Water Flashcards
Disadvantages of Hard Water
Sugar Industry - Contains sulphates, nitrates which interfere in crystallization of sugar.
Dyeing Industry - Causes imperfect shades and uneven spots.
Paper Industry - Causes pigmentation on paper due to presence of heavy metal ions and transition metals.
Boiler Trouble - Sludge formation, and scales which stick to the boiler surface and causes fuel wastage.
Laundry - Wastage of soaps and Water
Types of Hardness
Water containing hardness gives wine red color with Eriochrome Black –T indicator.
Temporary - Caused by carbonate, bicarbonate and hydroxide of calcium and magnesium ions in the water. It can be removed by boiling water or by the addition of lime (Ca(OH)2).
Boiling promotes the formation of carbonate from the bicarbonate and precipitates calcium carbonate out of solution.
Ca (HCO3)2 → CaCO3 ↓+ H 2 O + CO 2
Permanent - Caused by the presence of sulphates, chlorides, nitrates of calcium and magnesium and other metal ions
which become more soluble on boiling, can be removed using water – softener or ion-exchange column,
Units of Hardness
ppm - number of parts of CaCO3 equivalent in 10^6 parts of water.
mg/lit - number of grams of CaCO3 equivalent per 1 liter of water.
Clarke’s degree - No. of CaCO3 equi per 70000 parts of water
French degree - No. of CaCO3 equi per 10^5 parts of water
1ppm = 1 mg/lit = 0.1 0 Fr = 0.07 0 Cl
Preparation of Solutions
Standard Hard Water - 1 gm of dry CaCO3 is dissolved in minimum quantity of HCl and evaporate the solution to dryness on a water bath, and then diluted to 1 lit with water. Each ml of this solution then contains 1 mg of CaCO3 hardness.
EDTA solution: 4 gm of EDTA crystals + 0.1 gm MgCl2 in 1litre.
Indicator: 0.5 gm of EBT in100 ml of alcohol.
Buffer solution: 67.5 gm NH4Cl + 570 ml of Con. Ammonia solution diluted with distilled water to 1 lit.
Cold Lime Soda Process
Lime is added either as calcium hydroxide or calcium oxide, and soda is added as sodium carbonate.
Cold Lime Soda :
Water treated with mixture of lime and soda and coagulants at 25-30 degree centigrade.
Inner cylindrical tank ensures complete mixing of all elements.
Outer tank sludge settles.
Wood fiber filters used to completely remove sludge from water.
Slow Process.
Dissolved gasses not removed.
Difficult to filter.
Stirring is essential
Residual hardness is : 5-60 ppm
Hot Lime Soda Method.
At 94 - 100 degree centigrade.
Consists of :
Reaction Tank - Raw water and chemicals mixed
Conical sedimentation vessel : Sludge settles down
Sand filter : To ensure complete removal of sludge from softened water.
Coagulants not required.
Stirring not essential.
Fast process.
Filtration is easy.
Dissolved gasses get removed.
Residual hardness : 15-30 ppm
Advantages of hot lime-soda method over cold lime-soda method
Economical
pH value of softened water is higher hence lesser corrosion.
Alkaline nature of water reduces pathogens and bacteria.
Fe and Mn can also be removed to a certain extent.
Disadvantages of Lime-soda method
Softened water is 50 ppm hardness in cold
lime-soda method and 15-30 ppm hardness in hot lime-soda method.
Such water can not be used in high pressure boilers
Disposal of large quantity of sludge is a big problem
Zeolite softener method
Sodium-Zeolites which are hydrated sodium alumino silicates, capable of exchanging their sodium ions with hardness producing ions are used.
Na2O.Al2O3.xSiO2.yH2O
(Where, x= 2-10 & y= 2-6).
Process :
Hard water passed over zeolite bed at specific rate.
The hardness causing ions are replaced with sodium ions which do not cause hardness.
After a while zeolite is exhausted, to regenerate it 10% Brine is passed over the bed.
Advantages and disadvantages of Zeolite softener method
Advantages :
Water is 10 ppm
No sludge
Compact and easy equipment
Low Maintenance
Disadvantages :
Suspended impurities block zeolite pores
Colored ions like Fe3+ and Mn2+ form strong bonds with zeolite and cant be removed.
Mineral acids destroy zeolite.
Treated water high in sodium salts
Only cations removed, anions left behind.
Ion exchange softener method
Ion exchange resins are insoluble, cross-linked, long chain organic polymers with micro-porous structure. The functional groups attached to the chains are responsible for
ion exchanging property.
Cation Exchanger resins (RH+) :
Resins containing acidic functional groups (-COOH, -SO3H etc.)
Anion Exchanger resins (ROH-) :
Resins containing basic functional groups (-NH2-OH, =NH-OH, -NMe2-OH etc.)
Regeneration and softening process in Ion exchange.
Softening :
Hard water first passes over cation exchanger resin to remove Ca, Mg, these will be retained on resin column and are exchanged with H+
Then water is passed over anion exchanger to exchange So4, Cl, CO3, and exchanged with OH-.
Regeneration :
After certain age both resins get exhausted.
Cation exchange resin is regenerated by passing dil. HCL.
Cation resin is regenerated by passing dil NaOH.
Advantages and Disadvantages of Ion Exchange
Advantages :
Cn be used for highly alkaline or acidic water.
Produces water of 2 ppm
Softened water can be used in a steam generation boiler.
Disadvantages
Equipment is costly and expensive.
Turbid water could decrease the efficiency of this method hence turbidity has to be removed first by coagulation.
Biological Oxygen demand
It is a measure of amount of oxygen required for the biological oxidation of organic matter under aerobic conditions at 200 C for a period of 5 days.
Direct measure of extent of pollution in waste water.
Only applicable for biodegradable pollutants.
Drinking water should have BOD less than 1ppm
Initial oxygen content is measured, then sample is kept inside an incubator in the presence of micro organisms for 5 days at 20 degree centigrade. Decrease in dissolved oxygen is measured.
Formula :
BOD = [(DO)Blank – (DO)Incubated ] X Dilution Factor
Dilution factor : Volume after dilution/volume before dilution.
Chemical oxygen demand
It is a measure of amount of oxygen required for the chemical
oxidation of organic matter when refluxed in acidified potassium dichromate in the presence of Ag2SO4 or HgSO4 catalyst for 3 hours.
Applicable for both bio-degradable and non bio-degradable pollutants.
Higher the COD, higher will be the level of pollutants in water.
Drinking water should have COD less than 1 ppm.
Known water sample is refluxed with standard potassium dichromate and dilute sulfuric acid in the presence of a silver sulphate catalyst for 3 hours.
The unreacted potassium dichromate is titrated against Ferrous
ammonium sulphate (Mohr’s salt) with ferroin indicator till blue color
changes to wine red, the reading is (Vt).
This gives the amount of potassium dichromate consumed (in terms of
equivalent oxygen) required for degradation of organic pollutants.
Blank titration is performed, the reading is (Vb) (zero minute reading)
COD = [(Vb - Vt) * Nfas * 8 * 1000]/Y mg/Lit
Vb = Volume of FAS required for blank titration (Zero time)
Vt = Volume of FAS required for reaction mass after time (t)
Y = Volume of waste water sample taken
