Coagulation Assignment (S22)
Group Work — none
Write a memo report to your boss about the test you just conducted. For this report, pretend that you are
an engineer on a team developing the preliminary design for a proposed 6 MGD (million gallon per day)
water treatment plant at Johnstown, CA. The water source for the proposed plant is Cruz Creek. Part of
the design process for this plant is determining the chemical and power needs for the coagulation and
flocculation processes. Your laboratory test feeds into that effort. Based on your test results you want to
make recommendations about the best combination of chemical dose and velocity gradient, and scale up
those values to the full plant capacity, that is, the annual mass of chemical needed (dry kg/yr), the power
requirement (kw), and the annual energy requirement (kwh). For a preliminary design, it’s good to be
conservative, so assume the plant processes 6 MGD, operating 24 h/d, 365 d/y. The worst-case power
requirement is in winter when the water viscosity is highest. Assume the water temperature is 10 °C.
Let’s be clear about roles. You are on the Hornet Engineering design team, not in CE150 or at CSUS.
(Those days are behind you.) Your boss is your instructor. You did the testing in the Hornet Engineering
lab. The test water (what was in the white bucket) was a sample you collected from Cruz Creek.
Introduction – what, why
General description of your particular experiment. Your boss is familiar with jar testing
in general, so he/she does not need to be told how to do the test. Do not repeat detailed
lab procedures. On the other hand, even though your boss knows jar testing, he/she won’t
know how you ran your test so you’ll need to describe the important aspects (e.g., date,
source of water, jar volumes, doses, mixing times, settling times, mixing speeds and G
values). You should be able to do this in a paragraph, with reference to accompanying
tables of results.
Tabular and graphic presentations of your data (i.e., tables of data and the two graphs
mentioned in the procedure). All of the data that are in the graphs should be in tables.
G and P equations. Integrate these into your narrative. See notes below on format.
Your recommendations for dose and velocity gradient for the plant design and your
reasons for choosing them. Your estimate of the annual alum needed (in tons/yr) if the
water plant processes 10 million gallon/day on average.
A comment on your confidence in the experimental results. Did the data look like
what you expected based on your understanding of the mechanisms? Did you find a
dose/speed combination that was satisfactory? Do you think you need any
additional testing and why? Do you think the source water will always be the same?
12-pt serif font (Times New Roman or similar, not Arial or Calibri – look up “serif” if
you don’t know what it means)
Margins: 1.00 inch on both sides.
Pages numbered on the bottom, right edge.
Page 1 of 2
Double-spaced text except for tables which should be single or 1.5 spacing.
Memo heading – This is your company’s format. Use it. The top line should be all caps,
14-pt font, boldfaced. The rest of the heading is 12-pt regular font, single-spaced.
MEMO – HORNET ENGINEERING
your lab instructor
Subject: single-space if more than one line
Equations: Use the equation editing function in MS WORD (or equivalent). The
equations should look like algebra, not like Excel formulas. Define your variables in the
text. Incorporate the equations into the text, not in a separate figure. In the end, your
equations should look something like the equation in the lab procedures or ChemText.
All tables and figures should be referenced in the narrative. You can choose to
place them inside or at the end of the text. (Equations go inside the text.)
Tables: Title and number on top. Single-spaced contents (occasional blank lines for
spacing are OK). Format as shown below (with periods):
Table #. Title of the table.
Figures: Title and number on bottom. Size your figures to cover 30-50% of a page so
that you can get two figures and captions on a single sheet. Do not use the Excel title.
Format as shown below (with periods):
Figure #. Caption describing the figure.
Voice: For this assignment, use passive voice unless otherwise directed by your lab
Overall expected length – about 3 pages of text plus tables and figures.
