Aquaponic consists of a fish-farm (aquaculture) combined with soil-less grown crop plants (hydroponic).

The picture below shows a fish tank connected to a plant bed filled with gravel. The water flows from the fish tank to the plants. The water is then pumped back to the fish after it's been aerated and saturated with oxygen. Both fish and plants benefit from this method. The fish water contains a lot of precious nutrients which can be used to fertilize the plants. The plant roots, on the other hand, absorb the nutrients and thus clean the water to make it ready for reuse it in the fish tank. Another benefit is that no wastewater leaves the system.

Advantages of aquaponic compared to conventional fish farming systems:

  • Recycling of nutrients: The nutrients from the fish water are reused to fertilize the plants - therefore no additional mineral fertilizer has to be added.
  • Saving water: The plants and the biofilter on which they are growing clean the water so it can be reused in the fish tank.
  • No polluted outflow: Conventional fish farms have to deal with the outflow which is rich of nutrients and has to be treated as wastewater. In aquaponic there is no wastewater, for the water keeps circulating in the system.


The expression biomass refers to living material. It includes all kind of lives such as bacteria, plants, animals and human beings. Not only the salad you are eating belongs therefore to the world biomass, but also yourself, your dog and even the bugs that are responsible for your cold. It is the scientific expression for all living things.

Bulk material

The compost bacteria require carbon and nitrogen to work. Composting of carbon-rich materials such as leaves would therefore result in a poor decomposing process. That is one of the reasons that forest soils are often rich on organic matter and the decomposition of the autumn leaves can take several months.
Kitchen wastes or grass clips however are rich on nitrogen. There is not enough carbon to start a proper composting process. Such materials rather favour fermentation processes that result in ammonia and biogas emissions. Those emissions are responsible for the typical malodour of a bad compost. To get a proper composting process nitrogen rich materials such as kitchen wastes or grass clips need therefore to be bulked with a carbon rich bulk material such as leaves, saw-dust or newspapers.


Earthworm refers a group of worms that ar living in soil. Earthworms are related to essential soil processes such as airation, crumbling material and decomposition of organic material. Without earthworms many soils would not provide appropriate growing conditions for plants.
Earthworms are spread over the whole world and comprise on several hundred species. Lumbricus terrestis is the most common one and therefore also called the Common Earthworm. It usually lives in uppermost soil layers but has a high ability to adapt to environments with decaying organic material such as composts. It can therefore be used to support a composting process as an alternative to the compost worm.

Electronic wastes

Electronic wastes and old batteries contain a mix of different, partly high toxic materials. It need therefore to be treated at special treatment plants. Unfortunately get some our of the electronic wastes dumped to developing countries. Such kind of «recycling» as practiced by some electronic concerns will unfortunately not result in a proper reuse of the products. How should an african child use you old computer if there is no electricity available in the village. Most of the electronical «donations» from the industrial countries get therefore dumped into the environment and will threat the health and livelyhood of the local people. So-called «recycling» in developing countries is often cheeper than doing it correctly in Europe and unfortunately still practiced by several companies. It is therefore important to sensibilize pupils to be aware that the old computer or TV is getting correctly recycled in our part of the world rather than threatening the health of poor people somewhere in a developing country!

Electronic «donates» arriving in bags from Europe and Northern America. What should a child in a developing country do with this waste? Even if our old TV or computer is still working, there is often no electricity available to use it!

More information can be found in Schmidt Charles W. 2006

Household chemicals

A modern household is dealing with of chemicals, such as dishwasher, shower gel, cosmetics. Most of them cannot be decomposed by nature and some of them can even cause hazardous effects. It is therefore important that children learn to dispose such substances in the right way. Most critical are solvents, fossil fuels and motor oil. Such substances can cause serious damages already in low concentrations. But even less harmful products such as dishwashers can have hazardous impact on nature in higher concentrations and amounts. Old household chemicals should therefore be treated in special recycling stations or at least in a waste burning plant that is equipped with an appropriate filter system.
Such as the dishwasher is disturbing the composting process in a classroom compost box can household chemicals cause hazardous effects to nature.


Humus refers to decomposed organic matter. In other words it is the end product of a compost. Humus contains a lot of nutrient that are available to plants. Humus closes therefore the nutrient cycle between dead organic materials and growing of new biomass. Leaves that have fallen in the autumn get decomposed into humus that provides nutrients to the trees for shooting new leaves in the spring. Due to the high material turnover from the trees, forest soils usually contain a lot of humus.


In hydroponic systems, plants are growing on a medium different from soil (e.g. gravel, rockwool, expanded clay) and are usually fertilized using mineral nutrient solutions.

Plant fertilizer

Food for the plants is not just nitrogen: actually 20 mineral nutrients are necessary or beneficial for plant growth. Carbon (C), hydrogen (H), and oxygen (O) are supplied by air and water. There are six most important elements, also called macronutrients, because they are required by plants in large amounts: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Other elements are required in trace amounts (micronutrients). Essential trace elements include boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), sodium (Na), zinc (Zn), molybdenum (Mo) and nickel (Ni). Most of these minerals are present in fish excrements.


Polyculture is agriculture using multiple crops in the same space, in imitation of the diversity of natural ecosystems, and avoiding large stands of single crops, or monoculture. (Wikipedia)


The decomposing process is mostly anaerobic, exothermic process and the temperature inside the compost heap can reach up to 60 °C. The temperature is an important indicator for the stage and performance of the process. If it does not reach at least 40 °C, either the material did not contain enough nitrogen or it was too dry.