Investigation of the Physical, Chemical and Microbiological Parameters Influencing the Small-scale In-vessel Composting of Food Waste D. Orthodoxou, T.R. Pettitt, M. Fuller, M. Newton, N. Knight, S.R. Smith CYPRUS 2016
Food Waste (FW): a challenging feedstock ■ Needs bulking agents, otherwise it becomes highly homogeneous ■ High moisture content ■ Often very acidic ■ Collection in closed containers can lead to anaerobic conditions, further lowering pH ■ Must be treated in an enclosed environment, specified time- temperature conditions
The role of VFAs in FW composting ■ Initial decrease in pH is often observed after FW addition to IVC. In severe cases this can lead to significant decline in temperature and malodours ■ VFAs in compost mixture play key role in regulating pH during mesophilic phase ■ Acidic compost mix (pH<6) undissociated VFAs inactivation of aerobic microorganisms accumulation of VFAs further acidification ■ Mesophilic aerobes are more acid tolerant than thermophilic organisms ■ Maintaining the compost temperature below 46ºC until the pH value increases above pH 6.5, allows aerobic organisms to degrade VFAs, thus shortening the mesophilic phase and increasing the composting temperature in a shorter period of time.
In-vessel FW composting at the Eden Project ■ FW shredded to 20mm particle size ■ Retention time between 60 and 110 days (depending on feeding rate)
Feedstock Feeds eedstock ock pH pH Chara ract cteris eristics tics Food Waste (FW) 4.0-4.4 after shredding High MC, little porosity Green Waste (GW) 6.0-8.0 Increases C/N ratio. Adds microbial population Corrugated Board Dust 8.0 Low MC (~11%). Drying (CBD) agent Sawdust (SD) 5.6-5.8 Drying agent Sawdust Pellets (PEL) 4.7 Low MC (~15%). Expands on rehydration, increases free air space Finished Compost (FC) 7.5-8.5 Drying agent. Adds thermophilic organisms
Eight week feedstock management strategy Inp nput (kg) g): Week 1 2 3 4 5 6 7 8 FW FW 572 - - - - 14 224 279 GW GW 208 300 101 197 84 350 205 84 FC FC 80 172 681(565) - - - 115 47 CBD 99 125 34 132 119 - - - SD SD - 5 22 - - - - - PEL - - - 16 - 242 30 30 Tot otal al Inp nput ut: 959 602 838 345 203 606 574 440 Tot otal al Output put (kg) g): 194 - (55)* 388 179 579 189 - * Value in brackets represents the material removed from the discharge end of the vessel and recycled to the feed end.
Changes in temp., moisture content and pH Hatch 1 Hatch 2 70 7.0 70 10.0 65 65 Sample Temperature ( o C) / Sample Temperature ( o C) / 9.0 60 60 6.0 % Moisture Content % Moisture Content 8.0 55 55 50 50 7.0 pH pH 45 5.0 45 6.0 40 40 35 35 5.0 4.0 30 30 4.0 25 25 20 3.0 20 3.0 n n n n n n l l l l l u u u u u u u u u u u 2-Jun 5-Jun 11-Jun 16-Jun 22-Jun 26-Jun 1-Jul 7-Jul 13-Jul 17-Jul 23-Jul J J J J J J J J J J J - - - - - - - - - - - 1 7 3 7 3 2 5 1 6 2 6 0 0 1 1 2 0 0 1 1 2 2 Sampling Date Sampling Date Moisture Content % Temperature pH Moisture Content % Temperature pH Hatch 4 Hatch 3 70 10.0 70 6.0 65 65 Sample Temperature ( o C) / 9.0 Sample Temperature ( o C) / 60 60 % Moisture Content % Moisture Content 5.5 55 55 8.0 50 50 pH 45 7.0 pH 45 5.0 40 40 6.0 35 35 4.5 30 30 5.0 25 25 20 4.0 20 4.0 02-Jun 05-Jun 11-Jun 16-Jun 22-Jun 26-Jun 01-Jul 07-Jul 13-Jul 17-Jul 23-Jul 02-Jun 05-Jun 11-Jun 16-Jun 22-Jun 26-Jun 01-Jul 07-Jul 13-Jul 17-Jul 23-Jul Sampling Date Sampling Date Moisture Content % Moisture Content % Temperature pH Temperature pH
The effect of MC and pH on the temperature of the compost mixture B A Temp o C = 103.2 - 1.18 MC 65 R 2 = 0.75 65 P < 0.001 60 60 Sample Temperature ( o C) 55 Sample Temperature ( o C) 55 50 50 45 45 40 40 35 35 Temp o C = 25.81 + 18.12 pH - 1.03 pH 2 30 30 R 2 = 0.84 25 25 20 20 3.5 4.5 5.5 6.5 7.5 8.5 9.5 20 25 30 35 40 45 50 55 60 65 pH % Moisture Content
The effect of % moisture content on pH 10 9 8 7 pH 6 5 4 3 20 30 40 50 60 70 % Moisture Content
Interstitial gas concentrations
Relationship between temperature and C02 and temperature and 02 A B %CO 2 = 15.0 + 0.65 Temp o C 35 R 2 = 0.66 25 %O 2 = 32.1 - 0.56 Temp o C P < 0.001 R 2 = 0.69 30 20 P < 0.01 25 15 20 % CO 2 % O 2 15 10 10 5 5 0 0 30 35 40 45 50 55 60 65 70 30 35 40 45 50 55 60 65 70 Compost Temperature ( o C) Compost Temperature ( o C)
Conclusions ■ The rapid biodegradation of FW makes its composing susceptible to acid accumulation and pH decline. ■ The results show that process acidification is highly sensitive to increasing MC. ■ Possible reason: MC influences the thermodynamic balance of the process. A wet mixture leads to greater heat losses, causing a decline in temperature and a decline in VFA metabolism. Feedback mechanism leading to more VFA accumulation and pH reduction microbial toxicity process inhibition. ■ MC between 41-48% were associated with a marked transitional increase in compost pH above 6.0 and a concomitant rise in temperature to thermophilic values ■ Moisture contents over 48% caused severe acidogenesis and mesophilic temperatures ■ Under the conditions of this investigation the upper critical MC for in-vessel composting of food waste was 40% (whereas the recommended MC for composting is in the range of 40-60%).
Orthodoxou, D., Pettitt, T.R., Fuller, M., Newton, M., Knight, N., Smith, S.R. (2015). An Investigation of Some Critical Physico-chemical Parameters Influencing the Operational Rotary In-vessel Composting of Food Waste by a Small-to-Medium Sized Enterprise. Waste and Biomass Valorization, 6 (3) Thank you!
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