Water

Water is used extensively throughout textile processing operations. Almost all dyes, specialty chemicals, and finishing chemicals are applied to textile substrates from water baths. The amount of water used varies widely in the industry, depending on the specific processes operated at the mill, the equipment used, and the prevailing management philosophy concerning water use.

Textile operations vary greatly in water consumption. Following figure summarizes the water consumption of various types of operations. Wool and felted fabrics processes are more water intensive than other processing subcategories such as woven’s, knits, stock, and carpet. Water use can vary widely between similar operations as well. For example, knit mills average 10 gallons of water per pound of production, yet water use ranges from a low of 2.5 gallons to a high of 45.2 gallons.

Water consumption varies greatly among unit processes. Certain dyeing processes and print after washing are among the more intensive unit processes. Within the dye category, certain unit processes are particularly low in water consumption (e.g., pad-batch). Different types of processing machinery use different amounts of water, particularly in relation to the bath ratio in dyeing processes (the ratio of the mass of water in an exhaust dye bath to the mass of fabric). Washing fabric consumes greater quantities of water than dyeing.

Sources of Water:

1.     Rain water

2.     Surface water

3.     Sub soil water

4.     Deep well water

Rain water:

·        Rain collected immediately after precitation, is the purest of all natural waters.

·        It may contain traces of gases dissolved out of the atmosphere & possibly an almost infinitely small amount of finely divided solid matter derived from the air.

·        It also contain dissolve or suspended impurities such as shoot traces of sulphar di oxide or sulpharic acid, CO2, NH3, NO2 and other by products of industrialization.

·        Suspended impurities present in it can be filtered by using sand bed.

·        Suitable for boiling, washing & dyeing processes.

Surface water:

·        Surface water consists of rain water which has collected from streams, rivers or lakes.

·        These types of water contain organic & inorganic matter which are dissolved in it & also contains suspended impurities.

·        Then the nitrification bacteria will in time convert the organic substances into nitrates which are not objectionable in dyeing & finishing.

·        Surface water may receive considerable additions of dissolve material salts from shallow spring which feed the streams.

·        It contains chloride, sulphate, carbonate, bicarbonate of sodium, potassium, calcium and iron.

·        Not suitable for dyeing & finishing.

Sub soil water:

·        This type of water is collected from shallow springs & wells which are about 50 ft. or so deep.

·        It is usually free from suspended impurities because it has been filtered by its passage through the soil. It will however contain dissolve organic matter.

·        Subsoil water is often rich in dissolved CO2 as a gas abundantly present in the skin of the soli.

·        Subsoil water is very variable with regard to the impurities which they contain.

·         Not suitable for dyeing & finishing.

Deep well water:

·        These types of water are obtained 500 meter below the surface. It is free from organic matters.

·        The soluble impurities in water may be composed of a variety of substances. Soluble organic compounds, ammoniums salts, nitrates & nitrites of animal or vegetable origin may be found. If they are present in considerable quantities, the dirt contamination is undesirable for much textile process.

·        The presence of salts of calcium or magnesium in solution can be most undesirable in many finishing process.

Hardness of water:

The presence of Calcium, Magnesium salt of bicarbonates, sulfates, Chlorides in water causes hardness of water. The water containing these salts called hard water. The bicarbonate salts of calcium and magnesium are called Temporary hardness because boiling will liberate carbon dioxide and precipitate calcium carbonate. Chloride salts of calcium and magnesium are called Permanent hardness because boiling will not cause a precipitate.

CaSo4+2RCOONa        (RCOO)2 Ca + Na2SO4

MgSO4+2RCOONa        (RCOO)2 Mg + Na2SO4

Classification of water hardness:

Hardness is of two types:

• Temporary hardness
• Permanent hardness

Temporary hardness

Temporary hardness is a type of water hardness caused by the presence of dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate). When dissolved these minerals yield calcium and magnesium cat ions (Ca²⁺, Mg²⁺) and carbonate and bicarbonate anions (CO₃^2-, HCO₃^-). The presence of the metal cations makes the water hard. However, unlike the permanent hardness caused by sulfate and chloride compounds, this "temporary" hardness can be reduced either by boiling the water, or by the addition of lime (calcium hydroxide) through the softening process of lime softening.^[4] Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution, leaving water that is softer upon cooling.

