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RAW WATER ANALYSIS

Raw Water Analysis gives details about quantity of various impurities present inthe water. The quantities are normally given in terms of Parts Per Million (PPM).One part part million or one PPM means. I gram of the impurity in 1 Tonne or 1

M3

of water. One ppm also means 3 mg in one litre of water.

Substances are expressed as PPM as such or PPM as CaCO3.

1 PPM Ca as Ca means 1 m3 of water contains 1 gm of calcium.

1 PPM mg as Mg means 1 m3 of water contains 1 of magnesium.

Since the equivalent wts. of calcium and Magnesium are different, 1 gm ofCalcium will react with different quantity of HCl compared to 1 gm of Magnesium.Thus if quantity of the substances present in water are expressed as such (i.e. as

per their natural weights) then separate calculations would have to be done foreach element taking in account their eq. wts. etc., and the calculations willbecome cum so.

Hence to simplify the calculation ac substances are expressed in terms of PPMas CaCO3. CaCO3 has a molecular wt of 100 and equivalent wt of 50, henceconversion to CaCO3 becomes quite easy.

The situation is somewhat analogous to a person who has the money availablewith him in 4-5 different currencies. If he goes to market to buy commoditieswhose prices are quoted in some other 4-5 different currencies he will have a

very difficult time. It would be much simpler for him if he converts all his money inone currency and the prices are also quoted in the same currency. In watertreatment calculation, we are following the same logical. We expressedeverything as PPM CaCO3.

Following conversion table for common ionic species may be used :

To convert (PPM) of To PPM of CaCO3 divided by

C ++ 0.4

Mg ++ 0.24

N + 0.46

K + 0.78NH4 + 0.36

Cl - 0.71

NH4 -- 0.96

HCO3 1.22

SiO2 1.2

NO3 1.20

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Suppose you are given a typical raw water analysis which reads as under :

Calcium 60 ppm Ca

Magnesium 12 ppm Mg

Sodium 46 ppm NaM alkalinity 75 ppm as CaCO3

Total Hardness 200 ppm as CaCO3

Chlorides 71 ppm as CaCO3

Sulphates 24

SiO2 20 ppm as SiO2

First thing you must do is to convert everything to ppm as CaCO3. The analysis

will now read as under :

C ++ 150 ppm as CaCO3 (60 0.4)

Mg ++ 50 ppm as CaCO3 (12 0.24)Na ++ 100 ppm as CaCO3 (46 46)

M alk 175 ppm as CaCO3 --

Total Hardness 200 ppm as CaCO3 --

Cl - 100 ppm as CaCO3 (7l 0.71)

SO4 25 ppm as CaCO3 ( 24 0.96 )

SiO2 16.6 ppm as CaCO3 (20 0.93)

Following points may be noted.

I. Total hardness is sum of Calcium and Magnesium ions i.e. 150+50 = 200 ppm.

2. Total alkalinity or M alkalinity is given as 175 ppm CaCO3. Hence out of the 200ppm total hardness, 175 ppm hardness is associated with alkalinity. This 175ppm hardness is known as temporary hardness.

3. The difference between total hardness and temporary hardness is permanent

hardness.

4. In case the bicarbonate alkalinity or M alkalinity exceeds the total hardness,

then the entire hardness is temporary hardness and there is no permanent

hardness.

5. The sum of all the strong acid anions like Cl- SO4--, NO3- etc. is known as

equivalent mineral acidity or EMA.

6. Sum of all the cations and sum of all the anions is equal. In the above examplesum of all the Cations is [Ca( 150) + Mg(50) + Na (100) 1 = 300 ppm.

Sum of all the anions [HCO3 (175) + Cl (100) + SO4 (25)] = 300 ppm.

Sum of all the cations and anions as CaCO3 must be equal for natural waters.

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Other terms in Ion exchange

1. Ionic Load :

Quantity of ions, expressed as ppm CaCO3, taken up by the resin.

2. Regeneration Level :

Quantity of regenerant in grams / litre of resinExpressed as 100% acid or 100% alkali used per litre of resin.

3. Resin Capacity

Quantity of ions, expressed as gms CaCO3, taken up by one litre of resin.

4. Output Between Regeneration (OBR) :

Quantity of water treated by the unit between 2 regenerations expressed inm3.

Ionic load (ppm CaCO3) X OBR (M3)Resin Qty = ------------------------------------------------------in liters Resin Capacity (gins CaCO3/Litre resin)

As defined above Ionic Load means quantity of ions taken up by the resin ofthat particular unit. .pa.

Thus even if same water is passed through a cation exchanger and a softenerthe ionic load on them will be different. Since the softener will take up only thehardness, Ionic load on softener shall be equal to the total hardness i.e sumof Ca and Mg. ions.

In case of cation exchanger since the unit will take up all the cations, the ionicload will be sum of Ca, Mg and Na ions.

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For following water analysis, the ionic loads on different units shall be asgiven in the table.

Ca ++ 80 HCO3 150

Mg ++ 50 Cl 80Na + 120 SO4 20250 SiO2 10

Softener - Sum of Ca++ and Mg ++ = 80 + 50 = 130 ppmSAC - Sum of all cations = 80+ 50+120 = 250 ppmWBA - Sum of Cl and SO4 = 80 + 50 = 100 ppmSBA (without Sum of Hco3 + Cl--+SO4- + SiO2 = 150+80+20+10 = 260 ppmDegasser & WBA

SBA (with 6 ppm CO2 + Cl--+SO4- + SiO2 = 6 ppm Co2 + Cl + SO4 + SiO2Degasser without WBA) 6+80+20+10 = 116 ppmSBA (with 6 ppm CO2 + 2 (Cl) + SiO2 = 6 ppm Co2 + 2 (Cl) + SiO2Degasser with WBA) 6+20+10 = 18 ppm