Biofloc Technology (BFT) in Shrimp Aquaculture: Carbon-Nitrogen Ratio, Inoculation & Heterotrophic Management

In conventional aquaculture ponds, autotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter) oxidize toxic ammonia into nitrite and subsequently nitrate. This process is slow, requires high alkalinity (> 150 ppm), and generates substantial nitrate accumulation requiring water discharge.

Biofloc Technology (BFT) bypasses autotrophic nitrification by cultivating dense communities of heterotrophic bacteria. When provided with an external organic carbon source, heterotrophic bacteria synthesize new cellular protein directly from dissolved ammonium ions.

Because heterotrophic bacteria reproduce up to 10 times faster than autotrophic nitrifiers (generation time of 30 minutes vs 24 hours), total ammonia nitrogen (TAN) is stripped from the water column within hours of carbon administration.

Commercial shrimp feeds contain 32% to 38% crude protein. Since protein is approximately 16% nitrogen by weight, every 100 kg of feed introduced releases roughly 3.8 to 4.2 kg of nitrogen into the culture system.

Heterotrophic bacteria require approximately 15 to 20 grams of organic carbon for every 1 gram of nitrogen assimilated into microbial biomass. To maintain this balance, add organic carbon (such as sugarcane molasses containing 40% carbon) alongside daily feed rations.

Standard Practical Rule: For every 1.0 kg of 35% protein commercial feed administered, add 0.6 to 0.7 kg of jaggery/molasses dissolved in pond water to maintain the C:N ratio above 15:1.

Monitoring microbial density is critical to prevent biofloc systems from over-thickening. Collect 1,000 mL of pond water in a graduated Imhoff cone and allow it to settle undisturbed for exactly 30 minutes.

The target settleable floc volume for Vannamei culture is 15 to 25 mL/L. If floc volume drops below 10 mL/L, increase carbon dosing. If floc volume exceeds 30 mL/L, the microbial biomass consumes excessive dissolved oxygen and clogs shrimp gills. In high-floc conditions, operate central bottom drain purges or external settling clarifiers to remove surplus sludge.

Biofloc particles must remain in continuous suspension throughout the water column. If water velocity drops below 0.12 m/s, flocs settle onto the tank floor, creating dense anaerobic mats that generate lethal hydrogen sulfide (H2S) gas.

Circular HDPE tanks utilize a combination of continuous bottom air-diffuser grids (Roots blowers supplying 1.5 to 2.0 CFM per m² of tank floor) and directional Venturi injectors to maintain rotational kinetic velocity.

Do not stock shrimp post-larvae into raw, unconditioned water. Prepare a starter biofloc inoculum in a 1,000-liter conditioning tank 7 days prior to stocking.

Mix 500 liters of sterilized sea water, 5 kg of jaggery, 100 grams of Bacillus subtilis probiotic powder, and 1 kg of shrimp feed. Aerate vigorously for 5 to 7 days until water turns rich golden-brown with an Imhoff floc volume of > 5 mL/L before inoculating the main culture tanks.

While circular biofloc tanks require higher initial capital investment (HDPE liners, Roots blowers, and greenhouse roofing), they achieve production densities of 8 to 15 kg per cubic meter (equivalent to 40 to 80 tons per hectare).

With zero water exchange, biofloc facilities eliminate viral contamination risks from coastal tidal creeks, enabling reliable multi-crop harvesting year-round.