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.