A Bench Scale Anaerobic Digester Jacob Krall David Harrison Justin Ferrentino CEE 453

Introduction Aerobic vs. Anaerobic Treatment of Waste Aerobic vs. Anaerobic Treatment of Waste Rates Rates Aerobic bacteria grow much faster, consume waste faster Aerobic bacteria grow much faster, consume waste faster Cells Cells Much easier to grow aerobic bacteria, less sensitive to temperature, and other factors Much easier to grow aerobic bacteria, less sensitive to temperature, and other factors

Why Anaerobic Digestion? Energy Energy Cost to aerate tanks Cost to aerate tanks Cost to dispose of sludge (landfills?) Cost to dispose of sludge (landfills?) Anaerobic bacteria produce CO 2 and CH 4 (biogas) Anaerobic bacteria produce CO 2 and CH 4 (biogas) Chart from RudiThai Group, at

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

Plant Configuration

States and Logic State NameExplanationExit Condition (state exiting to) Fill With WastePump in 86 mL 20x waste to reactorTime>30 s (digestion startup) Digestion StartupGas production begins; pressureTime>1/2 day (gas production) or Allowed to build upGas Pressure>-10 kpa (gas production) Gas Productiongas production continues;Time>1 day (Settle) or Pressure builds upGas Pressure>-10 kpa (gas vent) Gas VentReactor vented to maintain vacuumGas Pressure<-40 kpa (gas production) SettleAll valves closed; sedimentation.Time> 1 hr (Drain) Drain360 mL drained from reactor via pumpTime> 10 min (fill with waste)

Results

Results, continued

In a given cycle, kpa of gas production attributed to anaerobic digestion (as opposed to endogenous respiration), equivalent to to mols of gas. In a given cycle, kpa of gas production attributed to anaerobic digestion (as opposed to endogenous respiration), equivalent to to mols of gas % of theoretical gas production given amount of waste being treated % 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. 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. Constant temperature and Constant Stirring: 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. 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. 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: 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. 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. Our Solution: Connect the collector to a vacuum line, and run at negative pressure.

Discussion Part 2 Loss of Cells during Draining: 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. 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. 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. 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: 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. 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.. A good experiment would be to further investigate using media to enhance settling and thus keep more solids in the reactor.. Experiment with Temperature: 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. 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.