Deactivation mechanism of calcified anaerobic granule: Space occupation and pore blockage

Calcification and deactivation of high rate sludge bed reactors is a common and serious engineering problem in the application of anaerobic bioreactor. In this study, the characteristics and deactivation mechanism of calcified anaerobic granules were investigated. The results showed that the calcium content of calcified anaerobic granules was ten times higher than that of control anaerobic granules. A large part of the calcium accumulated in the center of anaerobic granules in the form of calcite, and a small part of the calcium distributed in the outer layer of anaerobic granules in the form of Ca-P deposit. The calcium core occupied a large space which was available for the functional microorganisms. The calcium salts deposited in the outer layer of granular sludge which led to the significant reduction of macropore volume. The porosity of calcified anaerobic granules decreased by 13% compared with that of control anaerobic granules, causing generally the decline of methanogenic activity (for example, by 13% at influent organic concentration of 6.6 g COD L−1). The substrate gradient created by methanation of organic salts, including organic calcium salts, was deduced to be the driving force of anaerobic granule calcification, while the gradual accumulation of calcium salts in anaerobic granules was deduced to be the dominant factor for the decline of anaerobic granule activity. [Display omitted] •The methanogenic activity of calcified granules was 13% less than that of control.•Calcification of anaerobic granules led to a tenfold increase of calcium content.•A large part of calcium accumulated in the center of calcified granules as calcite.•A small part of calcium covered the surface of calcified granules as Ca-P deposit.•Space occupation and pore blockage proved to be responsible for the inactivation.