Utilization of waste water sludge for production of electricity is effective approach in many municipal wastewater treatment plants. Other methods are also used
The problem addressed
Every day approximatly 400 000 м3 waste water is treated and 1,7 mln tons of sludge had been produced for the last 30 years. This enormos ammount of sludge need to be treated as well and approprately utilized or dispozed. The best way to do that is to usethis rich on minerals and organic matter substance for fertilizer or utilization of the energy locked inside of the sludge and then used as fertilizer.
How does the practice reach its objectives and how it is implemented?
The good practice describes thecombined heat and power production (CHP) as well as the compost production facility by using the sewage sludge from the WWTP. WWTP is in charge of exploitation, maintenance and management of water supply, sewage systems and purification of waste water inMunicipality of Sofia. The plant disposes with technology line for mechanical and biological purification of the waste water and a technology line for sludge treatment, dewatering and energy production. The CHP installation has 3 co-generators producing1063 kWt electricity per co-generator and 1088 kWt heating power per co-generator. The daily capacity is 35.000 –39.000m3biogas. The sludge of the plant is used as fertilizer in agriculture.
Who are the main stakeholders and beneficiaries of the practice?
The Municipality of Sofia is contracting trough concession these activities to French company Veolia and more specifically local daughter company Sofijska voda

Resources needed

For its 33 year life, the Wastewater treatment plant Kubratovo has processed over 4 billion m3 of waste water. For the reconstruction and maintenance of the facility had been invested over 38 million leva (BGN) from Sofia municipality and another BGN 60 million under the ISPA program.

Evidence of success

The WWTP is fully energy independent. It contributes to cost efficiency and a greener city while protecting the environment by recycling waste water and reducing carbon emissions. In case the legislation develops further in the field of using fertilizers from sludge treatment, there could be much more incentives for their utilization in the agriculture. Social, ecological and environmental aspects of this project should be more exposed and become more visible to the population.

Difficulties encountered

It occurs that sometimes there are contaminations (substances) in the waste water, as the quality varies. This makes the purification process more difficult and challenge the production of fertilizer with high quality and level of safety.
A very close monitoring and control is applied.

Potential for learning or transfer

With good management and control of the polluters, the sludge could be very useful soil fertilizer. With the recent energy deficit, utilization of methane gas is very effective way. Methane gas is also very strong greenhouse gas and it should not be discharged in the atmosphere.
Main institution
Югозападен, Bulgaria (България)
Start Date
June 1984
End Date


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Good Practices being followed by

Demetrio Antonio Zema

Mediterranea University of Reggio Calabria