TPAD VS OTHER THERMOPHILIC ANAEROBIC DIGESTION PROCESSES: A RE-EVALUATION
Les Nemeth1, Reza Iranpour2, Huub H. J. Cox2, Steve Arant3, Brian Hystad5
and Tom Wilson4
1Earth Tech, 201-3275 Lakeshore Road, Kelowna, BC V1W 3S9, Canada.
2City of Los Angeles, CA,
3Earth Tech, Sheboygan, WI,
4Earth Tech, Alexandria, VA,
5Greater Vancouver Regional District, BC
(free)
Temperature Phased Anaerobic Digestion (TPAD) has been the most popular form of thermophilic
anaerobic digestion in the US for some time. In comparison to “conventional” thermophilic anaerobic
digestion (TAD), TPAD digestion is usually seen to offer several advantages, including better
dewaterablility and fewer odors. Today, there is sufficient full-scale operating experience and recent
research to verify whether or not the advantages have in fact been demonstrated and whether the
added complexity of TPAD is really necessary – this is the subject matter of this paper.
Recent data has shown many interesting and unexpected results, which warrant consideration:
• A TPAD plant in Minnesota reports relatively high polymer usage (Toffee) and odor problems
and has had to modify its centrifuges, resulting in lower cake solids (Krugel et al.).
• Another TPAD plant in the Midwest routinely gets less than 20% TS in its dewatered cakes
(Wilson et al).
• A third Midwestern TPAD plant routinely reports 20 -25 % TS in its sludge cake (Wilson et al).
• Research has reported an eastern TPAD plant that requires several times as much polymer
as mesophilic plants studied (Higgins et al).
• Recent WERF studies suggest that regrowth/reactivation of pathogens may be a problem in
cakes from some TPAD plants (refer to Figure 1) (Higgins 2006).
• One of the first TPAD plants in Germany reportedly has been converted to another
technology.
• Los Angeles’ Hyperion thermophilic digesters (Figure 2) routinely produce cake solids over
30% and have no cake odor or pathogen destruction issues (author’s observation from plant
operating data).
• The Annacis Island thermophilic digesters (Figure 3) routinely achieve low polymer use,
cake solids over 30% and no cake odor problems, and have shown no
regrowth/reactivation of pathogens.
