Out With Fe and Mn

A variety of treatments can remove iron and manganese. The choice depends on contaminant concentrations, the volume treated and other factors.
Out With Fe and Mn
This small community water treatment system designed for iron removal uses aeration to convert the iron to iron oxide, and then filters out the iron oxide. It also uses ion exchange to remove excess iron.

A small community asked Florida Gateway College for advice on resolving red water complaints from their customers.

High levels of iron and manganese appeared to be the problem. Iron is the more common of these contaminants, but they often occur together. High levels can discolor water, stain plumbing fixtures, and impart an unpleasant metallic taste. Iron deposits can also build up in pressure tanks, storage tanks, water heaters and pipelines, and can also reduce capacity, increase maintenance, and reduce pressure in the distribution system.

These metals do not pose health risks, but secondary maximum contaminant levels have been set for iron (0.3 mg/L) and manganese (0.05 mg/L). This community’s iron and manganese concentration was a combined 10 mg/L.

Treatment techniques’ ability to remove iron and manganese depends on hardness, pH, and the presence of iron bacteria, sulfur, silica, organic material and tannin. It also depends on the form and concentration of the iron and manganese.

Iron and manganese in water occurs in three forms. When water comes from the tap as clear, iron and manganese are in dissolved form — ferrous iron or manganous manganese. When the water comes out rust-colored, iron and manganese are in precipitate form — ferric iron or manganic manganese. When the water has a clear yellow tint, the iron and manganese have combined with organic material and are colloidal — the most difficult iron and manganese to remove.

Treatment options for iron and manganese include sequestration, oxidizing filters, ion exchange, and oxidation with settling or filtration. The most cost-effective and appropriate option depends on the form and concentration of the metals, the quantity of water to be treated, and the water chemistry.

Sequestration

Sequestration does not remove iron and manganese — it binds them in a soluble form and keeps them from oxidizing on contact with air or chlorine. This method only works with ferrous iron and manganous manganese, and the combined concentration must be less than 1 to 3 mg/L. Sequestering prevents staining of plumbing fixtures and discoloration of the water, but a slight metallic taste will remain.

Polyphosphates, followed by chlorination, can be an inexpensive method for sequestering iron and manganese. They are effective in a pH range of 5.0 to 8.0. Sodium silicate and chlorine are effective in sequestering iron, but less effective for manganese.

Ion exchange

Ion exchange units can remove small amounts of soluble iron and manganese. Potassium chloride can be used to regenerate the resin beads instead of sodium chloride if sodium is a concern. Water softening is usually considered the best treatment if water hardness is also a problem and the combined iron and manganese concentration is less than 2 to 5 mg/L. At higher concentrations, precipitated iron residue will build up on the softening resin, decreasing the unit’s efficiency. Ion exchange will not work if the iron has oxidized, if iron bacteria are present, or if the iron has combined with humic acid or organic material.

Oxidizing filters

Oxidizing filters can remove up to 15-25 mg/L of combined iron and manganese. Greensand, natural or synthetic zeolites, or anthracite sand are used in a pressure filter or in mixed media, and potassium permanganate is used to coat the greensand or anthracite sand with manganese oxide. The coating oxidizes and removes iron and manganese without an additional oxidation or precipitation step. The coating can be maintained by a continuous potassium permanganate feed or by backwashing with a potassium permanganate solution.

Synthetic and natural zeolite filter media have a catalytic effect that does not require chemical backwashing. The filter media can use a venturi air injection as an oxidant. The oxidation process is completed in the zeolite media filter, and the precipitate is filtered out. The filter is periodically backwashed.

Oxidizing filters can be used with ferrous or ferric iron, and manganous or manganic manganese. The minimum pH is 7.0, and a pH of 8.0 is needed when the manganese concentration is high. The rate of backwash is higher than for ion exchange and, unlike sodium salts, potassium permanganate does not pose an environmental issue. Hydrogen sulfides and tannins will foul oxidizing filter media, reducing its efficiency. Oxidizing filters work best with water low in organic material and phosphate. Chlorine adversely affects the catalytic property of the filter media and it should be added after filtration.

Oxidation

Oxidation is required before precipitation, filtration or settling. Soluble ferrous iron is oxidized to a ferric iron, which readily forms an insoluble iron hydroxide complex. Manganous is oxidized to manganic, which forms insoluble manganese dioxide. The insoluble metals can be removed by filtration or precipitated out in a settling tank.

Aeration can be an effective, low-cost method of oxidizing iron. Water is passed through a series of porous trays by gravity to provide contact between air and water. Air stripping towers can also provide aeration. The water trickles down through a tower packed with open plastic media while air is forced up through the media. Aeration towers also remove sulfides, radon and volatile organic chemicals. Aeration is not effective if humic materials or iron bacteria are present. The rate of reaction for manganese is very slow at pH values less than 9.5.

Chlorine can be used as an oxidant. The chlorine feed rate and contact time can be determined through jar testing. However, trihalomethanes can be a problem if organic material is present in the water when using chlorine.

Potassium permanganate is a very efficient oxidant for iron and manganese. It is more expensive than chlorine, but the capital equipment costs are less. The dose of potassium permanganate must be carefully controlled. Too little will not oxidize all of the iron and manganese, and too much will leave a pink hue in the water.

 

Settling or filtration

A detention tank can be used to allow contact time for the oxidation process to occur. The size of the detention tank depends on flow rate, tank configuration, oxidation method, and other contaminants in the water. A bench test or jar test can be useful in selecting an oxidation method and detention time.

Aeration oxidation followed by a settling tank is an inexpensive treatment option for small water systems that do not require chemical additions. It is not effective in all cases, and bench or jar tests must be done to determine the cost and feasibility of this method. Inclined plate settlers, baffles or tube settlers can be used in the settling tank to provide the maximum detention time, and the settling tanks must be designed to allow the precipitate to be removed periodically.

Filtration

Filtration is the most common method of removing iron and manganese after oxidation. Slow sand filters, pressure filters, bag or cartridge filters, or conventional filters can remove the oxidized contaminants. Conventional and slow sand filters are the most expensive, and they are not normally used for removing iron and manganese unless bacteria, colloidal particles or other filterable contaminants are present. Bag or cartridge filters have a very low capital cost but a higher maintenance cost. Automatically backwashing pressure filters have a higher capital cost and a lower maintenance cost.

Community leaders decided to use air stripping towers because the raw water also contained significant levels of volatile organic compounds, and trihalomethanes have been a problem at the facility in the past.

ABOUT THE AUTHOR

John Rowe, Ph.D., is a professor of Water Resources at Florida Gateway College in Lake City, Fla.
 



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