The RG7204 concentration was measured in suspended phase, to make sure whether the biomass attached to carriers or not. The concentrations of MLSS and MLVSS (Fig. 3) decreased sharply and maintained between 200 and 300 mg/L, respectively till stage III. This revealed that most of the biomass was attached to the carriers and only small fraction of biomass grew in the suspended phase. The attached growth system (fixed biofilm) prevents the biomass wash out and offered a suitable environment for microorganisms involved in the SNAD process. Between 40 and 80 d, unlike conventional SBR cycle, there were no settling phase in the reactor and even in this condition the effluent SS and VSS were still less than 10 mg/L (Fig. 3). Thus introduction of carriers into the reactor to support microbial growth was highly efficient in sludge retention. The concentrations of MLSS and MLVSS increased in stage IV (Fig. 3), which suggest that carriers were saturated with attached biomass and fresh biomass grown continuously in the suspended phase. The total amount of biomass attached to the carriers (measured in stage IV) was about 7080 mg (177 mg/carrier), which represents about 73% of the suspended biomass. This suggests that the major portion of the biomass was attached to the carrier.