The effect and biological mechanism of COD/TN ratio on nitrogen removal in a novel upflow microaerobic sludge reactor treating manure-free piggery wastewater

• An upflow microaerobic sludge reactor treating low COD/TN ratio piggery wastewater.
• Effect and biological mechanism of influent COD/TN ratio being investigated.
• Nitrifiers, hetero- and auto- trophic denitrifiers all thrived in the reactor.
• The nitrifiers and denitrifiers restricted by the increased influent COD/TN ratio.
• An influent COD/TN ratio less than 0.70 ensured a TN removal over 80% in the reactor.

A novel upflow microaerobic sludge reactor (UMSR) was constructed to treat manure-free piggery wastewater with high NH4+-N concentration and low COD/TN ratio, and the effect and biological mechanism of COD/TN ratio on nitrogen removal were investigated at a constant hydraulic retention time of 8 h and 35 °C. The results showed that the UMSR could treat the wastewater with a better synchronous removal of COD, NH4+-N and TN. The microaerobic UMSR allowed nitrifiers, and heterotrophic and autotrophic denitrifiers to thrive in the flocs, revealing a multiple nitrogen removal mechanism in the reactor. Both the nitrifiers and denitrifiers would be restricted by an influent COD/TN ratio more than 0.82, resulting in a decrease of TN removal in the UMSR. To get a TN removal over 80% with a TN load removal above 0.86 kg/(m3·d) in the UMSR, the influent COD/TN ratio should be less than 0.70.