Articles

AQUAPONICS – At a Glance

 

Aquaponics is the integration of recirculating aquaculture and hydroponics in one production system. In an aquaponic unit, water from the fish tank cycles through filters, plant grow beds, and then back to the fish tank. In the filters, the fish wastes are removed from the water, first using a mechanical filter that removes the solid waste and then through a bio filter that processes the dissolved wastes.

The bio filter provides a location for bacteria to convert ammonia, which is toxic for fish, into nitrate, a more accessible nutrient for plants. This process is called nitrification. As the water (containing nitrate and other nutrients) travels through plant grow beds, the plants uptake these nutrients and finally the water returns to the fish tank purified. This process allows the fish, plants and bacteria to thrive symbiotically and to work together to create a healthy growing environment for each other, provided that the system is properly balanced.

In aquaponics, the aquaculture effluent is diverted through plant beds and not released to the environment, while at the same time the nutrients for the plants are supplied from a sustainable, cost effective and non chemical source. The integration removes some of the unsustainable factors of running aquaculture and hydroponic systems independently. Beyond the benefits derived by this integration, aquaponics has shown that its plants an fish productions are comparable with hydroponics and recirculating aquaculture system.

Aquaponics can be more productive and economically feasible in certain situations, especially where land and water are limited. However, aquaponics is complicated and requires sustainable startup costs. The increased production must compensate for the higher investment costs needed to integrate the two systems. Before committing to a large or expensive system, a full business plan considering economic, environmental, social and logistical aspects should be conducted.

Although the production of fish and vegetables is the most visible output of aquaponics unit, it is essential to understand that aquaponics is the management of a complete system that includes three major groups of organisms: fish, plants and bacteria.

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ESTABLISHING BACTERIAL CULTURE

 

Aquaponics system may be established in a multitude of simple or complex manners, as long as the components of supporting the natural processes between fish, bacteria and plants are understood. Arguable the most important component of starting these systems is a well established culture of bacteria. The culture must contain species (Nitrosomonas) which convert ammonia waste into nitrites and the other species (Nitrobacter) that converts nitrites into usable nitrates for the plants.

There are many different ways to establish these important populations of bacteria; however, both populations must be present for plants to obtain their required nutrients and survive. The most efficient way to start a system is to inoculate it with bacteria from an existing system, an aquarium, or a pond. This can also be referred to as “seeding” the system. It is most important that the new system is seeded with bacterial populations from established, disease free communities. Aquarium filters contain high concentrations of the desired beneficial bacteria, as do samples of gravel. However, it must be from a disease free tank to avoid transferring diseases to the new seeding system.

The desired bacterial populations can also establish naturally in the system – it just takes longer. A source of ammonia must be present in the water to draw the desired bacterial species, such as food grade ammonia. A dead fish, or fish feed will also break down and release ammonia into the system.

In order to make sure the correct beneficial populations are established, it is recommended to purchase a water testing kit to test the levels of pH, ammonia, nitrite and nitrate periodically until the system is established in converting ammonia to usable nitrates. The amount of time required to establish efficient conversion of ammonia to nitrite and then to nitrate depends on how quickly the desired bacterial populations establish themselves. Establishment through seeding the system from an existing system or aquarium can take days, while drawing bacteria with just ammonia may take weeks. Once established, the water should be tested periodically. Then imbalances should be fixed before problems in fish death arise.

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WATER MOVEMENT IN AQUAPONICS

 

Water movement is fundamental for keeping all the organisms alive in aquaponics. Water moves from the fish tanks, through the mechanical separator and the bio filter and finally to the plants in their media beds, pipes and canals, removing the dissolved nutrients. If water movement stops, the most immediate effect will be a reduction in DO and accumulation of wastes in fish tank, without mechanical filter and bio filter, fish can suffer and die within hours.

Without water flow, the water in the media beds and deep water culture units will stagnate and become anoxic and Nutrient film technique systems will dry out.

A commonly sited guideline for better water flow in a densely stocked aquaponic system is to cycle the water two times per hour. For example, if an aquaponics unit has a total water volume of 1000 liters, the water flow rate should be 2000 liters per hour, so that every hour, the water is cycled two times.

However, at low stocking densities this turnover rate is not necessary, and the water only needs to be cycled one time per hour. The three commonly used methods of moving water through a system are submersible impeller pumps, airlifts and human power.

