Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
Aquaponics, the integration of recirculating aquaculture and hydroponics, promises sustainable food production by recycling nutrients from fish waste. However, conventional systems primarily recover dissolved nutrients, while particulate sludge—rich in organic matter and phosphorus—is often discarded, leading to low overall nutrient recovery and potential environmental pollution. Anaerobic digestion has been explored to mineralize sludge, but its long retention times and sensitivity to process parameters (e.g., pH, temperature, C/N ratio) limit its practicality, especially given the low carbon-to-nitrogen ratio of aquaculture sludge, which hampers methanogenesis. Consequently, there is a pressing need for faster, more robust conversion pathways that can unlock the nutrient value of sludge without the operational burdens of full digestion.
This study pivots to anaerobic acidification, a stage preceding methanogenesis, which rapidly hydrolyzes and acidifies organic matter into volatile fatty acids (VFAs) and releases soluble nitrogen and phosphorus. By deliberately arresting the process before methanogenesis, the conversion time is drastically shortened, and the resulting VFAs can be further processed in the biofilter or used as a carbon source for denitrification. The authors systematically compared two sludge loading rates to elucidate the dynamics of organic matter, nitrogen, and phosphorus release, and to identify optimal operational parameters. Their findings demonstrate that a low loading rate not only achieves higher nutrient conversion efficiencies but also fosters a microbial community dominated by Acinetobacter, which is advantageous for nutrient cycling. This work provides a practical framework for enhancing nutrient recovery in aquaponics, addressing a critical bottleneck in closed-loop system performance.
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ZI Yongxia, HE Xinmeng, GAO Yueshu, LI Chunjie (2026). Nutrient Release Characteristics of Aquaculture Sludge through Anaerobic Acidification in an Aquaponics System. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202511027
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Frequently Asked Questions
What are the key operational parameters (loading rate, HRT, temperature) that maximize nutrient release, and how do they compare to conventional anaerobic digestion?
The study identified an optimal sludge loading rate of 4.61 kg·kg−1 (low loading) and a hydraulic retention time (HRT) of 144 h (6 days) for anaerobic acidification. Under these conditions, organic solid conversion reached 68.2% at 70.5 h, ammonia nitrogen conversion was 64.8%, and orthophosphate conversion was 95.7%. In contrast, conventional anaerobic digestion typically requires 20-30 days for complete digestion, but this process achieves high nutrient release in only 6 days, significantly shortening the conversion cycle. The process was conducted at ambient temperature (not specified in the text), but the short HRT suggests it can operate without external heating, reducing energy costs.
How does the high loading rate (13.22 kg·kg−1) affect process stability and effluent quality, and what are the implications for scaling up?
High loading caused multiple SCOD peaks (three) with an average peak conversion efficiency of only 26.9% at 27.5 h, indicating process instability and incomplete hydrolysis. The residual SCOD concentration was higher (average 1794.52 mg·L−1) compared to low loading (peak 1257.96 mg·L−1), and the VFA profile shifted to iso-butyric and iso-valeric acids, which are more inhibitory to downstream biological processes. For scaling, high loading would require larger downstream treatment capacity and may lead to system upset. Therefore, low loading is recommended for stable operation and efficient nutrient recovery.
What is the fate of volatile fatty acids (VFAs) produced during acidification, and how do they impact the aquaponics system?
VFAs are intermediate products that can be further degraded in the biofilter or used as a carbon source for denitrification. Under low loading, VFAs were predominantly formic, acetic, and iso-valeric acids, which are readily biodegradable and less likely to accumulate to toxic levels. The study indicates that at the optimal HRT of 144 h, the SCOD and ammonia concentrations do not adversely affect water quality, as the biofilter can convert ammonia to nitrate. Thus, the VFAs are either mineralized or assimilated, posing minimal risk to fish and plants.
How does the microbial community structure differ between high and low loading, and what is the role of Acinetobacter in nutrient release?
Low loading promoted the dominance of Acinetobacter, a genus within Proteobacteria, reaching a relative abundance of 66.6%. Acinetobacter is known for its ability to degrade organic matter and release nutrients, likely contributing to the higher conversion efficiencies observed. High loading, conversely, led to a more diverse microbial community, which may dilute the activity of key functional species and reduce overall efficiency. This suggests that operational conditions can be tuned to select for beneficial microbial populations, enhancing process performance.
What are the practical implications for nutrient recovery in a full-scale aquaponics system, particularly regarding phosphorus recovery and plant fertilization?
The study achieved an orthophosphate conversion of 95.7% under low loading, indicating that nearly all particulate phosphorus is released as soluble phosphate. This phosphate can be directly used as a nutrient source for plants in the hydroponic unit, reducing the need for external fertilizers. The ammonia nitrogen (64.8% conversion) can be nitrified to nitrate in the biofilter, providing a balanced nitrogen source. With an optimal HRT of 144 h, a full-scale system could incorporate a sludge acidification reactor to recover nutrients, thereby improving overall resource efficiency and moving towards zero-discharge operation.
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