Nassau County wants to stop flushing money down the toilet.

A proposed tax will charge for sewer usage by public schools, fire departments, libraries and colleges — and is expected to collect $38 million a year.

The one-cent-a-gallon flushing fee would cost about $3,600 a year for a small library and a staggering $1.2 million for a large institution like Hofstra University, sources said.

Nassau County Executive Ed Mangano’s spokesman, Michael Martino, said the fee was needed because for eight years the prior Democratic administration “failed to reform our county’s finances, thus leaving behind the largest deficit in history.”