Nitrogen removal from low COD/TN ratio manure-free piggery wastewater within an upflow microaerobic sludge reactor

•An innovative upflow microaerobic sludge reactor was constructed.•Removing NH4+ and TN from manure-free piggery wastewater with COD/TN ratio 0.84.•The average TN load removal reached 0.94 kg/(m3 d) at NLR of 1.10 kg/(m3 d).•Anammox was the main pathway for NH4+-N and TN removal in the reactor.

An upflow microaerobic sludge reactor (UMSR) was constructed in treating manure-free piggery wastewater with high ammonium concentration and a COD/TN ratio as low as 0.84. The UMSR offered an outstanding removal of NH4+-N and TN at 35 °C and hydraulic retention time 8 h subsequent to inoculated sludge acclimation. A short NO2−-N accumulation phase was observed whenever there was a considerable increase in TN loading rate (NLR), but decreased rapidly along with an evident increase in TN removal. Fed with raw wastewater at a NLR of 1.10 kg/(m3 d), the average COD, NH4+-N and TN removal reached 0.72, 0.76 and 0.94 kg/(m3 d), respectively. Inference drawn from stoichiometry based on the potential nitrogen removal pathways and the C/N ratio required by denitrification indicated that anammox was the main mechanism for NH4+-N and TN removal in the UMSR.