A Bench Scale Anaerobic Digester

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

2. 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

3. 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

4. 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

5. Plant Configuration

8. State Name Explanation 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) Settle All valves closed; sedimentation. Time> 1 hr (Drain) Drain 360 mL drained from reactor via pump Time> 10 min (fill with waste) States and Logic

9. Results

10. Results, continued

11. 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.

12. 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.

13. 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.

14. 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.