Best Available Techniques for Reduction and Reuse of Emissions in Nordic Land-based Aquaculture

Callum Howard, Dr. Steven Prescott, Kristoffer Spigseth,
Dr. Ragnhild Inderberg Vestrum, Svein Martinsen, Iselin Evje, Dr. Adrian Love,
Finn Skjennum, Davide Sorella, Tamás Eisenbeck, Dr. Adrian Hartley, Freya Robinson

Contents

This publication is also available online in a web-accessible version at https://pub.norden.org/temanord2023-514.

Executive summary

This report describes methods for the reuse and reduction of waste streams from land-based aquaculture systems. It builds on a previous report describing BATs for Nordic aquaculture (Heldbo et al., 2013). Table 1 shows how the state of the art has changed in the intervening decade, highlighting not only technological improvements but also an increase in attention to the reuse of waste, as Nordic countries have become increasingly environmentally conscious. Land-based production is increasingly seen as an opportunity to reduce environmental damage compared to open-water alternatives. Escapes and interactions with wild populations can be avoided and the release of chemical therapeutants, such as antibiotics and anti-parasitics, can be reduced. If water is recirculated, its use can drop dramatically, which may increase energy demand, the environmental impacts of which might be mitigated through renewable sources. Wastes from land-based systems, rather than being discharged into the environment, can be contained and either processed to reduce harm or potentially valorised, turning the wastes into by-products.

The European Union has no dedicated legal framework for the regulation of aquaculture, with relevant legislation being spread across various areas of policy. Environmental legislation such as the Water Framework Directive and the Waste Framework Directive are relevant but only indirectly, and aquaculture does not fall within the scope of the newly revised Industrial Emissions Directive. The Faroe Islands, Norway, and Iceland are not Member States but implement some of EU legislations, particularly those relating to food safety and the use of chemical therapeutants. As the regulation of wastes and emissions from aquaculture is mostly through the legislation of individual nations, it differs across Nordic counties.

The major waste streams (including emissions that may become wastes or by-products) from land-based aquaculture are sludge, discharge water, mortalities, and gaseous emissions. Other potential waste streams common to many industries (e.g., plastics, metals, and wood) are also produced but are less specific to aquaculture and their reuse is covered in a large number of reports.

Sludge consists of faeces and uneaten feed. It can contain a large amount of water, or it can be thickened, dewatered, and dried. Sludge with various levels of moisture can be reused in a variety of manners. Uncontrolled discharge of sludge can cause environmental damage, such as contamination with antibiotics, heavy metals, and solids, and eutrophication due to its nitrogen and phosphorous content. These nutrients allow the sludge to be reused as fertiliser, although care must be taken to ensure that the bioavailability of these nutrients remains and that heavy metals are not in too high a concentration. Drying the sludge before use as a fertiliser reduces transport costs significantly, whilst it is also possible to desalt the sludge. Biogas production in a third-party biogas plant is another option, and drying reduces transport costs and gate fees but increases the capital expenditure (CAPEX) and operating expenditure (OPEX) of farms if conducted onsite. Biogas can be produced onsite and used to supply energy to the farm. Recent advancements in microbial culture selection allow for 100% fish sludge to be used without the addition of exogenous materials. Sludge can also be used to power parts of the farm through pyrolysis. Burning the sludge in the absence of oxygen creates large amounts of heat energy and turns the sludge into biochar, a charcoal-like substance, which, rather than being used as a fuel, can be used as a soil enhancer and filter material. Bio-oil and biogas are other potential products, but biochar is likely the most economically viable by-product. Dried sludge can be incinerated to produce electricity at third-party incineration plants or replace coal during the production of cement. The EU does not allow sludge to be fed to insects, but it is being explored experimentally, and insects could be fed to fish of another species or ruminants. Valorisation of saline sludge poses a larger issue than freshwater sludge Thus, fertilisation of halophyte crops is being explored, with a biogas plant with specially selected microbes is in the pilot stage. Pyrolysis is possible, as is incineration, whilst washing the sludge or using desalting technology allows sludge to be reused in the same manner as freshwater sludge.

Discharge water contains nutrients that can lead to eutrophication but also facilitate its reuse. Fertigation is the watering of crops with a fertiliser solution. It is more efficient than traditional irrigation and fertilisation, wasting less water and causing less nutrient run-off. The nutrients in aquaculture discharge water have proven useful in the cultivation of crops including cucumbers and tomatoes, but this reuse requires fish farms to be in proximity to a relevant agricultural user. Many farms in the Nordics only use fertiliser in certain periods of the year and the same is true for watering. Greenhouse-based agriculture is the most likely candidate for fertigation using aquaculture water. The nutrients in the water can also be used to grow algae. Algae have been used in the treatment of municipal wastewater. Depending on the species grown, algae can be used to produce high-value fatty acids, fertilisers, silicon, or as a bioenergy source. The economics of growing algae in this manner in the Nordics have not been proven. Large surface areas are needed to process discharge water in this manner, and efficient growth in winter typically requires additions of heat and light. Constructed wetlands are often used in Denmark, and if space is available these can present a simple method of discharge water treatment. Discharge water and nutrients can contain heat and physical energy, and capturing this energy can reduce costs on farms.

Almost every farm site we surveyed currently ensiles mortalities, which involves mixing mortalities in an acid solution to prevent bacterial activity. This process can be environmentally and economically expensive and poses a health hazard to workers, although the method is simple and well established. Ensiled mortalities can be processed into protein isolates and fish oils, but the market for these is limited, in part due to EU legislation. The fat content of the mortalities makes bioenergy production a reuse option; the mortalities can be transported to a biogas plant or turned into biodiesel. This option was until recently used in the Åland Islands to power buses in the capital city. The oil content of the mortalities can also be used to replace hydrocarbon oils in the leather industry, and purified fish oil can fetch a high price. Recent advancements in technology present the possibility of drying mortalities. As with sludge, drying decreases the transport costs significantly, sanitises the mortalities, and opens up other valorisation options such as use in cement production. Gate fees at biogas plants are reduced with dried mortalities. One possible avenue is pet food. This is not currently allowed due to EU regulations, but dried mortalities have proven to be stable, free from bacterial concerns, and nutritionally compatible. Mortalities can be fed to fur animals or animals in a shelter, but pet food regulations are stricter.

Gas emissions from land-based aquaculture are much less regulated or documented than the above-mentioned waste streams. Much of the greenhouse gas (GHG) emissions associated with salmonid aquaculture stem from the agricultural production of crops, and to a lesser extent, wild capture of fish, used to make fish feeds. The generation of electricity used to pump or heat water is a further source of GHG emissions, and increasing efficiency or using green energy sources can reduce this. Carbon dioxide (CO₂) is released by fish respiration, and whilst the amount is not comparable to heavy industry, reducing GHG emissions in any capacity is a noble goal. Two companies have been experimenting with capturing CO₂ from fish farms and using microbes to convert this into protein. This may reduce the reliance on exogenous food sources and associated GHG emissions. Hydrogen is an input into these systems and green energy must be used to produce the hydrogen; otherwise, this capture presents no environmental benefit. Nitrous oxide (N₂O) is another GHG that can be released from a fish farm, and an intermediate product in denitrification, N₂O release from land-based aquaculture is little studied or considered. Advancements in denitrification reactors that favour alternative chemical pathways of denitrification may help reduce this problem, but these reactors are not currently viable for use on a fish farm.

This report focuses on the reuse of waste streams, but it is easier and cheaper to reduce the streams rather than attempt to reuse or recycle them. Simple steps such as appropriate planning, recordkeeping, site selection, equipment choice, and farm practices can reduce the production of all forms of waste, and these should be considered first and foremost.

Table 1. Potential Best Available Techniques as featured in the 2013 and present report.

Sammendrag 

Denne rapporten beskriver ulike metoder for gjenbruk og reduksjon av avfallsstrømmer fra landbaserte akvakultursystemer. Rapporten bygger på en tidligere rapport som beskriver BAT for nordisk akvakultur (Heldbo et al., 2013). Tabell 1 viser hvordan det vi anser som best, har endret seg i løpet av tiåret som har gått siden den første rapporten. Det har ikke bare vært ved teknologiske forbedringer, det har også vært en økning i oppmerksomheten rundt gjenbruk av avfall etter hvert som de nordiske landene har blitt stadig mer miljøbevisste. Landbasert akvakulturproduksjon blir ansett for å være bedre for miljøet enn produksjon i sjø for enkelte miljøproblemer. Man kan unngå rømming og interaksjon med ville fiskebestander, og utslipp av kjemikalier som f.eks. antibiotika lusemidler og andre legemidler til miljøet kan reduseres. Ved resirkulering av vann vil vannbruken reduseres betydelig, men fordelene nevnt ovenfor kan føre til økt energibehov. Bruk av fornybar energi kan redusere påvirkningen på miljøet. I stedet for å slippe avfall ut i miljøet, kan man fra landbaserte anlegg samle det opp og prosessere det for å redusere effekten på miljøet, eller også bruke avfallet som nyttige produkter eller råstoff. Landbaserte oppdrettsanlegg vil kunne kreve varige fysiske inngrep, og dersom de eksempelvis legges i strandsonen kan det hindre allmennhetens tilgang. Slike anlegg kan også være avhengig av tilgang til ferskvannsressurser og da i noen tilfeller måtte konkurrere med annen vannressursbruk.

