This article was featured in Eurofish Magazine 1 2026.
Paras Aqua’s hybrid flow-through system challenges conventional RAS and other hybrid models.
Land-based fish farming is often held back by high investment costs, complex biological treatment systems, and strict environmental constraints. The Finnish company, Paras Aqua, a spin-off from the Natural Resources Institute Finland (LUKE), believes it has found a practical way forward. Its hybrid flow-through system (HFS) offers many of the environmental and operational benefits of recirculation aquaculture systems (RAS) while avoiding their cost and technical fragility.
Founded in 2023 by Tapio Kiuru, who doubles as the Chief Technology Officer, Paras Aqua emerged as a spinoff from LUKE, where Mr Kiuru worked for several years seeking to address the limitations of full RAS. The company’s aim, as he explains, is to make land-based aquaculture both sustainable and economically viable by reducing water use, cutting emissions, and improving fish welfare.
Removing the biofilter bottleneck
Mr Kiuru’s experience in designing and operating large-scale RAS projects in Finland revealed how expensive and demanding biofilters can be. They require constant oxygen input, release carbon dioxide, and are the main source of off-flavour compounds that later have to be removed through depuration. Paras Aqua’s system was conceived to eliminate this dependency.
In a conventional RAS facility, the biofilter is the heart of the operation, converting ammonia into nitrate through bacterial processes. But those microbes are slow to mature—taking months or even years to reach full efficiency—and are highly sensitive to variations in temperature, load, or disinfection. Because the biology cannot easily be interrupted, such systems are difficult to clean or restart. Paras Aqua’s hybrid flow-through model does away with biological
filtration entirely. Instead, water is mechanically treated and continuously refreshed at a low rate. Without the biofilter, farms can disinfect tanks at any time and resume full production immediately. This approach offers far greater biosafety than RAS, where pathogens can persist in shared water or infrastructure. The absence of a biofilter also means there is no need for separate depuration tanks. Fish grown in Paras Aqua systems do not accumulate off-flavours at concentrations detectable by humans, so the lengthy purging stage—typically one to two weeks in RAS—is unnecessary. Only a brief fasting period before harvest is needed to empty the gut for product quality. This saves labour, prevents biomass loss, and improves animal welfare by avoiding additional handling.
A high-reuse, low-impact concept
Although classified as a hybrid flow-through system, Paras Aqua’s technology reuses most of its water. Trials with rainbow trout have demonstrated reuse rates well above 90 percent, with as little as 1,000 litres of new water needed per kilogram of feed. The precise rate depends on the alkalinity of the water source, but even under less favourable conditions the consumption remains a fraction of that required by other hybrid systems or traditional flow-through farms. This efficiency has two immediate consequences. First, energy use is reduced because heating or cooling a small water volume is far less demanding than controlling temperature in an open-flow facility. Second, the environmental load from effluent discharge is easier to manage. The company’s approach to sludge removal is notably simple: solids settle in radial-flow cones directly beside each tank, are withdrawn automatically before nutrients dissolve, and can be used as agricultural fertiliser without chemical treatment. Compared with drum-filter systems that produce hundreds of litres of dilute sludge per kilogram of feed, Paras Aqua’s process yields a thick, nutrient-rich material of about 6.5 litres per kilogram.
Paras Aqua’s hybrid flow-through model distinguishes itself from other systems in that
each tank is a self-contained production unit with its own aeration, oxygenation, and control equipment.
The main trade-off is that ammonia remains the dominant nitrogen form in the effluent, whereas RAS typically discharge nitrate. This means that if farms must remove nitrogen biologically, an additional nitrification step is required before denitrification. Paras Aqua is developing solutions to make this process more cost-effective, but phosphorus removal is already highly efficient because the solids are captured so quickly.
Modularity and decentralisation
Beyond the water-treatment principle, the physical layout of the system is what truly distinguishes Paras Aqua from other land-based concepts. Each tank is a self-contained production unit with its own aeration, oxygenation, and control equipment. Water never mixes between tanks, so every group of fish receives a pristine flow of clean water. Centralised functions—such as temperature regulation—serve the farm as a whole, but almost all components related to biosafety are local to the tank. This decentralised structure reduces both biological and technical risks. A pump or aerator failure can affect only a single tank, and even in that scenario emergency oxygenation prevents losses. The modular design also allows farms to expand gradually: a ten-tank installation can be extended to fourteen without modifying the shared infrastructure. Temperature control adds further flexibility. Dual piping for inlet and outlet water enables precise thermal management in each tank. Farmers can acclimatise smolts to lower temperatures before transfer to sea -cages or slow the growth of fingerlings meant for later stocking. Such control is not possible in RAS, where a central biofilter requires uniform temperature and nutrient supply across the system.
