What is a sump tank?
A "sump" is an area where liquid run-off accumulates. For example, in a basement the sump is where water that seeps in drains to. In a boat the sump is what is also known as the "bilge". In an aquaponics system the sump sits at a lower point than the grow beds and is where the grow beds drain to. "CHOP", "CHOP2" (Constant Height One Pump) and CHIFTPIST (Constant Height In Fish Tank Pump In Sump Tank) designs employ a sump tank. The reason is to both maintain a constant height in the fish tank, which is best for the fish, and to add more water to the system overall, thus creating more system stability with regard to pH and temperature.
How large of a sump tank do I need?
- Since the sump tank is where the water flows in and out of the grow beds you want your sump tank to be big enough to handle the situation when all of your grow beds are full at once. You need to add the total water volume of your grow beds, then subtract the displacement effect of the grow media (for Hydroton we've found that 55% of the water is displaced, and 62% with 3/4" gravel).
- Once you have figured this out, then you also need to calculate the minimum amount of water must remain in the sump tank to cover the pump and insure that it does not run dry and burn out. We use 3", but your pump intake height may be different than ours. This volume should be added to the grow bed water volume calculation above and this total is the minimum volume you need for your sump tank.
- You can easily connect two sump tanks with a simple bridge siphon and create essentially a single, larger sump tank this way.
What else do I need to consider?
Pay close attention to the height and width of the sump tank to make sure that it will fit, unobstructed, under your grow beds. If you have a dirt floor then you can sink the sump into the ground, so height is less of a consideration.