The role of nitrate and nitrite in a granular sludge process treating low-strength wastewater

Granular sludge has recently come under study as an efficient technology in wastewater treatment. Different microorganisms coexisting within the granules allow simultaneous nitrification, denitrification and phosphorus removal (SNDPR). However, the behaviour of the process varies when nitrogen and phosphorus removal are coupled. The purpose of this paper is to study the SNDPR process in granular sludge with low-strength wastewater, focusing on the effect of nitrification products such as nitrite and nitrate on phosphorus removal. Results showed that aerobic granules allowed anoxic phosphorus uptake using either nitrite or nitrate as an electron acceptor. Phosphorus uptake rates had similar values using either product of nitrification (13.64 mg P g −1 VSS h −1 in the presence of nitrate and 12.20 mg P g −1 VSS h −1 in the presence of nitrite) without any observed nitrite inhibition of phosphorus uptake. However, different P uptake-N removal ratios were found in simultaneous denitrification and phosphorus removal. Nitrate was the best option for phosphorus-rich wastewater treatment (3.04 mg P mg −1N), while nitrite was more beneficial with the SNDPR of nitrogen-rich influents (1.68 mg P mg −1N).