Most aquaponic systems are single loop and fully recirculating. This means that the fish and plants exist in a single loop and share the same nutrient rich water. The reason these are the most popular is because they are simple to build and operate. But single loop systems are also a compromise because the plants, fish and bacteria must all be happy in the same solution. So for example, the temperature and pH levels must be maintained at levels that satisfy all the organisms, rather than being optimized for each.
|pH is maintained in a range to satisfy fish, plants and bacteria in a single loop system|
A typical decoupled system consists of a stand-alone RAS (recirculating aquaculture system) loop and a hydroponic plant loop. The sludge from the RAS system is digested in the biological waste system which provides the nutrients for the hydroponic system. Using a digester makes the system more efficient because the nutrients plants need (aside from Nitrogen) are trapped in the solid waste. The aerobic digester (a fancy term for a simple tank with air-stones in it) mineralizes the solid waste, releasing the nutrients the plants need.
A small decoupled system can be very easy to build and operate. If an AST bead filter is used, the fish loop is reduced to two components (fish tank and bead filter) because bead filter combines both bio-filtration and very efficient mechanical filtration. It also makes the system very easy to maintain and operate because it is self-cleaning. The following video shows simple the decoupled system can be with an AST bead filter.
The Heart of the System: AST Polygeyser Bead Filter
Although the video above shows a smaller endurance filter, the AST Polygeyser 3 cubic foot automatic backwash bead filter can be used to build a similar system that supports up to 60 m2 of plant area (900 gallon fish tank with up to 4.5 pounds of feed per day input).