Design and Operation of a
Bench Scale Continuous Flow
Waste Water Treatment Plant
By:
Stephen Mirabello
And
Allison Muehe
CEE 453 Fall 2004
ABSTRACT .................................................................................................................................................. 3
OBJECTIVE AND INTRODUCTION....................................................................................................... 3
Figure 1: Adopted diagram of effluent turbidity verses coagulant dose 1.................................... 4
Table 1: Summary of variables states for each trial.................................................................... 4
METHOD ...................................................................................................................................................... 4
Plant Set Up – Equipment and Flows ........................................................................................................ 6
Reactor Inflows ..................................................................................................................................... 6
Water ................................................................................................................................................ 6
Synthetic Waste ................................................................................................................................ 6
Aeration ............................................................................................................................................ 6
Recycle Stream................................................................................................................................. 6
Clarifier Flows ...................................................................................................................................... 6
Reactor Effluent ............................................................................................................................... 6
Clarifier Effluent .............................................................................................................................. 6
Pumps ................................................................................................................................................... 7
Peristaltic Pump ............................................................................................................................... 7
Centrifugal Pump ............................................................................................................................. 7
Mixers ................................................................................................................................................... 7
Stir Bar ............................................................................................................................................. 7
External Propeller............................................................................................................................. 7
Tanks/Reactors...................................................................................................................................... 7
Aeration Tank/Activated Sludge Reactor (reactor) .......................................................................... 7
Table 2: Mass balance of reactor, including parameters of the peristaltic pump ......................... 7
Clarifier/Settling Tank (clarifier) ..................................................................................................... 8
Table 3: Reactor and clarifier dimension summary .................................................................... 8
Plant Set Up – Process Controller Software ............................................................................................. 8
Black Control Box ................................................................................................................................ 8
Solenoid Valves ............................................................................................................................... 8
Stamp Box ........................................................................................................................................ 8
Sensors .................................................................................................................................................. 8
Pressure Sensors ............................................................................................................................... 8
Serial Port......................................................................................................................................... 9
States ..................................................................................................................................................... 9
Definition ......................................................................................................................................... 9
Pump Sludge .................................................................................................................................... 9
Clarifier Mixer On............................................................................................................................ 9
Centrifugal Pump Off ....................................................................................................................... 9
Completely Mix ............................................................................................................................... 9
Settle ................................................................................................................................................ 9
Off ...................................................................................................................................................10
Table 4: Summary of the process controller states for the short mix time .................................10
Table 5: Summary of the process controller states for the long mix time ..................................10
Effluent Analysis – Turbidity or TSS Measurements ................................................................................10
ANALYSIS ...................................................................................................................................................11
Figure 7: Percent removal of TSS for trials 1-4, respectively. ...................................................12
Figure 8: Turbidity of the Effluent for trials 1-4, respectively. ..................................................12
Figure 9 : Percent Change in Effluent (NTU)vs. Percent Change in Reactor (NTU) plus trend
line ..............................................................................................................................................13
Figure 10. Reactor Concentration (NTU) vs. Perecnt Removal plus trendline. .........................13
Table 6 : Summary of relevant data used in graphs ....................................................................14
DISCUSSION...............................................................................................................................................14
CONLUSION ...............................................................................................................................................16
SUGGESTIONS ..........................................................................................................................................16
BIBLIOGRAPHY .......................................................................................................................................16
ABSTRACT
The goal of this experiment was to turn a bench scale waste water treatment plant
that was batch processes into a continuous flow system by changing the equipment and
automated system. For our experiment, we varied two conditions: coagulant is in the
system, and complete mixing in the clarifier. Our hypothesis, based on research from a
previous experiment, was that we expected the best percent removal rate to occur when
the system had both complete mixing and coagulant. We tested the efficiency of our
system by measuring the turbidity of the clarifier effluent and the reactor effluent then
finding the percent removal of total suspended solids. While our hypothesis seems
correct, it is not thoroughly supported by the data. The total range for our percent
removal was 97.95% to 98.53% with the greatest change from the initial trial occurring in
the last trial. The last trial included coagulant in the system as well as complete mixing
in the clarifier. This change between these two values was 0.33 or 0.34%. This small
error can be attributed to an error in the measuring of the parameters. In order to obtain a
true understanding of how the system would react with the same parameters varied we
would have to keep the reactor concentration constant. With more data from both
experiments we possibly find which parameter has the strongest effect of the percent
removal, whether it is mixing, coagulant, or reactor concentration.
