Nutrient Release Characteristics of Aquaculture Sludge through Anaerobic Acidification in an Aquaponics System
Aquaponics systems integrate aquaculture and hydroponics to recycle resources, yet nutrient recovery from aquaculture sludge remains inefficient. This study investigated anaerobic acidification as an alternative to conventional anaerobic digestion, which suffers from long conversion cycles. Batch experiments compared two sludge loading rates: high (13.22 kg·kg−1) and low (4.61 kg·kg−1) (mass of sludge per mass of anaerobic inoculum). Under low loading, soluble chemical oxygen demand (SCOD) exhibited a single peak, reaching a maximum organic solid conversion of 68.2% at 70.5 h. In contrast, high loading produced three SCOD peaks with an average peak conversion efficiency of only 26.9% at 27.5 h and higher residual concentrations. Ammonia nitrogen conversion was slightly higher under low loading (64.8%) than high loading (62.2%), while orthophosphate conversion was markedly superior (95.7% vs. 76.4%). The optimal hydraulic retention time for low-loading operation was 144 h, corresponding to an organic loading rate of 0.77 kg·(kg·d)−1. Under these conditions, the produced ammonia and phosphate can be effectively recovered without adversely affecting water quality, as the biofilter converts ammonia to nitrate for plant uptake. Microbial analysis revealed that low-loading conditions favored the dominance of Acinetobacter (relative abundance 66.6%), which likely enhances organic degradation and nutrient release. These findings demonstrate that anaerobic acidification under low loading is a promising strategy for efficient nutrient recovery in aquaponics, offering a shorter conversion time and higher nutrient yields than traditional methods.