Aquacultural Engineering 41 (2009) 114–121

Coldwater RAS in an Arctic charr farm in Northern Norway

Steinar Skybakmoen a,*, Sten Ivar Siikavuopio b, Bjørn-Steinar Sæther b

a OppdrettsTeknologi (Fishfarming Technology), Nordslettveien 177, N-7038 Trondheim, Norwayb Nofima Marin, N-9291 Tromsø, Norway

A R T I C L E I N F O

Keywords:

Coldwater

Recirculating

Water quality

Water quality variations

A B S T R A C T

Use of coldwater recirculating aquaculture systems (RAS) are still very rare in Norway, and only two

farms are producing Arctic charr. This project took place in one of these commercial Arctic charr farms;

Villmarksfisk AS in Bardu, Northern Norway.

The farm gets its make-up water from ground water that holds 5 8C year around. Temperature in the

rearing water varies between 7.5 8C (‘‘low’’) to 12 8C (‘‘high’’) through the year. The biological filter in the

RAS seems to work stable at both ‘‘low’’ and ‘‘high’’ temperatures, including after incidents when feeding

has been stopped for a day and started on top again the day after. Such an extreme change in loading was

measured as a 70% increase in TAN concentration, with only minor changes in nitrite levels recorded. The

biofilter also kept the nitrite stable and low in spite of diurnal variation in TAN excretion at normal

feeding regimes (every day in three periods at day-time).

The farming concept is to stock the farm with wild caught juvenile fish for on-growing to market size

fish (0.75–1.00 kg). A drop in growth rate during early autumn has been a main concern for the farm. This

may reflect a seasonal shift in growth potential, sometimes referred to as ‘‘autumn depression’’.

Interestingly, there is little sign of seasonal changes in the growth of hatchery-produced fish tested in the

farm. Sampling of water quality through the seasons in tanks holding fish undergoing such growth

depressions indicate that TAN excretion is much higher per kg feed used in the wintertime than in the

springtime. This observation corresponds with the lack of weight gain during wintertime despite that the

fish is feeding. Thus, feed conversion calculations indicate that feed utilization also varies with season

reaching its nadir during this period.

Both challenges concerning RAS in cold water and strongly reduced growth in autumn and winter

time, have been investigated from 2005 to 2007 in a project financed by the Norwegian research council

and the partners in the agricultural framework agreement.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Arctic charr (Salvelinus alpinus L.) is the northernmostdistributed freshwater fish and is considered the most cold-adapted species within the salmonid family (Johnson, 1980). Thegrowth of Arctic charr populations is variable under naturalconditions (Johnson, 1980; Sæther et al., 1996). Arctic charrtolerate high-density culture conditions, have an excellent filletyield, are amenable to niche marketing, and are suitable forproduction within super-intensive recirculating systems (Jobling,1987; Johnston, 2002; Summerfelt et al., 2004a,b).

The company Villmarksfisk AS in Northern Norway (Barducommunity in Troms county) operate an on growing (grow out)farm based on wild caught Arctic charr from lake Altevatn situatedapproximately 40 km from the farm (Siikavuopio et al., 2009a). The

* Corresponding author. Tel.: +47 4804 7204.

E-mail address: steinar@oppdrettsteknologi.no (S. Skybakmoen).

0144-8609/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.aquaeng.2009.06.007

farm consists of a fully insulated building with two separatecompartments each of 200 m3 rearing volume (four tanks each50 m3; 45 m3 effective). Each compartment has their ownrecirculation system and both utilize the same source for make-up water; ground water from a borehole close to the building.Effluent water is discharged back to the ground in an infiltrationsystem.

A flow chart for the farm is shown in Figs. 1–3 show a simplifiedsketch of the farm and a picture inside the farm. The fish rearinginstallation and recirculation system were in place in 2002 and inoperation from 2003. First regular year of operation was 2004.

2. Technical installations

The rearing tanks are constructed as octagonal tanks wherefour tanks are put together in a square to share the inner walls. Thewalls are being built of vertical aluminium profiles on a concretebottom. The inside of the tank walls are covered with fibre glasssheets which are glued together in the corners by sheets of

Fig. 1. Process flow chart for Arctic charr farm in Northern Norway. This flow sheet represents one of two identical production systems (Compartment A and B).

