Transcript Bowling Green Satellite Aquaculture Center

Bowling Green Satellite
Aquaculture Center
Introduction to Recirculating
Aquaculture Workshop
Critical Considerations in
Recirculating Production Systems
Definition
An aquaculture
production system
that recycles and
renovates water for
the culture of aquatic
organisms
Categories of Recirc.
Systems
Semi- closed system
5% exchange per pass
•
120% exchange per
day
Closed system
•
0-20% volume change
per day (typical of
systems being
designed today)
•
Recirculating System Pro’s and
Con’s
PRO’S
• Less water needed
• Less land needed
• Temperature Control
• Water Quality Control
• Waste Retention
• Better feed Conversion
• Product Isolation
• Inventory Control
CON’S
• High Initial Investment
compared to other
technologies
• No existing standard
protocols
• Short Response time
THINGS GO WRONG
FAST !
• No track record
The Recirc. Golden Rule
Do Not Be Impressed By Fish Held at High
Densities.
Fish can be held at high densities, in even poorly
designed systems, if they are not fed .
Be Impressed By High Feed Rates per Day
Remember it takes Feed to Raise Fish
Daily Weight Gain = Daily Feed Rate / Feed Conversion
Ratio
ITS THAT EASY!!!
Feed Effects on Water Quality
Feed’s Impact On Water Quality Is Almost
Always NEGATIVE
Inputs and Outputs Based on the Input of
1Kg of Feed
But Why Mention Feed Now?
•
•
•
•
Feed is needed to grow fish (no brainer)
Feed will determine the inputs required to
maintain proper water quality
Feed will also determine the amount of
waste products that also need to be dealt
with to maintain water quality
So lets look at the general water quality
parameters we need to meet to insure a
healthy environment
General Water Quality Parameters
•
Dissolved Oxygen (O2) (mg/l)
> 6.0*
•
Carbon Dioxide ( CO2 )(mg/l)
pH
Alkalinity
(mg/l)
Ammonia
(NH3) (mg/l)
Nitrite
(NO2)(mg/l)
Nitrate
(NO3)
< 20
6.5-8.0
100-300
0.02-0.05
0.2-5.0
<1000
•
•
•
•
•
*Suggested average for culture tank, O2 should not fall below 4 mg/l
anywhere within the system
Feed and Water Contin.
Inputs
• 0.25-1kg Oxygen
• 0.18-0.4 Kg Alkalinity
(usually Sodium
Bicarbonate)
Outputs
• 0.35-1.38 Kg CO2
• 0.25-0.5 Kg Waste
Solids (dry weight)
• 0.025-0.055 Kg NH3
& NH4
Water Treatment
Solids Removal (Round Tank Hydraulics and
Filtration)
First, as always, definitions!
Settleable Solids: Under quiet conditions
these solids will settle from the water
column in 1 hour
Suspended Solids: Solids that will not settle
out in one hour under quiet conditions
Water Treatment Continued
Hydraulic Retention Time (HRT) = Tank
Volume Divided by Inflow Rate (Qin)
EXAMPLE: 20,000 liter (5,283 gal) / 333lpm
(88 gpm) = 60 minutes
In actuality this HRT is a mean or average
To turn the entire volume of the tank over
would take 1.6 hours!
How Do Round Tanks Work For
us?
•
•
•
•
Simple to maintain
Provide uniform water quality
Allow operation over a wide range of water
velocities to optimize health and condition
Settable solids can be rapidly flushed to
the center drain
Hydraulics
First lets look at water in flow and tank dimensions
The optimum tank should have a diameter to depth ratio of
3:1
With this ratio we also utilize a vertical manifold to deliver
water to the culture vessel
This combination allows for what is called a “tea cup effect”
The friction between the tanks walls and water form a
secondary rotation which will rapidly move settable
solids to the center drain
Now if we couple this effect with a double drain we can “decouple the HRT for suspended and settleable solids!!!!
Vertical Manifold
•
•
•
•
•
The vertical manifold
extends to the bottom of
the tank as shown right
This allows for better
mixing within the tank as
well as assisting in the
“tea cup effect”
Velocity should not
exceed Vs
Vs= Safe swimming
velocity in body lengths
per second
Vs< 5.25 L 0.37
•
Flow A = 85-90% of
flow
•
Flow B = 15-20% of
flow
So we have removed the solids
from the tank ! Now what?
Suspended Solids
One of the most
effect methods:
Drum Screen
filtration
Drum Screen Operation
Settleable Solids
•
15% of flow
• Referred to as a swirl
separator or
hydrocyclone
• Discharge from SS
re-enters flow to drum
screen filter
Lets look at it all together
Bio-filtration
The term bio-filtration refers to using a
biological process to remove or convert a
targeted substance
In the case of Recirc. systems we use
bacteria to deal with NH3 & NH4+ and
convert them to nitrate NO3
The Nitrification Process
Nitrification is a two step process
Nitrosomonas bacteria convert ammonia
(NH3) to Nitrite (toxic)
Nitrobacter bacteria convert nitrite (NO2-)
to nitrate (NO3) (virtually non-toxic)
Nitrification Equation
Nitrosomonas:
NH4+ + 1.5 O2  2 H+ + NO2Nitrobacter:
NO2- + 0.5 O2  NO3
The Take Home Message
Bio-filtration is all about
• Surface area
• Living space for the bacteria
• Competition for that space
• Food (ammonia or nitrite
• Good living conditions
O2 (enough), proper pH (6.8-7.5) and not
to much CO2
Lets Look at a Trickling Bio-filter
The Filtration Done, Now Let’s
Renovate
CO2 stripping:
CO2 is problematic in that it interferes with the biological
processes of both fish and nitrifying bacteria
CO2 is very volatile in water and can be stripped by
mechanical agitation
In the case of a trickling bio-filter the falling of
water through the substrate, as well as, air
diffusers in the bio-sump drive off unwanted CO2
Aeration (addition of O2 to the
system)
Any type of aeration attempts to increase
the surface contact area between the
water and the gas
The actual transfer occurs in a very thin area
known as the water/ gas interface
By increasing the surface area of that
interface we can increase the amount of
gas transfered
Aeration Continued
For a given volume of gas the smaller the bubble
the better the exchange
EXAMPLE
A gas bubble with a diameter of 20 mm has a
surface area of 12.6 cm3 and a volume of 4.19
cm3
296 3 mm bubbles could be made from the same
20 mm bubble. The total surface area of these
bubbles would be 83.6 cm resulting in an
increase of almost 7 times the surface area!!!
Oxygen Vs Air
(with air stones)
Airstones are very inefficient
With air only 3-4% actually goes into
solution
Pure Oxygen with the best of airstones in
1m of water is better but only 30-40 %
efficient
But we can do even better!!!!
Down Flow Bubble Contactors &
Speece Cones
How They Work
Water is flows into the top of the contactor/ cone
Oxygen in injected near the top as well
Water attempts to force the 02 down while the 02
attempts to rise
The result is a continuous contact between the gas
and liquid with no loss to the atmosphere
Oxygen Absorption efficiency = 80-90%!!!!
Oxygen Transfer Efficiency = 3.9 kg O2 / Kwh
Water Flow from Biosump to
Culture Tank
Review The Whole Cycle