Year Five of the Mucilage Crisis: Treatment Investments in the Marmara Basin Accelerate

From Crisis to Action Plan, From Action Plan to Investment

The mucilage layer that covered the Marmara Sea in 2021 became a turning point for Turkey's wastewater infrastructure debate. Five years later, the decisions taken under the Marmara Sea Action Plan have turned into a visible investment wave on the ground. According to data released by the Ministry of Environment, Urbanization and Climate Change, advanced biological treatment capacity in the basin has grown 62 percent since 2021.

The Marmara Basin in Numbers

The seven provinces in the basin (Istanbul, Kocaeli, Yalova, Bursa, Balıkesir, Tekirdağ, Çanakkale) host roughly a third of the country's population and account for a significant share of industrial production. Before the action plan, only 41 percent of the domestic and industrial wastewater discharged from the basin into the sea passed through advanced biological treatment. As of early 2026, this share has reached 67 percent. The target is to climb to 95 percent by the end of 2028.

Tightened Discharge Criteria

The new limits introduced in 2022 for plants discharging to Marmara reached their final stage at the end of 2025:

Total nitrogen: 10 mg/L
Total phosphorus: 1 mg/L
Chemical oxygen demand (COD): 100 mg/L
Suspended solids: 30 mg/L
Mandatory 24-hour composite sampling
Online monitoring station and data flow to the Ministry's central system

These criteria apply both to municipal wastewater plants and to joint treatment plants in organised industrial zones. Plants that fail to meet the limits face graduated administrative fines and operational restrictions.

Geographical Distribution of the Investment

While most of the investment in the basin is concentrated in Istanbul, other provinces have moved into a rapid catch-up phase. Major plants commissioned or under construction in the past three years:

Six advanced biological treatment projects on the Anatolian and European sides of Istanbul
Four OIZ joint treatment renewals around the Gulf of Kocaeli
Two new municipal plants in the Bursa Nilüfer basin
Three industrial basin plants on the Tekirdağ Çorlu-Çerkezköy line
Small-scale package treatment projects in Yalova and Çanakkale

Total investment volume exceeds 32 billion TL. Around 60 percent of this belongs to municipal plants, with 40 percent originating from industrial sources.

Technological Transformation

Much of the investment takes the form of upgrading existing plants to advanced treatment class. Leading technology choices in the sector:

Membrane bioreactor (MBR) systems
Anaerobic ammonium oxidation (Anammox) based nitrogen removal
Chemical phosphorus removal lines
UV and ozone disinfection stages
Energy recovery via biogas units

Capacity-based MBR installation volume has roughly tripled over the past three years. TUBITAK-supported domestic production projects continue, aimed at reducing import dependence in membrane equipment.

Financing and the Adequacy Debate

The most debated topic in the fifth year of the Action Plan is financing. Municipalities are asking for central support given the high operating costs of advanced treatment. Currently, around 6 billion TL of annual support is provided from the Ministry of Environment budget, but the total need across the seven provinces is reckoned to be twice that figure.

Will the Mucilage Return?

Despite improvements in discharge criteria, scientists stress that oxygen levels in the Marmara Sea remain in a critical band. The combination of rising temperatures, current dynamics and river inputs keeps the risk of mucilage reappearing on the agenda. Treatment investments are necessary but not sufficient on their own; an integrated strategy that includes basin management, fishing pressure and carbon-neutralisation targets is needed.

Bottom Line for the Sector

The transformation in the Marmara basin sets a reference point for similar basin-based action plans across Turkey. Similar programmes are expected to come online gradually along the Aegean and Mediterranean coasts from 2027. For the sector, this means a continuous and predictable demand base over the next five years.