The high degree of water reuse would not be feasible without precise automation. Paras Aqua and its development partner Schneider Electric have built a monitoring and control system that regulates oxygen, carbon dioxide, and pH in real time. Adjusting CO₂ and oxygen simultaneously allows operators to steer the acid-base balance in each tank
Early performance and first commercial use
The technology has been tested extensively at LUKE’s research facilities. Production-scale trials in 100-cubic-metre tanks achieved feed-conversion ratios (FCRs) close to 1.0 even for large rainbow trout and 0.8 for smaller fish.
Densities of up to 127 kilograms per cubic metre were reached with negligible mortality. Even European whitefish—a more delicate species—performed well. Paras Aqua’s first commercial installation is now being built for the Finnish company Nilakkalohi. It will produce 500–700-gram rainbow-trout post-smolts for grow-out in Baltic Sea net pens. The plant consists of multiple decentralised tank units and demonstrates the scalability of the concept for industrial use. Interest extends beyond Finland. Norwegian producers, familiar with large hybrid projects such as Salmon Evolution, are watching the Finnish development closely. Mr Kiuru points out that those systems reuse only a modest share of their water and rely on proximity to the sea, making them site-dependent and energy-intensive. Paras Aqua’s design, by contrast, can be operated almost anywhere freshwater or saline water sources are available.
In the Nordic context, where environmental licensing is stringent, reducing both water abstraction and effluent discharge is critical. Paras Aqua’s approach aligns with these regulatory pressures while maintaining profitability. The absence of biofilters lowers capital expenditure and simplifies operation, making land-based farming accessible to smaller operators as well as large investors. Energy demand remains moderate thanks to limited water volumes and effective temperature control. The company estimates that overall running costs are significantly lower than for comparable RAS, and the simplified waste-management system removes the need for chemicals or complex sludge-drying processes. At the same time, fish welfare indicators have been positive. Trials recorded very low mortality and good external condition, with minimal fin erosion or wounds. The tanks’ figure-of-eight flow pattern encourages varied swimming behaviour instead of continuous circular motion, which may help reduce stress and improve growth uniformity.
Research-driven development
Although the technology is already commercially viable, Mr Kiuru emphasises the company’s commitment to further research. Hybrid flow-through systems remain under-studied compared with RAS, despite being used in some of the world’s largest new aquaculture projects. Paras Aqua’s team, composed mainly of former researchers, intends to close that gap. Current priorities fall into three broad areas: environmental performance, farm economics, and fish welfare. Work is underway to reduce energy consumption and material use in construction, and to refine nitrogen-removal methods. On the welfare side, the company plans to investigate how hydrodynamics, lighting, stability of water parameters, and temperature regimes influence behaviour and health. Mr Kiuru argues that technological progress in RAS over the past two decades has been slow relative to its complexity, whereas hybrid systems are still at an early stage but already are delivering encouraging results. With more research and industrial feedback, he believes hybrid flow-through technology could become the dominant model for land-based production within this decade.
The first commercial installation of the Paras Aqua system is underway at the company Nilakkalohi’s farm.
The 1,200 sq. m unit will double existing capacity.
Paras Aqua’s technology reflects a pragmatic approach to aquaculture innovation: rather than pursuing maximum recirculation at any cost, it has balanced water consumption against the advantages of removing the biofilter. For investors and farmers, this offers a path to economically feasible, environmentally compliant land-based farming without the high capital intensity of RAS. For regulators, it demonstrates that reduced emissions and good fish welfare can coexist with industrial productivity. As the first Finnish facility nears completion and potential partners in Norway and elsewhere observe its performance, Paras Aqua stands at a decisive moment. Success would not only validate the company’s technology but could also shift perceptions of what land-based aquaculture should look like—simpler, cleaner, and more adaptive to local conditions.
