Effect of the gradual increase of salt on stability and microbial diversity of granular sludge and ammonia removal

Two sequential batch reactors were operated, aiming at forming aerobic granular sludge and studying the effects of the gradual increase of the NaCl concentration on the granule. structure and microbial diversity, and on the efficiency of ammonia removal. The reactors were fed with ammonia-enriched synthetic effluent and 5 g L−1 of NaCl per week were applied. A decrease in the size of the granules was observed until they were completely disintegrated as the salt concentration increased up to 10 g L−1. However, the ammonia removal efficiency remained high in all the salinities applied. By sequencing the 16S rRNA amplicon gene, the microbial community structure allowed the verification of the presence of several genera affiliated with the bacteria that perform both heterotrophic nitrification and aerobic denitrification, besides those involved in the conventional nitrification and denitrification and the ANAMMOX process. Salinity affected the microbial population related to the formation and stability of the granules. •With the increase of NaCl, the granules decreased in size until they disintegrated.•Ammonia removal remained efficient in all analyzed salinities.•There was no accumulation of nitrite or nitrate in the treated effluent.•The removal of nitrite and nitrate increased with the addition of NaCl.•Several genera of bacteria that perform the nitrification and denitrification.