Environmental Science & Engineering - September 2004

Athens treatment facilities provided offshore engineering challenges

By Alexander Farnsworth


The Athens Olympiad has been hailed as one of the most spectacular in the history of the Olympics with the stadium providing an elegant site for the athletic performances. While the world was enthralled by both the opening and closing ceremonies, a major engineering event - a wastewater treatment plant - played a vital but unheralded role in the Games.

The plant is situated on the island of Psyttalia, 2.2 kilometres south of the Athens suburb of Keratsini, in the Saronic Gulf.

“The island location was chosen because Athens, a city of almost 4 million people, is so densely populated that there was simply no room on the mainland,” says Dimitris Adraktas, project manager at Psyttalia B Consultants, a joint venture between a German and three Greek consultants. Adraktas is managing the plant’s upgrade on behalf of the Greek Ministry of Public Works.

The Psyttalia plant treats the lion’s share of Athens’wastewater, or about 1 million cubic metres of wastewater per day, about 12 cubic metres per second. Most of the waste is gravity fed to a huge pump station on the shore from where nine gigantic Archimedes’ screw pumps pump the waste to the island. As the Olympics returned to their origins, it was appropriate that modern pumps were installed in the land where the concept was dreamed up by Archimedes.

Before 1994, when the first phase of the Psyttalia plant was commissioned, most of Athens’ sewage and industrial effluent was released out to sea without any treatment.

After 1994, the plant provided primary treatment including screening, grit removal, primary sedimentation, anaerobic digestion, and mechanical dewatering for the sludge. The effluent was then discharged 2,000 metres out to sea at a depth of 64 metres.

The substantial upgrade was necessary in order to comply with stricter effluent limits for nitrogen in accordance with the European Union (EU) Urban Wastewater Treatment Directive. Prior to the EU-sponsored upgrade, which cost 200 million euros, the Saronic Gulf off the coast of Athens was designated as “sensitive” by the EU.

“Upgrading the Psyttalia wastewater treatment plant was used as an argument by Greece in its Olympic bid,” says John Margiolos, city project manager responsible for the final design and erection of the upgrade. “While officially we were not an Olympic project (the upgrade was planned before our Olympic bid), it was clear that we had to be ready for the Olympics,” he says.

So the upgrade included biological treatment of the waste to remove carbon, nitrogen and some phosphorus, the organic compounds that make wastewater detrimental to the environment. But this was easier said than done.

Psyttalia is a 57-hectare island made up primarily of limestone and clay. In ancient Greek, psytt-allos means literally “spit out from the sea.” As a proof of this, during extensive civil works on the island to make room for the biological treatment of Athens’ wastewater, many marine fossils were found in the limestone.

The island was also involved in vicious sea battles in the Greek and Persian War in 480 BC. The Persians were defeated, and the empire was prevented from expanding westwards.

In the mid 20th century, Psyttalia was Athens’ Alcatraz, a prison island for the Greek Navy. As a result, Psyttalia has some historical significance, most of which is preserved. A small archeological site remains untouched, as well as some graves.

The rest of the island, however, has been totally re-engineered since 1990 to accommodate sewage from the bustling metropolis of Athens.

First of all, a small bay on the north side of the island was filled in with 2 million cubic metres of excavated material. A total of 4 million cubic metres of earth was excavated to make room for the island’s biological treatment. This involved building large 9.4- metre-high bioreactor/aeration tanks on the island.

When the upgraded Psyttalia wastewater treatment plant is fully operational 800 tonnes of dewatered sludge a day (with 28 percent dryness) will be produced. This must then be shipped back to the mainland for disposal.

But depositing the sludge in landfill, as was done in the past, is becoming less of an option because of increased pressure from the EU wastewater and waste directives.

“As far as final disposal options, the most promising solution is to thermally dry the sludge to 90 percent dryness and to use the dry granules as a fuel in the cement industry,” says Adraktas. “This is our best option. The Greek government has applied for an additional 40 million euros in EU funding for a sludge-drying plant.”

Olympic upgrade
ITT Flygt played a vital role in upgrading the Psyttalia wastewater treatment plant in Athens to accommodate the Olympic Games.

The Psyttalia installation of Flygt pumps and mixers was the biggest order ever for ITT Flygt Hellas, the Greek subsidiary.

A large number of pumps are used to transport the wastewater to the island and throughout the different treatment processes. Seven submersible PL 7101 Flygt propeller pumps are used to pump the wastewater into the bioreactors.

Throughout the bioreactor tanks 36PP 4670 and 13 PL 7081 pumps are used for recirculation. Forty-eight SR 4410 Banana mixers are used to mix the bioreactor zones. An additional 24 SR 4650 mixers are used for degassing at the bioreactor outlet.


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