Mainstream nitrogen removal by partial nitritation/anammox (PN/A) offers energy and cost savings for sewage treatment. This pathway relies on two groups of microbes: aerobic and anoxic ammonium-oxidizing bacteria. Next to these essential bacteria, undesired groups of bacteria like nitrite-oxidizing bacteria (NOB) could proliferate in the system and reduce the overall efficiency. Selective suppression of NOB activity therefore remains a key bottleneck to robustly implement this technology and should further be researched.
In this study, multi-stressor sludge treatments were used and optimized to steer the microbial community towards the essential groups. These treatments consisted of sulphide-spiked starvation (2days) followed by a free ammonia shock (30mg NH3-N/L/d, 1hour). The effectiveness of different treatments was tested in two identical sequencing batch reactors at 19.5±0.5°C. Each treatment was tested in one experimental run, consisting of three repeated treatment phases in which all sludge was treated at once with 6-8 days of recovery in between. The inoculum was refreshed after each run.
All tested multi-stressor treatment resulted in selective NOB suppression, indicated by an elevated maximum nitrite accumulation ratio (NAR, nitrite produced/nitrite+nitrate produced). The standard treatment (150mg S2-/L/d at pH=7.4) achieved a maximum NAR of 33%. Reducing the starvation pH to 6.0 further increased the maximum NAR to 49% while doubling the sulphide spike at neutral pH reached 70%. The NOB suppression was however temporal as the NAR decreased within 4-9days. By applying a reduced aerobic, flocculent sludge retention time control (25±9 to 3±1days) in combination with the standard treatment, a maximum NAR of 72% was achieved which could be maintained until the run was completed (≥13days). This study demonstrated the potential of these improved multi-stressor treatments for selective NOB suppression and could be applied in the return-sludge line of a sewage treatment plant.
