To many, aquaponics may look like it uses less water. The reality is, this method of farming can be best described as growing more crop with the same drop. In most cases this can be represented as 20% to 25% more food from the same water source. When using separate loops, such as the way we design farms, balancing the water between the two growing cultures will increase or decrease that water efficiency and as previously discussed, the efficiency of nutrient use. How do we balance the water use One of the beautiful things about integrated aquaculture is when the plant side and the fish side of the system are sized and managed correctly, it works in perfect harmony in terms of water and nutrient use. This does not always happen because nature is such a flexible thing we need the capacity to adjust to her whims. A separate system allows for the capacity to manipulate the two growing cultures to adjust to changes in the environment. The following chart clearly shows the nutrient and water uses in balance between the two systems. Remember this is one climatic set of conditions. Sure I can make the data look like anything, but let me explain further below to remove that concern. The table below is a very simplified version of what really goes on behind the scenes but it is perfect for our example of water use between the two systems. The priorities of the two systems are to remove the solids and prevent Nitrate Nitrogen build up in the fish system and have sufficiently sized plant system to permit that exchange on the fish system without it overflowing. What is really interesting about these results above is the “Water Exchange per Day” is how much water is required by the fish system to maintain a Nitrate level of 80mg/L. The “Trans-evaporation” is the amount of water the plant system will need under a specific set of environmental conditions (these are very flexible). Note how close the two are with the plant system using slightly more water and the “Nitrate Level” indicates slightly lower than the desired 80mg/L Nitrate Nitrogen levels we want for the fish. Perfect… in this set of conditions. We have used a low 18 Mega Joules per m2 for the solar exposure. This is fairly low, allowing for winter climate say here where I live. If we increase to 22Mj/m2 we increase the water the plants transpire by around 25%. That is where the balancing act gets tricky. Standard plant spacing of 25/m2 of Lettuce in this case with a seedling cycle outside the system and a final 28 day growing cycle within the system. The seeding can happen within but it is not very productive when that space is used. The stocking density of the fish is per 1000 liters of water being fed a 32% protein feed at 1.2% body weight per day and a FCR of 1.5. Fish species was irrelevant to the calculations as long as they were fresh water. Happy to say this modeling works well in practice. However the amount of variables nature throws at you, like two weeks of cloud and rain will throw the water use out of whack. With two growing systems you have more control over what happens at both ends so any changes in the climate do not adversely effect either crop….
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