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The following decisions need to be made during the design stage:
Is a separate solid removal step necessary? In systems with a low fish stocking rate, a media growing bed can remove solids and act as a biofilter. However, over time, clogging and anaerobic areas will occur as the amount of solids increases.
What is the appropriate device for solids removal? Waste particles in the water can be of different sizes, which affects the technologies used to remove them. Systems with a lower stocking density (<10 kg/m3) may be able to use devices based on sedimentation for particle removal, while systems with a higher stocking density (>10 kg/m3) may need rotational drum filters (Figure 7).
How should the fish tank be connected to the solids removal device? The water should always flow by gravity from the fish tank to the solids separator and not be pumped, since the latter will only decrease the particle size and make it more difficult to remove. To avoid sedimentation the flow velocity in the pipe should be between 0.7 to 1.0 m/s.
What to do with the sludge? Fish sludge is rich in nutrients that can be reused as fertilizer. There are several alternatives to dumping it into the sewage system, including the following:
- storing and re-using it in traditional gardening and agriculture; however, this may be prohibited by law - co-composting with structurally rich green waste (tree cuttings, straw) - vermicomposting (composting process using various species of earthworm). - anaerobic digestion and reintroduction of digestate into the aquaponic system ([Goddek *et* *al.* 2016](https://www.mdpi.com/2073-4395/6/2/37/htm)). - Denitrification to shift the N:P ratio in the aquaponic system in order to reduce P limitation.
Most low-tech systems use gravitational sedimentation for the removal of particles. Filters in this category are: vortex filter, lamella separator, and radial flow separator (Figure 8). The low-tech sedimentation filters can normally only cope with particles of a size larger than 100 µm. However, due to the high flow and active mixing of the water column, the majority of particles in most modern intensive RAS will be smaller than 100 µm. Therefore, using sedimentation filters only is not an optimal solution for intensive RAS.
Figure 8: Diagram of a radial flow separator (adapted after www.garydonaldson.net)
Most modern and intensive RAS use microscreens, often applied as rotational drum filters for solids filtration (Figure 9). These drum filters work in the following way: water enters the drum filter and filters through the microscreens (usually with a filter cloth of 40-100 µm), solid particles are held back and then washed from the filter elements into the sludge tray, and the sludge water then leaves the fish system and enters the waste water treatment facility.
Figure 9: Diagram of a drum filter (www.nordicwater.com)
In addition to the drum filters, foam fractionators (also called protein skimmers) (Figure 10) are often used. These are mainly used to remove organic compounds such as proteins but they have also been reported to reduce a wide variety of other organic and inorganic molecules (e.g. fatty acids, detritus, bacteria, metals). Foam fractionators are mainly used in marine water, as their efficiency is very low in freshwater
Figure 10: Diagram of a foam fractionator (www.epd.gov.hk)
Table 5: Characteristics of different solids filtration systems
Sedimentation FilterDrum FilterFoam fractionatorPrincipleDensity (gravity)Filtration (size)Flotation (polarity/density)Size>100 µm>30-100 µm<30 µmPressure drop1Insignificant20 cmInsignificant
1 A pressure drop occurs when frictional forces, caused by the resistance to flow, act on a fluid as it flows through the tube. See the exercise in Module 2 – Aquaculture.
Figure 11. Different solids removal devices: (left) sludge trap; (centre) roughing filter; (right) rotational drumfilter at ZHAW (all photos by U.Strniša)
Figure 12: Sludge storage tank (left) (photo: U.Strniša) and compost (right) (photo: pixabay)
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