Paul
There is agreement that exothermic energy from aerobic processes exceeds anything that might theoretically be gained anaerobically by orders of magnitude. However, there are many aerobic effluent treatment systems in existence which demonstrate no appreciable temperature increase, i.e. sewage treatment works (approx 300mg/l COD), industrial effluent treatment plants etc. The only time an aerobic temperature rise is effectively detectable is when the energy density (COD or TVS) is great enough, i.e. compost systems. Some of the first autothermal aerobic systems were pioneered at Cornell University in the late 1980s as TAD processes and later developed further in the UK (Thermophyllic Aerobic Digestion) for very high strength liquid effluents. These could indeed maintain 55 deg C under aerobic conditions and specific operational circumstances. I work daily with anaerobic treatment of waste streams at 50,000-250,000mg/l COD ( very large scale: POME, Ethanol, Cassava; small scale: Catering Waste) and there is no observable temperature rise in anaerobic systems. In addition, this is at high ambient temperatures, so losses are at a minimum; further many are flexible liner reactors with low surface area to volume ratios (30,000 – 100,000m3), so if there was any observable impact this would be noted. These systems are also intensive monitored with daily profiles etc. Whatever the theoretical considerations, for design purposes we would be best advised to consider the process as neither positive or negative in terms of metabolic heat loss or gain. Regards Dr Stephen Etheridge PS an interesting contribution to the debate which might be explored further: “Long chain fatty acids degradation in anaerobic digester: Thermodynamic equilibrium consideration”, Oh, S.T. , Martin, A.D., Process Biochemistry, Volume 45, Issue 3, March 2010, Pages 335-345 Abstract The biological oxidations and reductions occurring in batch anaerobic digesters may approach equilibrium. The approach depends strongly on the activities of the micro-organisms present. The thermodynamics of linear long chain fatty acid degradation in a batch reactor was modeled. The substrates considered are the saturated fatty acids from acetic acid to stearic acid. From the thermodynamic perspective, the fermentation (acidogenesis and acetogenesis), decomposing the long chain saturated fatty acids to acetic acid though shorter chain acids, could not proceed spontaneously (ΔH ≫ 0 and ΔG ≫ 0). However the model suggests that the major driving force for the fermentation may be found in the methanogenesis. The model results show two distinct cases: (ΔS > 0 and ΔH > 0) and (ΔS < 0 and ΔH < 0), relating to spontaneous but endothermic and non-spontaneous but exothermic processes respectively. Where, spontaneous digestion is associated with high initial concentrations of LCFA and endotherm. This implies that the digestion of this type of substrate might be better facilitated by the supply of supplemental heat. The digestion of very low concentrations of LCFA is found to be non-spontaneous due in large part to the solubility of carbon dioxide. This implies that the digestion of this type of substrate might be enabled by selectively removing carbon dioxide. From: Digestion [mailto:[email protected]] On Behalf Of David Sent: 21 November 2012 06:21 To: For Discussion of Anaerobic Digestion Subject: Re: [Digestion] Reaction Kinetics for AD Paul, On 11/20/2012 12:43 AM, Paul Muthui wrote: G'day All, How much energy is given out in the digesters as a result of anaerobic reaction? I can't seem to get any publication with enough details on this. I would like to know the temperature rise that would be caused by the reaction. Dr. Fulford is correct, in essence, in that according to ordinary experience, no functional metabolic heat is available from anaerobic digestion, as contrasted to the very considerable metabolic heat available from aerobic digestion (at about 40% of the energy provided as methane in the former case). However: Self-heating <http://ebookbrowse.com/self-heating-of-anaerobic-digesters-using-energy-crops-pdf-d61331597> of anaerobic digesters using energy crops H. Lindorfer, R. Kirchmayr, R. Braun Water Sci Technol. 2006;53(8):159-66. Abstract With the increasing application of energy crops in agricultural biogas plants and increasing digester volumes, the phenomenon of self-heating in anaerobic digesters appeared in some cases. Until now this development was just known from aerobic systems. To get an idea of the thermodynamics inside an anaerobic digester, a detailed analysis of all heat fluxes in a full scale agricultural biogas plant was carried out. Several experiments were realised to quantify the influences of different internal and external energy sources. To estimate the impact of self-heating in anaerobic systems, data of other full scale agricultural biogas plants in Austria were collected. Alternatives to the cooling of the digesters are discussed basing on individual experiences of several plants. A connection between carbohydrate rich substrates, especially with high starch contents, and the self-heating could be shown. But from the results it can be assumed that heat enthalpy due to anaerobic microbial metabolism plays a key role in self-heating, which is in contrast to the current thermodynamic knowledge. And Prof. <http://www.matheon.de/about_us/people/list_members.asp?action=detail&id=812> Dr. Christof Schütte-- a mathematician, to be sure, but as applied to biological systems-- goes so far as to claim <http://www.matheon.de/research/show_project.asp?id=134> that "Knowledge of the net energy production of anaerobic fermenters is important for reliable modelling of the efficiency of anaerobic digestion processes. Furthermore energy balancing is crucial, because present biogas plants still suffer from severe instabilities caused by self-heating and full-scale plants typically have to be cooled down." These studies and statements, however, have a context, which is the Austrian/German biogas industry, where digestion is a high-tech (some might say gold-plated) affair in large part because of the previous regime of above-market price guarantees for electricity from biogas. That is to say, the digesters being analyzed and discussed above are very likely extremely well insulated. But as far as I know for the kinds of digesters with which you are working-- underground masonry, if I'm not mistaken-- I believe you can assume that the contents of the digester will be at ambient temperature six feet deep. d. -- David William House "The Complete Biogas Handbook" www.completebiogas.com Vahid Biogas, an alternative energy consultancy www.vahidbiogas.com "Make no search for water. But find thirst, And water from the very ground will burst." (Rumi, a Persian mystic poet, quoted in Delight of Hearts, p. 77) http://bahai.us/
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