A Recently Discovered 22-Year Old iAVs.

In 2017, a member of one of my Facebook groups unearthed a substantial article about an iAVs system that had been established in 1998.

While that was a surprise, it was nothing compared to the discovery that the system that was the subject of the article was still operating – 22 years after its establishment.

Long story short, I connected with its owner Gordon Watkins – an organic farmer and an avid tropical fish hobbyist – living in Arkansas.

He sent me a photographed copy of the article and what follows is a summary of its contents:

  • In 1990, he first learned of Mark McMurtry’s iAVs research project at North Carolina State University.
  • He invited Mark to Little Rock Arkansas to participate in a workshop on “Integrated Greenhousing.”
  • Following the workshop, GW began to explore the possibility of using iAVs to allow him to undertake year-round production…and to diversify from his main crop of organic blueberries.
  • GW built a small experimental iAVs to allow him to test the design and to assess local markets for tilapia and pacu.
  • His experimental prototype…housed in a 22’ x 14’ greenhouse was completed in October 1997.
  • Attached to the southern side of his house, the structure consisted of a cement block foundation with white oak and redwood framing…in preference to treated timbers.  He glazed the roof with twin-wall polycarbonate and the sides are recycled insulated glass panels.  The south wall is a series of top-hinged Thermopane windows with Bayless solar vent openers for ventilation.  A 20-inch thermostatically-operated exhaust fan – and motorised inlet shutters – provide additional ventilation.  Large sliding windows connect the greenhouse to the house…and the greenhouse serves as part of the passive climate control for the house.
  • GW’s iAVs system consists of a poured concrete vat – 22’ x 4’…and 2.5’ deep with a V-shaped bottom – that is set in the ground.  The tank is divided into five sections (with removable partitions) to allow the fish to be segregated by species, size and sex.  The tank is covered by 4’ x 2’ slatted panels made from black locust wood.  The tank covers also serve as a walkway.  They afford 50% shade to the tank and they’re easily removed when working with the fish…while still providing visual contact with the fish.
  • Having the fish tanks in the ground – with its slatted section walkway – is a particularly space-efficient feature of iAVs – not usually seen in aquaponics systems – while making use of gravity to return the water to the fish tanks.
  • Other systems utilise a double pump arrangement…with one pump in the fish tank to flood the beds – and the other to return the water to the fish tank…via a sump tank.
  • Adjacent to the walkway lies a 22’ x 8’ (12” deep) sand biofilter…filled with sand and gravel…a gently-sloping concrete floor leans toward the fish tank.  The sand bed walls were formed from cement blocks…and designed so that the outflow from the bed flows underneath the wall before draining back in the fish tank.  GW applied aquaculture-suitable epoxy to all concrete surfaces to prevent leaching of lime which would create problems with maintaining pH control.
  • He created furrows…to keep the plant crowns dry during flood cycles.  He planted water-loving plants like watercress, chives and emergent aquarium plants.
  • The sand/gravel media was selected so that 1 inch of water would drain through the 12” of sand in 15 minutes…providing for oxygenation for the bacteria and plant roots…while still retaining sufficient moisture for plant growth.
  • Each of the fish tank sections feeds an 8’ x 4’ bed.  Danner 1/20hp submersible pumps – one to a section – activated by a timer…every 90 minutes – for 30 minutes…sufficient to turn the fish tank volume over five times per day.
  • The pumps do not operate overnight – to help avoid fungal diseases from affecting the plants.
  • The water percolates through the sand media and along the sloping floor and back into the fish tank.
  • A PVC distribution grid is used to offset the tendency of the water to drain straight into the sand in a new bed.  The distribution grid is removed once the detritus layer and algal mat develop.
  • An air stone is suspended 6” above each pump…providing additional aeration and creating a current in the fish tank that causes the fish waste to accumulate at the pump intake.  The air pump is an L20NC floating-piston air pump.  Dissolved oxygen levels are kept at around 5ppm.
