the BIOLOGICAL COMPONENTS OF AQUAPONICS
Aquaponics is a form of integrated agriculture that combines two major techniques, aquaculture and hydroponics. In one continuously recirculating unit, culture water exits the fish tank containing the metabolic wastes of fish. The water first passes through a mechanical filter that captures solid wastes, and then passes through a biofilter that oxidizes ammonia to nitrate. The water then travels through plant grow beds where plants uptake the nutrients, and finally the water returns, purified, to the fish tank. The biofilter provides a habitat for bacteria to convert fish waste into accessible nutrients for plants. These nutrients, which are dissolved in the water, are then absorbed by the plants. This process of nutrient removal cleans the water, preventing the water from becoming toxic with harmful forms of nitrogen (ammonia and nitrite), and allows the fish, plants, and bacteria to thrive symbiotically. Thus, all the organisms work together to create a healthy growing environment for one another, provided that the system is properly balanced.
the nitrogen cycle
The most important biological process in aquaponics is the nitrification process, which is an essential component of the overall nitrogen cycle seen in nature. Nitrogen (N) is a chemical element and an essential building block for all life forms. It is present in all amino acids, which make up all proteins which are essential for many key biological processes for animals such as enzyme regulation, cell signalling and the building of structures. Nitrogen is the most important inorganic nutrient for all plants. Nitrogen, in gas form, is actually the most abundant element present in the Earth’s atmosphere making up about 78 percent of it, with oxygen only making up 21 percent.
Yet, despite nitrogen being so abundant, it is only present in the atmosphere as molecular nitrogen (N2), which is a very stable triple bond of nitrogen atoms and is inaccessible to plants. Therefore, nitrogen in its N2 form has to be changed before plants use it for growth. This process is called nitrogen-fixation. It is part of the nitrogen cycle, seen throughout nature.
Nitrogenfixation is facilitated by bacteria that chemically alter the N2 by adding other elements such as hydrogen or oxygen, thereby creating new chemical compounds such as ammonia (NH3) and nitrate (NO3 – ) that plants can easily use. Also, atmospheric nitrogen can be fixed through an energy-intensive manufacturing process known as the Haber Process, used to produce synthetic fertilizers. Animals produce waste (faeces and urine) that is largely made of ammonia (NH3). Other decaying organic matter found in nature, such as dead plants or animals, is broken down by fungi and different bacteria groups into ammonia. This ammonia is metabolized by a specific group of bacteria, which is very important for aquaponics, called nitrifying bacteria. These bacteria first convert the ammonia into nitrite compounds (NO2 – ) and then finally into nitrate compounds (NO3 – ). Plants are able to use both ammonia and nitrates to perform their growth processes, but nitrates are more easily assimilated by their roots.
Yet, despite nitrogen being so abundant, it is only present in the atmosphere as molecular nitrogen (N2), which is a very stable triple bond of nitrogen atoms and is inaccessible to plants. Therefore, nitrogen in its N2 form has to be changed before plants use it for growth. This process is called nitrogen-fixation. It is part of the nitrogen cycle, seen throughout nature.
Nitrogenfixation is facilitated by bacteria that chemically alter the N2 by adding other elements such as hydrogen or oxygen, thereby creating new chemical compounds such as ammonia (NH3) and nitrate (NO3 – ) that plants can easily use. Also, atmospheric nitrogen can be fixed through an energy-intensive manufacturing process known as the Haber Process, used to produce synthetic fertilizers. Animals produce waste (faeces and urine) that is largely made of ammonia (NH3). Other decaying organic matter found in nature, such as dead plants or animals, is broken down by fungi and different bacteria groups into ammonia. This ammonia is metabolized by a specific group of bacteria, which is very important for aquaponics, called nitrifying bacteria. These bacteria first convert the ammonia into nitrite compounds (NO2 – ) and then finally into nitrate compounds (NO3 – ). Plants are able to use both ammonia and nitrates to perform their growth processes, but nitrates are more easily assimilated by their roots.
Nitrifying bacteria, which live in diverse environments such as soil, sand, water and air, are an essential component of the nitrification process that converts plant and animal waste into accessible nutrients for plants. This natural process of nitrification by bacteria that happens in soil also takes place in water in the same way.
For aquaponics, the animal wastes are the fish excreta released in the culture tanks. The same nitrifying bacteria that live on land will also naturally establish in the water or on every wet surface, converting ammonia from fish waste into the easily assimilated nitrate for plants to use. Nitrification in aquaponic systems provides nutrients for the plants and eliminates ammonia and nitrite which are toxic
For aquaponics, the animal wastes are the fish excreta released in the culture tanks. The same nitrifying bacteria that live on land will also naturally establish in the water or on every wet surface, converting ammonia from fish waste into the easily assimilated nitrate for plants to use. Nitrification in aquaponic systems provides nutrients for the plants and eliminates ammonia and nitrite which are toxic
Nitrifying bacteria
Nitrifying bacteria are vital for the overall functioning of an aquaponic unit.