OBJECTIVE AND INTRODUCTION
The main purpose of this lab is to design, build, operate, and automate a miniature
wastewater treatment plant and conduct research to optimize plant performance8. To
begin with, the bench scale wastewater treatment plant was operating as a batch reactor.
For the size of our lab this size is practical, however, a real wastewater facility needs to
make the system continuous flow to maximize efficiency of the plant. Our goal was to
make the batch reactor system into a continuous flow system to make our wastewater
plant more realistic.
In converting the wastewater plant from a batch to a continuous flow system,
there are many equipment and automated differences. We knew the basic equipment and
software conversions by referring to a group’s report from the CEE 453 spring 2004 class
who tried to turn the batch reactor system into a semi-batch reactor system2. The largest
difference in terms of method and equipment is the separation of the aeration and settle
steps. In a batch reactor, both steps are done in one reactor; while for a continuous flow
system, our reactor only acted as the aeration reactor and we added a separate clarifier
tank.
Last spring’s group was our major reference and the starting block for our
2
project . They measured system efficiency by the amount of volatile suspended solids
(VSS) in the effluent. We decided to compare the total suspended solids (TSS) by
measuring the turbidity of the clarifier effluent and aeration reactor. Turbidity refers to
how clear the water is. The greater the amount of TSS in the water, the murkier it appears
and the higher the measured turbidity7.
In our experiment, we took turbidity measurements to compare conditions based
on two variables. Unlike the previous group2, we are comparing conditions quantitatively
and finding the most efficient TSS and percent TSS removal, instead of varying
conditions without quantitative analysis and comparing the ‘best’ continuous flow reactor
effluent VSS with the average batch reactor VSS. One condition that we varied was the
amount of mixing that was conducted in the clarifier. From the spring group’s report2,
we knew that we needed some kind of mixing to prevent the sludge draining in a cyclone
fashion and only pumping out clear water and not the sludge that would build up on the
side of the clarifier bottom. We had a short and long mixing time to either stir just the
bottom sludge up or to stir the entire tank. The other variable we incorporated was
adding a coagulant or not. The coagulant we used was aluminum sulfate, Al2(SO4)3.
Looking at Figure 1, we chose Al2(SO4)3 verses ferrous chloride, FeCl3 at a dose of 30
mg/L because we knew the effluent turbidity would not be above 10 NTUs. Table 1
shows each variable with it’s state for the different trials. Looking at the previous
group’s results2, we hypothesize that the best conditions would include coagulant and the
most mixing in the clarifier tank.
Dose (mg/L)
Figure 1: Adopted diagram of effluent turbidity verses coagulant dose1
Table 1: Summary of variables states for each trial
Trial
Coagulant
Mixing time
1
No
Short
2
No
Long
3
Yes
Short
4
Yes
Long
METHOD
The experiment consisted of three major set ups: the physical set up of equipment,
the animated part consisting of process controller software, and finally the calculation of
results by turbidity measurements. The actual set up can be seen in Figure 2.
Figure 2: A schematic drawing of our bench scale continuous waste water treatment plant
Plant Set Up – Equipment and Flows
Reactor Inflows
Water: The source of tap water is out of the room that is distributed at an
unknown flow rate through a tube that hangs from the ceiling of the bench area. We have
the water pumped in a size 15 PharMed tube by a peristaltic pump and connected with the
synthetic waste tube by a T-connector that has the outgoing tube entering the aeration
tank a.k.a. activated sludge reactor (reactor). Because of our continuous flow system, the
water is constantly flowing.
Synthetic Waste: The waste is originally at a concentration of 30X that is
stored in a refrigerated 1L bottle. A tube runs from the 1 L bottle and is also pumped in a
size 13 PharMed tube by the peristaltic pump to the connector piece that brings the water
and waste together to flow into the reactor. Since the concentration is at 30X, the tap
water must be combined at the connector to ensure a 1X concentration. The coagulant is
added to the system via the waste stream by adding the solid Al2(SO4)3 into the waste
bottle.
Aeration: The reactor has activated sludge that needs oxygen. There is an
inflow of air that has a source outside of the room to valves at each bench. From the
valve, the air is passed through a tube to an air accumulator (1L Nalgene bottle) and
enters into side of the reactor and disperses by a distribution stone. For the continuous
flow reactor, the air flow does not need to be regulated or turned off at any time, but in
case we do want to control the air flow there is a solenoid valve with a needle valve.
