Biogas production by anaerobic digestion: process basis and monitoring

1. Biogas production by anaerobic digestion: process basis and monitoring Università Degli Studi di Milano http://www.diprove.unimi.it/ reference: Fabrizio Adani fabrizio.adani@unimi.it Giuliana D’Imporzano giuliana.dimporzano@unimi.it

2. Anaerobic digestion biochemical basis • Process phases • Process parameter • Full scale application • Problems identification

3. Biochemical By a biochemical point of view anaerobic digestion is the breakdown of organic matter by bacteria in the absence of oxygen, where the final electron acceptor is not oxygen but carbon (carbon dioxide and acetic acid ) and the product is methane.

4. Industrial process By an industrial point of view anaerobic digestion is a monitored process run in anaerobic digestors to transform an organic substrate into biogas (methane and carbon dioxide) and a stabilized digestate (residual material). in lack of oxygen

5. Materials Almost any organic material can undergo anaerobic digestion • Animal waste • Vegetal by product • Sewage • Organic Fraction of Municipal Solid Waste • Energy crops

6. Biogas yield Source: Baserga 2000, Adani data not pubblished

7. AD promote a Closed Cycle of Carbon

8. AD promote a Closed Cycle of Carbon Vegetal and animal by products OFMSW Renewable energy CO2 saving atmosphere Fertilizer soil fertilizer saving and organic matter addition to soil

9. Phases and biochemical steps Fatty acids Aminoacids Source: T Al Seadi (2001)

10. Phases and biochemical steps Selonomonas Clostridium Desulfovibrio Syntrophomonas Eubacterium Clostridium Metanotrix Metanosarcina Metanobacterium Metanococcus Source: Lechner et al, 2000

11. Biochemical reactions coexist but at macroscopic level we detect: Prevalence of acidification reactions (hydrolisys, acidogenesis, acetogenesis ) in the first time Increasing and stabilization of methanogenic reactions in second time What actually happens when organic matter undergo anaerobic digestion:

12. During acidification phase: increase of organic acids and volatile fatty acids (VFA) and pH fall down pH fall support acid-generating bacteria, but inhibit methane-generating bacteria Biogas produced is rich in CO2 but not in methane Acidification phase

13. During methane phase: VFA concentration falls down pH value increases (7-8) pH increase support methane-generating bacteria growth biogas produced is rich in methane Methane phase

14. VFA concentration and pH trend during anaerobic digestion Acidification phase: low pH and high concentration of VFA Methane phase: pH value raises and steady around 7-8 VFA concentration drops.

15. Biogas production and methane concentration during AD phases Acidification phase: biogas production is low and methane concentration below 50%. Methane phase: biogas production increases and methane percentage raises (over 60%)

16. Different reactions coexist during AD Methane phase is the most sensitive, slow and limiting step In order to optimize an industrial AD process we have to maintain conditions favouring methane-forming bacteria (methane steady state) in the digester

17. To favour the growth of methane-forming bacteria we balance the rate betweendigestatematerial (e.g. material that previously underwent AD) andfresh material still to be degraded. Digestatematerial works like seeding formethane-forming bacteria and helps to buffer the acids deriving from the first step of organic matter degradation.

18. Each industrial process has its own strategy in order to balance the rate between digestate and fresh material and to keep the methane phase steady in the digester.

19. Process parameter to monitor in a digester Volatile Fatty Acids (VFA) VFA concentration is expressed as acetic acid conecentration. It depends on the quantity and quality of the material loaded in the digester and on the ballance between acid-forming bacteria and methan-forming bacteria. VFA concentration in a digester can range between 500 and 3000 mg Ac/l, but higer values are reported. More than the VFA concentration value is important the variation during time. Sharp increase of VFA points out that the AD process is turningtowards acid phase.

20. Neverless ammonia concentration is importan to buffer the system and ballance high accumulation of VFA CO2 + H2O HCO3- + H+ HCO3- + NH4+ NH4HCO3 Process parameter to monitor in a digester Ammonia concentration Ammonia is formed during the degradation of proteins. High concentration may inhibit acid and methane-forming bacteria. concentration ranges (van Velsen 1979): 200 e 1500 mg/l : never toxic, 1500 -3000 mg/l : inhibiting if pH is under 7.4 3000 mg/l : always inhibiting

21. Process parameter to monitor in a digester Total alkalinity Is the system capacity to neutralize protons and buffer the system. It is expressed as CaCO3mg/l Total alkalinity depends on ammonia, carbonate systems (CO2 HCO3-) and VFA. Total alcalinity values around 3000-5000 mg CaCO3/l (and more…) are reported in steady state digesters (Stafford et al., 1980).

