COMPONENTS in AQUAPONIC Units
All aquaponic systems share several common and essential components. These include: a fish tank, a mechanical filter, a biofilter, and hydroponic containers. All systems use energy to circulate water through pipes and plumbing while aerating the water. As introduced above, there are three main designs of the plant growing areas including: grow beds, grow pipes and grow canals. This section discusses the mandatory components, including the fish tanks, mechanical filter, biofilter, plumbing and pumps. The following sections are dedicated to the separate hydroponic techniques, and a comparison is made to determine the most appropriate combination of techniques for different circumstances.
1.1 - Fish tank
Fish tanks are a crucial component in every unit. As such, fish tanks can account for up to 20 percent of the entire cost of an aquaponic unit. Fish require certain conditions in order to survive and thrive, and therefore the fish tank should be chosen wisely. There are several important aspects to consider, including the shape, material and color.
Tank shape
Although any shape of fish tank will work, round tanks with flat bottoms are recommended. The round shape allows water to circulate uniformly and transports solid wastes towards the center of the tank by centripetal force. Square tanks with flat bottoms are perfectly acceptable, but require more active solid-waste removal. Tank shape greatly affects water circulation, and it is quite risky to have a tank with poor circulation. Artistically shaped tanks with non-geometric shapes with many curves and bends can create dead spots in the water with no circulation. These areas can gather wastes and create anoxic, dangerous conditions for the fish. If an odd-shaped tank is to be used, it may be necessary to add water pumps or air pumps to ensure proper circulation and remove the solids. It is important to choose a tank to fit the characteristics of the aquatic species reared because many species of bottom dwelling fish show better growth and less stress with adequate horizontal space.
material
Either strong inert plastic or fiberglass is recommended because of their durability and long life span. Metal is not possible because of rust. Plastic and fiberglass are convenient to install (also for plumbing) and are fairly light and maneuverable. Animal-watering troughs are commonly used, as they tend to be cheap. If using plastic containers, make sure that they are UV-resistant because direct sunlight can destroy plastic. In general, low-density polyethylene (LDPE) tanks are preferable because of their high resistance and food-grade characteristics. Indeed, LDPE is the most commonly used material for water storage tanks for civil uses. Another option is an in-ground pond. Natural ponds are very difficult to manage for aquaponics because the natural biological processes, already occurring within the substrate and mud at the bottom, can be hard to manipulate and the nutrients are often already used by aquatic plants. Cement or plastic-lined ponds are much more acceptable, and can be an inexpensive option. In-ground ponds can make plumbing operations difficult, and the plumbing design should be carefully considered before embarking on this option. One of the simplest fish tanks is a hole dug in the ground, lined with bricks or cinder-blocks, and then lined with a waterproof liner such as polyethylene plastic. Other options include secondhand containers, such as bathtubs, barrels or intermediate bulk containers (IBCs). It is very important to make sure the container has not been used previously to store toxic material. Contaminants, such as solvent-borne chemicals, will have penetrated into the porous plastic itself and are impossible to remove with washing. Thus, choose used containers carefully, and know the seller if possible.
Color
White or other light colors are strongly advised as they allow easier viewing of the fish in order to easily check behavior and the amount of waste settled at the bottom of the tank. White tanks will also reflect sunlight and keep the water cool. Alternatively, the outside of darker colored tanks can be painted white. In very hot or cold areas, it may be necessary to further thermally insulate the tanks.
Covers and shading
ll fish tanks should be covered. The shade covers prevent algal growth. In addition, the covers prevent fish from jumping out (often occurs with newly added fish or if water quality is sub-optimal), prevent leaves and debris from entering, and prevent predators such as cats and birds from attacking the fish. Often, agricultural shading nets that block 80–90 percent of sunlight are used. The shade cloth can be attached to a simple wooden frame to provide weight and make the cover easy to remove.
Fail-safe and redundancy
Do not let the fish tank lose its water; fish will die if the fish tank accidentally drains. Although some accidents are unavoidable (e.g. a tree falling on the tank), most catastrophic fish kills are the result of human error. Ensure that there is no way for the tank to drain without a deliberate choice by the operator. If the water pump is located in the fish tank, be sure to lift the pump off the bottom so that the tank can never be pumped dry. Use a standpipe inside the tank to guarantee a minimum water level.
