MnO4 – Fe(III) dosing to rapid sand filters for removing arsenic from drinking water to <1 µg/l

A case study in Katwijk (Dunea)

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Drinking water company Dunea N.V. produces drinking water by artificially infiltrating water into the dunes. During this dune passage arsenic is mobilized causing arsenic levels up to 4 µg/l in the influent of DWTP Katwijk. In this thesis the effectiveness of the MnO4 – Fe(III) dosing was evaluated on RSF influent of Dunea’s treatment plant Katwijk. The goal of the experiments was to reduce the residual arsenic concentration to <1µg/l while minimizing negative effects on further treatment steps. Experiments were performed in a pilot filter filled with filter material originating from the RSFs in Katwijk to ensure conditions were similar to RSFs in Katwijk. The performance of MnO4 – Fe(III) dosing was evaluated at different filtration velocities (4, 6, 10 m/h) and at different top layer materials (anthracite (1.6-2.5mm) and pumice (2.5-3.5mm)).
In this research multiple dosages were evaluated and residual As <1µg/l was consistently achieved by dosing 0.5 mg NaMnO4 /l and 1.0 mg Fe(III) /l. It was found that the effectiveness of the MnO4 – Fe(III) dosing was not influenced by the filtration velocity applied in the RSFs. For all the filtration velocities checked the removal efficiency ranged from 60-70%, resulting in As levels <1µg/l. The pumice used appeared to show slightly higher removal rates than the anthracite that was used. However, for full scale application purposes in Katwijk the used anthracite was better suited. The pumice filter clogged faster leading to a pressure loss of 95 cmWc after 6 days while the anthracite filter reached a pressure loss of 73 cmWc. Due to the coarse top layer less particles were removed in the top layer and caused the sand underneath to be loaded more and to clog more quickly.
The AOCF process did not negatively influence the filter performance. Mn-, Fe- removal and nitrification all still performed to standards and the turbidity of the RSF effluent decreased from 0.3 FTU to 0.03 FTU. Based on preliminary experiments the sedimentation properties of the backwash water changed and the sedimentation velocity increased.
The MnO4-Fe(III) dosage can effectively reduce the residual arsenic concentration to <1µg/l and is ready to be applied at Katwijk. However, the negative effects on the runtime of the RSFs can’t be overlooked. The runtime of the current filters is expected to decrease by about 50% causing a higher stress on the backwash water treatment.