S. Skybakmoen et al. / Aquacultural Engineering 41 (2009) 114–121 115

polypropylene with a back-side of polyester. Each tank has aparticle trap at the centre of the tank bottom with a sludgecollector outside the tank that also allows inspection of the sludgeproduction and feed loss (EcoTrap 250 with 70 L sludge collector;main outlet pipe dimension and outlet screen is Ø250 mm reducedto pipe Ø200 just after the trap; particle hose to the sludge collectoris Ø50 mm). The water then passes through a mechanical filterwith 60 mm openings (Hydrotech HDF 1202-1H, 0.37 kW). Fromthe mechanical filter, water flows into a common sump foraeration, trickling biofilter and sump for both pumps lifting waterto the top of the trickling biofilter and for the pumps lifting waterback to the tanks. This sump is made of polyethylene plates weldedtogether by extruder to a rectangular unit with a length of 9 m,

Fig. 2. Simplified sketch of th

water depth of 0.8 m and width of 1.4 m, total water volume of10 m3.

At the bottom of the common sump a number of 60 diffusorsprovide aeration/CO2-removal (Nopon MKP 600 Tube diffuser,Finland). These are fed by a blower (Venture HPB-200-XX-500T,5.5 kW, Sweden). Gas to water ratio is 5:1 and the degassed CO2 isremoved from the compartment through an outer wall ventilationchannel of 0.1 m2 situated at floor level. Two pumps (Grunfos NB80-160/177, 3 kW, Denmark) lift the water to the biofilter,installed for parallel operation and with stationary frequency(constant rpm). The trickling biofilter consist of 84 blocks each0.55 m � 0.55 m � 0.55 m size (Bio-blok 200 from Exponet, Den-mark). There are seven modules of blocks, in three layers and 2 by 2

e farm, Villmarksfisk AS.

Fig. 3. From inside one of the departments.

Fig. 4. Flow chart for the spreadsheet. Make-up water is actually not introduced

direct to the rearing tank, but in the common sump under the biofilter where CO2-

removal takes place.

S. Skybakmoen et al. / Aquacultural Engineering 41 (2009) 114–121116

in each layer. The water is distributed from the top of the biofiltervia seven rotating pipes, one for each module. These pipes turnaround as water hits the top of the biofilter. When both pumps areworking (normal operation), the hydraulic surface load is 472 L/min/m2.

The two recirculation pumps (Grunfos NB 100-200/100, 5.5 kW,Denmark) that lift the water back to the tanks, are installed at farend to the water inlet from the mechanical filter in the commonsump. One of these pumps has a frequency controller, the otherhave stationary frequency. To this time, it has not been necessaryto operate both pumps simultaneously. Before reaching the tanks,the water passes oxygen saturator units, one at each tank (InlineDown Flow Bubble Contactor, AquaOptima, Norway). Water isdistributed into the tanks through vertical pipes (Ø160, located25 cm from the tank wall) via 25 Ø18 mm holes. This provides aunidirectional circular water movement in the tanks enablingsome control of the water velocity and thereby the fish’sswimming speed.

Collected sludge from mechanical filters and sludge collectors isstored during winter time in two insulated under ground fibreglass tanks (35 m3 each). The sludge is utilized as fertiliser/soilimprovement on grass fields during the summer season.

3. Technical experiences

The main water flow through pumps (recirculation pumps) aredimensioned for 1000 L/min per tank. This flow is too high for theparticle trap main outlet screen, creating a very high swirl velocityin the tank surface centre and standing waves in the tank. Normalmain flow has been 350–550 L/min depending on loading. Theparticle trap system restrain only approximately 50% of feed lossand faeces from the fish, however, this is sufficient to get animpression of the appetite of the fish. The main problem with theparticle trap is the fouling inside the trap and in the small hosesbetween the trap and the sludge collector. The mechanical filterplays a key role for controlling organic matter and keeping the BOD

Table 1Day and night measurements series; date and general situation at the time.

