In 2023, 97 of Hamburg's 131 combined sewer overflows operated, releasing around 1,273,583 cubic meters of diluted wastewater into surface waters. In 2025, 49 operated and released around 559,739 cubic meters. HAMBURG WASSER's own environmental statements show that the city's wastewater system now swings with the weather far more than its drinking water supply does.
A sealed city meets heavier rain
Hamburg's environment authority calculated that 40% of the city area was sealed in 2021, up from 36% in 1999. The hydrological year from November 2023 to October 2024 brought almost 1,050 millimeters of rain at Hamburg-Fuhlsbüttel, about twice the roughly 550 millimeters of dry 2018. In the summer of 2024, four storms reached level 10 on the utility's 12-level heavy-rainfall index. On 27 June, 58 millimeters fell in 45 minutes.
The treatment plant feels it too
Volume at the Hamburg treatment plant rose to 183.4 million cubic meters in wet 2024, then fell by 26.9 million cubic meters in 2025. Between 2020 and 2024 the annual figure moved by 37.1 million cubic meters. Drinking water deliveries, by contrast, were 117.4 million cubic meters in 2025, against 114.4 million in 2016.
Pilots, not yet a program
HAMBURG WASSER and the environment authority run the RISA initiative to develop Hamburg as a sponge city, but the utility describes it as mainly pilot projects so far. The city's grant program covers 30% to 50% of eligible costs for cisterns, infiltration areas and unsealing on private land.
What the overflow figures do not show is how Hamburg Wasser's capital program, its treatment-plant projects and its water rights fit together, or which deadlines from 2027 to 2045 will set its investment. The full report, Water Systems Overview: Hamburg Wasser, is available for USD 499. For a free overview, read the Hamburg Wasser Utility Snapshot.