ENHANCED ARSENIC REMOVAL IN RAPID SAND FILTRATION – A PRACTICAL ASSESSMENT OF FE(II), FE(III) AND NAMNO4 DOSING FOR AS REMOVAL <1 ΜG/L AT WTP PRINSENBOSCH (Invited Presentation)

Arsenic is a toxic element. The WHO Standard is 10 μg/l, but since 2016, the Dutch drinking water companies adopted a goal of <1 μg/l. For Brabant Water, this resulted in the need to enhance As removal at seven water treatment plants (WPT’s). Earlier research had shown that dosing of NaMnO₄ to an aeration-filtration system efficiently reduces As concentrations, which lead to the implementation of NaMnO₄ at WTP’s Dorst and Prinsenbosch. However, also undesired operational side-effects occurred, like decreased filter run time, breakthrough of particles and poor thickening of backwash sludge.

Consequently, 3 alternatives (dosing of NaMnO₄, Fe(III) and Fe(II)) to enhance the As removal to < 1 μg/l at WTP Prinsenbosch were investigated. The assessment is based on the results of (full-scale and) pilot-plant research and includes not only the removal of As, but also the operational aspects.

It was found that Fe(III) was slightly more effective than Fe(II) and NaMnO₄ for the removal of As, but at doses of 1.3 mg/l NaMnO₄, 0.7 mg/l Fe(III) or 1.2 mg /l Fe(II) a water quality of 0.7 μg As/l could be obtained with all 3 chemicals. However, the impact on the operational aspects proved to be quite different. The dosing of Fe(II) lead to 50% longer filter run times and backwash production, no breakthrough and better thickening of the backwash water sludge (400%).

The differences in the impact on the filtration process and sludge properties between Fe(II) and Fe(III) and NaMnO₄ are most likely explained by a different densities and structures of the Fe(III)oxyhydroxides and Mn(IV) oxides formed in the filter

The conclusion of the research for WTP Prinsenbosch is that Fe(II) is the preferred chemical in view of the superior operational aspects. In Arpil 2019, Fe(II) was implemented at the full scale plant.