The Innovative Filter Media Revolutionising Aquaculture

By Mark Barrett, Director of Warden Biomedia, UK

More than 3 billion people rely on seafood, whether wild-caught or farmed. Aquaculture is the fastest growing food supply sector in the world, and the rapidly growing industry will help significantly improve local, regional and global food security - but does the future of aquaculture belong in the ocean?

Open-sea farming faces significant issues. Environmental pollution from fish waste and uneaten feed, harms marine ecosystems. High population densities in net pens are a thriving ground for diseases and parasite outbreaks, especially sea lice, necessitating the use of antibiotics and pesticides that can further damage the environment. Escaped farmed fish, which often occur during storms or net failures, threaten wild fish populations by competing for resources and interbreeding. And the industry suffers from high mortality, far greater than other traditional livestock farming.

Moving fish farming out of the ocean can protect the environment and its wildlife. The result, land-based aquaculture will play a key part in the future of the industry.

Going land-based

Onshore facilities utilise recirculating aquaculture systems (RAS) to carefully control the conditions, continuously recycling and cleaning the water, to ensure healthy and disease-free fish without antibiotics. This mitigates the risks of ocean farming, such as temperature swings caused by global warming, storms, and disease, while preventing environmental pollution into the ocean.

RAS recycle between 90-97% of the total water in the system. Water lost to evaporation, cleaning and sludge discharge is replenished from a borehole or even a municipal water supply, so the system can be located almost anywhere and does not need to be near the sea, river or lake.

Chemotherapeutical dosing is greatly reduced and overall fish health can be significantly improved due to the high level of biosecurity and disinfection of all intake water.

Maintaining water quality with RAS

The most important factor in successful RAS operation is control of fish health, and a vital factor is circulating water quality. High fish densities result in high loads of metabolic wastes – BOD and ammonia. Biological wastewater treatment processes use beneficial bacteria to purify the circulating water, and they are encouraged to grow on media in the treatment systems.

In aerobic conditions, bacteria like Zooglea, Achromobacter, Pseudomonas and Flavobacterium oxidise carbonaceous BOD to carbon dioxide, whilst Nitrosomas and Nitrobacter convert ammonia into nitrate. If nitrate accumulation becomes a problem, denitrification species like Thiobacillus denitrificans, can convert nitrate to nitrogen gas in anaerobic conditions.

The process of choice for biological treatment of RAS circulating water is the aerobic moving bed biofilm reactor (MBBR). The bacteria grow on the specifically-designed media, and consume the contaminants in the water as a food source, forming a deposit on the media surface, called a biofilm.

The media is free to move within the aeration reactor and is self-cleaning. Compared with technologies like trickling filters, BAFFs and SAFs, MBBRs give increased biomass retention time, higher loading rates and a smaller footprint. Providing these advantages is the function of the media, so selecting the right one is vital to optimising performance.

New sustainable media

New filter media developed after a three-year research partnership between Warden Biomedia and Cranfield University can help improve RAS sustainability and efficiency. Whilst a high protected surface area is desirable to maximise biofilm growth, the research found that it was not the governing factor for overall media performance. Laboratory testing identified shape and dimensionality as key parameters, alongside protected surface area.

Lower energy requirements

The Cranfield research showed that spherical-shaped media design can reduce the energy input required in the effluent treatment system. The design improves media mobility, requiring less energy to be agitated in solution, and therefore reduces aeration energy requirements and improving wastewater treatment efficiency.

The new media also showed improved Standard Oxygen Transfer Efficiency (SOTE) by around 30% in trials against other previous generation media, showing potential for higher operating dissolved oxygen (DO) for a fixed airflow. MBBR pilot trials confirmed these findings, with DO around 0.5 mg/l higher for the new spherical design, compared to previous-gen media.

The spherical-shaped media with large, protected surface area of 500m2/m3 and high voidage of 81%, is optimised for high biofilm attachment and biological treatment performance. It is manufactured in the UK from high-quality ‘A’ grade virgin HDPE (or recycled PP), contains no halogens or contaminants, and is ideal for aquaculture applications. The media is extremely durable, so never need to be replaced, and is fully recyclable at end of process life, helping contribute to a circular economy.

Looking forward

While opinions are very much divided in the tussle between land-based and ocean-based aquaculture, there are two things all experts seem to agree on. Aquaculture will continue to grow in importance to the global food supply. And land-based aquaculture will play a key role in the future of the industry.

Whether on land or in the ocean, the answer will always lie in sustainable practices, and developing more sustainable processes, systems and tools. Next-generation, efficient and recyclable filter media can reduce aeration energy requirements, improve process security and biological commissioning times, increase biomass retention and wastewater treatment capacity, reduce required tank volume, and promote generation of healthy biofilms. The new media, Biopebble, is now available to the market, after successful pilot and full-scale trials.

Original article was published in International Aquafeed September Issue 2024.