Rapid domestication of autotrophic nitrifying granular sludge and its stability during long-term operation

The nitrifying granular sludge process is a prospective technology for the efficient treatment of rare earth mine wastewater in southern Jiangxi, China. However, the long formation time of nitrifying granular sludge greatly restricted its application. In the present study, nitrifying granules were domesticated in a pilot-scale sequencing batch reactor by using heterotrophic granular sludge as carriers and adding exogenous nitrifying bacteria concentrate. According to variations of granular properties and ammonia removal, autotrophic nitrifying granules were successfully domesticated within 38 days using the strategy. It was found that the process involved secondary nucleation and microbial community evolution of the seed heterotrophic granules, and Nitrosomonas replaced most heterotrophic bacteria and became the dominant species with the largest relative abundance. During the subsequent 168 days of operation, the domesticated autotrophic nitrifying granules were stable, and their structures were denser than those of the inoculated granules. The ammonia nitrogen removal rate of the reactor was greater than 90% for a long period of time, and persistent partial nitrification was once achieved. However, severe fluctuation of influent ammonia nitrogen during the 120th to 206th day significantly inhibited the activity of nitrifying bacteria, which gradually led to the decrease of the ammonia nitrogen removal rate.