As(III) removal in rapid filters: Effect of pH, Fe(II)/Fe(III), filtration velocity and media size

J.C.J Gude,L.C Rietveld,D Van Halem

doi:10.1016/j.watres.2018.10.005

Abstract

In the top layer of aerated rapid sand filtration systems, uncharged As(III) is biologically converted to charged As(V). Subsequently, the main removal mechanism for As(V) is adsorption onto oxidised, flocculated Fe(III) (hydrous ferric hydroxides; HFO). The aim of this research was to understand the interactions between As and Fe in biologically active rapid filter columns and investigate the effect of different operational modes on Fe removal to subsequently promote As removal. For this purpose, different filter media column experiments were performed using natural, aerated groundwater containing 3.4 μg/l As(III). Results show that independent of the filter media size, complete (biological) conversion of As(III), manganese, ammonium and nitrite was achieved in approximately 70 days. After ripening, enhanced As removal was achieved with a top layer of coarse media or by dosing additional Fe(III). Addition of Fe(II) did not have the same effect on As removal, potentially due to heterogeneous Fe(II) oxidation in the upper layer of the filter, attaching rapidly to the filter grain surface and thereby preventing HFO flocs to penetrate deeper into the bed. Increasing the flow rate from 1 to 4 m/h did not improve As removal and lowering the pH from 8 to 7.4, resulted in an 55% increased removal of dissolved As. Altogether it is concluded that As removal in biologically active rapid sand filters can be improved by applying coarser filter media on top, in combination with dosing Fe(III) and/or pH correction.

Highlights

Arsenic (As) removal is increasingly important when treating groundwater for drinking water production
With 5%, 1%, 6% and 4% for fine sand, coarse sand, anthracite and pumice respectively. These results show that by increasing filtration velocity, As adsorption to hydrous ferric oxides (HFO) is slightly decreased
For this purpose the effect of filter media size, pH, Fe(II) and Fe(III) additions and filtration velocity were investigated in natural groundwater filter columns

Summary

Introduction

Arsenic (As) removal is increasingly important when treating groundwater for drinking water production. As is toxic and can cause adverse health effects even at concentrations below 10 mg/L (Roh et al., 2017; van Halem et al, 2009; WHO, 2011). Requiring a reduction in the range of 1e4 mg/L to below 1 mg/L (Vitens, 2014) in commonly applied aeration - rapid sand filtration systems. At these low concentrations, the challenge for effective As removal is rapid oxidation of As(III) to As(V) before iron (Fe) removal occurs, since As(V) is more effectively adsorbed onto flocculating hydrous ferric oxides (HFO) than As(III) (Bissen and Frimmel, 2003; Gude et al, 2016, 2017, 2018). Both reduced As(III) and Fe(II), react with O2 to As(V) and

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