The Environmental Effects of Organic
Waste in Waste Treatment Systems
By
Harrison Elba
SHWPCE Spring 2014
Abstract
The United States current waste stream is not necessarily the most environmentally or
economically beneficial utilization of what could be a valuable resource. A closer analysis of all options
using modern technologies is necessary, and potentially a combination of a number of technologies is
the best answer. To really determine what is the best method of waste treatment, the following criteria
need to be considered.
Energy harnessed- whether this is gas production from decomposition or heat from combustion, all
MSW has an energy value
Reuse potential- recyclability of content by composting or other method
Emission potential- CO2 and CH4 are the high volume greenhouse gasses emitted from most waste
treatment methods, however other toxic trace compounds can be found as well
Capital investment- what the technology or real estate costs
Revenue- whether it is from energy production or recycling, this figure should eventually be able to pay
for the investment
Life expectancy- how long the revenue will continue to flow and pay back for the investment
In this study we look at 3 forms of waste treatment- composting, landfilling, and incineration,
while examining the positives and negatives of each with respect to the above criteria.
Overall it is difficult to conclude which method yields the most beneficial waste stream.
Certainly a combination of all 3 methods would hedge all bets, and diversify the waste stream to be a
more balanced and sustainable infrastructure. Yet overall, it seems composting the organic content,
incinerating the rest and landfilling the ash yields the highest ratings for all criteria.
Introduction
The practice of composting biological waste has long been accepted as the premier method of
biological waste disposal. Dating back to 8,000 BC during the Akkadian Empire, the hunter gatherers
began to shift to pastoral farming noticing “that their plants grew better where there was manure and
started deliberately putting manure on their soil where they planned to cultivate crops.” (3) The benefits
are numerous; from production of nutritive and free planting soils to the reduction of nitrogen enriched
leachate in waterways. There are no real arguments against or downsides to composting all non toxic
bio waste. Yet for some reason, our nation still sends the majority of bio wastes to landfill and
incineration, causing greater emissions, more environmentally harmful leachates, premature site
capping, and ultimately ending the cycle of the nutrients that the waste may contain. Almost 30% of
MSW is organic content, and this is all content that can be both diverted from conventional waste
streams and utilized for a greater good.
If composting is easy, then why are we still contributing to organic landfills in such high
volumes? This is the underlying question that most activists and proponents of composting attempt to
answer and address. The root cause of our landfill obsession is the sheer convenience of a single stream
waste system. Nothing is easier than throwing all waste in one bin and having it carted away every week
by people you never even see. If we are to change our habits, then the rewards must outweigh the
effort required. The public must be made aware of the benefits of making your own compost, and it
must be presented in a plausible way. The biggest challenge to the feasibility of widespread adoption is
urban conditions. If the consumer does not have a garden, or any room to maintain a compost pile,
there is little incentive to manage a second waste stream. There are ideas that address this, but nothing
has really succeeded in reducing the amount of bio waste in MSW for urban areas. In fact it is often cited
that “composting sites located in rural-urban and urban areas often are forced to use high technology
options to minimize nuisance complaints.” (1) This high technology includes expensive equipment with a
high management intensity that is not plausible to urban consumers. The only successful urban systems
incorporate a localized storage system into a community operated composting center that collects the
bio waste from participating households in the area. Since there is no tangible direct benefit to the
participants, it is difficult to gain participants.
The benefits of composting bio waste are pretty obvious because it is such a tangible and
widespread practice and will be discussed in more detail below, however it is important to also argue
against diversion from waste stream. If all organic wastes were diverted from a landfill outfitted to
capture gas and extract methane, the methane production at landfill will be significantly reduced, since
it relies primarily on the decomposition of that organic matter. “As of January 2014, there are 636
operational LFG energy projects in the United States and approximately 450 landfills that are good
candidates for projects.” (2) There being 2400 operated landfills in the country, these 636 represent 25%
of all landfills. With a high governmentally subsidized capital investment that creates jobs and reduces
energy costs for local communities, significant push back can be expected to reducing their production.
In comparison, the 86 facilities in the United States for combustion of MSW with energy recovery
(mainly in the Northeast) may see an increase in production due to a lower moisture content in their
stream, yet if the majority of paper and wood are diverted also, would be negatively affected as well.