22. Process parameter to monitor in a digester pH In a steady state digester the pH value should range around 7-8. It indicates equilibrium in acid forming and methane forming methabolism. pH value depends, and to some extent resume, the parameters previously proposed (VFA, ammonia, alkalinity). pH fall below 7 indicates VFA accumulation (sometimes due to digester overloading) pH raise over 8, usually indicates ammonia accumulation

23. Process parameter to monitor in a digester Biogas quantity and methane concentration Variation in the production of biogas and in the concentration of methane indicates instability in the system (Stafford et al., 1980). If biogas quantity decrease and methane percentage falls down 50%, it indicates inhibition probably due to VFA accumulation. Methane concentration is the only parameter that shows digester instability faster than pH.Always to detect on line

24. Criteria to separate AD process Total solids (TS) content in the digester: • Wet process (TS = 5–10%), • Half- dry process (TS = 10–20%), • Dry process(TS > 20%). Biological phases: • One- stage system: all the process phases (acid and methane phase) are run in one digester • Multi-stage : hydrolitic and acid forming phase are run in different digesters than methane phase Feeding system: • continuous feeding system • Batch system Operating temperature: • 35–37°C • 55°C

25. Industrial process technologies widely applied in industrial scale: • Wetone stagecontinuous system • Dryone stagecontinuous system • Dryone stagebatch system • Drysequencebatchsystem Each tecnology run at anyoperating temperature (37-55 °C)

26. Continuous system Steady methane phase: pH around 7-8 Methane concentration over 50%

27. Continuous system Methane phase: biogas production increases and methane percentage raises (over 60%)

28. Batch system Acidification phase: pH around 5 methane concentratione below 50% Methane phase: pH around 7-8, methane concentration over 50%

29. Batch system

30. Wet one stage continuos system Digester design for this system is very simple and is called CSTR (Completely Stirred Tank Reactor). Digestate is continuously drawn from the digester Fresh material is continuously fed in the digester and completely mixed

31. Dry continuous system Total solids content is higher than 20%, so dry process are not run in completely stirred reactors but in plug-flow type.

32. Dranco system Totalal solid of feeding mixture20-50%. Feeding:from the top of the digester, extraction from the bottom Balance between digestate and fresh material: liquid digestate is recycled from the bottom and mixed with fresh material

33. Kompogas system Total al solid of feeding mixture 25-40%. balance between digestate and fresh material : inoculum recycling Mixing: slow rotating mixer

34. Valorga system • Total al solid of feeding mixture :30%. • balance between digestate and fresh materia:inoculum recycling • Mixing: biogas bubbling

35. Batch system The material is put into the digester in one single feeding In sequential bach system liquid digestate is recycled from digester with old material to the one with new material and from the one with new material to the one with old material after a stated period is drawn out

36. Eunomia 2005

37. Parameters value

38. Possible problems Biogas quantity Methane percentage pH 1. System overloading 1. System overloading pH Ammonia> 3000 2.ammonia accumulation Ammonia<3000 3. salinity pH VFA/total alkalinity> 0.3 4. Presence of inhibitors (antibiotics, pesticides, heavvy metals…) pH VFA/total alkalinity< 0.3

39. Possible solution 1. System overloading • Stop feeding • Dilute VFA concentration • digestate recirculation • Water addition • Buffer addition (NaCO3) • Stop digestate recirculation • Dilute concentration • enhance the feeding of fresh matherial (if condition allow) • Add water 2.ammonia accumulation 3. salinity 4. Presence of inhibitors (antibiotics, pesticides, heavvy metals…) Check inlet matherial

40. AD plant managing parameter Retention time (RT) it indicates the time (days) organic material resides in the reactor. In continuous feeding system RT is equal to: RT =V/Fv V reactor volume [m3]; Fv feeding volume for day [m3/day]. Organic Loading Rate (ORL) it indicates the loading of organic matter in the reactor OLR = F* C/V OLR [kg VS/m3 reactor/day]; F feeding volume for day [m3/day]. C substrate concentration in feeding , [kgVS/m3]; V reactor volume [m3];

41. Mass ballance Organic matter degradation during AD can reach 60-80% of VS content The digestate material present a relative enrichment in lignin and recalcitrant molecules Adani, data not pubblished

