A "fatberg" made up of grease, fat, wet wipes and other garbage that slowly grew in a Baltimore sewer main is the culprit of the release of more than 1 million gallons of sewage into a Maryland stream last week.

The Baltimore public works department says the massive plug near Baltimore Penn Station caused the overflow into Jones Falls on Sept. 21 despite dry weather. 

Baltimore has a fatberg in its sewer systems – a congealed lump of fat, along with wet wipes and other items that do not break down in sewer systems.

Baltimore's fatberg appears modest compared to monsters dredged up in Britain. In London, Thames Water engineers this month began a three-week sewer war against a fatberg almost the length of three football fields and weighing 130 tons — more than 10 of the city's iconic, double-decker buses.

In Baltimore, authorities began to smell trouble when overflows of the sanitary sewer started to become common following heavy rains. Finally, a dry-weather overflow convinced the department to take a peek. They sent a machine with a closed-circuit TV camera into the sewer, and soon discovered the walls of the sewer pipe were caked with congealed fats, oils and grease. The lovely concoction is sufficiently common to have drawn an acronym — FOG.

While working in the pipe, a backup occurred, leading to another sewer overflow. Engineers estimate 85% of the pipe, which is 24 inches across and more than 100 years old, is blocked.

Most of the gunk has been scraped away, and any repairs or replacement of the sewer pipe will be determined once the fatberg has been entirely removed, the department said.

The overflow is diverted into the stormwater system and then into the Jones Falls. The overflow system will be closed once a related wastewater treatment project is completed in late 2020, the department said. The city’s sewers are being repaired and replaced to avoid infiltration of stormwater. 

The department has a FOG program that includes monitoring food service outlets that discharge wastewater. Officials urge everyone to can the grease, trash and even so-called "flushable" wipes.

Says the department: "The only items that should be considered flushable are poo, pee and toilet paper."