electrodes in the biologically active filter bed instead of in the supernatant. Iron to arsenic ratios (Fe:As) were reduced from 18.3 to 15.0 when the electrodes were placed in the filter bed instead of in the supernatant (for a charge dosage of 6.4 C/L and 150 µg/L As(III) in the influent), showing an improved arsenic removal efficiency. The arsenic oxidizing bacteria (AsOB) on top of the electrodes oxidized As(III) to As(V) (over 85 to 97% oxidation), which was subsequently removed by the iron that got released (by the embedded Fe-EC) and formed precipitates in the filter bed. The major drawback of embedding electrodes in the filter bed is the increase in energy consumption due to the increase in operating voltage. As(V) flowthrough experiments showed that the arsenic removal can be improved by lowering the pH from 8.0 to 7.0 and by increasing the charge dosage from 6.4 to 9.4 C/L. It was shown that besides arsenic removal efficiency, parameters as iron dosage, energy consumption, sludge production, and clogging have a significant relevance. Iron-As(V) experiments showed the flaws of the integrated system related to a partly homogeneous iron release and improper mixing, resulting in a decrease in the arsenic removal efficiency. Further, ways to potentially improve the design of the system are discussed and evaluated, pointing the way forward for future research.