I stedet for å ha et eget lovverk for å regulere akvakulturindustrien i EU, er lovverket spredt på lover og regler som regulerer hver sine områder. Ulike typer miljølovgivning som for eksempel Water Framework Directive og Waste Framework Directive (rammedirektiver for henholdsvis vann og avfall) kan være relevante, men kun på en indirekte måte. Akvakultur blir ikke regulert av det nylig reviderte direktivet for industrielle utslipp (Industrial Emissions Directive). Verken Færøyene, Norge eller Island er medlemmer av EU, men noe av EU-lovgivningen, spesielt det som gjelder mattrygghet og bruk av kjemiske legemidler, implementeres også i disse landene. Ettersom utslipp fra akvakulturindustrien for det meste reguleres gjennom lover og regler i de individuelle landene, varierer dette mye mellom de nordiske landene.

De største utslippsstrømmene (inkludert utslipp som blir avfall eller biprodukt) fra landbasert akvakultur, er slam, utløpsvann, dødfisk og gassutslipp. Andre potensielle utslippsstrømmer som er vanlige i andre typer industri (f.eks. plast-, metall-, og treindustri), produseres også, men dette er ikke spesifikt for akvakulturindustrien, og gjenbruk av disse dekkes i et stort antall rapporter.

Slam består av fiskeavføring og fôrpartikler (fôrspill og ufordøyd fôr), det kan inneholde store mengder vann, men det kan også fortykkes, avvannes, og tørkes. Slam med forskjellig mengde vanninnhold kan brukes til ulike ting. Ukontrollert utslipp av slam kan føre til skade på miljøet, som f.eks. forurensing og eutrofiering på grunn av innholdet av nitrogen og fosfor i slammet. På grunn av at slammet inneholder disse næringsstoffene, er det mulig å bruke det som gjødsel, men man må sikre at biotilgjengeligheten til næringsstoffene ikke er forringet, og at slammet ikke inneholder høye konsentrasjoner av tungmetaller. Ved å tørke slammet før det brukes som gjødsel, vil man oppnå en betydelig reduksjon av transportkostnadene. Et annet mulig bruksområde for slammet er i biogassproduksjon. Dette gjøres som regel av en tredjepartsaktør, og tørking vil også her redusere transportkostnadene og avhendingsavgiftene, men øke CAPEX og OPEX til oppdrettsanlegget dersom biogassproduksjonen gjøres på stedet i stedet for hos en annen aktør. Biogass kan også produseres direkte på oppdrettsanlegget og brukes direkte som energi på anlegget. Nyere utvikling innen mikrobiell seleksjon/økologi gjør at man nå kan bruke 100 % fiskeslam i biogassproduksjon, uten at man må tilsette andre materialer. Slam kan også brukes til å drive deler av anlegget ved bruk av pyrolyse. Ved å brenne slam uten oksygen tilstede produseres store mengder varmeenergi som omdanner slammet til biokull – et materiale som minner om vanlig kull, men som i stedet for å bli brukt som en energikilde, kan brukes i jordforbedring og som et filtermateriale. Bioolje og biogass er andre mulige produkter, men biokull er mest sannsynlig det biproduktet som er økonomisk mest levedyktig. Tørket slam kan også brennes i forbrenningsanlegg for å produsere elektrisitet, og det kan dessuten brukes i stedet for kull i produksjon av sement. I EU er det ikke tillatt å bruke slam som fôr til insekter, men dette undersøkes nå videre, og insekter kan brukes som fôr til andre fiskearter eller til drøvtyggere. Marint slam inneholder salt og er mer problematisk enn ferskvannsslam. Gjødsel i produksjon av halofytter (saltelskende planter) blir nå undersøkt, og et biogassanlegg med spesielt utvalgte mikrober er i pilotfasen. Man kan bruke pyrolyse til forbrenning av marint slam. Vasking av slammet eller bruk av teknologi for avsalting av slammet, gjør at slammet kan brukes på samme måte som ferskvannsslam.

Avløpsvann fra oppdrett inneholder også næringsstoffer som kan føre til eutrofiering, men de samme næringsstoffene er det som gjør at gjenbruk er mulig. En mer effektiv måte å vanne og gjødsle avlinger på enn den tradisjonelle måten, er å vanne med en løsning av vann og gjødsel (dette kalles fertigation). Dette fører til redusert vannforbruk og redusert avrenning av næringsstoff. Næringsstoffene i avløpsvann fra akvakulturproduksjon har blitt brukt til å dyrke bl.a. agurk og tomat, men denne typen gjenbruk fordrer at oppdrettsanlegget ligger nært planteprodusenten. Mange gårder i de nordiske landene gjødsler og/eller vanner avlingene sine kun i enkelte perioder i løpet av året. Landbruk i drivhus som har produksjon året gjennom, er derfor den mest sannsynlige kandidaten til å bruke denne løsningen. Næringsstoffene i vannet kan også brukes til dyrking av alger. Alger har blitt brukt i behandling av kommunalt avløpsvann. Enkelte arter kan også brukes for produksjon av fettsyrer, de kan brukes som gjødsel, som en kilde til bioenergi, eller de kan produsere silikon som kan brukes industrielt. For å dyrke alger basert på avløpsvann må man ha områder med stort overflateareal, og effektiv dyrking i vintersesongen krever gjerne tilførsel av varme og lys. Denne typen algeproduksjon har ikke blitt bevist økonomisk levedyktig i de nordiske landene. I Danmark brukes ofte våtmarker, noe som kan være en enkel måte å kvitte seg med avløpsvannet på, dersom man har nok tilgjengelig areal. Både avløpsvannet og næringsstoffene kan inneholde varme og fysisk energi, og hvis man fanger denne energien kan man redusere kostnadene på oppdrettsanlegget.

Så å si hvert eneste oppdrettsanlegg ensilerer dødfisken. Det betyr at dødfisken blandes i en syreløsning for å hindre bakteriell aktivitet. Denne prosessen kan være en påkjenning både for økonomien og miljøet, og den utgjør også en helserisiko for de ansatte på anleggene. Metoden er imidlertid enkel og godt etablert i næringen. Ensilasje kan bli videre prosessert til protein og fiskeolje, men markedet for disse produktene er begrenset, delvis pga. lovgivning i EU. På grunn av høyt fettinnhold kan produksjon av bioenergi fra dødfisk være en mulig gjenbruksmetode. Dødfisken kan transporteres til et biogassanlegg eller gjøres om til biodiesel. Denne metoden brukes på Åland for å drive busser i hovedstaden. Oljeinnholdet i dødfisken kan også brukes i stedet for olje basert på hydrokarboner i lærindustrien, og renset fiskeolje kan selges for en høy pris. Nyere teknologiutvikling har sett på muligheten for å tørke dødfisken. Dette vil redusere transportkostnadene betydelig, det hygieniserer dødfisken og åpner opp for andre gjenbruksmuligheter. Avhendingsavgiftene ved biogassanlegg vil også bli redusert hvis dødfisken er tørket på forhånd, og det kan brukes i sementproduksjon. En annen mulighet er å bruke den som fôr til kjæledyr. Dette er per nå ikke tillatt i EU, men det er påvist at tørket dødfisk er stabil, uten bakterielle problemer, og næringsmessig tilfredsstillende. Dødfisk kan brukes som fôr til pelsdyr eller dyr i dyrehjem, men reguleringen av fôr til kjæledyr er strengere.

Gassutslipp fra landbasert akvakultur er mye mindre regulert eller målt enn andre utslipp. Mye av utslippene av drivhusgasser relatert til produksjon av laksefisk, stammer fra landbruksaktivitet som skjer i forbindelse med produksjon av fiskefôr. Mye mindre kommer fra fangst av villfisk for bruk i fiskefôr. Disse utslippene er like uansett om produksjonen foregår på land eller i sjø. Produksjon av elektrisitet som trengs for å pumpe eller varme vann, er også en kilde til utslipp av drivhusgasser, og dette kan reduseres ved å øke effektiviteten eller gå over til fornybar energi. CO₂ slippes ut som følge av fiskens respirasjon, og til tross for at mengden ikke kan sammenlignes med mengden som produseres i annen industri, er all reduksjon av utslipp av drivhusgasser positiv. To bedrifter har begynt å eksperimentere med CO₂ fra oppdrettsanlegg, og de bruker bakterier for å omdanne gassen til protein, som igjen kan brukes i fiskefôr. Målet er at utslipp av drivhusgasser i forbindelse med ekstern fiskefôrproduksjon da kan reduseres. Men hydrogen er også en innsatsfaktor i denne løsningen, og man må bruke energi for å produsere hydrogenet. Denne karbonfangsten utgjør derfor ingen gevinst for miljøet per i dag. N₂O er en annen drivhusgass som kan slippes ut fra oppdrettsanlegg. Gassen er et mellomprodukt idenitrifikasjonsprosessen, og utslipp av denne gassen er lite studert og hensynstatt når man snakker om utslipp fra landbasert oppdrett. Utvikling av bedre reaktorer for denitrifisering som favoriserer alternative kjemiske spor i denitrifiseringen, kan redusere utslippet av denne gassen, men slike reaktorer er per nå ikke tilgjengelig for bruk i oppdrettsnæringen.

