Sofiyska Voda commissioned a new digester at the wastewater treatment plant

The facility near Kubratovo is the second in terms of energy efficiency among more than 2,500 similar plants operated by Veolia, the leader in environmental services

In 2021 Sofiyska Voda, operated by Veolia, completed the construction and commissioned a new digester, thus increasing by 25% the capacity for green energy production in the treatment of the capital’s wastewater. The capacity of the digester is 7,000 m3 (as much as 3 swimming pools) and it is designed in line with the most modern technologies. Reliable sludge fermentation is guaranteed with that addition to the technological process, and higher volumes of biogas are expected, respectively. The processed sludge will have better parameters – less odor and organic matter, as well as more compact in size.


The main function of the plant near Kubratovo is to treat the wastewater of the city with a population of more than one million and return the water to the Iskar River with qualities identical to those of the pure river water. Along with that process, electric and heat energy is produced through stabilization of the sludge derived from the inflowing water. That happens through a long continuous process which includes all facilities of the plant located on 6,000 acres.



The sludge separated from the water through naturally-occurring processes, though speeded up by the plant facilities, reaches the digesters where the digestion takes place. During that process biogas – methane is released. The methane, which if released into the atmosphere causes a greenhouse effect, here is converted into electricity and heat energy through co-generation. After the sludge is stabilized in the digester, it is dewatered and consequently donated to farms for fertilizing certain types of crops. The obtained fertilizer is rich in phosphorus and nitrogen and is favorable for their raising. The result is a completely closed green energy loop with no waste.
Owing to its 5 digesters, Sofiyska Voda manages to produce even more green energy than necessary for the plant’s operation. This allows it to be energy independent and contributes to the company’s stability in times when the prices of energy resources are growing.

The digester is part of the investment program of Sofiyska Voda, operated by Veolia, from the past regulatory period 2017 – 2021. The company invested 6 M BGN, thus turning it into one of the first examples of green energy transition and circular economy in Bulgaria.

In March 2020, the plant generated record high amounts of energy in its 12-year history since the commissioning of the cogeneration process on its territory. Kubratovo WWTP produced 2,275 megawatt hours of electricity. With the operation of the new digester, the generated electricity is expected to be even more.



At the same time while the plant produces green energy for the needs of our treatment plant, more than 61 thousand tons of СО2 emissions are saved annually. In 2014 Sofiyska Voda received the first Gold Standard certificate for this project, in 2021 it is renewed with a focus on the contribution to the achievement of the UN Sustainable Development Goals.

With the implementation of this closed-loop production cycle, in the past year the wastewater treatment plant in Sofia was at the top of world efficiency rankings and is a convincing real-life example of a circular economy.

The WWTP in Kubratovo near Sofia processes more than 350,000 m3 of wastewater a day and more than 130 million m3 a year. Thus, for over 30 years of existence the plant has processed in total more than 4 billion m3 of wastewater, or nearly 8 full volumes of the main water source of Sofia – the Iskar dam.