A Lab Bench Scale Anaerobic Digester
Justin Ferrentino
David Harrison
Jacob Krall
CEE 453
Fall 2004
1
Table of Contents
Table of Contents ................................................................................................................ 2
Abstract ............................................................................................................................... 3
Objectives ........................................................................................................................... 3
Introduction ......................................................................................................................... 3
Methods............................................................................................................................... 4
Plant Configuration ......................................................................................................... 4
Reactor Process Control .................................................................................................. 4
Results and Analysis ........................................................................................................... 5
Discussion ........................................................................................................................... 9
Suggestions ....................................................................................................................... 10
Appendix ........................................................................................................................... 11
2
Abstract
A lab bench scale anaerobic digestion system was created using activated sludge
obtained from an anaerobic digester at the City of Ithaca Wastewater Treatment Plant.
An anaerobic digester was set up in the lab to treat synthetic waste at a BOD
concentration of 6500 mg/L and the system was automated using Process Controller
software. Results showed that anaerobic digestion was indeed occurring in the system
but suggested that many of the cells in the reactor were not viable. Future research
should include investigating ways to maintain higher cell populations and treat a waste
with a concentration closer to the typical wastewater BOD of 325 mg/L.
Objectives
Our objective was to explore the use of anaerobic digestion for wastewater
treatment with the use of a sequencing batch reactor. Synthetic waste was converted into
methane and carbon dioxide gas, and vented out of the system. To ensure optimum
growth conditions for the bacteria, the temperature of the reactor was maintained at 35°C,
and no oxygen was allowed in. We expected the treatment of the same volume of waste
to be slower under anaerobic conditions than aerobic conditions, but the goal of this
project is to lay some practical groundwork for further research projects in this area.
Introduction
The process of aerobic treatment of wastewater is well understood and widely
used, but suffers from two severe drawbacks. These drawbacks are that it requires large
amounts of energy to keep the reactor contents aerated, and that the process produces
large volumes of sludge that must often be landfilled. Anaerobic digestion of wastewater
is also widely used, as in the case of home septic tanks, but is not practiced at the
municipal scale. Anaerobic digestion offers several advantages over aerobic digestion, in
that it does not produce nearly as much sludge to be disposed of, and also that it yields
energy in the form of methane gas. The drawbacks include that the digestion proceeds at
a slower rate and that the bacteria favor temperatures of around 35°C. Despite these
challenges, if anaerobic bacteria cell concentrations in a reactor could be made high
enough, unheated municipal waste water could be treated. This project attempts to start
down that path.
3
Methods
Plant Configuration
Our plant was built around a 2.5 Liter glass jug with an inlet port at the bottom.
The jug was placed on a hotplate, with a temperature probe placed on its exterior, and
with a magnetic stirrer placed inside it. A rubber stopper with two holes was used to seal
the top, while the influent line came in at the bottom port. The drain was made by a long
steal tube that could be set to different heights to correspond to different recycle volumes
(for a higher recycle volume, the drain pipe was simply set at a higher level). A
Masterflex Model 7518-00 peristaltic pump with two drive heads and valves were used to
both fill and drain the reactor. The pump flow rate was set at 2.88 mL/s. The drain valve
was set at the desired height. In operation, the pump system worked by opening only the
valve that was required, as in the case of filling the reactor where the influent valve
would be opened, while the drain valve was left closed. On the influent end, a single line
was run through the pump head by having one valve control tap water and one valve
control the raw waste, but having both feed the same influent line. At the effluent end,
waste was simply pumped out to a drain.
The only other major system for the reactor is the gas collection/regulation
system. Gas regulation was achieved by having the second hole in the rubber stopper go
to a second 2.5 Liter glass jug, with a 200 kilo Pascal pressure sensor measuring relative
pressure between the gas collection jug and the atmosphere. The gas was vented to a
vacuum line with the use of another valve, and the system was designed and run under a
vacuum.
Vacuum Line
Reactor Effluent
Gas
Drain
Reactor
Peristaltic Pump
E-1
Gas Regulation Tank
Hot Plate
Influent Waste/Water
Reactor Influent
Figure 1: Process Schematic
Reactor Process Control
Our reactor was set up to be controlled via the Process Controller software.