Recycle Stream: From the bottom of the conical clarifier, there is a flow
of sludge that is pumped via a centrifugal pump to the side of the reactor. Incorporated in
the centrifugal pump is a solenoid valve that controls the power to the pump. Although a
continuous flow reactor requires the sludge to be pumped constantly, we could not do this
because of the unvarying power and tube size of the pump. Unfortunately, the peristaltic
pump would not be powerful enough, nor have the right size tubing for pumping sludge.
Therefore, through the process controller, the sludge is recycled for a small amount of
time for each cycle.
Clarifier Flows
Reactor Effluent: The reactor effluent empties by a weir in the form of a
½ inch plastic tube that tops off at about 5 L in the reactor and the bottom of the plastic
tube is attached to a right angle connector piece at the side of the reactor. From the side
of the reactor, the effluent travels to the peristaltic pump and is pumped in size 18
PharMed tubing up to the top of the clarifier. Reactor effluent is constantly pumped at a
faster rate than the combined water and waste flow because as long as the weir is set at
about 5 L, the reactor will not overflow and stay at the set volume.
Clarifier Effluent: The clarifier effluent is the final product. There is also
a weir in the same form as in the reactor, with the right angle connector at the top of the
clarifier tank. Although there is no pump for the effluent, the effluent flows because
gravity makes the effluent flow out through the tubes and into the lower drain. The weir
is kept at about 6 L in the clarifier.
Pumps
Peristaltic Pump: The pump by Cole Parmer is apart of the Masterflex
series with 3 easy-load pump heads3. Each of the pump heads had a different tube sizing
for the various flows to make the overall flows in the right proportions for making the
synthetic waste into a 1X concentration and for the reactor effluent to have a larger flow
rate than the combined flow of the tap water and waste.
Centrifugal Pump: The pump by McMaster is apart of the magnetic-drive
pumps that is used for high flow. Since the sludge had large particles, to ensure no
blocking of the tubing, the peristaltic pump could not be used. However, the centrifugal
pump had a relatively large value for the minimum flow rate and the tube size could not
be altered. Therefore, the process controller had to be utilized to turn the pump on for 3.5
seconds, a value found by trial and error, in the cycle of around 600 seconds.
Mixers
Stir Bar: A 1 ½ inch long magnetic stir bar mixed the contents of the
reactor. The stir bar mixed the solution constantly to imitate a continuously stirring tank
reactor (CSTR).
External Propeller: We utilized orthokinetic mixing, or forced collision
due to turbulent mixing1, with a mechanical propeller that has the prop located about half
a foot above the sludge exit in the clarifier. A solenoid valve controls the propeller.
Tanks/Reactors
Aeration Tank/Activated Sludge Reactor (reactor): The reactor consisted
of a 6 L Rubbermaid square box with three outlets through the lower part of two sides for
inflow tubes. Our reactor performed the function of an activated sludge reactor along
with an aeration tank. The reactor had a stir bar to mimic a continuously stirring tank
reactor, CSTR. Table 2 shows the mass balance of the reactor. The water, waste, and
recycle stream all entered the reactor with only the reactor effluent as the outflow. The
recycle stream had a variable flow rate, but made the reactor volume rise about ¼ L after
3.5 seconds of pumping, resulting in a flow rate of about 4000 mL/min. The reactor
effluent had a flow rate that was more than the summation of the inflows because the
reactor could never overflow. Therefore, the reactor effluent was more than 38 mL/min.
Table 2: Mass balance of reactor, including parameters of the peristaltic pump
Stream
PharMed Tubing Size
Flow rate (mL/min)
Time flow in 1 cycle (s)
Mass in 1 cycle (mL)
Water
15
14
600
140
30X Waste
13
.5
600
5
Recycle Stream
½ inch plastic tubing
~ 4000
3.5
~ 233
Reactor Effluent
18
>38
600
>380
Clarifier/Settling Tank (clarifier): The clarifier consists of a column with
a conical bottom; the dimension of the entire tank is shown in Figure 4. The top of the
clarifier is a column that holds 7 L, and the bottom cone of the clarifier holds 1 L, so the
total volume of the clarifier is 8 L. There are 3 outlets from the clarifier: 2 at the bottom
of the cone, 1 which is plugged and 1 which pumped sludge to the reactor; 1 at the 6 L
mark at the top of the column. The outlet at the top of the column also has a plastic weir
that tops off at 6.8 L and it empties the effluent into the drain by a ½ inch plastic tube.