This memo is worth more than a standard lab. Your instructor may use the Turnitin software tool to check
for similarities between documents and plagiarism, including copying of the lab procedures. So, use your
Advice – Do not give your instructor your first draft. Make sure it is as good as you can make it
— no format errors, no computational errors, and no grammatical errors. This is what you would do
in a professional job, right? It will not reflect well on you if your boss has to correct your math,
grammar and formatting.
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CE 150L – Environmental Engineering Lab
Chemical Coagulation Jar Testing (S22)
Drinking water treatment plants need to remove suspended solids and colloids before sending the water to
users. An important part of that treatment process is settling that is optimized by chemical addition. To
determine the optimum chemical dose and the optimum degree of mixing for coagulating and flocculating
a water sample, treatment plant operators do jar testing on a regular basis. In this lab, you will perform a
a typical jar procedure and use the results to make some design calculations. The ultimate goal of the test
is to determine the best combination of chemical dose and velocity gradient (mixing intensity) to use at a
drinking water treatment plant.
1. Describe in general terms (not the detailed reactions) the physical and chemical processes occurring
in coagulation/flocculation. (Knowledge)
2. Given a set of jar test results, recommend a chemical dose and velocity gradient for a treatment
process and justify your choice. (Comprehension, evaluation)
3. Calculate velocity gradient (G). (Application)
Reading: MZ Sections 4.4.2 (p 173+) and 8.5 (p 392 to 399) plus ChemText Chapter 9
Settling is the least expensive method of removing particles from water. So, we’d like to maximize
settling efficiency which means maximizing the number of particles removed in a tank with a given
hydraulic retention time. The settling velocity of spherical particles in a fluid under laminar conditions is
given by Stokes’ Law (MZ p. 174).
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Here vs is the settling velocity, ρ is the density of the particle and the fluid, µ is the viscosity and D is the
diameter of the spherical particle.
What you can see from the equation is that the settling velocity depends on the diameter of the particle
(Dp). Very small particles like colloids won’t settle quickly. For instance, using typical values for
viscosity (1×10-3 Ns/m2), density (998 kg/m3), and specific gravity (2.7), we would find that a sand grain
(1 mm diameter) would fall at 0.92 m/s and a clay particle (0.002 mm diameter) would fall at 4×10-6 m/s.
This means that a sand particle would take a little over a second to fall one meter, but a clay particle
would need more than 3 days. In a typical sedimentation tank with a fixed hydraulic detention time
(usually a couple of hours) and a depth of 3-4 m, we wouldn’t capture any clay particles. However, if we
can increase their size, their settling velocities will increase, allowing us to achieve a better capture
Increasing particle size is the goal of coagulation/flocculation. Briefly, in coagulation, chemicals are
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added to water to neutralize the surface charges of particles and/or to produce precipitates that can bind
particles together. In flocculation, the suspension is mixed so that particles come into contact with each
other and grow larger. For a more complete explanation, be sure to do the assigned reading.
For equipment reasons, the experimental work in this lab will be done by groups of six (i.e. by bench, not
Caution: You have barely enough test water to complete this lab. Don’t throw any out before you’ve
finished all the tests.
Summary of Doses and Mixing Speeds
Mixing speed (rpm)
5, 10, 15, 20, 25, 30
40 – Everyone
Chosen from Round 1 results
80 – Back bench
Chosen from Round 1 results
20 – Middle bench
Chosen from Round 1 results
10 – Front bench
Test Round 1 — Concentration Range-Finding
Initial measurements: water temperature,
paddle diameter, and initial turbidity of the
Measure out six samples into the square 2-L
test beakers (called “jars” in this test). Each
square test jar has a 2000 mL mark. You can
fill the jars using a large beaker and then top
off with water from a small beaker or you can
overfill the jar a little bit and drain down to
the mark using the sample port.