Denne rapporten fokuserer på gjenbruk av avfallsstrømmer, men det er billigere og enklere å redusere avfallsstrømmene enn å forsøke seg på gjenbruk eller resirkulering. Enkle trinn, som f.eks. tilstrekkelig planlegging, rapportering, valg av riktig lokalitet, valg av riktig utstyr og gode driftsrutiner, kan redusere mengden avfall som produseres i et anlegg, og bør vurderes før andre tiltak gjøres.

Table 2. Best available techniques identifisert i 2013 BAT-rapporten og i denne nye rapporten.

Glossary

Scientific Names

1. Introduction

Aquaculture is an important sector in the Nordic region, and recent developments suggest the number and size of land-based aquaculture facilities are likely to increase in the future. Across the Nordics, more than €13 billion worth of fish (including molluscs and shellfish) were produced in 2020 through a combination of fisheries and aquaculture. This production is not evenly spread across the region, with Norway producing more than double the amount of fish than all other Nordic countries combined, whilst Finland and Sweden are net importers of fish and fish products. Despite over 50% of fish produced worldwide being from aquaculture, the Nordic region, except for Norway, relies heavily on capture fisheries. Many Nordic countries have failed to establish an aquaculture industry equivalent to the scale of their marine-based economies. For example, Sweden produces fewer salmonids per year through aquaculture than is possible in a single large land-based facility. There are a multitude of reasons for this failure to meet potential. Environmental conditions give Norway an advantage compared to other countries such as Greenland and Denmark; an abundance of deep, sheltered fjords, and temperatures ideal for Atlantic salmon production allow the country to dominate production of this species. Instead, Denmark has focused on land-based and freshwater production.

The size of nations is relevant when comparing production volumes. The Faroe Islands produce more farmed Atlantic salmon per capita than any other country, despite a significantly lower overall production compared to Norway. Each country has unique factors influencing the growth of its aquaculture industry. Concerns of local stakeholders, including the tourism industry, anglers, and indigenous peoples, the presence or lack of specific legislation, concerns over interactions with wild populations and disease management, the prevalence of private ownership of aquatic spaces, and public suspicion over foreign ownership, all vary between countries. Each of these factors, whilst often valid and based on the wishes of users of shared space, places hurdles preventing the industry from meeting its full potential.

One of the largest and most consistent factors limiting the growth of the aquaculture industry across the Nordics emanates from environmental concern. The present trend in research and policy encourages reductions in environmental emissions from intensive aquaculture and an increased valorisation of waste streams. Nutrient discharge causing eutrophication, impact on benthic ecology, and the historic use of antibiotics have all caused concern. This is one of the appeals of land-based aquaculture systems over sea-based or lake-based cultures. Land-based culture of aquatic organisms, be it contained and tank based or of a more traditional pond or raceway design, gives a level of potential control over the outputs from the system not possible in open, cage-based culture systems. Wastes can be captured and turned into by-products, and control over inputs can reduce mortalities or the need to treat the fish with antibiotics or therapeutants. Whilst control of emissions is possible, land-based systems remain capable of generating large amounts of waste and emissions in various forms (sludge, water, mortalities, gas), and failure to handle waste streams appropriately can result in the emission of pollutants or at very least reduce the environmental benefits of using such a method of production. Landfill and discharge into the ocean or watercourse have been traditionally employed, but many more options are now available and allow for farms to both reuse waste and increase their green credentials.

A previous report highlighted Best Available Technologies (BATs) for waste capture and reuse relevant at the time (2013), but the capacity of land-based systems to capture waste streams and the potential for reusing them are improving through technological innovations. The present report aims to discuss some of the best currently available technologies at the time of writing (2022). It provides a brief overview of aquaculture in the Nordics and of land-based culture. Currently available and emerging opportunities to capture and reuse waste streams are discussed alongside a description of the current legislative requirements and hurdles. It is hoped the report will inform relevant stakeholders with an interest in this field. It will provide an overview of both emerging and established best available techniques for reducing emissions to the environment and for the management of wastes, with a focus on circular economy. Improving waste management might improve the viability of land-based aquaculture, supporting sustainable growth in the sector.

1.1. Aquaculture sectors in the Nordic countries

Table 3. Aquaculture production statistics for Nordic countries in 2020

1.1.1. Norway

1.1.2. Denmark

Table 4. Number of traditional and model trout farms in Denmark 2013 – 2020

1.1.3. Faroe Islands

1.1.4. Iceland

1.1.5. Finland

1.1.6. Åland Islands

1.1.7. Sweden

1.1.8. Greenland

2. Status of policy, legislation and certification

2.1. EU policy and legislation

Table 5. European Union policies, legislations, and regulations relevant or of potential relevance to the management of aquaculture wastes, by-products and emissions, and associated BATs

2.2. Norway

Table 6. Norwegian legislation and regulation relevant to land-based aquaculture

2.3. Denmark

Table 7. Danish legislation and regulation relevant to land-based aquaculture

Case Study 1 – Danish Model Trout Farms 

The story of how this regulation evolved and determined the trajectory of Denmark’s trout farming sector makes an interesting case study. In 1989, The Environmental Act was amended to include provisions for regulating the environmental impacts of freshwater trout farming. The subsequent Decree on Freshwater Pond Farming (BEK 224 of 05/04/1989) introduced new rules for limiting the level of nutrients in water discharged to the environment. A feed quota was introduced, restricting the quantity of feed used per tonne of fish (i.e., FCR), whilst a maximum limit was set for feed nutrient content. The use of feeds other than dry pelleted or extruded diets was prohibited. Additionally, the decree contains provisions that encourage a lower rate of water use, and it became mandatory to use sedimentation methods and sludge traps for reducing the suspended solid content of discharged water. The new rules had a profound impact on Denmark’s trout farming sector. Trout farms being typically of a flow-through, raceway-pond design, the mandatory use of sedimentation effectively entailed the construction of large settling ponds. In practice, the production of fish without exceeding the given feed quotas was difficult at best, and for production to increase or even be maintained, methods and strategies for reducing feed use were now needed. Some producers ceased to operate, and sectorial grow stagnated.  In 2000, an advisory committee was established by the former Ministry of Food, Agriculture and Fisheries. The committee prompted an investigation of methods and techniques that could be offered a standard model for producing trout that was economically viable and complied with stringent regulatory demands. The resulting ‘Model Trout Farms’ were a modification of traditional flow-through practices. The most distinct change was the reuse of water, facilitated through the addition of biofilters. Three types of model trout farms where established, each differing principally by the degree at which water is recirculated. Reuse of water reduces the rate at which water is discharged, enabling increased water residence times in sedimentation ponds. Trials undertaken by Danmarks Tekniske Universitet (DTU; Technical University of Denmark) demonstrated reduced emissions of dissolved and solid bound nutrients and all three model type farms (but only Type 1 and Type 2 were cost efficient). The outcome precipitated the most influential legislative shift since the curtailing measures introduced in 1989. Facilities meeting the specifications of a model farm no longer required a feed quota-based permit. Instead, permits could be granted according to maximum allowable nutrient emissions. This removed an upper limit on production to the extent that compliance with emission allowances could be maintained. 

The opportunity for increased production was a key driver behind the development and adoption of best technologies and techniques. Increased production required higher expenditure (CAPEX and OPEX), encouraging greater efficiency through economies of scale. Smaller farms merged, and the sector’s production grew without a concomitant increase in the number of farms.  Regulatory changes no longer classify farms as Type 1 or Type 3, but the importance of the model trout farm to the regulatory development of Danish aquaculture has been significant.

Below: Transition of a traditional flow-through trout farm to a Type 1 model farm, near Ringkøbing, Jutland, Denmark. Google Earth images show how the physical landscape of the farm has changed with the addition of technology, sedimentation lagoons, and the size and number of raceways.

 

2.4. Faroe Islands

Table 8. Faroese legislation and regulation relevant to land-based aquaculture

2.5. Finland

Table 9. Finnish legislation and regulation relevant to land-based aquaculture

2.6. Sweden

Table 10. Swedish legislation and regulation relevant to land-based aquaculture

2.7. Iceland

Table 11. Icelandic legislation and regulation relevant to land-based aquaculture

2.8. Certification schemes

Table 12. Main certification schemes of relevance to land-based aquaculture

3. Land-based systems

Aquaculture systems are often categorised according to production intensity and the degree of their dependence on the exogenous supply of inputs, especially feed. Aquaculture methods that exclusively use manufactured feed, such as pelleted and compound diets, are described as being intensive. Intensive feeding and the supply of other inputs are often associated with higher fish stocking densities and production volumes. Contrastingly, extensive methods rely almost entirely on natural production to supply nutrition to the cultivated species. The only exogenous inputs of economic consideration are fertilisers to support primary productivity. Mussel farming is a good example of extensive aquaculture. Mussels are often grown attached to ropes, and they feed by extracting seston from the surrounding seawater. Sometimes extensive farming methods are combined with the use of manufactured or hand-made diets. Such practices are referred to as semi-intensive or semi-extensive. This approach can be seen in some pond systems where herbivorous fish feed on naturally present aquatic plants but are also fed human-produced feed to increase the growth rate and/or stocking density. Intensive feeding may enable (and, indeed, the term is sometimes used to imply or denote) greater volumes of fish and stocking densities, and, thus, is often associated with a dependence on other inputs, such as oxygen and energy (e.g., electricity for operating equipment).

