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Running a recirculation system

2 months ago

21 min read

Figure 5.1 Water quality and flow in filters and fish tanks should be examined visually and frequently. Water is distributed over the top plate of a traditional trickling filter (degasser) and distributed evenly through the plate holes down through the filter media.

Moving from traditional fish farming to recirculation significantly changes the daily routines and skills necessary for managing the farm. The fish farmer has now become a manager of both fish and water. The task of managing the water and maintaining its quality has become just as important, if not more so, than the job of looking after the fish. The traditional pattern of doing the daily job on a traditional flow-through farm has changed into fine tuning a machine that runs constantly 24 hours a day. Automatic surveillance of the whole system ensures that the farmer has access to information on the farm at all times, and an alarm system will call if there is an emergency.

Routines and procedures

The most important routines and working procedures are listed below. Many more details will occur in practice, but the overall pattern should be clear. It is essential to make a list with all the routines to be checked each day, and also lists for checking at longer intervals.

Daily or weekly:

  • Visually examine the behaviour of the fish

  • Visually examine the water quality (transparency/turbidity)

  • Check hydrodynamics (flow) in tanks

  • Check distribution of feed from feeding machines

  • Remove and register dead fish

  • Flush outlet from tanks if fitted with stand-pipes

  • Wipe off membrane of oxygen probes

  • Registration of actual oxygen concentration in tanks

  • Check water levels in pump sumps

  • Check nozzles spraying on mechanical filters

  • Registration of temperature

  • Make tests of ammonia, nitrite, nitrate, pH

  • Registration of volume of new water used

  • Check pressure in oxygen cones

  • Check NaOH or lime for pH regulation

  • Control that UV-lights are working

  • Register electricity (kWh) used

  • Read information from colleagues on the message board

  • Make sure the alarm system is switched on before leaving the farm.

Weekly or monthly:

  • Clean the biofilters according to the manual

  • Drain condense water from compressor

  • Check water level in buffer tank

  • Check amount of remaining O~2~ in oxygen tank

  • Calibration of pH-meter

  • Calibration of feeders

  • Calibrate O~2~ probes in fish tanks and system

  • Check alarms – make alarm tests

  • Check that emergency oxygen works in all tanks

  • Check all pumps and motors for failure or dissonance

  • Check generators and make a test-start

  • Check that ventilators for trickling filters are running

  • Grease the bearings of mechanical filters

  • Rinse spraybar nozzles on mechanical filters

  • Search for “dead water” in system and take precautions • Check filter sumps - no sludge must be observed.

Figure 5.2 Oxygen generator. Control and service of special installations must be taken care of.

6-12 months:

  • Clean UV sterilizer, change lamps yearly
  • Change oil and oil-filters and air-filter on compressor
  • Check if the cooling towers are clean inside
  • Check if degasser is dirty and clean if necessary
  • Clean biofilter thoroughly if necessary
  • Service the oxygen probes
  • Change spraybar nozzles in mechanical filters
  • Change filter plates in mechanical filters.

Water quality

Managing the recirculation system requires continuous registration and adjusting to reach a perfect environment for the fish cultured. For each parameter concerned there are certain margins for what is biologically acceptable. Throughout the production cycle each section of the farm should if possible be shut down and started up again for a new batch of fish. Changes in production affect the system as a whole, but especially the biofilter is sensitive to dry outs or other alterations. In figure 5.3 the effect on the concentration of nitrogen compounds leaving a newly started biofilter can be observed. Fluctuations will occur for many other parameters of which the most important can be seen in figure 5.4. In some situations parameters may raise to levels which are unfavourable or even toxic to fish. However, it is impossible to give exact data on these levels as the toxicity

Figure 5.3 Fluctuations in the concentration of different nitrogen compounds from start-up of a biofilter.

A Guide to Recirculation Aquaculture

depends on different things, such as fish species, temperature and pH. The fish will most often adapt to the environmental conditions of the system and thus tolerate higher levels of certain parameters, such as carbon dioxide, nitrate or nitrite. Most important is to avoid sudden changes in the physical and chemical parameters of the water.

The toxicity of the nitrite peak can be eliminated by adding salt to the system. A salt concentration in the water of just 0.3 o/oo (ppm) is sufficient to inhibit the the toxicity of nitrite. Suggested levels for different physical and chemical water quality parameters in a recirculation system are shown in figure 5.4.