We will first look at the pros and cons of treating 1 metric ton of MSW by compost, landfill gas
energy, and combustion energy, focusing on comparative emissions and energy generated. We will then
examine the effect of diverting 100% of the organic content of MSW to composting on all three
methods. Our assumption is that 25% of MSW is readily separable organic content that is a proportional
blend of all common domestic organic waste. Using the widely accepted estimate of US domestic waste
production of 4.5 lbs per person per day, this ton represents the daily trash of 500 humans.
Composting
Composting is an aerobic process and a large fraction of the DOC is converted to CO2.
CH4 is produced in the anaerobic sections but are largely oxidized in the aerobic sections. In the end,
less than 1% of the initial carbon content is converted to methane. It is the large CO2 emissions that are
a problem, but in comparison with emissions from industry is a negligible figure. The decreased
methane emissions here in comparison to the emissions from anaerobic decomposition of organic waste
in landfill is a great boon.
The low capital investment for this waste treatment method allows facilities to lie dormant if
necessary while other methods handle higher volumes of waste without suffering depreciation. There is
really no limitation on how localized the facilities can be because of the low cost and low profile nature
of it. Therefore if distributed waste treatment is implemented, great savings on fuel can be expected.
The organic make up of 1 ton of MSW is approximately 560 lbs. The entirety is subjected to
complete and proper aerobic decomposition, and the mass is reduced by 20%. The mass loss is mainly
attributable to CO2 and H2O releases. In the end we are left with 448 lbs of completed compost, and
using the chart below we can see laboratory measured CH4 emissions per kg of material.
In our example, we have 255 kg waste to react, and choosing an intermediate value of 150 L/kg from
figure 1, we can calculate 1350 scf of CH4 emissions. For unknown reasons, the CO2 emissions per unit
of compost are not available and would be a good area to study.
Figure 1 (8)
Landfill Gas Energy
The organic content of MSW is solely responsible for all methane generating properties of
landfills. The amount of methane generated can be demonstrated using LandGem, assuming no
diversion. The energy potential of this practice is less than combustion, however it is a much cleaner
method, allowing for complete anaerobic decomposition and capture of all gasses and potential
pollutants, and only the methane produced is combusted- no plastics. If all organic waste was diverted,
this process would be essentially shut down- there would no longer be any anaerobic decomposition to
produce methane. All of the LGE projects rely on a constant stream of gas from the landfill to stay
productive, however they only produce fractions of community energy consumption and hardly pay for
themselves. Federal subsidy continues to play a large role in the development of new projects.
“In the event that the waste was landfilled, 1 metric ton (1.1 short tons) of MSW would produce
approximately 62 cubic meters (2,200 cu ft) methane via the anaerobic decomposition of
the biodegradable part of the waste” (6) Since methane has a much higher (2.5X) greenhouse gas
potential than CO2, it is important to capture these emissions where anaerobic decomposition occurs.
This 2,200 scf of methane has a 2,000,000 BTU heating content and can provide heat for an average
home for about 10 days.
Waste To Energy Combustion
The incineration of waste to produce energy is a practice much older than we realize. While the
first incinerators known as destructors were built in 1874, (6) the origins of burning waste dates back to
much earlier times. A burn pile or pit, simply a mound of combustible materials piled on bare ground
and lit on fire has been the “out of sight, out of mind” method of disposal for centuries. Only recently
have we been fussing over emissions caused from burning new industrial and commercial wastes that
produce toxic fumes. Incineration of MSW can reduce the mass by 80% and the volume by 90%.
The energy potential of WtE combustion technology varies significantly with the properties of
the waste stream, and the type of incinerator being used. For example, lower moisture content MSW
contains more carbon and has more combustion potential. Also it is worth noting that the electrical
efficiency of WtE incinerators is much lower than the cogeneration efficiency, so local heat production is
a more conscientious usage of the carbon content of MSW than fueling turbines.
The CO2 produced from combustion of 1 ton of MSW is approximately 1 ton. This is because the
average mass fraction of Carbon in modern waste is about the same as the mass fraction of carbon in
CO2- 27%. When comparing 1 ton of CO2 from combustion to 2200 SCF of methane from
decomposition, we see that the methane emitted has twice the warming potential than that of the CO2.
However, if this methane was captured and burned, the CO2 emissions would only be about 250 lbs,
and there would be no fine particulate, heavy metals, dioxins, or acid gasses.
