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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

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a bench scale anaerobic digester

A Bench Scale Anaerobic Digester

Jacob Krall

David Harrison

Justin Ferrentino

CEE 453

  • 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
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


  • Build an operational bench scale sequencing batch reactor
  • Attempt to characterize performance based on waste concentration and cell concentration
  • Achieve high solids retention rate
states and logic

State Name


Exit Condition (state exiting to)

Fill With Waste

Pump in 86 mL 20x waste to reactor

Time>30 s (digestion startup)

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

gas production continues;

Time>1 day (Settle) or

Pressure builds up

Gas Pressure>-10 kpa (gas vent)

Gas Vent

Reactor vented to maintain vacuum

Gas Pressure<-40 kpa (gas production)


All valves closed; sedimentation.

Time> 1 hr (Drain)


360 mL drained from reactor via pump

Time> 10 min (fill with waste)

States and Logic
results continued11
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 digestion- suggests not all cells were viable.
discussion an anaerobic sludge digester comes with some difficulty
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
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
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.