Kentucky State University Cooperative Extension Connections
Aquaponics is a hybrid growing system that combines fish and plant production in a recirculating water system (hydroponics). Fish are fed a commercial fish feed, plants use nutrients dissolved in the water for growth, and solid fish waste is removed.
Despite being resource efficient, there is a need to dispose of the concentrated fish effluent (mineralization). These solids contain valuable nutrients, including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and other micronutrients that can improve plant growth and limit the need for supplemental nutrients.
After 10 to 14 days, the aeration is shut off, the sediments are allowed to settle, and clean water can be transported directly to the sump of an aquaponic system.
Under anaerobic conditions, bacteria decompose organic matter in tanks with little to no oxygen. While this method can be useful for recovery of specific nutrients, if not managed properly, anaerobic digesters can result in toxic levels of ammonia for both fish and plants.
KSU studies demonstrate that a 14-day aerobic mineralization cycle significantly enhances nutrient availability, with increases of 143% in phosphate, 47% in nitrate, and ≥20% in calcium and potassium concentrations.
Mineralization is a simple process that recovers these nutrients by using native beneficial bacteria to convert nutrients bound within the solids into plant-available forms.
Mineralization of fish effluent can be compared to composting in soil. In aquaponics, concentrated fish solids are discharged into an offline holding tank. Nutrients are bound in their organic form and are not readily available to plants. Microbes degrade these organic molecules, releasing nutrients in an inorganic form that plants can use.
Increases availability of essential and limiting nutrients, especially for fruiting crops (Figure 1).
Enhances overall water quality and system stability.
Reduces dependence on external nutrient supplements.
Moves aquaponics toward a zero-discharge system through nutrient recovery.
Mineralization can occur in two ways:1. aerobic (with oxygen) and 2. anaerobic (without oxygen or low oxygen). In both types, waste sludge is collected from the solids filter and moved to a separate mineralization tank. Under aerobic conditions, the sludge is aerated continuously, promoting the growth of aerobic bacteria essential to the mineralization process. Over time, the microbes digest the solids, greatly increasing levels of essential nutrients like N, P, Mg, and sulfur (S), as well as limiting nutrients like K and Ca.
After 10 to 14 days, the aeration is shut off, the sediments are allowed to settle, and clean water can be transported directly to the sump of an aquaponic system.