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Recirculating Aquaculture Systems


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Title: Recirculating Aquaculture Systems

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Recirculating Aquaculture Systems
  • Recirculating aquaculture systems (RAS) are
    systems in which aquatic organisms are cultured
    in water which is serially reconditioned and

source Wik et al. (2009)
Why recirculate?
  • Conserves water
  • Permits high density culture in locations where
    space and or water are limiting
  • Minimizes volume of effluent, facilitating waste
  • Allows for increased control over the culture
    environment, especially indoors
  • Improved biosecurity
  • Environmentally sustainable

Recirculating System Applications
  • Broodstock maturation
  • Larval rearing systems
  • Nursery systems
  • Nutrition and health research systems
  • Short-term holding systems
  • Ornamental and display tanks
  • High density growout of food fish

Fish Food has an Impact (usually negative) on
Water Quality
0.35 1.38 kg CO2
1 kg Feed
0.025 - 0.055 kg NH3 NH4
Characteristics of Culture Tank Effluent
  • High concentrations of suspended
  • and dissolved solids
  • High ammonia levels
  • High concentration of CO2
  • Low levels of dissolved oxygen

Basic Components
  • Recirc systems maintain fish at high densities
    61-122 kg/m3
  • Water treated by several processes prior to
    recirc to culture units
  • Question exists which method is proven and
  • Main ones screening, sedimentation, media
    filtration, biological filtration, aeration,

Production Capacity DependsUpon Treatment System
Design Scale
Waste Solids Removal
Your Technology Maintains Life Support and Must
  • Remove Solid Wastes
  • Settleable, Suspended, and Dissolved
  • Convert Ammonia and Nitrite to Nitrate
  • Remove Carbon Dioxide
  • Add Oxygen
  • Maintain Proper pH
  • Control Pathogens
  • Keep up with generation of waste

Bacteria Are Important in a Recirculating System
  • Bacteria Can Cause Trouble
  • Consume Oxygen
  • Create Toxic Ammonia
  • Cause Disease
  • Bacteria Also Make the System Run
  • Biological Filtration

Bacteria Eat Wastes and Cause Changes in Water
  • Bacteria Break Down Uneaten Feed and Waste to
  • Ammonia (toxic to fish)
  • Consumes Oxygen (often referred to as BOD,
    BioChemical Oxygen Demand)
  • These Bacteria are called Heterotrophic

Ammonia is also Consumed and Converted by Bacteria
  • Bacteria (Nitrosomonas) Convert Ammonia to
    Nitrite (NO2)
  • Nitrite is also toxic to Fish
  • Other Bacteria (Nitrobacter) Convert Nitrite to
    Nitrate (NO3)
  • Nitrate is not generally very toxic to fish
  • The Process is called Nitrification
  • The Bacteria are called Nitrifying Bacteria
  • Also referred to as Autotrophic Bacteria

Very Important Water Quality Parameters
  • Dissolved Oxygen (continuously monitor)
  • Ammonia-Nitrogen (NH3 NH4)
  • Nitrite-Nitrogen (NO2-)
  • pH
  • Alkalinity

Biological Nitrification is a Two Step Process
Biological Nitrification Is All About
  • Surface Area
  • Living Space for the Nitrifying Bacteria
  • Competition for that Space
  • Food (ammonia or nitrite) gt 0.07 mg / L
  • Good Living Conditions
  • DO going into the biofilter gt 4 mg / L
  • pH (7.2 8.8 for nitrosomonas
  • 7.2 9.0 for nitrobacter)
  • Alkalinity gt 200 mg / L as CaCO3

Required Unit Processes
Unit Processes Waste Solids Removal
Suspended Solids Solids that will not settle out
in 1 hour under quiet conditions
Removal Mechanisms
  • Gravity separation
  • Settling tanks, tube settlers and hydrocyclones
  • Filtration
  • Screen, Granular meda, or porous media filter
  • Flotation
  • Foam Fractionation

Settling Basins Sedimentation Advantages
  • Simplest technologies
  • Little energy input
  • Relatively inexpensive to install and operate
  • No specialized operational skills
  • Easily incorporated into new or existing

Settling Basins Sedimentation Disadvantages
  • Low hydraulic loading rates
  • Poor removal of small suspended solids
  • Large floor space requirements
  • Resuspension of solids and leeching

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HydroTech Drum Screen Filter
Backwash Spray Nozzles
Waste Drain
Unit Processes Biofiltration
  • Biofilter operation for aquaculture production
    systems has only been studied for about 25 years
  • Earliest types were submerged filters, soon
    replaced by trickling filters, but same
    principles apply to all biofilters
  • Various types submerged, trickling, rotating
    biodisks, biodrums, fluidized beds, low-density
    media filters
  • Submerged biofilters are the simplest and come
    directly from the sewage treatment industry
  • Lately shown to be somewhat inefficient