42. References • Biocycle, April 2004. Kranert M, Hillebrecht K. (2000) – Anaerobic digestion of organic waste, process parameters and balances in practice – Internet Conference on Material Flow Analysis of Integrated Bio-Systems, Marzo-Ottobre 2000, www.ias.unu.edu/proceedings/icibs/icmfa. • Bolzonella D. Battistoni P. Susinii C., Cecchi F. 2006. “Anaerobic codigestion of waste activate sludge and OFMSW: the experiences of Viareggio and Treviso plants (Italy)” Water Science and Technology, vol 53 n 8 pp203-211. • Bolzonella D., Pavan P., Mace S., Cecchi F. 2006. “Dry anaerobic digestione of differently sorted organic municipal solid waste: a full-scale experience. Water Science and Technology, vol 53 n 8 pp 23-32 • Brummeler E.ten 2000. “Full scale experience with Biocel process”. Water Science and Technology, vol 41 n3 pp299-304. • Buekens A.(2005) – Energy recovery from residual waste by means of anaerobic digestion technologies – Atti della Conferenza “The future of residual waste management in Europe”, 2005. • Buffiere P.,Loisel D., Bernet N.and Delgenes J-P. 2006. “ Towars new indicators for the prediction of solid waste anaerobic digestione properties”. Water Science and Technology, vol 53 n 8 pp233-241. • De Baere L. “Anaerobic digestion of solid waste: state-of-the-art.” 2000. Water Science and Technology, vol 41 n 3 pp283-290 • De Baere L., 2000. Anaerobic digestione of solid waste: state of the art. Water Science and Technology, vol 41 no 3 pp 283-290 • Fruteau de Laclos H., Desboies S. and Saint-Joly C. 1997. “Anaerobic digestione of municipal solid organic waste:Valorga full-scale plant in Tilburg, the Netherlands”. Water Science and Technology, vol 35 n 6-7 pp457-462.. • Hartmann H., Ahring B. K. 2006. “Strategies for the anaerobic digestion of the organic fraction of municipal solid waste: an overview”. Water Science and Technology, vol 53 n 8 pp7-22. • Kubler H, Rumphorst M. (1999) – Evaluation of processes for treatment of biowaste under the aspects of energy balance and CO2 emission– Atti del II International Symposium on Anaerobic Digestion of solid waste, Barcellona, 15-17 June 1999. Blischke J. (2004) – Combining anaerobic digestion with enclosed tunnel composting – • Lissens G., Vandedivere P., De Baere L., Biey E M. and Verstraete W. 2001. “Solid waste digestors: process performance and practice for municipal solid waste digestion”. Water Science and Technology, vol 44 n 8 pp 91-102. • Observ’ER, 2006. Le baromètre du biogas. - Systèmes solaires, n. 173, Maggio 2006. • Pavan P., Battistoni P., Mata-Alvarez J., Cecchi F. 2000 “Performance of thermophilic semi-dry anaerobic digestion process changing the feed biodegradability”. Water Science and Technology, vol 41 n 3 pp75-81 • Six W. (2006) – Status and trends of anaerobic digestion in Europe – Atti della ISWA Beacon Conference on Biological Treatment, Perugia, 10-12 maggio, 2006. • Tilche A., Malaspina F. (1998) – “Biogas production in Europe: an overview”, Atti del 10° European Conference “Biomass for energy and industry”, Würzburg, Germania, 8-11 Giugno 1998. • Wang J.Y., Zhang H., Stabnikova O., Ang S.S., Tay J. H. 2005. “A Hybrid anaerobic solid-liquid system for food waste digestion”. Water Science & Technology. Vol 52 n1-2 pp 223-228. • Wellinger A. (2002) – Biowaste digesters in Europe – Atti del Convegno Biogas International 2002, 17 Gennaio 2002, JCC und Messe, Berlino.

43. Glossary Anaerobic Digestion: breakdown of organic matter by bacteria in the absence of oxygen, where the final electron acceptor is not oxygen but carbon (carbon dioxide and acetic acid ) and the product is methane It occurs in enclosed vessels (in this case, reactors, or digestion tanks). Biogas: The gas produced by anaerobic bacteria in the anaerobic digestion process. Typically, it is composed primarily of methane and carbon dioxide, with low levels of other gases such as ammonia, hydrogen sulphide, hydrogen and water vapour. Biogas Production Rate: The biogas production rate is the volume of biogas, at standard temperature and pressure, that can be produced per unit mass of digestion input. It is expressed in this report as m3 / tonne of waste to digestion. Digestate: The material resulting from anaerobic digestion of organic materials. Mesophilic: A specific temperature range in which AD reactions may take place, nominally 25°-40°C, but usually around 35° C Methanogenesis: The generation of methane in landfills and in AD by anaerobic processes. Retention Time: The average amount of time that material remains in the digester. It is a parameter used to determine adequate digestion. Organic Fraction of Municipal Waste (OFMSW): organic wastes (kitchen scraps, nonrecyclable paper, animal wastes and sanitary wastes) which are separated from mixed waste collected from householders. Thermophilic: A specific temperature range in which AD reactions may take place, nominally 45°-65°C, but usually around 55°C. Total Solids (TS): The amount of dry solids in a material. This includes both the organic and non-organic fractions of the solids. Volatile Fatty Acids (VFA): short chain organic acids such as acetic acid, formic acid, propionic acid. VFA are produced as intermediate during first stages of anaerobic digestion and can cause the pH fall in the medium. High accumulation of VFA in a digester can result in process failure, due to inhibition of methane forming bacteria. Volatile Solids (VS): The organic, or carbon-containing, fraction of TS. VS concentration is expressed as a percentage of the TS. It is determined by incineration of the sample at 550C; volatile solids burn off while the fixed solids (non-organic fraction) will remain in the sample.

44. Thank you for your attention For any further question please mail to giuliana.dimporzano@unimi.it