Structure of a methanogenic granular sludge from an industrial lagoon digester determined by different microscopies

Abstract

This is the first report describing the presence of granular sludge in an anaerobic digester lagoon (∼10⁴ m³, 200 m³/h continuous input flow since 14 years ago). The lagoon corresponds to a chicken slaughterhouse. The granules are similar to the ones generated in Upflow Anaerobic Sludge Blanket (UASB) or equivalent systems. Granules are elongated and flattened bodies (∼0.5–13 mm maximal length). The outer surface contains pores and, as well as the interior, it has a complex array of microorganisms, both single cells or aggregates and numerous filaments. Sections show concentric growth layers. In the center, there are structures of biological or inorganic nature, as woody tissue or crystalline components, including framboidal pyrite, which suggests an initial nucleating role. Energy Dispersive X-ray Spectroscopy shows that crystals contain C, O, Fe, S and/or Si. The vitality in ∼1 mm granules is high, whereas in larger ones decreases towards the center. Thus, most of methanogenesis would possibly take place close to the granule periphery where cavities highly coated with microbes are detected. The filamentous microorganisms and the presence of an extracellular polymeric substance rich in polysaccharides would contribute to the remarkable granule structural stability. The granules also have a strong functional stability.