A process of decomposition and stabilization of sludge under anaerobic conditions takes place in these tanks with the use of anaerobic microbes that develop in them. The anaerobic biological process in the tanks is comprised of two stages. In the first stage, the organic material decomposes into organic acids, while at the second stage the organic acids break down into gases – methane and carbon dioxide. All of the digester tanks at the plant are single-stage with mixers and heaters and are highly efficient. Numerous upgrades have been made over the years to the digester tanks.
In 2009, the development works for Digester Tank No. 5 were completed, in the framework of which the gas mixing system was replaced with an innovative pump-based mixing system. This pump discharges the sludge at a flow rate of 1000 m3/h through six spray nozzles at the bottom of the tank, which brings about more efficient mixing of the sludge in the tank. The pump can also grind rags in the sludge.
In 2012, an upgrade of Fermentation Tank No. 3 was performed, in the framework of which a new concrete roof was constructed, a new mixing system and new foam breaker system were installed and all of the piping in the tank was replaced. The upgraded digester will be operating by the end of the year.
Thorough emptying and cleaning of Digester Tank No. 7 was performed in 2014, constituting the first time it had been shut down since its initial operation in 2000. Piping works were executed in the tank and pneumatically-operated knife valves were installed enabling fast and efficient operation of the various systems. The roof of the tank was rehabilitated and sealed and a new pressure regulator system was installed in the tank supporting its safe operation.