Below is a table outlining the states of our reactor and our exit conditions for the final
reactor configuration.
4
Table 1. States and control logic for anaerobic digester.
State Name
Fill With Waste
Digestion Startup
Gas Production
Gas Vent
Settle
Drain
Explanation
Pump in 360 mL 20x waste to reactor
Gas production begins; pressure
Allowed to build up
gas production continues;
Pressure builds up
Reactor vented to maintain vacuum
All valves closed; sedimentation.
360 mL drained from reactor via pump
Exit Condition (state exiting to)
Time>30 s (digestion startup)
Time>1/2 day (gas production) or
Gas Pressure>-10 kpa
(gas production)
Time>1 day (Settle) or
Gas Pressure>-10 kpa (gas vent)
Gas Pressure<-40 kpa (gas production)
Time> 1 hr (Drain)
Time> 10 min (fill with waste)
During the gas production, digestion startup and gas vent states, the reactor was
kept at a temperature of 35°C using PID with both the I and D terms set at 0. Using only
the P term successfully kept the reactor very near the target temperature as the waste was
being digested. The digestion startup and gas production states appear to be very similar.
“Digestion Startup” was initially included to ensure that the reactor would be allowed to
treat the waste long enough to produce enough methane to necessitate venting at least
once. When it later became clear that given our volume of waste the reactor was unable
to produce that volume of gas, the time condition was added to allow the reactor to leave
the digestion startup state after one day. During much of our research, our reactor
included a “Fill with Water” state in order to dilute the 20x waste. However, we chose to
eliminate this state to allow our digester to treat a more concentrated waste.
Results and Analysis
In each cycle using our eventual reactor configurations, the reactor was given
0.0864 L of 6500 mg/L waste. We used several assumptions to obtain an approximation
of the theoretical volume of gas that should be produced during the digestion: 1g of
oxygen gas per gram of Carbonaceous Oxygen Demand (COD) and 1 mol of gas
produced per mol of Carbon. Our calculation appears below:
1gO2 1molO2 1molC 1mol _ gas
mgCOD
1g
0.0864 L * 6500
*
*
*
*
*
 0.018mol _ gas
L
1000mg 1gCOD 32 gO 2 1molO2
molC
Below we convert this value to an equivalent pressure produced (using a gas volume of
3L, equal to 1 full jug plus the volume of the second jug not filled with waste and solids):
P
nRT (0.018mol )(8.31kPa * L / mol * K )(308K )

 15.4kPa
V
3L
For most of the duration of our experiment, we were operating the reactor believing (due
to erroneous calculations) that the theoretical gas production was 0.11 mol, which
influenced the duration which we allowed the reactor to produce gas.
5
The following plot shows data obtained during a typical gas production phase for
our reactor. It can be seen that there are three major regions in the graph. At first, the
plot appears quite straight—the waste is being digested at an approximately constant rate.
Then, as the amount of waste available decreases, the graph starts to curve and the slope
becomes less steep. Finally, there is a second straight line region of the graph; this
represents endogenous respiration by the cells after all waste has been digested. This plot
gives a strong indication that there are two processes occurring that are producing gas:
waste digestion and endogenous respiration.
-20000
-25000
y = 13450x - 42209
R2 = 0.9826
-30000
Gas Pressure (Pa)
-35000
-40000
y = 34454x - 57029
R2 = 0.9992
-45000
-50000
-55000
-60000
0
0.2
0.4
0.6
0.8
1
1.2
Time (Days)
Figure 2. Plot showing the gas production of the anaerobic digester for a period of slightly more than
1 day.
If we take the slope of the second line, consider this the rate of endogenous respiration
and assume that this takes place at a constant rate throughout the cycle, then
(1.132674days*13.450kPa/day)=15.234 kPa of gas production can be attributed to this
process. This leaves the rest of the production, about 15 kPa of pressure, to digestion of
the waste. This value is very close to theoretical value giving an indication that we are
producing approximately our theoretical amount of gas.
