Startup and Stability of Thermophilic Anaerobic Digestion of OFMSW

Abstract

Anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) is promoted as an energy source and more recently as a greenhouse gas (GHG) mitigation measure. In this context, AD systems operating at thermophilic temperatures (55–60°C)—compared to mesophilic temperatures (35–40°C)—have the unique feature of producing hygienic soil conditioners with greater process efficiency, higher energy yield, and more GHG savings. Startup of AD systems is often constrained by the lack of acclimated seeds, leading to process instability and failure. The authors focus on strategies to startup thermophilic digesters treating OFMSW in the absence of acclimated seeds and examines constraints associated with process stability and ways to overcome them. Relevant gaps in the literature and future research needs are delineated.

KEY WORDS:

• anaerobic digestion
• mixing
• municipal solid waste
• stability
• startup

ACKNOWLEDGMENT

This work was supported by the National Council for Scientific Research and the Masri Institute of Energy and Natural Resources at the American University of Beirut, Lebanon.

Notes

1. Dead zones are areas that are not properly mixed and therefore do not receive fresh substrate that allows short circuiting and channeling of feedstock. Their presence is usually hydrodynamically detected using nontoxic tracers, such as Li+ (Olivet et al., 2005; Ward et al., 2008).

2. Resistance is the maximum accumulation of intermediate products upon disturbance.

3. Resilience is the time needed for the system to restore initial conditions after a shock.

4. Acetate-utilizing methanogens belong to either Methanosaeta spp., which are homoacetotrophic (i.e., can use only acetate), or Methanosarcina spp., which can utilize acetate, H₂, formate, and methanol (Batstone et al., 2004).

5. Reversibly transcribed mcrA mRNA.

6. The minimum time required for adequate separation of solids, t_sep, can be determined by column tests under anaerobic (active) conditions at thermophilic temperature as described by Kaparaju and Angelidakia (2008) for manure treating systems.