Nitrogen removal from manure-free piggery wastewater (MFPW) with high and low C/N ratio is a great challenge. A novel upflow microaerobic biofilm reactor (UMBR) was constructed to treat the MFPW in the present research. The reactor was operated at 27℃ and a hydraulic retention time of 8.0 h

in which the dissolved oxygen (DO) was less than 1.0 mg·L-1 controlled by refluxing aerated effluent with the reflux ratio decreased from 45:1 to 30:1 by stages. During the 79-day operation

the effect of wastewater characteristics and DO concentration on the UMBR' performance was investigated. The results showed that the internal DO decreased from 0.70 to 0.23 mg·L-1 following the decrease of reflux ratio from 45:1 to 30:1 by stages

and no negative impact on COD removal was found with a well NH4--N oxidation. But the anaerobic ammonium oxidation would be inhibited by a DO above 0.70 mg·L-1

resulting in a decrease in TN removal. Fed with raw MFPW characterized by a COD

NH4--N and TN of 271

336.7 and 337.4 mg·L-1 with the DO 0.40 mg·L-1 and the reflux ratio 35:1

the average pollutant removal load reached 0.60

0.94 and 0.91 kg·m-3·d-1

respectively. Though the COD/TN in the feed averaged 0.8

a removal of NH4--N and TN as high as 93.1% and 89.9% was obtained

respectively. Obviously

the filler allowed more activated sludge to grow as biofilm in the UMBR and could construct suitable microenvironments for chemoheterotrophic bacteria

ammonia oxidizing bacteria

autotrophic and heterotrophic denitrifiers

separately. The diversity of physiological groups of bacteria laid the foundation for the excellent pollutant removal in the microaerabic process.