Mankato's wastewater treatment plant uses physical, biological and chemical systems to treat wastewater.

How the wastewater process works

1. Wastewater is screened prior to pumping to the grit removal facility.
2. Primary clarifiers (tanks) allow gravity settling. Ferric chloride is added to reduce odors and decrease incoming phosphorus.
3. Flow from primary clarifiers is combined with return activated sludge in a splitter box and distributed to four aeration basins for biological treatment. These aeration basins are supplied with air for the active biomass to metabolize waste.
4. Ammonia removal is accomplished through extended aeration.
5. From the aeration basins, the flow is split between four secondary clarifiers for gravity separation.
6. Treated water from secondary clarifiers is sent to the water reclamation facility where a ballasted flocculation process includes the addition of ferric chloride, polymer and silica sand to remove  phosphorus.
7. A portion of the flow is sent through cloth media filters and chlorination for supply to a local industry and for internal plant processes.
8. Remainder of the flow is chlorinated with sodium hypochlorite and dechlorinated with sodium bisulfite and is discharged into the Minnesota River.
9. Solids removed from the system are thickened in a diffused air system to concentrate solids and then transferred to three primary anaerobic digesters where it is heated with methane gas, a process byproduct.
10. After the required detention time, sludge is transferred to a secondary digester for storage until pumped to two belt filter presses for dewatering.
11. Biosolids produced here are stored in a bunker and applied as fertilizer on nearby farmland in the fall. 

More information

• Mankato biosolids brochure
• Biosolids management program manual
• Biosolids program performance report
• National Biosolids Partnership audit report

For more information contact staff at 311 or 507-387-8600.