Introduction to Iron and Manganese Removal
In this section we will answer the following question:
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Iron and manganese occur naturally in water, especially groundwater. Water percolating through soil and rock can dissolve minterals containing iron and manganese and hold them in solution. Occasionally, iron pipes also may be a source of iron in water. Neither of the elements causes adverse health effects; they are in fact, essential to the human diet. However, water containing excessive amounts of iron and manganese can stain clothes, discolor plumbing fixtures, and sometimes add a "rusty" taste and look to the water.
Indications of Iron and Manganese
In deep wells, where oxygen content is low, the iron/manganese-bearing water is clear and colorless. In such water, the iron and manganese are in dissolved form. Water from the tap may be clear, but when exposed to air, iron and manganese are oxidized (combine with oxygen to become an oxide) and change from colorless, dissolved forms to colored, solid forms (often in the form of very small paticles).
Oxidation of dissolved iron particles in water changes the iron to white, then yellow and finally to red-brown solid particles (precipitates) that settle out of the water. Iron that does not form particles large enough to settle out and that remains suspended (colloidal iron) leaves the water with a red tint. Manganese usually is dissolved in water, although some shallow wells contain colloidal manganese (black tint). These sediments are responsible for the staining properties of water containing high concentrations of iron and manganese. These sediments or precipitates may be severe enough to plug water pipes.
Iron and manganese can affect the flavor and color of food and water. They may react with tannins in coffee, tea and some alcoholic beverages to produce a black sludge, which affects both taste and appearance. Manganese can be objectionable in water even when present in smaller concentrations than iron.
Iron will cause reddish-brown staining of laundry, porcelain, dishes, utensils and even glassware. Manganese acts in a similar way but causes a brownish-black stain. Soaps and detergents do not remove these stains, and use of chlorine bleach and alkaline builders (such as sodium and carbonate) may intensify the stains.
Iron and manganese deposits will build up in pipelines, pressure tanks, water heaters and water softeners. This reduces the available quantity and pressure of the water supply. Iron and manganese accumulations become an economic problem when water supply or water softening equipment must be replaced. There also are associated increases in energy costs from pumping water through constricted pipes or heating water with electric heating rods coated with iron or manganese mineral deposits.
Iron and manganese in water also promote the growth of iron bacteria, a group of organisms that obtains its energy for growth from the chemical reaction that occurs when iron and manganese mix with dissolved oxygen. These bacteria form thick slime growths on the walls of the piping system and on well screens. Such shines are rust-colored from the iron and black-colored from the manganese. Variations in flow can cause these slime growths to come loose, resulting in dirty water in the system.
The growth of iron bacteria can be controlled by chlorination. However, when water containing iron is chlorinated, the iron is converted from the ferrous state to the ferric state - in other words, rust - and manganese is converted into black manganese dioxide. These materials form a coating on the inside of the water main and, when they break loose, a customer will sometimes complain of "dirty" water.
The Safe Drinking Water Act secondary standards (aesthetic, not heatlth related) for iron in drinking water is 0.3 parts per million (ppm); for manganese it is 0.05 ppm. If the water contains more than 0.02 ppm of manganese, the operator should implement an effective hydrant-flushing program in order to avoid customer complaints.
Chemistry of Iron and Manganese in Water Systems
Iron (Fe) and manganese (Mn) can be present in water in one of three basic forms:
The predominance of one form over another is dependent on the pH, Eh (redox potential), and temperature of the water. Knowledge of the forms or states of iron and manganese can help finetune a given treatment practice for these metals.
Answer the questions in the Lesson 1 quiz . When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and submit your grade directly into the database for grading purposes.