Since 2006 there has been a 30% reduction in the amount of sludge generated in the treatment plants of the Community of Madrid, despite the general increase in population in the same period. This is explained by the improvements in the treatment and dehydration processes of WWTP sludge and by the economic crisis of recent years.

98% of sewage sludge is generated in the Canal de Isabel II WWTP and the Madrid City Council. The remaining sludge is generated in treatment plants in large urbanizations and in industries from various sectors (mainly agri-food).

After subjecting the wastewater to the corresponding purification treatments, a semi-solid residue is generated in the WWTP, mainly composed of a mixture of water, organic matter and various elements, some of interest as agricultural amendments (mainly Ca, P, N, Fe, K and vegetable micronutrients) and other potentially contaminants such as heavy metals (fundamentally Cd, Cr, Cu, Hg, Ni, Pb and Zn), pathogens and organic contaminants. In WWTPs, the sludge generated is subjected to various treatments in order to achieve its stabilization, thickening and sanitization. In this way, its subsequent handling and management is facilitated.

As a result of the analysis and characterization of these sludges, their management is determined:

• Almost all of the sludge generated can be considered as non-hazardous waste and, therefore, can be managed as such.
• Almost all of the sludge generated has a low content of heavy metals, which is why it is suitable to be managed by applying it to agricultural soils, either by direct application, or after subjecting the sludge to a prior process of thermal drying or composting. .
• To carry out the treatment of the sludge, the Canal de Isabel II has a composting plant, thermal drying with cogeneration and a landfill located in Loeches. The sludge with a higher content of heavy metals, and therefore not suitable for agricultural use, generated at the Canal de Isabel II facilities is preferably used for energy recovery in facilities not established in the Community of Madrid, after treatment in the drying plant Loeches thermal plant (commissioned in 2010), and occasionally deposited in a controlled landfill for non-hazardous waste.
• For its part, the Madrid City Council has two thermal drying and cogeneration plants. The treated sludge generated in the WWTP owned by the Madrid City Council has been entirely used for application in agriculture, either directly or having previously been subjected to thermal drying.
• The management of sludge generated by other producers outside the Canal de Isabel II and the Madrid City Council has been carried out through its application in agriculture.

Of the total sludge generated, more than 98% is being applied in agriculture (material recovery), so it is considered that the current management is in accordance with the objectives of the State Framework Plan for Waste Management (PEMAR) 2016-2022.

Problems for the future:

• The agricultural area available to receive sludge is reaching its maximum.
• There are periods in which the application of sludge cannot be carried out due to weather conditions (rainfall and inadequate temperatures) or due to agricultural conditions.
• The inadequate application of sludge in agriculture can cause problems due to bad odors and the risk of contamination of groundwater or soil masses.

This problem can be minimized with the proper operation, monitoring and control of the sludge drying facilities, the implementation of temporary storage and good agricultural practices in its application.