**4. Conclusions**

MSW is a worldwide problem, even if we are able to reduce the amount produced every year. However, developing countries are increasing the MSW production parallel to the effort of developed countries to reduce their production. In the EU countries, there is no harmonization of the treatments applied in each country, but all of them are promoting the reuse based on the circular economy. In the case of MSW, the composting process of the important organic fraction is one of the best strategies to improve soil properties, reduce the landfill disposal of this waste, and recycle the nutrients. However, two important environmental risks should be considered even if MSW is well pretreated and composted. The excess of nutrients can contaminate waters and the presence of trace elements can has a negative effect on the food chain.

 The risks can be minimized if pre- and posttreatments on MSW are applied and if the composted matter is added in a soil under adequate environmental conditions. This is a key factor to improve circular economy and ensure the use of these wastes. Nitrogen and other nutrients, especially micronutrients, are presented in MSW and can be bioavailable for plants. Nevertheless, in the same way, pollutants with emphasis in trace elements increase their availability.

The criteria for the recycling of MSW and compost-like output (CLO) derived as amendment should consider the end use but the previous treatments of MSW and have to look for a balance between the input of nutrients and the environmental risks associated with the soil conditions.
