Biology and chemistry: Key basics of aquaponics
Aquaponics couldn’t work without natural processes. Problematic of biology and chemistry in aquaponics systems is very extensive and complicated. Look at basic mechanisms of the most important processes, whose right adjustment and control is indispensable for good working aquaponics system.
Good knowledge of biological and chemical processes is the key of successful aquaponics growing. However that requires large volume of knowledge from the grower or serving staff of the farm that can be compared to professional personnel of large fish or hydroponics farm. Operating an aquaponics farm means putting those two professions together. This fact is the biggest peril of the whole aquaponics and its development.
There are two most important processes in aquaponics and we have to mention them. The first one is nitrification, which is done by special nitrification bacteria. It is a biological process of changing ammonia (NH4+) that fish release from their gills and its oxidization to nitrite (NO2-) a then to nitrate (NO3-), which can be absorbed by plants. Ammonia and nitrite is very toxic for the fish even in small doses and plats have hard time using them, meanwhile nitrate is ideal fertilizer and fish can tolerate quite large concentration of it. Nitrification goes on best in the dark with good oxygen supply, at the same time the nitrification bacteria are vulnerable to fast changes of pH and doesn’t work right in an unstable system. If lack of oxygen comes up for any reason then this process turns into denitrification. Bacteria start using nitrite as a source of oxygen and pure nitrogen begins to escape from the system and spreads out.
Nitrification bacteria aren’t pathogenic for a human. They can get to a new aquaponics system either spontaneously or put there artificially. If they have the right conditions then they’ll fastly populate the new environment and within moments the nitrification process gets started.
The other important process is mineralization. This is done by large scale of bacteria and other microorganisms that decompose sludge into simple compounds that are soluble in water and later are be used by the plants. This process is also aerobic; it requires enough oxygen for its running. In aquaponics systems the solid waste is usually separated in a special tank and intensively aired. Created solution that is rich with nutrients is then returned back into the system and is used by plants for their growth. This was it is possible to change the whole sludge info fertilizer for plants increasing efficiency of the aquaponics system. With lack of oxygen in the water, this process of mineralization turns into fermentation. This is undesirable in an aquaponics system, because it can strongly change chemical properties of the water, mainly its pH.
For good working of microbiological processes in an aquaponics system it is required to establish lots of areas for bacterial colonization. Nitrification bacteria like stable and dark environment. So an ideal place for them is in the filtration substrate. In the most cases it is gravel, ceramsite, lava gravel (on Hawaii), zeolite or special filtration matter and filling like in the aquarium filters. These materials have lots of small surfaces, where can the nitrification bacteria live. Amount of material needed depends mostly on its porosity, total volume of water in the system and amount of bred fish. In case of small garden aquaponics with gravel or ceramsite Australian growers recommend 1:1 volume of the filtration medium to volume of fish tank. In small aquaponics the required size of bio filter is usually exceeded, which raises the stability of the small system. In big aquaponics you have to thoroughly evaluate size and type of bio filter, eventually even their suitable combination.
Oxygen is the most important substance in the whole aquaponics, because it enables processes of changing waste into nutrients and is also used by fish and roots of plats to breathe. With oxygen in aquaponics the more the better applies. Oxygen naturally passes from air to water, where this is influenced mostly by temperature (warmer water can take less oxygen than the cold one) and area of surface between air and water. It is hard to check concentration of oxygen without special lab equipment, that’s why there are many ways in aquaponics to keep its levels in water as high as possible. The most common one, but not always best, is using air stones in fish tanks. Although it is the most efficient way of getting oxygen into water its disadvantage is big mixing of water and therefore pulverization of fish excrements into very small particles. Those are then faster clogging the bio filter and create sediments in aquaponics distributing pipes. Subtle parts of sludge settle down on biofilm made of stringy bacteria and can cause even total clogging of pipes. Better way of placing air stones are tanks with rafts or settling tank, from where is the collected water pumped back to fish tank. In well-designed systems, which have good hydrodynamics, airing usually isn’t needed, because there aren’t any blind spots in a tank, where the water would stay for a long time and frequent contact of water with outside air is ensured.
Water temperature affects speed of biological processes in a system. Nitrification bacteria work best in temperature between 25 and 28°C. With decreasing temperature their activity declines and at 4 °C it stops. If an aquaponics system freezes, nitrification bacteria will start to die. After unfreezing and reinitiating the system, their numbers will soon go back to the original.
Water temperature in an aquaponics system is mostly regulated for the needs of bred fish. They are prone to changing temperatures and can even die to heat shock if it changes too fast. This is true in case of releasing new fish into the tank and their acclimatization. Most bred fish in aquaponics the Nile Perch has quite high temperature requirements (26-28°C) and her breeding is hardly sustainable in temperatures under 20°C. This puts a big burden on breeder to heat the water, therefore rising running costs. Advantage of higher water temperature is its ability to heat an aquaponics greenhouse during winter months, reducing the need to heat the air itself (but it doesn’t disappear, more so if the system has to produce vegetables like salads or tomatoes even in winter months).
