Textile Solutions

Waste water treatment

2012-05-05T13:48:00.001-07:00

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Water hardness in different terms

2012-05-05T13:39:00.000-07:00

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Effluent treatment

2012-04-30T13:48:00.000-07:00

Any effluent treatment programme must consider whether the water contaminants are biodegradable, over what time scale, and – if persistent – how they will influence conditions down stream. Even the influence of subsequent water chlorination for drinking purposes on the potential formation of toxic organochlorine compounds must be considered. This a complex subject requiring detailed on-site analysis before arriving at specific recommendations and their implementation. One very important strategy in reducing water pollution from a textile finishing plant involves minimising waste and optimising process methods so that fewer contaminants are discharged in the effluent. The following list gives some examples of this.
(1) re-use of excess dye solution remaining at the end of a process – this can be incorporated into other dyeing recipes;
(2) mixing acidic and alkaline effluents before discharge to avoid excessive acidity or alkalinity;
(3) replacing starch-based sizing materials with polyvinyl alcohol, which can be recovered by membrane separation techniques and recycled;
(4) process changes and optimisation of the use of problematic chemicals – for example, for the oxidation of leuco vat dyes, hydrogen peroxide or sodium perborate can be substituted for sodium dichromate. Replacement of dyes that require aftertreatment with chromium or copper salts will have a beneficial impact on the immediate water environment. Glucose can replace sodium sulphide as a reducing agent for sulphur dyes. The dye manufacturers now offer ‘greener’ products and processes such as low salt/high fixation reactive dyes. Many companies are examining the possibilities of recycling water and chemicals, and of recovering heat from hot effluent. Various types of processes are used in textile effluent treatment. Dyehouse effluent has a composition that is highly time-dependent in terms of both the types and quantities of contaminants. A first stage in treatment is often an equalising lagoon. Equalisation involves holding the combined process effluents for a given period to allow stabilisation of pH and BOD, and time for sedimentation of some solids. This considerably reduces fluctuations in the composition of the water leaving the dyehouse, which can upset down stream processes such as activated sludge treatment. Biological treatment with organisms may be aerobic (with oxygen) or anaerobic (without oxygen). Many textile mills in urban areas discharge their effluent into municipal sewers. The sewage may be treated in an activated sludge plant. The effluent is mixed with micro-organisms, aerated and then the sludge allowed to settle. The phosphate and nitrogen nutrients needed for bacterial growth are not a problem if the industrial effluent is mixed with regular sewage. Activated sludge treatment considerably reduces the BOD by aerobic oxidation, and adsorption or coagulation of the contaminants. The effects are improved if the sludge is combined with activated carbon. This also protects the micro-organisms from heavy metals. They are sensitive to sudden changes in conditions and industrial effluents in the sewer system should be of relatively constant composition and concentration. The main problem is that of sludge disposal. After suitable treatment, it can be used as fertiliser, or for landfill. In a trickle filter plant, the effluent percolates through a filter bed with the bacteria growing on the surface of the filter medium. These aerobic microbial oxidation processes reduce the BOD, COD and TOC. The effect on colour from dyes, however, is often only marginal. Some dyes are adsorbed to some extent by the biological treatment. This has only limited effects in removing hydrolysed reactive dyes from the effluent. Because of the high levels of waste, textile auxiliary products should ideally biodegrade rapidly in water, although this is often not the case. Alkyl benzene sulphonate detergents, with a branched alkyl chain, such as that derived from propene tetramer, caused mountains of foam on rivers throughout the industrialised world in the 1960s because of their low rate of biodegradation. One of the most common effluent treatment methods is that of precipitation. This often involves a combination of precipitation of insoluble salts, coagulation of colloidal material and flocculation. This is similar in principle to the method used for clarifying water described in Section 8.1. Addition of lime (CaO) to the effluent is quite common. This neutralises any excess acidity and precipitates many types of anionic compounds. Treatments with alum or ferric chloride are also popular. The aluminium or ferric hydroxide, along with precipitated aluminium or ferric salts, removes colloidal matter and a number of anionic dyes. This reduces the COD, colour and suspended solids. Sedimentation is assisted by adding a flocculant such as a polyacrylic acid derivative, or a cationic polymer, the latter being able to bind hydrolysed reactive dyes. Again, the disposal of the sediment sludge needs to be considered. There are a variety of chemical technologies for effluent treatment. Few of these are used in the textile industry because of their cost. This situation may change as environmental protection becomes even more socially and politically acceptable. Chlorination with sodium hypochlorite and acid eliminates much organic material in waste water but may generate even more toxic organochlorine compounds. Oxidation by ozone is much safer but the cost of generating ozone by electrical discharge through oxygen gas is still prohibitive. Other technologies involve reverse osmosis and membrane filtration, adsorption on active carbon, or generation of coagulants by electrochemical techniques. The highest standards of effluent treatment require combinations of different types of treatment. These will probably become more significant, despite the expense, as regulatory controls are increasingly enforced.