Submerged Biofilters
  • Characterized as downflow filters (top to bottom)
  • Relegated to novice culture systems
  • Bacteria grow on a film at the surface of a sand
    substrate within a tank
  • The medium is continuously submerged
  • Most common medium is limestone rock (helps pH,
    until covered by bacteria)
  • Others oyster shell, clam shell, crushed coral,
    ceramic/plastic modules, glass/plastic beads
  • Particle must be large than 19-25 mm or will clog

Trickling Filters
  • Similar in design as submerged filters with one
    major exception medium is not submerged
  • Bacteria adhering to medium are kept moist and in
    a semi-aerobic environment
  • Seldom clog
  • Can only function in downflow mode
  • Media currently consist of plastic modules
    (light, large surface area)
  • Sand cannot be used due to small void area

Submerged vs. Trickling Filters
Rotating Media Filters
  • Also referred to as rotating biocontactors
  • biodisks or biodrums
  • Biodisks series of flat or corrugated disks
    mounted on a horizontal shaft
  • 40 of disk surface is submerged at a time, shaft
    and bearings above the water surface
  • Disks separated from each other by at least 13 mm
    (0.5 in.)
  • Most disks constructed from flat or corrugated
    fiberglass or plastic sheet material

Rotating Media Filters
  • Rotational speed 2-6 rpm, but no faster than
    1ft/sec (peripheral speed)
  • This is the generator of the previously mentioned

Rotating Media Filters
  • Biodrums are variations of biodisks
  • Cylindrical cages filled with media more
    surface area
  • Downside more energy required to turn them

Fluidized Bed Reactors
  • Contained within a vertical plastic tube
  • Sand media is supported by coarse gravel,
    supported by a perforated plate
  • Media kept in various degrees of suspension by
    upward flow of water
  • Usually pressurized and driven by a pump
  • Only used for NH3 removal (not solids)
  • Primary design criterion is upward flow rate and
    oxygen demand
  • Capacity is 10x that of static filters

Downside requires high upward Q (60-65 gpm/m2)
Floating Bead Filters
  • Low-density media filter
  • Use 3-5 mm poly beads in pressurized upflow mode
  • Beads float above injection point
  • Capable of solids capture and biofiltration
  • Traps suspended particles while enhancing
  • Can nitrify 270 mg TAN per m2 per day
  • 1.0 m3 of beads can provide complete water
    treatment of wastes generated from 12-16 kg feed
    per day (400-530 kg fish/m3 media)

Trickling Filter Typical
Design Nitrification Rate 0.45 g TAN / m2 /
day 90 g TAN / m3 / day (Losordo et al.) Approx.
3 kg feed per day per cubic meter of media (212
- 353 / cubic meter)
ExpoNet BioBlock 200 0.55m x 0.55m x 0.55 m
each 200 m2 / m3
Net 200
Biological NitrificationMoving Bed Reactors
Design Nitrification Rate 0. 10 - 1.0 g TAN /
m2 / day 50 - 500 g / m3 / day 1.66 - 16.66 kg
feed / day (Media Cost US1000 - 1500 / m3 )
B-Cell SSA 650 / m2 / m3
KMT SSA 500 / m2 / m3
KMT Copy SSA 850 / m2 / m3 ??
Biofilters Come in All Shapes and Sizes
Fluid Sand Beds are the most compact biofilter
Moving Bed Filters are low energy and compact
An RBC specifically designed for aquaculture
Bead Filters combine nitrification with solids
Trickling Filters are the work horse of
Biofilter Chemical Factors
  • pH nitrification inhibition commences below pH
    7 optimum slight gt 7.0
  • Alkalinity 20-50 mg/L
  • NH3 and NO2 NH3 inhibits Nitrosomonas sp. and
    Nitrobacter sp. at 10-150 and 0.1-1.0 mg/L,
  • O2 biofilter effluent gt 2.0 mg/L
  • Solids 1.4-2.7 µM best
  • Salinity normal culture ranges are OK, no
    sudden changes
  • Temperature 30-35C

Design Requirements
  • The Following Unit Process are required in any
  • Culture Tank Design
  • Circulation
  • Solids Removal
  • Biofiltration / Nitrification
  • Gas Transfer (Aeration / Oxygenation / CO2

Design Assumptions
For any design, some assumptions need to be made,
hopefully based either on actual experience or
reputable research.
Design Assumptions
  • Assuming 454,000 kg/yr production
  • Mean feeding rate rfeed 1.2 BW/day
  • Feed conversion rate FCR 1.3 kg feed/kg fish
  • Culture Density 80 kg fish/m3
  • Oxygen Demand 0.75 kg O2/ kg feed

(these rates are an average over entire year)
System Biomass Estimation
  • Estimate of systems average feeding biomass