You will test the 6 target concentrations (also
called doses) shown in the table above. In
your notebook, show the calculation of the
volume of stock solution needed for the
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To the best of your ability, keep the test water
bucket stirred while taking samples to assure
uniform concentrations of solids in your sample
jars over the course of the afternoon. Also,
return any uncontaminated sample water to your
Based on conservation of mass):
C stock Vstock = C jar (V jar + Vstock )
Cstock = concentration of stock solution, mg/L
Cjar = concentration alum desired, mg/L
Vjar = volume of the jar, mL
Vstock = volume of stock solution to be added to the
jar to achieve Cjar, mL
Because of the volumes and doses we are using in
this test, you will see that the volume needed (mL) is
just about equal to the desired dose (mg/L). Do the
best you can, but don’t be too concerned about small
errors in volumes. Below 20 mg/L, it is sufficiently
Measure out the appropriate volumes of alum
stock solution into 50-mL beakers, one for
each of the 2-L test jars.
Place the jars on the multiple-paddle mixer.
Center each one up so that the paddle does
not hit the jar wall.
Start the mixers on the highest possible
speed. Simultaneously add the pre-measured
alum stock to all the jars as quickly as
possible. Mix for 10 seconds, and then
reduce the mixer’s speed to the value shown
in the table above.
Flocculate (slow mix) for 15 minutes.
During the mixing watch your samples and
record the time required for flocs to form in
After flocculating for 15 minutes, turn off the
mixer. Before they have a chance to settle,
observe and record the relative sizes of the
flocs in the different jars (see scale to right).
Allow the samples to settle for 20 minutes
For each jar, flush the sampling tube with 3050 mL to remove any water from a previous
experiment. Then collect 30-40 mL of
sample for turbidity testing.
Measure the turbidities of these samples.
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accurate to use a stock volume in mL equal to the
intended concentration (e.g., 10 mL for 10 mg/L).
DO NOT ADD THE COAGULANT TO THE
WATER SAMPLES YET.
Position the mixer so that you can use the
sampling tubes without moving the jars. After
the flocs settle, you don’t want to disturb them.
This may require you to move the test apparatus
close to the edge of the bench.
Use your phone light to help you see the floc
Floc size scale
0 = Unable to discern individual floc particles
1 = Small (about the size of sand grains)
2 = Medium
3 = Large (size of eraser shavings or larger)
Don’t disturb the settled material.
Figure 1. Jar test mixer. You can collect the water directly into the turbidity sample bottle as shown or
into a beaker. Place labels on the cap of the turbidity bottles, not on the glass section.
Figure 2. Examples of well-flocculated and poorly flocculated samples
Preliminary Data Analysis
The first round of testing was six doses at one mixing speed. The second round will involve two doses at
multiple speeds. Select two coagulant doses for a second round of testing based on the first appearance of
floc, floc size, final turbidity (most significant factor), and coagulant dose.
Guide for choosing second round doses: The turbidity standard for drinking water is 0.5 NTU. It is not
necessary to achieve this value after settling because in a conventional treatment plant, settling is
followed by sand filtration. However, to assure that the filter can reliably produce high quality water
without excessive clogging, the settled water turbidity should be less than 10 NTU (rough rule of thumb).
So, you want to choose coagulation/flocculation parameters to meet the 10 NTU target at the lowest cost.
Costs include chemicals (dose) and energy for mixing (G) with chemicals being the greater cost.
1. You may have multiple doses that meet the target turbidity. Considering cost, which one would
be the best choice? You tested that dose at one speed. Could you meet the target with a different,
slower speed, which would save energy?
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2. You will also have a lower dose that almost meets the target turbidity at the speed you used in
Round 1. Could you meet the target with that dose, but a different speed?
Discuss your choices with your instructor before proceeding.
Test Round 2 — Determining Optimal Dose and Velocity Gradient
In this round, repeat the test at three different mixer speeds. Because of equipment and time, each bench
will run a different speed in Round 2 using samples from all the lab groups simultaneously. This will
require some coordination between benches.
Obtain six more samples from your bucket as
Calculate and measure out the stock solutions of
coagulant needed to dose three jars at each of
your two selected concentrations.
