Public administration bodies, or organisations that manage treatment plants on their behalf, must manage the sludge resulting from the process. Sewage sludge is a complex matrix containing heavy metals, thousands of organic pollutants and microbial pathogens, and it also contains phosphorus, one of three elements critical to plant growth (along with nitrogen and potassium). Phosphorus is usually sourced from phosphate rock, which is a non-renewable resource and on the list of 20 raw materials that are considered critical by the European Commission.
Phosphorous contained in sewage sludge can be recovered through a treatment process that ends in a mono-incineration phase.
The whole treatment process begins by de-watering the sludge, preferably by chamber filter presses achieving a dry matter content of 25–35 per cent. The organic compounds and pollutants of the partially de-watered sludge are then completely oxidised in a mono-incineration plant, meeting best available techniques according to the Industrial Emissions Directive. The incineration process generates steam and electricity and no additional fuel is required in this phase, except for the combustion start-up. Finally, the phosphorus contained in the ash from the mono-incineration process can be recovered through different techniques.
This last step makes this process preferred over the co-incineration of sewage sludge in power plants, cement plants or solid municipal waste incineration plants, because co-incineration does not allow phosphorus to be recovered.