Production systems may also be categorised by their level of connectivity with natural water bodies. Open systems present few barriers to the exchange of water from and to the natural environment. They rely on the surrounding ecosystem to supply water of a temperature and oxygen content suitable for cultivation and to remove and assimilate waste nutrients. Cage and net-pen farming in seas and freshwater lakes, cultivation of bivalves on drop-ropes and lantern nets, and seaweed production on rafts and longlines are examples of open systems. There are fewer examples of what can be considered a true, closed system, in which there is no exchange of water with the environment. Oxygen levels, temperature, and other water quality parameters are to be controlled within a closed system with minimal reliance on the provisions offered by the natural environment^{[1]I.e., their provision occurs within the economic sphere, which requires economic activity.}. Recirculation aquaculture systems are often described as closed systems. However, frequent water changes are often required to prevent the accumulation of nitrate. In semi-open (or semi-closed) systems, there is an exchange of water with the surrounding environment, but they also offer a degree of containment. Raceway flow-through systems are an example of land-based, semi-closed aquaculture. Water from a proximal natural source flows through the raceways before (usually) entering the same watercourse, albeit further downstream. Ponds are also frequently described as semi-enclosed. Water is let into the pond and discharged at the time of harvest.

There is sometimes debate about the categorisation of aquaculture systems, especially regarding the relative extent to which they can be considered open or closed. However, it is important to understand that these are useful but somewhat loosely applied terms, and they are not intended to be absolute.

3.1. Flow-through

Land-based flow-through systems are dependent on a continuous supply of suitable water from a natural source. Fish are reared in constructed ponds or tanks through which abstracted water makes a single pass before its subsequent release. They can be defined as semi-enclosed because, although water flows somewhat freely between the environment and the system, there is some, albeit limited, containment. The use of flow-through systems for cultivating freshwater trout proliferated in the 20^th century and the practice is still in use today. Trout grow-out farms are often located next to streams or rivers, from which water is abstracted and fed via gravity through elongated, rectangular ponds called raceways before being discharged into the same watercourse. Freshwater trout hatcheries consisting of much smaller raceway tanks and nurseries with circular or square tanks were also of a flow-through design. Raceways or tanks may be arranged in parallel or in series. The latter enables abstracted water to pass through two or more cultivation units before it exits the system, potentially lowering the volume of water required per unit of fish produced. Depending on characteristics such as the size and shape of raceways and the rate at which water flows, the proportion of nutrient-containing solids remaining within flow-through systems may be significant. This reduces the need for mechanical filtration, and water is sometimes discharged directly into the surrounding watercourse without prior treatment. As such, appropriate site selection is important to ensure that dissolved nutrient discharges do not exceed the assimilative capacity of the receiving environment. Phosphorous and nitrogenous nutrients can also be removed from outlet water using settling ponds and constructed wetlands.

Compared to traditional, relatively simple raceway designs, the operation of more modern flow-through systems incorporates greater use of technology. Typical examples can be found among salmon smolt production facilities. They consist of large, usually circular tanks that allow the fish to swim in shoals, which in turn enables higher fish densities. It is common to oxygenate the water to compensate for high biomasses and to lower water consumption. Water flow may be maintained via gravity alone or assisted by mechanical pumping. The intake water often gets filtered and treated with ultraviolet (UV) radiation before it comes into contact with the fish, as the system is vulnerable to diseases and the loss can be severe if pathogens get established in the plant. Other interventions may include degassing of carbon dioxide (CO₂) and the addition of minerals such as sodium chloride (NaCl) and lime (containing CaCO₃). Large biomasses entail large amounts of faeces and possible feed residues, so water might be subject to mechanical filtration before being discharged, perhaps as a condition for regulatory compliance. Captured solids may then be subject to thickening, flocculation, and dewatering, resulting in a condensed sludge (see Section 5.1.).

3.2. Recirculating aquaculture systems

Recirculation Aquaculture Systems (RAS) are similar to many other land-based systems in that the farmed species are cultivated in tanks, ponds, or raceways, through which water flows. However, unlike flow-through designs, water is recirculated throughout the cultivation system rather than being completely discharged after making a single pass. Instead, in RAS the percentage of water exchange typically varies from 5 to 20% according to various species-related parameters and according to production targets, water availability and budgets. Zero-water exchange systems have emerged in recent years and, rather than discharging water and exchanging it with freshwater, they replace only the water that is removed via evaporation or sludge removal. This results in around 2.5% replacement of system water per day. Owing to the degree of containment this ‘reuse’ of water can help maintain, RAS are often characterised as closed. They are also intensive, with feed and, most often, various other economic inputs and interventions being required to provide the suitable conditions for cultivation. Thus, RAS are frequently more dependent on technological processes than other types of aquaculture systems. Mechanical and biological water filtration technology is required to remove suspended solids and to prevent the accumulation of nutrients in their most toxic form. Oxygen must be added to replace oxygen consumed via fish respiration and other processes, and heating and cooling methods may also be required. The unit processes likely to be found in a basic RAS are:

• Mechanical filtration – Organic solids such as fish faeces and uneaten feed need to be removed to maintain appropriate water quality and enhance the efficiency of biological filtration. They are removed via a mechanical filter, the most common method being drum filtration. Cultivation water passes across the cylindrical screen (drum) which collects the solids. The solids are then removed from the screen via rotating the drum and backwashing and concentrated into an organic sludge.
   

• Biofiltration – Dissolved ammonia is released into the water as a product of fish metabolism and bacterial degradation of organic matter. Biofiltration reduces ammonia, which in specific concentrations and forms, especially the unionised form, can be toxic to fish. There are various forms of biofilters, although in general, they usually contain a form of media such as plastic beads, providing a large surface area upon which biofilm can be established. Nitrifying bacteria grow in the biofilm. These nitrifying bacteria convert ammonia in an aerobic two-step process, first to nitrite and then to nitrate. Nitrate is less toxic for fish than other forms. Nitrates are usually reduced by regular water exchange. In RAS facilities with a very high degree of recirculation, the removal of nitrate may also be necessary. This can be achieved with an anaerobic biofilter in which nitrate is converted to nitrogen gas (N₂) through a multi-step process facilitated by bacteria. The nitrogen gas can then be removed.
   

• Oxygenation – Oxygenation is required to maintain an appropriate level of dissolved oxygen in the water. This can be achieved via aeration, which is the mixing of air with water, for which a variety of methods exist, such as the use of air stones that send a stream of air bubbles through the water to its surface. Alternatively, pure oxygen can be used, supplied either by oxygen gas or liquid oxygen injection techniques. The use of pure oxygen is a highly effective and efficient technique. Oxygen can be dissolved into the water via a saturation cone or a venturi injection. Oxygen can also be produced onsite by a pressure swing adsorption generator.
   

• Degassing – At high stocking densities or low water exchange, carbon dioxide (CO₂), released via fish and bacterial respiration can reach toxic levels. To prevent this, excess CO₂ is stripped from the water. This can be achieved by passing water over a medium that diffuses the water, increasing its surface area, so that CO₂ transfers passively into the surrounding air. However, degassing units are not always required, as the aeration systems can be sufficient to strip CO₂ from the water, especially at lower stocking densities.
   

• Ozonation and UV disinfection – Ozonation is an effective method of destroying bacteria and viruses and is achieved by the mixing of ozone and cultivation water in a separate unit. Too high levels of ozone can be fatal to fish, with larvae being particularly sensitive. For this reason, ozone is not used in all RAS. UV treatment maybe be used alone or in conjunction with ozone. It is an effective method of water sterilisation and relies on exposing water to UV light. UV radiation ranges between 0 and 400 nm wavelength. However, 250-260 nm wavelength has the highest disinfection potential. UV-C radiation inactivates bacteria by creating thymine-thymine dimers inside the DNA. Effective UV sterilisation requires relatively clear water; a high level of organic content provides places for pathogens to hide. The most common type of UV lamp used in RAS is the low-pressure high-intensity mercury iridium amalgam vapour lamp.
   

• Monitoring – Maintaining appropriate water chemistry and quality with RAS requires a high level of monitoring. Probes and sensors are employed for continual monitoring of key variables such as oxygen level, temperature, salinity, pH and total dissolved gas pressure (TGP). In modern recirculating systems, these probes can automatically regulate O₂ dosing and pH level via programmable regulators and solenoid valves. Water chemistry, such as levels of ammonia, nitrite, and alkalinity, is tested daily using chemical test kits. More frequent testing is common during shifts in stocking intensity or when first starting up a RAS.