Date/period Department Water

temperature (8C)

Feeding

(kg/24 h)

2005 April 20–21a Hall A 10.6 57.6

2006 November 15–16 Hall B 7.5 16.0

2007 September 17–18 Hall B 11.6 70.4

a In 2005, the measurement was done the day after a complete feeding stop. In Nove

periods of 1–2 h duration).

as low as possible, and it is crucial that personnel operating thisattend to keep it clean and effective. To this point the biofilter hasbeen operationally very stable without any clogging problems.

The diffusors at the bottom of the common sump has been inoperation continuously without any cleaning and they still seemsto work properly. Although there is a tendency for higher CO2-levelwhich may be partly explained by clogged diffusors.

4. Biological experiences

The farm’s production is based on wild caught fish from anearby lake. Capture normally takes place from February untilMarch–April, or as long as the ice is strong enough to support a safeoperation. Some catching also takes place by boat during thesummer. Reduced growth in late summer and during most of theautumn and winter has been a major problem. High mortality hasalso been experienced (Siikavuopio et al., 2009b). The most severegrowth problem was experienced in a small scale highly controlledtrial at the farm in 2005–2006. Specific growth rate declineddramatically during the period from July until late October. FromNovember till January the growth increase, but is still lower thanwhat can be expected for hatchery-produced fish (Sæther et al.,1996; Siikavuopio et al., 2009a). It has also been observed fish thatapparently thrives well, based on appetite, still show poor growthrates combined with high mortality.

5. Investigation plan 2005–2007

The recirculating systems at the farm showed good perfor-mance from the very beginning. A concern, however, was thatthere was lack of practical experience on water quality and watertreatment efficiency in such systems with relatively cold water andhigh variation in organic loading from fish with un-known feedingcapacity and excretion. It was also a concern that water qualitycalculations were complicated for people with mainly technicalbackground and no comprehensive skills in water chemistry. Inorder to make some progress in this field, the project investigationplan was set up by a combination of:

Light/darkness

(h)

Make-up

water (L/min)

Tank volume

(m3)

Total flow

through (L/min)

L/D 24/0 133 4 � 45 1800

L/D 24/0 190 3 � 45 1500

L/D 24/0 120 4 � 45 1800

mber 2006 and in September 2007 feeding was regular at day-time (3 or 4 feeding

Table 2Measured pH and alkalinity, both measured and calculated the CO2-content and calculated difference between calculated and measured CO2.

Date Water type Measured

CO2 (mg/L)

Measured

pH

Measured

alkalinity

(mg/L CaCO3)

Calculated

CO2 (mg/L)

Difference between

calculated and

measured CO2

(% of measured value)

2006 November 15 Groundwater 13a 7.48c 180e 11.9f �8.5

2007 August 22 Effluent water 18b 7.29d 155e 15.9f �11.7

2007 September 18 Groundwater 13b 7.42d 165e 12.5f �3.9

2007 September 18 Effluent water 22b 7.06d 130e 22.6f +2.7

a Method: Oxyguard CO2 analyser.b Method: LaMotte carbon dioxide test kit, model PCO-DR, code 7297-DR.c Method: Oxyguard pH Manta.d Method: Hanna instrument pH/ORP Meter HI 98150, sensor HI 1618D.e Method: Merck alkalinity test 1.11109.0001.f Formula: CO2 = alkalinity � 10(6.3�pH).

Fig. 5. Variation in TAN content in treated water through 24 h; from 2007.

Fig. 6. Variation in TAN content in treated water through 24 h; from 2005.

S. Skybakmoen et al. / Aquacultural Engineering 41 (2009) 114–121 117

� Technical system input (as correct as possible by measurementsand registrations).� Water quality measurements (as correct as possible with field

test kits and methods).� Water quality calculations (as correct mass balances as possible

for TAN and CO2 combined with simplified formulas for thecarbonate system).

