12

off in 48-72 hours with aeration. Chloramines require chemical dissipation (ex. sodium thiosulfate) or charcoal filtration. Given the small volume of water exchange, chloramines typically do not negatively impact an aquaponic system. Typically, you can replace around 10% of the system water volume without treating or testing for chlorine/chloramines.

Surface water: Surface water includes ponds, lakes, rivers, and streams. Surface water can introduce pathogens, algae, snails, and other organisms. In addition, many surface waters are contaminated with pollutants or agricultural run-off that pose a food safety threat to the organisms in the system and to consumers.

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-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.

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 (CH₄) 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).