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The majority of water found on the earth, contained some form of dissolved, colloidal or suspended typed deleterious substances. Some of them are harmless others are nuisances for consumption or at the point of end use. Iron and manganese in mineral form are found in varying state in surface water, but proliferate in groundwater.
The sources of iron/manganese contamination are from:
Anoxic aquifers containing iron (Fe2+) and/or manganese (Mn2+) in soluble form
Oxidized iron and/or manganese forming hydroxide flocs from raw water
Natural organic matter containing complex iron substances
Hydroxide flocs forming from the coagulation process
The corrosion by-products from the materials of the distribution conduit.
These materials are removed from the water to a permissible level, by several methods, in order to achieve a certain standard for its end use.
Sources and Problems
Iron and manganese are common metallic elements found in nature. A natural process occurs underground on the minerals in soil, rocks, and sediments that contain iron and manganese. Water percolating through soil and rock dissolves iron and manganese, and these dissolved minerals are deposited in reservoirs, ground water aquifers, rivers, lakes and the oceans. Distribution iron pipes are also a source of iron in water.
Surface water does not usually contain high concentrations of iron or manganese, as both minerals settle out as sediments in the oxygen-rich water. The concentration of iron (Fe2+ ions) and manganese (Mn2+ ions) is higher in the anoxic lower levels of deep reservoirs.
The oxygen content in aquifers is low and in an anaerobic and low pH conditions. It dissolves the mineral oxides to a soluble form making the iron and manganese bearing water clear and colourless. As the iron and manganese become exposed to the oxygen in the atmosphere, their colourless, dissolved forms oxidized and changes to coloured solid forms of very small particles. Iron changes to white, yellow then to a reddish brown precipitate. Dissolved manganese in water when oxidized precipitates into a black tinted colloidal typed particle.
This is shown by the following equations.
Iron (Fe) and Manganese (Mn) are present in reduced forms, in Ground Water.
Fe2+ + O2 = Fe(OH)3 + 8H+ (Ferric hydroxide insoluble ââ‚¬" reddish colour)
Mn2+ + O2 = 2 MnO2 + 4H+ (Manganese dioxide-insoluble ââ‚¬"black typed tint)
These sediments deposit are accountable for the staining properties of water, which contains high concentration of iron and manganese. The sediments or precipitates are so severe that they clogged water pipes. Iron and manganese in the water system also affect the flavour and colour of food and water. They also react with tannin, a chemical derived from plants, producing a black sludge.
The standards prescribed by international bodies, such as, the World Health Organization (WHO) and agencies in different countries of the world, limit the amount of iron and manganese that should be presents in water for end use, are:
Iron = 0.30 mg/L (milligrams per liter, or parts per million)
Manganese = 0.05 mg/L (milligrams per liter, or parts per million)
Iron in excess of 0.3 mg/L will stain laundry and plumbing fixtures and cause water to appear rusty. When manganese is predominant, the stains will be brown or black.
There are both iron and manganese bacteria presence in water and a quantity of 0.1 mg/L of iron, will encourage the growth of iron bacteria leptothrix, gallionella and crenothrix. These non-pathogenic microorganisms feed on the iron and manganese present in water causing growth in the distribution systems. Taste, odour and colour in water are a result of these bacteria. Mains, service lines, meters, and pumps become clogged by these microorganism growths (see Photo 1.).
Effect of Iron Bacteria; Crenothrix, in Pipe Line
The presence of these elements in the water supply, at levels higher than the recommended limits set for iron and manganese are not considered potential health risks. These recommended drinking water standard are used to avoid the aesthetic nuisances and economic problems that caused the undesirable and unpleasantness of water for humans and animals use. There are other problems created by deposits of iron and manganese in pipelines, pumps, water heater and water softeners, that increase energy and maintenance costs and reduce the quality and quantity of the water supply.
Testing of Public Water Supplies Containing Iron and Manganese
The quality of water supplied by any Public Water System is regulated under legislature and is maintained by established standards. These standards are established on aesthetic factors such as taste, odour, colour, corrosivity and staining properties of the water supply. These standards are based on the protection of human health, and strict monitoring of the guidelines is maintained. Tests are done at established laboratory or on site, ensuring that the containment for end use is within the prescribed levels of 0.3 and .01 mg/L.
