Thermal Hydrolysis of Waste Activated Sludge Only at the Psyttalia WWTP in Athens: Operation Feedbacks

AbstractSummary

In order to improve the energy footprint of the WWTP of Psyttalia in Athens, Greece (3,500,000 E.P), the application of Thermal Hydrolysis Process (THP) was preferred to the option of constructing additional anaerobic digestion tanks. Since August 2015 approximately half of the generated Waste Activated Sludge (WAS) has been treated by a Cambi Thermal Hydrolysis B6-4 system, in line with the contractual obligations and the intended design basis, while the thickened Primary Sludge (PS) stream is by-passing THP and is mixed with the hydrolysed WAS stream before entering the 4 digesters. The 4 other conventional digesters have treated the remaining sludge.
The dewaterability of the mixed digested sludge has significantly been improved from 20-22%DS before THP installation, up to 29-31%DS after THP installation. This is providing substantial reduction of energy use and cost savings at the digested sludge drying plant of Psyttalia. In addition, biogas generation and digester efficiency in terms of solids destruction has been increased.
Since the introduction of Thermally Hydrolysed sludge in the system, the resulting high ammonia concentration in the anaerobic digesters required the adaptation of the methanogenic pathway of digestion to the new conditions, in order for the digesters to become stable and robust. Further improvements were also necessary during the ramping up and commissioning stages in order to optimize the treatment of the viscous surplus sludge stream, so that the sludge treatment is now working properly.

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