Table 3: Reactor and clarifier dimension summary
Equipment
Max
Weir
Length Width Height
Top
Bottom
Volume
Volume (L) (inch) (inch) (inch) Diameter Diameter
(L)
(inch)
(inch)
Reactor
6
5
8.5
8
7
Clarifier Column
7
6.8
11 5/8
6 3/8
6 3/8
Clarifier Cone
1
6
6 3/8
2
Typical Clarifier* 3,000,000
169
1,180
3.28
* Values taken from Metcalf and Eddy, see # 4 in the BIBLEOGRAPHY section
Plant Set Up – Process Controller Software
Black Control Box
Solenoid Valves: Solenoids comprises of a wire coil with a mobile
plunger seated against it and when an electric current is applied to the coil a magnetic
field is created. The magnetic field forces the plunger to either end of the coil, acting as
the valve in allowing or preventing the action6. For our experiment, we utilized 3
solenoid valves: 1 with the centrifugal pump, 1 with the airflow out of the accumulator,
and 1 with the peristaltic pump. With the batch reactor, there also needs to be a solenoid
valve for the stir bar operation and the airflow, but since we had a continuous flow
reactor the stir bar and air flow always operate.
Stamp Box: The box is an electrical device that can convert signals from
solenoid valves and a 24 V relay that controls a 110 V centrifugal pump. The stamp box
connects by a serial wire to the serial port on the bench top, which connects to the
computer9. Our stamp box had 2 ports filled, 1 from the peristaltic pump solenoid valve
and 1 that was a port splitter combining the centrifugal pump’s 24 V relay and it’s
solenoid valve.
Sensors
Pressure Sensors: Pressure sensors comprise of diaphragms that flex with
pressure and induces a stress to resistors, resulting in an electrical output5. The electrical
output is passed to the serial port on the bench top with the output as pressure readings
seen on the computer. In our experiment, we monitored the pressure of the air
accumulator, but did not need to monitor any other values. For the batch reactor system,
a pressure sensor is needed to measure the volume of the 1 L bottle of synthetic waste
and of the 6 L reactor. However, since we have a continuous flow system, the waste flow
is constantly running, controlled by the peristaltic pump, and the reactor has a weir at a
certain volume to control overflow, so neither volume needs controlling.
Serial Port: The serial ports located on every bench top act like data
converters by taking electric pulses and turning them into data for the computers to show
users information that they can understand. Our serial port had 2 inputs: 1 from the black
control box and 1 for the accumulator air pressure. The data from the serial port is
viewed through the process controller software Labview.
States
Definition: The process controller software has different ‘states’ for each
cycle in which each state specifies which valves are open and closed, for how long the
state lasts, and which state proceeds. In our experiment, we had 3 valves: the aeration
valve, the centrifugal pump, and the clarifier mixer. For every state except off the
aeration valve is on, or controlled by the air valve software. Each state is described
below, with trials 1 and 3 summarized in Table 4, and trials 2 and 4 summarized in
Figure 6.
Pump Sludge: During this state the centrifuge pump is turned on for 3.5
seconds. By this time, all the sludge has settled from the mixing of the clarifier and flows
out the bottom of the conical part of the clarifier.
Clarifier Mixer On: During this state the external propeller in the clarifier
tank is turned on for 0.25 seconds. With a short mixing period, there is only a small
amount of sludge that is mixed because the current in the clarifier does not travel above
the cone in the bottom of the clarifier. The length of time of this state is one of the
variables for our experiment. All trials incorporate the short mixing time because after
the sludge is pumped, the short mixing period must slightly disturb the remaining sludge.
Centrifugal Pump Off: During this state neither the centrifuge nor the
external propeller is operating. This state operates for the bulk of the time because this is
when the reactor activates and aerates the sludge while the clarifying tank settles the
sludge.
Completely Mix: This state has the same characteristics as the ‘Clarifier
Mixer On’ state but is added for trials 2 and 4 because the clarifier needed to become
completely mixed. With the long mixing period, the entire tank becomes mixed because
the current in the clarifier stirs up the entire tank.
