Performance study of the reduction of excess sludge and simultaneous removal of organic carbon and nitrogen by a combination of fluidized- and fixed-bed bioreactors with different structured macroporous carriers

Aiming for the reduction of excess sludge and the simultaneous removal of organic carbon and nitrogen, a new two-stage bioreactor system constructed by combining three-phase fluidized-bed (TFB) and sludge-reduction fixed-bed bioreactor (SFB) processes with different structured porous carriers was proposed. In this system, wastewater was first treated in the TFB bioreactor, in which the organic and nitrogenous compounds were simultaneously removed, and then the excess sludge produced in the TFB and residual organic substances flowed into the SFB for further degradation of suspended solids (SS) and nutrient removal. During a 470-day continuous operation, the average COD removal ratio reached about 95% at the influent COD of 500–600 mg/L. The removal ratio of T-N could reach up to 28–55% and was carried out through both microbial growth and simultaneous nitrification and denitrification (SND). The SS from TFB could be reduced in the following fixed-bed from 160 mg/L to 28 mg/L with two SFBs without the settling tank. Therefore, the simultaneous removal of organic carbon and nitrogen with an on-site reduction of excess sludge could be achieved in this two-stage bioreactor.