Nutrient Dynamics in Aquaponics

Chris Hartleb University of Wisconsin-Stevens Point

Northern Aquaculture Demonstration Facility Aquaponics Innovation Center

Aquaponics • Integrated & soilless • Free of biocides • Conservative use of water, space &

labor • Produces both vegetable & protein crop • Continuous year-round production • Meets socio-economic challenges

– Urban & peri-urban – Locavore movement

• Initial costs – Loans & financing

• Location – Zoning & permitting

• Market

Aquaponic Systems

Fish tanks

Clarifier (solids filter)

Mineralization tanks

Raft tank Water pump

Degassing tank & biofilter

Air pump

Plant Production Systems

• Raft (Revised agriculture float technology) – Deep water culture

• Large volume water • Root aeration • Nutrient uptake: High

• Nutrient film technique – Low volume water – Less system stability – Nutrient uptake: Low

• Media based – Biofiltration in media – Clogging & cleaning present – Nutrient uptake: High

Plant Nutrition

• Macro-nutrients • Micro-nutrients • Must be in fish food or water

supply

Element Concentration (ppm) Percent

Nitrogen 15,000 1.5

Potassium 10,000 1.0

Calcium 5,000 0.5

Magnesium 2,000 0.2

Phosphorus 2,000 0.2

Sulfur 1,000 0.1

Chlorine 100 <0.1

Iron 100 <0.1

Boron 20 <0.1

Manganese 50 <0.1

Zinc 20 <0.1

Copper 6 <0.1

Molybdenum 0.1 <0.1

Nickel 0.1 <0.1

Solids Filtration

Sources Fish wastes Uneaten food

Types Settable Suspended Fine and dissolved

Settable solids (gravity removal) Settling tank Hydrocyclone (swirl separator)

Aquaponic Mechanics • Feeding rate ratio: 60-100 g/day/m2

(leafy greens grown on rafts) • Nitrification • Mineralization • Why does it work? Similarities

Fish Plants

Organic (protein) Nitrogen

Potassium Potassium

Calcium Calcium

Magnesium Magnesium

Phosphorus Phosphorus

Sulfur Sulfur

Chlorine Chlorine

Sodium

Iron Iron

Boron

Manganese Manganese

Zinc Zinc

Copper Copper

Molybdenum Molybdenum

Nickel Nickel Iodine, Cobalt, Fluorine, Vanadium, Chromium, Selenium, Tin, Silicon

Food 100% N 100% P

Retained in Tissues 30% N 32% P

Dissolved 87% N 10-40% P

Solids 13% N 60-90% P

Effluent 70% N 68% P

87% Overlap

Biological Filtration

• Nitrification – Oxidizes ammonia and nitrite to nitrate – No light needed (photosensitive) – Oxygen required – Slimy, light brown, no bad odor

NO3- NH3

1½ O2 Ammonia-oxidizing bacteria (AOB)

Nitrite-oxidizing bacteria (NOB)

1½ O2 NO2-

4.3 g O2 and 7.14 g alkalinity as CaCO3 needed to oxidize 1.0 g NH3-N

Nitrogen Conversion

• Bacteria convert ammonia to nitrite to nitrate. – Ammonia-oxidizing bacteria (i.e. Nitrosomonas):

55NH4

+ + 76O2 + 109HCO3- → C5H7O2N + 54NO2

- + 57H2O + 104H2CO3

• Ammonium is combined with oxygen & hydrogen carbonate to produce nitrite, water & carbonic acid. – Nitrite-oxidizing bacteria (i.e. Nitrobacter):

400NO2

- + NH4+ + 4H2CO3 + HCO3

- + 195O2 → C5H7O2N + 3H2O + 400NO3-

• Nitrite is combined with ammonium, carbonic acid, hydrogen carbonate & oxygen to produce water & nitrate (nitrification).

Results of Conversion

• Approximately 4.3mg of O2 are consumed per mg NH3-N oxidized to NO3-N

• Conversion is an alkaline and acidic process

• Nitrate must still be converted to N2 (gas) for removal from the system (denitrification)

Ammonia

• Product of protein metabolism; major waste product of fish; excreted by gills as:

NH3 (unionized) + NH4 (ionized) = TAN

• Present as ammonia (NH3) and ammonium (NH4

+)

• pH and temperature determines the proportion of each

• Alkaline pH → more NH3 (toxic)

• Acidic pH → more NH4 (less toxic)

• Ammonia poisoning more common at alkaline pH

Ammonia Poisoning

• Can occur at levels of 0.1 ppm (acute); 0.06 pmm (chronic)

• Results in hemorrhaging & destruction of mucus membranes; noticeable around the gills; fish gasp for air & show rapid gill movement

• Ammonia is controlled by adjusting feeding levels & turnover of water

Nitrite • Nitrite, like ammonia, exists in a pH-dependent equilibrium with nitrous

acid (HNO2).

