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The Role of Microorganisms in a Full‐Scale Sequencing Batch Reactor Under Low Aeration and Different Cycle Times

This study describes the role of microorganisms in a full‐scale step‐feed sequencing batch reactor (SBR) system for urban wastewater treatment. Chemical profiles for three different cycle times were measured under low aeration conditions with a high carbon‐to‐nitrogen ratio. The applied organic load was above 1.0 g chemical oxygen demand (COD)/L·d. The removal efficiencies were higher than 81%, 93%, and 76% for soluble COD, N‐, and total Kjeldahl nitrogen, respectively. The ratio of volatile suspended solids (VSS) to total suspended solids was 78%, and the food‐to‐microorganism ratio was an average of 1.41 g COD/g VSS·d. The active biomass was comprised of 87.8% heterotrophic and 12.2% autotrophic organisms. Nitrifying organisms were found with a low amount of ammonia‐oxidizing bacteria (5%) and a much higher amount of nitrite‐oxidizing bacteria. Polyphosphate‐accumulating organisms (PAOs) were found at high amounts (25%) compared to glycogen‐accumulating organisms, even in a system with a high carbon to phosphorus ratio. The activity of denitrifying PAOs was 72%.