Recirculation systems are usually housed within a constructed shelter (e.g., insulated buildings and ‘polytunnel’ greenhouses), although there are examples of RAS being located outdoors (e.g., Type 3 model trout farms). Roofs, walls, and floors protect electrical and mechanical equipment and other infrastructure from damage through exposure to local environmental conditions. A suitably designed and constructed building can also support the maintenance of biosecurity by presenting a barrier to, e.g., birds and by facilitating measures for excluding other vectors of pathogens.

Over the past two decades, RAS have come to be seen as an emerging technological approach to fish farming that offers various advantages and opportunities. Certainly, Europe has witnessed a general increase in RAS, both in terms of the number of RAS facilities and the number of RAS proportional to other types of land-based systems. Technology is constantly improving, allowing for increased control, stocking densities, and water reuse. These advancements increase the viability of RAS but also the complexity. Various start-ups and small aquaculture businesses have chosen RAS for growing salmonids, pikeperch, carp species, African catfish, and other types of fish, mainly for local and niche-product markets. However, these endeavours have experienced limited commercial success, and the increase in the number of RAS facilities has come from the already established salmonid aquaculture sector, mainly to produce rainbow trout and Atlantic salmon smolts (previous to growing them in sea cages or net-pens) and some Arctic charr. During the last decade, the production of fish, e.g., Atlantic salmon, to slaughter size in RAS facilities has increased.

Figure 1. Schematic of a recirculating aquaculture system

3.3. Static ponds

3.4. Aquaponics

4. Emissions, wastes and by-products from land-based aquaculture

• Sludge (fish faeces and uneaten food)
• Discharge water
• Mortalities
• Gaseous emissions (mainly N₂O and CO²)

4.1. Definition of emissions, wastes, and by-products

4.2. Discharge water

4.3. Sludge

4.4. Mortalities

4.5. Gas emissions

Figure 3. Basic diagram showing the major production stages of an aquaculture value chain (blue circles) and some of the inputs that these stages require (such as energy and transport). All stages are associated with emissions to land, air, and water, as is the production and provision of inputs such as electricity.  

4.6. Sources of waste streams from RAS

Figure 4. Diagram of a simple RAS loop and the points at which waste is generated

5. Techniques for management of waste and reduction of emissions

5.1. Separation of solids and water

5.1.1. Sedimentation

5.1.2. Mechanical filtration

5.1.2.1. Drum filter

Figure 5. Diagram of a drum filter

5.1.2.2. Disc filter

Just like drum filters, disc filters are self-cleaning microscreen filters. The water flows through the unit and is passively filtered using cloth filters. An advantage of the disc filter is the use of several sequential filter cassettes, which increases the surface area of the unit, providing either more efficient filtration or a small unit footprint. A backwash system cleans the filters and the wastewater containing the concentrated suspended solids is removed for further processing. Finer filter meshes can be used in a disc filter than in a drum, and backwash frequency is reduced, resulting in backwash containing larger percentages of solids.

Figure 6. Diagram of a disc filter

5.1.3. Wastewater outflow

5.1.4. Sludge water processing

Figure 7. Diagram of a belt screen filter

Dewatering. Many reuse options require sludge to be further concentrated, which is achieved through dewatering. Decanter centrifuges, screw presses (Figure 8) and to a lesser extent filter presses are used to produce sludge with 20–30% TS. A screw press uses an Archimedes screw to move sludge along a permeable chamber. This chamber is normally inclined to assist with water drainage. The filter is typically wedge-wire or perforated metal, and this is less susceptible to clogging than cloth filters. A decanter centrifuge uses centrifugal forces to push solids towards the walls of the chamber, which are then moved along using a screw, the lighter water component flows freely towards the other end of the chamber. These systems must treat the sludge carefully as the lack of fibre makes it more delicate compared to municipal wastewater sludge. Dewatering beyond 30% TS is possible but the sludge is difficult to pump, and the benefits of such a practice are outweighed by the logistical and actual costs.

Figure 8. Diagram of a screw press used for sludge dewatering

Drying. Drying is the final stage that may be employed in aquaculture sludge processing. The TS of the sludge can be over 90% and this process can produce a hygienised and stable sludge that can be stored for extended periods and used in a larger number of reuse options compared to wetter sludge (see Section 5.2.4). Drying the sludge can be achieved through various methods including thermally, but this is a costly approach. The most cost-effective is the use of superheated steam, heat pumps, or thermo-mechanical drying. Drying is the most energy-intensive stage of sludge processing. It takes around 0,75kWh to evaporate 1kg of water (this decreases as starting temperatures increase), but many companies report values of more than 50% lower, thanks to efficient reuse of energy.

5.2. Possible reuse of sludge

5.2.1. Fertiliser

Table 13. Nitrogen inputs, outputs, and surplus in five Nordic countries in 2018 (FAO, 2021)

5.2.2. Energy production

Figure 9. Continuously Stirred Tank Reactor – taken from Banerjee et al., 2022

Up-flow Anaerobic Sludge Blanket Reactors (Figure 10) allow for lower operational costs whilst allowing for higher solid-removal efficiency from discharge water with a TS content of 3–4%. These systems use a suspended blanket of granular sludge through which discharge water flows upward, being processed by microorganisms along the way. The long retention times should be considered when choosing this system.

Figure 10. Up-flow Anaerobic Sludge Blanket Reactor.

Membrane Bioreactors (Figure 11) provide high effluent quality and high conversion of influent. They also come with high operational costs and the risk of biofouling. These systems use a submerged membrane to filter particulate waste from the major liquor solution, operating like a traditional activated sludge system.

Figure 11. Membrane Bioreactors – (a) Side-stream configuration; (b) Submerged or immersed membrane configuration. Taken from Jain et al., 2018.

Anaerobic Baffled Reactors are widely used in discharge water treatment and can treat water with high solid content. These reactors resemble modified septic tanks with a series of baffles along the length of the tank (Figure 12). As the influent travels across the tank, the sediment settles, therefore separating the solids retention time from the hydraulic retention time. These systems are robust against hydraulic and organic shock and are simple to build and operate. Technology of this type has been used to generate around 500.000 kWh/year from 260 tonnes of sludge (weight as dry matter) at a Norwegian hatchery (see Case Study 2). The heat and electricity generated by the biogas can be used to fuel the sludge dewatering, biogas production, and drying of the remaining sludge. These processes do not use all of the energy in the biogas, and over 50% of the heat and electrical energy is available for usage in additional farming operations.

Whilst dilute discharge water can be used in bioreactors, the efficacy of the process is reduced significantly compared to using dewatered and thickened sludge. A commercially active bioreactor on a salmon farm in Norway was reported to use sludge thickened to less than 5% TS before input into the bioreactor (Case Study 2). This system used microbes selected for high nitrogen tolerance and was able to operate on 100% fish sludge without inputs of an exogenous carbon source. Biogas production does not use the entire volume of the sludge and the remaining residue can be used as a fertiliser.

Figure 12. Anaerobic Baffled Reactor – Biogas extraction can occur from vent – Taken from Tilley, 2014.

Case Study 2 – Sterner

Sterner AS is a Norwegian-based company that has been offering water treatment solutions since the early 1990s. They offer two main solutions for aquaculture sludge treatment: sludge drying and on-site biogas production. Sterner’s approach to sludge dewatering and drying is mechanical, using sludge thickeners, dewatering screws, and belt dryers to create a sludge up to 90% TS that can be used as fertiliser or sent to an external biogas plant. Drying in this manner can use as little as 0,3 kW per litre of water removed. 

Biogas production takes place inside an anaerobic baffled reactor. Sludge thickened to 3-4%TS enters the reactors and the water remains for around six days. Solids settle out and are removed with a screw. This digestate can be mechanically dewatered and used as fertiliser. Inside the reactor, bacteria that have been specially selected for tolerance of a low carbon/nitrogen ratio can turn 100% fish sludge into biogas (CH₄ and CO₂). Sludge from marine aquaculture can also be used following novel dewatering techniques. A Sterner biogas plant of this type has been running for almost four years with a good production of biogas, which can be used to power parts of the farm. More than 50% of the electricity produced from the biogas is available for use on the farm, with the rest being used to power the sludge treatment process itself. The heat energy from the process can also be used to heat system water. Onsite biogas production is not applicable to every situation; farms below 5.000 tonnes may find that drying the sludge is more economical, depending on factors such as the remoteness of the farm and electricity costs.

Pyrolysis is also an option for the valorisation of sludge. Put simply, pyrolysis is the heating of organic material in the absence of oxygen. In general, pyrolysis uses sludge that has been dewatered and partially dried, although systems that can process sludge with <15% TS are available. These often use microwave heating and can incorporate drying, pyrolysis, and gasification into one process (Lin et al., 2017). Despite pyrolysis use dating back to ancient Egyptian times (Mohan et al., 2006), it was not until the 1980s that the pyrolysis of sewage sludge began to be investigated (Fonts et al., 2012), and only recently has this technology been used as a method of processing aquaculture sludge.

