Sludge volume ratio vol of sludge blanket in

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Sludge volume ratio : vol of sludge blanket in thickener divided by vol of sludge removed daily. 0.5 to 20 days with low values in warm weather to prevent gasification Blanket depths 0.5 to 2.5 m (2 to 8 ft) with shallow depths in warm weather Example 14-4 p. 1493 Stabilization, Anaerobic Digestion Anaerobic digestion can potentially result in enough recoverable energy to meet plant power requirements. Three reactions: hydrolysis, fermentation (acidogenesis/acetogenesis), and methanogenesis There is a minimum SRT for each of the 3 reaction types Important design factors: SRT, HRT, temperature, alkalinity, pH, inhibitory substances, and bioavailable nutrients and trace metals (e.g. cobalt, nickel, etc…) Most anaerobic digesters are single-stage, high-rate, reactors, operated in the mesophilic range (30 to 38 deg C). A total alkalinity of 2000 to 5000 mg/L is typical in anaerobic digesters. Figure 14-20a p. 1508 shows a single stage high rate digester. Most plants mix primary and secondary sludge for digestion, which degrades solid-liquid separation and slightly reduces reaction rates as well. Not much data on separate digestion but potentially could be beneficial EBPR sludge results in P bleedout. The supernatant can reintroduce a lot of P into the system if it goes to the head of the plant. Struvite precipitation may prevent P (and ammonia) from remaining in the liquid phase but excessive precipitation can block very large pipes. SRT method for digester design is shown in Example 14-5 p. 1511 SRT is the same as the HRT SRT range from 10 to 20 days typical (at 35 deg C) Design for peak hydraulic load Lower limit of 10 day SRT at 35 deg C
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The upper limit of VSS loading rates is due to accumulation of ammonia or other toxic materials, or washout of the methanogens. Excessively low loading rates can create problems also, and are very frequent in full scale systems Typical digester shapes Figure 14-21 p. 1516 Energy conservation/recovery has played a major role in the dominance of anaerobic digestion among sludge stabilization processes. Three basic processes: 1. Standard rate 2. High rate 3. Two-stage External heat exchangers used to heat sludge… growth and digestion kinetics very temp dependant. Standard Rate – digestion, sludge thickening, supernatant formation in one reactor. Only 50% of volume is active w.r.t. digestion though. Scum layer can be a problem sometimes also. Volumetric loading not as great as high rate. Generally used at smaller installations. High Rate – Solids loading rate much greater than standard rate digestor. Sludge mixed intimately by a variety of means. Sludge is heated. Sludge pumped continuously or on timer to maintain constant conditions. About 45 to 60% solids reduction. Solids go off as carbon dioxide and methane, typically about 65 to 70% methane and 25 to 30% carbon dioxide. The specific gravity of the off gas is about 0.86 relative to air. Measurement of gas production is one of the best measures of the progress of digestion. Since there is no solids-liquid separation effluent solids about 45 to 60% less than feed stream. Digesters have fixed roofs or floating covers.
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  • Spring '13
  • ........., Anaerobic digestion, VFAs, Anaerobic Wastewater Treatment, UASBs

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