The deficiencies of the aquaponics Sump Tank System were alleviated by the (re)discovery of the CHOP System.
CHOP is an acronym for Constant Height/One Pump. In some parts, the CHOP system will be known by the more vulgar acronym CHIFT PIST (constant height in fish tank – pump in sump tank).
The CHOP system is a variation on the single pump system used by the UVI Aquaponics Research Centre. Where the UVI model employs the raft growing system, the CHOP system is adapted for use with grow beds.
With the CHOP system, the water flows from the fish tank to the media-based grow beds before draining into a sump tank. A single pump in the sump tank then pumps the water back into the fish tank.
Importantly, the level in the fish tank remains constant because the only water that leaves the fish tank is that displaced by water being pumped up from the sump tank.
If you should develop a serious leak outside of the fish tank, the pump in the sump tank will eventually stop (because there is no more water flowing back into the sump tank) but your fish tank will remain full. This eliminates a prospective point of failure.
While grow beds usually flood and drain at different times, the sump tank has to be large enough to accommodate the reality that (sooner or later) the grow beds will all fill at the same time.
For design purposes, a media-filled grow bed will contain 30 – 40% of the media volume in water. For example, a 600 litre grow bed will contain 200 – 240 litres of water – assuming that it is filled to capacity with gravel or clay pebbles.
If we have four 600litre grow beds (each having a water capacity of around 200 litres of water), a sump with a capacity of around 1,000 litres will be required.
The good news is that, although a larger sump tank will be required, it can be set up in such a way that it can serve multiple purposes……including an ebb and flow water garden or a Moving Bed bio-filter.
The advantages of the CHOP system include:
- Flexibility in placement of the grow beds.
- Only one pump required – energy savings.
- No need for timers or float switches.
- No pump in the fish tank – avoids churning solids up.
- You can network two or more fish tanks so that nutrient levels remain constant right across the system.
- You can use a single mechanical and biological filtration system to serve several fish tanks.
- Water level in the fish tank remains at a constant level.
- Water tank remains full in the event of plumbing or equipment failure.
The disadvantages include:
- More expensive to set up than a basic flood and drain unit.
- Requirement for a larger sump tank – equal to the total volume of water able to be simultaneously contained in the grow beds.
A much smaller sump can be used if continuous flow watering (surface or sub-surface) is used in place of the usual flood and drain watering regime.
If you choose this option, I recommend that you incorporate a trickling bio-filter into the design to optimise nitrification and aeration.
The CHOP System has become the layout of choice for the more serious flood and drain aquaponicist……for the benefits that it offers.