Low pH Range

Under this category, iron is removed as Fe(OH)3 and manganese is removed as MnO2, which involves oxidation of the metal ions to their higher oxidation states. The unit operations under the previous category are applicable under the present category, only that under the present scheme, oxidation of the metals is involved. The unit process involves one of Eqs. (13.23) through (13.25) for the case of manganese and similar equations for iron. The equation designed into the unit operations depends upon the unit process contemplated. The unit process part, however, will only be at the mixing tank where the reactions should occur.

As in the previous category, the unit operations of flocculation and settling may not be necessary. Again, in all cases, the mixing chamber for the reaction to take place and filtration is always necessary. The reaction does not immediately produce particles capable of being filtered, therefore, sufficient detention time should be provided in the reaction chamber to allow the particles to grow into filtrable sizes. This normally ranges from 20 to 30 minutes. In any case, a pilot plant may be necessary to determine the exact detention time.

When oxidizing iron and manganese using dissolved oxygen, the process is usually carried out under catalytic reaction on some contact surfaces. To accomplish the reaction, the water is generally made to trickle over small rock surfaces such as limestone, coke, or pyrolusite (MnO2). Pyrolusite possesses high catalytic power, springing from the iron and manganese oxides precipitated from the raw water that coat over and around the rocks. These coatings act as catalysts between the reaction of the ferrous and manganous ions in the water and the oxygen from the air as they come into intimate contact over the contact surfaces of the rocks.

The contact medium sizes vary from 3.5 to 5 cm. The accumulated flocs produced from the deposition of the hydroxides or oxides are periodically flushed out by rapid drainage. This is done by filling the tank containing the bed of rocks to capacity and quickly releasing the water.

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