Lesson 15:
Coagulation and Flocculation Calculations
Objective
In this lesson we will learn the following:
Lecture
Introduction to Coagulation and Flocculation
Following screening and other pretreatment processes, the next unit process in a conventional water treatment plant is mixing, when chemicals are added during the coagulation process. The exception to this situation occurs in small systems using groundwater, where chlorine or other taste and odor control measures are often introduced at the intake and are the extend of treatment. The term coagulation refers to the series of chemical and mechanical operations by which coagulants are applied and made effective. These operations include two distinct phases: (1) rapid mising to disperse coagulant chemicals by violet agitation into the water being treated, and (2) flocculation to agglomerate small particles into well-defined floc by gentle agitation for a much longer time. The coagulant must be added to the raw water and perfectly distributed into the liquid. Coagulation results from adding salts of iron or aluminum to the water and is a reaction between one of the following coagulants and water:
Flocculation follows coagulation in the conventional system and is the physical process of slowly mixing the coagulated water to increase the probability of particle collision. Through experience, we have determined that effective mixing reduces the required amount of chemicals and greatly improves the sedimentation process, which results in longer filter runs and higher quality finished water. The goal of flocculation is to form a uniform, feather-like material similar to snowflakes - a dense, strong floc that entraps the fine, suspended, and colloidal particles and carries them down rapidly in the settling basin. To increase the speed of floc formation and the strength and weight of the floc, polymers are often added.
Let's watch a video that explains chemicals used in coagulation and why we use them.
Calculations
Calculations are performed during operation of the coagulation and flocculation unit processes to determine chamber or basin volume, chemical feed calibration, chemical feeder settings, and detention time.
Chamber and Basin Volume
It is important to know the volume of your basin, or the amount of water you will be dealing with in regards to how long to let it settle, the amount of chemicals needed, etc. To determine the volume of a square or rectangular chamber or basin, the following formulas are used:
Volume, ft3 = Length, ft x Width, ft x Depth, ft
Volume, gal = Length, ft x Width, ft x Depth, ft x (7.48 gal/ft3)
Example:
A flash mix chamber is 4 ft square with water to a depth of 2.5 ft. What is the volume of water, in gallons, in the chamber?
Volume, gal = Length, ft x Width, ft x Depth, ft x (7.48 gal/ft3)
Volume, gal = 4 ft x 4 ft x 2.5 ft x (7.48 gal/ft3)
Volume, gal = 299.2 gal
Example:
A flocculation basin is 35 ft long and 15 ft wide with water to a depth of 7 ft. What is the volume of water, in gallons, in the basin?
Volume, gal = Length, ft x Width, ft x Depth, ft x (7.48 gal/ft3)
Volume, gal = 35 ft x 15 ft x 7 ft x (7.48 gal/ft3)
Volume, gal = 27,489 gal
Example:
A flocculation basin is 45 ft long, 25 ft wide and contains water to a depth of 10 ft and 4 inches. How many gallons of water are in the tank?
For this problem you will have to convert the inches in the depth to feet:
That means the total depth of the basin is 10 ft + 0.33 ft = 10.33 ft. Now determine the volume of the basin:
Volume, gal = Length, ft x Width, ft x Depth, ft x (7.48 gal/ft3)
Volume, gal = 45 ft x 25 ft x 10.33 ft x (7.48 gal/ft3)
Volume, gal = 86,926.95 gal
Detention Time
Because coagulation reactions occur very quickly, detention time for flash mixers is measured in seconds, whereas the detention time for flocculation basins takes more time and is generally between 5 and 30 minutes. We can determine detention time with the following equation:
Example:
The flow to a flocculation basin 40 ft long, 15 ft wide and 12 ft deep is 2800 gpm. What is the detention time in the tank, in minutes?
First, determine the volume of the flocculation basin:
Volume, gal = Length, ft x Width, ft x Depth, ft x (7.48 gal/ft3)
Volume, gal = 40 ft x 15 ft x 12 ft x (7.48 gal/ft3)
Volume, gal = 53,856 gal
Now you can determine how long the water needs to stay in the flocculation basin, based on the flow rate to the basin:
Example:
Assume the flow is steady and continuous for a flash mix chamber 5 ft long and 3 ft wide with water to a depth of 2.8 ft. If the flow to the flash mix chamber is 5 MGD, what is the chamber detention time, in seconds?
