Sludge from wastewater treatment plants
Wastewater treatment plants (WWTP) produce considerable quantities of sludge, as :
Finally, for every 120 g of treated COD, or 1 person’s equivalent, 60 g of suspended solids in the form of sludge is being produced. At the same time, sewage treatment is a high energy consuming process.
In the past the produced sludge, rich of fertilizing elements, was used in the agriculture. Recently a certain number of undesirable compounds were found in the sludge (heavy metals, organic trace elements and residues of medicines), causing certain countries like Switzerland to prohibit sludge spreading in the agriculture. In these cases the sludge is being eliminated (incineration, wet oxidation, ..)
In countries where the use in agriculture of sludge from sewage plants is still allowed, the quality criteria are very strict.
Sludge digestion is a solution that has been used for decades to reduce the quantity of suspended solids in the sludge and to stabilise the remaining organic matter. Today, thanks to an improved control of the operational conditions (pre-thickening of the sludge, mixing of the digester, etc..) the digestion process can be optimised for energy production : up to 50% of the STPs consumption. On the other hand, the development of new technologies to enhance the sludge digestibility (thermolyse, ultra-sound, micro-crushing) allows even more the reduction of residual quantities and maximisation of the energy production (up to 30 – 100%).