Biofilters are commonly used in recirculating aquaculture systems (RAS), but because of their limitations, innovators are busy developing viable alternatives – such as electro-oxidation – or improvements, such as electrocoagulation and UASB reactors.
RAS is widely used for fingerling rearing, however its ability to efficiently produce market-sized fish such as salmon remains a subject of debate.
In a typical recirculating aquaculture system (RAS), a series of treatment processes are used to maintain the required water quality levels and also to sustain the growth of the cultured fish species, while maintaining a very low water exchange rate. An important aspect of the treatment process is biofiltration.
How does a biofilter work?
Biofilters use nitrifying bacteria to convert ammonia to nitrate and further break down dissolved and residual particulate matter. Biofilter performance is affected by many factors, including substrate type and water quality . Most modern systems are automated to maintain these key factors at precise, predetermined levels and prevent any rapid changes.
Bacteria form a biofilm on the surface of a carrier, known as the biomedia. Pore size, specific surface area, resistance to mechanical shear, and fill ratio (percentage of the biofilter volume that is empty) are important parts of how well a biofilter performs.
To increase the efficiency of the biofilter, solids are first removed from the process water, reducing the biochemical oxygen demand, nitrogen, and phosphorus loads of the system. Methods of rapid solids removal include screening, sedimentation, flocculation, and adsorption. These techniques reduce biological media clogging and the accumulation of suspended and settled particles in the biofilter.
Application of biofiltration in aquaculture
Biofiltration has been widely used in recent decades because of its effectiveness and relatively simple maintenance in removing nitrogen pollutants from process water in aquaculture.
This technique has been adopted from wastewater treatment plants where cultures of floc-forming microorganisms called activated sludge are used to treat water. In aquaculture, instead of using activated sludge, artificial biological media are often used as carriers for bacteria. Biological media come in many shapes and sizes and are usually made from food-grade polyethylene or polypropylene. The shape and pore size of the biological media are determined by the rate of organic loading and the type of biofilter in which it is used.
The two most common forms of biomedia biofilters used in RAS are the moving bed bioreactor (MBBR) and the fixed bed bioreactor (FBBR). FBBRs have lower energy usage, higher phosphorus and nitrate removal rates, while MBBRs have higher volumetric loading rates and lower solids accumulation (Choi, Lee, & Lee, 2012).
Other types of biofilters have been used in aquaculture, such as biofloc , wetlands, integrated multitrophic aquaculture , sand-gravel beds, and ion exchange membrane bioreactors. However, to date, none have been successful in commercial aquaculture operations, mainly because they tend to increase operating costs and carry a higher risk of failure.
Biofilters are the heart of RAS facilities
Limitations and challenges of traditional biofilters
One of the major challenges in biomedia biofilters is that their effectiveness varies with the type of substrate and depends on the levels of dissolved oxygen, organic matter, temperature, pH, alkalinity, turbulence, and salinity (Qi, Skov, de Jesus Gregersen, & Pedersen, 2022). Any rapid changes in these factors affect the microbial community and thus the efficiency of the filtration process. On top of that, biomedia can also harbor pathogens and bacteria that produce off-flavors.
Furthermore, bacteria in the biofilter consume a significant portion of the total oxygen demand in the RAS (20-30%), excreting CO2 and other metabolites. This increases the required capacity of the degassing and oxidation equipment as well as the operating costs in terms of energy requirements and maintenance.
Alberto Monteleone, R&D facility manager at the AquaBioTech Group in Malta, who oversees research trials at more than 30 RAS facilities – meaning he has more than 30 biofilters to maintain and restart regularly – explains that water changes, temperature fluctuations and pH changes have a negative impact on biofilter recovery times.
“One of the big problems is the long and unpredictable startup timeline, which makes planning research trials particularly difficult but crucial to success,” he notes.
Monteleone hopes that more reliable water quality sensors will become available to identify problems that could lead to bacterial population collapse in biofilters.
Biofilters also present some significant challenges from an engineering perspective. Typically, designs need to be integrated into existing structures, which imposes constraints such as structural loads and size constraints. These can then negatively impact the ultimate efficiency of the biofilter. Another issue is that aquaculture operators can vary feeding rates and stocking densities, which makes it challenging to design a stable and efficient biofilter.
“To address the space constraints, computational fluid dynamics simulations were used to come up with designs that provided the most suitable hydrodynamic profile for the required outlets and available space,” explains Michele Gallo, head of aquatic research facility design at AquaBioTech.
Additionally, microparticle filtration through methods such as protein separation, is used to improve biofiltration performance.
To complement the design, the production plan needs to be optimized in a way that provides a smooth transition between different growth stages and minimizes biofilter load fluctuations.
Gallo also noted that he has seen rapid growth in AI technology deployed in agricultural management software, where data collected by monitoring systems is analyzed using AI models to improve decision-making processes for operators and control systems.
New developments in filters
In recent years, several relatively new technologies have emerged as alternatives to conventional biofiltration, although these technologies have not yet been deployed on a large scale.
1. Electrooxidation process
Electrooxidation is an alternative to biological water treatment, removing particulate matter and eliminating odors and taste agents. Electrooxidation of ammonia occurs in a reactor through electrolysis, using an electric current to promote the formation of nitrogen gas from ammonia while removing total organic carbon (TOC) and reducing pathogen counts.
Electro-oxidation process in Eloxiras, a product developed by Apria Systems © Apria Systems
Electrooxidation promises low energy use in marine aquaculture systems, even at high stocking densities.
