EXERCICE ON LCA OF WASTE TREATMENT
Intensive composting versus controlled landfilling of organic waste
GOAL OF THE EXERCICE

Compare from an environmental point of view two ways of dealing with organic waste:
sanitary landfilling and intensive (tunnel) composting

Find out the contribution of the different processes to the overall impact

Discuss the limitations of the study
SYSTEMS TO ANALYSE
The first system to analyse is the management of organic waste by means of intensive
composting. The waste is collected from the bins and transported to a composting plant, where it
is introduced in controlled tunnels. In this tunnels the piles of waste are intensively aerated in
order to accelerate the process, and several parameters (humidity, oxygen, pH, etc) are
monitored to optimise it. The air passing through the tunnels is later treated in a biofilter, in order
to remove volatile compounds. After two weeks in these tunnels, the compost is extracted and
maturated in piles, which are turned periodically. Finally the material obtained is a humus-like
material, called compost, which is free of pathogens and ready to use for agricultural or garden
purposes (Figure 1).
Energy Resources
System boundaries
Households
O. waste
Waste
bin
Tr
Emissions
Composting
Compost
Waste
COMPOSTING SYSTEM
Figure 1. Composting system: Compost plant + Transport (Tr)
The second system corresponds to the management of organic waste by sanitary landfilling. In
this case, the waste is transported to a previously prepared area, in which the waste is covered
with inert materials and compacted by bulldozers. In this conditions, the organic matter degrades
slowly in anaerobic conditions, giving rise to biogas (with about 50% volume of methane) and
leachate. The biogas is partially collected and combusted in an engine to produce electricity,
which is mostly exported to the grid. The leachates are also collected and treated in a wastewater
plant, before discharging the treated water to the natural water stream (Figure 2).
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Energy Resources
System boundaries
Households
O. waste
Waste
bin
Tr
Emissions
Landfill
Electricity
Waste
LANDFILLING SYSTEM
Figure 2. Landfill System: Landfill + Transport (Tr)
FUNCTIONAL UNIT
The function we are interested in analysing is the management of organic waste, and thus we
choose the following functional unit:
“The management of 1000 kg of household organic waste until its biological stabilization”.
However, both systems provide more than this function of managing waste. In the case of
landfilling, the system provides also the function of producing electric energy, and in the case of
composting the system provides the function of producing compost, a substitute of mineral
fertilisers. This aspects will be taken into account in the inventory phase.
INVENTORY DATA AND HYPOTHESIS

Transport of waste to the composting plant: 50 km + 50 km (way back)

Transport of waste to the landfill: 30 km + 30 km (way back)

Fuel consumption of the truck: 0,05 litres/ton x km (45 MJ/ kg diesel fuel, with a density of
0,85 kg/litre)

Every kg of compost obtained displaces 78 g of mineral N fertiliser
Below are the balances for the unit processes:

Composting

Landfilling

Diesel fuel production and combustion

Electricity production

Mineral nitrogen fertiliser
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Table 1. Aggregated inventory for
production and combustion of diesel
fuel (1 MJ).
Table 2. Desaggregated inventory for an
intensive composting plant, per tone
INPUTS FROM NATURE
Resources
coal ETH
crude oil ETH
natural gas ETH
OUTPUTS TO NATURE
Emissions to air
ammonia
CO2-fossil
HCl
methane
NOx (as NO2)
SOx (as SO2)
Emissions to water
N-tot
NH4+
nitrate
phosphate
INPUTS FROM TECHNOSPHERE
Waste
Organic waste from households
1 Tm
Energy products
Diesel fuel
232 MJ
Electricity
108 MJ
INPUTS
450 mg
24 g
1,21 l
OUTPUTS
2,15 µg
79,1 g
162 µg
96,3 mg
1,42 g
119 mg
OUTPUTS TO NATURE
Emissions to air
CO2-re
ammonia
OUTPUTS TO TECHNOSPHERE
Waste
Compost
Refuse
2,58 mg
2,64 mg
793 µg
31,3 µg
0,25 Tm
0,05 Tm
Table 4. Desaggregated inventory for
sanitary landfilling of organic waste, per
ton
Table 3. Aggregated inventory for electricity
production in Spain (1 MJ)
INPUTS FROM NATURE
Resources
coal ETH
crude oil ETH
natural gas ETH
OUTPUTS TO NATURE
Emissions to air
ammonia
CO2
HCl
methane
NOx (as NO2)
SOx (as SO2)
Emissions to water
N-tot
NH4+
nitrate
phosphate
328 kg
57,7 g
INPUTS
INPUTS FROM TECHNOSPHERE
Organic waste from households 1.000 kg
Diesel fuel
38 MJ
58,2 g
6,57 g
7,82 l
OUTPUTS
572
131
27,9
466
317
574
µg
g
mg
mg
mg
mg
336
787
2,41
5,56
µg
µg
mg
mg
OUTPUTS TO NATURE
Emissions to air
ammonia
CO2-re
methane
NOx (as NO2)
SOx (as SO2)
Emissions to water
COD
NH4+
nitrate
P-tot
OUTPUTS TO TECHNOSPHERE
Electricity
Table 5. Aggregated inventory for production
of 1 kg of mineral nitrogen fertiliser.
INPUTS FROM NATURE
coal ETH
1,5 g
crude oil ETH
6,01 g
natural gas
0,126 m3
OUTPUTS TO NATURE
Emissions to air
ammonia
6,8 g
CO2
65,7 g
HCl
453 µg
methane
1,17 g
NOx (as NO2)
1,881 g
3
43
390
43
120
60
g
kg
kg
g
g
630
2,5
7,7
49
g
kg
kg
g
87 kWh
IMPACT ASSESSMENT
We will take into account 3 indicators: greenhouse effect, acidification and energy consumption.