DYEHOUSE EFFLUENT AND ITS TREATMENT

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Contaminants in dyehouse effluents For many years, the textile dyeing industry was a major source of water pollution. Increasing public awareness of this problem has resulted in much stricter legislation to protect the environment with controls on the types of contaminants and the amounts released. Governments have also become much more aggressive in updating legislation and in enforcing it. Water quality criteria are usually established for drinking water, and for surface and ground waters. The control of the discharge of effluent has become a major preoccupation for industries in Europe and North America. Legislation often sets maximum daily limits and a longer-term average limit for a whole range of contaminants. Controls in developing countries are much less stringent and, in some cases, almost non- existent. This has been a significant factor in the recent re-structuring of the dyestuffs manufacturing and textile finishing industries. There is little doubt that water quality standards will become increasingly important. Therefore, effluent treatment before discharge will be required, with increasing costs to the textile industry. The impact of textile effluent on aquatic life is an active field of research. There is considerable collaboration through ETAD (the Ecological and Toxicological Association of the Dyes and Organic Pigments Manufacturing Industry [3]) and ADMI (the American Dye Manufacturers Institute). Both have undertaken numerous studies in this area. Most textile effluent water is discharged into surface waters such as rivers and lakes, either directly or through municipal sewers. The main problem is the wide range of chemicals that it contains and the high level of dilution that usually exists. Table 8.4 lists some of the common pollutants. Of particular concern are those chemicals that are not degraded by water-borne bacteria, and which therefore persist and accumulate in the environment. Many such chemicals, such as polychlorobiphenyls, are extremely toxic and dangerous. Discharge of effluent to a municipal sewer has the advantage that the sewage treatment is able to remove many, but not all, of the contaminants. Provided the nature and approximate amounts of the contaminants are known, sewage treatment can reduce pollutant levels to the point at which discharge into surface water is then feasible. Because of the diversity of chemicals in textile effluent, it is usually characterised by a number of general criteria rather than in terms of specific contaminants. These include those described below.

(1) The volume of effluent.

(2) A measure of the amount of oxygen it will consume for oxidation of the organic chemicals it contains. This point is important because depletion of oxygen in water has a negative impact on aquatic life. The biological oxygen demand (BOD) [1] is the amount of o xygen (mg l–1 or ppm) consumed in 5 days at 20 ° C by growth of bacteria from a culture added to the water. The chemical oxygen demand (COD) [1] is based on a much faster chemical oxidation of organic compounds with hot sodium dichromate solution. The two values are often close but not equivalent. Many organic compounds are readily oxidised by hot dichromate but are resistant to microbial oxidation at ambient temperature. It is typical of textile effluent that the COD is much higher than the BOD. The COD is less affected by the usual effluent treatment processes, and is thus more persistent in the environment. Values range from 200 – 3000 mg O2 l–1 for BOD and from 500 – 5000 mg O2 l–1 for COD. The total organic carbon (TOC) in the water serves as an alternative to BOD and COD. All these can be determined by standardised analytical procedures.

(3) Floating insoluble chemicals, mainly insoluble oils and solvents.

(4) Suspended solid materials. These are quite diverse and include short fibres and insoluble dyes or compounds that have precipitated in the effluent because of a change in temperature or pH. Quantities range from 50 – 500 mg l–1. This can be estimated by filtration or by turbidity measurements.

(5) Colour. This is visible pollution. While it may not be toxic, colour does reduce light transmission into waters and limits photosynthesis. The dyeing industry discharges about 9% of the dyestuffs it consumes. This corresponds to a considerable degree of colour in a dyehouse effluent. Dyes are not easily biodegraded since, by design, they have good stability towards light and various chemical treatments. Most dyes are not of high toxicity and are eventually removed from water by oxidation or adsorption on sediment, but presence of colour in the water from a dyehouse is undesirable. It is a strong indicator of the presence of much higher quantities of dyeing assistants, almost all of which are present in the effluent. Even a simple chemical such as acetic acid can significantly increase the BOD.