German Santos Bregel, senior R&D engineer at Apria Systems , a company specializing in electrochemical oxidation in the aquaculture sector.
Implementation of electro-oxidation systems in RAS © Apria Systems
The most significant advantage of electro-oxidation is that it can be started and stopped without significant impact on the production cycle in the RAS.
“A notable advantage is the ability of the process to operate at full capacity, even at temperatures as low as 5°C,” says Bregel. Removing multiple contaminants in one process also means that capital costs for RAS can be reduced, as the need for ozone and UV treatment can be significantly reduced or even eliminated.
One of the major drawbacks of this process is the formation of trihalomethanes (THMs) – a byproduct that is toxic to aquatic organisms. A low pH is required during electrolysis to inhibit the formation of THMs (Ben-Asher & Lahav, 2016). There are two treatment processes: degassing and absorption of the byproducts. Degassing is already present in conventional RAS, while absorption is performed using granular activated carbon and is an additional component that needs to be considered in terms of investment and operation.
“We do not have enough references for our technology because the sector is still dependent on traditional filtration methods and because of the lack of funding to further improve, develop and integrate this technology for RAS,” notes Bregel. The system appears most promising for marine RAS, since their efficiency depends on the salinity of the water.
2. Electrolytic capacitor
The basic process in electrocoagulation is that an electric current is passed through electrodes made of the same material and immersed in treated water or some other electrolyte. During this process, metal oxides, hydrogen, and oxygen are formed. The metal oxides attract the pollutants and have a high tendency to form flocs with them. The lighter flocs then rise to the top of the water column with the help of hydrogen and oxygen gas produced during electrolysis. The flocs can then be easily removed by skimming. The heavier flocs sink to the bottom and are then removed as sludge by sedimentation or even mechanical filtration (Boinpally, Kolla, Kainthola, Kodali, & Vemuri, 2023).
In the RAS field, electrocoagulation is interesting in two respects: assisting in the removal of particulate matter and oxidizing dissolved pollutants such as various sulfur compounds and ammonia nitrogen. Another advantage is that the electrolytic cell can be easily adapted to different conditions such as feed rate and temperature changes by supplying more or less current to the electrodes.
This method has challenges, namely that the contaminant removal efficiency varies due to several factors such as pH, temperature, water velocity, and current density in the cell.
NaturalShrimp uses electrocoagulation to help purify water in shrimp tanks © NaturalShrimp Inc
In a recent development, this technology has been implemented in shrimp RAS by a company called NaturalShrimp . Their main goal was to increase the size of the total suspended solids so that they could be removed by the micro-drum filter. This process also increased the efficiency of removing chemical oxygen demand, ammonia and nitrite (Ben-Asher & Lahav, 2016). To address the ever-changing environment in aquaculture water, they measured the turbidity of the water and then varied the current applied to the electrodes accordingly.
The company aims to conduct several smaller trials with local shrimp farming companies to further validate its technology.
3. UASB reactor
Another recently developed potential process for RAS is the upflow anaerobic sludge blanket reactor (UASB). It is used to treat wastewater leaving the RAS facility (end of pipeline treatment) or as a polishing step to return water to the system.
RAS wastewater treatment results, Landing Aquaculture, 2024
The UASB integrates the bioreactor and the settling tank, making it very compact in the field of anaerobic reactors.
“A significant advantage is the ability to separate hydraulic retention time from sludge retention time, allowing for different filtration objectives. For example, maintaining a solids retention time of around four days facilitates the production of volatile fatty acids that support denitrification. Shorter retention times promote the production of organic acids, which help solubilize nutrients for aquaponic or integrated multitrophic aquaculture (IMTA) applications. Longer retention times are useful for biogas production. Additionally, the flow through the reactor can be controlled while retaining sludge, allowing for optimization of removal rates and effluent concentrations for specific parameters,” said Carlos Espinal, director of innovation at Landing Aquaculture , a consulting and engineering firm focused on intensive land-based fish farming.
The performance of the reactor is dependent on temperature. If operated at suboptimal temperatures (below 20 and above 35°C), the bacterial population will change and communities better suited to a given temperature will colonize the sludge bed. To maintain nitrogen removal rates, the temperature needs to be at an optimal level and the temperature change needs to be gradual. If a significant drop in temperature is anticipated, the reactor must be sized to accommodate those changes.
Additionally, Espinal highlighted the problem of sludge buildup in freshwater reactors and the high initial cost of the automation equipment required for proper control and monitoring. To prevent this, Landing Aquaculture is using an automatic drain system and designing a specific outlet. When the cost of additional automation is too high, manual monitoring and operation of these reactors can be implemented.
Unlike electrooxidation, UASB has been deployed in the field.
“These systems are already used in the Netherlands for end-of-pipe treatment in fish farms and are among the recommended practices according to the Dutch Sustainable Aquaculture Standard ,” says Espinal.
Future prospects
At present, it appears that conventional biofiltration will continue to exist due to its long tradition and proven effectiveness. Electrooxidation and electrocoagulation have the potential to replace biofiltration or at least alleviate the shortcomings of the filtration methods used in RAS. It appears that no other technology can compete with the unique role of nitrifying bacteria in biofiltration. Improvements in conventional biofiltration are expected in the form of improved biomedia and better flow configurations of biofilters.
UASB reactors are likely to find their way into downstream treatment as environmental standards become more stringent and RAS farms become larger, making these types of reactors more feasible. Further developments in water quality sensors will provide greater insight into microbial health and give farmers more opportunities to take corrective action.
THAN VUONG COMPANY
SOURCE: THE FISHSITE
