The location of each supplemental lighting luminaire was carefully calculated with the help of a computer software program for optimum light uniformity at plant level. An average light intensity of 200 Ámol-m-2-s-1 (1500 foot candle) reaches the plants, which are located 10.5 feet below the luminaires (600-watt high pressure sodium).
Each pond is equipped with a nutrient solution recirculation system (larger white PVC pipes) and heating system (smaller black plastic pipes looped on the bottom of the pond). Note the blue nutrient solution recirculation pump in the foreground to the right.
Each pond has a recirculation pump (blue) to circulate the nutrient solution continuously. Three (black) solenoid valves direct the water flow (after passing the pump) either to the fertigation station in the headhouse (the two horizontal valves are opened, the vertical valve closed) or back into the pond distribution system (by opening the vertical and closing the horizontal valves).


Three pumper head are used to pump the A and B stock solutions and nitric acid into the main nutrient solution stream as it is pumped by the fertigation station. Note the A and B stock tanks on the left of the picture.
The fertigation system uses an A and B stock tanks (larger black tanks in the foreground) in addition to an acid (nitric acid) tank (metal barrel to the left). Concentrated nutrient solution stocks are kept in tanks A and B. Mixing these stocks would likely result in precipitation, making the fertilizer unavailable to the plants.


The recirculating nutrient solution is checked with an EC (electrical conductivity) and pH sensor (top two sensors). The measurements are used to adjust the frequency of the fertilizer injection. In addition, a flow meter (bottom sensor) is installed to measure the flow rate of the nutrient solution flowing through the fertigation station.
A flow meter in the oxygen supply line allows for adjustment of the amount of oxygen gas flowing into the recirculating nutrient solution. Each pond recirculation system is equipped with such a flow meter.
A porous, stainless steel sparger is installed in the nutrient solution recirculation system of each pond. Oxygen gas is forced through the sparger, which releases tiny bubbles of oxygen into the nutrient solution as it is pumped around. These tiny bubbles dissolve quickly into the nutrient solution maintaining the appropriate dissolved oxygen concentration (4 - 8 mg/L).
The opening position of the ventilation window is controlled by a so-called differential pressure switch. The switch measures the static pressure difference between the inside and outside air in the greenhouse. By regulating the position of the vent window, a constant pressure difference can be maintained, allowing for sufficient airspeed of the incoming air and ensuring proper mixing with the greenhouse air.

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