Option study to remove Mn(2+) by KMnO4 at a water treatment plant

Abstract

Thermal stratification in reservoirs resuspends manganese (Mn) from the bottom layer to the top layer. When the water from this top layer is taken for the treatment of potable water supply, removal of Mn becomes essential. The Foster Water Treatment Plant (FWTP) situated in South Gippsland, Australia applies KMnO4 to remove Mn. It also uses powdered activated carbon (PAC) remove odour causing compounds such as geosmin and 2-Methyl-Isoborneol (MIB). This study aims to investigate an optimal KMnO4 dosing strategy including (i) optimizing the dosage of KMnO4, (ii) dosing order of PAC and soda ash/KMnO4 and (iii) the retention time required for the oxidation of Mn(2+) by KMnO4 and for the adsorption by PAC. The results showed that (a) the removal rate of [Mn(2+)+Mn(7+)] in the filtered water increased with higher dosage of KMnO4 in both dosing orders; (b) the removal of Mn(2+) neither at the entry point (after adding PAC and KMnO4) nor in the filtered water was impacted significantly by the increase in the dosage of KMnO4 when the dosage was 4 times higher than the concentration of Mn(2+) present in the raw water; (c) for a given KMnO4 dosage, there were no significant changes in the removal of [Mn(2+)+Mn(7+)] and Mn(2+) at the entry point and in the filtered water when the retention times ranged from 5 to 15 min in both order of dosing; (d) for a given KMnO4 dosage, Order 1 had higher removal of total Mn compared to that of Order 2 (Order 1 is the addition of PAC before adding soda ash to increase the alkalinity and then adding KMnO4 and Order 2 is the addition of soda ash and KMnO4 before adding PAC); (e) higher removal of [Mn(2+)+Mn(7+)] was obtained in Order 2 compared to that of Order 1 for a given dosage of KMnO4; (f) for the Mn(2+), the removal did not change with the order of dosing of KMnO4 and PAC. It could be concluded that dosing KMnO4 with four times the concentration of Mn(2+) present in raw water and providing 10 min retention time for the oxidation can be a suitable dosing strategy in FWTP and dosing order has no impact on achieving the Mn removal required.