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The partial nitritation-anammox (PN/A) bioprocess, a proven shortcut nitrogen removal technology in nitrogen-concentrated sidestream wastewater flows, has gained increased attention for the potential energy savings, organic carbon redirection, and reduced sludge production if applied to mainstream wastewater. Many challenges remain, including anammox biomass and activity retention, out-competition of nitrite oxidizing bacteria (NOB), competition from heterotrophic organisms, and sensitivity to aeration control. To address these challenges, integrated fixed-film activated sludge (IFAS) and suspended growth (SG) reactors have been operated in parallel for over 11 months under unusually dilute mainstream conditions (approximately 20 mg N/L total nitrogen in primary effluent) at the Terrence J. O’Brien (O’Brien) Water Reclamation Plant in Skokie, IL. The IFAS reactor biofilms achieved efficient anammox …