Lesson 4:
Chemical Dosages




In this lesson we will answer the following questions:

      • How do you calculate chemical feed rate for wet and dry chemicals?
      • How do you calculate how many chemicals to order?



Reading Assignment

Read online lesson.




You have a sore throat and go to the doctor, who diagnoses you with strep throat.  He gives you an antibiotic and tells you to take one pill three times a day. 

Treatment of water in the treatment plant follows a similar process to your treatment at a doctor's office.  The water is tested to determine what sort and amount of treatment will be required.  Then chemicals are added to the water in a set amount per day. 

The following table compares the two processes:



Treatment Plant


Tests are done to determine your state of health.  Examples: Looking in your eyes and ears, feeling your lymph nodes, etc.

Tests are done to determine the water's pH, turbidity, total alkalinity, temperature, and coliform bacteria.  Examples: Jar test, pH test, etc.


Your illness and any medication which should be used to treat it are determined.  Example: Strep throat is treated with antibiotics.

Characteristics of the water which will require treatment are determined.  Example: The water is determined to be extremely turbid, so extra coagulants will have to be used.

Volume to be treated

The doctor measures your weight to determine how many pills to give you.  Example: You weight 150 pounds.

The flow of water through the plant determines the volume of water to be treated.  Example: The flow of the plant is 3 MGD.

Chemical concentration

The doctor determines how much medicine is needed to treat the illness.  Example: The doctor decides you should take 200 mg of antibiotics per 100 pounds of your body weight.

Water tests determine the dosage of chemical which should be used to treat the water most effectively.  Example: In order to treat the turbidity, alum should be fed into the water at a concentration of 40 mg per gallon of water.

Chemical feed rate

The doctor tells you how many pills to take per day based on your weight and on the strength of the pills.  Example: The doctor tells you to take 300 mg of antibiotics per day.

You set the feed rate of the chemical feeder based on the flow and dose.  Example: The alum feed rate is set at 120,000,000 mg per day.


In this lesson, we will be concerned with three steps in this five step process - determining the volume to be treated (the flow of the water plant), the chemical concentration (the dose), and the chemical feed rate.  Each of these three factors is explained briefly below:

    • The flow is the rate at which water is moving through the treatment plant, commonly given in MGD.  The flow is measured with a flow meter, such as a Parshall flume or a pressure differential meter.  An increase in flow means more water is being treated, so more chemical must be added to the water to keep the concentration of chemical in the water at the appropriate level.

    • The dosage is the required concentration of the chemical within the water, commonly given in mg/L.  Tests of water characteristics determine the required dosage of each chemical.  A higher dose means that more chemical is getting into the water.

    • The chemical feed is the amount of chemical you add to the water, commonly given in lbs/day.  The chemical feed rate is typically set on a chemical feeder of some sort so that the chemical is automatically added to the water throughout the day.  A higher chemical feed rate means that chemicals are being added to the water more quickly. 



Dry Chemical Feed Rate

Determining chemical feed rate of a dry chemical is as simple as suggested in the example in the last section.  There, we found the feed rate by multiplying the flow of the plant by the required dosage:


Chemical feed = 3 MGD × 40 mg/gal

Chemical feed = 120,000,000 mg/day

However, since using a feed rate of several million milligrams per day is unwieldy, we usually report the feed rate in pounds per day.  In addition, the required concentration of the chemical is usually given as mg/L rather than mg/gal, so we must convert from liters to gallons.  As a result of these two changes, we add one more element to our equation to change the units, resulting in the following formula:

Chemical feed (lbs/day) = Flow (MGD) × Dosage (mg/L) × 8.34 (lbs/gal)


Let's consider a water treatment plant with a flow of 15 MGD.  We want to add alum to the water at a dosage of 18 mg/L.  What should the setting on the alum feeder be?

In order to find the answer, we make the following calculations:

Chemical feed = 15 MGD × 18 mg/L × 8.34 lbs/gal

Chemical feed = 2,252 lbs/day

And determine the alum feeder should be set to a rate of 2,252 lbs/day.

If we need to set the chemical feeder rate to pounds per hour or to pounds per minute, we merely convert units, as shown below:

2,252 lbs/day × 1 day/24 hr = 93.8 lbs/hr

93.8 lbs/hr × 1 hr/60 min = 1.56 lbs/min



Liquid Chemical Feed Rate

The situation becomes a little more complicated when liquids are being fed instead of dry chemicals.  In most cases, these liquids are not pure chemicals, but are a mixture of a dry chemical with water.  Since the chemical is not as concentrated in the liquid as it was in the dry form, a greater volume of the liquid chemical must be added to treat the raw water. 

The formula we use to determine the chemical feed rate of liquid chemicals is shown below:


You will notice that the formula is very similar to that used for dry chemicals, with the addition of four numerical terms used to convert units.  The only additional element which requires explanation is the liquid concentration.  The liquid concentration, given in mg/mL, is the amount of dry chemical (in mg) mixed with the amount of water (in mL):

Formula for calculating liquid chemical concentration.


So, if you added 230 mg of polymer to 100 mL of water, you would have a liquid polymer solution with a concentration of 2.3 mg/mL.

We'll consider a situation in which the flow of the plant is 5 MGD and the alum dosage is 10 mg/L.  The liquid alum has a concentration of 520 mg/mL.  The chemical feeder setting would be determined as follows:


The setting on the liquid alum feeder should be 252 mL/min.




Ordering Chemicals

As a water treatment plant operator, you will be expected to order chemicals for your treatment plant.  In order to do so, you will need to know how to calculate the amount of chemicals you will use during a certain period of time.  First, you should calculate the chemical feed rate for an average day.  Then you can determine the amount of chemical you will use in a certain period of time using the following formula:

Chemical used (lbs) = Chemical feed (lbs/day) × Time (days)


So, if you use 100 pounds of alum per day, how much alum should you order to last you for four weeks?

Chemical used = 100 lbs/day × 28 days

Chemical used = 2,800 lbs


You should order 2,800 pounds of alum.





The chemical feed rate is the rate at which chemicals are added to water in the treatment plant.  The chemical feed rate for dry chemicals is dependent on the flow of the plant and on the required chemical dosage.  For liquid chemicals, the chemical feed rate is dependent on flow, dosage, and on the chemical's concentration.

In order to order sufficient chemicals for use in the treatment plant, the operator must be able to calculate the amount of chemicals used over time.  This amount is dependent on the average chemical feed rate and on the number of days the chemical is required to last.




New Formulas Used

Dry chemical feed rate:

Chemical feed (lbs/day) =

Flow (MGD) × Dosage (mg/L) × 8.34 (lbs/gal)



Liquid chemical feed rate:


Liquid chemical concentration:

Formula for calculating liquid chemical concentration.



Amount of chemical used over time:

Chemical used (lbs) = Chemical feed (lbs/day) × Time (days)




Answer the following questions in Math Set 2. You may take the assignment online and submit your grade directly into the database for grading purposes.




Please take Quiz 4. When you have gotten all the answers correct, email or mail, the quiz to your instructor. You may also take the quiz online and submit your grade directly into the database for grading purposes.