Alton, Illinois sold its wastewater system to Illinois American and used the entire $53.8 million to chip away at its $134 million in pension liabilities. (Photo: Shutterstock)

(Bloomberg) — As cities across the U.S. struggle under the weight of ballooning pension costs, some are putting their public water and sewer systems on the auction block to come up with cash.

Municipalities took in more than a half a billion dollars over the past five years by selling their water and waste utilities to private companies, according to data compiled by Bloomberg. Such sales are increasing in places such as Illinois, where the state alone owes $137 billion to its pension funds and dozens of cities have saved far less than they need to cover all the benefits promised to police, firefighters and other public employees.

In the last decade or so, selling off municipal assets had become a tool of last-resort used by governments facing dire financial strain. Arizona famously sold its capitol in 2010 as it struggled to fill an historic $3 billion budget deficit. But water and sewer utilities offer municipalities a chance to come up with much-needed cash and relief from maintenance costs at a time when states and localities have a combined $4.2 trillion in unfunded pension liabilities.