Parameter Formula Unit Normal Unfavourable level
Temperature °C Depending on species
Oxygen O~2~ % 70-100 < 40 and > 250
Nitrogen N~2~ % saturation 80-100 > 101
Carbon dioxide CO~2~ mg/L 10-15 > 15
Ammonium NH~4~^+^ mg/L 0-2.5 (pH influence) > 2.5
Ammonia NH~3~ mg/L < 0.01 (pH influence) > 0.025
Nitrite NO~2~^-^ mg/L 0-0.5 > 0.5
Nitrate NO~3~^-^ mg/L 100-200 >300
pH 6.5-7.5 < 6.2 and > 8.0
Alkalinity mmol/L 1-5 < 1
Phosphorus PO~4~^3-^ mg/L 1-20
Suspended solids SS mg/L 25 > 100
COD COD mg/L 25-100
BOD BOD mg/L 5-20 > 20
Humus 98-100
Calcium Ca^++^ mg/L 5-50

Figure 5.4 Preferable levels for different physical and chemical water quality parameters in a recirculati on system.

Biofilter maintenance

The biofilter must be working at optimal conditions all times in order to secure a high and stable water quality in the system. The following is an example of procedures for maintenance of the biofilter.


Figure 5.5 Principle drawing of biofilter made of polyethylene (PE) plastic. Normally PE biofilters are placed above ground level fitted with a sludge discharge valve for easy flushing and cleaning. The sludge water is lead to the waste water treatment system outside the aquaculture recirculati on system. The picture on the right reveals the size of a large PE biofilter. Source: AKVA group.

Biofilter maintenance includes:

  • Brush the top plate every second week to avoid bacteria and algae developing and eventually blocking the holes in the perforated top plate

  • Brush and clean the microbubble diff users in the process water pipe from last biofilter chamber to microparticle fi lter every second week

  • Regular monitoring and cleaning schedule

Figure 5.6 The flow pattern in the shown multi chamber PE biofilter goes from left to right and upstream in each chamber. Most of the organic material is removed by heterotrophic bacteria in the first chamber. The consequent low organic load in the latter chambers secures a thin nitrifying biofilm for converting ammonia to nitrate. The last chamber is called a microparticle filter and is designed for removal of very fine particles that have not been removed by the mechanical filter. Source: AKVA group.

The following parameters should be checked regularly:

  • Check the distribution of air bubbles across each of the biofilter chambers. Over time the biofilter will accumulate organic matter, which will impact the distribution of air bubbles and increase the size of the bubbles

  • Check the height between the water surface level in the biofilter and the PE cylinder wall top edge to identify flow changes through the biofilter and microparticle filter

  • Regularly measure the water quality parameters that have most relevance to the biofilter

  • Closely monitor the remaining volume of base or acid used for dosing.

Cleaning and flushing for sludge removal in biofilter

A mix of inorganic material, dislodged biofilms and other organic matter that is difficult to break down by the microorganisms may accumulate below the biofilter. This should be removed by the sludge removal system placed in the chambers.

For sludge removal flush follow the protocol below:

  • Bypass the PE biofilter that is to be cleaned

  • Open outlet discharge valve for few seconds (approx. 10 sec.)

  • If sludge pump is installed: Pump the sludge from PE biofilter and check for a brown coloration in the water

  • Continue this procedure for all biofilters and microparticle filters (and turn off the sludge when finished). Ensure there is no siphoning from the biofilter chambers via the sludge pump. If there is a possibility of losing water this way, shut all the outlet discharge valves.

Simple cleaning of biofilter using air

Twice a week it is recommended to apply a simple cleaning protocol. In this procedure the PE biofilters are cleaned by air.

For simple biofilter clean follow the protocol below:

  • Do not change the flow to the biofilter

  • Open the air cleaning valves on the first PE biofilter

  • Check with that the cleaning blower is ready for operation. Turn this blower on

  • Direct all cleaning air to biofilter 1 for 10-15 minutes. The process water flow through the biofilter will transfer the loosened organic materials to the following chamber

  • Direct all cleaning air to the next PE biofilter for 10-15 minutes. Continue the procedure through to the last biofilter. Exclude the microparticle filter

  • All the loosened organic material finds its way to the microparticle filter.

Microparticle filter cleaning

The regularity of cleaning the microparticle filter depends on the loading on the system. As a guideline it is recommended to clean the microparticle filter every week.