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GROWING LETTUCE IN AQUAPONIC UNITS

 

Lettuce grows particularly well in the aquaponics owing to the optimal nutrient concentrations in the water. Many varieties can be grown in aquaponics, but four main types included here are: a) Crisp head lettuce (iceberg), which has tight head with crispy leaves, ideal for cooler conditions. b) Butter head lettuce, which shows that leaves that are loosely piles one on another and have no bitter taste. c) Romaine lettuce, which has upright and tightly folded leaves that are slow to bolt and are sweet in taste. d) Loose leaf lettuce, which comes out in a variety of colors and shapes with no head and can be directly sown in the media beds and harvested by picking single leaves without collecting the whole plant. Lettuce is in high demand and has a high value in urban and peri-urban zones, which makes it very suitable crop for large scale commercial production.

GROWING CONDITIONS:

Lettuce is a winter crop. For head growth, the night air temperature should be 3-12 degrees, with a day temperature of 17-28 degrees. The generative conditions (more than 18 degrees) at night cause bolting. Water temperature more than 26 degrees may also favor bolting and leaf bitterness. The plant has low nutrient demand; however , higher calcium concentrations in water  help to prevent tip burn in leaf in summer crops. The ideal pH is 5.8-6.2, but lettuce still grows well with a pH as high as 7, although some iron deficiencies might appear owing to reduced bio availability of this nutrient above neutrality.

GROWING INSTRUCTIONS:

Seedlings can be transplanted in aquaponics units at three weeks when plants have at least 2-3 true leaves. Supplemental fertilization with phosphorous to the seedlings in the second and third weeks favors root growth and avoids plants stress at transplant. Moreover, plant hardening through exposing of seedlings to colder temperature and direct sunlight for 3-5 days before transplanting results in higher survival rates. When transplanting lettuce in warm weather, place light sun shade over the plants for 2-3 days to avoid water stress. To achieve crisp, sweet lettuce, grow plants at a fast pace by maintaining high nitrate levels, in the unit. When air and water temperatures increase during the season. use bolt-resistant (summer) varieties. If growing media beds in, plant new lettuces where they will be partially shaded by taller nearby plants.

HARVESTING:

Harvesting can begin as soon as heads or leaves are large enough to eat. If selling to markets, remove the full plants and roots when harvesting as soon as they reach market weight (250 – 400 gm). Cut the roots out and place them in a compost bin. Harvest early in the morning when leaves are crisp and full of moisture and chill quickly.

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IS YOUR GREEN HOUSE OVER HEATING?

 

Plants produce their own form of air-conditioning through transpiration. Moisture evaporates off leaf surfaces much like sweat cools humans. When you place plants in a greenhouse, however, the confined area can become overheated if it is not properly ventilated and shaded. In general, you need to monitor your greenhouse for high temperatures throughout the spring, summer and fall.

VENTILATION:

A closed green house exposed to a clear, sunny day heats up quickly inside, coupling heat with humidity compounds the stress placed on indoor plants. To combat overheating, your greenhouse needs a combination of roof and side vents, along with louvers to control the airflow. You can go as far as installing a thermometer and automatic vent openers to fully automate your greenhouse while you are away from home.

Simply opening the greenhouse doors does not provide the air circulation needed to fully cool the interior. Plants need some circulation to transpire effectively. Depending on your plant species, heat damage occurs when the temperature rises above 81 degrees Fahrenheit.

DIRECT LIGHT:

Glass green houses allowing direct sunlight inside creates hots pots through out the day. Some plants receive too much or too little sunlight, while the entire structure over heats. As a solution, use a shade paint or install poly ethylene film to diffuse the light. Creating a translucent appearance across your greenhouse forces the incoming light to bend and bounce around the interior. You prevent the hot spots and actually increase photosynthesis abilities as light is available to both the upper and lower leaf surfaces. Your over heating problem is also avoided or solved, since some of the sunlight rays bounce off the translucent film and away from the greenhouse interior.

SHADING:

One of the simplest solutions to over heating is basic shading. South and south west facing green houses typically receive the most sunlight during the spring and summer, especially the hot afternoon sun. Blinds, netting and shade cloth placed on the green house, block the incoming light to reduce over heating. Green houses without strategic shading have reduced plant growth and high die back rates.

 

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ACCLIMATIZING FISH IN AQUAPONICS

 

The fish must be properly acclimatized to the new water. Acclimatizing fish into new tanks can be a highly stressful process for fish, particularly the actual transport from one location to another in bags or small tanks. There are two main factors that causes stress when acclimatizing fish i.e., changes in temperature and pH between the original water and new water. If the difference in pH values are more than 0.5; then the fish will need at least 24 hours to adjust. Keep the fish in a small aerated tank for their original water and slowly add water from the new tank over the course of a day. 

