Do you know what are the Types of Coagulants used in Water Treatment Process.
Coagulation is an important part of the water treatment process. Here we are going to discuss about the types of coagulants used in the water treatment process. The process of removal of suspended solids in water by use of chemical agents is known as coagulation. Coagulation is carried out for the filtration and purification of water.
Types of coagulants used in water treatment process.
Aluminium sulphate is the most widely used aluminium coagulant. It is available in a number of solid grades such as block, kibbled or ground and is also available as a solution. In waterworks practice aluminium sulphate is frequently but incorrectly referred to as ‘alum’. The solid form has the composition A12(SO4)3xH20 where x may range from 14 to 21 containing 14-18% w/w A1203 (alumina) or 7.5-9% w/w A1 (aluminium), depending on the number of molecules of water (x).
When dosed into water, the formation of an aluminium hydroxide floc is the result of the reaction between the acidic coagulant and the natural alkalinity of the water, which usually consists of calcium bicarbonate. A dose of 1 mg/1 of aluminium sulphate as A1 reacts with 5.55 mg/1 of alkalinity expressed as CaCO3 and increases the CO2 content by 4.9 mg/1.
Thus if no alkali is added the alkalinity will be reduced by this amount with a consequent reduction in pH. If a water has insufficient alkalinity or ‘buffering’ capacity, additional alkali such as hydrated lime, sodium hydroxide, or sodium carbonate must therefore be added; the alkalinity expressed as CaCO3 produced by 1 mg/1 of each chemical (100% purity) is 1.35, 2.5 and 0.94 rag/l, respectively.
The aluminium hydroxide floc is insoluble over relatively narrow bands of pH, which may vary with the source of the raw water. Therefore pH control is important in coagulation, not only in the removal of turbidity and colour but also to maintain satisfactory minimum levels of dissolved residual aluminium in the clarified water.
The optimum pH for coagulation of lowland surface waters is usually within the range of 6.5-7.2, whereas for more highly coloured upland waters a lower pH range, typically 5-6, is necessary. Lowland waters usually contain higher concentrations of dissolved salts, including alkalinity and may therefore require the addition of acid in excess of that provided by the coagulant. Under these circumstances it is usually more economic to add sulphuric acid rather than excess aluminium sulphate to obtain the optimum coagulation pH value.
2. Sodium aluminate
Another widely used types of coagulants is sodium aluminate. Sodium aluminate is prepared from aluminium oxide stabilised with caustic soda; it is used with aluminium sulphate to coagulate very cold waters which would not coagulate successfully with aluminium sulphate alone. It is also used in the ‘double coagulation’ of highly coloured waters; aluminium sulphate (with sulphuric acid) being added as the first stage to coagulate the colour at pH 4.5-5.0.
The resulting soluble aluminium in the settled water from the first sedimentation stage is precipitated in the second sedimentation stage using the alkaline sodium aluminate at pH 6.5. Sodium aluminate is also used in lime-soda softening in which insoluble calcium aluminate is formed, and in turn flocculates the precipitated calcium carbonate and magnesium hydroxide.
Iron coagulants in the ferric form behave similar to aluminium sulphate and form ferric hydroxide floc in the presence of bicarbonate alkalinity. A dose of 1 mg/1 of ferric sulphate or chloride as Fe neutralises 2.7 mg/1 alkalinity expressed as CaCO3 and increase the CO2 content by 2.36 mg/1. The ferric hydroxide floc is insoluble over a much broader pH range (4-10) than aluminium sulphate. The lower end of the pH range (4-5.5) is useful for treating highly coloured moorland waters.
The above three coagulants are the primary coagulants used in water treatment process.
4. Coagulant aids and polyelectrolytes
Coagulant aids are used to improve the settling characteristics of floc produced by aluminium or iron coagulants. The coagulant aid most used for a number of years was activated silica; other aids included sodium alginates and some soluble starch products.
These substances had the advantage of being well-known materials already used in connection with the food industry and were thus recognised as harmless in the treatment of water. Polyelectrolytes came later into use and were more effective. Originally of natural origin, for the most part they now comprise of numerous synthetic products: long chain organic chemicals which may be cationic, anionic, or non-ionic. The theory of their action has been reviewed by Packham.
Polyelectrolyte doses used are very small in relation to the dose of the primary coagulant. Natural polyelectrolyte doses vary between 0.5-2.5 mg/1 whereas polyacrylamide doses vary between 0.05-0.25 mg/1. Polyelectrolytes are added as a coagulant for turbid waters, or after the primary coagulant as a coagulant aid.
Sometimes they are added just prior to filtration in very small doses (less than 0.05 mg/1) to flocculate micro-floc particles carried over from the clarifiers and filter passing algae; care in control of the dose is necessary because excess polyelectrolyte could result in ‘mud ball’ formation and other problems in the filters.
These are the types of coagulants used in water treatment process.