Use of pH Control for Two-Phase Anaerobic Digestion of Municipal Solid Waste

Two experiments were conducted using pH-controlled recycle to accomplish two-phase anaerobic fermentation of simulated municipal solid waste (SMSW). Leachate was recycled from a solid bed reactor, operated as an acid phase, to a packed-bed methanogenic fermenter (methane phase). The SMSW was added to the solid bed at a solids concentration of 15 wt. % and maintained at a temperature of 25 °C. The methane phase, which was operated as an upflow packed bed reactor, was operated at mesophilic conditions (35 °C). Both experiments utilized recycle of solid bed leachate to the methane phase (inter-fermenter recycle), which was started and stopped based on a setpoint pH using a pH controller connected to a peristaltic pump. The objectives of the work were to:Evaluate pH control as a means to increase high solids MSW fermentation kineticsIncrease the high grade methane yield from the methane phase reactorAssess inhibition effects due to heavy metals during single phase MSW fermentationEstablish inhibition effects due to heavy metals during two-phase MSW fermentationExperiment 1 was operated as a two-phase system for 100 days. Methane yield from the methane phase reactor was 0.32 SCM/kg BVS when inter-fermenter recycle occurred at a setpoint pH of 5.5. The pH-controlled inter-fermenter recycle increased high-grade methane gas yield from the methane phase reactor when compared with previous research on SMSW of the same mass and composition. This is attributed to the pH-controlled operation, which allowed for acetogenesis and methanogenesis of substrates to be performed in parallel, rather than sequentially. The methane yield from the methane phase (high-grade methane) content was 75 mol % on average, which is greater than the 50-60 mol % observed for single-phase digestion.Experiment II was operated as a single-phase acid fermentation system with intra-fermenter recycle for 100 days followed by operation as a two-phase system with inter-fermenter recycle for 150 days. SMSW of the same mass and composition as Experiment I was used and included soluble amounts of Pb and Cd salts, which were added to the solid bed. The Pb and Cd concentrations were based on upper limit values for these heavy metals per USEPA Part 503 regulations (1994). During singlephase operation, the heavy metals caused accumulation of butyric acid in the solid bed suggesting uncompetitive inhibition of acetogenic bacteria. Also, two-phase operating time increased by 50% while high-grade methane yield was reduced by 11 %, which could indicate competitive or mixed competitive inhibition of methanogenic bacteria.This research demonstrates the successful application of pH control to enhance two-phase anaerobic fermentation of biodegradable fractions of solid wastes and improve high-grade methane yield from the methane phase. It also demonstrates the resilience of the two-phase fermentation process with respect to heavy metals in that the high-grade methane yield was slightly reduced at upper limit concentrations of Pb and Cd per USEPA 503. Pilot-scale and prototype designs should be developed to determine full-scale design and operating criteria.