Efficient removal of low-arsenic concentrations from drinking water by combined coagulation and adsorption processes

Combined separation and purification processes are gaining considerable attention in the water engineering community as they have the potential to integrate several treatment stages in a single, space-efficient and multifunctional process able to act as a multi-barrier against a wide spectrum of recalcitrant pollutants. In this paper, the efficiency of a combined physico-chemical process, previously validated as a tertiary treatment for municipal wastewater reclamation and successfully tested at pilot scale for the removal of total phenols, chemical oxygen demand (COD) and Escherichia coli, was tested for the precipitation of low-arsenic (V) concentration (<100 μg/L) from drinking water. The combined process, consisting of simultaneously dosing, in various proportions and according to a Latin square design-of-experiment scheme, aluminum polychloride (AP), zeolite (Z), powder activated carbon (PAC) and sodium hypochlorite (SH) into dechlorinated tap water spiked with arsenic (V), was assessed at laboratory scale in order to elucidate the mechanism of arsenic (V) removal as well as to identify the optimal mixing conditions using variable-speed jar-test experiments. Results indicated that the combined process was very effective in removing low arsenic (V) concentration from drinking water in the range of 25–100 μg/L. Moreover, it was found that, among the tested variables, high-velocity gradient conditions led to an improved removal efficiency which reached 89% under optimized process conditions. Although all treating agents played a statistically significant role in terms of process performance, arsenic (V) co-precipitation by AP was found to be the dominating removal mechanism contributing up to an 85% at 1400 rpm, with Z and PAC co-operating for the remaining 5% and mostly functioning as enhancing agents for ballasted settling. Notably, the process investigated in this study was also found to be robust against variation in initial arsenic concentration, showing similar arsenic (V) removal efficiency (85.9%) when the initial arsenic (V) concentration was further reduced from 100 to 25 μg/L. In conclusion, it was demonstrated that the combined treatment process was able to efficiently and simultaneously remove not only organic micropollutants such as phenols, COD and E. coli (as demonstrated in previous studies) but also inorganic contamination by arsenic (V) from a typical drinking water matrix via co-precipitation on aluminum polychloride, a treating agent that is worldwide accessible and typically used in water treatment applications.