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Approximately 850-1,000 tons of water is used to manufacture 1 ton of raw silk. This chapter covers water issues pertinent to silk production.

9.1   Type of Water

All natural water sources contain minerals. The mineral constituents found in greatest abundance are bicarbonates, sulphates, calcium chlorides, magnesium and sodium. Carbon dioxide contents may be greater than expected as a result of the decomposition of organic matter. Three water sources are described here: well/spring water, lake water and surface water/river water.

  1. Well water is usually clear with a consistent composition. It may contain sodium bicarbonate, calcium bicarbonate, magnesium bicarbonate, iron and carbon dioxide.
  2. Lake water can be clear but may be coloured and slightly acidic. High acidity combined with the presence of dissolved gases makes the source corrosive. Although lake water contains some calcium, magnesium chlorides and sulphates, the concentration of electrolytes is low.
  3. Surface water contains sulphates, chlorides, calcium bicarbonate and magnesium bicarbonate. The content of minerals and impurities is determined by the profile of the area from which the water originates, plus seasonal changes. Water quality is affected by the amount of industrial affluent discharged into canals and rivers.

9.2   Impurities

Impurities in water are discussed under the following headings"

    1. Turbidity and colour
    2. Alkalinity
    3. Hardness
    4. Iron and manganese
  1. Turbidity and colour. Turbidity may be caused by large or small mineral and organic particles suspended in the water. Mineral particles may include clay, silt, calcium carbonate and silica. Organic particles may include fine vegetable wastes, fats and microorganisms. Sedimentation in tanks or reservoirs is adequate to clear large particles, but filtration is needed for small or colloidal fragments. Filtration using sand or the use of coagulants followed by filtration may be indicated. The colour in water signals the presence of dissolved or colloidally dispersed organic matter of unknown composition augmented colloidal iron or manganese.
  2. Alkalinity. Raw water contains bicarbonates in amounts dependent on their origin. Often, water may contain small amounts of carbonate alkalines. The number of bicarbonate and carbonate ions present may be isolated by titration using phenolphthalein as an indicator and methyl orange as an indicator in the second stage.
  3. Hardness of water. Water hardness is caused by calcium and magnesium salts. Other contributing elements to hardness include metallic ions such as iron and strontium. Note that if metallic ions are present they will be found in minute quantities compared to salts.
  4. Hardness is described as permanent and temporary; where permanent is caused by nitrates, chlorides and sulphates, and bicarbonates, which may be boiled to precipitate into carbonates and removing the hardness, cause temporary.

    Ca(NCO3)2 CaCO3 + H2O + CO2

    Hardness can be measured when a given amount of potassium palmitate or oleate solution is titrated using a given amount of water, concluding when the shaken liquid produces a permanent lather.

    More sophisticated methods are now available where the concentrations of calcium and magnesium ions are quantified by titration with ethylene diaminetetra-acetic acid. Hardness is expressed in degrees, but the actual units of measure vary from country to country.

    1 unit (CaCO3 ppm) of United States is equivalent to 0.056 dH of Germany.

  5. Iron and manganese. Iron and manganese may be found in water depending on the source of the contamination. The three types of impurities are: (a) ferrous and manganous bicarbonates in well waters when high amounts of free CO2 are present, (b) ferrous and manganous sulphates in rivers containing acid mine-waste waters, or (c) iron and manganese mixed with organic waste. Iron is present in some waters at the source, but corrosion in pipelines and storage vessels may taint water.

If a given source is alkaline, iron in the form of ferrous bicarbonate may be removed by aeration when ferric hydroxide is precipitated and carbon dioxide released. Aeration also helps to raise the pH by reducing the content of dissolved carbon dioxide.

4Fe(HCO3)2 + O2 + 2H2O 4Fe(OH)3 + 8CO2

Manganese can be released in the same way but to ensure rapid oxidation, a pH greater than 10 is required through the addition of lime or caustic soda.

9.3   Methods of Water Softening

This section looks at industrial water treatment using lime-soda processing, base exchange and demineralization to remove all dissolved salts.

  1. Lime-soda process. In this method, hydrated lime and sodium carbonate are added to precipitate calcium and magnesium ions as compounds of low solubility. For temporary hardness arising from bicarbonates, the reactions are as follows:

  2. Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O

    Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 + 2H2O

    Calcium carbonate has a much lower solubility than magnesium carbonate. When water containing bicarbonates of calcium and magnesium is treated, it is the calcium carbonate, which is first deposited. For permanent hardness, the reaction is:

    CaSO4 + Na2CO3 CACO3 + Na2SO4

    MgC12 + Ca(OH)2 CaC12 + Mg(OH)2

    The CaC12 formed in the last reaction is removed as carbonate by the sodium carbonate. It may be noted that, when temporary hardness is eliminated by this method, no salts remain but permanent hardness, an equivalent quantity of sodium salts is left behind.

