Nutrient fluxes in aquaponics systems

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Nutrient fluxes in aquaponics systems

• Harry Ako and Adam Baker
• Molecular Biosciences and Bioengineering
• College of Tropical Agriculture and Human
  Resources
• University of Hawaii at Manoa

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I Definition. Aquaponics, our way of looking at
it.
Feed

• Feed fish.
• Fish metabolites remediated by bacteria.
• Fish water nourishes plants.
• And is recycled.

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I Definition. Benefits.

• 2 crops from 1 input
• no effluent (negligible environmental impact)
• productivity 6 times higher than soil agriculture
  very suitable for islands

Our experiment just before harvest
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II History. Prevailing system developed by
James Rakocy
http//rps.uvi.edu/AES/Aquaculture/basil2002.jpg
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II History. The system is very complex.

• Complex equipment necessitate high capital
  expense and constant electricity
• Operation and maintenance requires a trained
  staff
• Attempted and failed in Saipan

Fish tank
clarifier
sump
screen filter tank
degassing tank
Air pumps, water pump, and 237 air stones (not
shown)
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After we finished our work we discovered a nice
quote Estimates of nutrient uptake and a
deeper understanding of culture water nutrient
dynamics are required for design criteria Rakocy
and Hargreaves, 1993

• Hypothesis Plants have nutritional needs that
  can be discovered. Fish can supply these needs
  if their husbandry can be matched to the
  nutritional needs of the plants.
• What model is the starting point?
• The Virgin Island model of the 70s? Has
  problems. Failed once before.
• UHM hydroponics systems not only academically
  successful but also commercially successful?
• In some subject matter areas UHM and CTAHR are
  the places to be in the world as homes for
  intellectual property.

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III Determination of lettuce nutrients

• Used hydroponics nutrients (Kratky, a UHM
  colleague)
• Used ICP-AES to measure nutrients used up in
  intermediate (4 weeks) and full cycle (6 weeks)
  grow out
• In the early weeks not much used up.
• First benchmark, 48 heads lettuce.

• Nutrients used up at full cycle were hypothesized
  to be required nutrients (remember these words in
  future slides)

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III Testing the required nutrients hypothesis.
Try lower Ca and Mg from original formula.

• 1st column, required nutrients.
• 2nd column, lowered Ca and Mg in hydroponics mix
  should theoretically meet plant needs
• No reduction in yield found

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III Testing the required nutrients hypothesis.
Try higher nitrogen

• 1st column, required nutrients.
• High N trial theoretically exceeded plant needs
• N uptake was greater (not shown)
• But no benefit in yield, even when grown in
  better sunlight

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III Testing the required nutrients hypothesis.
Try lowering the K
a
b
Control
Low K

• 1st column, required nutrients.
• Lowered K level trial theoretically inadequate
  for lettuce plants
• Lettuce yields significantly reduced

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III Testing the required nutrients hypothesis.
Temporal experiment.

• If use ¼ nutrients, the required nutrient curves
  predict that they will run out by week 4
• Growth stunted at Week 4
• Biochemical approach not only valid in terms of
  nutrient amounts but also valid in terms of time

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III Testing the required nutrients hypothesis.
Temporal experiment.
Lettuce head weight (g)

• If use 1/2 nutrients, the required nutrient
  curves predict that they will run out by week 6.
• Growth stunted at Week 6
• Nutrient amounts defined as required nutrients
  seem accurate

a
b
½ nutrients
Control
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Footnote Supplemental Fe is required

• However, Mn supplementation was found to be
  unnecessary

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IV Determination of conditions to produce
nutritious fish water. The math

• Required nutrients from previous work
• Assumed that these will be satisfied by a 20 L
  daily exchange
• Second benchmark, 6 daily water exchange a day.
• We need to do more work with flowing systems.
  Marissas is a start.