All relevant information was summarized and controlled bymeans of a spreadsheet based on a flow chart (Fig. 4). The generalidea was to use it as a calculation control system at the same timeas the water quality measurement was done; establish anunderstanding of the importance of measured water qualityparameters. Table 1 shows an overview of the intensive day andnight measurements series performed at the farm. In April 2005,the measurement was done the day after a complete feeding stop.In November 2006 and in September 2007 feeding was regular atday-time (three or four feeding periods of 1–2 h duration).

6. Water quality considerations and calculations

In April 2005 the ground water was evaluated for total gaspressure (oxygen, CO2 and nitrogen). The water had very lowoxygen saturation (47%), high content of CO2 (2200%) and wassupersaturated with nitrogen (110%). The total gas pressure (97%)was close to barometric pressure. In February 2004 oxygensaturation was 57% and pH 7.24. Alkalinity was considered to bestable at 3.5 mmol/L (175 mg/L CaCO3) and CO2 was expected to behigh, around 20 mg/L. Other measurements taken are set up inTable 2. From these measurements it was considered to beacceptable to use the simplified formula CO2 (mg/L) = alkalinity(mg/L CaCO3) � 10(6.3�pH) for (Summerfelt, 1996) further calcula-tions of the relationship between pH, alkalinity and CO2.

7. Ammonium and CO2-excretion

As a starting point, it was assumed 32.5 g TAN excretion per kgfeed (3.25%) for the feed used (Nutra Parr, protein content 50%,Skretting). Further, it was assumed that this excretion is a mediumvalue over 24 h. More conservative estimations say 4.6% in TANexcretion (NRAC 2002), approximately 40% higher than thepresent start point. Our measurements in 2007 shows that theTAN content in the water was 37% higher at night-time ascompared to day-time when feeding took place at day-time. Fig. 5shows the variation in TAN content in treated water. The feedingperiods were 07:00–08:00, 13:00–14:00, 16:00–17:00 and19:00–20:00. From the measurement in 2005 the TAN contentwas 42% higher in the night than the medium level; however, it isimportant to recognise that at that day the feeding had beenstopped the day before. The regular feeding times were also

somewhat different: 06:00–08:00, 12:00–13:00 and 17:00–18:00, see Fig. 6.

In order to examine the TAN excretion, several calculations havebeen done based on measuring TAN, nitrite, nitrate, feeding andmake-up water use. Not surprising, there are periods when TANexcretion is much higher than the theoretical medium value.Reasons for this can be feed loss or that the fish is not able to utilizethe feed as expected. More surprising is the measurement from2007, when the TAN excretion was much below 3.25% (2.74%).However, this can be related to the fact that the measurement wastaken early autumn and that the fish still utilized the feed very well.Later in the autumn, e.g. as in November 2006, the TAN excretionrecording was much higher (8.1%) than the expected value of 3.25%.It is only the measurement in springtime 2005 that indicate a TANexcretion close to the expected medium value of 3.25%.

In order to decide CO2-excretion in a recirculation system,excretion from the bacterial activity in the biofilter should also beincluded. However, in a trickling filter the CO2-excretion in the

Fig. 8. Nitrification rate (TAN-removal rate) related to ammonium content in tank

effluent water at Villmarksfisk AS for 2005, 2006 and 2007-series (dots). The curve

in the diagram is a constructed line for preliminary dimensioning purposes.

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biofilter is likely to be released very quickly. For the purpose ofcalculations, the CO2-excretion is set to 1.3 g/g O2-consumed andthe O2-consumption set to 0.35 kg oxygen per kg feed offered tothe fish.

8. Water quality simulation

The main reason for assessing water quality in a recirculationsystem is to assure that the most important water chemicalparameters stays within acceptable levels, not compromisinganimal welfare (Colt, 2006). Also, it is important to use themeasurements to supervise the functionality of the systemenabling farmers to take correct action if one or more parametershave to be adjusted (Losordo et al., 2000; Losordo and Hobbs, 2000;Summerfelt, 2002; Summerfelt et al., 1993). By means of a simplespreadsheet it is possible to simulate the water quality in therearing tank volume and in the treated water. For the actual farm,treated water is similar to the effluent water (see flow charts,Figs. 1 and 4). The accuracy of this simulation depends onknowledge of the following parameters:

� Main water flow (L/min).� Make-up water flow (L/min).� TAN excretion (% per kg feed, etc.).� Removing efficiency of the water treatment processes.� Make-up water quality (alkalinity, pH and CO2).� Interactions between removing efficiency and water tempera-

ture.� All parameters involved in and influencing the carbonate system.