Permanent hardness

Permanent hardness is hardness (mineral content) that cannot be removed by boiling. When this is the case, it is usually caused by the presence of calcium sulfate and/or magnesium sulfates in the water, which do not precipitate out as the temperature increases. Ions causing permanent hardness of water can be removed using a water softener, or ion exchange column.

Total Permanent Hardness = Calcium Hardness + Magnesium Hardness

The calcium and magnesium hardness is the concentration of calcium and magnesium ions expressed as equivalent of calcium carbonate.

Total permanent water hardness expressed as equivalent of CaCO₃ can be calculated with the following formula: Total Permanent Hardness (CaCO₃) = 2.5(Ca²⁺) + 4.1(Mg²⁺).^{[citation needed]}

Problems with Hard Water

Mineral deposits are formed by ionic reactions resulting in the formation of an insoluble precipitate. For example, when hard water is heated, Ca²⁺ ions react with bicarbonate (HCO₃^-) ions to form insoluble calcium carbonate (CaCO₃), as shown in Equation 1.

(1)

This precipitate, known as scale, coats the vessels in which the water is heated, producing the mineral deposits on boiler. In small quantities, these deposits are not harmful, but they may be frustrating to try to clean. As these deposits build up, however, they reduce the efficiency of heat transfer, so boiler may not work as evenly or quickly in pans with large scale deposits. More serious is the situation in which industrial-sized water boilers become coated with scale: the cost in heat-transfer efficiency can have a dramatic effect on your power bill! Furthermore, scale can accumulate on the inside of appliances, such as dishwashers, and pipes. As scale builds up, water flow is impeded, and hence appliance parts and pipes must be replaced more often than if Ca²⁺ and Mg²⁺ ions were not present in the water.

Methods of expressing hardness of water

1.     PPM (parts per million)

2.     English degree

3.     German degree

4.     French degree

Hard/soft water classification:

Classification	hardness in mg/L	hardness in mmol/L	hardness in dGH/°dH	hardness in gpg
Soft	0–60	0–0.60	0.3-3.00	0-3.50
Moderately hard	61–120	0.61–1.20	3.72-6.75	3.56-7.01
Hard	121–180	1.21–1.80	6.78–10.08	7.06-10.51
Very hard	≥ 181	≥ 1.81	≥ 10.14	≥ 10.57

 Standard water parameter of water in wet processing:

Characteristic	Permissible Limit
Color	Colorless
Smell	Odorless
pH value	Neutral pH 7–8
Water hardness	< 5°dH (6.25°eH; 8.95°fH; 5.2°USA)
Dissolved solids	< 1 mg/l
Solid deposits	< 50 mg/l
Organic substances	< 20 mg/l (KMnO4 consumption)
Inorganic salts	< 500 mg/l
Iron (Fe)	< 0.1 mg/l
Manganese (Mn)	< 0.02 mg/l
Copper (Cu)	< 0.005 mg/l
Nitrate ( NO3-)	< 50 mg/l
Nitrite ( NO2- )	< 5 mg/l

Quality of water used in boiler:

1.     Appearance: clear, without residue.

2.     Residual hardness: <0 .05="" dh="" span="">

3.     Oxygen: <0 .02="" mg="" span="">

4.     Temporary CO2: 0 mg/L

5.     Permanent CO2: <25 mg="" span="">

6.     Iron: <0 .05="" mg="" span="">

7.     Copper: <0 .1="" mg="" span="">

8.     Phosphate: 4-5 mg/L

9.     pH: >9

10.                         conductivity: 2500 us/cm

11.                         Temperature of boiling feed water: 90’C

Potential problem caused by hard water in textile wet processing:

Process	Problem
Desizing	Deactives enzymes & insoluble size materials such as Starch, PVA
Scouring	Combine with soap, precipitate metal organic acids.  Produce yellowing or off white shades.
Bleaching	Decompose bleach bath.
Mercerizing	Form insoluble metal acids, reduce absorbency & luster.
Dyeing	Combine with dyes, changing their shades, insoluble dyes, cause tippy dyeing & reduce dye diffusion.
Printing	Break emulsion, changes thickness, efficiency & viscosity and those problems associated for dyeing.
Finishing	Interfere with catalysts, cause resin & other additives to become non reactive break emulsion & de-active soap.

Removal of water hardness:

Permanent hardness of water can be a removed by the following ways:

(a) By the use of soda:

Soda removes both temporary and permanent hardness. It is also inexpensive and easy to use. This makes it the ideal substance for softening water in the home.

