Mechanical Filtration
Hugh S. Hammer, PhD GSCC
Ron Malone, PhD LSU
Joe Fox, PhD Texas A&M
Total Solids
 The amount of solid material left in a container after the
water has evaporated.
 Total Solids = Total Suspended Solids (TSS) + Total
Dissolved Solids (TDS)
 Total Suspended Solids (TSS) are solids that can be
trapped by a filter. Examples: silt, decaying organic
material, industrial wastes, sewage
 Total Dissolved Solids (TDS) are solids that pass through a
filter (0.45 microns). Examples: carbonates, bicarbonate,
chloride, sulfate, phosphate, nitrate, calcium, magnesium,
sodium and other ions.
 TOTAL SOLIDS ARE INDICATORS OF POLLUTION
Sources of Total Suspended Solids
 High flow rates from fast moving water, silt,
sand, clay, organics
 Soil erosion (non-point source)
 Urban runoff (non-point source)
 Waste water and septic effluent
 Decaying organic matter
 Fish that stir up sediments (carps)
Problems with TSS
 Increased biotic and abiotic turbidity
– Reduced light transmittance and photosynthesis
– Unstable dissolved oxygen
– Increase water temperature
– Abiotic sources can clog gills and increase
disease
– Smother eggs, filter feeding animals, and
aquatic insects
– High TSS is often an indicator of other types of
pollutants and toxins (mercury and PCB)
Testing TSS
 A water sample is filtered through a preweighed filter (0.45 microns)
 The residue retained in the filter is dried in
an oven at 103 to 105 C
 The sample is dried to constant weight and
the weight is recorded
 Reported as grams per liter (ppt)
Total Dissolved Solids
 The water sample is passed through a 0.45
micron filter
 The water that passes through the filter is
dried in a pre-weighed dish at 180 C
 The sample is dried to constant weight
 TDS is reported as milligrams per liter (ppm)
 This is directly related to the conductance of
water (dissolved ions)
 EPA standard of 500 ppm for drinking water
Sources of TDS
 Geology and sediment composition
 Fertilizer run-off
 Waste-water and septic effluent
 Soil erosion
 Urban run-off
*** The TDS frequently includes
phosphorous, nitrate, and other nutrients
Aquaculture Solids
FEED
FECES
Uneaten Feed
Mechanical Filtration
 Solids removal employs systems from the
wastewater treatment industry
 Screening, gravity separation (sedimentation,
centrifuging, hydrocycloning) or adsorption
between particulate beds
 Processes designations for RAS
– Primary: one or more gravity methods
– Secondary: biological filtration
– Tertiary: ion exchange, reverse osmosis, foam
fractionation, carbon adsorption, sometimes
disinfection
Solids Characterization
 Three means of classification:
 Solid materials are further classified as being either
settleable, suspended, dissolved or colloidal
 Difference between settleable and suspended solids is
a matter of practicality
 Most settleable: > 10 µM (settle in an Imhoff cone in
less than 1 hr)
 Particles passing through a 1.2 µM membrane filter
are dissolved, suspended are trapped
 Dissolved particles consist of some organic and
inorganic ions and molecules present in solution
Particle Size Distribution (microns)
Settleable
10-4
100
Dissolved
10-3
10-2
Colloidal
10-1
1
10
Suspended
SOLIDS REMOVAL PROCESSES AND
PARTICLE SIZES
Coarse
Screens
Cartridge Filter
Plain
Sedimentation
Tube Settler
Microscreen
Granular Filter
Foam Fractionation
100
30
75
Particle Size, microns
10
Impact of Solids on Recirculating Systems
 Increased BOD: causes oxygen availability
problems with animals and biofilters
 Organic wastes (feces) build up increasing
ammonia and nitrite levels (toxic)
 Increased system turbidity, decreased water
clarity (fine particles)
 Gill damage in fish (fine particles) can create
opportunities for diseases
Waste Solids Become Chemical Problems
 Both uneaten feed and fecal material
become toxic ammonia through the action of
decomposing bacteria.
Uneaten Feed
Feces
Heterotrophic Bacteria
Ammonia NH3/NH4
Increased Biochemical
Oxygen Demand (BOD)
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
Tilapia
No Fine Solids Capture
Tiger Barbs
Settleable Solids Removal
 If screens aren’t used, wastewater is first treated by
simple sedimentation (primary treatment)
 Separation is via gravity settling
 As with ponds, the principle design criteria are the
basin’s cross-sectional area, detention time, depth and
overflow rate (refer to previous notes)
 Ideal sedimentation basins don’t exist in the real world
due to a variety of particle sizes, composition, etc.