For simple micro-particle filter cleaning follow the protocol below:

  • Stop the flow through the PE biofilters

  • Reduce the water level to 100 mm below the top plate of the microparticle filter using the sludge discharge valve (use the sludge pump if available)

  • Shut the air cleaning valves on all PE biofilter chambers. Open the microparticle filter chamber air cleaning valve

  • Check with the engineer that the cleaning blower is ready for operation. Turn this blower off

  • Direct all cleaning air to the microparticle filter for 30 minutes. This volume of air raises the water level to near the outlet boxes. The foul water should not be allowed to exit the outlet box

  • Following the cleaning discharge the entire microparticle filter volume using the protocol described for the sludge removal flush.

Deep cleaning of biofilter

If the head difference between biofilter and/or microparticle filter chambers is increasing and the normal head difference cannot be re-established by normal cleaning, then a biofilter deep clean procedure is required. Use regular measurements in each biofilter chamber, between the top of the water level and the PE cylinder top edge to identify flow problems through the biofilter and microparticle filter.

Before completing a deep rinse shut the aeration off in the given chamber for two hours before completing the clean. The given chamber will then act like a microparticle filter for this short period collecting extra waste which is to be discharged during the cleaning process. As a guideline it is recommended that all areas of the biofilters are deep cleaned every month.

For deep biofilter filter cleaning follow the protocol below:

  • Stop the flow through the PE biofilters

  • Use heavy aeration for 30 minutes in the filter(s) to be cleaned. Then completely empty the given filter(s) using the protocol described for the sludge removal flush.

Sodium hydroxide (NaOH) cleaning

If severe blocking in biofilter system is identified, complete a sodium hydroxide cleaning. Severe blocking may be identified by continuous problems with head difference between the chambers, signs of uneven aeration across the top of the chamber and/or reduced biofilter performance.

For a sodium hydroxide cleaning follow the protocol below:

  • Empty the filter section

  • Refill with freshwater and a sodium hydroxide solution (NaOH, adjusted to pH 12)

  • Leave this to work for an hour with aeration and then empty the filter again using the protocol described for sludge removal flush.

This treatment should only be necessary if the biofilter has not received maintenance regularly. It will take several days (app. 10-15 days) until the sodium hydroxide cleaned chamber is back at full capacity.

Trouble shooting biofilter problems:

Problem Reason Solution Increased turbidity Too much aeration Lower aeration Reduced flow rate to biofilter Open valve between degasser and biofilter, increase flow Increasing TAN level Too much aeration, reduced nitrification performance due to damage to the biofilm Lower aeration Increasing nitrite & TAN levels Too high organic loading Make sure feeding does not exceed system specs. Check mechanical filter function. Decreasing nitrate level Anaerobic activity Increase aeration, clean biofilter Hydrogen sulphide (H2S) production (smell rotten egg when cleaning) Anaerobic activity Increase aeration, clean biofilter Increasing alkalinity Anaerobic activity Increase aeration, clean biofilter Reduced flow to biofilter Closed inlet valves partly Open valve between degasser and biofilter, increase flow blocking of biofilter, insufficient cleaning of the biofilter Clean biofilter according to schedule & production specific demands Reduced or no aeration Blower failure Check blower, intake air filter, fuse and power

Figure 5.7 Table of problems with reasons and possible solutions.


Water that is under aeration has a lower density than normal water making swimming impossible!

An operator should only walk on the biofilter top plates whilst wearing a safety harness! Correct footwear must be worn, and care must be taken on the extremely slippery surface!

Follow all instructions with regards to safety procedures for the use of tools, chemicals, machines or any other!

Oxygen control

Dissolved oxygen (DO) is one of the most important parameters in fish farming, and it is important to understand the relationship between % saturation and mg/l. When water is saturated with air it has a DO of 100% saturation. A correct monitoring of the oxygen levels on the farm is vital for the overall performance of the fish.

The oxygen content in milligram oxygen per litre of water depends on the temperature and barometric pressure. At a barometric pressure of 1 013 mbar 100% saturation equals 14.6 mg/l at 0°C, but only 6.4 mg/l at 40°C. This means that in cold water there is much more oxygen available for the fish to consume than in warm water. Thus farming fish in warm water requires even more intense oxygen monitoring and control than farming in cold water.