(b) Other softening agents in the home:

It is difficult for the housewife to be very precise in the use of soda and the water softened by soda may often contain an excess of it, which even if it is slight, may damage certain fabrics. Hence, other softening agents could be used. They are:

(i) Soap:

Soap is used as a softening agent. However, the use of soap as a softening agent is extravagant on account of its high cost compared with soda.

(ii) Caustic soda:

t removes temporary hardness but reduces permanent hardness only when the lather is very slight.

(iii) Solution of Ammonia:

It may be used for softening water, when the fabrics to be treated would be harmed by soda. If used in excess, ammonia may destroy the Iustre of rayon's, discolour and injure animal fabrics and loosen the dyes of coloured articles. Since, it is not possible to be very certain of the quantity to be used; this is not practicable for softening water.

(iv) Borax:

It is useful for softening water containing over 20% of hardness. Borax is usually used to reduce the alkalinity of soap solution rather than to soften water.

(c) Removal of Permanent Hardness by the Base-Exchange Process:

Base exchange process' is a chemical method by which, softening of permanent hardness in water can be done on a large scale or for household purposes. It is the most popular and effective means of softening hard water. It was discovered by Dr. Robert Gans, who found out the natural minerals called 'Zeolites', which is very effective in softening water,

The Base-Exchange Process includes the following procedures:

The water passes through specially prepared zeolite- a sodium compound, called base-exchange compound. it is has the property of being able to exchange its sodium base for another. When hard water passes through the zeolite, the hardening compounds of calcium and magnesium are caught up by the zeolite and become compounds of sodium. Since sodium salts in water do not precipitate out on heating or form soap curds the water is called 'soft'.

When a given quantity of water, determined by the size of the appliance, has been softened, the zeolite becomes depleted; having parted with all its sodium, but this can be remedied, as the substance has the property of being able to exchange its base again and to take back sodium in place of calcium and magnesium. This process is called 'regeneration'.Zeolite water softeners made for domestic use are either connected with the men water-supply or fixed on to a water tap.

Basic ion exchange processes in water treatment:

The ion exchange technology is used for different water treatment applications:

• Softening (removal of hardness)
• De-alkalisation (removal of bicarbonate)
• Decationisation (removal of all cations)
• Demineralisation (removal of all ions)
• Mixed bed polishing
• Nitrate removal
• Selective removal of various contaminants

Softening

Natural water contains calcium and magnesium ions (see water analysis) which form salts that are not very soluble. These cations, together with the less common and even less soluble strontium and barium cations, are called together hardness ions. When the water evaporates even a little, these cations precipitate. This is what you see when you let water evaporate in a boiling kettle on the kitchen stove.

Hard water also forms scale in water pipes and in boilers, both domestic and industrial. It may create cloudiness in beer and soft drinks. Calcium salts deposit on the glasses in your dishwasher if the city water is hard and you have forgotten to add salt.

Strongly acidic cation exchange resins (SAC, see resin types) used in the sodium form remove these hardness cations from water. Softening units, when loaded with these cations, are then regenerated with sodium chloride (NaCl, table salt).

Reactions

Here the example of calcium:

2 R-Na + Ca⁺⁺ R₂-Ca + 2 Na⁺

R represents the resin, which is initially in the sodium form. The reaction for magnesium is identical.

The above reaction is an equilibrium. It can be reversed by increasing the sodium concentration on the right side. This is done with NaCl, and the regeneration reaction is:

R₂-Ca + 2 Na⁺ 2 R-Na + Ca⁺⁺

What happens to the water

Raw water

SAC (Na)

Softened water

The water salinity is unchanged, only the hardness has been replaced by sodium. A small residual hardness is still there, its value depending on regeneration conditions.

Uses

Examples for the use of softeners:

• Treatment of water for low pressure boilers
• In Europe, most dishwashers have a softening cartridge at the bottom of the machine
• Breweries and soft drink factories treat the water for their products with food grade resins

Softening the water does not reduce its salinity: it merely removes the hardness ions and replaces them with sodium, the salts of which have a much higher solubility, so they don't form scale or deposits.

