Guide to Recirculation Aquaculture: Chapter 6

Chapter Six: Waste water treatment

Farming fish in a recirculation system where the water is constantly reused does not make the waste from the fish production disappear. Dirt or excretions from the fish still have to end somewhere. The biological processes in the system will to a certain extent reduce the amount of organic compounds, because of simple biological degradation or mineralisation within the system. However, a significant load of organic sludge from the farm will still have to be dealt with.

Waste leaving the recirculation process typically comes from the mechanical filter, where faeces and other organic matters are separated into the sludge outlet of the filter. Cleaning and flushing biofilters also adds to the total discharge volume from the recirculation cycle.

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Figure 6.1 The pathways of sludge and water inside and outside a recirculation system. The higher the rate of recirculation, the lower the amount of water let out from the system (dotted line), and the lower the amount of waste water to be treated.


Different ways to treat waste

Treating the waste leaving the recirculation system can be accomplished in different ways. Quite often a secondary mechanical water treatment is installed in order to concentrate the sludge in the waste water. The sludge fraction will go on to a sludge accumulation facility for sedimentation or further mechanical dewatering, before it is spread on land, typically as fertiliser on agricultural farms. Mechanical dewatering also makes the sludge easier to handle, and minimises the volume whereby disposal or possible fees becomes cheaper. On the downside, mechanical dewatering is associated with higher investment and running costs.

Figure 6.2 Hydrotech belt filter used for dewatering the sludge. 

Source: Hydrotech

The cleaned waste water from the secondary treatment will usually have a high concentration of nitrogen and phosphorous. This so-called overflow or reject-water, can be discharged to the surroundings, river, etc., or it can be returned into the recirculation system. The content of nutrients in this overflow water can be removed by directing it to a plant lagoon, a root zone or seepage system, where phosphorous and nitrogenous compounds are absorbed. The content of nitrogen in the overflow water can also be removed by denitrification. As described in chapter 2, methanol is most commonly used as the carbon source for this anaerobic process. The reason for using denitrification inside the recirculation system is to reduce the amount of nitrate in the process water in order to minimize the need for new water in the system. The reason for using denitrification outside the recirculation system is to reduce the discharge of nitrogen into the environment. As an alternative to the use of methanol, sludge from, for example, mechanical filters can be used as the carbon source. Using sludge requires tight management of the denitrification chamber, and back-washing and cleaning the chamber becomes more difficult. In any case, an efficient denitrification chamber can reduce the nitrogen content in the effluent water to a minimum.

Understanding how fish excrete

Figure 6.3 A plant lagoon placed after a recirculation trout farm in Denmark – before and after overgrowing.

Source: Per Bovbjerg, DTU Aqua.

It is important to notice that fish excrete waste in a different way than other animals such as pigs or cows. Nitrogen is mainly excreted as urine via the gills, while a smaller part is excreted with faeces from the anus. Phosphorous is excreted with the faeces only. The main fraction of the nitrogen is therefore dissolved completely in the water and cannot be removed in the mechanical filter. The removal of faeces in the mechanical filter will catch a smaller part of the nitrogen fixed in the faeces, and to a larger extent the amount of phosphorous. The remaining dissolved nitrogen in the water will be converted in the biofilter mainly to nitrate. In this form nitrogen is readily taken up by plants and can be used as fertilizer in agriculture or simply be removed in plant lagoons or root zone systems.

It is important that faeces from the fish tanks are carried immediately to the mechanical filter without being crushed on the way. The more intact and solid the faeces are, the higher the level of removed solids and other compounds. Figure 6.5 shows the estimated removal of nitrogen, phosphorous and suspended solids (organic matter) in a mechanical filter of 50 micron.

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Figure 6.4 Excretion of Nitrogen (N) and Phosphorus (P) from farmed fish. Note the amount of N excreted as dissolved matter. Source: Environmental Protection Agency, Denmark.

The higher the rate of recirculation the less new water will be used, and the less discharge water will need to be treated. In a growing number of cases, no water at all will return to the surrounding environment such as a nearby river. After a first step waste water treatment, the small amount of water remaining can simply be allowed to seep into the ground in a nearby area. In any case, the total volume discharge water will be significantly lower than that from a traditional fish farming system – see Figure 6.6.


Source: Fisheries Research Station of Baden-Württemberg, Germany.Figure 6.5 Removal of Nitrogen (N), Phosphorous (P) and Suspended Solids (SS) from mechanical filter.
Parameter Race-way Race-way Race-way Self cleaning tank Self cleaning tank Self cleaning tank
40 μ 60 μ 90 μ 40 μ 60 μ 90 μ
Efficiency, % Efficiency, % Efficiency, % Efficiency, % Efficiency, % Efficiency, %
Tot-P 50-75 40-70 35-65 65-84 50-80 45-75
Tot-N 20-25 15-25 10-20 25-32 20-27 15-22
TSS 50-80 45-75 35-70 60-91 55-85 50-80

Combining intensive with extensive production can help deal with waste

Recirculation is an efficient way of reducing the impact from fish farming on the surroundings, but the waste water treatment requires tight management on a daily basis to make the treatment system work efficiently. Combining intensive fish farming, whether recirculation or traditional, with extensive aquaculture systems, such as for example traditional carp culture, can be an easy way to handle biological waste. The nutrients from the intensive system are used as fertilizer in the extensive ponds when the excess water from the intensive farm flows to the carp pond area. Water from the extensive pond area can be reused as process water in the intensive farm. Growth of algae and water plants in the extensive ponds will be eaten by the herbivorous carp, which in the end are harvested and used for consumption. Efficient rearing conditions are obtained in the intensive system and the environmental impact has been accounted for in combination with the extensive pond area.

Source: Danish AquacultureFigure 6.6 Example of discharge from traditional flow-through, semi-recirculation, and full recirculation model farming.
Discharge from different types of fish farms at 1,000 tonnes production per year

Nitrogen discharge


Water consumption


Traditional  flow-through 38,000                      250,000
Semi-recirculation 2,000 10,000
Full recirculation 250 1,500

For the innovative entrepreneur there are several opportunities in this kind of recycled aquaculture. The example of combining different farming systems can be developed further into recreational businesses, where sport fishing for carp or put & take fishing for trout can be part of a larger tourist attraction including hotels, fish restaurants and other facilities.


Figure 6.7 Combined intensive-extensive fish farming systems in Hungary. The number of opportunities seems unlimited.
Source: Lazlo Varadi, Research Institute for Fisheries, Aquaculture and Irrigation (HAKI), Szarvas, Hungary.