Lesson - Treatment for Iron and Manganese

Let's begin this page by watching an American Water College video called Iron and Manganese Treatment where we review the concept of Iron and Manganese Treatment.

Application of Treatment for Iron and Manganese

As an Operator you will understand, if the water contains manganese up to 0.3 mg/L and less than 0.1 mg/L of iron, an inexpensive and reasonably effective control can be achieved by feeding the water with one of the three polyphosphates. Chlorine usually must be fed along with the polyphosphate to prevent the growth of iron bacteria. The effect of the polyphosphate is to delay the precipitation of oxidized manganese for a few days so that the scale that builds up on the pipe walls is reduce.

Polyphosphate Treatment

The chlorine dose for phosphate treatment should be sufficient to produce a free chlorine residual of 0.25 mg/L after a five-minute contact time. Any of the polyphosphates can be used; however, sodium metaphosphate is effective in lower concentrations than the other polyphosphates. The proper phosphate dose is determined by laboratory bench-scale tests.

Polyphosphate treatment to control iron and manganese is most effective when the polyphosphate is added upstream from the chlorine. The chlorine should never be fed ahead of the polyphosphate because the chlorine will oxidize the iron and manganese to insoluble precipitates.

Ion Exchange

Iron and manganese ion exchange units are similar to a down flow pressure filters. The water to be treated enters the unit through an inlet distributor located in the top. The water is forced down through the ion exchange resin into an underdrain structure. From the underdrain structure, the treated water flows out of the unit to the next treatment process.

The location of the ion exchange resins with respect to other water treatment processes will depend on the raw water quality and the design engineer. If the water contains no oxygen, iron and manganese may be removed by ion exchange using the same resins that are used for water softening. If the water being treated contains any dissolved oxygen the resin becomes fouled with iron rust or insoluble manganese dioxide. The resin can be cleaned; however, this process is expensive.

The primary advantage of ion exchange for iron and manganese removal is that the plant requires little attention. The disadvantages are the danger of fouling the ion exchange resin with oxide and high initial cost.

To operate an ion exchange unit, operate as close as possible to design flows. Monitor the treated water for iron and manganese daily. When iron and manganese start to appear in the treated water, the unit must be regenerated. Regenerate with a brine solution that is treated with 0.01 pounds of sodium bisulfite per gallon (1.2 g/L) of brine to remove oxygen present. After regeneration is complete, dispose of the brine in an approved manner.