Treating Water with Ozone to Increase Productivity in Recirculating Aquaculture Systems (RAS)

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Introduction
Real Case – Increased Production at Swiss RAS Plant
The Effect of Ozone
     Removal of Colloidal Solids, Dissolved Organic Compounds and Nitrite
     Disinfection
The Importance of System Design
     Reliability
     Monitoring and Measurement
     Individual Dosing per Culture Tank
     Low Life-Cycle Costs

Most of the studies investigating the use of ozone for enhancing the quality of water in recirculating aquaculture systems (RAS) reiterate the effectiveness of ozone in creating an ideal water environment. Furthermore, the water environment in ozone-treated culture tanks is conducive for the improved growth, health and survival of fish.

Ozone water treatment at RAS enables better fish performance and cleanliness of water, thereby increasing the production capacity.

Driven by the need for more sustainable and water-economic systems, the aquaculture industry is on the quest for modern water treatment methods. Despite the high sustainability of low water exchange RAS and its suitability for areas where water supply is limited it has not gained popularity due to reduced fish performance.

It is challenging to control the quality of water when feed loads are high and the water exchange rate is low. Other issues faced in low water exchange RAS is the accumulation of metals and high concentrations of fine colloidal solids and nitrite.

The ability of ozone to improve the water quality and to control various other water parameters in municipal and aquaculture tanks has been established. According to a study conducted at the Freshwater Institute, rainbow trout (Figure 1) exhibited more growth in an ozonized low exchange RAS than in a non-ozonized system. The study focussed on investigating the ability of ozone to create a favorable water environment for salmonids.

The study established that ozone was successful in creating an optimal water environment that fosters improved survival, feed conversion, growth and other conditioning factors for rainbow trout. The high risk of heavy metal accumulation in non-ozonized low exchange RAS was also established by the study.

Figure 1. Growth of rainbow trout in an ozonized low exchange RAS

Valperca, which is a newly established Swiss fish farm, was facing issues with perch production because of fungi and a high organic load. The existing water treatment system was unable to clean water thoroughly, resulting in very low redox and ORP values and a very high organic load.

The fish farm installed a well-designed ozone system (Figure 2) along with ozone generators, a reaction tank and a dissolution system; which resulted in a significant improvement in water quality.

The Redox-ORP probe helped to control ozone generation for maintaining the water quality level between roughly 270 and 290 mV. Armed with the new ozone system, which increased redox values, Valperca was able to produce fish at design capacity for the first time, without compromising on fish health and performance.

Figure 2. Primozone

The performance of RAS tanks is often affected by high organic loads, particularly in cases with high fish density and low water exchange rates. There is also a risk of heavy metal and nitrite accumulation. Ozone brings about micro flocculation, or clumping, which enables easy removal of impurities by sedimentation and filtration.

Ozone alters the strucutre of dissolved organic compounds (DOC) via oxidation and precipitation, which enables hassle free removal of DOCs via bio filtration and sedimentation. A reduction in the organic load in turn reduces the nitrite content, The nitrite contained in water is oxidized to nitrate by ozone.

A fish farmer needs to implement methods to control the growth of bacteria found in dirty water because their growth can cause serious issues. Growth of bacteria can be curbed effectively by using ozone, which is a reliable disinfectant due to its high oxidation potential. Although the chief advantage of using ozone in aquaculture is its micro-flocculation ability, its disinfecting properties can also be used to improve water quality.

The amount of ozone (dosing) used for disinfecting purposes is calculated based on the total ozone demand, which includes dissolved organics, nitrate colloidal solids and disinfection.

An environment that discourages bacterial growth is automatically created when the solids and DOC are reduced, thereby, minimizing the need for disinfection. During the study conducted at the Freshwater Institute, it was observed that although the ozone dosage was not intended for disinfection, there was an indirect reduction in levels of heterotrophic bacteria.

High concentrations of ozone can cause harm due to its high oxidizing potential. Therefore, extreme caution must be exercised in the construction of an ozone system. In the past there have been instances of reduced reliability in the performance of ozone generators. Recent advances in technology eliminate this issue.

The reliability of ozone reactors made of stainless steel is much higher than those made of glass. Another important aspect in the construction of ozone generators is their redundancy. In-built redundancy of ozone generators ensures safety of the live fish stock during any failure.

An ideal way of measuring the ozone demand is by using oxidation-reduction (ORP or Redox) probes. The total capacity in mV can be determined by using an ORP or Redox meter. The ozone systems must be designed in such a way that the optimal ozone dosage can be automatically determined based on actual ORP (Redox) levels. The optimum ORP-Redox levels in freshwater RAS is approximately 300 mV, however this value depends on the species.

One of the main difficulties in aquaculture is the variation in fish loads for different tanks, which in turn cause variations in the ozone demand for creating an ideal growth environment. In order to ensure the right ozone dosage is incorporated, an advanced distribution module that distributes ozone to different treatment lines with specific dosing on every line is required.

The specific dosage for each tank is governed by the actual ORP-Redox rates of each tank. By using an ozone distribution module, the number of ozone generators required can be minimized and, as a result, costs can be reduced.

Thanks to advanced technology, highly efficient ozone generators, which require minimum maintenance throughout their life cycle are available at low costs. Choosing an optimally constructed ozone system that dispenses accurate dosage can contribute towards fish growth and health by creating an ideal water environment, which eventually enhances the productivity of the system.

This information has been sourced, reviewed and adapted from materials provided by Primozone.

For more information on this source, please visit Primozone.

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