Examples:

� By increasing main flow through, all excretion product con-centration will be reduced as more water flow, e.g. to the CO2-stripping unit hence more CO2 are being removed. This brings thepH up.� Reduced make-up water will reduce the alkalinity import to the

system, and this will bring the pH down.

9. TAN-removal rate

In a biological filter, bacteria convert ammonia (TAN) to nitrite(NO2) and nitrite is further converted to nitrate (NO3) (Timmonset al., 2002; Eding et al., 2006). The second step work faster than thefirst step and this is convenient because the acceptable limit ofnitrite is much lower than for nitrate (Timmons et al., 2002). Lookingat the TAN conversion separately, the TAN-removal rate isinfluenced strongly by the temperature and the TAN content of

Fig. 7. Nitrification rate (TAN-removal rate) related to water temperature at

Villmarksfisk AS for 2005, 2006 and 2007-series (dots). The lines in the diagram are

a summary from different papers (Bovendeur, 1989; Eikebrokk, 1988; Rusten,

1986; Speece, 1973), see Gebauer et al. (2005) (textbook in Norwegian).

the water. At Villmarksfisk the TAN-removal rate is calculated bymeans of the earlier mentioned spreadsheet made for simulation ofthe systems functionality and water quality. Fig. 7 shows the TAN-removal rate relative to temperature for the three investigationseries performed in 2005, 2006 and 2007. Fig. 8 shows the TAN-removal rate related to the TAN content in the outlet water from thetanks, i.e. before the biofilter. The TAN-removal rate is generally asexpected, except for the series from September 2007, with theremoval rate being less than expected. This may be due to high CO2

levels, but it can also be other reasons not yet examined orunderstood. For further information on biofilter and water qualityfactors affecting nitrification rate, see Chen et al. (2006).

10. Operation and documentation from April 2005

Average water temperature was 10.6 8C and the feeding ratewas approximately 60 kg per 24 h in compartment A where dayand night measurements were performed. The purpose of themeasurements was to examine how the biofilter reacts to variationin TAN excretion over a period of 24 h. In addition thesemeasurement series should also reflect the biofilter reaction tothe feeding stop the day before. The most important parameter in arecirculation system is the level of nitrite. Some have experiencedsudden peaks in nitrite (e.g. in a pilot-scale farm at Villmarksfiskusing submerged biofilter) resulting in instant and high mortality.The commercial scale system operating now does not give thesame surprises (Fig. 9), despite sudden increases in organic load.Over a period of 12 h (3–15 h after start of feeding) the TAN contentin the system increased 200%, whereas the nitrite increased only by12%. The nitrate-producing bacteria in the biofilter had no problemto handle the increased production of nitrite as the nitrate content

Fig. 9. Ammonia content strongly increased in treated water, but not the nitrite

content.

Fig. 10. Approximately 24% increase in nitrate content the first 12 h period, next

12 h period a 15% slow down.Fig. 11. Normal variation in TAN content in treated water, very little variation in

nitrite.

Fig. 12. Weak increase in CO2 level in effluent water from the tanks in the afternoon,

decreasing from midnight.

S. Skybakmoen et al. / Aquacultural Engineering 41 (2009) 114–121 119

increased with 24% over the same 12 h period (Fig. 10). In the nextperiod of 12 h (15–27 h after feeding start), the TAN content stillincreased but at a slower rate (67%); the nitrite content increasedonly 3%, and the nitrate content started to fall (15% reduction).

