Water Movement
Water movement is fundamental for keeping all organisms alive in aquaponics. The flowing water moves from the fish tanks, through the mechanical separator and the biofilter and finally to the plants in their media beds, pipes or canals, removing the dissolved nutrients. If water movement stops, the most immediate effect will be a reduction in DO and accumulation of wastes in the fish tank; without the mechanical filter and biofilter fish can suffer and die within a few hours. Without water flow, the water in media beds or DWC units will stagnate and become anoxic, and NFT systems will dry out. A commonly cited guideline for densely-stocked aquaponic systems is to cycle the water two times per hour. For example, if an aquaponic unit has a total water volume of 1 000 liters, the water flow rate should be 2 000 litres/h, so that every hour the water is cycled two times. However, at low stocking densities this turnover rate is unnecessary, and the water only needs to be cycled one time per hour. There are three commonly used methods of moving water through a system: submersible impeller pumps, airlifts and human power.
Submersible impeller water pump
Most commonly, an impeller-type submersible water pump is used as the heart of an aquaponics unit.
External pumps could be used, but they require further plumbing and are more appropriate for larger designs. High-quality water pumps should preferably be used in order to guarantee a long life span and energy efficiency. Top-quality pumps will maintain their pumping capacity and efficiency for least 1–2 years, with an overall life span of 3–5 years, whereas inferior products will lose their pumping power in a shorter time leading to significantly reduced water flows. Regarding flow rate, the small-scale units described in this publication need a flow rate of 2 000 litres/h at a head height of 1.5 meters; a submersible pump of this capacity would consume 25–50 W/h. A helpful approximation to calculate energy efficiency for submersible pumps is that a pump can move 40 litres of water per hour for every watt per hour consumed, although some models claim twice this efficiency. When designing the plumbing for the pump, it is important to realize that pumping power is reduced at every pipe fitting; up to 5 percent of the total flow rate can be lost at each pipe connection when water is forced through. Thus, use the minimal number of connections between the pump and the fish tanks. It is also important to note that the smaller the diameter of the pipes, the larger the water flow loss. A 30 mm pipe has twice the flow of a 20 mm pipe even if served from pumps with same capacity. In addition, a larger pipe does not require any maintenance to remove the buildup of solids accumulating inside. In practical terms, this results in significant savings on electricity and operating costs. When installing an aquaponic unit, be sure to place the submersible pump in an accessible location because periodic cleaning is necessary. Indeed, the internal filter will need cleaning every 2–3 weeks. Submersible water pumps will break if they are run without water; never run a pump dry.
External pumps could be used, but they require further plumbing and are more appropriate for larger designs. High-quality water pumps should preferably be used in order to guarantee a long life span and energy efficiency. Top-quality pumps will maintain their pumping capacity and efficiency for least 1–2 years, with an overall life span of 3–5 years, whereas inferior products will lose their pumping power in a shorter time leading to significantly reduced water flows. Regarding flow rate, the small-scale units described in this publication need a flow rate of 2 000 litres/h at a head height of 1.5 meters; a submersible pump of this capacity would consume 25–50 W/h. A helpful approximation to calculate energy efficiency for submersible pumps is that a pump can move 40 litres of water per hour for every watt per hour consumed, although some models claim twice this efficiency. When designing the plumbing for the pump, it is important to realize that pumping power is reduced at every pipe fitting; up to 5 percent of the total flow rate can be lost at each pipe connection when water is forced through. Thus, use the minimal number of connections between the pump and the fish tanks. It is also important to note that the smaller the diameter of the pipes, the larger the water flow loss. A 30 mm pipe has twice the flow of a 20 mm pipe even if served from pumps with same capacity. In addition, a larger pipe does not require any maintenance to remove the buildup of solids accumulating inside. In practical terms, this results in significant savings on electricity and operating costs. When installing an aquaponic unit, be sure to place the submersible pump in an accessible location because periodic cleaning is necessary. Indeed, the internal filter will need cleaning every 2–3 weeks. Submersible water pumps will break if they are run without water; never run a pump dry.
airlifts
Airlifts are another technique of lifting water. They use an air pump rather a water pump. Air is forced to the bottom of a pipe within the fish tank, bubbles form and burst, and during their rise to the surface the bubbles transport water with them. One benefit is that airlifts can be more electrically efficient, but only at small head heights (30–40 cm). Air lifts gain power in deeper tanks, and are best at a depth greater than one metre. An added value is that airlifts do not clog the way that submersible impeller-type pumps do. In addition, water is also oxygenated through the vertical movement operated by the air bubbles. However, the volume of air pumped should be adequate to move the water along the pipe. Air pumps generally have a longer life than submersible water pumps. The main benefit comes from an economy of scale – a single air pump can be purchased for both aeration and water circulation, which reduces the capital investment in a second pump.