Development of a Biofloc System

for the Production of Tilapia

James Rakocy, Donald Bailey,

Charlie Shultz and Jason Danaher

University of the Virgin Islands

Agricultural Experiment Station

Objectives

Evaluate the production of tilapia in a 200-m3

tank employing aeration, solids removal, mixing,

bacterial-based treatment in the water column,

and denitrification.

Modify and improve the system during the course

of four production trials.

Initial Tank Design

Size: 200 m3, 16 m diameter, 1 m mean water depth

Surface area: 200 m2 (0.02 ha or 1/20 acre)

Bottom: 3o slope to center

Center clarifier: 1 m3, 45o slope, fiberglass, 10-cm drain

Outside standpipe for solids removal

Aeration: three ¾-hp Kasco aerators

Water movement: one ¾-hp Kasco aerator angled

Biofloc Tank

16 m

Sludge Cone Volume = 1.0 m3

1.09 m 0.15 m Freeboard Total Culture Volume = 200 m3

Sludge Removal Line

3º Slope

Biofloc Tank

To storage lagoon

Flow

Base addition tank Central cone

Drain

Aeration device

Sludge collection

and measurement

Procedures

Aeration continuously

Mix continuously to maintain suspension of biofloc

Remove settleable solid waste daily

Feed twice daily with floating feed (32% protein)

Feed ad libitum for 30 – 40 minutes

Monitor pH, maintain pH 7.5 with Ca(OH)2

Add CaCl2 to prevent nitrite toxicity

Monitor important water quality parameters

Trial 1

Stocking rate: 20 fish/m3

Solids removed from central cone

Production

Trial Stocking

Rate

(#/m3)

Initial

Size (g)

Final

Size

(g)

Culture

Period

(d)

Growth

Rate

(g/d)

Final

Biomass

(kg/m3)

FCR Survival

(%)

1 20 214 912 175 4.0 14.4 2.2 78.9

2

3

4

Trial 2

Stocking rate: 25 fish/m3

Solids removed from central cone

In last 3 weeks solids also removed by external

clarifier

Clarifier effluent

Culture tank water

Sludge from clarifier

After 10 minutes of settling

Clarifier Efficiency

Total Suspended Solids

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Week

Co

nc

en

tra

tio

n (

mg

/L)

Trial2

Production

Trial Stocking

Rate

(#/m3)

Initial

Size (g)

Final

Size

(g)

Culture

Period

(d)

Growth

Rate

(g/d)

Final

Biomass

(kg/m3)

FCR Survival

(%)

1 20 214 912 175 4.0 14.4 2.2 78.9

2 25 73.6 678 201 3.0 13.7 1.9 81.0

3

4

Trial 3

Solids removed solely by external claifier

Denitrification tank employed

Bird netting at edge of tank

Biofloc Tank Trial 3

Flow

Base addition tank Aeration devices

Sludge removal to storage lagoon

Clarifier

Pump

Horizontal mixer

Nitrate-Nitrogen – Trial 1 - 3

0

100

200

300

400

500

600

700

800

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Week

Co

ncen

trati

on

(m

g/L

)

Trial 1

Trial 2

Trial 3

TSS – Trial 2 and 3

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

Week

mg

/L

Trial 2

Trial 3

Production

Trial Stocking

Rate

(#/m3)

Initial

Size (g)

Final

Size

(g)

Culture

Period

(d)

Growth

Rate

(g/d)

Final

Biomass

(kg/m3)

FCR Survival

(%)

1 20 214 912 175 4.0 14.4 2.2 78.9

2 25 73.6 678 201 3.0 13.7 1.9 81.0

3 25 70.0 707 182 3.5 15.3 1.8 86.0

4

Trial 4

Complete bird netting barrier

Production

Trial Stocking

Rate

(#/m3)

Initial

Size (g)

Final

Size

(g)

Culture

Period

(d)

Growth

Rate

(g/d)

Final

Biomass

(kg/m3)

FCR Survival

(%)

1 20 214 912 175 4.0 14.4 2.2 78.9

2 25 73.6 678 201 3.0 13.7 1.9 81.0

3 25 70.0 707 182 3.5 15.3 1.8 86.0

4 25 154 745 183 3.2 18.6 2.0 99.7

Major Inputs and Outputs

Trial Initial

Water

(m3)

Makeup

Water

(L/day)

Sludge

(L/d)

Feed

(kg/day)

Base

Addition

(kg/day)

Electricity

(kWh/day)

1 200 880 470 25.4 1.5 52.8

2 200 401 366 23.0 1.7 52.8

3 200 588 213 27.3 0.9 58.9

4 200 577 240 33.0 1.3 71.7

Water Quality

Parameter (mg/L) Trial 1

Mean

Trial 2

Mean

Trial 3

Mean

Trial 4

Mean

DO 5.5 7.9 5.3 5.3

Water Temp (C) 26.8 28.5 26.1 26.4

NH3-N 1.2 1.8 2.0 2.3

NO2-N 1.5 2.7 1.9 5.6

pH 7.8 7.8 7.7 7.5

Total Alkalinity 224 204 247 211

Water Quality

Parameter (mg/L) Trial 1

Mean

Trial 2

Mean

Trial 3

Mean

Trial 4

Mean

Chlorophyll a (ug/L) 1895 924 820 937

COD 353 363 292 315

Settleable solids (ml/L) 29 48 23 10

TSS 476 855 317 368

Ortho-Phosphate 16.9 19.2 34 32

Cl 301 317 95 370

Total Ammonia-Nitrogen – Trial 1 - 4

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30

Week

Co

ncen

trati

on

(m

g/L

) Trial 1

Trial 2

Trial 3

Trial 4

Nitrite-Nitrogen – Trial 1 - 4

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30

Week

co

nc

en

tra

tio

n (

mg

/L) Trial 1

Trial 2

Trial 3

Trial 4

Biofloc Culture Advantages

Simple management

Low water requirements

Algal die-offs do not cause mortality

Algae and bacteria supplement tilapia diet

No off-flavor detected

Production ~ 37 times higher than ponds for tilapia

No recruitment problem

Wastewater used to irrigate and fertilize field crops or return to the system

Solid waste used as soil amendment and potting mix

Biofloc Culture Disadvantages

Requires a 6-week period to establish bacterial populations

Suspended solids nitrification less stable than fixed-film nitrification

Feeding response fluctuates

Algal die-off reduces feeding response temporarily

High energy input

Reliance on continual aeration and mixing requires backup power

Key Results

Total tilapia production: (3711 kg in 1/50-ha tank)

or (8,163 lbs in 1/20-acre tank)

Potential from trial 4 of 16,326 lbs per year in 1/20-acre tank

Daily makeup water averaged 0.29% (0.58 m3) of total water volume

Recovered approximately 40% (0.23 m3) of daily makeup water.

Conclusions

This biofloc system was 37 times more productive than a

standard earthen pond (18.6 vs. 0.5 kg/m3)

External clarification simplifies tank construction,

improves solids removal and water quality and increases

production

Simple open channels with solids accumulation can

provide adequate denitrification

This production technology conserves water and land by

97% and recovers solids and nutrients.

All waste products of system can be reused.

Future Research

Scaleup (1,000 m2?, 4,000 m2?)

Aeration requirement

Mixing technique

Reduce energy costs

Solids removal

Return Geotube® filtrate to system

Species

Economics