Water Treatment and dosage calculations

 

For the safe water intake human beings have adopted various processes of water treatment and have determined chemical dosage calculations to treat the water before use for removal of physical, biological and chemical contamination. 

Number of researches culminating in theories have been introduced to curb the menace of water borne diseases due to intake of contaminated water.

Water is an imperative for all forms of living beings. Almost 71% of our globe is water and 97% of which is found in the oceans meant to of no use (unless radically treated) for drinking, agriculture and other needs that a fresh water serves to living beings especially human beings. Only 3% is the quantum of fresh water on the earth. 

This least proportion of available fresh water is also inflicted with variety of contaminants which are essentially needed to be removed for safe drinking water intake of human beings.

For the treatment of water various methods and articles (such as Alum, Chlorine, HTH and other chemical solutions) are used.

This blog intends to provide information about chemical dosage calculations and its application for water treatment.

Chemical dosages for water treatment are measured in ppm (parts per million) or mg/l (milligram per liter). The metric term of ppm is mg/l and it is equal to ppm.

1 ppm = 1 mg/l

Parts per million (ppm) is elaborated in terms of weight as pounds per million pounds. It means one pound of chemical added to one million pounds of water.

Since each gallon of water weighs 8.34 pounds, thus one million gallons of water weighs 8.34 million pounds and would require 8.34 pounds of chemical to obtain the dosage of 1 ppm.

In this way the number of pounds of chemical required to achieve a certain dosage can be determined by multiplying the ppm by number of million of gallons to be treated and then by 8.34 lbs/gal (as weight of 1 gallon of water is 8.34 pounds).

It must be noted that while calculating the chemical under certain dosage the amount of water to be treated is always taken in terms of million gallons or it may be million gallons per day (mgd).

Thus the formula to determine chemical quantity for certain dosage is as given below;

ppm x mgd x 8.34 = pounds of chemical per day

or 

mg/l x mgd x 8.34 = pounds of chemical per day

See the example how this formula works in calculating chemical quantity.

Example: If the dosage is 2.4 mg/l in 1200,000 gal/day, how much chlorine in lbs is needed per day?

Convert gal/day to mgd

1200000/1000000 = 1.2 mgd

Now put figures in formula

mg/l x mgd x 8.34 = pounds per day (Chlorine)

2.4 x 1.2 x 8.34 = 24.01 lbs/day

It means 24 lbs of chlorine per day is required at the dosage of 2.4 ppm for flow of 1.2 mgd of water.

If instead of chlorine gas, HTH is used that constitutes 65% - 70% chlorine in each pound of it. In such case the amount of HTH must be calculated by dividing the pounds of chlorine needed by the proportion of chlorine in it i.e 65% - 70%.

Example: A tank is 60' in diameter and 20' high and is dosed with 60 ppm chlorine. How many pound of 70% HTH is needed?

It tells we have to determine the quantity of HTH in pounds which contains 70% chlorine.

First we have to determine the quantity of chlorine needed

Formula is same as

ppm x mgd x 8.34 = pounds of chlorine

here ppm is given but mgd is required to be calculated first from tank dimensions

Volume of tank = 60 x 60 x 0.785 x 20 = 56,520 cubic feet

convert cubic feet to gallons

56,520 x 7.48 = 422,770 gallons

convert gallons to million gallons

422,770/1000000 = 0.422 million gallons

Now put them in formula

ppm x mgd x 8.34 = pounds of chlorine

60 x 0.422 x 8.34 = 211 lbs of chlorine

HTH contains 70% of chlorine

Find lbs of HTH

Lbs of chlorine / 0.70

211 / 0.70 = 301 lbs of HTH

Example: A 12" pipe is 1900 feet long and to be disinfected with 40 ppm of 65% HTH. How many pounds of HTH are needed?

Here HTH constitutes 65% of chlorine

Find gallons of water in pipe to be disinfected

12 x 12 x 0.0408 x 1900 = 11,163 gallons

convert gallons to million gallons

11,163 / 1000000 = 0.011 million gallons

Now put them in formula for finding chlorine quantity

ppm x mgd x 8.34 = pounds of chlorine

40 x 0.011 x 8.34 = 3.66 lbs of chlorine

Now find HTH amount

3.66 / 0.65 = 5.63 lbs of HTH

In water treatment and dosage calculations, many a times you have to come across with chemical solutions instead of raw chemical. 

When chemical solutions are used, the weight of that solution will be more than the weight of a gallon of water. The weight of a gallon of liquid is found by multiplying weight of gallon of water (8.34 lbs) by the Specific Gravity of the solution. 

If S.G is not given then weight of solution is assumed to be 8.34 lbs/gallon same as of water.

Example: A chlorine pump is feeding 10 % bleach at a dosage of 5 mg/l. The S.G of the bleach is 1.14. If 2,000,000 gallons are treated in 16 hours, how many gallons per hour pump is feeding?