De-alkalisation

This particular process uses a weakly acidic cation resin. This resin type is capable of removing hardness from water when it also contains alkalinity. After treatment, the water contains carbon dioxide, that can be eliminated with a degasifier tower. The cation resin is very efficiently regenerated with an acid, usually hydrochloric acid and the hydrogen cations combine with the birarbonate anions to produce carbon dioxide and water. Here the example of calcium:

2 R-H + Ca⁺⁺(HCO₃^–)₂ R₂-Ca + 2 H⁺ + 2 HCO₃^–

H⁺ + HCO₃^– CO₂ + H₂O

What happens to the water

Raw water	WAC (H)	Decarbonated water
Recombination of hydrogen and bicarbonate and removal of carbon dioxide with the degasifier:
Decarbonated water	DEG	Degassed water

The salinity has decreased. Temporary hardness is gone.

Uses

De-alkalisation is used:

• In breweries
• In household drinking water filters
• For low pressure boilers
• As a first step before the SAC exchange in demineralisation

De-alkalisation reduces the salinity of water, by removing hardness cations and bicarbonate anions.

Decationisation

The removal of all cations is seldom practiced, except as a first stage of the demineralisation process, or sometimes in condensate polishing where the decationiser precedes a mixed bed unit. A strongly acidic cation exchange resin (SAC) is used in the H⁺ form.

Reactions

Here the example of sodium, but all cations react in the same way:

R-H + Na⁺ R-Na + H⁺

The equilibrium reaction is reversed for regeneration by increasing the hydrogen concentration on the right side. This is done with a strong acid, HCl or H₂SO₄:

R-Na + H⁺ R-H + Na⁺

What happens to the water

Raw water

SAC (H)

Decationised water

DEG

Decat + degassed water

In the second step, a degasifier is used again to remove the carbon dioxide formed by combining the bicarbonate anions and the released hydrogen cation. The water salinity is reduced, and the water is now acidic. A small sodium leakage is shown.

Demineralisation

For many applications, all ions in the water must be removed. In particular, when water is heated to produce steam, any impurity can precipitate and cause damage. As there are cations and anions in the water, we must use two different types of resins: a cation exchanger and an anion exchanger. This combined arrangement produces pure water, as presented in the general introduction. Demineralisation is also called deionisation. The cation resin is used in the hydrogen form (H⁺) and the anion resin in the hydroxyl form (OH^–), so that the cation resin must be regenerated with an acid and the anion resin with an alkali.

A degasifier is used to remove the carbon dioxide created after cation exchange when the water contains a significant concentration of bicarbonate.

The cation resin is usually located before the anion resin: otherwise if the water contains any hardness, it would precipitate in the alkaline environment created by the OH^— form anion resin as Ca(OH)₂ or CaCO₃, which have low solubility.

Layout SAC – (DEG) – SBA

Let us first consider a simple demineralisation system comprising a strong acid cation exchange resin in the H⁺ form, a degasifier (optional) and a strong base anion exchange resin in the OH^– form. The first step is decationisation as shown above:

R_SAC-H + Na⁺ R_SAC-Na + H⁺

With calcium insead of sodium (also valid for magnesium and other divalent cations):

2 R_SAC-H + Ca⁺⁺ (R_SAC)₂-Ca + 2 H⁺

In the second step, all anions are removed with the strong base resin:

R_SBA-OH + Cl^– R_SBA-Cl + OH^–

The weak acids created after cation exchange, which are carbonic acid and silicic acid (H₂CO₃ and H₂SiO₃) are removed in the same way:

R_SBA-OH + HCO₃^– R_SBA-HCO₃^– + OH^–

And finally, the H⁺ ions created in the first step react with the OH^– ions of the second step to produce new molecules of water. This reaction is irreversible:

H⁺ + OH^– H₂O

What happens to the water

1: Cation exchange removing all cations (as in decationisation) followed by degassing:
Raw water	SAC (H)	Decationised water	DEG	Decat + degassed water
2: Anion exchange removing all anions (strong and weak acids):
Decat + degassed water	SBA (OH)	Demineralised water

Demineralised water is completely free of ions, except a few residual traces of sodium and silica, because the SAC and SBA resins have their lowest selectivity for these. With a simple demineralisation line regenerated in reverse flow, the treated water has a conductivity of only about 1 µS/cm, and a silica residual between 5 and 50 µg/L depending on the silica concentration in the feed and on regeneration conditions.
Note that the pH value should not be used as a process control, as it is impossible to measure the pH of a water with less than say 5 µS/cm conductivity.