Figure 2 (7)
Figure 3 (9)
Given the average MSW BTU value of 4500 by figure 2:
4500*2240=10,000,000 BTU’s per ton of MSW incinerated.
This figure and simple calculation show us that incineration creates 5X the energy derived from
anaerobic decomposition.
Diversion
By diverting 100% of organic content to compost practices, we are substantially increasing
compost production in the United States. This has many positive benefits for the country. The most
obvious without analysis required are the wider availability of organic farming nutrients, as well as the
returning of organic compounds to the earth.
As an estimate on the annual amount of waste to be diverted, we consider population size of
314,000,000 and MSW production of 4.5 lb/day
314M*4.5*.25*365= 129 billion lbs/ year
Estimating a mass reduction of 20% after decomposition
103 billion lbs
Estimating density of 1200 lb/cy
86 million cubic yards
Estimating application depth of 1 ft
18,000 acres of farmland fertilized with compost
While this amount does not cover our 408 million acres of farmland (4), it does make a substantial
economic impact because it is free and no longer has to be processed by other methods. In addition if
the practice of widespread application continues, eventually most commercial farmland will be covered
and it will greatly increase crop production.
So we realize the benefits of increased compost production, but how does this affect the landfill
and WtE businesses? It is difficult in this aspect to argue in favor of diversion to compost.
Landfills will see a decrease in daily input of 25% mass. This will essentially prolong the life of
the landfill by the same percentage. For example, landfills originally designed to take 500 tons/day and
accept waste for 20 years will now take 375 tons/day and last 25 years. Gas emissions from anaerobic
decomposition of the organic matter will cease. This is an environmental boon where LGE projects are
not capturing the emissions and utilizing the methane content, but on the other hand this will eventually
shut all LGE plants down when their current stock is depleted of reactive waste. The capital investments
for blowers, piping and all other required materials will depreciate and eventually be worthless.
Incineration plants will suffer a more negative reaction to the diversion than landfills, because
they rely heavily on carbon laden paper products having a high heat value of 7000 BTU/ dry lb (figure 4).
Looking at figure 5 we can see the heat content of the organic portion of the MSW is a little over half.
We can therefore expect the energy potential of the remaining waste stream to be halved as well.
However, the average heat value per ton of combusted waste will remain similar because the organics
have approximately the same value as the proportional combination of plastics and other waste. Plastics
alone have a value 2-5X that of organics, but are balanced out by non reactive materials like glass.
(figure 6) The emissions are another story. The organic portion is higher in carbon content than the rest
of the waste, so the CO2 emissions will be reduced, but the other harmful emissions are mostly
resultant from the non biogenic portion and will remain the same.
Figure 4 (9)
Figures 5 and 6 (10)
Conclusion
From this study we can begin to assemble a more informed opinion on the realistic value of
composting organic wastes. Not all of the data needed for a completely comprehensive analysis of the
subject is available, but the general comparative values are possible to simulate using simple
calculations.
A metric ton of MSW containing 25% of 560 lbs of proportionally blended organic matter has 4
paths. Path 1: Divert organics to compost and landfill inorganics, Path 2: landfill all MSW, Path 3: divert
organics to compost and combust inorganics, Path 4: combust all MSW. If asked to decisively choose one
path for maximum environmental, and economic benefit, I would pick Path 3. This is because the
benefits of benefits of composting the organic waste is utilized, maximum energy conversion is achieved
from incineration of the inorganics, and maximum landfill life due to 80-90% volume reduction of waste.
Sources
1. http://www.ewp.rpi.edu/hartford/~ernesto/S2014/SHWPCE/Papers/SWBiochemicalTreat/UWisconsin-Art-and-Science-of-Composting.pdf
2. http://www.epa.gov/lmop/basic-info/index.html
3. http://www.carryoncomposting.com/142941469
4. http://www.epa.gov/oecaagct/ag101/landuse.html
5. http://en.wikipedia.org/wiki/Waste-to-energy
6. http://en.wikipedia.org/wiki/Incineration
7. http://www-tnswep.ra.utk.edu/activities/pdfs/mu-W.pdf
8. http://www.stopwaste.org/docs/greenhouse_gas_balance_for_composting_operations__sally_brown.pdf
9. William Worrel, “Solid Waste” pg 226
10. http://www.eia.gov/totalenergy/data/monthly/pdf/historical/msw.pdf