Wastewater treatment plants (WWTPs) in the EU produce more than 10 million tonnes per year of dry sludge.
Article 14 of the Urban Waste Water Directive promotes the reuse of sludge, as long as there are no adverse effects on the environment. In Spain, WWTPs generate about 1.2 million tonnes of sludge; up to 82.6% of this is used as fertiliser in agriculture, 7.9% is landfilled and 5.1% is incinerated.
The Water Framework Directive promotes the recovery of the costs of water-related services. The transportation and disposal of sewage sludge accounts for 13% of a WWTP's total operating costs. These costs can be reduced through better drying of the sludge. If sludge is dryer the odour nuisance is also greatly reduced.
Current commercial techniques can achieve sludge with a dryness of 15-18%. Techniques that could increase the dryness percentage consume large amounts of energy and are therefore currently not commercially viable. A low-energy alternative sludge drying beds takes too long and is not effective at reducing odours.
Energy recovery is the preferred alternative when dry sludge cannot be used in agriculture. Gasification is the most environmentally-friendly means of energy recovery, with lower carbon dioxide (CO2) emissions than incineration.
LIFE DRY4GAS proposes an innovative and highly replicable solution for the management and reuse of sewage sludge generated by wastewater treatment plants. It will develop a pilot plant to dry the sludge for use in energy generation, the construction industry and agriculture. The plant will incorporate a solar drying system and a thermochemical gasification process to dry the sludge and recover 120 MWh/year of energy by means of an organic Rankine cycle. The project will also evaluate the reuse of gasification ashes mixed with sewage sludge to make a better quality agricultural additive and study ways of reusing the ashes in the construction industry. The pilot plant will be built at an existing wastewater treatment plant in Torrepacheco, Murcia. This produces 2 200 tMH/year of sludge.
The project will contribute to the implementation of relevant EU policy and legislation: