Concurrent nitrification and denitrification (CND) are natural phenomena in the soil environment that can be applied in wastewater treatment for enhanced total nitrogen removal. However, significant renovation is necessary for existing plants to equip them for nutrient removal. At a domestic wastewater treatment plant, we performed a pilot test by installing bioplates with entrapped biomass in a conventional aeration basin for CND, and investigated the effects of bioplate packing ratio (PR), hydraulic retention time (HRT), dissolved oxygen (DO) level, on/off aeration mode, and supplemental carbon and alkalinity on nitrogen removal. With the pilot aeration basin of 1.3 m 3 loaded with mixed liquor suspended solids of 1,500-2,500 mg/L and bioplates at PR of 3.2% INTRODUCTIONEngineers employ bacteria to treat human and industrial wastes to preserve environmental quality and ecosystems. The goal was to demonstrate TN removal that could be implemented in municipal WWTPs without significant interruption and renovation of existing facilities. METHODS Pilot system setup and operationsThe bioplates were prepared by immobilizing activated sludge in cellulose triacetate as a carrier as originally described The amendment of carbon and alkalinity was evaluated for the bioplate-augmented aeration basin under varied PR of 3.2, 6.5, 9.1, and 11.4% while holding HRT constant at 6 h and DO at 4-6 mg/L via intermittent aeration of 1 h on/ 1 h off. Table 2 presents When increasing presence of bioplates to a PR of 9.1% without supplemental carbon or alkalinity, the residual TN remained at about 11 ± 0.3 mg/L and the residual TKN at about 3.9 ± 0.5 mg/L (reduced from an influent TN of 25 ± 1.9 mg/L). COD removal was 72 ± 3.0% and TN removal was 57 ± 4.5% with 84 ± 2.7% nitrification. When supplemented with carbon or with both carbon and alkalinity, the effluent TN was reduced to 4.7 ± 1.5 and 3.6 ± 1.9 mg/ L, respectively. At the same time, the effluent TKN was reduced to 2.9 ± 1.2 mg/L, indicating excellent nitrification performance. The results show that increasing the PR of bioplates has not led to proportionally increased TN removal.Further increasing bioplate PR to 11.4% did not result in TN removal beyond those with PR of 6.5 and 9.1%. Without supplemental carbon or alkalinity, the effluent TN was 8.5 ± 1.0 mg/L from an influent TN of 27 ± 0.1 mg/L. With supplemental carbon or with both supplemental carbon and alkalinity, the effluent TN lowered to 5.2 ± 2.3 mg/L.The results in Table 2 showed no enhanced TN removal with increased packing of bioplates. Although improved treatment using entrapped biomass had been studied since 1985, much work had focused on the performance and mechanisms of simultaneous nitrification and denitrification with little attention on the extent of immobilized biomass.Morita et al. (, ) developed the packed gel envelopes system using PVA-SbQ gel that occupied 25% of the reactor volume, and they achieved 95% TN removal with supplemental carbon. In the present case, extensive biopla...