Two stage high rate in series with a second digestion

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Digesters have fixed roofs or floating covers. Two Stage – High rate in series with a second digestion tank that can be an open tank, unheated, or a sludge lagoon. First tank for digestion. Second tank for storage & concentration of digested sludge and for solids-liquid separation (i.e. formation of a clear supernatant). Sometimes primary and WAS sludges are digested separately to retain the superior solids-liquid separation characteristics of digested primary sludge. Sometimes the primary sludge is digested anaerobically and the WAS aerobically. Most WWTPs digest both primary and WAS together to avoid additional capital costs. Anaerobic Digestor Design Four methods of design : 1. MCRT 2. Volumetric loading
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3. Observed volume reduction 4. Loading factors based on population HRT and MCRT are equal for complete mix digesters like the high rate processes (no solids- liquid separation so operates like a chemostat). Both are critical variables. Typical yields on A.S. sludge are about an order of magnitude less than A.S. yields from wastewater. 0.06 versus 0.6 lbVSS/lbBOD 5 . Nitrification falls in between at 0.2 lbVSS/lb ammonium-N. A typical volumetric loading is 0.3 lb ultimate BOD per cubic foot per day. Conventional A.S. loadings are 20 to 40 lbs BOD 5 /cubic foot per day (p. 550) which is roughly 29 to 59 lbs ultimate BOD/cubic foot per day or 0.029 to 0.059 lb ultimate BOD per cubic foot per day. About 1 order of magnitude less. Anaerobic processes in general can accept much higher volumetric loadings. In addition they have much lower yields. Thus for a given mass of organics to be treated anaerobic processes will require a significantly smaller reactor and will produce less biomass. There is a similarity between volumetric loading and F:M ratio to the parameter values used in digester design from loading factors. HRT is a significant variable and is considered in design methods. Standard rate digesters = 30 to 90 day HRT; 0.03 to 0.10 lb/cubic ft/day VSS loading High rate digesters = 10 to 20 day HRT; 0.10 to 0.30 lb/cubic ft/day VSS loading. Mixing difficult above 0.30 loading. Tracking of VSS production is important. Alkalinity and VSS content should be checked daily to ensure the stability of the digestion process. Typically an alkalinity of 1000 to 5000 mg/l and VFAs of less than 250 mg/l are desirable. Sufficient N&P for biological growth must also be present. When the alkalinity is depleted the pH will become lower due to the production of VFAs and decreasing methanogenesis. This is known as “going sour” and typically alkalinity must be added to recover the reactor. Typically when this happens methanogenesis has been lost and it takes some time for the reactor to recover since the methanogens are slow growers. Recall that neither the acid fermentation nor the methanogenesis will work efficiently in isolation since they are dependant on inter-species hydrogen transfer to remain thermodynamically favorable. Thus the trophic levels are interdependent and if methanogenesis is interrupted or inhibited, this will cause the accumulation of VFAs with the eventual shut down of the reactor.
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  • Spring '13
  • ........., Anaerobic digestion, VFAs, Anaerobic Wastewater Treatment, UASBs

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