Disposal of sludge from wastewater treatment plants in large cities is an environmental problem , however there are solutions.

In Copenhagen, a new sludge incineration plant has been put into operation in 2011 at the plant Lynetten owned by BIOFOS, jointly owned by 15 municipalities in the central part of the Copenhagen region.

As part of the new project, a fluid-bed incinerator has been installed together with a modern flue gas cleaning system. Furthermore a flue gas condensation step has been added. Thereby, with this project, the process of disposal of sludge has been turned from an energy consuming to an energy producing process. There is much more surplus heat available to the district heating system and there is a lot of biogas available. The main part of the high temperature heat from the boiler is first used for pre-drying of sludge and the rest of it plus the energy from the condensation, is used for the district heating in the Copenhagen Region.

But what to do with the surplus biogas? Detailed analysis showed that the best for the whole society would be to clean the biogas and use it directly to produce city gas, which today is produced by mixing natural gas with air.

Thus, the energy generated from the waste water is now used for heating and for cooking in thousands appartments and for process gas to industries mainly in the municpalities of Copenhagen and Frederiksberg.

Some key figures for the new sludge treatment plant in Copenhagen are:

• Capacity: 2,35 tons of Dry Solids per hour, (corresponding to sludge from 1.000.000 Person Equivalents)
• Boiler production: 3,8 MW, (used for drying the sludge and for district heating)
• Flue gas condensation : 2,8 MW, (used for district heating)
• Biogas production on digesters: 900 Nm³/h, (used for city gas production)
• The main part of the residues (fly ash) can be used for building material

Other well known sustainable solutions for the Greater Copenhagen are:

• 98% district heating,
• integrated regional district heating system
• 95% share of CHP in the district heating,
• several new district cooling systems,
• 20% of district heating from waste to energy,
• huge share of bikes in local transport
• off shore wind farms
• a new metro system

Over the recent 10 years Ramboll have been involved in a number of other sludge incineration projects of which can mentioned Skt. Petersburg South; Skt. Petersburg North; Hong Kong (in progress).