Aeration towers

Where soluble iron is present in condensate, it often is associated with copper. When the problem is considered serious, it usually is removed by a condensate polisher. Soluble iron in MU often is associated with manganese and usually (but not totally) is removed either by ion-exchange resins (often inadvertently) or in an aeration tower, where the process employs a combination of air or chlorine oxidation, followed by precipitation and filtration. 

Where present in boiler MU water, both iron and manganese may present fouling and deposition problems in the pre-boiler section. These problems may extend to the boiler section, and therefore these metals must be removed at source. Typically, this is achieved by oxidation followed by filtering off the flocculated iron. (Process examples are aeration towers, contact with chlorine, pressure filters with BIRM media, manganese greensand filters, etc.)... 

Aerators are often used for iron and manganese removal from deep well waters. Dissolved sulfur gases and some carbon dioxide are also removed, resulting in a slight elevation in pH levels. There are various designs but pressure aerators, aeration towers, and spray ponds, are common. 

Aeration towers are often simply constructed wooden towers with or without the benefit of a forced-air draft, over which the well water is distributed. Typically, the tower holds 8 to 10 trays, each with a 3- to 4-inch (7.6-10.2 cm) bed depth of volcanic lava or coke media to provide the total 30 to 36" (76-91 cm) depth required. The lava or coke acts as a catalyst for the further precipitation of iron and manganese salts. These salts can be readily oxidized provided there is sufficient alkalinity present. If in doubt, ensure a minimum of 120 to 150 ppm total alkalinity (T Aik or M Aik) as CaC03. 

In practice, aeration towers using coke or volcanic lava tend not to be as efficient as spray ponds in facilitating the precipitation of ferrous hydroxide consequently, there is usually a requirement for a cationic flocculant to aid the precipitation of the insoluble materials into a larger floe or denser sludge that can be removed by clarification or sand filtration. 

Kavanaugh, M.C., Trussell, R.R. (1980) Design of aeration towers to strip volatile contaminants from drinking water. J. Am. Water Works Assoc. 72, 684-692. 

The produced water is a combination of water from a free-water knockout and from heater treaters. An aeration tower is used to oxidize the iron and hydrogen sulfide, and chlorine is also added to the tower. The chemical treatment utilizes 45 to 60 ppm alum, 75 ppm chlorine, and a corrosion inhibitor. The finished water quality is as follows ... 

A common form of pretreatment is to remove the risk of fouling and staining by iron and manganese using an aeration tower with a coke or media catalyst, onto which the aerated water falls, promoting the oxidation and precipitation of insoluble ferric hydroxide and manganese dioxide. 

The principal reactions for the oxidation of dissolved iron and manganese in aeration towers are as follows ... 

J. C. Little and R. E. Selleck, Evaluating the performance of two plastic packings in a cross-flow aeration tower. J. Am. Water Works Assc. 83(6), 88 (1991). 

---