Give two test jars and stock solution aliquots
(one for each dose) to each of the other two lab
Collect samples and aliquots of coagulant from
the other two benches. Set up to run all six
samples (including yours) simultaneously.
Repeat the jar test procedure used in Round 1,
including the 10-sec rapid mix and 15-min of
slow mixing at the speed specified in the table
At the end of the settling period, collect samples
for turbidity testing from all of your jars,
wherever they are located.
Measure the turbidities of our samples.
Mix the bucket first!
Total = 6 jars and 6 beakers (3 for each alum
dose). Don’t add the coagulant yet!
Your sample water (white bucket) is different from
the other groups, so prepare your own samples
and do your own analyses. You are asking the
other groups to only mix the samples.
Remember, you are providing mixing for the other
You do not need to record the time to first floc
formation, or the relative floc sizes.
Tell your classmates when you are finished mixing
so that they can collect their settled water
1. Using a spreadsheet, plot the data from Round 1 using coagulant dosage as the abscissa (x) and
turbidity as the ordinate (y). Be sure your graph is properly and completely labeled. (See example
graph below.) Include the zero dose (white bucket sample) turbidity on your graph.
2. Calculate the velocity gradients at the four mixing speeds using the equations in the ChemText.
P = power (ft-lb/s)
n = rotational speed (rev/s, not rpm)
D = diameter of paddle (ft)
γ = specific weight of water (lb/ft3)
g = acceleration due to gravity (32.17 ft/s2)
KT = empirical constant = 2.4 for Phipps and Bird jar test mixer used in this lab
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Convert the power to SI units using 1.3558 W per ft-lb/s.
G = velocity gradient (s-1)
P = power dissipated in the water (W)
µ = viscosity (N-s/m2)
V = volume (m3)
= � �
Settled turbidity (NTU)
Settled turbidity (NTU)
3. Plot the data for the doses tested at the speeds tested. Use velocity gradient (G) as the abscissa (x)
and turbidity as the ordinate (y). (See example graph below.)
Alum dose (mg/L)
Figure 3. Examples of results graphs. The number of data points and graph shapes shown may differ
from your results.
Assignment — Individual
1. Complete the calculations shown in the template spreadsheet. Show formulae in all appropriate cells.
Pay attention to significant figures. Please name your file as instructed. Upload your file to the Canvas
assignment drop box.
2. Write a memo report as described in a separate handout. Upload your file to the Canvas assignment
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Study Guide and Quiz Prep (Suggestions for your notebook)
1. The volume of coagulant stock solution needed for a given jar volume, target dose, and stock solution
2. Velocity gradient, including the power provided by this type of mixer.
3. Mass of coagulant needed at full scale. (See hints in template spreadsheet.) (see guidelines in the
4. Power requirement for a full-scale unit, given G, mixing time, and flow.
Concepts and discussion
Why are we interested in G rather than rpm? Remember the horizontal shaft mixer at the Fairbairn WTP
shown in the background video? The P equation in your lab does not apply to that device. There are
many different kinds of mixers and each has its own equation relating power to rpm. The G equation is
universal. You can produce the same mixing intensity (G value) with different kinds of mixers, but their
rotational speeds will be different. Therefore, if specifying full-scale operations from lab data, G is the
factor to use.
What’s happening to the settled turbidity at high and low G values? In flocculation, if the mixing is
insufficient, there won’t be enough collisions between microflocs which limits the growth of larger flocs.
On the other hand, if the mixing is too intense, the flocs will be torn apart by hydrodynamic forces. Some
treatment plants use tapered flocculation – high to medium to low G values. At the high end, the flocs are
small, but dense. At the low end, the flocs are larger but fluffier. Tapered flocculation produces large
flocs built around dense cores. This increases both the density of the particle (ρp) and its diameter (Dp),
both of which increase the settling speed as per Stokes Law.
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