For a tray of 48 lettuce heads
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IV Determination of conditions to produce
nutritious fish water

• Stocked tilapia in 200 L of water. Fed and
  removed 20 L daily.
• Daily requirement in first data column
• When fish biomass was such that they ate 14 or 20
  g of feed daily, several nutrients would be
  deficient
• When fish biomass was such that they are 40 g of
  feed daily, all requirements would be met (except
  iron and Mn).
• Another consequence is that nitrogen may be used
  as a proxy for all nutrients

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IV Determination of conditions to produce
nutritious fish water.
The previous data suggested that 40 grams of feed
per day provided to 2.3 kg of tilapia maintained
target nutrient concentrations of 47 mg/L
nitrate-N in a 200 L tank with a 20 L of water
removed daily. Shown below.
The above was replicated in 5 week experiments.
As before 20 L of water were removed daily from a
200 L tank. The following resulted.
Alternate third benchmarks, 44-49 mg nitrate N/L,
40-59 g feed/day, and 2.3-2.5 kg fish. Additional
benchmark, have to bring the biofilter up slowly
and carefully. Fish rearing the hard part.
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V Aquaponics aquaculture hydroponics,
integration. Verification of predicted lettuce
needs

• If benchmarks can be hit, aquaponics lettuce
  heads were not significantly different in size to
  hydroponics lettuce heads.

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V Aquaponics aquaculture hydroponics,
integration. Fish growth parameters
During the 10 week aquaponics trial, fish growth
was measured (tanks proportionate to 1.5 lettuce
trays)
Can be used to predict fish yields in aquaponics
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V Aquaponics aquaculture hydroponics,
integration. Aquaponics is environmentally
friendly

• Of total nitrogen input into the system as feed,
  about 27 is captured as fish flesh, about 43 is
  captured as lettuce biomass, and a small
  fraction is lost as nitrogen gas or as solids
  used to fertilize garden plants
• None released into the environment

Denitrification a problem.
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Midterm conclusions

• Our nutrient fluxes are for trays with 48 heads
  of lettuce.
• Fish are held in 200 L (50 gallon) tanks at about
  12.5 kg/m3 and are fed 40-60 g of feed a day.
  This is 5 times less than Rakocys.
• Hence, our system proven with only one moving
  part, an air pump (which we are trying to get rid
  of) and is very simple and very inexpensive.
  Some people are using it with great success.

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VI Scenarios. Single family size

• Components

• Specifications

• One lettuce tray (1.2 X 2.4 m)
• One fish tank (200 L)
• One small air pump
• Shade cloth

• Water transfer, manual
• Fish biomass, about 2.5 kg
• Daily feed, 40-59 g
• Iron chelate, 0.25 g/week
• 1.4 heads lettuce/day 1.8 kg tilapia/10 weeks
• Cost, 250 USD

Other designs are permissible as long as the
basic specifications are followed. In this
instance fish are under the plants, water flows
constantly, etc.
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VI Scenarios. Micro-farm size

• Components
• 8 linked lettuce trays
• one 1600 L fish tank
• one air blower
• water pump
• Shade cloth (50)
• Specifications
• stock about 19.2 kg of fish
• feed 0.32-0.47 kg/day
• iron chelate 2 g/week
• annual production, 3300 heads of lettuce and 75
  kg tilapia
• annual income about 8600 USD at Hawaii farmgate
  pricesratio of lettuce to fish income
• cost of construction, 2500 USD

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VI Scenarios. Small farm, 0.1 hectare

• Components
• equivalent of 270 lettuce trays
• 54,000 L in tanks
• air blower
• recirculate water with a water pump
• Specifications
• stock about 648 kg of fish
• daily feed, 11-16 kg
• annual production, 112,000
• heads of lettuce, 2,500 kg tilapia
• Income 234,000 USD/year
• Cost, lt80,000 USD

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Summary

• Fine tuned lettuce nutrient requirements
• Set fish parameters that provide optimal
  nutrition to plants
• Verified results in several aquaponics trials
• These fluxes eliminated all electrical components
  but aeration in fish tanks
• Rational parameters will allow for flexible
  aquaponics design to accommodate different needs
  and physical environments weidenbach, koch,
  Mays, Ho Farms, Dave Campbell
• The methodology described can easily be applied
  to grow other crops

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Thank you

• This work was funded by the United States
  Department of Agriculture (USDA) Center for
  Tropical and Subtropical Aquaculture (CTSA)
  through Grant No. 2004-38500-14602