Selective enrichment and granulation of ammonia oxidizers in a sequencing batch airlift reactor

Sequencing batch airlift reactor (SBAR) was developed to selectively enrich and granulate ammonia oxidizers from wastewater nitrification. Partial nitrification and reduced settling time were the selection pressures for the selective enrichment and granulation of ammonia oxidizers in the SBAR operation. Sludge flocs became granulated and the sludge volume index (SVI) decreased sharply from 130 to 40 in 50 days. Most of the ammonia was oxidized to nitrite and nitrate was hardly produced during the nitrification in the SBAR after 70 days. Ammonia oxidizers became the dominant nitrifying bacteria in the granulated sludge as characterized by fluorescence in situ hybridization (FISH) and confocal laser scanning microscope (CLSM). Nitrosomonas spp. (Nsm156) was the most abundant ammonia oxidizer and the representative nitrite oxidizers, Nitrobacter (Nit3) and Nitrospira (Ntspa662), were rarely detected in the granules after 100 days of the SBAR operation. It seems that the inhibition by free ammonia and nitrous acid selectively suppressed the growth of nitrite oxidizers and washed them out of the SBAR. The SBAR successfully enriched ammonia oxidizers as the dominant nitrifying bacteria in granules and also achieved stable partial nitrification to accumulate nitrite.