Clogged drains.

Via Dan Savage at the SLOG:

"To explain this letter," writes a "Savage Love" reader/tipster, "a greywater recycling system is where you take shower and wash basin water, treat it, and use the treated water to flush your toilet. Recently a university that uses one of these systems ran into some problems. Filters clean the water before it is recycled and, well, university students were doing what university students do and the filters were getting clogged."

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