The three major products of pyrolysis are biochar, bio-oil, and syngas. Pyrolysis of biosolids typically occurs between 300 and 900 °C, with the temperature and duration of the process determining the amount of each product. Slow pyrolysis increases the production of biochar to the detriment of biogas and bio-oil production. Fast pyrolysis increases bio-oil production, whilst flash pyrolysis increases bio-oil production even further (Elkhalifa et al., 2022). Some emissions of H, CO, CO₂, and CH₄ occur during sludge pyrolysis and commercial units often include gas cleaning and purification systems to remove NO_x, CO_x, particulates, and sulphur. Pyrolysis equipment can be purchased from aquaculture suppliers, including the option to handle all processing such as sludge drying and dewatering (see Case Study 3).

Bio-oil and synthetic gas (syngas) are two of the three main products of pyrolysis. The energy content of syngas is considerably lower than that of natural gas (~6 MJ/kg-¹ and ~54 MJ/kg-¹ respectively) (Laird et al., 2009). This energy can, however, be used to power the pyrolyzer itself, reducing the costs associated with this process. This option is already being offered in some aquaculture pyrolysis ovens.

The heating value of bio-oil varies depending on the nature of the feedstock, but aquaculture sludge will likely produce heating values around half that of No. 2 fuel oil (Laird et al., 2009). Bio-oil cannot be used as a direct replacement for diesel or petrol and must first be refined increasing the cost and complexity of using bio-oil as a fuel source (Gupta et al., 2021), although it can be burned in industrial boilers (Laird et al., 2009). The use of aquaculture sludge in this manner will not only reduce the sludge emissions of a farm but also decrease reliance on fossil fuels. There has also been recent interest in the chemical composition of bio-oil with the possibility to extract a wide range of value-added products from the oil, which can have economic value and industrial applications (Brodie et al., 2018), but the low quantity of these compounds makes the use of these resources from aquaculture sludge a challenge. Burning syngas and bio-oil emits greenhouse gases and toxic compounds, including CO, CO₂, NO_x, SO₂, and in the case of bio-oil, particulate matter (PM<2,5). Despite these emissions, because the carbon in these fuels is sequestered from the environment, they are considered by some to be carbon neutral.

Biochar production results in much of the carbon content of sludge being captured in solid form. Compared to incineration, pyrolysis emits considerably less carbon into the atmosphere, and the carbon can remain stored for hundreds of years. Biochar can be used as a soil enhancer, increasing the nutrient uptake efficiency of plants by trapping nutrients in the soil for longer and decreasing run-off (Carey et al., 2015; Figueiredo et al., 2021). Biochar can also be used to neutralise the soil, with acidic or alkaline biochar being produced from the same urban discharge water sludge depending on the temperature of the pyrolysis (Hossain et al., 2011). Biochar may also be used as a fertiliser, but without the addition of chemicals such as potassium acetate (Buss et al., 2020) or calcium oxide (Liu et al., 2019), the bioavailability of the phosphorous will be low. The temperature of pyrolysis can alter the concentration and bioavailability of nutrients. In municipal discharge water sludge, the total nitrogen content decreases with rising temperatures, as does the bioavailability of the remaining nitrogen (Hossain et al., 2011). Conversely, total and plant-available phosphorous increased with pyrolysis temperature as did the concentration of assessed micronutrients (Ca, Fe, Mg, S, Cu, and Zn) (Hossain et al., 2011). The heavy metal content of the biochar also increased with pyrolysis temperature. However, the availability of these to plants (DTPA-extractable concentration) decreased with temperature (Hossain et al., 2011).

The use of biochar has also been suggested in water treatment systems, with the highly porous nature of biochar, particularly biosolid biochar (Elkhalifa et al., 2022) allowing the removal of chemical, biological, and physical contaminants (Gwenzi et al., 2017). Removal of H₂S and NO_x from gas streams is another use, as is the sorption of metals, phenols, and dyes (Paz-Ferreiro et al., 2018). Due to its porosity, biochar is a good catalyst during biodiesel production, or for the removal of tar from bio-oil and syngas (Lee et al., 2017). Activated carbon is a further valorisation option. This requires additional processing, but prices can range from €670–2000/tonne (Alibaba, Dec 2022). One company offering pyrolysis to aquaculture (see Case Study 3) reported tuning their system to only produce biochar due to the larger market and increased usage of the product. Pyrolysis occurs at high temperatures, and much of this heat can be recaptured and fed back into the system or used to heat system water. It can also be used to generate electricity to power farming operations. Using this heat decreases the costs associated with pyrolysis. Compared to bioreactors, pyrolysis units are smaller and can process sludge much more quickly. Purchasing and the initial start-up process of a pyrolysis unit can have significant costs and a large amount of sludge is needed to make the operation economically viable. For large farms (10.000 to 15.000 tonnes/year), they can present a viable solution for sludge management.

Case Study 3 – Blue Ocean Technology

One supplier offering pyrolysis-based solutions for aquaculture sludge is Blue Ocean Technology. Started in 2012, Blue Ocean Technology offers a systems approach to sludge management capable of handling all facets of sludge thickening, dewatering, and drying, and offers pyrolysis systems for further valorisation. Specially designed belt filters and screw presses handle the sludge more delicately than those designed for municipal wastewater sludge. These systems require little maintenance and few worker hours, with only ten minutes twice a day being needed. These containerised solutions can be placed anywhere onsite, increasing flexibility and reducing wasted space.  

The company can capture sludge from beneath sea cages and boasts the ability to remove 60-70% of the salt, allowing saline sludge from marine or land-based facilities to be used as fertiliser or for biogas production. Blue Ocean Technology states that biogas production onsite generally becomes viable once production is above 10.000 to 15.000 tonnes/year. An alternative for both larger and smaller farms is pyrolysis. 
 
Blue Ocean Technology calculates that on larger farms the total cost of processing the sludge including pyrolysis was reported as 200-400NOK/tonne of fish produced. The pyrolysis system requires heating to 400 °C before the heat from the process itself increases temperatures to 700–900 °C. Too small a volume of sludge makes this process less cost-effective. The pyrolysis system is tuned to produce biochar, which can be used as a soil enhancer, or for other applications such as air or water filtration. The pyrolysis unit produces up to 1300 kWh of heat constantly, based on approximately 4000 tonnes of sludge with 15 MJ energy content. If the sludge possesses a larger energy content (20 MJ) then it is possible to produce 1750 kW of heat or 525 kW of electricity constantly. This makes the system more than self-sufficient as the unit needs only 65 kW to operate. The remaining energy can be delivered to the farm. At 150 m2 to 200 m2 , the pyrolysis unit is smaller than many biogas reactors in general.

Burning – biofuel for the cement industry
In addition to biogas and pyrolysis, the energy in the sludge can also be harnessed through incineration. Dried sludge can be transported to plants which incinerate the sludge alongside waste products from other industries and municipal sources. The burning of sludge compared to microbial digestion prevents the emission of methane but emits large volumes of CO₂ into the atmosphere. Cleaning (scrubbing) of the air is common, but carbon capture is not, with only one known carbon capture and utilisation system operational on a European incinerator. Compared to energy produced from renewable sources, e.g., wind or hydroelectric, burning of waste is less efficient and produces much higher emissions of CO₂, but when incineration of biomass replaces a hydrocarbon source of fuel, CO₂ emissions can be reduced overall. The cement industry uses large amounts of coal every year and emits huge amounts of CO₂. The coal is used to burn limestone (CaCO₃) to calcium oxide (CaO). Both the combustion of coal and CaCO₃ release CO₂, but renewable sources of energy cannot currently be used during the combustion process. Replacing coal with biomass is an option however and is an option being explored by a cement manufacturer in Norway (see Case Study 4).

Case Study 4 – Norcem AS

Norcem is one of the world’s largest manufacturers of building materials, including cement. Norcem is a good example of a company considering circularity as part of its core business. Cement is a notoriously “dirty” industry, burning large amounts of coal and emitting large amounts of CO₂. Since 1996, Norcem has reduced the percentage of coal used to power Kjøpsvik cement plant from over 95% to just over 50% in 2022. Located in Norway, Kjøpsvik is the northernmost cement plant in the world, and this presents unique challenges including high transport costs for fuels. The plant is located in a region rich in aquaculture farms and routinely uses aquaculture sludge as a fuel source for cement production. During the calcination stage, the dried sludge is used alongside coal and other alternative fuels including used tires, municipal waste, and spoiled fish feed. The plant routinely receives sludge from a number of different farms and is upgrading onsite facilities to accommodate tankers of sludge rather than the current supply in big bags. Sludge and animal meal combined make up 2.000 tonnes of fuel used in the plant each year, and it is hoped this can be increased to 10.000 tonnes of fish sludge in the future. 

Sludge is not the perfect fuel source for cement production, and whilst it could replace large volumes of coal, running a plant entirely on fish sludge is unlikely, even if a sufficient supply could be guaranteed. The phosphorus content of the sludge can result in high levels of phosphorus pentoxide (P2O5) and high nitrogen levels can increase NOx emissions that may require additional processing, whilst the chlorine content, especially in marine sludge, can present an issue. Removing phosphorous and nitrogen from the sludge can reduce the issue and allow for larger percentages to be used. Despite these issues, the plant hopes to use large amounts of sludge in the future and sees both environmental and economic benefits in reducing coal usage and replacing it with locally sourced fuel. Should the 10.000-tonne target be met, this would remove the need to burn around 5.000 tonnes of coal. The carbon present in fish sludge was sequestered from the atmosphere and it is therefore a carbon-neutral fuel. Coal (fly) ashes are often added to cement to increase the strength and durability of concrete and the ashes from the burning of the fish sludge can also be used for this purpose. 

