Comparative Study on Ammonia-Nitrogen Removal from Aquaculture Effluent by Fixed-Bed Bioreactors

With the rapid development of intensive aquaculture, the demand for efficient and cost-effective ammonia–nitrogen (NH4+–N) removal technologies is increasing. In this study, two fixed-bed biofilm reactors were operated for 154 days to treat low-ammonia-load aquaculture effluent under red-light and dark conditions. The NH4+–N removal performance, biochemical characteristics of biofilms, and microbial community differences between algal–bacterial and bacterial biofilms were compared. The results showed that, throughout the operation, the red light group achieved a minimum effluent NH4+–N concentration of 0.15 mg/L (97.78% removal), while the dark group reached 0.37 mg/L (93.94% removal). The red light group also exhibited greater stability in NH4+–N and nitrite (NO2−–N) concentrations. During the stable operation period, the NO2−–N concentration was 0.015±0.012 mg/L in the red-light group, compared with 0.018±0.024 mg/L in the dark group. Red light promoted the enrichment of algae such as Gomphonema and Ancylothrix_8PC and enhanced biofilm stability and stress resistance by regulating extracellular polymeric substance composition and increasing antioxidant enzyme activity. High-throughput sequencing revealed that the red-light reactor was dominated by Cyanobacteriota (>75%), indicating strong phototrophic selection that may support nitrification via an improved algal–bacterial microenvironment, whereas the dark reactor was dominated by Pseudomonadota (49.16%) with notable Chloroflexota (12.97%) and Nitrospirota (5.17%). The typical nitrite-oxidizing bacteria Nitrospira was detected in both systems but was more abundant in the dark reactor (5.17% versus 1.02%), suggesting different nitrification-supporting pathways under light versus dark conditions. For practical applications, intermittent light and moderate aeration are recommended to ensure long-term, stable operation with lower energy consumption. These findings provide insights into optimizing NH4+–N removal in aquaculture effluent.