BASIC SIZING CRITERIA FOR BIOFILTERS USED IN RECIRCULATING AQUACULTURE SYSTEMS

by

Douglas G. Drennan II

Biofiltration is defined as:

• A technique for pollution control using living material to capture and biologically degrade process pollutants.

• A filtration method that uses bacteria to break down waste by means of the nitrogen cycle

• An emission control device that uses microorganisms to destroy volatile organics compounds and hazardous air pollutants.

• Biofiltration is a pollution control technique using living material to capture and biologically degrade process pollutants.

RAS TYPICALLY UTILIZES FIXED FILM

BIOFILTERS

Fixed Film Biofilter

Emerged

Rotating Biological Contactor

Trickling Filter

Submerged

Packed

Submerged Rock

Plastic Packed Bed

Shell Filters

Expandable

Upflow Sand Filter

Floating Bead Bioclarifier

Foam Filters

Expanded

Fluidized Bed

Downflow Microbead Filter

Moving Bed Bioreactor

Malone and Pfeiffer, 2006

EXAMPLES OF BIOFILTERS

BEAD FILTERS

EXAMPLES OF BIOFILTERS

POLYGEYSER BEAD FILTERS

EXAMPLES OF BIOFILTERS

MOVING BEAD BIOREACTORS (MBBRS)

EXAMPLES OF BIOFILTERS

FLUIDIZED BEADS

BIOFILTER SIZING

In its most basic form, all biofilters perform the same function; the removal of toxic TAN from system water.

Biofilters are typically sized based on either the volumetric TAN conversion rate (g TAN m-3 filter d-

1), or the Areal TAN Conversion Rate (g TAN m-2

filter d-1).

There is no universally accepted methodology for sizing and comparing biofilters.

CUSTOMER REQUIRED INPUTS

Volume of System

Number of Systems

Flushing Rate

Flow Rate/Turnover per Hour

Fish Species

Loading Regime Broodstock Display Fingerling/Ornamental Holding Purging Growout

Max Weight of Animals

Max Daily Feed Rate

Feed Protein Content

Max Operating TAN

Max Operating Nitrite

Max TSS

Temperature/Salinity

pH & Alkalinity Source Water

Physical Characteristics Loading & Water Quality

MAX DAILY TAN PRODUCTION ESTIMATE

AST 13.63 g TAN/lbs feed-day * lbs feed * Actual Feed

Protein/35% (protein correction factor) = g TAN Produced/day (29.99 g TAN/day) (3%)

Timmons and Ebeling, 2007 Kg feed/day * % Protein Content * 0.092 kg NH3 /kg

protein = kg TAN produce/day * 1000gm/kg = g TAN Produced/day (32.20 g TAN/day) (3.2%)

Malone and Beecher 2000 30 g TAN/kg Feed-day * kg feed per day* Actual Feed

Protein/35% (protein correction factor) = g TAN Produced/day (30 g TAN/day) (3%)

Losordo and Hobbs 2000* Kg feed per day* Feed Protein* 651 (calibrated constant)

= g TAN Produced/day (22.75 g TAN/day) 1 Assumes 2.5% converted to TAN but gives citation for

2.0% to 3.5%

BIOFILTER PERFORMANCE IS TYPICALLY BASED

ON

The Volumetric TAN Conversion Rate (VTR), (g TAN m-3 d-1) is the rate at which a biofilter can remove TAN from a recirculating system based on the volume of media within the filter.

OR

The Areal TAN Conversion Rate (ATR), (g TAN m2 d-1) is the rate at which a biofilter can remove TAN from a recirculating system based on the surface area of the media within the filter.

VTR is be related to ATR through the specific surface area of the media

VTR CALCULATION

m

eiR

V

TANTANQVTR

(Malone and Beecher 2000)

VTR

QR

TANi

TANe

Vm

=

=

=

=

=

Volumetric TAN Conversion Rate (gm m-3 d-1

Volumetric Flow Rate through Filter (m3 d-1)

TAN Concentration Entering Filter (gm m-3)

TAN Concentration Exiting Filter (gm m-3)

Volume of Media within Filter (m3)

SAMPLE VTR-BASED SIZING PLOT

Note: the data presented hereare hypothetical and are to beused for explanatory purposesonly.

