Sludge granulation in ASBR: Reactor performance, sludge physiochemical properties evolution and the unique clustered structure of the granular sludge

Abstract

Anaerobic granular sludge was successfully cultivated in the anaerobic sequencing batch reactor (ASBR) after 45 days of operation. The results showed that the mature granular sludge exhibited better physicochemical characteristics: granule percentage of 62.6 ± 8.8 %, 0.8–1.2 mm in diameter, with smooth edges and regular shapes, and sludge volume index of 20.0 mL/g SS. 16S rRNA-targeted fluorescence in situ hybridisation (FISH) and scanning electron microscope (SEM) examination revealed that the granular sludge had a unique clustered microbial community structure, which would to be an optimal microbial arrangement for syntrophic colonies. The large substrate concentration gradient that derived from the feast-famine alternation regime of the substrate in a running cycle of ASBR was the major cause of the clustered microbial distribution structure. Lots of syntrophic consortia formed by acetogens and methanogens randomly distributed throughout the interior of the granules, furthermore, the cavities existing amongst the syntrophic consortia were convenient for substrate transfer and biogas liberation, thus enhancing the digestion performance of ASBR and preventing granule disintegration caused by gas scour. With sludge granulation, the methane yield of ASBR increased from the initial 0.23 L/g COD to 0.28 L/g COD, meanwhile, the maximum specific methanogenic activities (SMAs) of acetoclastic and hydrogenotrophic methanogens respectively increased from 0.51 gCOD/(gVSS·d) to 0.73 gCOD/(gVSS·d) and from 0.35 gCOD/(gVSS·d) to 0.62 gCOD/(gVSS·d). The exploration of granular sludge formation, sludge characteristic evolution and treatment performance in ASBR reactor has theoretical value for its engineering promotion.