Polyhydroxyalkanoate Production from Food Waste Coupled with Aerobic Nitrate Reduction

Polyhydroxyalkanoate (PHA) production from wastes by mixed microbial cultures has been developed into an integrated process to couple with nitrate removal due to its economic and environmental benefits. However, the anoxic conditions for denitrification result in low productivity of PHA production due to the limited energy supply. In this work, the fully aerobic feeding regime uncoupling carbon and nitrogen supply was applied to enhance the culture enrichment and productivity while being accompanied by nitrate removal. The enriched cultures showed a nitrate assimilation rate of 2.35 ± 1.10 mg of NO₃^––N per gram of active biomass (X_a) per hour; meanwhile, the specific growth rate (μ_Xa, 0.019 ± 0.002 h^–1) and biomass yield on PHA (Y_Xa/PHA, 0.68 ± 0.15 g X_a/g PHA) were higher than those of a classic anoxic process (μ_Xa, 0.011 ± 0.001 h^–1 and Y_Xa/PHA, 0.38 ± 0.09 g X_a/g PHA). Furthermore, ammonia and oxygen were confirmed to have significant impacts on regulating nitrate utilization by directing nitrate flow to assimilatory nitrate use, aerobic denitrification, or anoxic denitrification. Metagenome analysis revealed that the overall function was mostly undertaken by Thauera aminoaromatica, and a nitrate metabolic model driven by PHAs was proposed. In summary, these findings widened application scenarios for PHA production coupled with nitrate removal.