Morphological characteristics of microbial sludge performing enhanced biological phosphorus removal in a sequencing batch reactor fed with glucose as sole carbon source

Electron microscopic analysis was used to analyse the morphological characteristics of microbial sludge performing enhanced biological phosphorus removal (EBPR) in an anaerobic/aerobic sequencing batch reactor (SBR) fed with glucose as the sole carbon source. The amounts of phosphate release and uptake during the SBR cycle gradually increased with operation time and complete EBPR was achieved after about 90 days. Scanning electron microscopy (SEM) showed that the initial sludge inoculated into the SBR consisted of various microorganisms such as coccus-, rod- and bacillus-shaped bacteria, but after extended operation (more than 650 days) perpendicular cuboidal bacteria of eight coccus-shaped cells dominated the microbial sludge in the SBR reactor. The cell size of the cuboidal bacteria was about 0.7 μm in diameter. In the sludge, coccus- and rod-shaped bacteria also existed but at much lower frequency. Transmission electron microscopy (TEM) also revealed that the cuboidal bacteria dominated the sludge, but they did not contain polyphosphate granules or glycogen inclusions. The rod-shaped bacteria did not contain polyphosphate granules or glycogen inclusions either. Only coccus-shaped bacteria with a diameter of about 1.2 μm contained small black polyphosphate granules and a large white inclusion. Based on previously proposed metabolic pathways and electron microscopic results, it was inferred that the dominating cuboidal bacteria were lactic acid producing organisms (LPO) and the coccus-shaped bacteria were lactate-using phosphorus accumulating organisms (PAO).