- Acidogenesis
Acidogenesis represents the second stage in the four stages of
anaerobic digestion :
*Hydrolysis : A chemical reaction whereparticulate s are solubilized and largepolymer s converted into simplermonomer s;
* Acidogenesis: A biological reaction where simple monomers are converted into volatilefatty acid s;
*Acetogenesis : A biological reaction where volatile fatty acids are converted intoacetic acid ,carbon dioxide , andhydrogen ; and
*Methanogenesis : A biological reaction whereacetate s are converted intomethane and carbon dioxide, while hydrogen is consumed.Anaerobic digestion is a complex biochemical process of biologically-mediated reactions by a consortium ofmicroorganism s to convertorganic compound s into methane and carbon dioxide. It is a stabilization process, reducing odor,pathogen s, and mass reduction.Hydrolytic
bacteria form a variety of reduced end-products from the fermentation of a given substrate. One fundamental question which arises, concerns the metabolic features which controlcarbon andelectron flow to a given reduced end-product duringpure culture , and mixed methanogenic cultures of hydrolytic bacteria. "Thermoanaerobium brockii " is a representative thermophilic, hydrolytic bacterium, which fermentsglucose , via the Embden-Meyerhof Parnas Pathway. "T. brockii" is an atypical hetero-lactic acid bacterium because it forms molecular hydrogen (H2), in addition to lactic acid andethanol . The reduced end-products of glucose fermentation are enzymatically-formed frompyruvate , via the following mechanisms: lactate by fructose 1-6 all-phosphate (F6P) activatedlactate dehydrogenase ; H2 by pyruvateferredoxin oxidoreductase andhydrogenase ; and ethanol viaNADH - andNADPH -linkedalcohol dehydrogenase [Marchaim, U. (1992). FAO Agricultural Services Bulletin – 95:Biogas process for sustainable development, FAO – Fodd and Agriculture Organization of the United Nations, ISBN 92 – 5 – 103126, http://www.fao.org, (1/9/2003)] .By its side, the acidogenic activity was found in the early 20th century, but it was not until mid-
'60s that the engineering of phases separation was assumed in order to improve the stability and waste digesters treatment [Alexiou, I.E. and Panter, K. (2004). A review of two phase applications to define best practice for the treatment of various waste streams. Anaerobic Digestion 10th World Congress, Sept. 2004. Montreal, Quebec, Canada] . In this phase, complexmolecule s (carbohydrate s,lipid s,protein s) are depolymerized into soluble compounds by hydrolytic enzymes (cellulase s,hemicellulase s,amylase s,lipase s andprotease s). The hydrolyzed compounds are fermented into volatile fatty acids (acetate,propionate ,butyrate , and lactate), neutral compounds (ethanol,methanol ),ammonia , hydrogen and carbon dioxide [Cairó, J.J. and París, J.M. (1988). Microbiología de la digestión anaerobia, metanogénesis. 4o Seminario de Depuración Anaerobia de Aguas Residuales. Valladolid. F.F. Polanco, P.A. García y S. Hernándo. (Eds.) pp. 41-51] [Dinopolou, G., Rudd, T. and Lester, J.N. (1987). Anaerobic acidogenesis of a complex wastewater: I. The influence of operational parameters on reactor performance. Biotech. And Bioeng. 31: 958 – 968] [Laroche, M. (1983). Metabolisme intermediaire des acides gras volatils en fermentation methanique. These de Docteur – Ingenieur en Sciences Alimentaires_Fermentations. Institut National de la Recherche Agronomique, France]Acetogenesis is one of the main reactions of this stage, in this, the intermediarymetabolite s produced are metabolized to acetate, hydrogen and carbonic gas by the three main groups of bacteria:
*homoacetogen s;
*syntrophe s; and
*sulphoreductor s.For the
acetic acid production are considered three kind of bacteria:
* "Clostridium aceticum ";
* "Acetobacter woodii "; and
* "Clostridium termoautotrophicum ". Winter y Wolfe, in 1979, demonstrated that "A. wodii" in syntrophic association with "Methanosarcina " produce methane and carbon dioxide fromfructose , instead of three molecules of acetate [Winter, J.U. and Wolfe, R.S. (1979). Complete degradation of carbohydrates to CO2 and methane by syntrophic cultures of Acetobacterium woodii y Methanosarcina barkeri. Arch. Microbiol. 121: 97 – 102] . "C. thermoaceticum " and "C. formiaceticum " are able to reduce the carbonic gas to acetate, but they do not havehydrogenase s which inhabilite the hydrogen use, so they can produce three molecules of acetate from fructose. Acetic acid is equally a co-metabolite of the organic substrates fermentation (sugars ,glycerol ,lactic acid , etc.) by diverse groups of microorganisms which produce different acids: *propionic bacteria (propionate + acetate);
* "Clostridium " (butyrate + acetate);
*Enterobacteria (acetate + lactate); and
* Hetero-fermentative bacteria (acetate, propionate, butyrate,valerate , etc.).References
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