(6) Acidity. The pH may vary from about 4 up to near 12. The acidity of water affects aquatic life and effluent must be neither too acidic nor too alkaline on discharge. This also applies for discharge into a municipal sewer because the micro-organisms used in sewage treatment are equally susceptible.

(7) Toxic chemicals. For the textile industry, the major offenders here are heavy metals such as chromium and copper, organochlorine compounds from insecticides or moth-proofing agents, and sulphides from dyeing with sulphur dyes. We also study about- Effluent treatment, Dye-house water treatment, Water treatment in dyeing, importance of water treatment in the dyeing, Water quality in dyeing, benefits of treated water in dyeing, different types of water treatment plant, Methods of water treatment.

BOILER WATER

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Boilers for steam generation have varying operating pressures and capacities. The use of a high pressure boiler allows a greater generation capacity but to minimise some of the problems discussed below requires a much higher water quality than a low pressure system. Temporary hardness in boiler feed water gives an accumulation of chalk scum in the boiler and scale on the walls and heating tubes. Deposition on the latter greatly reduces the rate of heat transfer and the boiler becomes increasingly less efficient. At high temperatures and pressures, both calcium and magnesium carbonates and magnesium hydroxide are much less soluble than under ambient conditions and contribute to scaling. Simple phosphates such as Na2HPO4, added to boiler feed water, will precipitate insoluble calcium and magnesium phosphate in a form that does not form a crust on the boiler walls and pipes. Polyphosphate sequestrants are less helpful since they tend to hydrolyse rapidly to simple phosphates in boiling water. Treatment with simple phosphates also ensures absorption of colloidal silica. This is significant, since silica scale on pipes and walls is very difficult to remove. The precipitation of calcium and magnesium in a non-crusting form in the boiler produces suspended material, the rate of accumulation being greater the higher the pressure and the capacity of the boiler. It is quite common to add dispersants such as polyacrylates to the feed water as these keep the precipitates well dispersed to prevent scaling. Discharge of the sludge is necessary from time to time to avoid excessive accumulation and to keep the salinity of the water within reasonable limits. In addition, dispersants and anti-foam chemicals prevent carry- over of sediment and foam with the generated steam. It is vital that there is minimum corrosion of the boiler and piping. This can be caused by acids, and by dissolved carbon dioxide and oxygen. Iron fittings corrode rapidly if boiler feed water is too acidic, so it is usual to condition the water to about pH 8 – 9 by addition of NaOH. This also ensures that all carbon dioxide is converted into bicarbonate. Too high a pH causes caustic embrittlement of non-ferrous metal fittings such as rivets. On the other hand, at pH values below 6, carbon dioxide can attack iron to form ferrous bicarbonate. This can occur in the boiler and in the piping for steam distribution and condensate return. Sometimes, volatile amines, such as ammonia or cyclohexylamine, are added to the water to neutralise any acidity and prevent this. Dissolved oxygen is a major source of corrosion. Preliminary heating removes most oxygen, since this gas is much less soluble in hot water. Alternatively, it reacts with reducing agents such as sodium sulphite or hydrazine added to the feed water. Sulphite is oxidised to sulphate but hydrazine has the advantage that it does not produce any ionic products.

Following matters are important in dye-house- Steam making with boiler, steam produce with boiler,The quality of steam produced from boiler, The capacity of boiler, pressure in the boiler, Mechanism of steam producing in the boiler, dye-house steam produce with boiler, End use of the boiler steam, The quality of water required to produce boiler water.

