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Recovery and digestion of fish effluent is more important in aquaponics than waste disposal. A large portion of feed is excreted as solid waste. Nutrients essential for plant growth are trapped within this concentrated slurry and should be recovered to reduce production costs and limit the need for nutrient supplementation. Recovery of these nutrients moves aquaponic production towards a zero-discharge system. Nutrients can be recovered through aerobic or anaerobic digestion of solids. Direct application of nutrients to crop land or composting sludge may be appropriate.
Mineralization: Approximately 20% of the N and 50% of the P from the feed is utilized by the fish for their growth (Timmons et al. 2018). The remainder of the N and P (70% and 30%, respectively) is excreted as a waste product by the gills and as particulate waste (10% and 20% for N and P, respectively). Particulate waste also contains macro- and micro- nutrients not absorbed by the fish. Recovery of these nutrients can improve plant growth and limit the need for supplemental nutrients.
Mineralization of fish effluent functions similarly to the processes that occur in soil. In AP, concentrated fish effluent is discharged into an offline holding tank. Microbes aerobically (or anaerobically) degrade organic solid materials, releasing soluble inorganic nutrients into the water, which are then available for plants to use (Delaide et al. 2018, Goddek et al. 2018). Only in an inorganic form are nutrients available to plants. Under aerobic conditions, heavy aeration is applied to concentrated solids (Figure 10). After 8-10 days, aeration is turned off, solids are allowed to settle, and clarified water is released into the system (Pattillo 2017). Under anaerobic conditions, bacteria decompose organic matter in environments with little to no oxygen. Anaerobic digestion produces methane gas (CH~4~) that can be utilized as biofuel (Dana 2010) and concentrated digestant that can be applied to greenhouse crops (Pickens 2015) or used for seedling production (Danaher et al. 2009, Pantanella et al. 2011). Anaerobic digestion of fish solids is more complex to manage than aerobic digestion and may be cost prohibitive due to the large digester volume needed (Chen et al. 1997).
Limited information exists on microbial contribution or environmental processes that underlay effective aerobic mineralization of fish effluent; however, studies suggest that nutrient recovery from fish solids can be significant (Cerozi and Fitzsimmons 2017, Cerozi and Fitzsimmons 2016, Goddek et al. 2018, Rakocy et al. 2016, Tyson et al. 2011, Yogev et al. 2016, Khiari et al. 2019, Graber and Junge 2009). Preliminary results from on-site AP research systems at Kentucky State University (KSU) show that aerobic mineralization of fish effluent for 14 days resulted in a 143% increase (7.61 to 18.5 mg/L) in phosphate (PO~4~), a 47% increase in nitrate (NO~3~-N; 28.5 to 41.7 mg/L), and ≥ 20% increase in Ca (57.97 to 74.23 mg/L) and K (27.38 to 32.7 mg/L) compared to system water (unpublished). However, even if nutrients are recovered from effluent and provided in the right form and quantity, interactions with other nutrients and water chemistry can sometimes make them unavailable to plants (Bryson and Mills 2014).
Direct application: Waste can also be applied directly as a soil amendment, composted through traditional heat-treatment methods, or via vermicompost (worm composting). Direct application should be used as a low-grade fertilizer or if the slurry is less than one percent solids. Heat-based composting of dewatered fish solids requires additional expertise and labor cost but can add an important additional income stream. Vermicomposting uses similar methods to traditional composting but does not rely on heat to process waste. Worms consume organic matter, fragment and aerate the solid material, and can potentially provide a supplemental live feed for fish (Yeo and Binkowski 2010). Compost can include vegetable waste or other compostable materials from production. It is not uncommon for mineralized effluent to be bottled and sold directly to home gardeners or small greenhouse operations; however, some restrictions may apply depending on your local regulations.
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.