1. Freshwater recirculation systems

2. Recirculating system - 60%
System components
solids filter
tanks
aeration

3. Recirculating system 90%
System components
solids filter
tanks
Aeration
Degasser (CO2)
biological filter
buffering systems

4. Recirculating system 98%
System components
solids filter
tanks
Aeration
Degasser (CO2)
biological filter
buffering systems
Anaerobic biofiltration
disinfection

5. System Components Primary Secondary Tanks Solids filter Biofilter
Aeration
Secondary
Pumps
Lighting
Disinfection

6. Components of a Recirculating Aquaculture System continued
2. Biofiltration continued
Engineered forms:
Submerged
Fluidized beds
Emerged
Trickling/RBC
Combinations
Submerged biofilters can either be up-, or down-flowing. Over time, submerged biofilters may block due to accumulation of sediments and must therefore, be flushed. Trickling biofilters are similar to submerged biofilters except that water trickles downwards over the media, keeping bacteria wet but not entirely submerged. Rotating biological contactors are circular media that turns like a wheel, thereby alternately bringing bacteria in contact with water and then air. They are cheap and also can act to oxygenate the water column while stripping gasses.

7. Pressurized downflow sand filter
Solids Filtration
Sources
Fish wastes
Uneaten food
Types
Settable
Suspended
Fine and dissolved
Pressurized downflow sand filter

8. Solids Filtration Settable solids (gravity removal) Sediment trap
Settling tank
Hydrocyclone (swirl separator)
Gravity Sump

9. Solids Removal Suspended Solids (non-gravity) Screen filtration
Expandable granular media
Downflow (fine sand)
Upflow (course sand)
Suction
Screen
Screen filters

10. Solids Filtration Fine and dissolved solids Principle
Foam Fractionation
Principle
Particles attach to bubbles that rise through tube.

11. Biological Filtration
Sources
Fish metabolism
Function
Nitrification
Oxidizes ammonia and nitrite to nitrate
NO3-
NH3
1½ O2
nitrosomonas
nitrobacter
NO2-

12. Types of Bio-filters Submerged filters Trickling filters Simple
Inefficient
Trickling filters
Aerates
Submerged filter

13. Oxygenation Non-pressurized DBC U-tube Downflow bubble contactor (DBC)
Counter current diffusion column
U-tube diffusers
U-tube
DBC

14. Degassing
spray tower
packed column
packed column

15. Bicarbonate Drip Rational Purpose CO2 removal (<20 mg/L) Buffering
from respiration
Buffering
nitrification is an acidifying process
Purpose
Adds alkalinity to water

16. Recirculating Aquaculture System Components continued
2. Biofiltration continued
Ideal characteristics:
Large surface area: volume ratios
Long-lasting
Cheap
Light
B-cell
Kaldnes
Commonly used media
                                                               i.      Sand – cheap, high specific surface area, low void, heavy
                                                             ii.      Gravel – cheap, low specific surface area, medium void, heavy
                                                            iii.      Rock (limestone) – cheap, v. low specific surface area, low void, heavy
                                                            iv.      Crushed oyster shell - cheap
                                                              v.      Plastics (KMT, RBC) - expensive
198 m2/m3
152 m2/m3

17. Recirculating Aquaculture System Components continued
2. Biofiltration continued
TAN
NO2
NO3
time (days)
mg l-1 40 15
The speed of maturation of the nitrosomonas and nitrobacter colonies can be decreased with addition of ammonia (1-5 mg/l as ammonium chjloride, hydroxide or phosphate) to the water column or by mixing with old media.

18. Particulate removal Removal by: Gravity separation Filtration
Flotation
0.001
0.01
0.1 1 10
100
1000
supra colloidal
settleable
colloidal
dissolved
Particle size m
Comprising uneaten feeds, fecal materials, mucus, bioflocs, fish scales, etc.

19. Most effective solids removal processes and particle range
Particulate removal
Most effective solids removal processes and particle range 10 25 50 75
100
fractionation
sedimentation
granular filters
microscreens
Particle size m
cartridge filter
Optional treatment Main treatment Pretreatment
coarse screen

20. Recirculation systems

21. Biological filtration
Biological filtration is the use bacteria to removal of waste metabolites that accumulate from keeping, feeding and growing fish in a closed recirculating system.
After oxygen, it is the accumulation of ammonia - the waste metabolite from protein digestion - that is probably the most limiting factor affecting the success of operating closed recirculation systems.

22. Biological filtration
The removal of ammonia (NH3) by oxidation to nitrite (NO2) and finally nitrate (NO3) is carried out by the bacteria Nitrosomonas sp. and Nitrobacter sp. respectively. This process is known as nitrification.
Un-ionised ammonia and nitrite are both toxic to fish at relatively low concentrations whereas nitrate, the end product of nitrification, is relatively non-toxic at even quite high concentrations.

23. Biological filtration
The rate of nitrification is optimal around 30°C.
The particular size and type of filtration equipment needed will depend on a number of factors including,
the type of fish,
the biomass of fish,
the amount of food fed
the temperature of the water.