Sequestering agents

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Transition metal ions in the water supply can pose difficult problems in a dyehouse. Firstly, many of these ions catalyse the decomposition of hydrogen peroxide in bleaching baths. In addition, transition metals often give insoluble salts with dyes, or form complexes that are invariably duller and even different in shade. If the amounts are excessive, the water may be unusable without treatment. On adequate aeration of the water at pH values around 7, iron precipitates as insoluble Fe(OH)3 and can be removed. There is always concern about the possibility of iron from corrosion inside the water pipes in a textile plant. Addition of a sequestering agent to the water avoids many problems from relatively low concentrations of undesirable metal ions. Sequestering agents react with the metal ions to form very stable complex ions. Examples of such chemicals used in textile processing include EDTA, and related aminocarboxylic acids, as well as polyphosphates such as sodium tetrametaphosphate Na4P4O12. EDTA is a most effective sequestering agent, particularly in neutral or weakly alkaline solution. It forms such stable complexes with metal ions that it often removes the metal ion from a metal– complex dyestuff molecule, to give the EDTA – metal complex and uncomplexed dye. Polyphosphates can bind alkaline earth metal ions and thus decrease the effects of water hardness. The product Calgon, sodium hexametaphosphate Na6P6O18, is widely used for this purpose. Calcium ions replace sodium in the hexametaphosphate, forming a stable complex. The free calcium ion concentration in the water is then so low that calcium soaps do not precipitate. Polyphosphates are frequently present in domestic washing powders for the same reason.

Water softening with Ion exchange methods

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A zeolite is a naturally occurring insoluble mineral of the sodium aluminosilicate type. When hard water passes through a bed of small particles of such a mineral, an ion exchange reaction takes place (Scheme 8.10). As more and more alkaline earth metal ions are retained by the zeolite, its exchange capacity gradually decreases. Regeneration of the sodium salt of the zeolite involves passing a concentrated solution of NaCl through the zeolite. The calcium and magnesium ions are displaced and leave with the solution (the reverse of Scheme 8.10). After rinsing with water to eliminate the excess salt, the zeolite is ready for another cycle of softening.

Many zeolites occur in nature but they can also be manufactured. The removal of the alkaline earth metal ions is more effective the greater the surface area of the zeolite particles in contact with the water. For this reason, softening involves percolation of the water down a packed column of the finely ground zeolite, followed by periodic regeneration. Although the calcium and magnesium ions in the water are replaced by sodium ions, these are relatively harmless in textile processing.The chemical structures of aluminosilicates are based on the structure of silica.This consists of a three dimensional network of SiO4 units, in which the oxygen atoms have a tetrahedral arrangement around the central silicon atom. These tetrahedra may have common corners or faces. In an aluminosilicate, a number of aluminium atoms replace silicon atoms in the silica structure. The aluminium atoms are bonded to four tetrahedral oxygen atoms but because their atomic number is one less than silicon, each aluminium atom introduced has a negative charge, balanced by incorporation of a cation such as Na+ or K+. It is these cations that are available for exchange. The newer synthetic polymer ion exchangers are much more versatile than the zeolites and are widely used for water softening and demineralisation. They are often called ion exchange resins. Many are based on polystyrene that has been partly crosslinked by incorporation of a small amount of divinylbenzene (2 – 10%). Suspension polymerisation of the styrene and divinylbenzene produces the crosslinked polymer in the form of small beads. These have the appearance of a fine white sand. Sulphonation of some of the benzene rings, mainly on the bead surfaces, provides anionic sites suitable for cation exchange (Figure 8.2). Since a sulphonic acid is comparable in strength to a mineral acid, these are called strong acid exchangers. They may absorb cations and release hydrogen ions, or, in the form of their sodium salts, exchange sodium ions for other cations in the water

Weak acid cation exchanger, containing acrylic acid (ethenoic acid) units ( 1 ); strong basic anion exchange resin, containing quaternary ammonium hydroxide groups (2 );weakly basic anion exchange resin with protonated secondary amino groups (3 ) The so-called weak acid cation exchangers are usually polymers containing carboxylic acid groups, such as vinyl polymers containing acrylic acid (ethenoic acid) units (1, in Figure 8.3). These are used in the form of their sodium salts. They do not exchange hydrogen ions for cations below pH 5 since the free carboxylic acid groups are barely dissociated at low pH values (Scheme 8.12). Other types of ion exchange resins are available for anion exchange. These may be of the strong basic type, containing quaternary ammonium hydroxide groups (see Figure 8.2, and 2 in Figure 8.3), or weakly basic with protonated secondary amino groups (3 , in Figure 8.3). Either type will have associated counter anions that exchange with anions in the solution in contact with the resin. The weakly basic types have ammonium ion groups that can lose a proton in contact with an alkaline solution and therefore are ineffective above pH 10 (Scheme 8.13). The ion exchange processes are again reversible so resin regeneration simply involves treatment with a concentrated solution of a salt containing the appropriate anion.