Convert gallons to million gallons

2,000,000 = 2 mgd

Find chlorine amount

5 x 2 x 8.34 = 83 lbs of chlorine

Find bleach amount

83 / 0.1 = 830 lbs of bleach

Find the weight of a gallon of bleach

1.14 x 8.34 = 9.50 lbs/gal

Find the gallons of bleach

830 / 9.50 = 87.3 gal. of bleach

Find gallons per hour

87.3 / 16 = 5.45 gal/hr

Example : A chlorine pump is feeding 14% bleach at a dosage of 2 mg/l or 2 ppm. If the flow is 1,250,000 gallons per day (gpd) , how many gallons per hour is the pump feeding? S.G 1.14

Convert gpd to mgd

1,250,000 = 1.25 mgd

Find chlorine required

2 x 1.25 x 8.34 = 20.85 lbs of chlorine

Find bleach amount for getting chlorine required

20.85 / 0.14 = 149 lbs of bleach

Find weight of a gallon of bleach

1.14 x 8.34 = 9.5 lbs/gal

Find gallons of bleach

149 / 9.5 = 15.68 gal

Find gallons per hour

15.68 / 24 = 0.65 gal / hour

Now you know how to find quantity of chemical required for disinfection. Also you know how much gallons of chemical solution are required per day or per hour. 

After getting these quantities, now you need to know, how a chemical feed pump is to be calibrated to feed the dosage in ml/min in water on the basis of determined calculations of solutions in gallon per day.

Remember 1 gallon = 3.785 liters = 3785 milliliters

1 day = 1440 minutes           

So If you take 3785 ml/gal and divide it by 1440 min/day, the conversion for gal/day to ml/min can be determined.

3785 / 1440 = 2.6 ml/min/gal/day

ml/min = gal/day x 2.6

Example: A 20% available flouride solution is used to dose 2,500,000 gpd at 450 ppb (parts per billion). The S.G is 1.26. How many ml/min is the pump feeding?

Convert ppb to ppm

1 ppm = 1000 ppb

450 ppb = 0.45 ppm

Convert gpd to mgd

2,500,000 = 2.5 mgd

Note: Here flouride is referred instead chlorine

Find lbs of flouride

ppm x mgd x 8.34 = lbs of flouride

0.45 x 2.5 x 8.34 = 9.38 lbs / day

Find lbs of flouride solution

9.38 / 0.2 = 47 lbs of flouride solution

Find weight of a gallon of flouride solution

1.26 x 8.34 = 10.5 lbs/gal

Find the gallons of flouride solutions

47 / 10.5 = 4.48 gallons per day

Find pump feed in ml / min

ml/min = gpd x 2.6

ml/min = 4.48 x 2.6

11.65 ml/min 

Thus the pump is required to be calibrated to feed the solution in water at 11.65 ml/min.

Example: An 18% available Alum solution is used to dose 800,000 gpd at 25 mg/l. Determine the pump feeding of solution in ml/min? 

No S.G is given so assume weight of solution is 8.34 lbs/gal. 

Convert gpd to mgd

800,000 gpd = 0.8 mgd

Find lbs of Alum

25 x 0.8 x 8.34 = 167 lbs/day of Alum

Find lbs of Alum solution

167/0.18 = 928 lbs/day of Alum solution

The weight of gallon of Alum solution is assumed to be 8.34 lbs/gal

Find the gallons per day of Alum solution

928/8.34 = 111 gpd

Find pump feed in ml/min

ml/min = gpd x 2.6

ml/min = 111 x 2.6

288 ml/min

Example: A chlorine pump is feeding 12% bleach at a dosage of 2.4 mg/l. If flow is 1,250,000 gpd. how many ml / min the pump would be feeding? S.G = 1.14.

Convert gpd to mgd

1,250,000 gpd = 1.25 mgd

Find lbs of chlorine

2.4 x 1.25 x 8.34 = 25 lbs/day of chlorine

Find lbs of bleach

25/0.12 = 208 lbs/day of bleach 

Find weight of gallon of bleach

1.14 x 8.34 = 9.5 lbs/gal

Find gallons of bleach solution 

208/9.5 = 22 gpd

Find ml/min

22 gpd x 2.6 = 57 ml/min

Example: A system has a well that produce 200 gpm and a 1500 gallon storage tank.There are 120 homes on the system and the average daily consumption is 350 gal/home. A chlorine dosage of 1.3 ppm is maintained using 65% HTH.

How many pounds of HTH to be purchased each year?

Find system consumption 

120 homes x 350 gal/day/home = 42000 gpd

Convert gpd to mgd

42000 gpd = 0.042 mgd

Find lbs/day of chlorine

1.3 x 0.042 x 8.34 = 0.45 lbs of chlorine

Find lbs of HTH for each day

0.45 / 0.65 = 0.7 lbs/day of HTH

Find lbs/year of HTH

0.7 x 365 = 255.5 lbs/year.

I hope this will serve the purpose of imparting knowledge and remain beneficial for the students and water operators in their practical application of water treatment processes with the knowledge of dosage calculations involved in it.

I have made it so simple and articulate in order to enable the students or other interested groups to understand the entire concept and dynamics involved in water treatment and dosage calculations. I welcome any further query in this regard by your comments and make sure to respond accordingly.

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