Assessing the efficiency and potential for internally reusing nitrogen-containing effluent in the PHA accumulation stage under low C/N conditions in a mixed-culture process.

Polyhydroxyalkanoates (PHAs) are biodegradable polyesters poised to replace plastics. Mixed culture (MC)-based three-stage processes are effective for carbon recovery from waste biomass, but the energy-intensive PHA synthesis is negatively affected by ammonia nitrogen, inhibiting PHA yield. This study aims to reuse ammonia nitrogen efficiently to mitigate its impact and prevent secondary pollution. PHA production assays under varying MC types, substrate types, feeding modes, and oxygen levels showed that the butyrate type substrate-enriched, high-load, low-oxygen mode (R_{BC(4)P(1)O(+)}) achieved a PHA conversion ratio of 0.45 g COD/g COD, 1.8 times higher than R_{BC(2)P(5)O(++)}, with reduced energy consumption and CO₂ emissions. Ammonia uptake was 0.06 g NH₃-N/g PHA at a productivity of 4.54 g/L, showing improved nitrogen recycling. Direct recycling of ammonia nitrogen-containing effluent in the PHA-producing MC enrichment system was performed, and no significant decrease was observed in either the physical properties of the MC flocs or the metrics related to PHA synthesis capacity. These results highlight the feasibility of ammonia reuse and indicate that the soluble microbial products in the effluent have minimal impact on MC enrichment.