Total Oxygen Requirements
  • Estimate the oxygen demand of systems feeding
  • where
  • RDO average DO consumption Rate
  • kg DO consumed by fish per day)
  • aDO average DO consumption proportionality
  • kg DO consumed per 1 kg feed
  • Ranges from 0.4 to 1.0 kg O2/kg feed cold
    water to warm water

Total Flow Requirement Oxygen Load
  • Estimate water flow (Q) required for fishs O2
  • Assuming oxygen
  • DOinlet 18 mg/L
  • DOeffluent 4 mg/L (_at_ steady state)

Total Tank Volume Requirements
  • Assume an average fish density across all culture
    tanks in the system
  • culture density 80 kg fish/m3

Check Culture Tank Exchange Rate
  • Rule of Thumb

a culture tank exchange every 30-60 minutes
provides good flushing of waste metabolites while
maintaining hydraulics within circular culture
Number of Tanks Required
  • Assuming 9 m (30 ft) dia tanks
  • water depth
  • 2.3 m
  • 7.5 ft
  • culture volume per tank
  • 150 m3
  • 40,000 gal
  • 10-11 culture tanks required
  • Assuming 15 m (50 ft) dia tanks
  • water depth
  • 3.7 m
  • 12 ft
  • culture volume per tank
  • 670 m3
  • 177,000 gal
  • 2-3 culture tanks required

Tanks Design Summary
  • Ten Production Tanks
  • Diameter
  • 9.14 m ( 30 ft )
  • Water depth
  • 2.3 m (7.5 ft)
  • Culture volume per tank
  • 150 m3 (40,000 gal)
  • Oxygen Demand
  • 117 kg O2/day (257 lbs/day)
  • Flow Rate (30 min exchange)
  • 5,000 Lpm (1,320gpm)
  • Biomass Density
  • 86 kg/m3 (0.72 lbs/gal)

Removal solids design
  • Settling Basin
  • Dual-drain System
  • Swirl Separator
  • Microscreen Filter
  • Propeller Washed Bead Filter

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Terms Used To Describe Biofilters
  • Void Space / porosity
  • Cross-sectional Area
  • Hydraulic Loading Rate
  • Specific Surface Area

Biofilter Design Step 1
Step 1 Calculate the dissolved oxygen
requirement (RDO).
Assume a DO consumption of 1.0 kg/kg feed Both
the MBB and Trickling Tower provide O2 for
Nitrification or approximately 0.25 kg. Thus
0.75 kg O2 /kg feed.

Biofilter Design Step 2
Step 2 Calculate water flow requirement (Qtank)
required for fish DO demand. Assume DOinlet
18 mg/L (pure oxygen aeration system) DOtank
4 mg/L (warm water 24 Deg. C, Tilapia!!)  

Biofilter Design Step 2 (cont)
Step 2 Check the Exchange rate (2-4

A tank exchange rate of 2 exchanges per hour is
Biofilter Design Step 3
Step 3 Calculate TAN production by fish
(PTAN) (Note Feed is 35 protein)
PTAN F PC 0.092 F 0.35 0.092
0.032 where PTAN Production rate of total
ammonia nitrogen, (kg/day) F Feed
rate (kg/day) PC protein concentration in
feed (decimal value)

Ammonia Assimilation Rates

Biofilter Design Step 4 (MBB)
Step 4 Calculate volume of media, Vmedia based
on the Volumetric nitrification rate (VTR)
Consider a Moving Bed BioReactor (MBB) Curler
Advance X-1 has a 605 g TAN/m3 (17.14 g TAN/ft3).

Biofilter Design Step 4 (MBB)
Step 4 Calculate volume of biofilter, Vbiofiler
based on a fill ratio of 65.

This would require a tank (3200 gal) 7 ft in
diameter and 11 ft tall.
Biofilter Design Step 4 (Trickling Tower)
Step 4 Calculate the surface area (Amedia)
required to remove PTAN from the Areal TAN
removal rate (ATR) (0.45 g TAN/m2 day) 

Biofilter Design Step 5 (Trickling Tower)
Step 5 Calculate volume of media based on the
specific surface area (SSA), example BioBlock
200 m2/m3 (61 ft2/ft3)  

Biofilter Design Step 6 (Trickling Tower)
Step 6 Calculate the biofilter cross-sectional
area from required flow for the fish oxygen
demand (Qtank) and the hydraulic loading rate,
HLR of 250 m3/m2 day (4.4 gpm/ft2).

Biofilter Design Step 7 (Trickling Tower)
From high school math class area ? (Dia)2
/ 4 diameter 4 area / ?1/2
The diameter of a two trickling towers,
Dbiofilter, with this cross sectional area is

Biofilter Design Step 8 (Trickling Tower)
Step 8 Calculate the biofilter depth
(Depthmedia) from the biofilter cross-sectional
area (Amedia) and volume (Vmedia).

The final Trickling Tower is 15 ft in diameter
and 12 ft tall plus distribution plate, etc.