  • Supplemental heat is provided to keep the greenhouse at 80oC. (27oC)…through a two-zone hydronic heating system – sometimes known as a radiant slab heating…comprising special rubber tubing embedded in the concrete floor of the fish tank and buried in the gravel beds – connected via a manifold to a 40-gallon natural gas water heater.  Thermostatically controlled circulation pumps moved the hot water…140oC (60oC)…through the tubing as required.  The fish tank was kept at 80oC during the day and 64oC at night.  GW asserts that the hydronic heating had the advantage of keeping the heat where needed rather than at the peak of the greenhouse.  By way of a supplementary heating system, hot air ducted from GW’s wood furnace to offset the possibility that climbing plants may succumb to freezing…notwithstanding that their roots remained comfortably warm.
  • One downside to the iAVs system is that construction is a little more involved and viewing the fish is a bit more difficult than with purchased prefabricated tanks.
  • iAVs comprises three primary interdependent living components that are crucial to its operation…beneficial bacteria, fish and plants.  All three must be carefully monitored and maintained in equilibrium.
  • On 10th October 1997, GW introduced 12lbs (250 individuals) of hybrid all-male tilapia into the system.  As of 1stJanuary 1997, he estimated the fish load to be 60lbs.  He planned to use fish spawned in his system to stock the system until it reached the expected carrying capacity of 0.5lbs/gallon – or 750lbs of fish was reached.
  • GW fed fry and fingerlings a 42% protein feed until they were about an inch long…at which point they were transferred to the iAVs and switched to a 32% grow-out ration.  He fed three to four times daily at approximately 5% of their bodyweight…or as much as they’d eat in 15 minutes…with a view to harvesting 1.5lb fish at 9 – 12 months from spawning.
  • GW noted that, with few exceptions, almost any plant suitable for greenhouse production can be grown in an iAVs.  He trialled tomatoes, cucumbers, melons, eggplant, peppers, lettuce, spinach, oriental greens, watercress, chives, basil, citrus, several emergent aquarium plants and numerous house plants.  The only complete failure was spinach – probably caused by the high bed temperature.
  • The temperature was to impact the fish in the system, too.  Finding that it was uneconomical to heat the water for the fish, GW switched to hybrid bluegill.
  • He transplanted – and direct-seeded – crops with equal success.  He also propagated a variety of house plants and shrubs with hardwood and softwood cuttings.
  • Achieving the proper balance between the biological load (quantity of fish produced) and the biofilter capacity is critical to the successful operation of the iAVs.  If biofilter volume or plant densities are too low, toxins will accumulate and result in reduced fish production or death.  Similarly, if fish densities are too low or plant densities are too high there will be insufficient nutrients to grow healthy plant growth.
  • GW regarded that a 1:1 ratio was optimum for fish and plant production.  He noted (from McMurtry’s work) that “fish stocked at 2.75lbs/m3 – at this ratio – yielded approximately 10lbs of fish per cubic metre per month. Tomatoes – planted at the rate of 4 plants per square metre – yielded 25lbs/m2 in 132 days.”  He stated that his system was designed on that data.
  • An additional variable that had a direct bearing on the component ratio…and one that McMurtry didn’t address…was that of crop choice.  Different crops absorb different amounts of nutrients (lettuce versus tomatoes, for example)…and this will affect the nutrient cycle.
  • GW believes that, if a flexible ratio is used, the system is flexible, forgiving and responsive enough to allow some variation in stocking rates, planting densities and crop choices.

Fast forward 22 years…

  • This iAVs is still operating…albeit in ‘maintenance mode’…and is currently planted to ferns and a few other perennials.
  • The greenhouse temperature is now maintained solely by passive solar heat.
  • There has been little noticeable reduction in percolation rates.

Gordon Watkin’s 22-year old iAVs puts to bed the old myth about sand clogging.  Its story also adds an important second voice about what happened back at the time the method was developed.


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A Recently Discovered 22-Year Old iAVs.