Ozone in recirculating aquaculture systems

Introduction

Research indicates that ozone is increasingly recognized for its ability to enhance water quality in Recirculating Aquaculture Systems (RAS). These systems offer flexibility, reduced environmental impact, and heightened production intensity. However, the challenges associated with waste accumulation and environmental control necessitate advanced waste management solutions.

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To address waste accumulation in RAS, conventional removal methods like microscreen filters and sedimentation tanks only provide partial solutions. They fail to eliminate fine colloidal solids, which can lead to significant inefficiencies in biofilter systems. As production intensity increases, the ability of bacteria to efficiently convert nitrite to nitrate can diminish, leading to elevated nitrite levels. Ultimately, this buildup of organic waste creates a less stable and less productive system.

Increasing the daily water exchange rate in RAS improves waste removal but adversely affects operational costs such as heating or cooling. Therefore, utilizing an oxidizing agent, such as ozone, becomes imperative for waste breakdown. Additionally, ozone plays a critical role in sterilizing supply and effluent water in RAS, effectively eliminating pathogens. This paper explores the various applications, benefits, and risks of ozone use in aquaculture systems.

The Chemistry of Ozone

Ozone is a highly reactive and unstable gas, identifiable by its distinct odor at low concentrations. Formed when diatomic oxygen (O2) bonds with an extra oxygen atom (O), ozone's instability makes it an exceptional oxidizing agent ideal for water treatment. Nonetheless, on-site generation and immediate use are necessary due to its hazardous nature.

Uses of Ozone

Ozone can be employed in water treatment for several key purposes:

Removal of fine and colloidal solids

Small particles, ranging from 1-30 microns in size, are classified as fine solids, while colloidal solids measure between 0.001-1 micron. Their minute size allows them to remain suspended, making mechanical removal methods ineffective. Accumulation of these solids can impede biofilter efficiency and stress aquatic life.

Ozone effectively aggregates fine and colloidal solids via microflocculation, enhancing removal through filtration, foam-fractionation, and sedimentation.

Removal of dissolved organic compounds

Dissolved organic compounds (DOCs) produce a tea-colored hue in the water and are non-biodegradable, accumulating based on feed input and water exchange. Elevated DOC levels can stress fish and lower biofilter nitrification efficiency.

Ozone oxidizes dissolved organics into more easily nitrifiable products while enabling the precipitation necessary for waste removal through conventional filtration.

Removal of Nitrite

As production intensifies, nitrite levels can rise due to increased organic load. Although the bacteria responsible for converting ammonia to nitrite (Nitrosomonas spp) thrive under heavy loads, those converting nitrite to nitrate (Nitrobacter) do not, leading to harmful nitrite accumulation.

Ozone eliminates nitrite through:

• direct oxidation to nitrate;
• reducing organic loads to enhance biofiltration efficiency.

Disinfection

High stocking densities and increased nutrients in RAS create favorable conditions for pathogens. Employing quarantine procedures for introduced fish is essential. Furthermore, disinfection of effluent water before environmental release is necessary to prevent transferring diseases.

Ozone efficiently inactivates various bacterial, viral, protozoan, and fungal pathogens, with its efficacy dependent on concentration, exposure time, pathogen loads, and organic matter levels.

Production of Ozone

Modern ozone generators typically utilize corona discharge or ultraviolet (UV) light to generate ozone. Corona discharge systems create a high-energy electric field to convert oxygen into ozone, while UV light breaks down oxygen molecules. Though UV generators are initially cheaper, they are less energy efficient compared to corona discharge systems.

Ozone Application

System specifications

The design of the ozone reactor is crucial for effective and safe ozonation. Numerous reactor types facilitate the ozone transfer, including fine bubble diffusers and static mixers, each with distinct advantages and drawbacks. Key considerations when selecting a reactor include:

• ozone transfer efficiency;
• leak-free design and assembly;
• materials that resist ozone corrosion.

Poorly constructed materials may erode and lead to dangerous leaks, diminishing overall ozone efficiency. Suitable materials, such as stainless steel, are recommended for ozone systems.

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Treatment Regimes

Ozone can be applied continuously or in batches based on feeding schedules. Concentrations of ammonia and other wastes peak several hours post-feeding, making timed ozone applications effective in targeting waste increases. Continuous ozonation maintains stable water quality; however, cost-effective batch treatments can also be employed.

Recommended ozone rates typically range from 10 to 15 grams ozone per kilogram of feed, considering any background organic loading present in the source water.

For disinfection purposes, the required ozone amount greatly depends on the baseline organic load. In pristine water, concentrations between 0.01 and 0.1 ppm remain effective against bacterial loads within minimal exposure durations. Conversely, in organic-laden waters, higher residual concentrations are essential for efficient disinfection.

Site of Application

Caution is required when introducing ozone due to its potential toxicity at low residual concentrations. Various application points within a RAS can yield different results:

• Oxygen feed gas - Injecting ozone with the oxygen transfer systems occurs after the biofilter, reducing fish exposure risks.
• Pre-biofilter - Introducing ozone before the biofilter allows any residuals to be utilized in biofilm oxidation while protecting fish stocks.
• Incoming water supply - Disinfecting incoming supply water prevents pathogen ingress.
• Effluent - Ozone can also disinfect effluent to mitigate pathogen release into the environment.

Ozonation of coarse solids is inefficient and not recommended. Direct treatment in culture tanks poses significant risks to fish stocks.

Saltwater ozonation generates toxic byproducts, requiring careful management and filtration to remove residual ozone before reintroduction.

Measuring Ozone in RAS

Standard measurement techniques, including colorimetric tests and spectrophotometry, may inadequately capture low residual ozone levels. Alternatively, oxidation-reduction potential (ORP) probes offer a continuous indirect monitoring solution by assessing the oxidizing capacity based on total oxidants. A safe ORP level for freshwater fish is generally considered to be 300 mV.

Many systems automate ozone application using ORP-linked generators, allowing real-time adjustments based on the determined oxidizing potential. Nonetheless, reliance on ORP introduces some inaccuracy limitations, making additional monitoring of water quality parameters essential.

The Risks

While ozone proves effective for wastewater treatment, its usage introduces significant risks. These include:

• Reduced nitrite levels could disrupt the biofilter system, leading to potential spikes in nitrite concentration.
• Excessive ozone can cause harm to both fish and beneficial bacteria.

To counteract residual ozone hazards, the installation of de-ozonation units is paramount. These systems permit ozone degradation and the safe venting of any residual gas.

Human exposure to ozone can provoke severe health issues, including respiratory complications. Legal standards dictate exposure limits, underlining the necessity of maintaining leak-free ozone reactors and adequate ventilation within the RAS environment.

Test kits for airborne ozone assessment are highly recommended to ensure the safety of all operators working within RAS.