5.2.3. Insects

5.2.4. Costs and benefits of sludge drying

Figure 13. Reuse options for land-based aquaculture sludge and the level of drying required

Case Study 5 – Alternative drying solutions

Various methods of drying aquaculture sludge are commercially available. In earlier case studies we have mentioned mechanical drying such as belt dryers. These can dry sludge to >90%, and when using heat capture this can be achieved with relatively low power consumption. Alternatives to this technology include heat pumps, thermo-mechanical drying, and superheated steam. (This is discussed in the mortalities section.)

Fjell Technology Group AS offers solutions for sludge thickening, dewatering, and drying. A combination of band thickeners and bespoke polymers can effectively capture and thicken suspended solids. Electrocoagulation is installed in two facilities and work is being undertaken to further refine the technology. Decanter centrifuges dewater the sludge before the 20–25% TS sludge is pumped into the drying unit. The patented thermo-mechanical dryer is a type of hammer mill that uses friction to dry the sludge and produces a hygienised and stabilised sludge between 92 and 95% TS. The rotor spins at 600–720 rpm and the internal friction of the materials generates temperatures of 105–120 °C. Fjell reports significant savings compared to the disposal of wet sludge despite the increased energy costs of drying compared to wet disposal. The dry sludge is in the form of a fine powder, and this can improve its use for industries such as fertiliser. Turbo disc dryers are also available, and these use a steam-heated disc to apply indirect heat over a large surface area.

Drying Matter AS use heat pump technology to dry sludge from 15–40% dry matter and up to 95%. Dry and slightly heated air generated using the heat pump flows over the sludge and becomes saturated with water. The cooling side of the heat pump condenses the water, which is drained away. The de-watered air then flows back over the heating element and the cycle begins again. Low temperatures (50–80 °C) and the recycling of air within the system reduce odour from the unit and eliminates emissions. Energy consumption can be as low as 0,3 kWh per litre of water removed. The residency time in the dryer is 60 to 90 minutes, thereby ensuring hygienisation, and the product is a dry sludge of a consistent size. For processes where an external heat source is available, Drying Matter AS has a product range allowing for the substitution of internal heat pumps or a hybrid energy mix using both heat pumps and external sources. A medium-sized unit can process 12 tonnes of mechanically dewatered sludge per day, producing three tonnes of dried sludge. Parallel lines can easily be added if a larger capacity is required.

5.2.5. Saline sludge

5.3. Discharge water

5.3.1. Treatment of discharge water

Figure 14. Illustration of DAF process, taken from Ebrahiem et al., 2021

• Microfiltration - Membrane pore size ranges from 0,1 to 10 µm. Such membranes are mainly used for the removal of larger particles, but bacteria, algae and fungi can also be removed by this technique.
• Ultrafiltration – Membrane pore size ranges from 0,01 to 1 µm. The smallest pore sizes will filter out bacteria, viruses, algae, fungi, and polypeptides. This technique is widely used in wastewater treatments.
• Nanofiltration – Membrane pore size ranges from 1 to 10 nm. Typical applications for nanofiltration may include the removal of colour and total organic carbon (TOC) from surface water and the removal of hardness from well water. Nanofiltration removes all of the above-mentioned agents plus some multivalent ions.

Figure 15. Diagram of a packed media degassing tower

5.4. Reuse of discharge water

5.4.1. Wetlands

Figure 16. Model trout farm and constructed wetland near Aalborg, Denmark. Google Earth

5.4.2. Fertigation

5.4.3. Algae

5.4.4. Heat/energy recovery

5.4.5. Chemical discharges

5.5. Mortalities

5.5.1. Ensiling

Case Study 6 – The Åland Islands: biodiesel and waste reduction

Historically, the region’s aquaculture production has been involved in several initiatives that together minimise environmental impacts, such as AQAFIMA, AQUABEST and HELCOM’s Baltic Sea Action Program for the Baltic Sea aquaculture sector. The latter project focused on reducing the net load of waste inputs and creating a nutrient-neutral aquaculture sector using Baltic raw material in fish feed for fish. These initiatives complement Åland’s Environmental Law and Water Law. 

Despite the widespread acknowledgement of the issues, consensus to resolve them, and resultant policies and action programmes, the most recent 2021 review observed that targets have generally not been achieved as envisioned and that the Baltic Sea is still heavily affected by multiple pressures caused by human activities (HELCOM, 2021). According to Åland Environmental and Health Protection Authority, the biggest concern is the excessive emission of nutrients to water. Though cognisant of and potentially regulated by BAT, cage farms have as yet made no efforts to move towards land-based operations where effluent is more easily monitored and controlled. This reticence is due to market forces: there is no premium or value added for RAS produced fish and the competition from Norwegian cage farms is considerable. The only RAS producer in the region has yet to make a profit. Nevertheless, this and another aquaculture producer are pursuing innovative steps to reduce their waste production.

Storfjärdens fisk process their rainbow trout and were aware that the fish oil from the trimmings was a potential commodity, so they began investigating how to make biodiesel from it in 2009. The technology is simple and had existed for decades; fish oil has been used as a fuel for boats in the past. The company invested in a processor for refining the oil, and from this design built a second one themselves.  

The plant can produce approximately 400 litres of fish biodiesel a day. In a year, the plant can turn 15–20 m³ of fish oil into biodiesel. The farm has been able to successfully use this in its own operations and in the public transport system in Mariehamn, the nearest town (population 11.000). From 2013 until recently, the town ran all of its public transport vehicles solely on fish biodiesel. Sixten Sjöblom, the CEO of Storfjärdens fisk, estimates that it has reduced the farm’s waste production by 45% and instead added value from a by-product.

Fifax Eckerö is the region’s only RAS facility, with a capacity of 3200 tonnes of rainbow trout per year. Driven by the aforementioned concerns of eutrophication in the Baltic Sea, the RAS was designed to meet exceptionally high environmental standards. One of the issues with RAS is the buildup of nitrates and phosphates, which typically are removed by dilution – changing perhaps 10% of the water each day (see Section 4.2). Not only is this a waste of resources (particularly as the water will have to have been treated before and after use), but it is a source of waste. By designing specialist water treatment facilities, Fifax are able to reduce their wastewater to less than 1% of total volume; nitrogen and phosphorous emissions are a very small percentage compared with “traditional RAS”.

5.5.2. Drying

Case Study 7 – Waister AS

Waister AS is based in Åslyveien, Norway, just 45 km from Oslo, and offers an alternative to traditional ensilage. Waister’s patented drying technology turns mortalities and discarded fish into a dried and finely ground powder. This approach avoids the use of toxic chemicals such as formic acid and the downstream applications for the dried product range from fertiliser and biogas production to pet food. The €55.000 Waister 15 can process 500kg of mortalities per day in just 2,4 m². The company also produces sludge drying machines in a range of sizes, and these can be used to dry mortalities should the capacity of the Waister 15 be too low. Employees must fill the machine with fish, but the machine then operates self-sufficiently. Mortalities are dried using superheated steam and the heat can be reused within the system, resulting in energy consumption of ~0,35 kWh per kg of water removed.
     
By drying, the weight and volume of the mortalities is reduced by 85% compared to ensilage, cutting down on transportation costs and gate fees at biogas plants. Current legislation preventing use as pet food means that biogas will remain the most likely source for dried mortalities. Drying using this method produces a sanitised and stable product. After six months at room temperature, samples still had no measurable bacteria (Enterobacteriaceae, Salmonella spp, sulphite-reducing clostridia). There has been some interest from pet food producers. However, it is not an option under current EU rules even after testing to ensure the product is disease free. Legislation not designed to fit aquaculture remains, in the eyes of Waister CEO Hallstein Baarset, the biggest challenge to the circular reuse of aquaculture waste products. However, he remains confident that, despite limited downstream application options, the economic, logistic, and safety benefits of drying waste make it the best option. Waister currently supplies mortality drying machines to farms in Croatia and Australia and has plans to build a Nordic demonstration plant in the near future. 

5.6. Gas emissions
 

5.6.1. Carbon capture

Table 14. Typical reported nutritional profile of feed produced using microbial fermentation supplied with captured CO₂ to technology

This technology is still very much in the pilot stage, and it remains to be seen if capturing CO₂ in this manner would be economically viable. The disposal of sludge, discharge water, and mortalities are required and regulated facets of a fish farming operation. Because the processing of these waste streams is necessary, there is an incentive to valorise and reuse the waste. Gas emissions face no such restrictions and therefore there is less incentive to spend money reusing the emissions. The benefits of capturing the carbon would need to outweigh the cost of the capture, the capture would need to provide an economic benefit, or legislation would need to change to require the capture or reduction of CO₂ emissions. Aquaculture farms are not currently covered by the EU emissions trading system (ETS) but should this change, they would need to pay for all CO₂ released beyond a set threshold. The CO₂ permit costs reached a record high of almost €98 per tonne in August 2022. Including feed production and operational emissions, a salmon RAS facility will produce roughly seven tonnes CO_2eq/tonne of live weight (Liu et al., 2016).

