The formula for the calculation of chemical dosages and chemical poundage is the single most important formula that introductory-level water treatment facility operator students must learn to perform well on the math portion of introductory-level water treatment facility operator exams. In all likelihood, four to five questions on any introductory-level water treatment facility operator exam will involve questions on the calculation of chemical dosages and chemical poundage.
The formula for calculating chemical dosages and chemical poundage is "lbs. = MG x mg/l x 8.34." The term "lbs." in this formula is the total bulk poundage of a chemical used in a specific timeframe, which is usually a 24-hour period. The term "MG" in this formula does not necessarily mean MGD, but for purposes of introductory-level exams, it is used as MGD. The term "mg/l" in this formula can be used interchangeably with ppm, which may allow some students to better understand its context.
Practically everyone who is studying this website has completed one week of water treatment training school. In the math classes at that school, handout sheets containing a pie chart of the chemical dosage and poundage formula were distributed. This chart may prove helpful in performing chemical dosage and poundage calculations.
Nearly all chemical dosage and poundage problems involve solving the problem for dosage or poundage. Very few of these problems involve a calculation in which the MG part is the answer. Thus, the two most common versions of the dosage and poundage formula are "lbs. = MG x ppm x 8.34" and "ppm = lbs. / (MG x 8.34)". The third version, for those rarer times when it is needed, is "MG = lbs. / ppm x 8.34".
1. A water treatment plant uses 1245 pounds of alum to treat 2.65 MG of water. What is the plant's alum dosage?
Step one: ppm = 1245 divided by (2.65 x 8.34). Step two: ppm = 1245 divided by 22.1 = 56.3 ppm
2. A water treatment plant uses a caustic dosage of 14.8 ppm to treat 12.7 MG of water. How many pounds of caustic did the plant use?
Step one: lbs. = 12.7 x 14.8 x 8.34. Step two: lbs. = (187.96) x 8.34 or 12.7 x (123.432) = 1568
lbs.
3. A water treatment plant has only 320 lbs. of coagulant-aid polymer left in its inventory. If the plant feeds this polymer at a dosage of 1.5 ppm, how many gallons of water can the plant treat before running out of polymer?
Step one: MG = 320 divided by (1.5 x 8.34). Step two: MG = 320 divided by 12.51 = 25.6 MG
A SPECIAL VARIATION OF THIS FORMULA: A special variation of this formula is used to calculate the available poundage of a treatment chemical when only a portion of the total chemical poundage of that chemical is available for actual treatment usage. For most introductory-level water treatment math purposes, the chemicals to which this refers will be HTH (calcium hypochlorite) and bleach (sodium hypochlorite).
Whenever chlorine gas is used to disinfect water, 100 percent of the chlorine in the gas is available for treatment purposes. Whenever calcium hypochlorite is used to disinfect water, only about 65 to 70 percent of the total calcium hypochlorite poundage is converted to usable chlorine poundage. Whenever commercial-grade sodium hypochlorite is used to disinfect water, the available chlorine poundage is only about 12 to 15 percent of the total sodium hypochlorite poundage. With laundry-grade sodium hypochlorite bleach, the available chlorine poundage is only 4 to 6 percent of the total sodium hypochlorite poundage.
A typical test question involving this variation of the poundage formula might read, "How many pounds of 12 percent pure sodium hypochlorite will be needed to treat eight million gallons of water with a chlorine dosage of 6 mg/l?" On introductory-level exams, these types of questions will almost always start with "How many pounds", since poundage is the only variable that is calculated with introductory-level questions of this type. Also, the word pure, which is not really a very descriptive term, is commonly used on test questions to refer to the "available treatment chemical" involved in the question.
To solve these special types of problems, first calculate the total chemical poundage as if the percentage factor didn't even exist. Then, divide the total poundage answer by the percentage in a decimal form. The final poundage answer will always be greater than the original poundage answer. Below are two examples to fully illustrate this procedure.
1. How many pounds of 70% pure HTH will be needed to disinfect a newly installed 1.5 MG water storage tank with a 50 mg/l dosage of chlorine? (In this question, the term " 70% pure " denotes that the total HTH poundage yields 70 percent of itself to available chlorine poundage.)
Step one: lbs. = 1.5 x 50 x 8.34 = 625.5 pounds. Step two: 625.5 divided by .70 = 894 pounds.
2. How many pounds of sodium hypochlorite with 12.5% available chlorine will be needed to treat 10,000,000 gallons of water with a chlorine dosage of 5.5 ppm?
Step one: lbs. = 10 x 5.5 x 8.34 = 458.7 pounds. Step two: 375.3 divided by .125 = 3669.6 pounds.