1.2 Filtration – mechanical and biological
Mechanical filtration
For RASs, mechanical filtration is arguably the most important aspect of the design. Mechanical filtration is the separation and removal of solid and suspended fish waste from fish tanks. It is essential to remove these wastes for the health of the system, because harmful gases are released by anaerobic bacteria if solid waste is left to decompose inside the fish tanks. Moreover, the wastes can clog systems and disrupt water flow, causing anoxic conditions to the plant roots. Small-scale aquaponics typically has lower stocking densities than the intensive RAS methods for which these mechanical filters were originally designed, but some level of mechanical filtration is essential for healthy aquaponic tanks, regardless of the type of hydroponic method used. There are several types of mechanical filters. The simplest method is a screen or filter located between the fish tank and the grow bed. This screen catches solid wastes, and needs to be rinsed often. Similarly, water leaving the fish tank can pass through a small container of particulate material, separate from the media bed; this container is easier to rinse periodically. These methods are valid for some small-scale aquaponic units, but are insufficient in larger systems with more fish where the amount of solid waste is relevant. There are many types of mechanical filters, including sedimentation tanks, radial-flow clarifiers, sand or bead filters and baffle filters; each of them can be used according to the amount of solid wastes that needs to be removed. However, as this publication focuses on small-scale aquaponics, clarifiers, or mechanical separators, are the most appropriate filters. Clarifiers, in general, can remove up to 60 percent of the total removable solids.
Mechanical separators (clarifiers)
A clarifier is a dedicated vessel that uses the properties of water to separate particles. Generally, water that is moving slower is unable to carry as many particles as water that is flowing faster. Therefore, the clarifier is constructed in such a way as to speed up and slow down the water so that the particles concentrate on the bottom and can be removed. In a swirl clarifier, the water from the fish tank enters near the lower middle of the clarifier through a pipe. This pipe is positioned tangentially to the container thereby forcing the water to swirl in a circular motion inside the container. The centripetal force created by the circular motion of the water forces the solid waste in the water to the center and bottom of the container, because the water in the center of the vortex is slower than that on the outside. Once this waste is collected on the bottom, a pipe attached to the bottom of the container can be periodically opened, allowing the solid waste to flush out of the container. The clarified water exits the clarifier at the top, through a large slotted outlet pipe covered with a secondary mesh filter, and flows into the biofilter or into the media beds.
Biofiltration
Biofiltration is the conversion of ammonia and nitrite into nitrate by living bacteria. Most fish waste is not filterable using a mechanical filter because the waste is dissolved directly in the water, and the size of these particles is too small to be mechanically removed. Therefore, in order to process this microscopic waste an aquaponic system uses microscopic bacteria. Biofiltration is essential in aquaponics because ammonia and nitrite are toxic even at low concentrations, while plants need the nitrates to grow. In an aquaponic unit, the biofilter is a deliberately installed component to house a majority of the living bacteria. Furthermore, the dynamic movement of water within a biofilter will break down very fine solids not captured by the clarifier, which further prevents waste build up on plant roots in NFT and DWC. However, some large aquaponic facilities following the design of the system developed at the University of the Virgin Islands do not use a separate biofilter as they mostly rely on the units’ wet surfaces, on plant roots and direct plant uptake to process ammonia. Separate biofiltration is unnecessary in the media bed technique because the grow beds themselves are perfect biofilters.
The biofilter is designed to have a large surface area supplied with oxygenated water. The biofilter is installed between the mechanical filter and the hydroponic containers. The minimum volume of this biofilter container should be one-sixth that of the fish tank.
The biofilter is designed to have a large surface area supplied with oxygenated water. The biofilter is installed between the mechanical filter and the hydroponic containers. The minimum volume of this biofilter container should be one-sixth that of the fish tank.
One commonly used biofilter medium is Bioballs® a proprietary product available from aquaculture supply stores, although similar generic brands exist. These are designed to be an ideal biofilter material, because they are small, specially shaped plastic items that have a very large surface area for their volume (500–700 m2/m3). Other media can be used, including volcanic gravel, plastic bottle caps, nylon shower poufs, netting, polyvinyl chloride (PVC) shavings and nylon scrub pads. Any biofilter needs to have a high ratio of surface area to volume, be inert and be easy to rinse. Bioballs® have almost double the surface area to volume ratio of volcanic gravel, and both have a higher ratio than plastic bottle caps. When using suboptimal biofilter material, it is important to fill the biofilter as much as possible, but even so the surface provided by the media may be not sufficient to ensure adequate biofiltration. It is always better to oversize the biofilter during the initial construction, but secondary biofilters can be added later if necessary. Biofilters occasionally need stirring or agitating to prevent clogging, and occasionally need rinsed if the solid waste has clogged them, creating anoxic zones.
