Denitrification by Purple Phototrophic Bacteria: A Carbon-Neutral and Resource-Efficient Route for Nitrogen Removal

Purple phototrophic bacteria (PPB) have great potential in treating nitrogen-contaminated wastewater. Unlike conventional heterotrophic denitrification, PPB-driven denitrification utilizes light-driven metabolism, concurrently improving nitrogen removal and carbon management efficiency. This work aimed to develop a PPB denitrification process for nitrogen removal, carbon emission mitigation, and resource recovery. The PPB growth was first optimized and the most desirable light and carbon sources (i.e., incandescent light and sodium acetate) were pinpointed. PPB denitrification could reach a nitrate removal rate of 0.68 mg N/L/h, while no nitrite was detected during the process, regardless of the amount of external electron donors. This was attributed to the fact that the true reduction rate of nitrite (4.42 mg N/gVSS/h) was significantly higher than that of nitrate (1.51 mg N/gVSS/h). In the presence of a sufficient carbon source, PPB denitrification was found to be a low-carbon process, with only ~0.17% of converted nitrate being emitted as nitrous oxide. Meanwhile, PPB biomass for denitrification was rich in value-added products (e.g., protein and pigment), which potentially generated additional benefits over the biomass valued at USD 17 kg−1. These results provide a theoretical basis for implementing PPB denitrification for carbon-neutral and resource-efficient wastewater treatment.