Global Warming Potential 100 years (kg eq CO2/kg)
(IMPORTANT: CO2 generated by the degradation of biomass (CO2-re) is not taken into
account in GWP, according to IPCC1 guidelines)
Compartment
Air
Air
Air

Substance
CO2
CO2-re
methane
Acidification Potential (kg eq SO2/kg)
Compartment Substance
Air
ammonia
Air
HCl
Air
NOx (as NO2)
Air
SOx (as SO2)

Factor
1
0
21
Factor
1,88
0,88
0,7
1
Primary Energy Consumption (MJ)
Compartment
Substance
Factor
Raw
coal ETH
18
Raw
crude oil
41
Raw
natural gas ETH
35
Unit
kg
kg
m3
INSTRUCTIONS
1. Think about the functions provided by both systems: How many functions are there for
landfilling and composting? and which are them? How can we compare systems providing
different functions?
Each system provides two functions, which are discussed below.
The functions for landfilling are quantified as follows: managing 1000 kg waste and producing
87 kWh of electricity. As we are interested only in the first function, we use the substitution
approach to eliminate the second function; in this way, we can say that this system is “saving”
the production of 87 kWh by conventional power plants. Therefore, in the inventory we will
subtract to the system the environmental impact of producing this amount of electricity using
the conventional mix of technologies, in this case the mix used is that of Spain (about
20%hydropower, 30% nuclear power and 50% thermal power) (see Table 4.).
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Intergovernmental Panel on Climate Change.
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In the case of composting, the functions are: managing 1000 kg waste and producing 250 kg
compost (Table 2). As we are interested only in the first function, we also use the substitution
approach to eliminate the second function. If we assess the quality of the compost by its
nitrogen content, and we compare it with that of conventional mineral nitrogen fertilisers, we
can say that every kg of compost displaces the use of 78 g of mineral fertiliser. In this way,
we can say that our composting system “saves” 19,5 kg of fertiliser thanks to the 250 kg of
compost produced. Therefore, in the inventory we will subtract to the system the
environmental impact of producing this amount of mineral fertiliser.
With this approach, we “eliminate” from our system the functions we do not want to include.
Now both systems provide the same function (managing waste) an can be fairly compared.
2. Calculate an inventory of the relevant substances for each unit process (be careful with the
units)
First we have to identify the unit processes or subsystems included in each system.
For landfilling we have:
-
Transport to the landfill
Landfilling process, including the energy consumption of the machines and
the emissions related to waste degradation
“Credit” for electricity produced with the biogas
For composting we have:
-
Transport to the composting plant
Composting process, including the energy consumption and the emissions
related to waste degradation
“Credit” for compost used as fertiliser
Now, using the mass and energy balances for each process, and the hypothesis assumed, we
have to compile the inventory for each system.
The energy consumed in the transport to the composting plant is calculated as follows:
(0.05 l diesel/Tm x km) x (1 Tm) x (50 + 50 km) x (0,85 kg diesel/ l) x (45 MJ/kg diesel) = 191 MJ
For landfilling only the distance has to be changed.
Once the energy consumption has been calculated, we must multiply this amount by the unitary
emissions per MJ of diesel, displayed in Table 1.
Then we have the treatment processes, landfilling and composting. The mass and energy
balances are in Table 2 and 4, and are already expressed per tone of waste input. However, both
processes have, as inputs, diesel fuel and/or electricity, so we must calculate the emissions
related to these energy inputs with Tables 1 and 3.
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Finally we have the credits or environmental savings for both systems, thanks to electricity
production in the landfill (87 kWh)2, and to compost production (equivalent to 19,5 kg N fertiliser)
in the other system. Using Tables 3 and 5, we can calculate the savings for both systems.
Finally, the overall inventory for each system and substance is calculated as:
For landfilling:
For composting:
Transport + landfill – electricity
Transport + composting – fertiliser
3. Perform the impact assessment by multiplying the overall amount of each substance by its
characterisation factor, and then calculate the environmental profile for both systems.
For the detailed calculations and results, see the excel file.
4. Discuss the results and the limitations of this approach:
a. Which option appears to be better from an environmental point of view?
The preferable option depends on the impact category considered. Composting is
preferable if the greenhouse effect is considered, but from the acidification and
energy point of view, landfilling obtains better results.
b. Which processes and environmental interventions are the main contributors to these
impacts?
Methane is by far the main contributor to greenhouse effect in the landfilling process.
In the case of composting the main contributions come from the energy consumed in
transporting and treating the waste. The saving of resources and emissions due to
the valorisation of waste is important in both systems, but more in the case of
electricity production in the landfill.
c. Think about aspects or impacts not taken into account.
d. Think about the simplifications we have made.
The following aspects could be considered in further studies on this issue:
-
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Other relevant impact categories, such as eutrophication, toxicity on
humans and ecosystems, summer smog, land use, etc.
Compost should be followed to the grave, including the transport to
agricultural soil and spreading on the field.
Compost can be regarded not only as a substitute of nitrogen fertiliser, but
also as a substitute of phosphorus and potassium fertiliser. Its value as soil
conditioner, displacing other organic substrates, neither has been
included.
1 kWh corresponds to 3.6 MJ.
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