|Figure 1: Typical potassium permanganate application points.|
Permanganate is a strong oxidant used primarily to control taste and odors, remove color, control biological growth in treatment plants, control zebra mussels in intake structures and pipelines, and remove iron and manganese. Permanganate can also be used for controlling the formation of trihalomethanes and other disinfection byproducts by oxidizing precursors and reducing the demand for other disinfectants. Permanganate has also shown to lower coagulant dose requirements and improve clarification.
At full-scale water treatment applications, potassium permanganate is most commonly added early in the treatment process at the raw water intake. Permanganate oxidation may be followed by conventional treatment or granular activated carbon (GAC), membrane filtration, and chlorine disinfection. Other possible points of application are at the rapid mix tank in conjunction with coagulants, or at clarifiers upstream of filters. In direct filtration plants, this oxidant is typically added at the raw water intake to increase the contact time upstream of the filter units. In all cases, potassium permanganate is added prior to filtration.
As previously indicated, permanganate is widely used for iron and manganese removal. With permanganate application, ferrous iron is oxidized to ferric iron (oxidation state +3), forming solid ferric hydroxide (Fe(OH)3); and manganese (Mn+2) is oxidized to manganese at a higher oxidation state (Mn+4), forming solid manganese dioxide (MnO2). These precipitates are removed by subsequent treatment steps, such as filtration or GAC, to avoid aesthetic problems related to staining or colored water at the customer's tap.
Due to the limited disinfection capabilities of permanganate it is not commonly used as a disinfectant. Further, it is not desirable to maintain a permanganate residual because of its tendency to give water a pink color.
Dose, contact time, pH and temperature affect the effectiveness of permanganate application. Typical doses are approximately 1 mg/L, but can vary depending on water quality and the application point (generally ranges between 0.1 and 5 mg/L). Higher potassium permanganate dose increases its oxidation capabilities. Dosage needs to be carefully controlled so that permanganate residual does not remain in the water. Sufficient contact time must be provided to exhaust the permanganate residual before reaching the distribution system. Powdered activated carbon, iron salts and hydrogen peroxide can be used for removing excess permanganate. Higher temperature slightly increases the disinfection efficiency of potassium permanganate. Higher pH tends to increase the oxidation capabilities of potassium permanganate, however, better disinfection is achieved at lower pH.