 Once settling velocity is known, basic dimensions can
be estimated
Sedimentation
 Advantages:
– Inexpensive
– Works by gravity and doesn’t require energy
 Disadvantages:
– Only gets largest solids
– Takes a lot of space
– Labor intensive to clean
SEDIMENTATION
Vs
OUTFLOW
Settling Zone
(Vs > Overflow Rate
to settle)
Sludge Zone
Outlet Zone
INFLO
W
Inlet Zone
Vh
Sedimentation
Tanks and Basins
Sedimentation Tank
Plate and Tube Separators
 Also work on principle of
gravity
 Actually enhance settling
capacity of basins
 Typically shallow settling
devices consisting of
modules of flat parallel
plates or inclined tubes of
various geometric design
 Used in primary thru
tertiary treatment
 Limited success
Centrifuges and cyclonic separators
 Increase gravitational force on particles
via spinning motion (i.e., settling rate
increases)
 Many devices rated at different g
forces
 Work best on freshwater systems due
to many particles having similar
densities to that of seawater
 Most practical are hydrocyclones or
cyclonic separators
 Heavy particles are moved by higher
outside velocity to outside and
downward
 Underflow exiting unit is very small and
high density, “cleaner” water exits top
Under-gravel Filters
 Advantages:
– Easy to build and operate
– Inexpensive
– Does both mechanical and biological filtration
 Disadvantages:
– Needs to be vacuumed regularly (lots of maintenance)
– Clog easily
– Can’t handle big loads (mainly for aquariums and not
practical for aquaculture production)
Airlifts Perform Several Functions
Air
Air
–
–
–
–
Circulation
Aeration
C02 stripping
Foam control
Circulation
Circulation Options
Air
Airlift
Pump
Air
Screens
 Simplest, oldest method, pre-treatment prior to
primary treatment
 Placed across flow path of RAS water
 Coarse screens handle raw effluent, biofloc; fine
screens for tertiary treatment
 Many materials: fibers to A/C filters; cost increases
with decreased mesh size
 Static vs. rotary screens (0.25 to 1.5 mm; about 4-16
gpm flow per square inch of screen; removal efficiency
around 5-25%
 Rotary screens for fine solids removal are 50-70%
efficient; 15-60 µM
Screens
 Disadvantages:
–
–
–
–
–
May be difficult to remove and clean
Labor intensive to clean
Auto wash micro-screen filters use a lot of water
Some Units very expensive ($10,000)
Get mainly large solids and clog quickly
 Advantages:
– Simple concept
– Can be inexpensive and simple to build (socks, pantihose, furnace filters, mesh bags)
Micro-screen Filters
Over-Drain Flow
Captured Solids
Microscreen Cleaning Jets
Granular Media Filters
 Commonly referred to as “sand” or “bead” filters
 Two types “slow” and “rapid” filters
 Advantages:
–
–
–
–
–
–
Less labor is required (typically only to backwash)
Gets a wide variety of solid sizes (down to 20 microns)
Require less water than some units
Mechanical and Biological filters (depending on the media)
Best all-around mechanical filters
Capable of handling large loads (production aquaculture)
 Disadvantages:
–
–
–
–
Requires a lot of pressure for some (pumps)
Expensive
Can be more complex to operate
Can clog quickly depending on the media
Slow Sand Filters
 Usually custom-built, open
to atm
 Loading rates are slow,
0.6-0.7 lps/m2
 Particle size: 30 µM max
 For this reason require
more floor space
 Used in gravity flow
situations
 Downside: cleaning
O pen
T op
O PEN
C LO SED
P e r of ra et d
G ra ve l
S uppo r tP al et
F LI TR A T OI N M O D E
BAC KW ASH NI G M O D E
Rapid Sand Filters
 Typically closed,
pressurized units
 Handle high flow rates: 20
gpm/ft2
 Downside: very high head
loss (30-90 ft)
 Only really good for low
solids process streams with
some sort of pre-trt
 Backwashing can be made
automatic
Granular Filters
Important Point
 Sand filters can be used in series to filter out
different size particles so that they don’t clog
quickly.
– Large gravel
Small gravel
sand filter
– This is frequently used for facilities that bring in
natural water (such as seawater)
BEAD FILTERS
(a) Propeller-washed
(b) Bubble-washed
Propeller-washed Floating Bead Filters
F iltra toi n
B ackw ash ni g
Broodstock
Return
Bypass
Anti-siphon
valve
Sludge View Port
Pressure Gauge
Sludge
Intake
ADM System Prop-Washed Bead Filters
Motor and Backwash Propeller
4/13/2015
Pump
Filter Mode
Drop Filters :
Low Water Loss
Floating Bead Bioclarifiers
(2 )
W a te r F low
A
B
(1 )
W a te r F low
(7 )
(5 )
(6 )
C
E
(4 )
A ir F low
A ir
W a ter
Settled Backwash
Waters returned to
system
D
Air Bleed
Builds Charge
W a ter
S lu d g e
P a ten t # 5 7, 7 0 0, 8 0
(3 )
Backwash
mode
(2 )
Drop Filters : Low Water
Loss
Floating Bead Bioclarifiers
Released Air Washes Beads
(1 )
W a te r F low
(7 )
(5 )
(6 )
C
E
(4 )
A ir F low
A ir
W a ter
Internal Sludge
Capture
P a ten t # 5 7, 7 0 0, 8 0
(3 )
Circulation
Aeration
Degassing
Solids Capture
Biofiltration
Inlet
Airlift
Cartridge Filters
 Consist of cannister and replaceable cartridge
 Advantages:
– Removes very small particles
– Max particle retention is 0.01 µM (0.00001 mm)
– Very high water clarity
– Great for aquariums
 Disadvantages:
– Can be expensive
– Can clog quickly
– Can’t handle large volumes
– Not practical for production aquaculture
Sock and Canister Filters
Diatomaceous Earth (DE) Filters
 Granular material
composed of diatom
skeletons (frustules)
 Can serve as
replacement for
cartridge filters, but
require pre-filtration
 Fine grade DE can
filter down to 0.1 µM
Factors to Consider




Particle size to be removed
Amount of energy required to operate
Labor and maintenance
Amount of bio-load the filter can handle
(pounds of fish and pounds of feed)
Separate Units Strategy
 Partitions water treatment into a series of
individually steps
 Optimizes each step to meet the narrow
objective
 Integrates steps to develop a “treatment
train”
Consolidation Strategy
 Utilize multi-functioning components to:
 Minimize the number of components
 Improve the stability
 Reduce costs of components and energy
 Smaller footprint (less space)
 Disadvantage is that neither process is
optimized
 If you have space and money the separate
units strategy is better