Final Clarification

Four final clarifiers (105 ft. x 105 ft.) providing 7.25 hours detention at average daily flow, settle out the Activated Sludge and other heavy solids by gravity, while floatables are removed by skimming. A major portion of the activated sludge is returned to the plant flow upstream of the aeration basins. The activated sludge is returned to the plant flow in order to maintain a high concentration of bacteria and microorganisms within the process. The excess portion of activated sludge, known as waste activated sludge, is pumped to a point upstream of the primary clarifiers where it is co-mingled with the raw wastewater and settles with the primary clarifier sludge.

Effluent Disinfection

The treated wastewater is disinfected with a 15% chlorine bleach solution in the chlorine contact basins. After chlorination, a sodium bisulfite solution is added to neutralize any residual chlorine in the wastewater before it is discharged to Lake Michigan. The now clean water flows through a 60‑inch diameter concrete outfall pipe that extends 1,570 into Lake Michigan. Detention time in the contact basins is 1.75 hours at average daily flow.

 

Effulent disinfection is continuously monitored by two online chlorine residual analyzers. Bleach dosage is automated to maintain a steady residual setpoint. Sulfite and Phosphorus levels are also monitored and both systems have been automated. The plant has seen substantial chemical savings since automating chemical feed.