Therefore, a strongly basic quaternary ammonium ion resin is regenerated using NaOH solution, and a secondary amine type with NaCl or HCl solution. The two most important properties of an ion exchanger are its capacity and its selectivity. The capacity is the number of ions that a given mass of resin is capable of binding by exchange, normally expressed in milliequivalents per gram of dry or of wet resin. For example, 1.0 mmol of Na+ is the same as 1.0 mequiv, but 1.0 mmol of Ca2+ is 2.0 mequiv. Therefore, a resin with a capacity of 15.0 mequiv g– 1 would be capable of binding 15.0 mmol g– 1 Na+ or 7.5 mmol g–1 Ca2+. The selectivity of the resin determines how strongly it binds a given ion and therefore its ease of exchange. Clearly, for water softening a cation exchange resin should have a higher selectivity for Ca2+ and Mg2+ than for H+ or Na+. Fortunately, the selectivity is often greater for ions of higher ionic charge. Besides these two properties, the degree of swelling of the resin in contact with the water must be limited. The higher the degree of crosslinking, for example from incorporation of more divinylbenzene in the polystyrene, the lower the extent of swelling. It is also important that the resin particles have a large surface area and that water is able to penetrate into the surface pores. Water can be totally demineralised by firstly exchanging all cations using a strongly acid form of a cation exchanger. Thus, a solution of salts M+X– becomes a solution of acid H+X–, the M+ ions being retained by the resin. Subsequent percolation through a packing of a strongly basic form of an anion exchanger absorbs the X– ions and liberates HO– ions into the water. These then neutralise the H+ ions from the first step. The result is retention of all anions and cations and the neutralisation of H+ and HO– to form water (Scheme 8.14). Thus, the

water has been demineralised. It may, however, still contain organic material and dissolved carbon dioxide from the reaction of carbonate and bicarbonate with the acid from the resin. A thorough aeration eliminates the carbon dioxide. Demineralisation is important for water fed to very high pressure boilers. The use of ion exchange resins for water treatment is relatively simple. The resin is packed into a column containing water and treatment simply involves flowing water up or down the column. The capacity of the resin and the ionic content of the water determine when regeneration will be required. One problem with beds of ion exchangers is the retention in the column of suspended matter and living organisms in the water. Countercurrent rinsing and occasional treatment with a bactericide minimise these problems. For removal of both cations and anions (demineralisation), two columns in series are used, the first for strong acid exchange and the second for strong base exchange. It is even possible to mix anion and cation exchangers in the same bed. If the different types of particles have different densities, they can be separated by sedimentation in a counterflow of water, regenerated separately, and then re-mixed. Figure 8.4 shows a typical series of processes for water softening.

Water Softening with lime-soda process

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The old lime-soda process is now obsolete but was very useful for the treatment of large volumes of hard water. Addition of lime (CaO) and soda (Na2CO3) to the hard water precipitates calcium as the carbonate, and magnesium as its hydroxide. The amounts of the two chemicals required are easily calculated from the analysis of the water and stoichiometry of the reactions (Scheme 8.9). Since calcium

carbonate and magnesium hydroxide are not completely insoluble, the water obtained has a residual hardness of not less than 20 ppm CaCO3. A major problem of this type of process is the disposal of the sludge of precipitated calcium carbonate and magnesium hydroxide.

WATER SOFTENING

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Soft water is a desirable prerequisite for all textile wet processes, except for bleaching with solutions of hydrogen peroxide stabilised by sodium silicate. Modern synthetic detergents do not form precipitates in hard water containing calcium and magnesium ions so a certain degree of hardness is tolerable provided that other dyes and chemicals are not seriously affected by this. In general, however, if a mill is in a region where the water is hard, a softening pretreatment is essential for at least part of the water used. The objective of this is simply to reduce the concentration of the alkaline earth metals to a level at which the water has the desired quality. In many cases, the softening process may reduce the calcium and magnesium ion concentrations to zero.