Settle: This state has the same characteristics as the ‘Centrifugal Pump
Off’ state but is added for trials 2 and 4 because the clarifier needed time to settle sludge
after the complete mixing of the tank.
Off: This is the default state that the process controlling software
automatically enters if there is a problem; i.e. power failure.
Table 4: Summary of the process controller states for the short mix time
State
Pump Sludge
Clarifier Mixer On
Centrifugal Pump Off
Off
Parameters turned on
Air Valve;
Centrifugal Pump On
Air Valve; Clarifier
Mixer on
Air Valve
No parameters on
Time in State (sec)
3.5
Next State
Clarifier Mixer On
0.25
Centrifugal Pump
Off
Pump Sludge
600
Default state
Table 5: Summary of the process controller states for the long mix time
State
Pump Sludge
Clarifier Mixer On
Centrifugal Pump Off
Completely Mix
Settle
Off
Parameters turned on
Air Valve;
Centrifugal Pump On
Air Valve; Clarifier
Mixer on
Air Valve
Air Valve; Clarifier
Mixer on
Air Valve
No parameters on
Time in State (sec)
3.5
Next State
Clarifier Mixer On
0.25
Centrifugal Pump
Off
Completely Mix
Settle
400
2
198
Default state
Pump Sludge
Effluent Analysis – Turbidity or TSS Measurements
Turbidity is measured easily with a turbidimeter in units of NTU, or
Nephelometric Turbidity Units. A turbidimeter works by estimating how light is
scattered by suspended particulate material in the water. Usually, we measure turbidity
to provide a cheap estimate TSS concentration (mg/L). However, TSS measurement
requires you to filter a known volume of water through a pre-weighed filter disc to collect
all the suspended material (greater than about 1 micron in size) and then re-weigh it after
drying it overnight at ~103°C to remove all water in the residue and filter. This is tedious
and difficult to do accurately for low turbidity water - the reason why a turbidimeter is
often used7. With our turbidimeter, we filled 7 vials: 1 standard of distilled water, 3
effluent samples, and 3 reactor samples. For each of the 4 trials, we did 3 measurements,
1 each day for 3 days. The steps for each measurement were the same for each vial.
First, the vials were wiped with a Kim wipe to minimize the fingerprints on the vial that
would increase the TSS value. Since the reactor samples had such high turbidity that
decreased with time, or as the particles settled, we had a standard 15 seconds of wrist
shaking the vials, then writing down the NTU value after 20 seconds of placing them in
the turbidimeter. Also, each vial number was kept the same for the bottle to decrease
discrepancy within the experiment. We are looking at two numbers from the
turbidimeter: the actual turbidity reading, and the percent of TSS removal. The clarifier
effluent turbidity reading gives a portrayal of the final TSS concentration, while
comparing the reactor and clarifier effluent turbidity readings give an efficiency reading
of percent TSS removal. Equation 1 shows how to calculate the percent TSS removal.
ANALYSIS
The primary form of data analysis for our experiment is percent removal of Total
Suspended Solids (TSS). The equation for the removal of TSS is given below
%removal 
TSSr (Re actor )  TSSe( Effluent)
*100
TSSr (Re actor )
(1)
Where TSSr is in mg/L
TSSe is in mg/L
An empirical relationship exists that allows the turbidity of a sample, measured in
NTU to be converted into TSS. It is written below
TSS  TSSf * T
Where TSS is in mg/L
TSSf is a conversion factor (mg/L/NTU)
T is turbidity in NTU
(2)
(Metcalf & Eddy, 2003)
Equations (1) and (2) can be combined to yield
%removal 
TSSf * Tr  TSSf * Te
Tr  Te
*100 
*100
TSSf * Te
Te
It can be seen that that conversion factor for the turbidity cancels out and the
percent removal can be viewed as just a function of the turbidity. The results of this
analysis are displayed in the graph below.
(3)
100.00
99.50
% Reduction
99.00
98.53
98.50
98.20
97.98
98.00
97.95
97.50
97.00
1
2
3
4
Trial #
Figure 7: Percent removal of TSS for trials 1-4, respectively.
It is also important to look at the effluent of each trial individually as well. The
effluent is measured in Nephelometric Turbidity Units (NTU). It should be noted that the
effluent concentration does not vary directly with the percent removal for the respective
trial. This diagram is provided below
12.50
12.04
12.00
11.50
Turbidity (NTU)
11.00
10.55
10.50
10.00
9.54
9.50
9.00
8.82
8.50
8.00
1
2
3
4
Trial #
Figure 8: Turbidity of the Effluent for trials 1-4, respectively.
