The implementation of two interim “treatment stages” in the technological process of the plant started on the eve of the 40 th anniversary of the establishment of the Kubratovo Wastewater Treatment Plant (WWTP). Act 16 was issued for the facilities and their commissioning is to take place by early 2025, as scheduled.

Thus, the treatment process of the sewage sludge will be improved, as well as the quality of the treated wastewater, in conformity with the recent environmental standards of the EU. Thanks to the two new facilities, more biogas will be produced from the sludge, and the amount of the residual product will be drastically reduced at the end of the process. The treatment of the water extract, released in sludge dewatering, will be significantly improved. That will also facilitate the next stage in the treatment of wastewater before its discharge into nature.
The first facility introduces the thermal hydrolysis technology. In the standard treatment process, the sludge is thickened before entering the digesters where biogas is released. An interim stage before the digesters is introduced with the newly constructed capacity, where the microorganisms in the sludge decompose under high temperature and pressure.

That brings several benefits. First, the volume of sludge at the end of the treatment process is reduced, a residual product in practice. Until now, the ratio of sludge converted into biogas and residual sludge has been 40/60, now the new technology has optimized it to 60/40 in favor of the sludge converted into biogas. Moreover, the need for a 6-month sludge storage will be eliminated, as it will be suitable for use in agriculture right after the process of its digestion and stabilization. All that will also lead to optimization of its utilization.

The entire process will be energy independent in view of the used green energy generated from the increased amount of biogas extracted from the sludge. Thus, the energy balance of the plant and Sofiyska Voda’s ambition to become entirely energy neutral by 2025 are maintained. The second new facility treats the liquid released in the sludge dewatering process. Until now, the liquid has flowed directly into the aeration tanks for the next treatment stage. However, the nitrogen content of the liquid entering the aeration tanks was high. According to the latest environmental requirements of the EU, the nitrogen in the treated wastewater has to be reduced, and the newly constructed anammox plant will help for that. Thanks to it, the amount of nitrogen in the water from the dewatered sludge has to decrease by 90%, thus making the treated water discharged into nature fully complaint with the latest European requirements.
With the implementation of the two new technologies, the WSS operator in Sofia is again a pioneer in the technological development of the sector. The facilities are something new not only for Bulgaria, but they are also rare for the network of the concessionaire of Sofiyska Voda – Veolia, operating in 58 countries around the world.

The construction of a thermal hydrolysis facility and an anammox plant at the Kubratovo WWTP is implemented under a project of the Municipality of Sofia under the OP Environment. It is part of the Sludge Management Strategy of the Municipality of Sofia, developed in partnership with Sofiyska Voda, part of Veolia.