A hybrid system has been developed for biological nitrogen removal through nitrification-denitrification. The system includes an aerobic tank and an anoxic tank with an intermediate sludge settler connected to a membrane bioreactor (MBR) with a submerged 0.4 μm hollow-fiber membrane module. The laboratory system has a total working volume of 6.5 L treating a glucose-based synthetic wastewater. The experimental results demonstrate that the new process is highly effective for simultaneous organic and nitrogen removal. During the stationary operation, a sludge SS (suspended solids) concentration of 6 g/L or higher can be maintained in the reactors. The system has a COD (chemical oxygen demand) loading rate of up to 2,100 mg/L-d and a total nitrogen loading rate of up to 170 mg N/L-d. More than 95% COD can be degraded, and the total nitrogen removal efficiency can be 90% or higher as the nitrogen is reduced from 100 to around 7.5 mg/L. A high quality effluent is produced with a SS of less than 1 mg/L. With the MBR, organic degradation, nitrogen removal and sludge-liquid separation can be well achieved within a short HRT of about 10 hr.

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