Remediation of Iron and Manganese in Water
There are a number of methods used in the removal of iron and manganese in water. These methods depend on the bacteria, quantity of water and the form and concentration of the metals.
The four most common methods used are:
Water Softening (Ion Exchange)
Oxidation (Aeration or/and Chemical) Followed by Filtration
Water Softening (Ion Exchange
The water softeners (ion exchange process) are used by substituting the iron and manganese in the water by sodium for the exchange process. The iron and manganese are removed from the softener resin bed by the backwashing and regeneration methods. The raw water is not allowed to come into contact with oxidizing agents, such as, air or chlorine, as the oxidized iron and manganese will polluted the softener resin. If oxidized iron and manganese are present in the raw water, then, filtration is recommended for their removal. The efficiency by softeners depends on the concentration of the metals being less than 5 mg/L, water hardness between 50 - 350 mg/L and the water pH greater than 6.7.
Polyphosphate process is used for water that contains dissolved concentrations, of 2 mg/L of iron or less. This process is not effective when treating manganese. The phosphate is combined with the water by using controlled doses, sequestering, stabilizing and dispersing the iron in the water. It also keep the iron in its soluble form and the water clear and not causing stains during use. However, the metallic taste is not eliminated and the water tends to have a soapy feel and if ingested could cause diarrhea. The polyphosphate will be degraded in water heaters, resulting in the iron becoming solids.
Oxidizing filters are used to oxidize and filter iron and manganese when their concentration is in the range of 3 to 10 mg/L. The filter media comprised of manganese treated greensand, treated with potassium permanganate. It forms an outside layer that oxidizes the dissolved iron and manganese which is filtered out of the water. One of the drawbacks of this system is the high maintenance and regular regeneration, using a potassium permanganate solution. The solution is used up during oxidation of the dissolved metals. The filters in this process require periodic backwashing to remove the residues of oxidized iron and manganese particles.
Oxidation (Aeration or/and Chemical) Followed by Filtration
Oxidation of iron and manganese that exceed levels of 10 mg/L, followed by a filtration treatment involves the addition of chemical to convert dissolved iron and manganese into their solid, oxidized forms. Chlorine (sodium hypochlorite) in solution is one of the oxidant that is fed into the water stream to precipitate iron and manganese. Activated carbon filters are installed to remove the residual chlorine taste and odour. This process is high maintenance, as the filters are backwashed on a regular basis, to remove accumulated iron and manganese particles. Chlorination is the preferred treatment when iron-bacteria are present as it kills the bacteria and removes the iron.
When the levels of iron and manganese exceed 10 mg/L, the most effective treatment, involves filtration preceded by oxidation. The use of aeration units is used by cascading, bubbling, or stripping the gas from the water. Disinfectant would be used in the water to prevent bacteria from inhabiting the aerator. Aeration does not add chemicals to the water and its maintenance costs are low. Aeration system includes filtration units that use backwashing for removal of the oxidized iron and manganese.
In Trinidad, 24% of the Water and Sewerage Authority (WASA) distributed water are from groundwater. Iron and manganese are present in the water from wells and springs, with high concentrations of iron, found in water supplies sampled from the central and southern areas. The WHO with WASA internal standards are adopted in maintaining the prescribed levels of Iron and manganese in the water for its end use. WASA had constructed eleven iron and manganese removal plants, to supply potable water to their costumers, within the permissible limits. These Plants used the aeration method for iron and manganese removal together with sedimentation, disinfection and filtration.
Iron and manganese are water contaminants with little or no known adverse health effects at the intensity found in domestic water supplies. Their presence in water is of nuisancesââ‚¬â„¢ value resulting in staining and clogging of plumbing fixtures as well as unpleasant tastes and odours. Treatment of these minerals depends on the amount and structure of the elements in the raw water; hence, accurate testing of the water supply is important before selecting treatment options. These options follow the same process that causes the iron and manganese to come out of the water and cause plumbing and laundry problems. Oxidation and ion exchange are methods used to remove these elements from the water.