Water alkalinity

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Various dissolved salts in natural water give it an alkaline pH. These are mainly carbonates, bicarbonates and phosphates (Scheme 8.7). The alkalinity of a water sample can be determined by titration with standardised sulphuric acid solution. With phenolphthalein as the indicator, the equivalence point occurs at about pH 8.3. At this point, all free hydroxide ion is neutralised, carbonate converted into bicarbonate, and phosphate (PO43–) into monohydrogen phosphate (HPO42–). The alkalinity to pH 8.3 is calculated in ppm CaCO3 based on Scheme 8.8. Therefore, each millimole of sulphuric acid required corresponds to 1.00 mmol CaCO3 in the water sample. If the titration of a 100.0 ml water sample with 0.01 M sulphuric acid consumes V ml of acid, the alkalinity is given by:

At the phenolphthalein end-point of pH 8.3, the water is still slightly alkaline and it is therefore common practice to titrate the sample to a methyl orange end-point, corresponding to about pH 4.5. This ensures that all carbonate is in the from of carbonic acid and all phosphate is present as dihydrogenphosphate (H2PO4–). The alkalinity to pH 4.5, often called the total alkalinity, is given by the same equation as above, but, of course, the volume of sulphuric acid solution may be greater.

Table 8.3 relates the total (T) alkalinity and phenolphthalein alkalinity (P) of various types of water to the dissolved salts that are present. In the absence of phosphate ions, there will be some correlation between the alkalinity caused by bicarbonate and the temporary hardness. For example, if the alkalinity to

phenolphthalein is zero (P = 0), the total alkalinity, assuming no phosphates or weakly acidic pollutants, will be caused by bicarbonate ion and the total alkalinity in ppm CaCO3 will equal the temporary hardness. The alkalinity of water fed to boilers must not be too high because of the risk of corrosion of non-ferrous fittings.

Determination of water hardness

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The hardness of a water sample can be determined by direct titration of a known volume with a standardised solution of a soap such as potassium oleate. Initially, this soap will precipitate, and the calcium and magnesium salts and the water will not produce any foam on shaking. Gradual addition of the potassium oleate solution is continued until the agitation of the solution produces a reasonably permanent foam. F rom Scheme 8.4, 1.00 ml of 0.02 M potassium oleate solution 2R– K+ (aq) + Ca2+ (aq)= R2Ca(s) + 2K+ (aq) R= Oleate radical is equivalent to 0.01 mmol Ca2+. Thus, if a 100.0 ml sample of water is titrated with 0.02 M potassium oleate solution to just give a permanent foam after addition of V ml of soap, equivalent to V 0.01 mmol Ca2+, the hardness willbe V 10 ppm CaCO3.Despite the simplicity of this method, it is not always possible to detect precisely the point at which a layer of foam persists. Hardness is therefore more usually determined by titration with a standardised solution of the disodium salt of ethylenediamine tetra-acetic acid (EDTA). EDTA forms very stable, soluble complexes with calcium and magnesium ions (Figure 8.1). EDTA is a tetraprotic acid and the various acid–base forms are usually abbreviated by H4Y, H3Y–, H2Y2–,HY3–,and Y4–, as the degree of dissociation progresses. Scheme 8.5 illustrates complex formation with calcium and magnesium ions. The equilibrium constants are large and ensure quantitative formation of the colourless complexes on addition of EDTA to a solution containing calcium and magnesium ions. The titration involves gradual addition of standardised EDTA solution(Na2H2Y) to the water sample, buffered at pH 10 with an ammonia–ammonium

chloride solution. The indicator is a dye that changes colour on complexing with calcium or magnesium, such as Eriochrome Black T (CI Mordant Black 11). At the beginning of the titration, the large concentrations of calcium and magnesium ions ensure that all the dye indicator is in the form of the red complexes with these metals (Ca–Dye–, Scheme 8.6). Very close to the equivalence point, the free Ca2+ and Mg2+ ion concentrations become so low that the added EDTA begins to remove the metals from their red complexes with the dye. This liberates the blue dye ion. The colour of the solution therefore changes from red (dye–metal complex) to blue (free dye anion) at the end-point.

In the titration of calcium ions alone at pH 10, the red calcium–dye complex(Ca–Dye–) is not sufficiently stable and the calcium transfers to EDTA liberating the blue dye anion (H–Dye2–) slightly before the true equivalence point. For titrations of magnesium ion, the magnesium–dye complex is more stable than that of calcium and the blue colour does not occur until all the Mg2+ ions in solution have reacted with the EDTA. Thus, the red-to-blue colour change occurs at the true equivalence point. Towards the end of the titration of hard water, as long as calcium ion is present, it will liberate magnesium ion from the magnesium–EDTA complex because the calcium–EDTA complex is more stable. The liberated magnesium thus ensures that the more stable magnesium–dye complex is always present, until all the calcium has been complexed with EDTA. The last addition of this reagent then removes the magnesium from the red magnesium–dye complex and the solution becomes blue. To ensure that a water sample contains a sufficient quantity of magnesium, it is common to add a little to the EDTA solution before standardisation with a known calcium ion solution.

