Athens Water Supply and Sewerage Company EYDAP S.A. has launched a €9.31 million engineering-study tender that will define the next phase of the Psyttalia Wastewater Treatment Plant’s modernisation.
The contract, formally titled Study of New Works for the Optimal Utilisation of the Psyttalia Wastewater Treatment Plant — Psyttalia Phase 3, has an estimated duration of 35 months and will establish the technical framework for the plant’s future development and upgrade. Bids are due electronically by 10:00 on 16 October 2026.
The tender covers the study and planning stage rather than construction itself. Its purpose is to assess Psyttalia’s existing infrastructure, identify the works required over the coming decades and prepare selected interventions for subsequent procurement.
Psyttalia is the principal wastewater treatment facility serving the wider Athens basin and one of the largest plants of its kind in Europe.
EYDAP’s current tender documents put its design capacity at 5.6 million population equivalents, with an average daily inflow of around 680,000 cubic metres of wastewater. The plant currently removes approximately 93% of organic load and around 80% of nitrogen.
The facility has played a central role in Athens’ sewerage system and in the environmental recovery and protection of the Saronic Gulf. Parts of its infrastructure, however, were originally designed in the 1980s and 1990s, making a new investment cycle necessary as environmental and operational requirements become more demanding.
A key driver of the programme is the revised EU Urban Wastewater Treatment Directive, Directive (EU) 2024/3019.
The new framework introduces stricter requirements for nutrient removal, advanced treatment of micropollutants, wastewater reuse and energy performance. At EU level, it also establishes a pathway towards energy neutrality for the wastewater treatment sector by 2045.
EYDAP’s technical brief says the Psyttalia study will determine what investments are required to comply with the new rules, including possible additional removal of nitrogen and phosphorus, quaternary treatment for micropollutants, lower greenhouse-gas emissions and greater reuse of treated water and sewage sludge.
This makes the programme more than a conventional refurbishment of ageing equipment. The study is intended to establish how Psyttalia can move towards a more resource-efficient operating model while maintaining the reliability of a facility that provides critical environmental infrastructure for metropolitan Athens.
Circular-economy technologies form an important part of the engineering brief.
Among the options to be assessed are advanced treatment systems that could produce higher-quality reclaimed water for industrial use and irrigation, as well as technologies for recovering or producing resources such as biomethane, hydrogen, phosphorus and nitrogen.
The tender documents also call for an assessment of alternative approaches to sewage-sludge treatment and for measures to increase energy production while reducing consumption. Around 35% of Psyttalia’s current energy needs are generated on site, primarily through cogeneration using biogas from sludge treatment.
These technologies remain options to be evaluated, not approved construction projects. The winning consultant will compare their technical and economic performance before EYDAP decides which interventions should move forward.
The scope of the tender is deliberately broad.
It includes engineering, mechanical and electrical studies, civil works, environmental studies, geological and geophysical investigations, topographical services, energy-efficiency analysis and the design of technical installations. The open procedure is intended to attract engineering groups with experience in large wastewater treatment facilities and complex environmental infrastructure.
Price will not be the dominant factor in selecting the consultant.
Under the tender documents, the technical offer carries 80% of the overall evaluation score, compared with 20% for the financial offer. EYDAP will assess bidders’ understanding of the project, proposed methodology, timetable and the qualifications and experience of their specialist teams.
The weighting underlines the technical complexity of an assignment that must combine ageing infrastructure, tighter environmental regulation, energy performance and future investment planning within a plant that must remain operational.
The €9.31 million contract marks the beginning of Psyttalia’s next investment cycle rather than the construction phase itself.
The consultant will first assess the existing plant and compare alternative solutions. The preferred interventions will then move into engineering and environmental design, followed by preparation for future procurement.
The final construction scope — and therefore the overall capital cost of the next Psyttalia programme — has not yet been fixed by this tender.
What the M-217 study will determine is the roadmap: which projects EYDAP should pursue, which technologies offer the best technical and economic case, and how one of Europe’s largest wastewater treatment plants should adapt to the environmental requirements of the coming decades.
