N. Defoer & H. Van Langenhove Technology, Ghent University
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
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