When analyzing the data it was noted that often when the reactor concentration
decreased, the effluent concentration also decreased. An analysis of the change in
effluent concentration to the change of reactor concentration was conducted. The data
points from the first trial was used as a reference to measure percent change. The
equation used for percent change is of the same form as equation (1), but the turbidity
values for the specific trials are used. The graph is shown below
50.00%
40.00%
30.00%
% Change R
20.00%
10.00%
-20.00%
-15.00%
-10.00%
-5.00%
0.00%
0.00%
5.00%
10.00%
15.00%
20.00%
-10.00%
-20.00%
-30.00%
-40.00%
% Change E
Figure 9 : Percent Change in Effluent (NTU)vs. Percent Change in Reactor (NTU) plus trend line
It was also noted that there could also be a relationship between reactor
concentration and the actual percent removal of TSS. In general as the reactor
concentration increased, so did the percent removed. This relationship is given in the
graph below.
98.60
98.50
% Removal
98.40
98.30
98.20
98.10
98.00
97.90
300.00
400.00
500.00
600.00
700.00
Reactor NTU
Figure 10. Reactor Concentration (NTU) vs. Perecnt Removal plus trendline.
800.00
900.00
All of the data used to make the graphs can be summarized in the following table.
Table 6 : Summary of relevant data used in graphs
Trial
1
2
3
4
Effluent
10.55
8.82
9.54
12.04
Reactor
587.00
436.78
466.00
817.67
% Change E
-16.45%
-10.55%
14.10%
% Change R
-25.59%
-20.61%
39.30%
% Removal
98.20
97.98
97.95
98.53
DISCUSSION
As stated in our hypothesis, we expected the best percent removal rate to occur
when the system had both complete mixing and coagulant. While this turned out to be
the case, it is not thoroughly supported by the data. The total range for our percent
removal was 97.95 – 98.53 with the greatest change from the initial trial occurring in the
4th trial. This change between these two values was 0.33 or 0.34%. This small error can
be attributed to an error in the measuring of the parameters.
One parameter that we should control in the future to get better results is the
amount of sludge in the reactor. During the duration of the experiment we had mass
shifts of the sludge from reactor to clarifier in addition to the growth of the bacteria itself.
When the experiment was started we had approximately 1.2 L of sludge in the reactor.
When we concluded we had approximately 1 liter of sludge in the clarifier and
approximately 2 L of sludge in the reactor. This is evidence that we had significant
growth of the bacteria during the experiment that would account for mass increase. But if
this was the only cause for mass change then the reactor should have been constantly
increasing in concentration, which according to Table 6, is untrue. The average
concentration for trials 2 and 3 are less the initial concentration, which indicates a
transfer of mass other than growth.
The other reason for mass shifts has to do with the clarifier. Initially a large
portion of mass from the reactor moved into the clarifier where ideally it would settle and
then provide a good transfer of sludge back into the reactor. Over time we observed that
much of the sludge stuck to the sides of the clarifier and could therefore not be
transferred back to the reactor. In addition to the mass sticking to the sides, during trial 3,
there appeared to be a cavern of sludge forming on the bottom. During this trial
coagulant was added, but there wasn’t any mixing step for the clarifier. Initially the
sludge settled, but as it was pumped out the remaining sludge adhered to each other and
prevented new sludge from going to the bottom. Since there wasn’t an extended mixing
step, no shear force was provided to break up the floc and let it settle. This problem was
solved during the last trial, when we had both the coagulant and the mixing step present.
This actually provided a very good transfer of solids, in fact at the end on the experiment
there were very little solids in the bottom of the clarifier, indicating that it had been
transferred to the reactor. This was also indicated by the rapidly increasing turbidity in
the reactor for the final trial.
As mentioned, some of sludge was sticking to the sides or the clarifier and could
therefore not be transferred back into the reactor. This was a big factor due to the
dimensions of the clarifier. Typical reactor dimensions are 4.3 m deep, 30 m wide, and
have a bottom slope of 1/12. This ensures that the surface area of the walls is negligible
with respect to the surface area of the bottom. With the height of our column, about 1.5
feet, the approximate width of our scaled-up clarifier would have to be 10 feet. In fact,
the surface area of our walls was much greater than that of the bottom, making the
amount of sludge that stuck to the walls very significant. Unless good data is obtained,
we can not apply it to real world situations. In order to get results that can be used in a
real plant, we will need to construct a clarifier that is more similar to typical ones. This
could be partially achieved by limiting the depth of the clarifier column.
