Sofia wastewater plant expands biogas production unit

Sofiyska voda presented its new digester at the wastewater treatment plant in Sofia. The tank, which cost EUR 3.1 million, increased the biogas production capacity by 25%.

The wastewater treatment facility near Kubratovo in Bulgaria’s capital city added a digester of 7,000 cubic meters for sludge fermentation. It is operated by Veolia’s subsidiary Sofiyska voda, which produces drinking water and manages wastewater for 1.4 million people in Sofia.

The French company said the plant is the second in terms of energy efficiency among its 2,500 units. The construction of the digester, a tank for the extraction of biogas from waste sludge, cost EUR 3.1 million and it will increase the capacity of green energy production by 25%, according to the announcement.

Contribution to circular economy

The firm already has four digesters, built in 1984, OFFNews.bg reported, adding the new tank would enable the company to upgrade the old ones. The methane from the sludge fuels a cogeneration system, which produces 24 GW of electricity per year.

Sofiyska voda operates four drinking water and two wastewater treatment plants as well as three combined heat and power units. It said it plans to build another cogeneration unit.

After the sludge is stabilized in the digester, it is dewatered and donated to farms for fertilizing certain types of crops, the firm said and added the material is rich in phosphorus and nitrogen. It sends the purified water to the Iskar river, completing the circular economy cycle.

More than self-sufficient

Sofiyska voda stressed its biogas and cogeneration system protected it from the recent spike in electricity prices as the output is 10% higher than the consumption of the wastewater treatment plant. The firm added it aims to become fully energy independent. The Kubratovo facility processes more than 350,000 cubic meters of wastewater a day.

The Sofia Municipality owns 22.9% of the Sofiyska voda. The utility asked the citizens not to throw wet wipes into the sewer, explaining that the material causes congestion in wastewater treatment plants.

A similar wastewater treatment plant with a biogas cogeneration system was recently built in Kruševac in neighboring Serbia.