Water PurificationA plentiful supply of suitable water is essential for a dyeing and bleaching plant. Before modern methods of water purification were purification were available, the textile industries tended to congregate in areas where the natural water supply was plentiful and sufficiently pure. Water with a high degree of purity is rarely obtainable from Natural sources. A knowledge of he impurities which will probably be present and how to remove them is, therefore, desirable. Supplies of water may be classified as follows:

(i) rain water

(ii) surface water, which consists of rain which has collected into streams, rivers, or lakes;

(iii) subsoil water, which has percolated a small distance into the ground. It is collected from shallow wells or surface springs;

(iv) deep well waters, which have usually percolated through several strata.

Rain water:-

Rain, collected immediately after propitiation, is the purest of all natural waters. It may contain traces of gases dissolved out of the atmosphere, and possibly an almost infinitely small amount of finely divided solid matter derived from the air.

In the neighborhood of towns rain may collect dissolved or suspended impurities such as soot, traces of sulphur dioxide, or sulphuric acid, and other by-products of industrialization. Rain water from such sources is improved by filtration through a bed of sand.

Although the collection of rain for industrial purposes dose not warrant serious consideration, there are occasions when small quantities of it may be useful for special purposes. If storage is undertaken it is desirable to have some device which rejects the first few gallons which will be delivered from the surface after a dry period, because they will contain all the solid matter which has been deposited from the atmosphere.

Further purification, if it be desired, is easily achieved by passing the water through a filter made by filling a barrel or small tank with alternate layers of pebbles and fine sand.

Surface waters:-

Rain inevitably finds its way into steams, rivers, and lakes, all of which are classified as surface water. As it passes over the surface of the earth it caries with it organic matter in various stages of decomposition. It may also dissolve a certain amount of mineral matter, depending upon the nature of the soil or rock with which it has come into contact.

When it reaches a site where it can stagnate, such as a lake, it deposits the suspended matter but retains the dissolved organic and inorganic constituents.

Nitrifying bacteria will in time convert the organic substances into nitrates which are not objectionable in dyeing and finishing. Surface waters, however, may receive considerable additions of dissolved mineral salts from shallow springs which feed the streams.

Surface water is sometimes distinctly discolored by colouring matter derived mainly from peat but also from other decaying vegetable sources.

Subsoil water :-

Water in this classification is collected from shallow springs and wells which are about 50 (15 m) or so deep. It is, therefore, surface water which has predated a short distance through the soil or rock formation.

It is usually free from suspended impurities because it has been filtered by its passage through the soil. It will, however, contain dissolved organic matter which was either collected whilst it was surface water or was extracted from the zone in the soil which is densely occupied by the roots of vegetation.

Subsoil water is often rich in dissolved carbon dioxide, a gas abundantly present in the skin of the soil because it is by product of the metabolism of vegetable life. Water rich in carbon dioxide will convert insoluble calcium carbonate.

Subsoil waters are very variable with regard to the impurities which they contain because pollutions is determined by the nature of the surface on which the rainfalls and the composition of the soil or strata through which the water has percolated.

Deep well water:-

The deep wells are those which are bored through the subsoil into the water-bearing strata beneath. Water from such sources is generally free from organic matter because it has percolated through a considerable depth of rock and soil, and this type of impurity has been removed by filtration activity.

The mineral content, however, may be very high because the water has had abundant opportunity to extract soluble inorganic constituents from the strata through which it has The impurities in water may be either suspended or dissolved.

Filtration is necessary to remove matter held in suspension. The addition of a precipitant to the water before it is filtered greatly improves the efficiency of the process.

Aluminum hydroxide forms a flocculent precipitate which retains colored matter and colloid ally suspended impurities. It is common practice to add aluminum sulphate in the form of alum, to

the water but, because this salt gives an acidic solution, no aluminum hydroxide will be precipitated until the P H has been adjusted to between P H5 and P H7 .

The presence of salts of calcium or magnesium in solution can be most undesirable in many finishing processes. These salts are responsible for hardness of water an they lead to the formation of insoluble precipitates with soaps which may be deposited on the goods, causing discoloration.

Calcium and magnesium compounds are commonly present as soleplates and chlorides. Bicarbonates produce what is known as ‘temporary’ hardness; soleplates and chlorides are the cause of ‘permanent’ hardness.

Temporary hardness:-

Calcium and magnesium carbonates are virtually insoluble in water. The carbon dioxide which is always present in natural waters will convert calcium or magnesium carbonates in the rock formations with which contact is made, into soluble bicarbonates:

CaCo,+CO,+H,O Ca(HCO,);

MgCO,+CO,+H,OMg(HCO,);

When water is boiled the bicarbonates decompose with liberation of carbon dioxide and precipitation of the insoluble carbonates which are

Ca(HCO,); CaCO,+CO,+H,O.

