Richard Lab:Review of biodigesters

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basics After hydrolysis of biodgeradable waste, acidogenic bacteria break it down to acetic and propionic acids, among other compounds. This is ideally done at a low pH and with a high throughput of waste. Then methanogenic bacteria use the acetic acid (mostly) as a substrate to produce methane. The process consists in cleavage of the carboxyl group in acetic acid, then reduce the resulting CO2 to methane, using hydrgen resulting from enzymatic breakdown of organic compounds. Methanogenesis is ideally done at neutral pH and requires a lower throughput of feed than acidogenesis. Biodigesters can be single or 2-phase. In the 2-phase digester acedogenesis and methanogenesis occur in 2 separate containers maintained at different, in 2 batch reactions. Digester can be mixed a a certain frequency to improve mehane production, and mixing is either mechanical or consists in re-circulating biogas, or effluent. Hala
McCarty, P. 1964. Anaerobic waste treatment fundamentals. Part one. Chemistry and Microbiology. Public Works. efficient anaerobic digestion depends on a balance between methane and acid forming bacteria. The establishment and maintenance of this balance is indicated by the concentration of volatile acids (short chain organic acids). Formic, acetic, propionic, butyric, valeric, isovaleric, caproic acids. 1-6 C carbon chains. When the system is in balance methane bacteria use the acid intermediates at the same rate of production, otherwise VA concentration increases. A volatile acids analysis would show which methane bacteri is under-producing. (check??) it does not show which acid forming bacteria in under-active. acetic and propionic are the major ones. They're precursors of methane. Methane is produced by cleavage of acetic acid. C*H3COOH -> C*H4 + CO2 then CO2 is reduced: CO2 + 8H -> CH4 + 2 H2O . CO2 acts as the electron or hydrogen acceptor. It's in abundance in anaerobic decomposition, so it's never a limiting factor in creating CH4. H2 is created by enzymatic degradation of organic compounds. Complex waste follows a pathway to propionic and acetic acid (and others) than to methane (link to pic). Propionic and acetic acid are the main intermediates before methane. 72% of methanogenesis = from acetic acid cleavage. Most of the methane results from fermenting acetic acid, the prevalent VOC from fermentation of carbs, proteins and fats. Propionic is formed mainly during fermentation of carbs and fats. The acetic acid and propionic acid formers are thus the most important bacteria to methane formation. They're also the slowest growing and most sensitive to environmental conditions. Acetic acid can be collected and burned to CO2 and water (and heat). Advantages of anaerobic digestion: 90% of biodegradable waste can be stabilized in anaerobic treatments as opposed to 50% in aerobic treatments; little sludge is produced; no O2 or aeration is required (and the associated power expenditure). Biodigestion disadvantages: temperatures in the range of 85 - 95 F are preferred for operation; % solids are an issue: dilute wastes don't produce enough heat to heat the digester; slow seeding time for methanogens; for concentrated wastes, BOD < 10,000 mg / l. Typical retention time: 5 days at 95 F < detention time < 30 days. Minimal retention time = shorter than which bacteria will be removed faster than they can reproduce. The more dilute the waste the shorter the retention time should be. Waste stabilization is directly related to methane production. Buswell et al derived an equation to predict methane production (link) . The most important advantage of anaerobic waste treatment is high rate of stabilization and low rate of of conversion to biological cells. Different amounts of biosolids (suspended solids) are produced depending on the type of waste. Fatty acids produce the lowest growth, and carbohydrates the highest. Sludge build up is reduced as retention time increases, because cells consume themselves in time: so high retention time = more stabilization, more efficiency, and less biological production. Hala
Smith et al. (1996) an intermediate degree of mixing leads to optimal methane production. Hala
Brade and Noone (1981) Mechanical mixers were found to be the most efficient as to power consumption. Hala
Lee et al. (1995) Mechanical mixers were found to be the least efficient as to power consumption. Hala
Casey (1986). Biogas or liquor re-crculation the least equipment-intensive (Casey, 1986). Hala
Karim, K., Klasson, T., Hoffmann, R., Drescher, S.R., David W. DePaoli, D.W., Al-Dahhan M.H. 2005. Anaerobic digestion of animal waste: Effect of mixing. Bioresource Technology, 96, 1607–1612 Found that mixing by biogas recirculation, at different rates and the gap between the draft tube (through which biogas is recirculated) and the bottom of the biodigester did not affect methane production, but this could be due to how diluted the slurry was (50 g biosolids / liter). Hala
Keshtkar, A., Meyssami, B., Abolhamd, G., Ghaforian H., Khalagi Asadi, M. 2003. Mathematical modeling of non-ideal mixing continuous flow reactors for anaerobic digestion of cattle manure Bioresource Technology, 87, 113–124 An imperfect mixing model was developed based on the stoichiometry of anaerobic digestion of manure (Hill, 1982; Angelidaki et al., 1993), and on a 2 region mixing model. The kinetic model includes the hydrolysis of organic substrates by extracellular enzymes, formation of acids by acidogenic bacteria, formation of acetic acid from propionic acid, VFA, and other intermediates and the formation of methane from acetate by methanogenic bacteria. The 2 region mixing model assumed that in a digester there a flow-through region and a retention region, and various degrees of mixing change the proportion of each. Considering variable a as the volume fraction of flow-through region, and b the fraction of added feed that is transfered between retention region (dead zone) and flow through region. Keshtkar et al. (2003) compared the following a,b combinations in a simulation; 0.9, 10 = perfectly mixed reactor; 0.6, 0.5 = imperfectly mixed reactor; and 0.2, 0.2 = incompletely mixed reactor. The simulated imperfect mixing model outputs were the levels insoluble substrates, total actetate, total propionate and total ammonia concentrations, in both regions. Results showed that in the well mixed reactor the concentrations of the outputs were the same throughout the residence time, because the material was quickly transfered from the flow-through to retention region. In the poorly mixed reactor = non-homogeneous distribution of material, less volume for active digestion because of limited feed exchange between regions. The model simulation also showed that methane yield is very dependent on pH. This model was successfully tested against real life data. Hala
Banister, S.S. & Pretorius, W.A. 1998. Optimization of primary sludge acidogenic fermentation for biological nutrient removal. Water S.A., 24(1). The abscence of mixing improved the volatile fatty acid yield by 70%, in an acid-phase biodigester. Hala
Stroot, P.G., 1, McMahon, K.D., Mackie, R. I., Raskin, L. 2001. Anaerobic codigestion of municipal waste and biosolids under various mixing conditions. I. Digester performance. Water Resources, 35 (7), 1804-1816. Stroot et al. (2001) compared continuous versus minimal mixing at various biodigester loading rates and solids levels. Minimal mixing consisted in manually shaking the 1 l pyrex bottles (lab scale digesters) for 2 minutes every day: 1 min before wasting and 1 min after feeding. Minimal mixing led to good performance at all loading rates. Continuous mixing consisted in placing the bottles on a shaker table. It led to unstable performance. Unstable continuously mixed digesters were quickly stabilized after reducing mixing. This showed that continuous mixing destabilizes digesters. Hala