Various units used for water hardness

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Various units used for water hardness Unit of water hardness ppm CaCO3 1 British degree 14.3 1 American degree 17.2 1 French degree 10.0 1 German degree 17.9 1 mmol/L CaCO3 100

Definition of Hardness

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Soft water is relatively free of calcium and magnesium ions. It produces a rich foamy lather with soap. This is essential for the soap to be an effective emulsifying agent for oils and dirt. With hard water, the soluble sodium salt of soap reacts with the alkaline earth metal ions and precipitates as the useless and undesirable calcium or magnesium soap (Scheme 8.2). The cleaning ability is lost. Hardness is defined as the presence of soluble calcium and magnesium salts in the water. If these are present in the form of bicarbonates, the hardness is temporary. Heating hard water containing bicarbonates eliminates dissolved carbon dioxide and the reverse of Scheme 8.1 causes precipitation of calcium carbonate. Magnesium carbonate is slightly soluble in water but heating will cause its hydrolysis into the much less soluble magnesium hydroxide (Scheme 8.3). Simply boiling and filtering the water therefore eliminates temporary hardness. In regions where water has high temporary hardness, and is used directly without treatment, it is not uncommon to see hot water rinsing and washing baths with a generous crust of chalk (CaCO3) on the inner surfaces. This type of precipitation inside a boiler is also undesirable because the scale reduces the efficiency of heat transfer. Mg(HCO3)2(aq)= MgCO3(s) + CO2(g) + H2O MgCO3(aq) + H2O =Mg(OH)2(s) + CO2(g) Permanent hardness arises when water contains soluble salts of calcium and magnesium such as chlorides and sulphates. It is unaffected by boiling the water. The total hardness of water is determined by the amount of dissolved calcium and magnesium, but expressed in the form of CaCO3. This is possible because calcium and magnesium carbonates have identical stoichiometry. Therefore, water that contains 1.0 mM calcium and 0.5 mM magnesium ions, has a total concentration of 1.5 m M . Since the molecular weight of CaCO3 is 100 g mol–1, 1.5 m M corresponds to 150 mg l–1 of CaCO3, or 150 ppm (parts per million), assuming that 1.0 l of the water has a mass of 1.0 kg. The permanent hardness is that remaining after the water has been boiled and filtered to remove the precipitated calcium and magnesium carbonates and magnesium hydroxide. The temporary hardness is the difference between the total and permanent hardness.Occasionally, hardness due to magnesium alone is of interest, again expressed as the equivalent amount of CaCO3. For the water mentioned above, 0.5m M of magnesium ions is equivalent to 50 mg l–1 or 50 ppm CaCO3. Understanding water hardness values is often a tedious exercise in manipulation of units. In the past, the hardness of water was given in units called degrees. The definition of one degree of hardness, however, varied from one country to the next (Table 8.2). It is much simpler to express hardness in mg l–1 or ppm of CaCO3.

Consequences of using hard water

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Hard water using in a textile dyeing or finishing mill can have some serious consequences. Below some consequences- (1) precipitation of soaps; (2) redeposition of dirt and insoluble soaps on the fabric being washed – this can cause yellowing and lead to unlevel dyeing and a poor handle; (3) precipitation of some dyes as calcium or magnesium salts; (4) scale formation on equipment and in boilers and pipelines; (5) reduction of the activity of the enzymes used in desizing; (6) decreased solubility of sizing agents; (7) coagulation of some types of print pastes; (8) incompatibility with chemicals in finishing recipes.

Typical Dye House Water Quality

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Dyeings Water Quality Component Permissible Concentration mg/L Hardness 0-25 CaCO3 Iron .02-.1 Manganese .02 Silica .5-3.0 Alkalinity to Ph 4 35-65 CaCO3 Dissolved Solids 65-150 The water may pass from the primary source into a settling pond, from which water can be drawn after any small particles have settled to the bottom, or it may be filtered by passage through a bed of fine sand. For particles smaller than about 0.02 mm, a coagulant aids sedimentation and clarification of the water. Addition of small quantities of sodium aluminate (NaAlO2) or aluminium sulphate, and adjustment to around pH 7, give a gelatinous precipitate of aluminium hydroxide that imprisons the small particles and accelerates sedimentation or filtration. Addition of a flocculant, such as a polyacrylic acid–polyacrylamide copolymer, gives better particle adhesion and a higher rate of sedimentation. This type of treatment also improves the colour of the water. Soluble impurities in the water, such as iron, pollutants, and organic matter, are a real problem. Good aeration of the water will usually convert iron into Fe3+, which precipitates as ferric hydroxide unless the water is quite acidic.

