Recirculating Aquaculture System Fundamentals (Part 3 of 6)

Recirculating Aquaculture System Fundamentals (Part 3 of 6)

There are two main forms of bio filters, aerobic and anaerobic

Ammonia and Nitrite

There are two main forms of bio filters, aerobic and anaerobic. There are many filters that perform specific tasks and filters that multitask. For me, a moving bed bio filter is the first choice as it is self-cleaning and if it is working correctly it can deal with NH₃, NO₂ and to a lesser degree NO₃. Remember, if fundamentals have been followed there should be no or little solids entering the filter. Sludge and anaerobic areas should be none. The intake to this filter should be above the half-way mark of the tank. Filter media can be made-to-order to your specifications and needs. High volume low electricity pumps can be used with MBR and is a huge saving if used correctly. Aeration can be added by a blower or a simple LP100 Air pump. This would be the main bio filter on the system. Conditions need to be perfect. PH between 6.8 and 7.8 with good general hardness (GH) and alkalinity (KH). The temperature should be between 25°C and 29°C and dissolved oxygen (DO) above 5mg/l entering the filter.


Nitrate seems to be forgotten when it comes to tilapia. South Africans have the idea that Tilapia can and will survive in any water conditions, so why go to such great lengths and expense to have perfect water. Every farmer who harvests tilapia knows the fish is a diehard species. Because of this psychological reasoning tilapia should thrive in any body of water. Temperature and quality of water is often sacrificed. Nitrate is generally dealt with by partial water changes, assuming that a 10% to 20% water change a month would be sufficient. If the tank volume is small, 10 000l, a 2 000l water change a month is no big deal, however, if the overall system involves 1 000 000l, 200 000 l of water would have to be thrown away. This would be environmentally unsatisfactory when we are meant to be practising sustainable aquaculture processes. Zero waste RAS systems should be our aim of every responsible farmer.

When a system is designed with all aspects of water quality in mind, anaerobic filters can be installed in line after other aerobic filters using little or no electricity. These filters need to be designed with cleaning and efficiency in mind.

The addition of Bacillus spp bacteria can and will improve all aspects of water quality. Biological treatments are becoming affordable and will help grow fish for the farmer to reach their targets financially and ecologically.

Dissolved oxygen

Testing dissolved oxygen (DO) levels in a system is generally the fastest way to see if a RAS system is performing adequately and stocked correctly. In general DO is the first sacrifice made. All Tilapia require oxygen to live but only perfect conditions will allow Tilapia to thrive and grow at a commercial rate. Because of the hardiness of Tilapia and because oxygen cannot be seen we so often allow suboptimum levels of oxygen to become the norm. O2 levels between 5mg/l and 9mg/l are optimal. Oxygen levels of under 3mg/l can sustain life but will become a breeding ground for pathogens and disease. No fish can be expected to grow in such poor conditions.

The general thinking is that if a ton of aeration is added the levels of DO will increase but sadly the cause of the problem is still there and DO will not increase significantly the higher the altitude the less atmospheric oxygen available. Generally the cause of low DO is over stocking within a warm water system. The thinking is that if the farmer adds more fish to the system he/she will have larger returns. However, the reality is that if they have less fish in the system the farmer will have better water quality where a fish can thrive and grow, resulting in a quality fish with good financial returns.

Altitude, Water temperature and salinity will determine the oxygen solubility.

Only systems where all fundamentals are followed and merged perfectly, will allow stocking densities above 15kg/kl.

The addition of liquid oxygen can solve low DO in a system, but is complex and has several new issues that will need to be overcome.

Please buy a DO meter as soon as you are able. There is no other tool more important to a commercial fish farmer. Oxygen needs to be measured before sunrise as this reading will be the lowest reading of DO. Generally a test at 10pm and 6am will reveal the true levels of DO your fish experience. There are always algae in a system, either in the water or the surfaces within the water. Once sunlight enters the water the process of photosynthesis begins and the algae consume CO2 and releases O2. Potentially a system with algae could have levels of DO at 7mg/l or more during the day but at night when the process of photosynthesis reverses, algae then consumes O2 and releases CO2. Levels of DO could drop to life threating low levels, less than 1mg/l.

DO levels above 5mg/l will give you a financial return over a period, because the fish will grow within a set period and achieve targets. Bio filters can perform their functions optimally and deliver good quality water. Nitrite seems to be the hardest hit when it comes to not having enough oxygen in filters. The filter and media may be adequate in size but cannot support enough bacteria because of DO. Whenever DO is compromised the entire operation is compromised and will not achieve the results expected.

In the next part of this series, I will be taking a closer look at Stocking Density and Harvesting.

Written by:  - 21 Oct, 2020  
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