Two major groups of nitrifying bacteria are involved in the nitrification process: 1) the ammonia-oxidizing bacteria (AOB), and 2) the nitrite-oxidizing bacteria (NOB) (Figure 2.6). They metabolize the ammonia in the following order: 1. AOB bacteria convert ammonia (NH₃) into nitrite (NO₂- ) 2. NOB bacteria then convert nitrite (NO₂- ) into nitrate (NO₃- )
Two major groups of nitrifying bacteria are involved in the nitrification process: 1) the ammonia-oxidizing bacteria (AOB), and 2) the nitrite-oxidizing bacteria (NOB) (Figure 2.6). They metabolize the ammonia in the following order: 1. AOB bacteria convert ammonia (NH₃) into nitrite (NO₂- ) 2. NOB bacteria then convert nitrite (NO₂- ) into nitrate (NO₃- )
As shown in the chemical symbols, the AOB oxidize (add oxygen to) the ammonia and create nitrite (NO₂- ) and the NOB further oxidize the nitrite (NO₂- ) into nitrate (NO₃- ). The genus Nitrosomonas is the most common AOB in aquaponics, and the genus Nitrobacter is the most common NOB; these names are frequently used interchangeably in the literature and are used throughout this publication. In summary, the ecosystem within the aquaponic unit is totally reliant on the bacteria. If the bacteria are not present or if they are not functioning properly, ammonia concentrations in the water will kill the fish. It is vital to keep and manage a healthy bacterial colony in the system at all times in order to keep ammonia levels close to zero.
MAINTAINING A HEALTHY BACTERIAL COLONY
The major parameters affecting bacteria growth that should be considered when maintaining a healthy biofilter are adequate surface area and appropriate water conditions. As shown in the chemical symbols, the AOB oxidize (add oxygen to) the ammonia and create nitrite (NO₂- ) and the NOB further oxidize the nitrite (NO₂- ) into nitrate (NO₃- ). The genus Nitrosomonas is the most common AOB in aquaponics, and the genus Nitrobacter is the most common NOB; these names are frequently used interchangeably in the literature and are used throughout this publication. In summary, the ecosystem within the aquaponic unit is totally reliant on the bacteria. If the bacteria are not present or if they are not functioning properly, ammonia concentrations in the water will kill the fish. It is vital to keep and manage a healthy bacterial colony in the system at all times in order to keep ammonia levels close to zero.
The major parameters affecting bacteria growth that should be considered when maintaining a healthy biofilter are adequate surface area and appropriate water conditions.
Surface area Bacterial colonies will thrive on any material, such as plant roots, along fish tank walls and inside each grow pipe. The total available area available for these bacteria will determine how much ammonia they are able to metabolize. Depending on the fish biomass and system design, the plant roots and tank walls can provide adequate area. Systems with high fish stocking density require a separate biofiltration component where a material with a high surface area is contained, such as inert grow media – gravel, tuff or expanded clay.
The major parameters affecting bacteria growth that should be considered when maintaining a healthy biofilter are adequate surface area and appropriate water conditions.
Surface area Bacterial colonies will thrive on any material, such as plant roots, along fish tank walls and inside each grow pipe. The total available area available for these bacteria will determine how much ammonia they are able to metabolize. Depending on the fish biomass and system design, the plant roots and tank walls can provide adequate area. Systems with high fish stocking density require a separate biofiltration component where a material with a high surface area is contained, such as inert grow media – gravel, tuff or expanded clay.
water pH
The pH is how acidic or basic the water is. The pH level of the water has an impact on the biological activity of the nitrifying bacteria and their ability to convert ammonia and nitrite (Figure 2.8). The ranges for the two nitrifying groups below have been identified as ideal, yet the literature on bacteria growth also suggests a much larger tolerance range (6–8.5) because of the ability of bacteria to adapt to their surroundings. However, for aquaponics, a more appropriate pH range is 6–7 because this range is better for the plants and fish (Chapter 3 discusses the compromise on water quality parameters). Moreover, a loss of bacterial efficiency can be offset by having more bacteria, thus biofilters should be sized accordingly.
water temperature
Water temperature is an important parameter for bacteria, and for aquaponics in general. The ideal temperature range for bacteria growth and productivity is 17–34 °C. If the water temperature drops below 17 °C, bacteria productivity will decrease. Below 10 °C, productivity can be reduced by 50 percent or more. Low temperatures have major impacts on unit management during winter.
Dissolved oxygen
Nitrifying bacteria need an adequate level of dissolved oxygen (DO) in the water at all times in order to maintain high levels of productivity. Nitrification is an oxidative reaction, where oxygen is used as a reagent; without oxygen, the reaction stops. Optimum levels of DO are 4–8 mg/litre. Nitrification will decrease if DO concentrations drop below 2.0 mg/ litre. Moreover, without sufficient DO concentrations, another type of bacteria can grow, one that will convert the valuable nitrates back into unusable molecular nitrogen in an anaerobic process known as denitrification.
Ultraviolet light
Nitrifying bacteria are photosensitive organisms, meaning that ultraviolet (UV) light from the sun is a threat. This is particularly the case during the initial formation of the bacteria colonies when a new aquaponic system is set up. Once the bacteria have colonized a surface (3–5 days), UV light poses no major problem. A simple way to remove this threat is to cover the fish tank and filtration components with UV protective material while making sure no water in the hydroponic component is exposed to the sun, at least until the bacteria colonies are fully formed. Nitrifying bacteria will grow on material with a high surface area, sheltered using UV protective material, and under appropriate water conditions