The following plot is similar to the previous one, but represents a cycle that was
started immediately after adding additional cells to the reactor (after centrifuging raw
sludge to concentrate them). Using the same analysis as above but using an endogenous
respiration slope of 11.943 kPa/day (from the plot below), we attribute 11.7 kPa of gas
production to digestion of the waste. This is closer to our theoretical value than is the
value from the previous cycle, and it is fairly close to the previous value, strengthening
6
the evidence that anaerobic digestion in our reactor is indeed occurring and is producing
gas at approximately the rate we would expect.
In addition to showing that anaerobic digestion is taking place, it is of that we
added cells to the reactor between the trials shown in figures 1 and 2, but saw no increase
(and in fact a decrease) in initial rate of gas production. The turbidity of the reactor was
measured to be 320 NTUs, as compared with 350 NTUs for raw sludge obtained from the
Ithaca anaerobic digester. This indicates that the cell concentration in the reactor
remained significant and combined with the lack of effect on gas production rate after
increasing cell concentration appeared to indicate that waste concentration was the
limiting factor.
0
-5000
-10000
Gas Pressure (Pa)
-15000
y = 11943x - 34671
R2 = 0.9954
-20000
-25000
7
-30000
-35000
-40000
y = 31038x - 46611
R2 = 0.9959
-45000
-50000
0
0.5
1
1.5
2
2.5
Time (Days)
Figure 3. Plot showing Gas Production over a complete treatment cycle immediately after adding
additional cells to the reactor.
However, in a further attempt to discover what parameter was limiting our
methane production, cell population or amount of waste, we allowed the digester to run
several cycles using waste 5 times more concentrated than in our previous runs (32,500
mg/L or 100 times the normal wastewater concentration). If the initial rate of gas
production increased with the higher waste concentration, this would suggest that the
concentration of the waste is the limiting factor in our system. Conversely, no increase in
initial gas production rate would suggest that the cell population is the limiting factor. A
plot of the beginning of a cycle using 100x waste is below.
7
-40000
-44000
Gas Pressure (Pa)
y = 37801x - 60215
R2 = 0.998
-48000
-52000
-56000
-60000
0
0.1
0.2
0.3
0.4
0.5
Time (Days)
Figure 4. Plot showing gas production over the first 0.5 days of a cycle using 100x waste.
The initial slope is 37.8 kPa/day. This represents a 22% increase over in the rate
as compared to the run immediately prior to changing to 100x waste. If the only limiting
factor was the waste concentration, we would expect that a five fold increase in waste
concentration would yield a five fold increase in gas production rate. One possible
explanation is that when provided with 20x rate the cells were functioning at nearly their
maximum rate and with the 100x waste the maximum rate was reached and the cell
concentration became the limiting factor. However, since increasing the waste
concentration made only a relatively small change in production rate, a more likely
explanation is that the cells added were not viable, due to being “starved” in the weeks
since the sludge was acquired from the Ithaca Wastewater Treatment Plant, so their
addition made no difference. Thus, we feel that the limiting parameter was the amount of
viable cells in our reactor.
Our analysis shows that our primary goal, creating an anaerobic digester capable
of producing methane gas from degradation of organic waste, was achieved. We would
like our reactor to be able to treat waste more rapidly and efficiently and to degrade a less
concentrated waste (like the concentration treated at a typical wastewater treatment
plant). In the next section, we discuss these future goals as well as some of the
difficulties we faced during this project.
8
Discussion
In setting up our plant we encountered numerous problems of configuration and
automated control, mostly due to the fact that it was to be an anaerobic plant. These
problems can be classified as pertaining to heat control, gas collection/regulation, flow
control, and cycle control.
Complications with heat control stemmed from the fact that we used a hot
plate/magnetic mixer to both stir our reactor and to maintain it at 35°C. As we wanted to
have the hot plate to be able to be cycled on and off to maintain proper reactor
temperature, but also wanted the stirrer to be constantly on during the digestion startup
and gas production states, we needed to be able to control them independently. To
overcome this, we controlled power to the whole stirrer/hot plate with one relay, and
using the internal relay in the hot plate/stirrer to control power to heating element.
Gas collection was one issue that threatened the stability of the experiment from
the beginning; since gas production would be capable of producing high pressures in the
sealed reactor, explosions were a serious risk. This whole issue was avoided by running
the reactors at negative pressure off of the vacuum line in the lab. By doing this, there
was no risk of explosion, or even of foul smells from small leaks.
