Spontaneous Mainstream Anammox In A Full-Scale Wastewater Treatment Plant With Hybrid Sludge Retention Time In A Temperate Zone Of China

Spontaneous anammox bacteria enrichment at mainstream conditions was reported in a full-scale Wastewater Treatment Plant (WWTP) in a temperate zone of China. The mainstream anammox was observed after WWTP process retrofit, which constructed a hybrid sludge retention time (SRT) system by providing moving carriers in the anaerobic/anoxic tank and was initially designed to enhance the denitrification process in a conventional anaerobic/anoxic/oxic process. The hybrid SRT system achieved 86.0 +/- 4.6% total nitrogen (TN) removal via combined mainstream anammox and conventional denitrification. Autotrophic denitrification via mainstream anammox was confirmed by various shreds of evidence including high-throughput sequencing, specific anammox activity test, and N-15 isotopic tracing. Long-term anammox bacteria existence in the biofilm of the carrier in anoxic zones was detected in a much higher relative abundance compared with other spots. The contribution of anammox activity to TN removal was estimated at around 20%-30%. The reasons leading to spontaneous anammox enrichment were mainly attributed to the carriers for slow-growing bacteria growth and dissolved oxygen gradient in the anoxic tank (caused by intermittent aeration) for nitrite production. The insights of this full-scale case study provide important perspectives for future mainstream anammox application, and also the design of an energy-neutral WWTP process.Practitioner pointsSpontaneous mainstream anammox in a full-scale WWTP after its retrofit in a temperate zone of China was reported.Anammox bacteria enrichment and long-term stability on moving carriers at mainstream conditions was achieved by modified hybrid SRT system.The hybrid SRT system achieve stable nitrogen removal even in cold winter and high BOD/N situation by combining mainstream anammox with conventional denitrification.Long term full-scale operation demonstrated excellent nitrogen removal with about 20%-30% contribution of mainstream anammox.This full-scale case study provided perspectives for future optimizing mainstream anammox application, and also energy-neutral WWTP process design.