Ammonia removal and microbial characteristics of partial nitrification in biofilm and activated sludge treating low strength sewage at low temperature

•PN performance under low temperature were investigated in an SBR and SBBR.•Activated sludge system showed higher PN efficiency but worse stability.•AOB presented a higher biodiversity but lower proportion in biofilm system.•Biodiversity of Nitrobacter was lower in biofilm while Nitrospira was higher.•Feasible strategies for the operation of different systems were proposed.

In this study, the ammonia removal and stability of partial nitrification (PN) were investigated in an activated sludge system (R1) and a biofilm system (R2). The two systems were operated with identical setup and seeded with same sludge, to treat the sewage with ammonia of 60–65 mg L−1 at 14–16 °C. R1 performed a higher ammonia removal ability but a lower stability, while R2 showed a contrary result. In R1, both ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) showed higher bioactivity, AOB and Nitrospira presented lower biodiversity while Nitrobacter showed higher result than those of R2. FISH results indicated the lower proportion of AOB in R2, and relative low proportion of NOB in both the two systems. Activated sludge system should be operated as half PN or treat high ammonia sewage under low DO, while biofilm system could run as totally PN or treat low ammonia sewage with high DO.