Biofloc Technology (BFT) Aquaculture: A Guide
How biofloc technology works: heterotrophic bacteria turn waste nitrogen into microbial protein, C:N ratio control, near-zero water exchange, heavy aeration, and the trade-offs for tilapia and shrimp.
Overview
Biofloc technology (BFT) is an aquaculture method that uses a dense, managed community of microorganisms in the culture water to recycle waste nitrogen into microbial biomass. The heterotrophic bacteria amalgamate with other microbes and organic particles to form suspended aggregates called bioflocs, which both improve water quality and serve as a supplementary food. BFT is operated with minimal or zero water exchange, which is its defining feature.
How biofloc works
Fish and shrimp excrete most of their nitrogen as ammonia, which is toxic. In BFT, adding a carbohydrate source raises the carbon-to-nitrogen ratio in the water, stimulating heterotrophic bacteria to take up inorganic nitrogen and convert it into new microbial cells. Reviews report that heterotrophic bacteria can immobilize ammonia far faster than nitrifying bacteria, producing a large increase in microbial biomass. The resulting flocs are then grazed by the cultured animals, recycling waste back into feed.
C:N ratio management
Control of the carbon-to-nitrogen ratio is central to BFT. The immobilization of inorganic nitrogen into microbial protein takes place when the C:N ratio of the organic matter exceeds about 10, and reviews recommend operating ratios on the order of 12–15:1, with 15–20:1 cited for greater stability. The ratio is raised by adding external carbon sources.
- Common carbon sources include molasses, dextrose, starch, wheat flour, tapioca and rice bran.
- Carbon is dosed relative to the nitrogen entering the system from feed.
- Too little carbon leaves ammonia high; excess carbon raises oxygen demand and solids.
Aeration, mixing and solids
Because the floc community consumes oxygen and must be kept suspended, BFT requires strong, continuous aeration and mixing; reviews cite aeration requirements on the order of 25 horsepower per hectare, far above conventional pond systems. Suspended solids must be monitored: target total suspended solids are generally kept below about 500 mg/L, with settleable floc volume often held around 5–15 mL/L for shrimp and higher for tilapia. Excess solids are removed with settling devices when they accumulate.
Benefits and challenges
| Benefits | Challenges |
|---|---|
| Recycles waste nitrogen into edible microbial protein | High and continuous oxygen demand |
| Improves feed conversion and lowers feed cost | Requires constant monitoring of C:N, solids and water quality |
| Near-zero water exchange saves water and improves biosecurity | Suspended solids must be controlled to avoid overload |
| Floc acts as a natural probiotic and food source | Sensitive to power and aeration failure |
Suited species
BFT works best with species that tolerate high stocking density and feed on suspended particles. The most common are whiteleg shrimp (Litopenaeus vannamei, also written Penaeus vannamei) and Nile tilapia (Oreochromis niloticus); other species studied include black tiger shrimp (Penaeus monodon) and the giant freshwater prawn (Macrobrachium rosenbergii). These omnivorous, particle-feeding animals can use the floc directly as food.