Figure 5.8: Concentration in mg/l at 100% saturation of dissolved oxygen (DO) in fresh water. The concentration is higher in cold water than in warm water.

Dissolved oxygen in fresh water mm Hg 700 710 720 730 740 750 760 770 780 790 800 mbar 933 946 960 973 986 1000 1013 1026 1040 1053 1066 Temperature °C °F 0 32 13.4 13.6 13.8 14.0 14.2 14.4 14.6 14.8 15.0 15.2 15.4 5 41 11.8 11.9 12.1 12.3 12.4 12.6 12.8 12.9 13.1 13.3 13.4 10 50 10.4 10.5 10.7 10.8 11.0 11.1 11.3 11.4 11.6 11.7 11.9 15 59 9.3 9.4 9.5 9.7 9.8 9.9 10.1 10.2 10.3 10.5 10.6 20 68 8.4 8.5 8.6 8.7 8.8 9.0 9.1 9.2 9.3 9.4 9.6 25 77 7.6 7.7 7.8 7.9 8.0 8.1 8.2 8.4 8.5 8.6 8.7 30 86 6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 35 95 6.4 6.5 6.6 6.7 6.8 6.8 6.9 7.0 7.1 7.2 7.3 40 104 5.9 6.0 6.1 6.2 6.2 6.3 6.4 6.5 6.6 6.7 6.7

Figure 5.9 Dissolved oxygen in fresh water in mg/l at 100% oxygen saturation.

There is also a difference of the availability of dissolved oxygen in fresh water versus saltwater. In fresh water the availability of oxygen is higher than in saltwater (see figures 5.9 and 5.10).

Dissolved oxygen in saltwater Salinity parts per thousand 0 10 20 30 40 Temperature °C °F 0 32 14.6 13.6 12.7 11.9 11.1 5 41 12.8 11.9 11.2 10.5 9.8 10 50 11.3 10.6 9.9 9.3 8.7 15 59 10.1 9.5 8.9 8.4 7.9 20 68 9.1 8.6 8.1 7.6 7.2 25 77 8.2 7.8 7.4 7.0 6.6 30 86 7.5 7.1 6.8 6.4 6.1 35 95 6.9 6.6 6.2 5.9 5.6 40 104 6.4 6.1 5.8 5.5 5.2

Figure 5.10 Dissolved oxygen in saltwater in mg/l at 100% oxygen saturation.

Modern equipment has sensors for temperature and barometric pressure to give you correct values at all times. If you are measuring oxygen in saltwater, simply write in the level of salinity in the menu of the oxygen meter and the meter will automatically adjust accordingly.

This means that calibration of for example a hand held oxygen meter is quite simple.

Turn on the Polaris. It should show 100.5%. Small variations from this can be due to changes in humidity or in the actual oxygen concentration of the air. If calibration is needed and wiping the membrane does not help select “Calibrate” and press “OK” to start. Progress is shown on the display. When “Calibration done” is shown press “OK”. If calibration is blocked and an error message appears you can either choose “Field” calibration precision or force a calibration by holding “OK” depressed when “Calibrate – Please wait” is shown. The result will not necessarily be precise – “Calibrate”

Figure 5.11 Handy Polaris oxygen meter for measuring oxygen content of the water in mg/l and % saturation.

Source: Oxyguard International.

will blink in the display when making measurements. Re-calibrate under more stable conditions when possible.

Set the salinity by using the arrow buttons, “OK” and “Esc” to set the salinity to that of the water you measure in. Then both mg/l and % sat measurements are correct.

To measure, turn the Polaris on and immerse the probe in the water. In still water move the probe, 5-10 cm/sec is enough. After use rinse the probe in clean water and wipe the meter dry if it is wet. If an error occurs then “Error”, “Warning” or “Calibrate” will blink in the display. More information is shown in the status list – see “Status List”.

Polaris will block calibration if conditions are unsuitable – an error message will be displayed. Changing or low temperature can, for example, make it difficult to calibrate outdoors. The sensitivity of the automatic check can be changed – see “Calibration Precision”.

Accurate measurements need accurate calibration, which in turn needs stable conditions. Polaris checks and only permits calibration if conditions are stable.

The sensitivity of this check can be changed – see “Calibration Precision”.

When not in use store Polaris in its pouch in a place where the temperature is moderate and stable. It will then be easy to check calibration and, if needed, re-calibrate with the probe in the pouch in the same place before taking Polaris into use.