Another required component for the biofilter is aeration. Nitrifying bacteria need adequate access to oxygen in order to oxidize the ammonia. One easy solution is to use an air pump, placing the air stones at the bottom of the container. This ensures that the bacteria have constantly high and stable DO concentrations. Air pumps also help break down any solid or suspended waste not captured by the mechanical separator by agitating and constantly moving the floating Bioballs®. To further trap solids within the biofilter, it is also possible to insert a small cylindrical plastic bucket full of nylon netting (such as Perlon®), sponges or a net bag full of volcanic gravel at the inlet of the biofilter. The waste is trapped by this secondary mechanical filter, allowing the remaining water to flow down through small holes drilled at the bottom of the bucket into the biofilter container. The trapped waste is also subject to mineralization and bacterial degradation.
Another required component for the biofilter is aeration. Nitrifying bacteria need adequate access to oxygen in order to oxidize the ammonia. One easy solution is to use an air pump, placing the air stones at the bottom of the container. This ensures that the bacteria have constantly high and stable DO concentrations. Air pumps also help break down any solid or suspended waste not captured by the mechanical separator by agitating and constantly moving the floating Bioballs®. To further trap solids within the biofilter, it is also possible to insert a small cylindrical plastic bucket full of nylon netting (such as Perlon®), sponges or a net bag full of volcanic gravel at the inlet of the biofilter. The waste is trapped by this secondary mechanical filter, allowing the remaining water to flow down through small holes drilled at the bottom of the bucket into the biofilter container. The trapped waste is also subject to mineralization and bacterial degradation.
Mineralization
Mineralization, in terms of aquaponics, refers to the way that solid wastes are processed and metabolized by bacteria into nutrients for plants. Solid wastes that are trapped by the mechanical filter contain nutrients; although processing these wastes is different from biofiltration and requires separate consideration. Retaining the solids within the overall system will add more nutrients back to the plants. Any waste that remains on the mechanical filters, within the biofilters or in the grow beds is subjected to some mineralization. Leaving the waste in place for longer allows more mineralization; longer residence time of the waste in the filters will lead to more mineralization and more nutrients being retained in the system. However, this same solid waste, if not properly managed and mineralized, will block water flow, consume oxygen and lead to anoxic conditions, which in turn lead to dangerous hydrogen sulphide gas production and denitrification. Some large systems therefore deliberately leave the solid waste within the filters, ensuring adequate water flow and oxygenation, so that a maximum of the nutrients is released. However, this method is impractical for small-scale NFT and DWC systems. If it is decided to deliberately mineralize these solids, there are simple ways to facilitate the bacterial breakdown in a separate container, simply storing these wastes in this separate container with adequate oxygenation using air stones. After an indefinite amount of time, the solid waste will have been consumed, metabolized and transformed by heterotrophic bacteria. At this point, the water can be decanted and re-added to the aquaponic system, and the remaining waste, which has decreased in volume, can be added to the soil. Alternatively, these solid wastes can be separated, removed and added to any in-ground agriculture, garden or compost bin as a valuable fertilizer. However, losing these nutrients can cause deficiencies in the plants which may then require supplementation of nutrient
Using a media bed for a combination of mechanical and biological filtration
It is also possible to use a media-filled bed for mechanical and biofiltration in NFT and DWC units.
This can be important where it is not possible to obtain the materials needed for a swirl separator and/or separate biofilter. It is sufficient to say that for every 200 g of fish feed per day the biofilter needs to be 300 litres in volume. This small gravel would provide adequate biofiltration for about 20 kg of fish. Although this media bed would provide adequate biofiltration for an NFT or DWC unit as well as capturing and retaining solid wastes, an additional solids capture device placed into the bed is sometimes recommended in order to prevent the media bed from clogging with fish solids. The bed will need rinsing periodically to remove solid wastes.
This can be important where it is not possible to obtain the materials needed for a swirl separator and/or separate biofilter. It is sufficient to say that for every 200 g of fish feed per day the biofilter needs to be 300 litres in volume. This small gravel would provide adequate biofiltration for about 20 kg of fish. Although this media bed would provide adequate biofiltration for an NFT or DWC unit as well as capturing and retaining solid wastes, an additional solids capture device placed into the bed is sometimes recommended in order to prevent the media bed from clogging with fish solids. The bed will need rinsing periodically to remove solid wastes.
In summary, some level of filtration is essential to all aquaponics, although fish stocking density and system design determines how much filtration is necessary. Mechanical filters separate solid wastes to prevent toxic build up, and biofiltration converts dissolved nitrogenous wastes into nitrate. The media beds themselves act as both mechanical filters and biofilters when using that technique, but additional mechanical filtration is sometimes necessary for higher fish densities (15 kg/m3 ). Without the media beds, such as in NFT and DWC units, standalone filtration is necessary. Mineralization of solid wastes returns more nutrients to the system. Mineralization occurs in media beds, but within NFT and DWC systems separate apparatus are needed.