What is a Sump tank and why do people use them?
This is a common question I’ve seen in aquaponics forums. There are a few possible reasons for using a sump tank but they are primarily a plumbing device and water level fluctuation control device.
For instance, in an aquaponics system design, if your grow beds are not up high enough to drain into your fish tank, you need a tank lower down to collect the water which can then be pumped back up to your fish tank. Many systems have been built where a sump tank is used to collect the water draining from the grow beds and there is a float switch that will turn the pump on and move water back to the fish tank when the water level rises in the sump tank. This is a common example of a two pump aquaponics system design.
Now Having the sump tank can definitely reduce the amount of water fluctuation in the fish tank too. There are even examples of Aquaponics system designs where the water level doesn’t fluctuate much at all in the fish tank. These are often called CHIFT PIST (constant height in fish tank pump in sump tank) or CHOP (constant height one pump) systems. In these systems the sump tank holds the pump and the sump tank must be big enough to flood all the grow beds at once (unless an indexing/sequencing valve is used, will get to that in another blog post.) So now the sump tank is handling all the water level fluctuations and the fish tank has no danger of being pumped dry. But it does mean you need a big tank down low or even dug into the ground. Heck if you are doing a 2:1 grow bed to fish tank ratio flood and drain aquaponics system design, you need a sump tank as big as your fish tank! But don’t take that as all bad, more water does mean more system stability though digging a big tank into the ground can be a pain. And keep in mind the danger of what if all beds are flooded at the same time and there is a heavy down pour that fills the ground with water and floats the sump tank up? This is a real danger in some places.
So, sump tanks can take care of water level fluctuations in the fish tank and allow the grow beds to be set lower than the fish tank.
Here are some diagrams that might help people visualize some of what I’ve been saying.