You need to determine the correct flow and volume of the chamber before you can determine the detentino time.
First convert the flow rate from million gallons per day (MGD) to gallons per second (gps):
Next, determine the volume of the flash mix chamber:
Volume, gal = Length, ft x Width, ft x Depth, ft x (7.48 gal/ft3)
Volume, gal = 5 ft x 3 ft x 2.8 ft x (7.48 gal/ft3)
Volume, gal = 314.16 gal
Now you can determine how long the water needs to remain in the flash mix chamber:
Let's watch a video showing the importance of adding chemicals for coagulation/flocculation to occur.
Determining Dry Chemical Feeder Setting, lb/day
When adding chemicals to the water flow (known as dosing), a measured amount of chemical is required which is dependent upon such factors as the type of chemical used, the reason for dosing, and the flow rate being treated. To convert from mg/L to lb/day, the following equation is used:
Chemical added, lb/day = Chemical, mg/L x Flow, MGD x 8.34 lb/gal
Example:
Jar tests indicate that the best alum dose for a water is 9 mg/L. If the flow to be treated is 1,800,000 gpd, what should the lb/day setting be on the dry alum feeder?
The flow rate needs to be converted from gallons per day (gpd) to million gallons per day (MGD):
Now you can determine the lb/day setting on the feeder:
Chemical added, lb/day = Chemical, mg/L x Flow, MGD x 8.34 lb/gal
Chemical added, lb/day = 9 mg/L x 1.8 MGD x 8.34 lb/gal
Chemical added, lb/day = 135.11 lb/day
Example:
Determine the desired pounds per day setting on a dry chemical feeder if jar tests indicate an optimum polymer dose of 11 mg/L and the flow to be treated is 2.2 MGD.
The flow is already given in MGD in this problem, so the conversion isn't needed. Plug the values straight into the formula:
Chemical added, lb/day = Chemical, mg/L x Flow, MGD x 8.34 lb/gal
Chemical added, lb/day = 11 mg/L x 2.2 MGD x 8.34 lb/gal
Chemical added, lb/day = 201.83 lb/day
Determine Chemical Solution Feeder Setting, gpd
Sometimes the solution concentration is expressed as pound of chemical per gallon of solution and the required feed rate can be determined using the following equation:
Chemical, lb/day = Chemical, mg/L x Flow, MGD x 8.34 lb/gal
Once you determine the pounds per day of chemical needed you must convert it to a gallons per day solution:
Example:
Jar tests indicate that the best alum dose for a water is 8 mg/L. The flow to be treated is 1.85 MGD. Determine the gallons per day setting for the alum solution feeder if the liquid alum contains 5.22 lb of alum per gallon of solution.
First we need to determine the lb/day of chemical needed for this flow:
Dry alum, lb/day = Chemical, mg/L x Flow, MGD x 8.34 lb/gal
Dry alum, lb/day = 8 mg/L x 1.85 MGD x 8.34 lb/gal
Dry alum, lb/day = 123.43 lb/day
Now that we know we need 123.43 lb/day of dry alum, we need to determine what this is equivalent to in gallons of solution per day:
Determining Chemical Solution Feeder Setting, mL/min
Some chemical solution feeders dispense chemicals in milliliters per minute (mL/min) instead of gallons per day. To calculate the required dosage for these types of feeders use the following equation:
Example:
The desired solution feed rate was calculated to be 23.65 gpd. What is this feed rate expressed as mL/min?
Sometimes you will need to know the solution feed rate in mL/min, but you don't know the gallons per day solution feed rate. In these cases you would have to determine the gpd solution feed rate first.
Example:
Determine the feed rate, in mL/min, if jar tests indicate the best alum dose to be 6.2 mg/L for a flow of 2.85 MGD using an alum solution that has 4.85 lb of alum per gallon of solution.
