Thermal hydrolysis of waste activated sludge only at the Psyttalia WWTP in Athens: operation feedbacks

Zikakis D 1, Chauzy J. 2, Droubogianni I.1 and Georgakopoulos A.1, 1 Aktor S.A., (Greece) 2 Cambi AS (Norway)


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

The wastewater treatment plant of Psyttalia, is the final recipient of domestic sewage and pre-treated
industrial wastewater of the greater Athens area in Greece, receiving an average wastewater flow of
730.000 m3/d, with a biological treatment capacity of 3,500,000 Equivalent Population (Figures 1 and
2). The Plant’s wastewater treatment line consists of pre-treatment facilities, primary sedimentation and
biological treatment. Secondary treatment is facilitated by 12 activated sludge tanks (bioreactors) with
pre-anoxic denitrification for nitrogen removal. The total volume of the activate sludge bioreactor tanks
is approximately 300.000 m3. Secondary clarification takes place in 64 rectangular final settling tanks
(type Gould II) with an approximate effective surface of 52.000 m2.

The Psyttalia WWTP was constructed in separate implementation Phases. The First (A) Phase, which
was finalised in 1994, facilitated the primary treatment of the incoming wastewater loads and the
stabilization of the produced Primary Sludge via four mesophilic Anaerobic Digesters (10.000m3 each).
The Plant was upgraded in 2004 with the construction of the Second (B) Phase treatment facilities
which incorporated additional treatment stages in order to comply with the EU Directive 91/271. Phase B included secondary treatment, with the installation of the biological treatment stage for the wastewater
line (primary effluent lift pumping station, bioreactors, final settling tanks, process water disinfection
etc), and mechanical thickening for the Waste Activated Sludge as well as four additional digesters
(10.000m3 each) for the stabilization of the resulting quantities of Primary Sludge (PS) and Waste
Activated Sludge (WAS) and a new dewatering installation for the sludge line. Additional expansion
works took place in 2007 and 2008 that include the installation of a Thermal Drying Plant for the
Dewatered sludge (nominal capacity 8.625kh H2O evap/h, for each line with four lines in total) and the
construction of additional cogeneration facilities with Biogas and one with Natural Gas.
Figure 1: Location of the Psyttalia WWTP Figure 2: Aerial view of Psyttalia Island with
wastewater treatment work prior installation of THP.

Due to that staged construction, the Plant consists of two parallel sludge treatment lines with mesophilic
anaerobic digesters. Each line has four 10,000 m³ digesters that originally treated a mixture of thickened
Primary Sludge (PS) and thickened Waste Activated Sludge (WAS). The PS passes through strain
press fine screens and is thickened in picket fence gravity thickeners whereas WAS is thickened by
gravity belt thickeners. Digested sludge is dewatered by decanter centrifuges and is finally dried at four
Thermal Rotary Drum Drying lines to a concentration of >92% DS. The generated Biogas from the
digestion process is used primarily as fuel (powering) for the sludge drying process and the remaining
biogas is used at the cogeneration plants to produce electricity.

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