The current legislation in terms of digested sludge waste water treatment plants recommends its application as fertilizer. But, the presence of heavy metals may limit this application. This limitation is even more important in the case of industrial waste water treatment plants.
Another of the possible solutions is its stabilization and composting by mixing the sludge with other materials which have an elevate content in carbon until reaching a C/N acceptable ratio. Nevertheless, with this kind of mixing the heavy metals problem is not avoided.
The third possibility to eliminate the heavy metals is the energetic valorisation of the sludge. To make it possible, the sludge must be dry and must be assimilated as a solid waste derived fuel if its calorific power is more competitive than other fuels.
In this work, the energetic balance of digested sludge coming from 7 different waste water treatment plants of several productive sectors has been analysed. So, first of all, it has been necessary to know the sludge humidity to be able to calculate the energy used in the sludge drying process. Next, with an isoperibolic calorimeter the sludge calorific power is calculated and it is compared with another fuels. Finally the samples are calcinated and the content of ashes of every kind of sludge is determined.
With all the data exposed, on the one hand it is determined the elemental analysis to determine the content of essential nutrients in the soil (C, H, O, N and S) and on the other hand the content of heavy metals to accept or not the sludge as a fertilizer and/or to identify and analyze the pollutant power of its combustion gases.