Nutrient fluxes in aquaponics systems

Harry Ako and Adam BakerMolecular Biosciences and Bioengineering

College of Tropical Agriculture and Human ResourcesUniversity of Hawaii at Manoa

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

• Feed fish.• Fish metabolites remediated

by bacteria.• Fish water nourishes plants.• And is recycled.

Feed

Plants take up metabolites to grow

Bacteria remediate toxic N species

bioremediatedwater

Fish grow and excrete metabolites

cleaner water

Feed

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

http://rps.uvi.edu/AES/Aquaculture/basil2002.jpg

II History. Prevailing system developed by James Rakocy

•Research began in the 1970’s•Plants (in raceways) added to a fish tank system (under black tent)

• Complex equipment necessitate high capital expense and constant electricity

• Operation and maintenance requires a trained staff

• Attempted and failed in Saipan

II History. The system is very complex.

clarifier

degassing tank

sump

screen filter tank

Air pumps, water pump, and 237 air stones (not shown)

Fish tank

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.

III Determination of lettuce nutrients

Nutrients Hydroponics nutrients

Remaining after 4 weeks

Remaining after 6 weeks

Manganese (mg) 307 2% 1%

Nitrogen (g) 33 70% 5%Potassium (g) 53.7 74% 14%Calcium (g) 31.1 83% 51%

Magnesium (g) 16.1 91% 53%

Phosphorus (g) 11.2 90% 45%Iron (mg) 337 123% 83%

Zinc (mg) 77.7 97% 62%

Copper (mg) 55.5 61% 56%

Boron (mg) 322 86% 59%

•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)

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

Nutrients Required nutrients

Low Ca & Mg

HydroponicsManganese (mg) 305 307Nitrogen (g) 31.6 33.3Potassium (g) 46.0 53.7Calcium (g) 15.2 18.8Magnesium (g) 4.72 6.97Phosphorus (g) 6.12 11.2Iron (mg) 58.1 688Zinc (mg) 29.6 77.7Copper (mg) 24.6 55.5Boron (mg) 133 322

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

a a

III Testing the required nutrients hypothesis. Try higher nitrogen

Nutrients Required nutrients

Provided to High N

Manganese (mg) 305 307Nitrogen (g) 31.6 62.2Potassium (g) 46.0 53.7Calcium (g) 15.2 31.1Magnesium (g) 4.72 16.1Phosphorus (g) 6.12 11.2Iron (mg) 58.1 688Zinc (mg) 29.6 77.7Copper (mg) 24.6 55.5Boron (mg) 133 322

•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

III Testing the required nutrients hypothesis. Try lowering the K

Nutrients Required nutrients

Provided to Low K

Manganese (mg) 305 307Nitrogen (g) 31.6 33.3Potassium (g) 46.0 34.3Calcium (g) 15.2 31.1Magnesium (g) 4.72 16.1Phosphorus (g) 6.12 11.2Iron (mg) 58.1 688Zinc (mg) 29.6 77.7Copper (mg) 24.6 55.5Boron (mg) 133 322

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

a b

Control Low K

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

1/4th nutrients, Week 4 Control, Week 4

III Testing the required nutrients hypothesis. Temporal experiment.

•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

Lettuce head weight (g)

a

b

Control ½ nutrients

Footnote: Supplemental Fe is required

•However, Mn supplementation was found to be unnecessary

Control, Week 3

Aquaponics (no iron) Week 3

Aquaponics, Week 4

Fe chelate

With Mn

Lettuce head weight (g)

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

Nutrients

Required nutrients (g; determined previously)

Daily requirement from 20 L

(mg/L)Manganese 0.305 0.36Nitrogen 31.6 37.6Potassium 46.01 54.8Calcium 15.2 18.1Magnesium 10.6 12.6Phosphorus 6.117 7.28Iron 0.058 0.49Zinc 0.03 0.036Copper 0.025 0.03Boron 0.133 0.015

•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. Marissa’s is a start.

For a tray of 48 lettuce heads

IV Determination of conditions to produce nutritious fish water

Nutrients(mg/L)

Daily requirement from 20 L

(mg/L)

Fish water 14

g feed daily

Fish water20 g feed daily

Fish water 40 g feed daily

Manganese 0.36 0 0.002 0.001Nitrogen 37.6 30 34 47Potassium 54.8 101 100 105Calcium 18.1 22.5 46.2 33.9Magnesium 12.6 13.5 18.6 21.0Phosphorus 7.28 4.46 6.36 10.7Iron 0.49 0.001 0.011 0.038Zinc 0.036 0.01 0.021 0.095Copper 0.03 0.04 0.02 0.059Boron 0.015 0.05 0.09 0.079

•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

IV Determination of conditions to produce nutritious fish water.

Tank Daily feed input (g)

Tilapia biomass (kg)

Nitrate N (mg/L)

1 59 2.5 44

2 54 2.5 49

Nitrate N (mg/L)

Tank size (L)

Daily water exchange (L)

Daily feed input (g)

Tilapia biomass (kg)

47 200 20 40 2.3

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.

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.

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.

V Aquaponics = aquaculture + hydroponics, integration. Fish growth parameters

TankFish

recovered/stocked

Fish biomass (kg) Feed input (kg)

FCR

Mean weight (g)

Start End Gained Start End

T1 51/51 3.35 5.84 2.49 4.84 1.9 66 114T2 48/48 3.38 6.32 2.94 4.72 1.6 71 132

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

TankFish

biomass (%)

Lettuce biomass

(%)

Denitrification or solids (%)

T1 26 40 34T2 32 41 27T3 22 49 29

Mean 27 43 30

•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

V Aquaponics = aquaculture + hydroponics, integration.

Aquaponics is environmentally friendly

Denitrification a problem.

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 Rakocy’s.

• 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.

VI Scenarios. Single family sizeComponents

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

Specifications

Other designs are permissible as long as the basic specifications are followed. In this instance fish are under the plants, water flows constantly, etc.

•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

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 prices…ratio of lettuce to fish income

• cost of construction, 2500 USD

VI Scenarios. Small farm, 0.1 hectareComponents• 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, 112,000heads of lettuce, 2,500 kg tilapia• Income 234,000 USD/year• Cost, <80,000 USD

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, May’s, Ho Farms, Dave Campbell

• The methodology described can easily be applied to grow other crops

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