Flow control wound up being one of the most challenging parts of this
experiment, as many physical problems presented themselves. The first of these
problems was the fact that the mixture in the reactor did not settle readily; digester feed
waste from the Ithaca Wastewater Treatment Plant had only partial settling after spending
several weeks untouched in a refrigerator. Our concern was that without effective
settling, the anaerobic bacteria in the reactor would be wasted during draining, and gas
production would eventually drop. We attempted to overcome this problem by adding
particles that would be in suspension due to the magnetic stirrer, but would then quickly
settle out when the stirrer was shut off. The hope was that attached growth on the
particles would be the main population of bacteria in the reactor.
To select our media, we performed informal experiments in an identical reactor
filled with clean water. We tested sand, glass beads of different diameters, and activated
carbon. The results of these observations are summarized in Table 2 in the appendix.
Activated carbon seemed to have the most promise, so approximately 50 mL were
added to the reactor. The ultimate effect on reactor performance was never determined
because of the lack of good pre- and post-addition gas production data. One problem was
that over a five day break when the plant was left to run, a drain valve clogged, resulting
in waste overflow into the gas collection jug. We have hypothesized that much of the
activated carbon was drained from the reactor in this failure. Despite these problems, the
high recycle rate that we eventually set the reactor at probably helped mitigate problems
with settling and cell retention.
Cycle control wound up being an issue for our reactor that was never really solved
with our process control or physical setup. The main issue with cycle control was when
exactly to exit from gas production to drain and start the next cycle. Since we determined
that gas production seemed to occur at two relatively distinct rates, a higher rate followed
by a lower rate, we hypothesized that the lower rate was to a large degree endogenous
respiration. Although we would have liked to exit the gas production state after gas
production dropped to the lower rate, there was not the means in our process control
9
software to do so(at least not without significant software modifications). The solution
wound up being that group members were required to check the reactor at least once per
day in order to determine if a new cycle should have been started.
Overall, we would consider our project a success. We were able to get the plant
running, demonstrate that digestion of the waste was occurring and do some initial
investigation into the limiting parameters for our reactor. However, if we were to do this
experiment again a few things would be different. For one, we would have the great
advantage of having struggled with it once before. Much of our time was spent trying to
get our reactor running smoothly, and having a little experience would make our time
spent much more effective. Also, our goal would change from just being able to run the
plant to treat waste to being able to achieve high solids retention. This would probably
be examined in the context of attached bacterial growth.
Suggestions
The cells in the reactor we used did not settle under standard conditions. Even if
left in a container for weeks, they remain in a homogeneous mixture. This causes a
problem because whenever we drained some of the reactor, we lost some of the cells.
Some of the volume needs to be drained, before more waste is added, so that the volume
does not accumulate. Therefore, it would be beneficial if the cells could be made to settle
quickly after the stirring is finished. One possible solution would be the attached
bacterial growth that is discussed earlier. A possible future experiment would be to
determine what kind of particles would work best to culture an attached growth bacteria
population.
Another possible experiment would be to determine the effects of temperature.
Constant heating is a major drawback of this experiment and unrealistic at the municipal
scale. A possible future experiment would be to determine the effects of temperature on
reaction rate. As the bacteria are known to operate outside their optimum temperature
range, this experiment could be tied to that of the attached growth problem by the fact
that if the cell concentration could be made high enough in the reactor, non-ideal
temperatures would not be a significant problem in treating the waste.
10
Appendix
Table 2. Levels of suspension achieved for different media and sizes.
Media
Media
Water
Media
Volume (L)
Diameter (in)
Volume (L)
glass beads
0.05
0.0083-0.0165
2
glass beads
0.05
0.0083-0.0166
2
glass beads
0.05
0.0083-0.0167
1
glass beads
0.05
0.0083-0.0168
1
sand
0.05
1
sand
0.05
1
Activated Carbon
20-40
0.05
2
Activated Carbon
20-40
0.05
2
Stir
Rate
6
8
6
8
6
8
6
Level of
Suspension
very low
10-15%
very low
10-15%
very low
<5%
90% (near bottom of
reactor)
8
>90%
11