Don't throw anything down drains or into canals.
tom-BEE-ni — stress the second syllable; 'tombino' is a manhole cover or street drain.
Use this as a warning or instruction, especially near water (canals in Venice, rivers in Florence) or when explaining environmental rules to children or visitors unfamiliar with Italian infrastructure.
Tombini are street drains connected to the sewage or rainwater system. Throwing waste into them — especially wet wipes, cigarette butts, food scraps, or liquids — pollutes waterways, blocks drainage systems, and causes flooding. It is an administrative offence with fines.
Le salviette umide non si buttano nel water.
Wet wipes must not be flushed down the toilet.
A related issue; flushing wipes causes sewer blockages across Italy.
Le cicche di sigaretta sono rifiuti: usate il posacenere.
Cigarette butts are waste: use the ashtray.
One of Italy's most common forms of littering; cicca is the colloquial word.
Non gettare rifiuti nei corsi d'acqua.
Do not throw waste into waterways.
Formal register; corsi d'acqua means streams, rivers, canals.
Venice faces a particularly acute problem: anything thrown into the canals or street drains ultimately enters the lagoon. The city has installed CCTV cameras near canal bridges and uses drone patrols to catch and fine people who dump waste into the water — fines start at €500.