Note that if “Renovate Probe” flashes in the display the probe must be renovated.

Education and training

Management of the fish farm is just as important as having the right technology installed. Without properly educated and trained people the efficiency of the farm will never become satisfactory. Fish farming in general requires a wide range of competencies from broodstock and hatchery management, weaning and nursing of fish larvae, fry and fingerling production to grow-out of market size fish.

Training and education is available in many forms from practical hands-on courses to academic studies at universities. A combination of theory and practice is the best combination to gain an all-round understanding of how to run a recirculation aquaculture system.

The following is a listing of the areas that should be considered when building up an educational program.

Basic water chemistry

Understanding the basic chemical and physical water parameters important for the farm operation, such as ammonium, ammonia, nitrite, nitrate, pH, alkalinity, phosphorus, iron, oxygen, carbon dioxide and salinity.

System technology and management in general

Understanding different system designs, primary and secondary water flows. Production planning, feeding regimes, feed conversion rate, specific growth rate relations, registration and calculations of fish size, numbers and biomass.

Knowledge of emergency installations and emergency procedures.


Understanding fish feed compositions, feeding calculations and distribution, water consumption levels and sources, electricity and oxygen consumptions, pH adjustments by the use of sodium hydroxide and lime.

Parameter readings and calibration

Understanding readings from sensors of oxygen, carbon dioxide, pH, temperature, salinity, pressure, etc. Ability to test and calculate levels of ammonia, nitrite, nitrate, TAN and understanding the nitrogen cycle. Calibration of devices for measuring oxygen, pH, temperature, carbon dioxide, salinity, waterflow, etc. PLC and PC settings for alarms, emergency levels, etc.

Machinery and technical installations

Understanding the mechanics and maintenance required for the system, such as for the mechanical filter, the biofilter system including fixed bed and moving bed, degassers, trickling filters and denitrification filters. Operational knowledge of UV systems, pumps, compressors, temperature control, heating, cooling, ventilation, oxygen injection systems, emergency oxygen systems, oxygen generator and oxygen back-up systems, pH regulation systems, pump frequency converter systems, electrical generator systems, PLC and PC systems, automatic feeding systems.

Operational knowledge

Practical knowledge from working on a fish farm including handling of broodstock, eggs, fish larvae, fry and fingerling and grow-out of larger fish for market. Hands-on experience from fish handling, grading, vaccination, counting and weighing, mortality handling, production planning and other daily work at farm level. Understanding the importance of biosecurity precautions, hygiene, fish welfare, fish diseases and correct treatment.

Management support

When starting a recirculation system there are many things to attend to and it can be difficult to prioritize and focus on the right items. To have the system up and running at optimal level and at full production is most often extremely challenging.

Supervision or management support of the day-day production conducted by a professional end experienced fish farmer can be a way to overcome the starting phase and to avoid mismanagement. Also continuous education and training on site of the farm personnel can be a part of the support.

The fish farmer should build a team of skilled personnel to run the fish farm 24 hours a day 7 days a week. The team members will most often work in shifts to account for night watch and work on weekends and holidays.

Personnel in the team should consist of:

  • One manager with overall responsibility for the day-to-day practical management on the fish farm

  • Assistants referring to the manager with responsibility for practical work on the farm with special emphasis on the husbandry of the fish

  • One or more technicians with responsibility for maintenance and repair of technical installations

  • Other workers for miscellaneous work will most often have to be hired.

It is important to make sure that the team actually has the time available to undergo training on site in order to optimize their skills. Quite often training is neglected because the daily work has higher priority and there seems to be no time at all for learning. This is however not the right way to build a new business. Any chance of increasing knowledge and working in a more efficient and professional way should have the highest priority.

Service and repair

A service and maintenance program should be made for the recirculation system to ensure that all parts are working at all times. In the beginning of this chapter routines have been listed and care should be taken on how to solve any malfunctions. It is recommended to make service agreements with suppliers of different equipment to have professional service at hand and at regular intervals.

It is also important to secure efficient sparepart deliveries together with the service regimes. A complete sparepart package for the most important items together with redundancy machinery such as water pumps and blowers should be stored at the farm for immediate use.

Source: Food and Agriculture Organization of the United Nations, 2015, Jacob Bregnballe, A Guide to Recirculation Aquaculture, Reproduced with permission.

Food and Agriculture Organization of the United Nations