Even grown plans, their roots respectively, are affected by water temperature. The optimal temperature for roots of many vegetables to grow properly is a bit lower (about 18-24°C), than for some kinds of fish (Nile Tilapia). It doesn’t mean that plants will not grow in warm water, but rate of their growth slows down.
PH values are the most important parameter after the temperature. Checking pH is very easy, with many available tests in paper or solution type. Breeder’s goal is to ensure the most stable pH in an aquaponics environment, because nitrification and even availability of trace elements for roots of plants and satisfaction of fish depends on its value. Appropriate pH can differ from system to system. It is said that the most ideal interval is 6,5 – 7,5. The very best pH value should be 6,8, nevertheless it isn’t easy and desirable to keep it at that point. Every system can have slightly different value. The widest value that shouldn’t be crossed in an aquaponics system is 6,0-8,0.
Refilling of trace elements
Fish organism requires less iron, potassium, calcium or manganese than plants to grow. Therefore ratio of those elements in fish feed is lower (to nitrogen) than would be ideal for plants. This often times causes deficit of mostly iron and calcium in aquaponics solution and plats can suffer from lacking them. That can manifest by bad growth or by various defects of leaves like faded color, spots or withered parts. The most supplied element in aquaponics is iron, its lack manifests by bright spots on leaves of plants.
Elements are supplied in form of powder or solution. Those can be bought in chemically pure form. However their consumption in an aquaponics system is very small and they don’t represent sizable financial unit. An aquaponics can be operated even without supplying those elements; however that requires bigger ratio of fish to plants so that the small amount of those elements in fish feed would be delivered in surplus. This also requires integrating of denitrification part into the aquaponics cycle, where the excess nitrogen is turned into gas and released.
Plants unlike fish require large volume of light for their growth. Ensuring sufficient lighting of grown plants is the basic assumption for successful growing. This could be a problem in an interior aquaponics system that has lack of natural lighting and requires artificial lighting. Illumination of grown plants that are very light demanding (salads, paprika, tomatoes), is required to keep reasonable production during winter month on our latitude. Alternative is to grow less light and heat demanding plants. Those are for example spinach, broccoli or kale. Appropriate lighting is the same as the one used in hydroponics. The choice of right variant of lighting source is decided by financial resources and total form of the aquaponics system.
In greenhouse systems the best type is sodium-vapor lamps, which have sufficient lighting power and can illuminate sizeable area. However they make a lot of heat and have to be kept at distance from plants. Fop smaller interior placed systems fluorescent lights are better choice, or even LED panels, which are nowadays still more expensive, but provide specific part of light spectrum for photosynthesis and have much longer longevity than fluorescent lights. Those have to be replaced about every 12 months because their power wanes with time.
Right ratio of fish and plants
Ratio of bred fish and grown plans is another big topic of talks in aquaponics, which is very important and the right setting is required to keep this „ecosystem“ running. In reality this is about ratio of given feed to amount of grown plants, because we are trying to reach a balanced movement of nutrients from fish to plants.
On many different forums you can read various approaches how to count the best ratio between fish and plant, nevertheless every guide isn’t based on real knowledge of operating an aquaponics system. Basically it is one-way motion of nutrients that change inside the system, from complex organic substances (fish feed) to simple elements dissolved in watery slope, which can be consumed by plants (for example nitrates). If we want to design a system with the right ratios, we have to think in this plane of nutrients movement from one form to the other, from fish to plants. Dr. Wilson Lennard from Australia is working with approx. 16 g of fish feed on 1 square meter of growing area daily in his aquaponics systems. He defines this area as 30 plants (salad) in bed of floating rafts. This ratio means that 30 pcs of salad grown in 1 square meter are able to absorb nitrogen contained in 16 g of fish feed daily. However this ratio is only tentative for every other aquaponics system other than the one that it was counted for (Lennard uses whole mineralization of excrements). Every single one aquaponics system has a different dynamic of fish feed transformation into available nutrients for plants. Among the factors that affect this ratio belongs: type of fish feed and its nutrient content, design of the system, volume of water, rate of nitrification and mineralization, kinds of bred fish and efficiency of their digestion, water temperature and amount of dissolved oxygen. Last but not least also type of fish feed and its nutrient content. Ratio of feed dosage to growing area has to be counted for every system individually.
Let’s make an example of counting the right ratio of feed dosage to growing area. If we take the data from Lennard’s system, which uses the whole mineralization of excrements, therefore changes 100% of feed to available nutrients, every square meter of growing area will require minimally 16 g of feed. In the case of having growing area of 10 square meters, we’ll have to supply 160g of feed into system every day. Imagine we breed a carp that has daily feed dosage of 2-3% of his weight in temperature around 22 ° C. In lower temperatures will his metabolism slow down and he’ll consume less feed.
160 g therefore represents daily feed dosage for approximately 5,5 kg of carps. In a tank of 1000l volume we would get fish density of 5,5 kg/m3.
As is obvious from above text the problematic of nutrient cycle in an aquaponics system is very complex. Understanding and using it right is the most essential part to reach success in aquaponics growing, because it affects every other aspect of aquaponics.