A Bench Scale Anaerobic Digester
Jacob Krall
David Harrison
Justin Ferrentino
CEE 453
Introduction

Aerobic vs. Anaerobic Treatment of Waste

Rates


Aerobic bacteria grow much faster, consume waste faster
Cells

Much easier to grow aerobic bacteria, less sensitive to
temperature, and other factors
Why Anaerobic Digestion?

Energy
Cost to aerate tanks
 Cost to dispose of sludge (landfills?)
 Anaerobic bacteria produce CO2 and CH4 (biogas)

Chart from RudiThai Group, at
www.draaisma.net/rudi/anaerobic_wastewater_treatment.html
Objectives

Build an operational bench scale sequencing batch
reactor

Attempt to characterize performance based on
waste concentration and cell concentration

Achieve high solids retention rate
Plant Configuration
Vacuum Line
Reactor Effluent
Gas
Drain
Reactor
Peristaltic Pump
E-1
Hot Plate
Influent Waste/Water
Reactor Influent
Gas Regulation Tank
States and Logic
State Name
Fill With Waste
Explanation
Exit Condition (state exiting to)
Pump in 86 mL 20x waste to reactor
Time>30 s (digestion startup)
Gas production begins; pressure
Time>1/2 day (gas production) or
Allowed to build up
Gas Pressure>-10 kpa
(gas production)
gas production continues;
Time>1 day (Settle) or
Pressure builds up
Gas Pressure>-10 kpa (gas vent)
Reactor vented to maintain vacuum
Gas Pressure<-40 kpa (gas production)
Settle
All valves closed; sedimentation.
Time> 1 hr (Drain)
Drain
360 mL drained from reactor via pump
Time> 10 min (fill with waste)
Digestion Startup
Gas Production
Gas Vent
Results
-20000
-25000
Gas Pressure (Pa)
-30000
y = 13403x - 42160
R2 = 0.9824
-35000
-40000
y = 34401x - 57019
R2 = 0.9994
-45000
-50000
-55000
-60000
0
0.2
0.4
0.6
Time (days)
0.8
1
1.2
Results, continued
-10000
y = 11946x - 34676
R2 = 0.9951
-15000
Gas Pressure (Pa)
-20000
-25000
-30000
-35000
-40000
y = 30852x - 46598
R2 = 0.9962
-45000
-50000
0
0.2
0.4
0.6
0.8
1
Time (days)
1.2
1.4
1.6
1.8
2
Results, continued

In a given cycle, 11.7-15.0 kpa of gas production
attributed to anaerobic digestion (as opposed to
endogenous respiration), equivalent to 0.0140 to 0.0175
mols of gas.

76-97% of theoretical gas production given amount of
waste being treated.

Adding additional cells and increasing concentration of
waste did not significantly increase rate of digestionsuggests not all cells were viable.
Discussion: An anaerobic sludge
digester comes with some difficulty.

Constant temperature and Constant Stirring:



The hot plate and the stirrer are a part of the same unit. However,
the stirrer must be left on constantly while the hot plate must be
cycled on and off.
Our Solution: Two connections to the unit. An external one to
leave it on constantly, an internal one to control the heating.
Gas collection and Pressure Buildup:


As pressure builds up within the collector, there is the risk of an
explosion, or at least a foul-smelling gas leak.
Our Solution: Connect the collector to a vacuum line, and run at
negative pressure.
Discussion Part 2

Loss of Cells during Draining:
The cells would not settle in the reactor, so when the
reactor is drained, some cells are drained with it.
 If we were to add particles, we would not be able to
keep the particles suspended and evenly distributed
while stirring.
 Our Solution: Drain as little of the reactor as
possible. This also causes a high recycle rate and cell
retention.

Suggestions for Future Experiments

Experiment with Settling:



We experimented with different media for enhanced settling and
solids retention. Keeping media in suspension, even at the fastest
stirring speed possible was a challenge.
A good experiment would be to further investigate using media
to enhance settling and thus keep more solids in the reactor..
Experiment with Temperature:

Our reactor relies on constant heating, to maintain an optimum
temperature. This is a major drawback. A good experiment
would be to determine how well the reactor works at less
optimum, more realistic temperatures.