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3.1.4 Utilization of Sewage Sludge

The example describes combined heat and power (CHP) production as well as compost production facility using the sewage sludge of the WWTP, who is in charge of the exploitation, maintenance and management of the water supply, sewerage system and purification of the waste water of the municipality of Sofia.

The plant uses a technology for mechanical and biological purification of the waste water and a technology for sludge treatment that ensures dewatering and energy production. The CHP installation has 3 co-generators producing: 1063 kWt electricity per co-generator and 1088 KWT heating power per co-generator. The daily capacity is 35 000 – 39 000m3 biogas. The sludge of the plant is used as fertilizer in agriculture.

It occurs that sometimes there are contaminations (substances) in the waste water (as its quality varies) that make the purification process more difficult and challenge the production of fertilizer with high quality and high level of safety. A very close monitoring and control system is exerted giving input on the sludge treatment process as a whole to prevent dangerous substances being present in the produced fertilizer. A special legislation regulates all processes of using sludge as fertilizers.

The WWTP is fully satisfying its own needs and provides energy independency by the plant. The graph below shows the tendency in energy consumption and energy independence. The produced fertilizer is used for re-cultivation of areas around Sofia and its neighborhood.


Tracking the quality of the sewage sludge at every stage; Energy independency of the plant contributes to cost efficiency and a greener city while protecting the environment by recycling smartly 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 population29.