Simultaneous nitritation, anammox, and denitrification (SNAD) is an emerging alternative to the conventional nitrification-denitrification process for complete removal of ammonia. Innovative designs are needed to develop SNAD in constructed wetlands that are designed as ecologically engineered treatment systems. This study investigated the enhancement of SNAD by varying unsaturated layer depth, vegetation, and pH adjustment with furnace slag in recirculating vertical flow wetlands. Eight wetlands were set up in four configurations, i.e., two vegetated wetlands having a 5-cm unsaturated layer (W-5cm,W- plants), two non-vegetated wetlands having a 5-cm unsaturated layer and a furnace slag bed in a recirculation line (W-5cm,W- slag), two vegetated wetlands having a 20-cm unsaturated layer (W-20cm,W- plants), and two non-vegetated wetlands having a 20-cm unsaturated layer and a furnace slag bed (W-20cm,W- slag). Dissolved oxygen concentration was increased more by recirculation through a 20-cm than a 5-cm unsaturated layer. Recirculation through furnace slag maintained pH levels favorable for SNAD, but could result in ammonia inhibition. Active plant growth promoted denitrification due to supply of organic substrate and suppressed anammox in the vegetated wetlands with a 5-cm unsaturated layer. Anammox and denitrification played complementary roles for efficient ammonia removal via SNAD, depending on the naturally varying conditions. W-20cm,W-plants and W-20cm,W-slag with balanced anammox and denitrification removed nitrogen as much as W-5cm,W-plants with denitrification-dominated SNAD. The significantly lower nitrogen removal rates of W-5cm,W- slag were attributed to inhibition of free ammonia (7.1-21.3 mg N/L) to anammox and nitritation at pH 7.7-8.0. (C) 2017 Elsevier B.V. All rights reserved.