[USCC] Using Wastewater Digesters for food waste diversion

[Jim McNelly]

At 09:38 AM 9/10/2006, Nelson wrote:



>I was intrigued by the concept of using existing wastewater 
>digesters for anaerobically digesting food wastes.While there may be 
>a number of equipment conversion issues, if the tankage is 
>available,it would seem like something worthwhile pusuing.
>   However, Mr. Andrew Niv's input confused me.He seems to be saying 
> that using an existing WWTP digester would be uneconomical  vs. 
> purchasing the Masstech(TM) process. The web site he refers us to 
> indicates that the Mastech(TM) processor has a capacity of 100kg. 
> per day. So the theoretical 500 TPD of food waste he uses in his 
> example would require 5000 of his units. It is hard for me to 
> envision how that is more economical than using pre-existing, paid 
> for anaerobic digesters at a wastewater treatment facility.I'd like 
> to hear more from Mr. Niv regarding mass balance,moisture and cost 
> comparisons. Nelson Widell


Hi Nelson,

My experience with anaerobic digestion along the lines of WWTP 
digestion with 3% to 5% solids feedstocks has shown that the biggest 
problem is with dewatering and handling the high nitrate and BOD load 
in the filtrate.  With dewatering costs often running over $70 per 
dry ton and the virtual inability to store the filtrate, sent it to a 
POTW or land apply it in most circumstances, the liquid management 
issue alone makes this type of anaerobic digestion often out of the 
question.  To me, it is a simple rule of organics management not to 
liquify material if one can possibly avoid it.  While decomposing 
mass needs water, that does not mean that we need to add  water to 
the point of pumpability!

If there is an abandoned digester at a wastewater treatment facility, 
representing low or no capital cost and the POTW will take the 
filtrate, then liquid digestion might make sense.  But there is often 
a reason why a particular digester at a POTW is abandoned, which 
might be a failure of pumps, corroded pipes, poor insulation, sensors 
needing replacement etc.  Managing a liquid digestion system is not 
for the faint of heart even if it is working well.

Concerning a new design, the desire to optimize gas production and 
keep the CH4 percentage high has resulted in anaerobic digestion 
system complexity that does not lend itself to the typical compost 
facility operator.  Most composting operations are windrow systems, 
often stating that forced aeration technologies are too "high tech" 
for their operators.  If they have a hard time running simple blower 
systems and keeping air plenums and lines clean, I can not imagine 
them spending the effort required to keep a much more complex 
anaerobic digestion system working properly.  I have seen anaerobic 
digestion presentations at BioCycle Renewable Energy Conferences that 
suggest such complexity that one would need advanced degrees both in 
mechanical engineering and microbiology to operate them properly.

My thoughts at the moment on the issue include moving toward aspects 
of "dry digestion" methods at 20% or more solids, eliminating the 
dewatering problem, settling for less gas yield efficiency, while 
still focusing on optimizing the aerobic composting aspect.  One 
problem with anaerobic digestion is that the substrate requires 
composting anyway, resulting in dual technology costs.  Class B 
biosolids, for example, are often the product of anaerobic 
digestion.  If they are to be marketed successfully, they are 
typically treated to Class A using composting, a requirement in many 
parts of the country anyway.  I am working on methods to recover the 
heat of composting from a second stage of decomposition to support 
anaerobic digestion in a high solids first stage.  But still, the 
capital costs and operating costs have to be kept well below the 
value of the gas. As I see it, the gas will invariably need be an 
incidental byproduct rather than the primary focus of the 
decomposition process.




Jim~ McNelly
Renewable Carbon Management LLC 320-253-5076
NaturTech, NaturSoil, CompostMan
jim at composter.com
www.composter.com