From Pollution to Resource: Algal–Bacterial Symbiotic Systems for Swine Wastewater Treatment and Resource Recovery—A Review

Swine wastewater is a high-strength agricultural effluent characterized by high organic loading, elevated ammonium nitrogen and phosphorus concentrations, and frequently low C/N ratios, which make simultaneous pollutant removal and resource recovery challenging. Conventional physicochemical, anaerobic, and aerobic treatment technologies are widely used, but they are often constrained by high energy demand, ammonia inhibition, insufficient nitrogen recovery under low C/N conditions, and limited resource valorization. This review comparatively evaluates these conventional technologies alongside microalgal and algal–bacterial symbiotic (ABS) systems for swine wastewater treatment and resource recovery. Particular attention is given to algal–bacterial interactions, oxygen and carbon exchange, nitrogen and phosphorus removal pathways, reactor configurations, key operational parameters, and biomass valorization routes. The reviewed evidence shows that conventional anaerobic–aerobic systems generally achieve stable COD removal (>80%) but often provide limited nitrogen recovery, whereas microalgal systems can remove 80–90% of nitrogen and phosphorus but remain restricted by ammonia toxicity, light attenuation, and biomass harvesting costs. Under optimized conditions, ABS granular systems have achieved >90% COD removal, >80% total nitrogen removal, and 70–95% total phosphorus removal, while also improving biomass settleability and process stability. Overall, ABS systems offer a promising route to shift swine wastewater treatment from discharge-oriented pollution control toward resource-oriented management. Future research should prioritize reactor scale-up, long-term operational stability, biological monitoring, and economically viable biomass valorization.