N. Defoer & H. Van Langenhove
Research Group of Environmental Organic Chemistry and
Technology, Ghent University
Introduction
• biological waste treatment is a growing sector in Europe
• 30% of urban waste/industrial residuals is biodegradable
or 60 million tons/year
• Flanders (2000):
347 000 tons greenwaste
286 000 tons VFG (vegetable, fruit and garden waste)
Consequence of this growth: odour is becoming an
increasingly sensitive issue!
Introduction
Aim of this presentation:
Giving an overview of the present knowledge on odour
emissions and possible reduction techniques at
composting facilities
Odour generation
Important mechanisms in odour generation:
• anaerobic conditions
• high temperatures
• stripping
Odour generation
1. anaerobic conditions
aerobic: organics + O2 → CO2 + H2O
anaerobic: instead of O2 other electronacceptors like NO3-, SO42- ,
oxidized matter and CO2
Þ results in formation of N2, H2S, organic sulphur compounds,
reduced organic compounds and methane
→ efficient aeration, optimal moisture content and appropriate
porosity will contribute to less odour production
Note: Anaerobic microsites are hard to avoid in practice!
Odour generation
2. High temperatures
Development of typical process phases with own temperatures and
production of specific odorants
• mesophilic startphase: biogenic odour components (terpenes)
• selfheating phase: volatile biogenic odours
• thermophilic phase: decrease of biogenic odours
abiogenic odours by pyrolyse, auto-oxidation,
Maillard reactions
• maturation phase: decline in temperature,
decrease of microbiological activity
Odour generation
• Activity of thermophilic organisms optimal at 52 to 60°C
• Macgregor et al. (1981), Finstein et al. (1986): reduction in
odour production when T < 60°C
• Miller (1993), Toffey et al. (1995), Hentz et al. (1996):
increasing odour emission at increasing composting temperatures
Odour generation
3. Stripping
Aeration of biowaste
→ improves input of oxygen
→ promotes output of volatile odorous substances
Odour sources
Flanders: 2 types of waste are composted separately
VFG waste
greenwaste
(vegetable, fruit & garden
waste)
(organic waste from gardens,
parks & roadsides)
composting in closed
buildings
composting outside
Odour sources
Continuous odour sources
Discontinuous odour sources
- storage of waste
- dumping of waste
- composting process
- shredders, mills, …
- leachate
- turning and moistening of compost
- sieving
- digging up compost
Odour sources
Diffuse odour sources
• leachate, open hall doors, cracks, failure to properly
clean facility areas, …
• Kuchta (1994): diffuse odour sources can be up to 10%
of total odour emission at composting facilities
Odour sources
Bidlingmaier (1993)
Composting stage
Receiving area
Pretreatment
Composting area
-sucking
-blowing
Storage
Final treatment
Air flow
(m3/h)
45320
14825
Odour emission
(ou/h × 104)
2131
156
13450
6800
3400
3300
15380
3400
34
39
• composting area (including turning) most important odour source
• immediately prior to turning: 1000 ou.m-3
during turning: 5000 ou.m-3
• relative values more important than absolute values
Odour sources
TNO (1994)
• research at greenwaste composting facilities with
different turning frequencies
• turning most important odour source followed by
continuous emission of composting piles and storage of
biological waste
• contribution of different sources (in percentage)
dependent on method of composting
Factors influencing odour emission
History of waste
• storage, frequency and way of collecting
• material too long in collection containers
Þ trash and putrefying odours
• De Bo & Van Langenhove (1999):
one week storage of material that was collected every two
weeks gave rise to an odour emission of 2 to 6 times higher
Conclusion: it is really important to handle the waste the same
day as it is received!
Factors influencing odour emission
Feed composition
• high levels of specific feed like grass can be problematic
• grass fraction > 10%: odour problems are possible
• mechanism: high grass levels (nitrogen source)
Þ no lack of nitrogen
Þ higher rate of oxygen depletion
Þ anaerobic conditions
Possible solution: addition of grass in phases during the composting
process
Factors influencing odour emission
Seasonal influence
• cumulation of different effects:
- higher waste supply (from June to September supply 2
or 3 times as high as in January and February)
- feed composition (more grass)
- higher temperatures
Þ higher odour emission during spring/summer
Factors influencing odour emission
Moisture content
• one of the most common factors leading to anaerobic
conditions
• moisture < 35-40%: reduction of decomposition
• upper limit varies with different materials (function of particle
sizes and structural characteristics)
• for most compost mixtures: 55-60% is recommended
Odour reduction techniques
Turning frequency
• turning releases odorous compounds trapped within piles
• lack of turning allows anaerobic conditions Þ increases
severity of odours when piles are eventually disturbed
• Defoer & Van Langenhove (2000): research on the influence
of turning frequency on odour emission at 3 greenwaste
composting plants with different turning frequencies
( plant A: every 6 weeks, plant B: every week, plant C:
every 8 weeks)
Odour reduction techniques
Turning frequency
RESULTS
- odour emission high and nearly the same during first 10 days
- afterwards fast decline to the same continuous odour emission
- peak emissions after every turning were lowest for plant B
with the highest turning frequency
Odour reduction techniques
Turning frequency
CONCLUSIONS
- a complete odour balance could not be made
- the plant with the highest turning frequency had the lowest
odour emission
- other factors (height of compost piles, turning equipment, …)
could have played a role
Odour reduction techniques
Aeration
inadequate oxygenation (aeration) leads to development
of anaerobic conditions
Þ active aeration has pronounced effect on both rate of
composting and odour production
Influence type of aeration
research of Bidlingmaier (1993)
sucking aeration: 15380 ×104 ou.h-1
blowing aeration: 3400 ×104 ou.h-1
Odour reduction techniques
Odour masking
• overpowering of the odorous molecules with stronger, more
pleasant molecules
• Defoer & Van Langenhove (2000): sniffing team measurements
around a greenwaste composting plant with and without odour
masking products
- large variation in the results (from -5 to +73% reduction
with a mean value of 17%)
- ‘soap’ odour is also annoying
- no ecologically sound reduction technique
Odour reduction techniques
Air treatment
• air from closed buildings can be removed by ventilation
• used as process air for aerating compost piles (odour reduction)
• odour removal by chemical or biological treatment
• biological treatment is the most cost-efficient technique
• in Flanders used by all VFG composting facilities
Odour reduction techniques
Air treatment
Defoer & Van Langenhove (2000): monitoring campaign of
biofilters at 5 Flemish aerobic VFG composting plants
- 3 of the 5 biofilters: VOC removal efficiency > 94%
- 1 biofilter lower efficiency of 80% due to low influent
concentrations
- 1 biofilter VOC removal efficiency of only 64%
(reason was not clear)
Odour reduction techniques
Air treatment
Note: results were expressed in VOC removal efficiency but
Defoer et al. (2002) found a good correlation between odour
and VOC concentration for the effluent of biofilters at VFG
composting plants
Þ biofiltration is a reliable technique for odour removal at
composting facilities
Final conclusion
• considerable research on odour production has been done
• importance of factors like turning frequency and aeration are
difficult to describe quantitatively
• effects of odour masking products are ambiguous
• VFG composting: biofiltration is a reliable technique for
odour removal
• greenwaste composting: not in closed buildings
Þ biofiltration not possible
Þ other methods like adaptation of aeration and
turning frequency must offer a solution