Real-Time PCR Quantification of Ammonia-Oxidizing Bacteria (AOB): Solids Retention Time (SRT) Impacts during Activated Sludge Treatment of Industrial Wastewater

A bench-scale activated-sludge system, consisting of four treatment trains operating at solids retention times (SRTs) of 2, 5, 10, and 20 days, was used to study effects of SRT on ammonia-oxidizing bacteria (AOB) population structure dynamics during industrial wastewater processing under two different influent ammonium levels (low-N: 20.2 ± 5.9 mg/L NH₃-N, and high-N: 54.9 ± 10.0 mg/L NH₃-N). Based on sequence analysis of the amoA gene, three distinct AOB groups including Nitrosomonas nitrosa [AJ238495], uncultured AOB RI-27 [AF532311], and uncultured AOB B2–3 [AF293067] were identified in the activated sludge from this experimental system. Specific real-time PCR assays using TaqMan^® probes were used to quantify these three AOB groups. Results indicated that N. nitrosa (3.4 × 10⁹ cells/L) and RI-27 (5.3 × 10⁹ cells/L) were the dominant AOB in the activated sludge used to seed the reactors. The RI-27 group remained dominant in the 5-, 10-, and 20-day SRT trains (≍1.0–6.4 × 10¹⁰ cells/L) under both low-N and high-N conditions while the N. nitrosa group became a minor component of the AOB population. In contrast, the B2–3 group population in the 2-day SRT train significantly increased (5.8 ± 3.8 × 10⁹ cells/L) under high-N condition, and thus dominated the overall AOB population. These results suggest that the operating parameters, SRT, and ammonium loading, may produce selective pressures on the AOB population structure during wastewater treatment.