5.6.2. N₂O reduction

Figure 17.  Anammox process

This process has been used in industrial discharge water treatment for quite some time, but because anammox bacteria are easily inhibited by pollutants and environmental parameters (Massara et al., 2017) wide-scale usage of this process has not resulted (Cho et al., 2019). Despite anammox reactor prototypes being reported for both cold and seawater aquaculture applications (Espinal and Matulić, 2019), commercial application of this technology in an aquaculture setting has not been demonstrated. Outside of a specific reactor, anammox still often occurs in RAS. Yogev et al. (2017) reported that 10-20% of the nitrogen removal in the denitrification reactor was by anammox. This phenomenon was also reported in a small number of additional studies (Tal et al., 2003; Klas et al., 2006; Lahav et al., 2009), with anammox bacteria from the gut of the fish accessing the denitrification reactor through the addition of filter backwash (Lahav et al., 2009). Adding filter backwash, which can serve as a carbon source (Suhr et al., 2013), ensures the regular resupply of anammox bacteria. Anammox in the denitrification reactor can be encouraged by increasing solid retention time (Lahav et al., 2009).

Whilst in its early stages in aquaculture, the use of this process would present a good opportunity to remove the emission of a volatile pollutant and increase the green credentials of the industry.

5.6.3 Methane

Like N₂O methane is a potent greenhouse gas. In land-based aquaculture, methane is typically released by microbial digestion either of sludge or suspended solids. If solids are collected and prevented from accumulating at the bottom of tanks or raceways, then methane release is likely to be low. Emissions can, however, be significant from ponds if they are not regularly dredged. As described in Section 5.2.2., biogas plants aim to produce methane to use as a source of energy. This methane is contained and used, but if sludge is stored in sludge lagoons the emission of methane into the environment will be unrestricted. The release of methane from land-based aquaculture (other than ponds and sludge lagoons) has not been well studied. The capture and reuse of any emissions are unlikely. Instead, best practices should be employed to ensure that sludge and solids are not allowed to accumulate and that feeding regimes prevent wasted feed. Mortalities should be handled in such a way that they are not allowed to decompose and release GHGs.

5.6.4 Odour

Odour emissions from land-based aquaculture are less well studied than other forms of emissions. Odour emissions in general can affect the quality of life of neighbouring populations, lead to complaints to local councils, and negatively impact public perception of the relevant industry. In Nordic countries, regulations regarding the emissions of waste minimise the emissions of odour. Immediate and appropriate handling of mortalities prevents these from becoming a health hazard or odour issue. Collecting sludge prevents uncontrolled decomposition, whilst keeping water quality high ensures it remains low in odour. Bad aquaculture practices can generate disagreeable odour, but a combination of legislation and good standards make odour emissions to surrounding areas an issue of minimal concern in Nordic land-based aquaculture production.

5.7. Other forms of waste

5.8. Reduction before reuse

Table 15. Methods for reducing waste production in land-based aquaculture

6. Results from stakeholder engagement

A large number of stakeholders (Table 16) were approached to gain a better understanding of the current state of the industry and the use of BATs. An initial questionnaire was used to determine participation interest, and a more detailed questionnaire followed for those who responded positively (Appendix 1). The questionnaire was distributed in English, Norwegian, Danish, and Icelandic. Responses from waste producers and other stakeholders were fairly limited; around half of the fish farmers responded. Other stakeholders included universities and research institutions, associations, egg suppliers, national authorities, and waste users. Of those that did respond, not all responses could be used for analysis. Many responders failed to answer every question; this was particularly apparent in questions relating to the handling of specific waste streams. Other responders gave vague or incomplete answers. No farms surveyed reported methods to mitigate gas emissions. Answers that could be used have been organised in Figures 18 – 20, showing the rough location of the responders and the method of waste treatment that they employ. Of the farmers that responded, farm size ranged from 300 kg in a small-scale aquaponics system to farms with over 20.000 tonnes per year. Recirculation levels ranged from 0% (flow-through) to 99% high-intensity RAS. Three-quarters (75%) of the farms solely produced Atlantic salmon, although three responders produced Arctic charr and three produced trout. The life stage of production varied, with grow-out farms alongside hatchery and nursery facilities. The source of water in the facilities varied; 50% of responders reported using borehole water, 27% seawater, 22% lake water, and 17% both river and reservoir water. Some responders reported on multiple farms and therefore the total number of farms is more than 100% of the responders. One responder reported using municipal water whilst another used repurposed industrial wastewater. Of the farms that responded, 38% reported belonging to an environmental certification scheme (ASC, GGAP, Debio).

Table 16. Response rate from stakeholders in Nordic countries.

Figure 18. Location of fish farms and the method of sludge treatment employed

Figure 19. Location of fish farms that responded to the survey and method of water discharge treatment

Figure 20. Location of fish farms and the method of mortality treatment employed 

7. Recommendations and considerations

Section 5 highlights multiple approaches for the management of waste streams from land-based aquaculture. For each form of waste (e.g., sludge, water, mortalities) there are several methods of waste reduction and reuse to choose from. Each of these methods has benefits and restrictions and no single method presents the best solution in every scenario. Even limited to the Nordics, land-based aquaculture occurs in multiple forms (pond, flow-through, raceway, and RAS), with different species being cultured in facilities ranging in size from less than a hundred tonnes to tens of thousands of tonnes. These facilities are distributed across the Nordics from remote regions with limited infrastructure or local industry to urban areas around major cities. Most aquaculture farms are bespoke constructions making use of the best available system technologies at the time and being designed to fit the unique requirements of the site and farm operator. All these factors combine to make choosing the best available technology for a specific site more difficult than in a more standardised or homogenous industry.

One of the major restrictors on the method of reuse chosen is farm size, as such, Table 17 lists possible BAT choices and indicates potential feasibility based on farm size. The restriction column goes into detail on factors that can limit the viability of the technology such as location, local industry or farm type. Most of the technologies are readily available from commercial suppliers or easily implementable on farm. Others are only recently being developed or tested, require legislative change to become feasible, or require adapting to the aquaculture industry. Key points to consider when defining BATs for the treatment and reuse of waste from land-based aquaculture are described in Table 18.

Table 17. Potential techniques that can be applied as BATs for the treatment and reuse of waste from Nordic land-based aquaculture

Table 18. Points to consider when defining BATs for the treatment and reuse of waste from land-based aquaculture

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Personal communication – Case studies

• Blue Ocean Technology – Hans Runshaug (CEO) and Jan Henning Legreid (Development/Sales manager)
• Danish Aquaculture Association – Lisbeth Plesner (Chief Advisor)
• Drying Matter – Jonas Christensen (Director/co-owner) and Thomas Eilkær (Director/co-owner)
• Fjell – Roy Olav Hovlid (CCO)
• Norcem – Annika Steien (Process and Environmental manager)
• Sterner – Robert Eliassen (project manager) and Dr Arne Hjalmar Knap (Co-founder/R&D)
• Storfjärdens fisk - Sixten Sjöblom (CEO)
• Waister – Hallstein Baarset (CEO)

Appendix 1

Aquaculture guidelines

Water Framework Directive, Environmental Quality Standards Directive, and other relevant water-related EU legislation

Industrial Emissions Directive and the European Pollutant Release and Transfer Register

Waste Framework Directive and the European List of Waste

• Its further use is certain.
• It can be used directly without any further processing other than normal industrial practice.
• It is produced as an integral part of a production process.
• Its further use is lawful, i.e., the substance or object fulfils all relevant product environmental and health protection requirements for the specific use and will not lead to overall adverse environmental or human health impacts.

Appendix 2

Aquaculture guidelines

Water Framework Directive, Environmental Quality Standards Directive, and other relevant water-related EU legislation

Industrial Emissions Directive and the European Pollutant Release and Transfer Register

Waste Framework Directive and the European List of Waste

• Its further use is certain.
• It can be used directly without any further processing other than normal industrial practice.
• It is produced as an integral part of a production process.
• Its further use is lawful, i.e., the substance or object fulfils all relevant product environmental and health protection requirements for the specific use and will not lead to overall adverse environmental or human health impacts.

About this publication

BAT for Reduction and Reuse of Emissions in Nordic Land-based Aquaculture

Callum Howard, Dr. Steven Prescott, Kristoffer Spigseth, Dr. Ragnhild Inderberg Vestrum, Svein Martinsen, Iselin Evje, Dr. Adrian Love, Finn Skjennum, Davide Sorella, Tamás Eisenbeck, Dr. Adrian Hartley, Freya Robinson

ISBN 978-92-893-7591-7 (PDF)
ISBN 978-92-893-7592-4 (ONLINE)
http://dx.doi.org/10.6027/temanord2023-514

TemaNord 2023:514 
ISSN 0908-6692

© Nordic Council of Ministers 2023

Cover photo: AquaBioTech Group 

Published: 22/6/2023

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