Recirculating Aquaculture Systems (RAS) – Pros and Cons

In recent years, aquaculture is one of the fastest developing sectors in the Middle East and the Arabian Gulf region. Various projects are at different stages of development or already producing variety of aquatic organisms and Saudi Arabia, Oman, Dubai and the Emirates are all encouraging foreign investments in aquaculture.

However, whilst the aquaculture industry is rapidly developing and promises huge opportunities, it also presents with challenges and constraints.

The availability of water for farming fish and other commercially valuable aquatic

organisms limits the extent to which this sector can develop in these

regions. This is especially true when fresh or brackish water species are targeted.  

Therefore, developing aquaculture in harsh environmental physical conditions, typical of

deserts and arid lands, dictates the adoption of production strategies and systems focused on water management and environment control.

Recent projects are aimed at producing salmon, a cold-water species which requires complete recirculating systems and environmental control in the ambient desert temperatures, which can soar up to 50 °C. Rearing salmon in the desert can become a reality!

Recirculating Aquaculture Systems (RAS) are new to the region. However, there is huge interest in such systems, from small-medium production units (50-400 MT) up to very large systems, which are capable (theoretically!) of producing over 10,000 MT.

From only 18% in 2019, more than 30% of CPF’s (Charoen Pokphand Foods, the largest penaeid shrimp producer in South East Asia) shrimp are now raised in RAS facilities. CPF hope to raise ALL its Thai shrimp in bio-secure, resource-efficient RAS farms by 2023.

RAS offers many benefits to the aquaculture industry:

  • Fully controlled environment for the fish and shrimps
  • Low water use coupled with high quality water
  • Efficient energy use
  • Efficient land use
  • High density culture (Fig.1)
  • Healthier and stronger fish
  • Minimizes volume of effluent, facilitating waste recovery
  • Allows for increased control over the culture environment
  • Improved biosecurity and disease control
  • Environmentally sustainable

However, there are some constraints with RAS:

  • High upfront investment in materials and infrastructure
  • High operating costs
  • A need for highly trained staff to monitor and operate the system
  • Necessity for electricity 24/7
  • Necessity for oxygen supply 24/7 (in high biomass)
  • In case of pathogen infection, the whole system must be emptied and disinfected
  • Very short response time in case of technical/human failures

Due to these constraints, the adaptation and development of RAS is still slow. Moreover, while commercial companies offer RAS for almost any cultured species, most of the RAS projects around the world are aimed at fresh water species such as salmon, tilapia, catfish etc. Development of systems for marine species is still lagging behind.

‘One size does not fit all!’ – Different species, even within the same temperature range may need different designs to deal with various issues such as increased CO2, increased ammonia, stress etc. One of the reasons for RAS projects failures is the use of a ‘standard’ design and the expectation of having similar output from a system based on results from different species.

Recirculating Aquaculture Systems can be designed as a complete closed (or almost closed) system with minimal top-up water (Fig 2) or a simple system adopting just a few of the RAS components.

RAS can be designed and built from the ground up or retrofitted to an existing system (Fig 3).

For example, in South East Asia, the ornamental fish industry in many cases comprises of small, ‘back-yard’ systems made of small concrete tanks with static or very low flow of water. A simple apparatus comprising of an integrated biofilter can reduce the ammonia in the tank and increase water quality. Such biofilters can be made locally from simple materials and have significant impacts on fish growth and survival.

Small and compact RAS can be suitable to the Middle East / Arabian Gulf region, offering a small footprint and ease of use (Fig 4). Such systems can offer an easy installation and low maintenance and can be retrofitted to existing tank systems or built from the ground up.

RAS are ideal for sensitive stages in aquaculture production. Broodstock facilities can significantly benefit from RAS as controlled environments are essential for broodstock, especially when out-of-season groups are being held (Fig 5). Removing risk of pathogen infection in broodstock facilities, which are the most important part of any aquaculture project, can enhance the economic security of a project. RAS are also ideal for hatchery and nursery stages where the larvae are very sensitive to pathogens and need stable conditions.

Moreover, culturing non-endemic species requires tight control on effluent and, in many cases, the use of RAS is one of the culture permit’s requirements.

However, as aforementioned, RAS technology is relatively new. Whilst there are successful projects which grow fish and have a good return on investment, there are many other projects that are either not reaching their designed capacity or are simply not profitable due to higher production costs,  low ROI etc.

RAS Components

Recirculating water process has several steps and components:

  1. Removing solids such as feed leftover and feces from the water which is achieved via mechanical filtration. There are different solutions for solid removal including drum filters, parabolic screens, swirl separator and others (Fig 6).
  2. Removing and transforming harmful soluble nutrients such as ammonia and nitrite through a biological process. This process is called ‘biofiltration’ and it is carried out by using specific bacteria that transfer harmful ammonia to nitrite (nitrosomonas bacteria) and then to nitrate (nitrobacter bacteria) which is harmless metabolite. The process requires a large number of bacteria, hence, there are specifically made apparatus which have large surface to volume ratios (‘bio-balls’) which creates a surface area for the bacteria. Water passes through the bio-balls and the bacteria ‘strip’ the ammonia from it.
  3. Removing excess CO2 and oxygenating the water through ‘de-gassing’ via exchange and passing of atmospheric air and water. This can be followed by further oxygenation of the water with pure oxygen. De-gassing columns are used, in which a large volume of air is pushed up by blowers and passes through the water droplets (Fig 7).
  4. Disinfecting the water to remove and eliminate pathogens using UV radiation and/or ozone. This is an essential stage that removes all or most of the bacteria and viruses from the water.

In many cases some of these processes are combined into one apparatus such as a protein skimmer which removes organics, disinfects the water (when ozone is used) and enriches the water with oxygen.

One aspect of RAS that is very important is the monitoring and management of the systems and their parameters. Each stage must be closely monitored to ensure optimal water parameters. As the system is based on many sub-systems, pumps, electrical devices etc. that tend to fail, it is essential to have a real-time monitoring and alarm system for oxygen, water flow, ammonia, CO2, pH etc. Backup systems including electrical and oxygen are essential due to the limited response time (high fish biomass will strip the oxygen from the water within minutes) the system has.

Aqua Feed.
Feed for aquatic organisms are, in many cases, made with the assumption that the fish and/or shrimp are growing in an environment that can contribute to their nutrition. For example, ponds develop a natural biota with time after inoculated. This biota contributes to the nutrition of the culture organism. By nature, RAS is a ‘clean’ system without any nutritional input aside from the feed. Moreover, especially in shrimp culture, the feed needs to be stable in the water for a longer period of time to prevent increased organic load on the system. However, feeds designed for RAS have recently started to become available and, in most cases, standard feeds are still used in RAS.

In some case, It was identified that fish reared in RAS have a muddy off-flavor. The amount of off-flavor will vary with the system, water quality parameters, species and size of fish, and even individual variations. The source of the off-flavor is accumulation of the cyanobacteria metabolites geosmin (GSM) and 2-methyl-isoborneol (MIB) in the fish. Eliminating the off-flavor is slower than uptake, and the rate of elimination is reduced as water temperature decreases and tissue lipid content increases. Management of off-flavors caused by aquatic microorganisms is difficult under commercial conditions. Strategies include eliminating odor-producing cyanobacteria from culture systems (using ozone) or purging the fish without feed in clean water before harvesting. In many cases, new RAS are designed to deal with this issue.

Recirculating Aquaculture Systems technology is fast developing around the world. It presents many advantages over traditional culture systems such as ponds and cages. While it seems that RAS is the way forward for certain species and locations, it is still relatively new and may not result in good Return On Investment.