Influence of substrate concentration on microbial selection and granulation during start-up of upflow anaerobic sludge blanket reactors
Abstract:Microbial granulation was examined in four lab-scale upflow anaerobic sludge blanket (UASB) reactors at 35°C. The reactors had working volumes of 3.2, 5.0, 7.6, and 9.4 L. A synthetic wastewater of glucose, peptone, and meat extract was used as substrate and anaerobic digested sludge as seed. The reactors were fed 1 000, 2 000, 5 000, and 10 000 mg COD/L. The start-up was followed by maintaining a high microbial load index (the ratio of sludge loading rate applied to specific methanogenic activity measured) of ∼0.8 in each reactor. Microbial granulation was initiated within 1 month and matured within 4 mo despite different influent substrate concentrations. The granulation process was characterized by acclimation, granulation, and maturation. However, the growth rates and the granules cultivated differed significantly. Large Methanothrix-like species (thrix granules) were cultivated with 1 000 to 5 000 mg COD/L influents, and small Methanosarcina like species (sarcina granules) were cultivated with 10 000 mg COD/L influent. The thrix granules with median diameters of 2.5–3.4 mm exhibited better settleability, higher substrate affinity, and slightly higher bioactivity than the 0.54-mm sarcina granules. With thrix granules, the reactors operated efficiently and stably at a volumetric loading rate (VLR) of 30–40 g COD/L · d and/or a hydraulic retention time (HRT) of 1–2 h. However, the sarcina granules proved too small to be retained effectively in the reaction zone. At a moderate VLR of 10–20 g COD/L · d, the sludge granules expanded into the settling zone and accumulated there; at 25 g COD/L · d, the sludge granules were lost excessively with effluent, and the reactor was unstable. Influent with concentrations of 1 000–5 000 mg COD/L is desirable for cultivation of Methanothrix-dominated, well-settling, large granules.
Document Type: Research Article
Publication date: 1996-11-01
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