Composting can be carried out in different ways:
• in large centralized facilities;
• in farms or agricultural or farming;
• in small units of the family (and in this case is called home composting).
In the case of composting in centralized facilities, we must distinguish clearly between two possible situations:
• the case where the waste is collected in the conventional way, all mixed together, then separated partly by mechanical and manual processes in the sorting stations before being sent to composting
• the case in which the selective collection at the door of each house of biodegradable waste that can be directly diverted to composting.

The first situation has the advantage of not to make any change in the system of collecting rubbish and consequently no need for investment by municipalities or associations of municipalities in the education of citizens for the separation of waste.
The disadvantages associated with this process are clear: after the waste has been mixed at home it’s very difficult to make proper separation, and it is even impossible to remove certain waste of small dimensions. Several scientific studies show that the compound produced in these situations is of poor quality and found to be contaminated, for example, by heavy metals in batteries, plastic, pieces of metal and glass.
Besides the problem of poor quality of compost produced, we must emphasize the costs of the proceedings and the problem of alienation of the population of the solving environmental problems.
The costs for public health are serious and so far, not made by the competent authorities.
In the second situation, since the waste is separated at source, no longer poses the problem of contamination, at least in a way so serious. But for the separation at source result, it is necessary to rethink the circuits of waste collection and invest in awareness campaigns and environmental education of the population.
Most municipalities still do not recognize the organic matter, the importance and priority that is needed.

On farms there is a big production of organic waste which if not treated properly can lead to serious pollution sources. The droppings, remains of plants and wastewater compounds with hay, straw, wood chips can be used as natural fertilizer in agriculture.




Sources: http://architecture.myninjaplease.com/wp-content/uploads/2007/10/composting.jpg
http://www.esb.ucp.pt/geo/myfiles/compostagem/compost/intro3.html