Degradation/landfill gas Flashcards
During aerobic degradation the share of the substrate that goes into new biomass is up to
about 40%, yielding a BOD/COD of 0.6. If you see a BOD/COD of 0.1 (despite of the waste
being a typical compost), what can that mean?
*The organic material is not readily biologically degradable, but oxidizable.
* There are non-organic components in the material, which consume chemical oxidant.
* Something is affecting the BOD-assay, e g toxicity
Why does aerobic pre-treatment before landfilling promote a swifter establishment of
methanogenesis?
- Because easily degradable material is degraded during pre-treatment which otherwise
would have formed organic acids and alcohols, thus lowered the pH (acidogenic
degradation) and prevented the development of archae (methanogens). - There are anaerobic niches in the waste also during composting, and anaerobic biota
can develop there and thus a higher concentration of anaerobic microorganisms will be available at the onset of anaerob conditions.
Why is it that the gas permeability in a landfill is higher in the horizontal direction than in
the vertical?
Because compaction and intermediate covers area applied horizontal or at least more
horizontal than vertical and settlements tends bring objects such as plastic bags to a more
horizontal orientation (settlements are not lateral but vertical).
Landfills typically go through different phases.
1. Relate the phases to the degradation of organic material leading to LFG formation.
2. What can be delaying the beginning of the methanogenic phase?
- Aerobic -> Acidogenic -> Methanogenic.
The landfill conditions at the time of collection influence the LFG composition, since
acidogenic and methanogenic degradation yield different gaseous products. For example, at the beginning of the methanogeneous activity, there is a very low methane
content and a high carbon dioxide content. - too much easily degradable organic matter or also very fast filling of such material –
leading to acidification and low pH. Also, very wet material and low temperatures can delay or
even hinder an increase of pH and so the onset of methanogenesis.
Landfills typically go through different phases.
1. Relate the phases to the degradation of organic material leading to LFG formation.
2. What can be delaying the beginning of the methanogenic phase?
- Aerobic -> Anaerobic -> Methanogenic
- too much easily degradable organic matter or also very fast filling of such material –
leading to acidification … low pH. Also, very wet material and low temperatures can delay or
even hinder an increase of pH and so the onset of methanogenesis.
LFG is often assumed to contain 50% CH4 and 50% CO2.
* Is this a reasonable assumption?
* Will it change over time?
Motivate your choice with respect to the waste composition, the degradation products of
different organic compounds, and the landfill conditions at the time of collection.
No. LFG is not the same as biogas. Landfill gas is a mixture of gases from three main sources:
1. biogas from the degradation of biopolymers in landfilled wastes,
2. atmospheric air that is landfilled with the waste or enters the landfill and/or the LFG
collection system.
3. gases or vapours from landfilled materials, e.g. vinyl chloride from PVC (compare
table 8.2 compendium) and water vapour (due to increased temperature)
Yes it changes over time.
The LFG composition depends on the waste composition and the type of organic matter.
For example, waste that mainly contains carbohydrates or proteins will generate a biogas containing about 50% CH4 and 50% CO2. But for waste containing more fat, fatty acids or higher alcohols, the gas can contain as much as 70% CH4 and only 30% CO2.
The time of LFG collection matter depending on the phase of the landfill. The age of the landfill also matters (varying waste in old landfills)
Describe in principle how one can determine the methane oxidation potential of a
material in the laboratory. What do you have to do and how do you express the result?
Methane oxidation can be measured by the drop in pressure resulting of the reaction:
CH4 + 2 O2 => CO2 and 2 H2O
Methane oxidation consumes three molecules of gas, while only one molecule of gas is
produced, and therefore a decrease in pressure occurs.
Apart from the oxidation, also the assimilation of methane into cell carbon results in a
pressure drop, since two molecules of gas are consumed and none produced:
CH4 (g) + O2 (g) → CHOH + H2O (l)
Add material (soil, waste), water, and oxygen in closed bottle. Note the weight of everything.
Add known amount/volume of CH4. Measure (with a syringe) and record the decrease in
pressure over time. Add more CH4 until the pressure keeps stable.
The consumed methane is then calculated and the potential is expressed in gram or litre CH4 per gram of dry solids or per gram of volatile solids of the input material.
What is the purpose of a LFG flare?
To decrease the greenhouse emissions from a landfill by oxidising CH4 into CO2.
Why is the heat release from biological oxidation of methane lower than from
combustion (chemical oxidation) of methane? How much lower is it theoretically?
Because some of the substrate goes into new cells (and hence, is not oxidised). 40%
lower.
How can one assess the remaining gas formation potential of an old landfill? Describe
briefly two in principle different methods.
1) look at the documentation – what has been landfilled? And then based on the waste
composition (+age of the landfill, +gas produced so far) assess the remaining potential.
2) Take samples and measure the BMP or the COD.
3) Perform emission measurements and compare parts of the landfill with different age
What is landfill gas?
A mix of biogas, water vapour, air and fumes that have evaporated from material in the
landfill.
Why is water essential for the degradation of organic material?
Because all biological reactions occur in water solution
: How can you improve the capture of landfill gas without increasing the suction
(negative pressure) in the collection system (and thus increase the risk of sucking air into the
landfill)?
You increase the surface of the gas collectors
Why is COD higher than BOD7?
Because chemical oxidation also affects organic matter that is not or hardly
biodegradable.
What are the main pathways for carbon during aerobic and anaerobic waste
degradation?
Aerobic: CO2 and biomass, Acidogenic: leachate, methanogenic: LFG