11. Operation and documentation from November 2006

During this period the farm struggled with very high mortalitydue to saprolegnia (fungal infection). Water temperature wasdropped to 7.5 8C and feeding reduced. Fig. 11 shows the TANvariation during a 24 h period of the measurement series. Feedingtime was 06:00–08:00, 12:00–13:00 and 17:00–18:00. The nitritecontent was low and stable and apparently the biofilter handledthe diurnal variation also at this low temperature. The content ofnitrate in the outlet water from the tanks where on average only 6%higher than in the treated water. This indicate that the nitrification(TAN-removal) mainly occurs in the biofilter and also that there isvery little suspended biofilm in the system (passive nitrification is

Fig. 13. Spreadsheet for water quality simulation; situation from 2007-series. All bold figures are input/assumed values, all other figures are calculated. The assumed values

for removal rates are adjusted in order to make the calculated values correspond with the measured values for TAN, CO2, alkalinity and pH.

Fig. 14. TAN variation day/night in effluent water from the tanks.

Fig. 15. Alkalinity variation day/night.

Fig. 16. pH variation day/night.

Fig. 17. CO2 variation day/night.

Fig. 18. TAN and nitrite variation day/night in treated water.

S. Skybakmoen et al. / Aquacultural Engineering 41 (2009) 114–121120

low). The CO2 content varies over 24 h in the effluent water fromthe tanks (Fig. 12). The same picture appeared also in treatedwater, however, with less variation, indicating difficulties withremoving CO2 when the CO2 content is low.

12. Operation and documentation from September 2007

During the autumn 2007, growth rates were good andmortalities low as compared to the previous year. The fish weretreated with formaldehyde (1:3000) in order to reduce thesaprolegnia spp. infection, and this is likely to have contributedto the improved performance. The previously mentioned simula-tion shows that CO2-level was high and this was attributed toreduced efficiency of the diffusors (Fig. 13). This can affect thenitrification rate in a negative way, which at this point was as low

as 0.3. On the contrary, during spring 2005 the CO2 removal ratewas 0.71 and the nitrification rate was 0.64. Another reason for lownitrification rate can be high content of organic matter (BOD). Thiswas not measured, only calculated, and can therefore be under-estimated. Compartment B operates as the fish entrance area. Thispart of the farm has the most severe problems with fungus andperiodically very low feed utilization. This situation can result inhigh organic load on the biofilter and, thus, reduced nitrificationrate.

Figs. 14–17 show the dynamics of TAN, alkalinity, pH and CO2.The CO2 level was calculated whereas the other parameters aremeasured. Despite some variation, the nitrification process is verystable, as indicated in Fig. 18. The nitrite content was under 0.5 mg/L-N, which is considered to be acceptable.

13. Summary

The biofilters at Villmarksfisk AS are traditional trickling filters;very robust filters that handle large variations in load. However,compared with other filter media trickling filters are sometimesconsidered as old fashioned and too space demanding. Because ofthe very short water retention time in the filter, a trickling filterneeds a very high hydraulic flow through to maintain nitrificationstability. Biomedia that is submerged all the time usually have alonger water retention time (contact time), and for theseapplications it is easier to obtain a lower TAN and nitrite contentthan in trickling filters. There are concerns, however, on howsubmerged biofilters responds to large variability in nutrition load,e.g. following a period with low load. Available literature dealingwith, e.g. moving bed biofilters points to sudden increase in TANload as critical events for these filters to handle (Rusten et al.,2006). The trickling biofilter at Villmarksfisk seems to react fast

S. Skybakmoen et al. / Aquacultural Engineering 41 (2009) 114–121 121

enough at a relative high increase in TAN load, for instance such asrepresented by feeding stop, and the nitrification rate issatisfactory and stable at low temperature.

Acknowledgements

This paper is based upon work supported by the Norwegianresearch council, the partners in the agricultural frameworkagreement and Villmarksfisk AS. We want to thank EspenHaugland at Norwegian Institute for Agricultural and Environ-mental Research (Bioforsk Nord), Tromsø, who acted as projectmanager and his colleges Hallvard Jensen and Christian Uhlig.Special thanks to Dagfinn Lysne and Nils Steien at Villmarksfisk ASfor all assistance and advice during the full duration of the projectperiod.

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