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Urban Aquaponics
There’s nothing quite like eating home grown food, but it seems the rewards are often overshadowed by low
crop yield and endless weeding. Perhaps a solution lies amongst green-fingered city dwellers and the new
techniques that have spawned from the restrictions of urban life.
Aquaponics incorporates aquaculture and hydroponics to create a highly efficient and sustainable means of
food production by combining the disciplines of rearing fish and growing plants in water. Although both
processes have their problems with hydroponic systems requiring expensive fertilisers and fish tanks needing
regular cleaning, it is in unison that the system can flourish. In aquaculture the accumulation of effluents
increases the toxicity of the water to the tank’s inhabitants, whether it be fish, crustaceans or aquatic plants.
This water is pumped to the plants and drained through the bed to allow the roots to assimilate. An ebb and
flow method is adopted here; the beds are flooded before being quickly drained. A bell siphon is a nonmechanical way of ensuring that not only do the beds not overflow but that the clean water is returned to
the fish quickly and well oxygenated. A mature aquaponic system is self-sufficient and highly productive, and
with simple set ups comfortably fitting onto a balcony it certainly provides a wise alternative to old fashioned
farming.
The Nitrogen Cycle, more specifically the nitrification cycle, can be used to explain aquaponics. All animals
produce ammonium ions, along with larger organic solids, in their waste. However unlike other creatures,
aquatic animals are capable of directly excreting ammonia into the water, due to the compound’s high
solubility and the abundance of water available for dilution. The break-down of nitrogenous proteins and
amino acids to ammonium is known as ammonification. It is important to note that the toxicity of ammonia
increases with pH and temperature. The unionised form, NH3, is far more toxic than NH4+; therefore toxicity
to fish is lower in acidic conditions. With the additional H3O+ ions present the equilibrium shifts to the right,
this results in the formation of the relatively harmless NH4+.
H30+ + NH3 ⇌ NH4+ + H2O
Ammonia is first oxidised by nitrosomanos bacteria to nitrite (NO2-), which is then converted to Nitrate (NO3-)
by nitrobacter. The bacteria will naturally form in a fish tank due to the presence of ammonia, but it’s in the
grow beds where the primary nitrate production occurs. The plants will need to be grown in a media, often
gravel or clay pellets, to act as a biofilter. With the roots constantly submerged in water, the gravel
encourages the growth of these vital bacteria. As nitrification is aerobic, oxgenating the water will maximise
the nitrate yield. This can be furthered by allowing the water to drop into the grow beds from a height,
mimicking a waterfall.
2NH3 + 3O2 + nitrosomanos  2NO2- + 2H2O + 2H+
2NO2 + O2 + nitrobacter 2NO3In term of produce, plants ranging from leafy greens to fruiters can prosper. Types of fish, on the other hand,
depend on climate. Nevertheless, aquaponics insures that all produce is entirely organic. Though still in its
infancy, perhaps the science will inspire a generation of urban farmers.
Bibliography
http://www.kohalacenter.org/HISGN/pdf/Lesson8ChemistryandMicrobiologyofAquaponics.pdf
http://farmerbrownsaquaponics.blogspot.co.uk
http://www.fao.org/docrep/field/003/AC183E/AC183E18.htm
http://fins.actwin.com/mirror/begin-cycling.html#speed-cycling