First, determine the lb/day needed of dry alum:
Chemical, lb/day = Chemical, mg/L x Flow, MGD x 8.34 lb/gal
Chemical, lb/day = 6.2 mg/L x 2.85 MGD x 8.34 lb/gal
Chemical, lb/day = 147.37 lb/day
Now that we know we need 147.37 lb/day of dry alum, we can determine the gallons per day needed of alum solution:
The last conversion can now be made converting gallons per day (gpd) to milliliters per min (mL/min):
Determining Percent of Solutions
The strength of a solution is a measure of the amount of chemical solute dissolved in the solution. We use the following equation to determine
Example:
If a total of 8 ounces of dry polymer is added to 12 gallons of water, what is the percent strength, by weight, of the polymer solution?
The first step is to convert ounces of dry polymer to pounds:
Next determine the weight of the 12 gallons of water:
12 gallons x (8.34 lb/1 gallon) = 100.08 lb
Now determine the percent strength of the polymer solution:
Example:
If 80 grams of dry polymer are dissolved in 5 gallons of water, what percent strength is the solution?
*Hint: 1 gram = 0.0022 lb
The first step is to convert grams of polymer to pounds:
80 grams x (0.0022 lb/gram) = 0.176 lb polymer
Next, determine the weight of 5 gallons of water:
5 gal x (8.34 lb/gal) = 41.7 lb
Now you can determine the percent strength of the polymer solution:
Determining Chemical Usage
One of the primary functions performed by operators is the recording of data. Chemical use in lb/day or gpd is part of that data. From the data gathered, the average daily use of chemicals and solutions can be determined. This is important for forecasting expected chemical use by comparing it with chemicals in inventory and determine when additional chemicals will be required. To determine the average chemical use for your plant, use one of the following formulas:
Then you can calculate the number of days of supply you have left in inventory:
Example:
The chemical used for each day during a week is given below. Based on the data, what was the average chemical use during the week, in lb/day?
Monday: 72 lb/day
Tuesday: 85 lb/day
Wednesday: 77 lb/day
Thursday: 82 lb/day
Friday: 85 lb/day
Saturday: 90 lb/day
Sunday: 89 lb/day
To determine the average use, you must first add all the amounts and divide by the number of days (7 in this case):
72 + 85 + 77 + 82 + 85 + 90 + 89 = 580 lbs
Now determine the average use for the week:
Example:
The average chemical use at a plant is 82.86 lb/day. If the chemical inventory is 3400 lb, how many days of supply is this?
Summary
Following screening and other pretreatment processes, the next unit process in a conventional water treatment plant is mixing, when chemicals are added during the coagulation process. The term coagulation refers to the series of chemical and mechanical operations by which coagulants are applied and made effective. Flocculation follows coagulation in the conventional system and is the physical process of slowly mixing the coagulated water to increase the probability of particle collision. Through experience, we have determined that effective mixing reduces the required amount of chemicals and greatly improves the sedimentation process, which results in longer filter runs and higher quality finished water. The goal of flocculation is to form a uniform, feather-like material similar to snowflakes - a dense, strong floc that entraps the fine, suspended, and colloidal particles and carries them down rapidly in the settling basin. To increase the speed of floc formation and the strength and weight of the floc, polymers are often added. It is important to know the volume of your basin, or the amount of water you will be dealing with in regards to how long to let it settle, the amount of chemicals needed, etc. Because coagulation reactions occur very quickly, detention time for flash mixers is measured in seconds, whereas the detention time for flocculation basins takes more time and is generally between 5 and 30 minutes. When adding chemicals to the water flow (known as dosing), a measured amount of chemical is required which is dependent upon such factors as the type of chemical used, the reason for dosing, and the flow rate being treated. The strength of a solution is a measure of the amount of chemical solute dissolved in the solution. From the data gathered, the average daily use of chemicals and solutions can be determined. This is important for forecasting expected chemical use by comparing it with chemicals in inventory and determine when additional chemicals will be required.
Assignment
Complete the math worksheet for this lesson. You must be logged into Canvas to submit this assignment. Make sure you choose the appropriate semester.