The process of obtaining a constant concentration in the reactor is very difficult
and would not be feasible due to time constraints for the lab. First you would have to
ensure a good transfer rate of solids back into the reactor. This would require much time
and modification of the logic to get the concentration within an acceptable range.
Further, you would also have to model the growth rate of the bacteria and add a sludge
removal step to account for any excess that would develop. The other option instead of
adding a sludge removal step would to modify the influent feed concentrations so that the
growth rate would be the same as the death rate. Both of these options would require the
collection of data to obtain a growth kinetics model, which would require more time.
Once a steady concentration is obtained we can then collect data and then change the
parameters to the next trial. Since the parameters change, the transfer rates would also
change. This would again require the modification of the logic and would be very time
consuming. For the purposes of this lab it could be very difficult to get a constant
concentration in the reactor. If we could ensure that the reactor concentration would
remain constant we could they ensure that changes in effluent quality were due to
changing the mixing and coagulation parameters alone.
It is possible however, that the turbidity of the effluent is just direct function of
what is in the reactor. This analysis was done by taking the data from the first
experiment and using it as a base measurement to which later trials could be compared.
The percent change for both the effluent and reactor were taken and then plotted. If the
effluent is dependant upon the reactor then a straight line should result. This was not the
case, as could be seen in Figure 9. Though a line with a good correlation was fir, it is
inaccurate due to asymmetrical data. This problem could be corrected by obtaining data
with an even range, but as mentioned above could be difficult due to time constraints.
A similar analysis was also conducted that compared reactor concentration and
percent removal. In general we observed that as the reactor concentration increased so did
the percent removal, as can been seen in Figure 10. This could be do to the fact that a
higher concentration of particles enter the clarifier and can then flocculate and settle at a
faster rate than at a low concentration. This could also be due to fact that the change in
effluent concentration is much smaller than the change in reactor concentrations. The
would account for the percentage removed to increase even though the effluent quality
has not changed much. Once again with the analysis graph, the data is not symmetrical
so even though a good trend line is found it is not entirely accurate. The best way to test
for the relationship would to do an even distribution of reactor concentrations that would
start at zero.
In general the performance curve for a system should show the removal rate
approach a value asymptotically as the initial concentration increases. It is possible that
for our data, we were in the range where the value had already approached the asymptotic
value, and that any increase in the concentration would not be significant in the overall
percent removal rate. This can be tested by using reactor concentrations that are much
lower, and seeing how the system responds. It is entirely possible that our data is very
good, but rather incomplete, and therefore inconclusive.
CONLUSION
We were able to see in this lab how the percent removal of total suspended solids
for a clarifier would change if you change system parameters, such as amount of mixing
and addition of a coagulant. From this data we were able to see what other parameters
we need to fix in order to obtain better conclusive results. In order to obtain a true
understanding of how the system would react with the same parameters varied we would
have to keep the reactor concentration constant. This variability in reactor also raised
questions for some other research that can be done. By doing experiments with an even
distribution of concentrations we can then see if the effluent concentration is independent
of the mixing time or coagulant. With more data from both experiments we possibly find
which parameter has the strongest effect of the percent removal, whether it is mixing,
coagulant, or reactor concentration.
SUGGESTIONS
Overall, this lab was very helpful for understanding the trials and tribulations that
occur at a wastewater treatment plant, at least on a small scale. The data acquisition
software was a little difficult to understand initially, but after we got used to Labview it
was not too difficult to configure our automated system. The only way to get used to
Labview is to practice with it, maybe an interactive tutorial at the beginning of the course
that everyone has to complete would be helpful. The equipment for the experiment was
difficult to organize, but amusing to assemble. We found all the equipment satisfactory
for our uses. All of the helpful links online make the goals and requirements for the
experiment easy to understand. Since each group is allowed to change to experiment, we
have no modification suggestions except for the continuous flow reactor. Another
parameter that we did not try is to vary the clarifier column height by having the effluent
weir come through the plugged hole in the bottom and topping off at various heights.
BIBLIOGRAPHY
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1. Bisogni, James. “Coagulation”. Viewed November 2004. CEE 656 Class
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