Sources of Hardness Minerals in Drinking Water
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 Fe 2+ + O 2 + 10 H 2O 4 Fe ( OH) 3 + 8 H +
4×56 mg Fe 2 = 2x 16 mg O 2
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 m 3/m 2/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.
Iron Removal: A World Without Rules
Iron can often be detected visibly in water or by staining on plumbing fixtures.
There is one rule to keep in mind when selecting a method for iron removal — and that is there is no rule. You will find — as with all problem water applications — the solution is 50 percent science and 50 percent experience.
The following information describing the different types of iron removal process applications are the basics. Before using any of these applications, it’s good to have an understanding of the type of iron present; the equipment and its limitations; and the product and processes involved with method.
Care must be taken when considering iron removal advice from different regions of the country as water temperature, pH, alkalinity, dissolved oxygen content and other factors will affect the actual results.
Most application failures are caused simply by not selecting the right equipment for the water conditions present. It is important to follow manufacturer’s guidelines regarding flow rates, backwash rates, pH levels, maximum iron input levels, water temperatures and any other application limitations that the manufacturer has noted in order for the equipment and media to deliver their best result as designed.
Most iron filtration systems operate on the principal of oxidizing the iron (oxidation) to convert it from a ferrous (dissolved or soluble) to a ferric or undissolved state. Once in the ferric state, iron can be filtered.
Water filters are the most widely used equipment in removing iron. Its popularity comes from its versatility due to the various media products available and the process involved with each media.
The most common reasons for filter failure are a lack of flow in backwash or a lack of frequency of regenerations. Low pH levels when using filters are another reason for unsatisfactory results.
Water softeners exchange ions by design. When used in iron removal, the softener uses a cation resin to exchange iron for sodium, in addition to the calcium and magnesium exchanged for sodium in the softening process.
Softeners are commonly used in removing low levels of ferrous iron (1-3 ppm), though it is not uncommon to remove 10 or more ppm depending on water conditions and control settings.
The last thing a water softener needs is for the ferrous iron to oxidize and convert to a ferric state. Since pH plays a big part in how quickly this conversion takes place, it is important to note that softeners perform better on low pH, which will also prolong bed life.
In the ferric state, iron will coat the resin, plugging the exchange sites and fouling the resin. Iron fouling will eventually happen in any iron application and requires replacement of the media.
High saltings, longer backwashes, frequent regenerations and the use of iron cleaners are keys to longer bed life. However, even after taking these steps to prevent the bed from fouling, the resin will eventually succumb to the iron and require replacement.
Each type of treatment has its own strengths and weaknesses. As in the selection of equipment, it is important to follow manufacturers’ recommendations and note any application limitations such as water temperature, pH alkalinity and dissolved oxygen content to get the best result.
To do this, water treatment professionals need a clear understanding of all limitations of the product and equipment selected.
Filtration using various means of oxidation is the most common method of iron removal. Depending on the media selected, other common processes such as ozone, aeration, chlorine or peroxide injection may be used to boost the oxidizing properties of the water being treated.
Greensand is one of the oldest but proven oxidation technologies. Potassium permanganate, itself an oxidizer, is used to regenerate the greensand.
In this application, potassium permanganate produces manganese dioxide on the surface of the mineral and — once the water comes in contact with it — any iron is immediately oxidized. The iron can be filtered and then cleaned away in the backwash cycle. Greensand is also effective with low levels of H 2S (hydrogen sulfide) and manganese.
Synthetic greensand is a granular mineral with a manganese dioxide coating having the same ability as regular greensand. It is much lighter and requires less of a backwash rate than standard greensand.
Manganese dioxide is a naturally mined ore with the ability to remove iron, manganese and hydrogen sulfide. The hydrogen sulfide capability exceeds that of either greensand or synthetic greensand and requires no chemicals to regenerate.
It does, however, require adequate amounts of dissolved oxygen in the water as a catalyst and may require some type of pre-oxidation to achieve its maximum ability.
Birm has the ability to remove iron and manganese and has no effect on hydrogen sulfide. Like manganese dioxide, birm also uses dissolved oxygen as a catalyst and may require some type of pre-oxidation in cases where the dissolved oxygen content is too low to affect a maximum iron removal result.
Redox media, which requires adequate dissolved oxygen to be effective, consists of two metals – 85 percent copper and 15 percent zinc. These two dissimilar metals create a small electrical field in the bed that will not allow bacterial growth in the media.
This property earns redox the unique distinction of being effective on bacterial iron without the use of chlorine injection and being rated as bacterial static.
Effective on removal of iron and hydrogen sulfide, able to reduce chlorine and heavy metals such as lead and mercury, redox is not effective with manganese.
The biggest drawback for this media is its weight. Being almost twice as heavy as other minerals, it requires more than twice the backwash rate of other minerals. Sizing mineral tanks is crucial.
Catalysts & Considerations
Once you have identified the enemy and selected the equipment with compatible backwash and flow rates for the media selected, the water itself must be scrutinized.
Check for dissolved oxygen and pH levels and determine what, if any, pre-treatment is necessary for the selected application to deliver maximum iron removal efficiency.
What is the role of pH?
The pH of a given water source plays an important role in how quickly ferrous (dissolved) iron converts to a ferric (solid) state. The higher the pH, the faster iron will convert to the ferric state that can then be filtered.
This is good in all equipment selections with the exception of a water softener where the ferric iron plugs the exchange sites and fouls the resin.
When using an iron filter a pH above 6.5 is necessary for iron to properly convert and is the recommendation of most manufacturers. However, most experienced water treatment professionals agree that a pH above 7.0 is a must and an 8.0 to 8.5 pH greatly enhances the chance of a successful application.
If it is necessary to increase the pH level, chemical feed of either sodium carbonate (soda ash) or sodium hydroxide (caustic soda) is preferred over a filter filled with calcium carbonate or magnesium oxide, as the filter method may foul quickly
IMost chemical-free iron filters and several chemical filter media require some dissolved oxygen in the water to act as a catalyst. Pre-oxidation is required in cases where the dissolved oxygen content is too low.
IPre-oxidation can come from aeration, chlorine or peroxide injection, ozone and other methods.
There are several types of chemical feed applications. Using sodium carbonate or sodium hydroxide to raise pH is common. Using 5 percent to 10 percent chlorine or 7 percent hydrogen peroxide as oxidizers to the water before a filter is also widely used.
Different rules apply to each of these methods, from retention or contact tanks to using static mixers. When using different chemicals together, it’s important to understand the compatibility of the chemicals and the safety considerations.
For greater success, follow the manufacturers’ recommendations closely regarding proper feed rates and installation when injecting chemicals.
When aeration is used as a pre-oxidizer it is generally done with either an air inductor or an air pump.
An air inductor is a venturi installed inline. The water flowing through the inductor creates a vacuum and sucks air into the water line. The faster the water flows, the more air induced into the water.
Watch for pressure drop and perform routine maintenance of the inductor, as they will clog with iron over time.
The air pump method allows more air induced into the water, as a mechanical pump is used to force air into the water. A contact tank is often used.
This method has proven effective with the only cautions being maintenance to the pump and injection fittings.