Wastewater Treatment Process
The sewage flows through the treatment processes in the following order:
Raw Wastewater Pumps – Influent wastewater enters the treatment plant through a 60”gravity interceptor sewer. Three 84” spiral screw pumps raise the sewage into the preliminary treatment building. The pumps are based on Archimedes principle. The screw turns at a constant rate and pump output is based on the amount of submergence at the bottom of the screw. Each pump has a rated capacity of 16.8 million gallons per day (MGD). A grease pump runs continuously to provide lubrication for the submerged bottom bearing. At peak flows, two screw pumps run with the final pump to serve as a backup.
Step Screens – Step screens remove debris that can cause clogging to pumps & piping. The step screens remove all debris larger than 1/8” and deposit it to a washer compactor that washes organic material out and squeezes the water out of the collected debris. The water is dropped into the flow for treatment and the screenings are disposed of at a sanitary landfill.
Grit Removal - Grit is removed by two Pista Grit systems, which create a vortex to remove heavy inorganic material, such as sand, silt and egg shells. This material is not organic and is removed because it is not biologically treatable. Removing grit helps minimize abrasion and wear on pumps and equipment further down the treatment line. This material is taken to the landfill.
Primary Clarifier Tanks - Wastewater flows from the preliminary treatment building into four primary clarifiers, three rectangular and one circular. The velocity slows down in these tanks and allows solids to fall to the bottom of the clarifier. This settled material, called sludge, is scraped into a hopper and pumped to the anaerobic digester. The scrapers also skim the surface to remove scum, grease and other floating material, which is also pumped to the digesters. The liquid from the primary clarifiers passes over weirs to the secondary treatment process. Manitowoc uses a trickling filter process consisting of two stack filters and two rock filters in series.
Stack Filters - Two high-rate filters with synthetic media in a 20-foot deep honeycomb pattern provide a large surface area for the growth of aerobic bacteria. Three pumps are used to pump the primary effluent from the wet well to the center column and rotary distributors. Aerobic bacteria grow on the wetted media and consume the BOD as the water trickles down through the filter. Four large fans supply air to the stack filters to ensure optimal conditions for the bacteria. The stack filters were added in 1976 because the rock filters alone could not meet the future BOD loadings to the treatment plant.
Rock Filters – The rock filters were built in 1954, are 8 feet deep, and continue in service to provide polishing for the remaining BOD and also remove ammonia during the summer months. Instead of plastic media, rock provides the substrate for the bacteria to adhere to. Over time, the bacteria grow on top of one another and eventually smother out the lowest layer of biomass. The biomass dies off and separates from the rocks and flows to the final clarifiers where it settles out by gravity and is pumped to the anaerobic digester.
Phosphorus removal – Ferric chloride is a chemical coagulant that is added prior to the final clarifiers. It bonds with the phosphorus compounds and creates a floc allowing the phosphorus to settle and be removed along with the sludge in the final clarifiers. The chemical is precisely dosed based on the output from an online orthophosphate analyzer that samples every 15 minutes and adjusts the chemical pump output if needed.
Final Clarifier Tanks - Four tanks receive the effluent from the trickling filters. This effluent contains solids made by the action of the microorganisms. These solids settle to the bottom of the tanks forming a sludge that is removed by a rotating arm collector mechanism and are pumped into the primary digester. Phosphorus compounds that have been precipitated with chemicals also settle in this tank.
Tertiary Filters - Four tertiary sand filters remove the fine solids, which do not settle out in the final clarifiers. The media used is manufactured to a uniform particle size and density so it doesn’t wash through the filter or is lost during the backwash sequence. When the filters begin to clog, they are backwashed using effluent water which is isolated and pumped back to the head of the plant for treatment. The filters removal of suspended solids also lowers the effluent BOD and a small amount of phosphorus held within the solids.
Disinfection – Chlorine is added to the tertiary filter effluent to meet the disinfection permit requirement. The chlorine contact chamber provides the contact time necessary to reduce the pathogenic organisms. Excess chlorine may have deleterious effects in the receiving water. To prevent this, sodium bisulfite is added at the end of the chlorine contact tank to remove any excess chlorine prior to release back into Lake Michigan.
Anaerobic Digesters – The sludge from the primary and secondary clarifiers is pumped to the anaerobic digesters. The digesters are heated and held at a constant 98 degree Fahrenheit which selects and allows the anaerobic methane and acid forming bacteria to thrive. These bacteria decompose the sludge into stabilized humus like material called biosolids. In the process, they produce methane gas which is fired in large boilers and used to heat the digesters and the rest of the plant buildings. The biosolids are then trucked off site and held in clay lined lagoons until they can be land applied.
Land Application – The lagoons are large enough to hold a full year’s worth of sludge and are emptied in fall and injected into the soil. The solids handling process is complete when the sludge is applied at the agronomic rate indicated by the soil analysis testing and the future crop to be grown. Site requirements do not allow the sludge to be spread on slopes or if the land is too sandy or has too much clay. Set back requirements eliminate spreading near homes, schools, lakes and streams, and other environmentally sensitive areas.