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.