Hardness caused by bicarbonates is called temporary because it disappears on boiling. It is a contributory factor in the formation of scale in boilers, which is a deposit, amongst other things, of calcium carbonate and magnesium carbonate:

MgCO,+H,O Mg (OH),+ CO,

Permanent hardness:-

Calcium and magnesium chlorides or sulphates are soluble in water both in the presence an in the absence of carbon dioxide. Expulsion of dissolved carbon dioxide makes no difference to their solubility. They are not, therefore, precipitated when the water is boiled, but remain in solution, and therefore hardness due to these salts is called permanent hardness. Most waters contain both temporary and permanent hardness, but the proportions of the two vary wields.

Water softening:-

If water contains more than 5 parts per 100.000 of hardness it is generally accepted that softening is desirable. It must be borne in mind, however, that for many processes in a dyeworks hard water no disadvantage. This applies to a grater extent now than in the past because synthetic detergents, which are stable in the presence of calcium and magnesium ions, are used to such a

large extent. Softening can be quite expensive, and the use of softened water where it is unnecessary is wasteful. Temporary hardness is removed by boiling, but this is impractical in daily use.

The carbon dioxide can, however, be extracted from the bicarbonate by the action of an alkali, calcium hydroxide being the one which is commonly used. The reaction is as follows;

Ca(HCO3) 2 +Ca(OH) 2 MgCO3 +2H 2 O.

Thus the whole of the temporary hardness due to calcium is precipitated as calcium carbonate. According to the equation it flows that 100 parts of temporary hardness require 7+ parts of calcium hydroxide or 56 parts of calcium oxide. The reaction follows a slightly different with magnesium bicarbonate.

The first stage is the conversion to magnesium carbonate:

Mg(HCO 3 ) 2 +Ca(OH) 2 MgCo3 +2H 2 O.

The reaction, in this case, however, does not suffice to soften the water because magnesium carbonate is sparingly soluble.

A second molecule of calcium hydroxide must therefore be added to precipitate the insoluble magnesium hydroxide:

MgCO 3 +Ca(OH) 2 Mg (OH) 2 + CaCO 3 .

In order to calculate the exact quality required to remove temporary hardness, the carbon dioxide must be known and allowed

Permanent hardness is removed by converting the calcium and magnesium soleplates into carbonates by the action of sodium carbonate.

CaSO,+Na 2 CO 3 Na 2 SO,+ CaCO 3

MgSO,+ Na 2 CO 3 Na 2 SO,+ MgCO 3 .

The calcium soleplate is thus removed as calcium carbonate, an equivalent quantity of sodium soleplate being left in solution.

Magnesium sulphate would be converted into magnesium carbonate and this would this would require to be precipitated as Mg(OH), by means of lime. If enough lime be added to precipitate all the magnesium present, the magnesium soleplate is converted into calcium soleplate which, in turn, is precipitated as its carbonate:

MgSO 4 +Ca(OH) 2 +CaSo 4

It is apparent that in order to calculate the quantities of lime and sodium carbonate necessary for softening any given volume of hard water, we must know:

i. the temporary hardness;ii. the hardness due to magnesium whether temporary or permanent;

iii. the permanent hardnessiv. the free carbon dioxide.

Water softeners:-

Water softening is carried out on an industrial out an industrial scale by means of automatic apparatus. There are two kinds, namely, intermittent and continuous. In the former the water is mixed with the necessary chemicals, the precipitated hardness is allowed to settle, and the clear softened water drawn off. In the continuous form the precipitate is removed partly by gravity but chiefly by means of a filter.

The essential parts of a water softener are

i. a reagent tank, ii. a reaction tank,

iii. a precipitation tank connected generally with a filter,iv. a soft- water collecting tank.

The calcium compound formed does not precipitate soaps, nor does it confer any of the other prosperities of hardness upon the water.

Determination of hardness:-

Total hardness is the factor which is most commonly required in routine testing. The simplest method is based on titration with a standard soap solution which depends upon the reaction.

2C 17 H 33 COOK+CaCO 3 (C 17 H 33 COO) 2 Ca+K 2 CO 3 ,

from which it follows that 2x282g of oleic acid are equivalent to 100 g of calcium carbonate. Hence, I ml of a solution containing 5.64 g of oleic acid per litre would precipitate 100x5.64+ 1000x2x282= 0.001 g of hardness expressed as calcium carbonate.

Purification of effluents:-

The removal of impurities from effluents is a specialized subject. The permissible degree of pollution acceptable for discharge into either sewers or rivers varies from one locality to another.

As a guide the old Royal Commission standard specified a maximum of 30ppm of suspended matter and 20ppm of B.O.D. It may well be that river authorities will require higher standards than this such as,

for example, 20ppm of B.O.D. It may well be that river authorities will require higher standards than this such as, for example, 20ppm of suspended solids and 15ppm of B.O.D.

References:

 Filtration of water supplies (PDF), World Health Organization Zagorodni, Andrei A. (2007).  Ion exchange materials: properties and

applications. Elsevier.  ISBN 978-0-08-044552-6.