Regeneration

The SAC resin is regenerated with a strong acid, HCl or H₂SO₄:

R-Na + H⁺ R-H + Na⁺

And the SBA resin is regenerated with a strong alkali, NaOH in 99 % of the cases:

R_SBA-Cl + OH^– R_SBA-OH + Cl^–

Layout WAC/SAC – DEG – WBA/SBA

Because weakly acidic and weakly basic resins offer a high operating capacity and are very easy to regenerate, they are used in combination with strongly acidic and strongly basic resins in large plants. The first step with the WAC resin is dealkalisation (removal of bicarbonate hardness), and the second step with the SAC removes all the remaining cations. A WAC resin is used when both hardness and alkalinity are present in large relative concentrations in the feed water.

WBA resins remove only the strong acids after cation exchange. They are not capable of removing the weak acids such as SiO₂ and CO₂. In the regenerated, free base form, they are not dissociated, so no free OH^– ions are available for neutral anion exchange. On the other hand, their basicity is enough to adsorb the strong acids created after cation exchange:

R_WBA + H⁺Cl^– R_WBA.HCl

In the last step, a SBA resin is thus required to remove the weak acids, as shown in the preceding section:

R_SBA-OH + HCO₃^– R_SBA-HCO₃^– + OH^–

What happens to the water

1 & 2: Cation exchange beginning with the removal of temporary hardness (WAC, as in dealkalisation) followed by the removal of all remaining cations (SAC):
Raw water	WAC (H)	Decarbonated water	SAC (H)	Decationised water
3 & 4: Anion exchange begining after degassing with the removal of strong acids (WBA) followed by the removal of weak acids (SBA):
Decat + degassed water	WBA (FB)	Partially demineralised	SBA (OH)	Demineralised water

A full demineralisation line is shown below, with a cation exchange column (WAC/SAC), a degasifier, an anion exchange column (WBA/SBA), and a polishing mixed bed unit. The use of a weakly acidic resin and the degasifier column are conditioned by the presence of hardness and alkalinity in the feed water, as explained in the previous sections.

A demineralisation line (click to enlarge)

Regeneration

Regeneration is done in thoroughfare, which means that the regenerant first goes through the strong resin, which requires an excess of regenerant, and the regenerant not consumed by the strong resin is usually sufficient to regenerate the weak resin without additional dosage.

The cation resins are regenerated with a strong acid, preferably HCl, because H₂SO₄ can precipitate calcium.
The anion resins are regenerated with caustic soda.

Regeneration of the demineralisation line (click to enlarge)

The quality obtained is the same as in the simple SAC-SBA layout, but because the weak resins are practicallly regenerated "free of charge", the regenerant consumption is considerably lower. Additionally, the weak resins have a higher operating capacity than the strong resins, so the total volume of ion exchange resins is reduced.

Uses

Examples of demineralisation:

• Water for high pressure boilers in nuclear and fossil fuelled power stations and other industries
• Rinse water used in production of computer chips and other electronic devices
• Process water for many applications in the chemical, textile and paper industries
• Water for batteries
• Water for laboratories

Mixed bed polishing

Mixed bed unit in service
and in regeneration

The last traces of salinity and silica can be removed on a resin bed where highly regenerated strong acid cation and strong base anion resins are mixed.

Mixed bed units deliver an excellent treated water quality, but are complcated to regenerate, as the resins must first be separated by backwashing before regeneration. Additionally, they require large amounts of chemicals, and the hydraulic conditions for regeneration are not optimal. Therefore, mixed beds are usually only used to treat pre-demineralised water, when the service run is long.

What happens to the water

Practically nothing is left:

Demineralised water

SAC (H) + SBA (OH)

Nothing is left

Mixed bed polishing produces a water with less than 0.1 µS/cm conductivity. With sophisticated design and appropriate resins, the conductivity of pure water (0.055 µS/cm) can be achieved. Residual silica values can be as low as 1 µg/L.
The pH value should not be used as a process control, as pH meters are unable to operate at 1 µS/cm conductivity or below.

Uses

• Treatment of water pre-demineralised with ion exchange resins
• Polishing of reverse osmosis permeate
• Polishing of sea water distillate
• Treatment of turbine condensate in power stations
• Treatment of process condensate in various industries
• Production of ultra-pure water for the semiconductors industry
• Service de-ionisation (with off-site regenerated columns)