Even if the pH values of the two environments are fairly close, the fish still need to acclimatize. The best method to do this is to slowly allow the temperature to equilibrate by floating the sealed transportation bags containing the fish in the culture water. This should be done for at least 15 minutes. At this time small amounts of water should be added from the culture water to the transport water with the fish. Again, this should take at least 15 minutes so as to slowly acclimatize the fish. Finally, the fish can be added to the new tank.

 

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HARVESTING FISH AND STAGGERED STOCKING

 

A constant biomass of fish in the tanks ensures a constant supply of nutrients to the plants. To achieve a constant biomass in the fish tanks, a staggered stocking method should be adopted. This technique involves maintaining three age classes or cohorts within the same tank. Approximately every three months, the mature fish (500 gm each) are harvested and immediately restocked with new fingerlings (50 gm each). This method avoids harvesting all the fish at once, and instead retains a more consistent biomass. If it is not possible to obtain fingerlings regularly, an aquaponics system can be still managed by stocking a higher number of juvenile fish and by progressively harvesting them during the season to maintain a stable biomass to fertilize the plants. 

If the fish are mixed-sex, the harvest must firstly target the females to avoid breeding when they reach sexual maturity from the age of five months. Breeding depresses the whole cohort. In the case of mixed-sex tilapia, fish can be initially stocked in a cage and males can then be left free in the tank after sex determination.

Remember that adult tilapia, catfish and trout will predate their smaller siblings if they are stocked together. A technique to keep all of these fish safely in the same fish tank is to isolate the smaller ones in a floating frame. This frame is essentially a floating cage, which can be constructed as a cube with PVC pipe used as a frame and covered with plastic mesh. It is important to ensure that larger fish cannot enter the floating cage over the top, so make sure that the sides extend at least 15 cm above the water level. Each of the vulnerable size classes should be kept in separate floating frames in the main fish tank. As the fish grow large enough not to be in danger, they can be moved into the main tank. With this method, it is possible to have up to three different weights in one tank, so it is important that the fish feed pellet size can be eaten by all sizes of the fish.

 

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RULES OF THUMB IN AQUAPONICS

 

Starting your aquaponics unit at home? Developing a large scale aquaponics project? Championing small scale aquaponics units in the classroom? Here are the rules to follow:

Choose tank carefully:

Fish tanks are a crucial component in every aquaponics unit. Any fish tank will work, but round tanks with flat or conical bottoms are recommended because they are easy to clean. Try using strong inert plastic or fiber glass tanks, because of their durability and long life span.

Ensure adequate aeration and water circulation:

You should use water and air pumps to make sure that the water has high levels of dissolved oxygen and good water movement so that your animals, bacteria and plants are healthy. Electricity costs are a significant portion of the system budget so choose the pumps and power source wisely and consider photo voltaic power if possible.

Maintain good water quality:

Water is the life – blood of an aquaponics system. It is the medium through which all essential nutrients are transported to the plants, and it is where the fish live. Five key water quality parameters that are important to monitor and control: Dissolved oxygen (5 mg/Lt), pH (6-7), Temperature (18 – 30 degrees Celsius), total nitrogen and water alkalinity. The water chemistry may seem complicated, but the actual management is relatively simple with the help of common test kits.

Do not over crowd the tanks:

Your aquaponics system will be easier to manage and will be insulated against shocks and collapse if the stocking density is kept low. The recommended stocking density is 20 kg/1000 Lt, which will still allow for substantial plant growing area. Higher stocking densities can produce more food in the same space, but will require much more active management.

Avoid overfeeding and remove any uneaten food:

Wastes and uneaten food are very harmful for aquatic animals because they can rot inside the system. Rotting food can cause disease and can use up all of the dissolved oxygen. Feed the animals everyday but remove any uneaten food after 30 minutes and adjust next days portion accordingly.

Choose and space the plants wisely:

Plant vegetables with short grow-out periods between plants with long term crops. Continued replanting of tender vegetables such as lettuce in between large fruiting plants naturally shaded conditions. In general, leafy greens plants do extremely well in aquaponics along with some of the most popular fruiting vegetables including tomatoes, cucumbers and peppers.

Maintain balance between plants and animals:

Using a batch cropping system can help keep a steady harvest of both aquatic animals and vegetables to keep a consistent production level and maintain a constant balance between the fish and plants. A secure source of young plants and young fish is important, so make sure that the supply is considered during the planning phase.

 

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