GHG sources in composting

Composting may result in emissions from various sources, such as

• biogenic processes during composting,

• process gas cleaning and process control,

• collection and transportation of the raw material and the compost,

• the application of compost in agriculture.

Main gas components to be considered are CO2, CH4, N2O, and NH3. A qualitative review of the emissions includes the following emission types:

• Emissions from the process itself mainly consist of carbon dioxide which is the result of the aerobic decomposition. Depending on the type of raw material, the duration of the composting process, as well as other bioprocess characteristics, different amounts of CO2 are emitted per ton of composted raw material. Because CO2 in this case is biogenic in origin, this emission is not counted in greenhouse gas inventories. Nevertheless capturing of emitted CO2 and its use instead of carbon dioxide from fossil sources will improve the anthropogenic greenhouse gas balance (see chapter 12.3.4).

• In a well-managed composting process, CO2 is the only process gas. If aeration in the compost heap is poor, or the material is too wet, an anaerobic situation may occur, which is accompanied by methane development and the liberation of emissions of odor. Emission factors of methane are different for biowastes from households and from green wastes. The values are 2.5 and 3.36 kg methane per ton of bio-waste treated, respectively (UBA, 2007).

• Nitrous oxide (N2O) has to be taken into account. It results from the oxidation of ammonia which is another by-product of the composting process. Emission factors are different for biowaste from households and green wastes. The values are 83 and 60.3 g N2O per ton of bio-waste after experimental results in Germany. The total N2O emissions from composting in Germany are about 0.25 Mio t CO2-eq. or 0.02 percent of total GHG emissions (in 2004; UBA, 2007).

• Another source of N2O are biofilters which are a component of composting facilities and aim to reduce or eliminate odors. They are applied also in other processes where organic emissions occur. N2O in this case is the result of microbial conversion of ammonia. Therefore biofilters may act as climate gas sources if ammonia is not eliminated from the waste gas stream before entering the filter. As an example data of an experimental biofilter (in the case of MBP technology -see chapter 12.6.2) are displayed in table 12.7: N2O concentration is raised from 19 to 130 g, measured as a specific amount per ton of waste. Such effects can be avoided if ammonia is eliminated from the waste gas stream by use of an acid washer and scrubber.

Table 12.7 Nitrogen balance in a biofilter (g/t biowaste) (Soyez, 2001)

Substance

Raw gas input

Clean gas after biofilter

N2O

19

130

NO

1

190

nh3

500

200

Norg

100

100

Percolation

0

<1-10

Closed reactor, mature compost

Closed reactor, raw compost

Open windrow, mature compost

Open w indrow, raw compost

Home composting, open heap

0 50 100 150 200

GHG emission potential (kg CO2-eq./t)

Figure 12.4 GHG emissions by composting technologies (after Knappe, 2004)

Besides direct process related emissions other technological steps contribute to GHG emissions. During collection of biowaste and its transportation to the composting facility, as well as during turning of compost and aeration, CO2 and methane emissions take place.

The overall greenhouse gas emissions, both antropogen and natural, amount up to about 150 kg CO2-eq. per ton of waste treated (see figure 12.4), depending on the technology and the type of the compost produced.

Obviously the emission values are quite similar for the technologies compared, with highest values for mature compost the production of which normally comprises an extra maturation step. Home composting represents the lowest value, since practically no energy consumption in transportation and handling is necessary. Thus home composting from climate perspective would be a favourable composting option if processed properly.

Additionally a third factor which is the application of compost as a soil fertilizer has to be taken into account. However though it results in GHG emission, it isn't counted in GHG balances due to its biological origin.

A specification of the contribution of the steps of processing and application is given in figure 12.5.

□ Agricultural application

□ Disposal of residues

□ Main composting step ■ Mechanical treatment

0 Waste collection and transport

Figure 12.5 GHG emissions of composting steps (after Knappe, 2004)

Waste collection and mechanical treatment as a step of the composting process contribute by only 10 percent, the main process phase by not more than 25 percent. Largest effect is by agricultural application. If matured compost is produced this value is cut in half. However, in this case, the production effort is higher, so that the benefits are equalized and the total GHG effect is nearly unchanged. More than 40 percent is not counted in the GHG balance which is due to its biological origin.

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