Mind Your Solids – An Important Tool for Grow-Out Projects
By Philip Haargaard Gyldenkærne, Biologist and aquaculture engineer at CM Aqua.
For the past 10 years, CM Aqua has produced and delivered HEX drum filters and Ratz Protein Skimmers to the Norwegian and global RAS industry, focusing on durability and long-lasting products made from stainless steel and PE (polyethylene) for both fresh and seawater applications in any environment.
Mechanical filtration design
It has always been our top priority to advise and support our partners on the small details that a designer must include in a RAS system to optimize mechanical filtration and ensure gentle handling of particles as they travel from the fish tank to the HEX drum filters.
In the design between the tank and the mechanical filtration, attention should be given to the flow to reduce unnecessary turbulence that can break particles and create fine particles below the fine mesh (40-80 microns) of the drum filter in the RAS.
It is well proven that every step in the supply chain strives to design feeding systems, tanks, piping, and filtration systems to minimize fine particle buildup in the system water.
However, completely avoiding particle breakage in a RAS system is impossible—some turbulence from fish, piping, and weir level control will contribute to the background level of fine particles found in RAS water. Additionally, there is a natural contribution from the leaching of DOM and fats from feces, feed pellets, and dust formation from feeding systems.
RAS has proven its value by providing a steady supply of high-quality juvenile fish of various species for net pen grow-out systems. Grow-out RAS projects for Atlantic salmon, which are leading the way for other species, have experienced some setbacks over the years. However, we see more and more projects being planned and built. Some are running successfully, thanks to years of dedication to developing system setups and tweaking design parameters. It would be unwise not to learn from the system designers and operators of these projects to understand the ingredients of their success.
Optimising ozone and skimmer use
The use of ozone is considered a standard component in the RAS loop, but there is still debate on how to properly dimension and dose it. Many factors come into play, with feed load being the most important from the system designer’s perspective. The rule of thumb is 15 g O3/kg feed, and many systems are installed with safety levels up to 20-25 g O3/kg feed. Yet, we come across systems performing grow-out RAS with much lower ozone levels, down to 10-12 g/kg feed, at least in seawater operations.
The common factor across these systems is the extensive use of protein skimming and detailed attention to its operation. Protein skimming, or foam fractionation, with ozone is a combined solution where the two technologies enhance each other’s performance.
Ozone works by direct and indirect oxidation of dissolved or particulate organic matter, altering the molecular structure or completely oxidizing it. This can kill bacteria and remove color from the system. If the ozone does not fully oxidize the organic component, the protein skimmer takes over, lifting the once-dissolved organics out as absorbents attached to the rising bubbles. Ozone enhances the lifting performance by altering the chemistry of the organics.
This presents an opportunity: we only need partial oxidation of organic matter, which can then be completely removed from the RAS loop through the foam fraction. As a result, the oxidized organic matter will not serve as a nutrient source for heterotrophic bacteria downstream.
A RAS biofilter would typically be designed to handle the background level of organics and could still perform its primary task of converting TAN (total ammonia nitrogen) to less harmful substances, forming the core of the full RAS cycle. This approach has been successfully implemented for many years.
Challenges in grow-out systems
However, challenges in grow-out systems can arise on harvest day, when planned purging may prove insufficient to remove a potential buildup of off-flavors (such as geosmin or MIB) in fish tissue. These off-flavor compounds can ruin the planned harvest, drastically reducing sales prices and frustrating designers, operators, buyers, and investors alike.
Heterotrophic bacteria will always be present on surfaces in RAS, but the above management strategies can help reduce the bacterial load that can cause off-flavors and increase oxygen competition within biofilms.
Looking at the industry, it is capable of growing large fish, raising funds, and reducing mortality to levels comparable to other farming practices. Moving forward, the industry must work together to develop more robust water treatment designs, which may differ slightly from smolt or juvenile operations. This could be seen as unnecessary by some who have successfully raised smolt in “RAS water” for 20 years without issues.
On the other hand, the relatively low levels of ozone usage with protein skimming, compared to other injection technologies, could be a future path for RAS management. Chemical treatment of organics with ozone creates new, sometimes unspecified or even unwanted organic compounds that could reach the consumer’s dinner plate. As pointed out at a NOFIMA event in Sunndalsøra, we do not yet have the full picture of this. The food safety implications of ozone are still speculative, but reducing any technical input into the system can be seen as an asset that every RAS designer would appreciate. From an economic perspective, reducing power and oxygen consumption is always beneficial.
Approaches for industry advancement
The strategies outlined here cannot be considered a silver bullet for managing off-flavors and DOM, but they should be seen as essential tools for reducing the organic load in the system. NOFIMA has already shown that lowering TSS levels significantly improves growth and health metrics for salmon smolt, meaning this approach serves multiple purposes, including promoting welfare and growth.
Theoretical studies of ozone and protein skimming support this development. Publicly available information from RAS suppliers shows that extensive protein skimming is already being implemented, though design parameters are still being debated.
While no one can provide a simple and definitive answer on how to dimension these systems, we strongly believe that when applying ozone in low-pressure systems like protein skimmers, we must respect the laws of physics. Degassing is inevitable according to Fick’s law and should be considered during the design phase. Dosing should also be based on the volume of water being treated, instead of a one sided focus on ozone to feed ratio.
Below is our best guidance for dimensioning these systems without wasting too much excess ozone. Ultimately, it is the RAS operators who will adjust settings based on water matrix readings. With each visit, we continue to learn more about RAS management.
Skimmer sizing is often a number based on % of flow, while feeding, stocking density, ozone dosage and tank retention time is not considered. The above formula can be used to give a calculated answer to the skimmer sizing. Retention time will play a role in how polished should be when water exits the skimmer. Designing with a higher retention time gives the possibility to adjust flow up in case more water should be treated in peak load situations.
In collaboration with CM Aqua’s team, RAS designers, and operators, we are pushing the industry forward together.
For additional information feel free to contact CM Aqua at info@cmaqua.dk
