school energy&environment

Organic waste

What happens to a banana skin when we throw it away? If we want to try an experiment, and we leave it in a garden, we will notice that in a short period of time the skin will transform and disappear completely or almost completely, leaving a new organic substance in its place which is then absorbed by the ground. This happens because the banana is an organic waste and it is biodegradable like kitchen leftovers and garden cuttings, and therefore decomposes easily and is transformed by saprophytic bacteria. So can we also think of recovering organic waste? And if so, how? Organic waste is transformed by means of a biological treatment, composting, in order to recover the organic material that is present in this waste and to obtain a new material called compost. Compost is not a fertilizer, but is defined an organic amendment, because it adds an organic substance and nutrients to the soil (nitrogen, phosphorus and potassium), consequently leading to a decrease in the use of chemical fertilizers. The process consists of the decomposition of the organic substance by microorganisms, in aerobic conditions, i.e. in the presence of oxygen. The principal products obtained from the compost reaction are CO₂, water and heat. This is a natural phenomenon that is forced by insufflation of air and by periodically turning over the material, in order to accelerate the reaction. Compost production times vary depending on the material and the period of the year, indicatively from 2 to 6 months. Microorganisms are the main promoters of the process, and they are many and of different strains – bacteria, fungi, algae, protozoa, etc.) and usually they are naturally present in sufficient amounts in leftovers; however, so that they can carry out their function correctly, they must be in optimum conditions. Therefore, in the production of compost, it is important to pay attention to some parameters: oxygen, sufficient porosity of the material in order to guarantee circulation, humidity and the Carbon/Nitrogen ratio.
The starter materials, which must be used in compliance with the law, are: the organic fraction of MSW collected separately; plant waste from agricultural crops, sawdust, wood chips, wood fragments, zootechnical sewage, paper and cardboard (in small quantities), mud from civil sewage purifiers and discarded wood that has not been used and has not been treated.
Dangerous waste and materials which have undergone chemical treatments are strictly prohibited and, lastly, also inert substances that would hinder the degradation process. In fact it is very important that the compost does not contain polluting substances, heavy metals and pathogenic agents. During the composting process, the materials are suitably mixed in order to obtain an optimum C/N ratio.
For example, humid materials have a low C/N ratio while dry materials that act as structuring layers have a high ratio. Two main stages are identified in the composting process: the first phase, ACT (active composting time) is an accelerated bio-oxidation phase in which the waste is highly putrescible and the metabolic process is very rapid and there is a large consumption of oxygen, a maturing phase in which the metabolic process is slowed and the consumption of oxygen decreases, besides any refining process pre-treatment or post-treatment. Depending on the quality of the material, it is used in different ways: to fertilize the land (mixed with manure), mulching, as soil for covering waste dumps, etc. Composting can be carried out on a domestic scale with small volumes of individual humid waste collection plus other selected materials, or on an industrial scale where large volumes are used and all the physical and chemical parameters are suitably monitored in order to obtain a good quality compost that can be sold in the market. Domestic composting can easily be carried out in composting bins of various sizes (usually 30 or 60 l) which are sold in the market.