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Hi Jeff ~
This is a highly complex issue, with a lot of variables and uncertainties,
so I hope that you will bear with me.
At least four issues need to be covered: (1) gas generation and its
disposition, (2) the acid phase of landfills, and (3) the impact of lignin
on degradation under anaerobic conditions, (4) the value of the compost.
First, the gas generation disposition.
Dane County explored this issue when it did its initial study on diverting
food from landfills. Our landfills recover methane gas and generate
electricity, selling over $1 million in power a year and the question was
about the effect of diverting food on electricity revenues. For over 10
years, we have not taken yard materials, so that was not an issue.
What we found was that the situation varies considerable with the
infrastructure of the gas collection system, but in our case, the
environmental benefits of diversion are great.
The issue hinges upon the relative time for materials to degrade, and is
based on the concept of a half-life. The basic idea is -- just like
radioactivity -- the decomposition of materials under either aerobic or
anaerobic conditions are likely to follow a first order decay curve. The
degradation under "average" conditions in a landfill according to a 1988
seminar presentation by Professor Bob Ham is that the half life of different
food, garden debris 0.5 to 1.5 years
paper, wood 5 to 25 years
In a more recent study done for the Norwegian government, (Miljøkostnader
ved avfallsbehandling [Environmental Costs from Solid Waste Management],
ECON Senter for økonomisk analyse, December 2000), more detailed estimates
of half lives of solid waste in landfills are provided on page 74 as:
wet organics 2.8 years
paper 8.4 years
wood 10.5 years
textiles 10.5 years
For our existing landfill, we install the gas recovery system so that the
openings in the pipes to recover the gas are only in the layers that are at
least 5-7 years old. We do not want to draw oxygen through the surface of
the landfill into the lower layers because this will change the landfill
from an anaerobic to an aerobic situation and only under anaerobic
conditions will methane be produced.
Given the short half life of food and garden debris, much will start to
degrade quickly and will do so aerobically, and then anaerobically. If the
gas extraction system is only recovering gas from layers that are at least
five years old, under the assumption of a half life of 1 year for food and
grass clippings and other wet organics, less than 6% of the original
material will typically be available, and we concluded for our system that
the contribution of this gas to our recovery system was negligible. Instead,
this gas would largely escape to the atmosphere and contribute to global
warming. Thus, under our gas collection system configuration, diversion of
food and garden debris is beneficial from a gas and environmental viewpoint.
Of course, a bio-reactor may have an entirely different gas collection
configuration, which would change the conclusions. Thus, the actual design
must be known before a definitive conclusion can be reached.
The acid phase of landfills.
The degradation of solid waste in a landfill goes through a series of very
distinct phases. To simplify things, one of the early stages is called the
"acid phase", because at this stage (soon after aerobic decomposition has
switched to anaerobic decomposition), the conditions in the landfill become
very acidic, dissolving out heavy metals in a leachate flush. This is a
condition that landfill operators would very much like to avoid if they can.
The acids are basically caused by the decomposition of the materials in food
and garden debris and grass clippings. After the acid phase, the landfill
becomes basic and continues there for a long period of time.
Wisconsin has a work group to look at how to achieve quicker stability of
organics in landfills to reach the point where the organics would not be an
environmental risk. In looking at rapid decomposition in landfills, three
options are available -- (1) pump air into the landfill for the entire time,
using an aerobic (i.e., composting) approach, (2) keep the air out, having
the landfill operate as an anaerobic system, or (3) do a hybrid, pumping air
into the site to get past the acid stage and then go anaerobic. It seems
that the hybrid approach has a lot of advantages over the anaerobic approach
because of the avoidance of the acid phase, but, it is more technically
complex and costly, and reduces the methane generation and income from the
degradable material. Thus, there are no clear winners. Viewed another way,
there is no clear advantage to having the food and garden debris in the
site, since it is these materials that produce the acid phase.
Lignin and decomposition
The third issue to highlight is the impact of lignin on anaerobic
decomposition and whether a bio-reactor will work at all. There are many
stories of old landfills being dug up and the waste layers dated by reading
the dates from the newspapers. This is because lignin is extremely resistant
to degradation in anaerobic conditions and newspapers have very high levels
of lignin, on the order of 25-30%. A really good paper on the impact of
lignin to inhibit degradation is found on the Internet at
http://compost.css.cornell.edu/calc/lignin.html. Articles specifically on
lignin and degradation in landfills by Ham, et. al., can be found in
Environmental Science and Technology, 1995, pages 2305-2310 and the Journal
of Environmental Engineering, December 1998, pages 1193-1202.
What the research suggests is that bio-reactors -- being anaerobic systems
-- are not going to achieve breakdown of the organics in newsprint and other
high lignin materials, and thus are not going to achieve true organic
stability. Thus, one of the major reasons for bio-reactors -- stability of
the organics and less long term care -- is very questionable.
Incidentally, the lignin issue also needs to be considered when calculating
the potential methane generation from landfills. While the newsprint and
other lignin-containing materials could theoretically produce methane, the
actual methane generation is minimal from these substrates and thus the
total potential generation must be correspondingly reduced. This will effect
both the economics of the sites and as well as the calculations of
efficiencies. The literature that I have seen on both methane potential and
efficiencies, unfortunately, ignore this issue.
The economics of composting
In our area, we have found some interesting results vis-à-vis the economics
issues. First, we have encouraged leaving grass clippings on the lawn and
home composting of garden debris, food and leaves. This has saved our
collection systems enormous amounts of money. On the other hand, we also
operate three yard material composting sites (mainly for leaves) and find
that there is extremely strong competition for our finished compost, with
some landscapers wanting to buy our sites so that they can have all the
compost. While in might be theoretically possible to recover compost from
bio-reactors, I am not aware of any successful operations of this type.
So, in closing, this is a very complex subject. And, while I know that these
points do not completely answer your questions, I hope that they are of some
John Reindl, Recycling Manager
Dane County, WI
> -----Original Message-----
> From: Aluotto, Jeff [mailto:Jeff.Aluotto@no.address]
> Sent: Monday, December 20, 2004 7:26 AM
> To: 'email@example.com'
> Subject: [greenyes] Composting vs. Landfillling
> I know this has been a frequent topic of discussion on this list, but I
> hoping someone might have some information I could use. With bioreactor
> landfills (or landfills that attempt to pass for them) becoming more
> commonplace, I believe that those of us putting public money into
> and organics composting programs will come under increased pressure to
> justify the reasons for doing so. The argument is typically posed as "Why
> spend thousands of dollars on collecting and composting yardwaste and
> organics, when the stuff is useful as a means of producing energy from
> landfills and biodegrades quite easily?".
> Many organizations such as NRC have done a good job promoting
> the financial
> and environmental benefits of recycling. Does anyone have
> any information
> that does the same for composting? I know Peter has posted
> some good info
> on the relative inefficiency of gas collection systems - but
> I was wondering
> if anyone had any sort of life-cycle assessment on composting vs.
> Jeffrey W. Aluotto
> Hamilton County Solid Waste District
> 250 William Howard Taft
> Cincinnati, OH 45219
> 513-946-7719 phone
> 513-946-7779 fax
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