[Digestion] Digesting MSW

[Zietsman, Rex]

Bjorn wrote:

"Hi

If you are not particularly interested in precisely methane for some
special purpose or in the N P K content of food waste but more in the
energy content, than it is more efficient to burn it in a large
incineration plant and produce hot water or steam and electric power.
The total energy efficiency will be higher than what you can achieve in
a digestion plant. The hot water can be used for district heating, steam
can be used for district heating or sold to some nearby industries.
Electricity can be sold on the market for electric power. However
incineration plants can not be very small and the moisture content can
not be very high. Usually the technical maximum limit for the moisture
in the total waste stream going into a modern incineration furnace is
about 65% and above that you may have problems with the ignition and the
required temperature level (850 C) of the combustion. Waste with a
moisture content of 38-40% is very good fuel unless most of it is inert
material. If a limited waste stream in the municipal solid waste has
higher moisture content than approximately 65% you can still extract a
lot of heat from it by co-combustion with dryer waste followed by flue
gas condensation. Flue gas condensation is a common method for
increasing the heat production from combustion and standard equipment in
Scandinavia to day on incineration plants and plants for burning wood
residues (branches and tree tops) from the forest industry. The
absolutely theoretical upper moisture content limit for extracting heat
from solid waste in a co-firing plant is around 80%. It is above that
limit that digestion of food waste is most competitive from energy point
of view. Also it can be competitive for cases where the waste supply is
small, where the transport distance to a big incineration plant is too
long and where the methane market is willing to pay a high price (e g
city busses) and where there is no market for district heating. So when
it is question about municipal solid waste there is room for both
incineration and digestion but one has too look carefully at the market
conditions in each case and also consider the societal cost of
separating food waste from other waste at the source, which is necessary
if you want to digest in closed containers. Municipal solid waste is one
of the most difficult energy sources to use irrespective of method but
anything is better than landfill without treatment.  (Landfills can of
course produce methane but the efficiency is lower and the time required
to complete the process is extremely long.) The best use of digestion
seems to be in agriculture and for treatment of high moisture waste in
the food industry.

Bjorn Dahlroth"

Bjorn,

There is an Israeli technology that takes unsorted MSW, sorts the
"heavies" from the "lights" by dropping the MSW into a water bath -> the
plastics float off while the heavies are sucked out and subjected metal
and glass recovery. The organics are broken up by shear in the water
bath using water jets. The organic laden water is then sent to a two
stage AD process of which the second stage is a UASB treating filtered
water from the first stage. This system works remarkably well but, as
you noted, does not generate as much energy (plastics are not burnt and
the co-production of CO2 leads to energy loss). 

For more information, contact Mel Finstein: see contact details below

Kind regards
Rex


Melvin S. Finstein, Ph.D.
Emeritus Professor of Environmental Science
Rutgers, The State University of New Jersey
Head, ArrowBio USA
Email: finstein at envsci.rutgers.edu