Wastewater

Wastewater is collected through the sanitary sewer system that consists of approximately 80 miles of sewer main. There are 19 lift stations throughout the system that pump the wastewater up to allow gravity to take over again and flow to the plant.

Once at the plant, the wastewater goes through pre-treatment that consists of a fine bar screen and grit removal system. After that, it flows to anaerobic selectors to allow for some biological phosphorus removal before flowing to the oxidation ditch. There, it is mixed with bacteria and air to allow the bacteria to break-down nutrients and some contaminants.

Once the wastewater flows out of the oxidation ditch it flows to the final clarifiers where the bacteria used for treatment form heavy solids that sink and filter the water. The clean water then flows out of the top of the clarifier to UV treatment before being discharged to the Mississippi River.

The solids collected at the bottom of the clarifier are then returned to the beginning of the process in the oxidation ditch or wasted into one of the four aerobic digesters. The digesters use air, time, and temperature to breakdown the bacteria and nutrients. After digestion is complete, the sludge is pumped to a centrifuge that turns the sludge into a solid, like black dirt. The black dirt, or biosolids, is then land applied for beneficial reuse as a nitrogen and phosphorus fertilizer on local farm fields that are approved sites.

There are 5 main steps to treating domestic and industrial wastewater.

• Preliminary treatment
• Primary treatment
• Secondary treatment
• Final clarification
• Solids processing

Preliminary Treatment

The majority of the wastewater flows to the main lift station. The main lift station pumps the water to the treatment facility. 

Primary Treatment

Primary treatment is the separation of liquids and solids. The system consists of a bar screen and aerated grit chamber to remove sand and other debris.

Secondary Treatment

Secondary treatment is an activated solids process used to remove suspended solids, dissolved solids, nutrients, and metals. The system is a continuous flow, biological process where microorganisms use the organic material in the wastewater as an energy source. During secondary treatment, organic matter is reduced to carbon dioxide and water.

Final Clarification

Similar to primary treatment, final clarification is where solids are allowed settle out. A portion of the solids are used for "seed" and recycled back to the activated solids process. The remaining solids are further processed. The liquid portion is disinfected using chlorine and then discharged to the Mississippi River.

Solids Processing

Solids are generated at 2 stages during wastewater treatment; primary treatment and secondary treatment.

The two solids are mixed together and dewatered. The liquid portion is returned to the beginning of the treatment process and the solids portion is sent to an aerobic digester.

During the digestion process, solids are mixed for a minimum of 15 days. The digestion process produces liquid, solids and carbon dioxide gas. The liquid is returned to the head of the treatment process, and the solids portion, called biosolids, are sent to a centrifuge. The centrifuge further dewater the solids before they are land applied.