Emerging trends in biofloc aquaculture

Pranav Rachh – AirOxi On Aeration requirement for BFT

Pranav Rachh

Key points of aeration in BFT aquaculture - Aeration needs to be intensive , at all points - Vigorous movement of bio floc - Aeration to be calculated for the nitrifying bacteria as well as marine life - Too low aeration – hypoxia will kill the nitrifying bacteria - Too high aeration has also been shown to reduce effectiveness of the nitrifying bacteria (when balance of oxygen and ammonia / nitrogen compounds is imbalanced)

2 uses of Dissolved Oxygen (DO) • Biological Oxygen Demand (BoD) This is the DO needed by marine life and other micro organisms • Chemical Oxygen Demand (CoD) This is the DO needed for organic and inorganic waste for oxidation. For example to convert what we call as ‘sludge’, waste, excess feed, minerals and other chemicals added to the water and also present in soil and water.

BFT v/s Traditional v/s RAS • In traditional aquaculture, the natural system of DO consumption and replenishment is available, so natural DO fulfils almost 75% or more of the oxygen requirement. And depending on density of fish cultivation, additional aeration may need to be given • In RAS the water is constantly filtered, so except the species being cultivated, very little DO is required as BoD or CoD. Hence it is easier to maintain DO levels • In BFT there is an additional load of BoD created by added nitrifying bacteria. And also there is very little water exchange so CoD is also little higher even after accounting for the biofloc.

Factors affecting aeration in aquaculture • Species reared through BFT • Concentration of micro organisms.

• Aerobic (utilizing DO) • Anaerobic (utilizing nitrogen compounds).

• Photosynthetic organisms like algae, that produce DO in presence of sunlight

• Water temperature • Surface area of water v/s water volume • Salinity • Amount of organic and inorganic waste in the system • External aeration provided

Oxygen saturation at various temperatures

t °C` 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10

mg O2/l 14.65 14.45 14.25 14.05 13.86 13.68 13.49 13.31 13.13 12.96 12.7 12.62 12.46 12.3 12.14 11.99 11.84 11.7 11.55 11.41 11.27

t °C

mg O2/l

t °C

mg O2/l

10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 18 19.5 20

11.14 11 10.87 10.75 10.62 10.6 10.38 10.26 10.15 10.03 9.92 9.82 9.71 9.61 9.5 9.4 9.3 9.21 9.12 9.02

20.5 21 21.5 22 22.5 23 23.5 24 24.5 25 25.5 26 26.5 27 27.6 28 28.5 29 29.5 30

8.93 8.84 8.76 8.67 8.58 8.5 8.42 8.33 8.25 8.18 8.1 8.02 7.95 7.87 7.8 7.72 7.05 7.58 7.51 7.44

Source – FAO website

Oxygen consumption vis a vis water temperature

Source FAO website

DO saturation - mg O2/l vis a vis temperature at sea level in fresh water 16 14

Temperature degree C

12 10 8 6 4 2 0

0

5

10

15

20

25

30

35

Points to consider in artificial aeration • Biomass / Stocking density, salinity, temperature, species, water movement.

• Low water movement may lead to reduced aeration of nitrifying bacteria. Very high water movement will lead to unnecessary movement and fatigue of the animal, increasing their oxygen requirement.

• Depth of water and hence aeration • Drop / bubble size • Path of bubble and time in water, in case of diffused aeration • Duration for which aeration is provided

Independent study on biofloc aeration A study on aeration methods for biofloc culture of L. Vannamei published in Aquaculture International February 2017 found diffused aeration to be the best aeration method for biofloc Stocking Density - 40 / sq mt 9 tanks of 35 sq mtr size (4900 pieces per tank) Initial weight – about 4.5 gram Study period

Aeration method

– 33 days.

Diffused aeration with Blower

Vertical Pump

Propeller aerator

DO – mg /L (Avg)

4.91

4.49

4.50

Final weight (Avg)

12.96

10.93

12.81

C o m p a r a ti v e E ffi c i e n c y o f a e r a ti o n m e t h o d s kg/kwh lb/hp-h

General Oxygen Transfer Efficiency (standard conditions)

6 5 4 3 2 1

11 10 9 8 7 6 5 4 3 2 1

Source - Environmental Dynamics International, Aeration efficiency guide 2011

High-Density / Low-Flux – Aeration Systems (fine bubble diffuser arranged in high density layout)

Typical Fine Bubble Diffuser System

Jet Aerators & Coarse Bubble Diffuser Systems Mechanical Floating Aerators

Aspirating Floating Aerators

Eventual Benefits to Farmers Higher and more uniform DO, Faster DO leading to: • Higher Stocking Density – increase by 40-50% • Higher Immunity • Higher Survival – 85% to 95% • Faster Growth (saving of 10-20 days, even one additional crop/ year) • Low FCR • Reduced cost of power used for aeration – saving of 40-60%

MOBILE AERATION UNIT

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