N. Defoer & H. Van Langenhove

Research Group of Environmental Organic Chemistry andTechnology, 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 anincreasingly sensitive issue!

Aim of this presentation:

Giving an overview of the present knowledge on odouremissions and possible reduction techniques atcomposting facilities

Introduction

Odour generation

� anaerobic conditions

� high temperatures

� stripping

Important mechanisms in odour generation:

Odour generation1. anaerobic conditions

aerobic: organics + O2 → CO2 + H2O

anaerobic: instead of O2 other electronacceptors like NO3-, SO4

2- , oxidized matter and CO2

� results in formation of N2, H2S, organic sulphur compounds,reduced organic compounds and methane

→ efficient aeration, optimal moisture content and appropriateporosity will contribute to less odour production

Note: Anaerobic microsites are hard to avoid in practice!

Odour generation2. High temperatures

Development of typical process phases with own temperatures andproduction 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

3. Stripping

Aeration of biowaste

→ improves input of oxygen

→ promotes output of volatile odorous substances

Odour generation

Flanders: 2 types of waste are composted separately

Odour sources

VFG waste

(vegetable, fruit & garden waste)

composting in closedbuildings

greenwaste

(organic waste from gardens, parks & roadsides)

composting outside

Odour sources

Continuous odour sources

- storage of waste

- composting process

- leachate

Discontinuous odour sources

- dumping of waste

- shredders, mills, �

- 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 Air flow

(m3/h)Odour emission

(ou/h × 104)Receiving area 45320 2131Pretreatment 14825 156Composting area -sucking -blowing

134506800

153803400

Storage 3400 34Final treatment 3300 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 � 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)� no lack of nitrogen� higher rate of oxygen depletion� 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

� 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 � 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 developmentof anaerobic conditions

� 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 ofbiofilters 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 butDefoer et al. (2002) found a good correlation between odourand VOC concentration for the effluent of biofilters at VFGcomposting plants

� 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 � biofiltration not possible

� other methods like adaptation of aeration and turning frequency must offer a solution