In 1981, Hannibal's new Wastewater Treatment Plant was put into operation. The facility is an aerobic plant which means that only oxygen (no chemicals) is used. Renovations to the existing plant were completed to reroute the Plant's outflow from Bear Creek to the Mississippi River and make other improvements.

The wastewater treatment process has many stages.  First the wastewater flows through the grit and dewatering area where it is screened or filtered to remove rags, sticks, and other large debris that would damage equipment or interfere with the treatment process.  Then the wastewater goes through a grit removal system that allows sand, gravel, and other particles to settle out, while organic matter remains suspended in the wastewater.  From there, the raw wastewater gets oxygen enriched and flows into the reactor/digester before proceeding to the primary clarifier where it comes into contact with activated sludge, which contains microorganisms that decompose the organic material.  Next the wastewater passes into secondary clarifiers where sludge settles to the bottom and the liquid continues to the Mississippi River.  Sludge is removed from the clarifier and sent to an aerobic digester where it is heated for several weeks to destroy any remaining pathogens.

The first sewers in Hannibal were constructed around the turn of the century and consisted of combined systems carrying both storm and sanitary waste. In 1968, a sewer bond was passed by the voters of Hannibal to separate the storm and sanitary waste by adding several thousand feet of new sewer lines and constructing lift stations to carry sewage to the Wastewater Treatment Plant.

Currently the processing average per day is 3 million gallons with a capacity of 6 million gallons per day. Preliminary studies are taking place to determine the improvement processes that will be phased in over the next several years through multiple projects.