Nutrient Release Characteristics of Aquaculture Sludge through Anaerobic Acidification in an Aquaponics System

• • Low sludge loading (4.61 kg·kg−1) achieved a peak organic solid conversion of 68.2% at 70.5 h, whereas high loading (13.22 kg·kg−1) yielded only 26.9% at 27.5 h, indicating that lower loading prevents overloading and enhances hydrolysis efficiency. • • Ammonia nitrogen conversion reached 64.8% under low loading versus 62.2% under high loading, but orthophosphate conversion was significantly higher (95.7% vs. 76.4%), demonstrating that low loading facilitates near-complete phosphorus release for plant uptake. • • The optimal hydraulic retention time (HRT) for low-loading anaerobic acidification was 144 h, corresponding to an organic loading rate of 0.77 kg·(kg·d)−1, which balances nutrient release and reactor productivity for practical application. • • Low-loading conditions promoted the dominance of Acinetobacter (relative abundance 66.6%), a genus known for organic degradation and nutrient cycling, whereas high loading led to a more diverse but less efficient microbial community, underscoring the importance of operational control for microbial selection.