WATER QUALITY FOR THE DYEHOUSE

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During the Industrial Revolution, the textile industry invariably grew in regions having an abundant supply of soft water such as in West Yorkshire. Such water is relatively free from calcium and magnesium salts. It does not precipitate soap or other chemicals during washing, or form a scale on the boiler walls when generating steam. Water for a textile plant may come from various sources. These include surface water from rivers and lakes, and subterranean water from wells. The water may be obtained directly from the source or from the local municipality. Natural and pretreated water may contain a variety of chemical species that can influence textile wet processing in general, and dyeing in particular. The various salts present in water depend on the geological formations through which the water has flowed. These salts are mainly the carbonates (CO32–), hydrogencarbonates (HCO3–, more commonly named bicarbonates), sulphates(SO42–) and chlorides (Cl–) of calcium (Ca2+), magnesium (Mg2+) and sodium(Na+). Although calcium and magnesium carbonates in limestone are relatively insoluble, the action of dissolved carbon dioxide in rain water can leach them out in the form of the more soluble bicarbonates (Scheme 8.1). Hard water contains calcium and magnesium ions and gives an immediate precipitate with soap. Soap is a water-soluble sodium salt of a high molecular weight carboxylic acid, such as stearic acid. Its calcium and magnesium salts are much less soluble and give a gummy precipitate with a soap solution in hard water (Scheme 8.2). When this happens, the cleaning efficiency of the soap decreases dramatically. The dirt in suspension, as well as the precipitated calcium and magnesium soaps, can deposit back onto the material being washed. This gives a dingy fabric with a poor handle that will be difficult to dye in a level shade. CaCO3(s) + CO2(aq) + H2O(l) = Ca(HCO3)2(aq) Scheme 8.1 2CH3(CH2)16CO2Na+ (aq) + Ca2+ (aq) = (CH3(CH2)16CO2-)2 Ca2+ (s) + 2Na(aq) Scheme 8.2 Besides dissolved salts of natural origin, water may contain a variety of other salts from human or industrial activity. These include nitrates (NO3–), phosphates(HPO42– and H2PO4–) and various metal ions. The ions of certain transition metals, such as those of iron, copper and manganese, can precipitate dyes, or form complexes with them, dulling or even changing their shade. In addition, many of these ions catalyse the decomposition of hydrogen peroxide in bleaching baths. It is not uncommon for a particle of iron rust on the surface of a cotton fabric to generate such rapid peroxide decomposition that the cellulose is totally oxidised and a hole results. These transition metal ions may be present in the natural or municipal water, or, in the case of iron, come from corrosion within pipelines. Both dissolved and colloidal silica in boiler feed water can lead to the formation of a hard resistant scale on the boiler’s internal surfaces, considerably decreasing heat transfer efficiency. Organic compounds from the decomposition of vegetable matter in the water, or from sources of pollution, can be very varied. They can have such high concentrations that the water is coloured. Table 8.1 illustrates the quality of water usually considered acceptable for textile processing and steam generation. Water entering a textile dyeing or finishing plant usually passes through a grill to eliminate floating debris. Small particles in suspension must also be removed.

Extraordinary video

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Water treatment

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Water treatment In textile finishing, water consumption is far greater than the amounts of fibres processed. It is the ubiquitous solvent for the solutions of chemicals used. Rinsing and washing operations alone consume enormous amounts of water. Steam is still the major heat-transfer medium for many processes and the quality of water fed to boilers is often critical. The volume of aqueous effluent leaving a textile finishing plant is very close to the total input for the many different wet processes. The effluent consists of a relatively dilute solution of a considerable number of chemicals, including acids, alkalis, salts, dyes and other organic chemicals. Any presentation of textile dyeing would be incomplete without some discussion of the quality of water required for textile wet processing, and of the treatment of waste water so that it can be discarded with minimum environmental impact. Important topics for water treatment- Water treatment in the textile, Water treatment in the leather, Reprocessed water using in the dyeing, Total amount of water required in knit dyeing, Procedure of water treatment, The result of water treatment in dyeing.