Sludge is the thick, malodorous slurry left behind in a sewage treatment plant after human and industrial chemical wastes have been bio-chemically treated and the water discharged. With a booming global population (reaching over 7 billion in 20141) and with over 2.1 billion people living in cities by 20302, the volume of municipal sludge being produced worldwide is increasing exponentially: If all the sewage water from the world’s human population were collected and treated, in one year there would be enough dry sludge to cover an area equivalent to Singapore with a 10 mm thick layer.
Therefore, there is a clear need for solutions to treat this increasing amount of sludge. The European Directive 91/27/EEC obliges agglomerations of > 2,000 population equivalents4 to treat its wastewater and has improved the general quality and management of water across the member states. Nevertheless, it has also created challenges for the cost-effective management and disposal of the increasing sewage sludge, as traditional methods for sludge management, i.e. sea disposal, landfill, incineration and agriculture recycling, are running obsolete in the current legal framework, e.g. EU legislation banned sea disposal of sewage sludge in 1998; sludge deposits in landfills (accounting for 35% to 45% of the overall sludge volume) have been drastically restricted and will be phased out in the near future5; Incineration (the main sludge disposal strategy in Western European countries6) is highly contaminant as hazardous substances like dioxins, furans and heavy metals are released to the environment, and costly (due to the treatment of flue gases and ashes); and society is becoming increasingly concerned over the agricultural use of sludge as fertilizer for crops as it contains heavy metals, pathogens and other toxic substances that are transferred through the trophic chain
• Why is it important for society?
Valorisation of wastes is becoming a necessity for sewage WWTPs (i.e. those treating municipal sludge), backed up by the 2008/98/EC Waste Framework directive, and the transformation of waste/biomass into valuable materials and energy is emerging as a strong need. Valorisation of the sludge would transform the disposal costs, which currently are 30%-50% of sewage WWTPs operation costs into a source of income.
• What are the overall objectives?
SSOP is a profitable green technology that transforms 95% of the solids in sewage sludge to fuel (oil + gas + char) and reduces the disposed sludge volume down to 5%. In this way SSOP minimizes the environmental impact and disposal costs of sludge and valorises this residue transforming it into a fully renewable source of energy (bio-crude).
