Iron is an objectionable constituent of a drinking water. Appreciale amounts of Iron in water impart a bitter characteristic, metallic taste and cause oxdized precipitate.Coloration of water which may be yellowish brown to reddish brown and renders the water objectionable or unsuitable for domestic purpose. In addition Iron stain everything with which it come in contact. In hotels,hospitals, clubs, Institutions, office buildings and homes, Iron-bearing water stain wash basins, toilets, urinals, bath tubs, showers, tiled floors and walls.Iron tolerances for municipal or house hold use should not exceed 0.3 ppm. Concentration of Iron in excess of 0.2 to 0.3 mg/l may cause nuisance even though its presence does not affect the hygienic quality of water.
Source and Nature
Iron exists in water in two levels.One as the bi-valent , Ferrous Iron ( Fe ++) and the second one as the tri-valent, Ferric Iron (Fe+++). The Ferric Iron generally occuring in the precipitated form. Iron forms complexes of hydroxides and other in-organic complexes in solution with substantial amounts of bi-carbonate, sulphate, Phosphate, Cyanide or Halides. Presence of organic substances induces the formation of organic complexes which increase the solubility of Iron. The waters of high alkalinity have lower iron than waters of low alkalinity.
The commonest form in which Iron is found in water supplies is as Ferrous bi-carbonate which is a soluble, colourless salt and exists only in solution. Its solubility is increased by increasing the free Carbon-di-oxide content of the water. The unaerated water is clear and colourless. It develops a slight whitish haze, which on longer standing turns yellowish and then forms yellowish brown to reddish brown deposits of hydrated Ferric oxide after aeration..
Removal of Iron
Oxidation by aeration or use of chemicals like chlorine, chlorine di-oxide or potassium permanganate followed by filtration alone or by settling and filtration can bring about the precipitation of iron and its removal. Use of zeolites as well as catalytic oxidation also serve the purpose.
Iron in water in the reduced form is converted to ferric compound by oxidation and these are removed by filtration alone or by sedimentation and filtration. The reaction period is about 5 minutes or less at a pH of 7.0 to 7.5 and 0.14 mg of oxygen is needed to convert 1 mg ferrous iron to ferric hydroxide as indicated below :
4 Fe2+ + O2+ 10 H2O
4 Fe ( OH)3 + 8 H+
4x56 mg Fe2 = 2x 16 mg O2
The rate of oxidation of ferrous iron by aeration is slow under conditions of low pH and is fast under high pH conditions. Rate of precipitation and filtration are accelerated in practice by contact and catalysis. Water is allowed to trickle over coke or crushed stone. The deposition of hydrated oxides of iron and bacteria on the contact media is believed to act as catalysis which accelerate the oxidation of Iron.
Simple Technique for Iron removal
A simple and inexpensive treatment unit for the removal of iron is suggested so that the difficulties of operation and maintenance can also be minimized.
When the source is a well or a sump and the water consumption rate is in the order of 40 lpcd and when hand pump is used, a tray type aerator with two trays operated at an aeration rate of 1.26 m3/m2/hr are employed and the water aerated. Then the water is settled in a sedimentation basin having a detention period of 3 hours and the clarified water passed through a sand filter having a depth of 0.3 m supported by gravel 3-6 mm in size 0.1 m deep. The effective size of the sand is 0.30-0.45 mm and its uniformity coefficient 2-3.Sand is cleaned by manual scraping.