[Digestion] Environmentalists Anti-AD Issues

[Jim McNelly]

Greetings,

I have been in the organics management industry since 1975 and had written
off low solids AD in 1980 as a technology that had more problems than it was
worth due to the cost of solids separation, the high BOD of filtrate, and
the phytotoxicity of digestate, all of which, collectively, in my opinion,
precluded low solids AD as a viable option for operations that did not have
year round storage and land application options, which in and of themselves,
would not result in value added markets for digested solids.  In my years of
land application contracting with POTW biosolids, I have frequently observed
farms form cartels that not only did not pay for the residuals, but they
required land application at the generator's expense, even charging $10 or
more per wet ton as the disposal fee for "dumping waste" on their farms,
even though the nutrients are worth $50 per wet ton or more. Simply put, wet
digested solids are typically a liability, not an asset.

Consequently, I have focused on value added composting options, which result
in marketable compost products.  Eventually I founded the US Composting
Council and continue to promote the beneficial use of bio-based nutrients in
many circles of endeavor. The evolution of "dry digestion" approaches,
meaning solids over 15% intrigues me, but even then, the solids still need
to be aerobically conditioned after the AD process as AD solids are
phytotoxic, meet only PSRP (process to substantially reduce pathogens)
regulations, not PFRP,  (process to further reduce pathogens, hence
unrestricted use) and are not yet suited for value added markets.

In various narratives concerning the net cost of AD for foodscraps
composting, a dual approach, dry AD plus aerobic composting is my
recommendation. The question, I suggest, is, "what percentage of
decomposition is best served by generating biogas and what percentage
composting?" Recovering heat from composting to supplement dry AD makes
sense. But I suggest that the focus on biogas instead of overall cost
benefit analysis may be shortsighted beyond what biofuel is used on-site.

Anyway, the discussion asked for negatives regarding AD.

I am developing a "brief phase" dry AD stage to glean the "easy pickings" of
AD, but then shutting down AD digestion and converting over to aerobic
conditioning, perhaps only ten days of single stage dry AD, leaving the
aerobic stage to produce heat to warm the AD stage.

Any thoughts?

Jim McNelly

www.composter.com
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