• Nitrous acid is freely diffusable across the gill membrane, therefore, more toxic to fish.

• However, nitrous acid is rarely present at pH values acceptable to fish.

• Nitrite is not freely diffusable across the gill, but can be pumped by the chloride uptake mechanism.

Bio-Planning • Feeding rate controls the growth of

the fish added to the system. • Rate of nitrogen excretion increases

with feeding rate. • For every 1.0 kg of feed consumed

– 0.25 – 0.5 kg of O2 are consumed – 0.18 – 0.4 kg alkalinity consumed – 30-40 g ammonia (NH3) produced

• Every 10 mg/L of O2 consumed will produce:

– 9.6 mg/L CO2

– 7.2 mg/L TSS – 5.9 mg/L COD – 1.2 mg/L TOC (Total Organic Carbon) – 0.8 mg/L TN – 0.58 mg/L TAN

• Biofilter produces approximately 37% of the total CO2 in an RAS

• Heterotrophic respiration produces 1 mg/L CO2 for every 1 mg/L consumed (mineralization)

• Degassing must occur after the biofilter

New Tank Syndrome

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Ammonia Nitrite Nitrate pH

Denitrification

• Microbially facilitated – Facultative anaerobic heterotrophic bacteria – Best when oxygen <10% – More important in larger systems

What Base do you Use to Raise pH?

• Potassium hydroxide (KOH)

• Calcium hydroxide (Ca(OH)2) = (hydrated lime)

• DO NOT use a sodium-base (for instance sodium bicarbonate)

What Acid* do you Use to Lower pH?

• Sulfuric acid • Phosphoric • Citric acid

• *Since nitrification creates nitric acid, it is quite uncommon to

have to lower pH in aquaponics. But, if your source water has a high pH, high alkalinity and high hardness, it might be required.

Mineralization

• Decomposition of organic matter into elements • Heterotrophic bacteria, fungi & microorganisms

– Aerobic; Reproduce rapidly (hours) • Concentrated where solid waste is abundant • In addition to mineralization, this tank is also de-nitrifying (reducing nitrogen). • Reducing the amount of nitrogen increases the ratio of the other elements.

Nutrient Availability for Plants

• The frequency of cleaning the mineralization tank results in the ability to vary the ratio of nitrogen to other elements.

• When growing fruiting crops, you clean the netting less often so you have less nitrogen and more of the other elements.

• When growing leafy crops, you clean the netting more often so you have more nitrogen (less de-nitrification).

Nutrient Deficiencies & pH

• Calcium & Potassium: Adjust pH with calcium hydroxide and potassium hydroxide.

• Iron: Not usually deficient with mineralized well water. Can be deficient in rain water or municipal water. Supplement with chelated iron.

Alkalinity

The capacity of water to buffer against wide pH swings. Bicarbonate: CO2 + H2O H+ + HCO3

-

Carbonate: HCO3- H+ + CO3

-

At pH = 7-8, bicarbonate dominates. At pH > 9, carbonate dominates.

Can be removed from water through boiling & distillation (white scale seen on dishes).

Acceptable range: 40-400 mg/L If need to increase alkalinity, pass water through limestone or crushed oyster shells. Addition of calcite lime:

CaCO3 + CO2 + H2O Ca+2 + 2HCO3-

Hardness

• Amount of divalent cations such as: calcium, magnesium, etc. – Measured as mg/L CaCO3

• Often used as an indicator of alkalinity but hardness is not a measure of alkalinity

(magnesium or calcium sulfate increases hardness but has no affect on alkalinity).

• Hard water is well buffered; while soft water is less well buffered.

• Water hardness affects fish because it influences osmoregulation.

– Each species has an optimum range. Water Type CaCO3 (mg/L)

Soft 0 - 75

Moderately hard 75 - 150

Hard 150 - 300

Very hard >300

Coupled and Decoupled

• Raise cool and cold water fish • Greater management