The water in an aquaponic system not only contains nutrients and minerals for plants but also dissolved oxygen which is required by plants, fishes, and nitrifying bacteria to carry out their biological functions.
Dissolved oxygen in aquaponics can be increased by using a properly sized air pump with air stones strategically placed both in the fish tank and the biofilter. Also, a venturi attached to the water pump and designing the system such that there is maximum water turbulence can help to add more dissolved oxygen.
Oxygen is not supplied directly to the water by the bubbles, instead, its the actual agitation of the water which causes the oxygen to be dissolved. Low concentrations of dissolved oxygen in an aquaponic system can cause a number of problems because it affects all living organisms within the system.
Knowing how to increase the oxygen is therefore necessary when it comes to addressing the demands of the system. There are a couple of ways to achieve this and we will show how in this article.
See our complete guide on aquaponics and what you need to know to run a successful system.
What is the Oxygen Demand in Aquaponics?
The oxygen demand is the amount of oxygen required by the organisms in the system to carry out their biological functions.
Dissolved oxygen is required by all living organisms within the system for respiration and energy. The bacteria, in particular, require this much-needed oxygen to carry out the nitrification process which drives the entire aquaponic process.
Fishes and Oxygen Demand –
Different fish species need a varied range of dissolved oxygen. Fishes normally require 45 mg/liter of dissolved oxygen. This may vary from species to species and should be a factor to consider when choosing fishes for your system.
A clear indication that there is a lack of oxygen in the system is when you see the fishes gasping for air at the waters surface.
Plants and Oxygen Demand –
Plants not only absorb oxygen through stems and leaves during respiration but through the roots. Most plants require high levels of Dissolved Oxygen which may exceed > 3 mg/liter dissolved oxygen.
Plant root systems need oxygen for aerobic respiration that is an important process to release metabolic energy for nutrient uptake and root growth.
Healthy roots with an adequate supply of oxygen can absorb the required nutrients from the water to ensure proper plant growth. Healthy roots also take up enough calcium minerals required for the growth and development of new tissues.
Nitrifying Bacteria and Oxygen Demand –
Nitrification is a biochemical reaction that occurs inside bacteria whereby the ammonia from the fish’s waste is oxidized into other forms of nitrogen such as Nitrites and Nitrates.
Nitrifying bacteria requires a healthy amount of dissolved oxygen to live and are killed off by anaerobic conditions.
Maximum nitrification occurs at a D.O. (Dissolved Oxygen) level of 3.0 mg/l. Significant nitrification occurs at a D.O. level of 2.0 to 2.9 mg/l. Nitrification ceases at D.O. levels of <0.5 mg/l. Approximately 4.6 kg of oxygen are required for every kg of ammonium ions oxidized to nitrate
To ensure nitrification is occurring, always maintain a D.O. level of ?1.5 mg/l.
see our detailed post on how to protect an aquaponic system in the event of a power failure.
How to Choose an Air pump for Aquaponics?
An air pump supplies oxygen to water by pumping air from the atmosphere into the water in the form of bubbles. The air pump works well with air stones which help to further break up the bubbles into smaller ones for maximum oxygen to be dissolved into the water.
Several factors are used when determining the size of an air pump for an aquaponic system
These factors are –
- The Amount of Fishes in the System (Fish Density)
The fishes use up the majority of the dissolved oxygen in an aquaponics system. Depending on the number of fish there are living in the tank, you can have a better idea of the aeration requirements.
The total biomass of the fishes increases as time passes this is because as they eat food they grow bigger and would, therefore, require more air.
- The Size of the Fish Tanks
From the size of the fish tank in your system, you would have an idea of how much water there is circulating in the system. Water is the medium by which oxygen is carried throughout the system to the different users.
The rate at which oxygen is dissolved into the water should be sufficient to ensure the fishes, bacteria, and plants, have a sufficient amount of oxygen to live healthy.
- The Depth of the Water in the Fish Tank
The depth of the water also plays a part in choosing the size of the air pump you may need. The deeper the placement of air stones the more force would be required to push air out of the stone and into the water.
- The Amount of Plants in the Grow beds
Plants too require oxygen to survive since they themselves respire. Plants undergo aerobic respiration which uses oxygen and produces carbon dioxide. Photosynthesis however uses carbon dioxide and produces oxygen.
The more plants in the system the more oxygen they would require. As plants grow over time, just like the fishes the oxygen demand will also increase and this should also be taken into consideration
How can I Aerate Fish Water Without an Air Pump?
Turbulence of the Water
The water in an aquaponic system can stay oxygenated without an air pump if there is enough turbulence in the system to provide oxygen absorption.
The number of fishes, in this case, would therefore have to be significantly less than if there was an air pump attached to the system.
Fishes can eat up a great deal of dissolved oxygen depending on their size and stress levels, so keeping the fish happy is a must. Even if it means getting an additional air pump.
Designing a system such that it runs on a single pump, the water should only have to be lifted once and this is done by the water pump.
The rest of the system should be designed such that the water flows sequentially through all the components by gravity flow. You can now use gravity to allow the water to have a cascading effect on each component.
The cascading effect is similar to that of a waterfall and creates maximum turbulence as the water circulates through the system.
This type of system is known as a
CHOP system (Constant Height One Pump)
Adding a Venturi Aerator
Another way the water can be aerated without an air pump is by use of a venturi.
A venturi aerator supplements the oxygen demand of a system by providing additional dissolved oxygen. This is done by use of the Bernoulli principle whereby air is sucked into a high-velocity area of a pipe and injected into the water stream.
A venturi is created by a restriction in the piping which speeds up the flow of water at the restriction causing a lower pressure at that point.
This lower pressure is utilized by placing a hose that is open to the atmosphere. Air is then sucked in into the hose causing air bubbles to go along with the stream of water and in turn aerating the water.
Want to learn how to make a venturi for Aquaponics? I have written an entire article on how you can easily do it using simple materials.
Air pumps with air stones as we know are the most popular way to supply that much-needed oxygen for an aquaponic system. But what if you dont have an air pump or your current air pump isnt supplying the amount of oxygen required for the plants and fish to grow properly?
In this case, a venturi would help add that extra dissolved oxygen into the water.
How does a Venturi Work?
The Bernoulli principle
Bernoullis principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluids potential energy.
The bernoulli equation – P1 + V1 = P2 + V2
The pressure, P1 is greater than P2
The waters velocity, V2 is greater than V1
What this simply means is that the water pressure (P1), before the reduction is greater than the water pressure (P2) at the reduction.
This results in the water velocity (V2), being greater than the water velocity (V1).
The reduction in pipe size causes the water to speed up resulting in a pressure to drop at that point, pulling air into the pipe from the outside.
Once the air is sucked into the water stream it is carried into the fish tank as tiny bubbles which causes agitation. This agitation in turn helps oxygen to be dissolved into the water. The oxygen is then used by both fishes, plants and nitrifying bacteria to carry out their daily functions.