Field evaluation of the sustainability and efficiency of an arsenic removal plant based on the zero-valent iron technology

BERARDOZZI, E.; ARTURI, T.S.; MOSQUEIRA S.; DORADO R.; GARCÍA EINSCHLAG, F. S.

CABA

Congreso; WCCE11, XI Congreso internacional de Ingeniería Química; 2023

Arsenic (As) is a contaminant widely distributed in groundwater in many regions of Argentina where the concentration levels exceed the values recommended by the WHO (MCL = 10 μg.l−1). Many different technologies have been developed to remove arsenic However,Zero-valent iron (ZVI) technique has gained attention during the last decades for the removal of arsenic due to its applicability under different geochemical conditions, operational simplicity, low-cost maintenance, and minimal environmental impact. The removal mechanism of As by ZVI involves complex processes of adsorption and/or co-precipitation, which are closely related to the corrosion products generated by iron oxidation. The purpose of this work was to evaluate both the performance and treatment costs in field applications of a full-scale ZVI-based water treatment plant.The treatment system designed by our research group involves three stages. Water initially flows through four tubular reactors filled with ZVI. Commercial iron wool was the selected iron source due to its low cost and easy accessibility. The pH of the inlet water is controlled automatically by dosing hydrochloric acid. Along the first stage, ZVI reacts with dissolved oxygen (DO) naturally present in the influent water producing soluble Fe(II) species. Part of Fe(II) also reacts with DO to produce Fe(III) species that form corrosion products able to co-precipitate or adsorb a fraction of the As present. During the second stage, an oxidation and flocculation tank, with chlorine and flocculant manual dosage, is used to oxidize the remaining soluble Fe(II) and to induce additional precipitation and co-precipitation processes, which improve the As and Fe removal efficiencies. Finally, a sedimentation tank and two pressure sand filters remove the iron-precipitated particles.The latter removal plant was installed in Pipinas (Buenos Aires) to provide drinking water to 1,000 inhabitants. The arsenic content in the raw groundwater is about 140 μg.l−1. The system was operated and monitored for 10 months by local operators and samples were taken daily at different stages of the process The consumption of reagents and the percentage of water discarded during the backwashing of the filter were evaluated. The iron and arsenic contents in the samples were determined by colorimetric techniques.Results show that removal efficiencies in this period attained 86-99%. Considering the days of stoppages due to technical problems the average treated water flow rate was 11.93 m3/day, with the maximum value being 14.64 m3/day. The average fraction of treated water used for filter backwashing was 5.1%. Iron concentrations in the treated water slightly higher than the maximum recommended (0.3 ppm) were occasionally detected. This problem was solved by incorporating an automatic ORP controller in the Fe(II) oxidation stage and reducing the steel wool replacement frequency. Reagents consumption showed values of 92 ml/m3 (acid), 64 ml/m3 (chlorine), 4.3 ml/m3 (flocculant), and 15 g/m3 (iron wool). Power consumption was 2 .2 kW/m3 obtaining a total cost expressed in local currency of 40 $/m3 which is adequate to be faced by a small community.