Ak

AVTRVTR

A max

Volumetric TAN Conversion Rate (gm m-3 d-1)Maximum VTR (gm m-3 d-1)TAN concentration (gm m-3)Half-Saturation Constant (gm m-3)

VTRVTRmax

AkA

====

BIOFILTER SIZING

Using the VTR or ATR values, the volume or surface area, of the filter can be determined based on the previous estimates of TAN generation

Vm = PTAN/VTR

Where Vm is the volume of media (m3) PTAN is the mass of TAN produced in the system (g d-1)

OTHER CONSIDERATIONS

Safety Factors

In-situ Nitrification

Flushing

Factors that Affect VTR/ATR

Trophic Level

Salinity

Temperature

SAFETY FACTORS

o Safety factors protect the manufacturer and client

o Procedures between manufactures vary

o Be careful not to add safety factors on top of safety factors.

o Safety factors ultimately increase capital cost but mitigate risk.

AST procedures

Run the bead filter to the maximum sustainablefeed rate under laboratory systems (2.25 lbs/ft3-media)

Divide by 1.5 1.5 lbs/ft3-media is the design load

Verify with commercial experience

IN-SITU NITRIFICATION

Defined as Nitrification that occurs in the tank and on the walls of pipe in a RAS.

Can account for 30-70% of nitrification in a RAS

Vm = (1-Is) PTAN/VTR

Where Is=0.3 (conservative)

Is can be often neglected adding to the safety factor

FLUSHING

Ammonia mass removal by flushing can be defined as removal=Q*TANtank

Generally ineffective when TANtank <1 ppm-N

Generally neglected and adds a little to safety factor

TROPHIC LEVELS

Limnologists classify lakes by trophic levels to distinguish their level of nutrient enrichment (Holum, 1977; Wetsel, 1983).

By analogy Malone and DeLosReyes (1997) proposed that recirculating production systems can be classified as Oligotrophic, Mesotrophic or Eutrophic by the level of the water’s enrichment as driven by feed application rates and defined by the water quality objectives

RECIRCULATING

SYSTEM

CLASSIFICATION

RecirculatingSystems

Freshwater

Marine

Warmwater

Oligotrophic

Mesotrophic

Eutrophic

Coldwater

Oligotrophic

Mesotrophic

Eutrophic

Warmwater Mesotrophic

Eutrophic

Coldwater

Oligotrophic

Mesotrophic

Eutrophic

Oligotrophic

Malone and DeLosReyes (1997)

BIOFILTER CLASSIFICATION BASED ON TROPHIC

LEVEL

Water Quality ParameterOligotrophi

c

Mesotrophi

c

Eutrophi

c

Loading Regime BroodstockFingerling/

OrnamentalGrowout

Total Ammonia (mg-N/L) <0.3 <1.0 <2.0

Nitrite (mg-N/L) <0.3 <1.0 <2.0

Nitrate (mg-N/L) <50 <200 <500

Dissolved Oxygen

(mg/L)>6.0 >5.0 >4.0

Carbon Dioxide (mg/L) <1.0 <5.0 <25

Biochemical Oxygen Demand (mg-

N/L)<5.0 <10 <20

Total Suspended Solids (mg-N/L) <5.0 <15.0 <25.0

Modified from Malone and DeLosReyes (1997)

SALINITY EFFECTS ON VTR/ATR

There have been some reports that salinity adversely impacts nitrification rates.

There are definitely impacts on acclimation rates for the nitrite oxidizing bacteria.

In AST’s experience the freshwater design values used for design have been readily obtained in saltwater applications but acclimation times are extended.

TEMPERATURE EFFECTS VTR/ATR

Temperature has a predictable impact on bacteria growth rates.

Biofilms tend to correct for slower kinetics in cold waters by increasing the density of bacteria in the film.

At AST we normally do not reduce VTR in our sizing criteria until the temperature approaches 10oC however, acclimation times are greatly extended and backwashing procedures may need to be modified to avoid excessive biofilm removal.

DIFFERENCES IN SIZING METHODOLOGIES

Inclusion of In-Situ Nitrification

Inclusion of TAN flushed from system during water changes

Differences inherent to filter used. Fluidization velocities are required for fluidized beds,

but have no allegory in packed beds.