  3. Base or cation exchange. This process relies on the exchange of calcium and magnesium ions, which occur when water is passed through a bed composed of complex aluminium silicates commonly known as Zeolites.
  4. If Z represents the silicate, the reaction may be represented as follows:

    Temporary hardness           Ca(HCO3)2 + Na2Z CaZ + 2NaHCO3

                                              Mg(HCO3)2 + Na2Z MgZ + 2NaHCO3

    Permanent hardness            CaSO4 + Na2Z CaZ + Na2SO4

                                              MgSO4 + Na2Z MgZ + Na2SO4

    In contrast to the lime-soda process, the exchange process replaces the calcium and magnesium ions by an equivalent of sodium even when the hardness is temporary. The anions present are unaffected. The bed is made from Zeolite in granular form, sieved to a particle size around 1-2 mm in diameter; its capacity is limited and will eventually be depleted.

    However, the process is reversible and employing a concentrated salt solution can regenerate the bed:

    CaZ + 2NaC1 CaC12 +Na2Z

    When the calcium and magnesium ions are removed as soluble chlorides, they are replaced by sodium. A cyclic procedure is created of softening, regeneration, water wash (to remove excess salt), softening, etc.

    Between the softening and regeneration, it is sometimes desirable to backwash by passing water upwards through the bed for a short time to remove any matter, which has filtered out of the bed. Water from this process may have hardness values as low as 0.5 ppm.

  5. Demineralization. If two exchange processes are used, one for the anions and one for the cations, the content of mineral salts may be reduced to that present in distilled water. Removal of the metal ions may be carried out with one of the sulphonated resins in the "Hydrogen" form, so that water running out contains hydrogen instead of metal ions.

          CaC12 + H2R CaR + 2HCL

          Ca(HCO3)2 + H2R CaR + 2CO2 + 2H2O

The resultant carbonic acid breaks down into water ad carbon dioxide, which can be removed by aeration, but the mineral acids remain. Regeneration of the exchange process may be carried out by the use of hydrochloric acid.

Table 29. Ion-exchange types and the quality of water treated (K.E. Song and Y.W. Lee, 1973)

Type of treatment

Original water

Base type

Hydrogen type





M-alkalinity (CaCO3 mg/1)




Acidity (CaCO3 mg/1)




Whole hardness (CaCO3 mg/1)




9.4  Quality Standard of Water

It is essential to carefully select the quality of reeling water, as it impacts reeling efficiency, raw silk percentage of cocoons and the raw silk quality. Table 31 gives a standard for quality of reeling water and Table 31 shows the pH values and water hardness desirable for cocoon cooking water.

Table 30. Standard quality of reeling water (B.H. Kim, 1983)


Standard concentration

Range of concentration

(1) Colour and cleanness

Colourless and clear

(2) smell

No smell

(3) Suspension and sediment


(4) pH of water



(5) pH of water after being boiled



(6) specific electro conductivity (micrombo/cm)



(7) Hardness ( dH) dH x 17.85 – CaCO3 ppm



(8) M-alkalinity (CaCO3 ppm)



(9) Total acidity (CaCO3 ppm)



(10) Heavy metal iron (Fe2O3 ppm)

Under 0.1


(11) Residue after evaporation (ppm)




Table 31. pH values and water hardness suitable for cooking parts



Good reelable cocoons

Poor reelable cocoons

Dipping part




Water hardness ( dH)



Low temperature permeating part




Water hardness ( dH)



Cooking adjusting part




Water hardness ( dH)



Finishing part




Water hardness ( dH)




9.5   Analysis of Reeling Water

  1. pH. A pH meter can measure the value of pH.
  2. Acidity. A 50 ml water sample is taken from the original water and 5 drops of phenolphthalein indicator added. Titrate with the N/50 of NaCO standard solution until the yellowish colour of the sample water changes to purple.

  3. Notice:  a, Total acidity (CaCO3)
                 b, Amount of N/50 NaOH sol. Consumed for titration (ml)
                 c, Amount of sample water (ml)
    * Phenolphthalein indicator: 0.15g phenolphthalein and 0.05g thymol blue
    dissolved in 100 ml of 50% ethanol.

  4. M-alkalinity. A 50 ml of water sample is taken from the original water and 5 drops of methyl red indicator added. Begin titration by using the N/50 N2SO4 standard solution. Terminate when the bluish colour of the water sample changes to purple (pH 4.8).

  5. Notice:  a, M-alkalinity (CaCO3)
                 b, Amount of N/50 H2SO4 sol. Consumed for titration (ml)
                 c, Amount of sample water (ml)

    *Methyl red indicator: 0.02g methyl red and 0.1g bromcresol green dissolved
    in 100 ml water

  6. Water hardness. A 50 ml water sample is poured into a beaker. pH 10 is reached by adding 1 ml of Buffer solution and a few drops of E.B.T. indicator to the water sample, shaking it for a while and then begin the titration by using the E.D.T.A. standard solution. Terminate when the reddish colour of the water sample changes to a bluish colour.

dH = a x 0.056

           |Notice:   a, Total hardness (CACO3)
                          b, Amount of N/50 H2SO4 sol. Consumed for titration (ml)
                          c, Amount of sample water (ml)

* Buffer solution: the total volume to be 1 litre by adding the distilled
water after 67.5 g of NH4C1 is dissolved into 570 ml of NH4OH.
*E.B.T. indicator: the total volume to be 100 ml by adding the ethanol after mixing 9.5g of Eriochrom Black T. and 30 ml of triethanolamine.
*E.D.T.A. Standard solution: 3.7225g of the purified Ethylene diamine Tetraacetic Acid disodium salt is dissolved into distilled water and until a total volume of 1 litre is achieved.

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