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205 South Lincoln Ave.
Beaver Dam, WI 53916
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    (920) 887-4600

Wastewater Treatment Plant Process

Preliminary Treatment
The preliminary treatment processes include raw wastewater pumping, bar screens, grit removal, and comminution. Wastewater flows through a 48-inch interceptor sewer to the influent pump station located in the service building. Screw pumps lift the wastewater about 30 feet to the bar screens, also located in the service building. The two mechanically cleaned bar screens remove large objects from wastewater, including branches, wood, rags, and plastic items. The items, which are removed by the bar screens fall by gravity into dumpsters and are hauled to a landfill. Continuing from the bar screen, the wastewater flows by gravity to the vortex-induced grit removal units located in a nearby building. Grit that is removed by this process is augured to dumpsters and is also hauled to a landfill. Currently, the communitors are bypassed. The wastewater is monitored by a Parschall flume to determine flow rates and sampled before entering the primary clarifiers.
 
Primary Treatment
Screened and de-gritted wastewater flows by gravity through three 60-foot square primary clarifiers. Wastewater flow is quieted in the large clarifiers, and most of the particulate material settles to the bottom, where the sludge is mechanically scraped off the bottom. The clarifiers have a skimmer and a scum collection box to remove floating material. About 60% of the Total Suspended Solids (TSS) and 30% of the Biochemical Oxygen Demand (BOD) is removed in this process.

Primary clarifiers receive waste activated sludge (WAS) from the secondary treatment system for co-settling with primary sludge. Co-settled sludge is pumped to the primary anaerobic digesters for further treatment. Primary effluent flows by gravity to a splitter box, where flow is divided between the aeration tanks.

Secondary Treatment
Secondary (biological) treatment is accomplished with an extended air activated sludge process. Each of six aeration tanks has a volume of about 460,000 gallons, with a total volume of 2.77 million gallons. At the design average flow rate of 3.5 million gallons per day (mgd), the aeration tanks have a hydraulic retention time (HRT)) of about 19 hours. The aeration tanks contain a microbial population of bacteria, protozoa, rotifers, and other organisms that consume primarily soluble solids in wastewater. The mixture of water and microbiological organisms is called "mixed liquor." The aeration tanks have a grid system of diffusers on the bottom of the tanks where air is bubbled through the mixed liquor. The air is necessary for the aerobic biological process and keeps the mixed liquor well mixed.

Three final clarifiers allow the activated sludge to settle and separate from the water. Most of the activated sludge is pumped back to the aeration tank influent, and is called return activated sludge (RAS). Recycling the activated sludge increases the lifespan of the microbiological organisms. The average time that biomass spends in the aeration tanks is called the sludge age, or mean cell residence time (MCRT). Under existing conditions, the WWTP has a sludge age of about 14 days. Since the microorganisms are constantly reproducing, a certain amount of activated sludge has to be removed from the aeration, or "wasted," to maintain a steady population. The waste activated sludge (WAS) can be dewatered in the gravity thickener, but is currently pumped to the primary clarifiers and co-settled with primary sludge. Final effluent flows to the chlorine contact chambers for disinfection.

Disinfection
Before discharge to the river, the final effluent is chlorinated for disinfection. The Beaver Dam WWTP uses chlorine gas for disinfection and sulfur dioxide for dechlorination. Post aeration increases the dissolved oxygen (DO) level above the minimum permitted limit. The effluent flow rate is determined with a Parshall flume.
 
Sludge Handling
Sludge from primary and secondary treatment, co-settled in the primary clarifiers, is stabilized in two primary anaerobic digesters, where about half of the volatile (organic) solids are biologically converted to methane gas, carbon dioxide, and water. The digestion process reduces odors and makes the sludge less desirable to rodents, insects, and other pests. The digested sludge is pumped to secondary digesters for temporary storage, where excess water is decanted. The methane gas produced during digestion is used as fuel in two of the four blowers supplying air to the activated sludge process. The methane gas is temporarily stored under a floating cover within one of the secondary digesters.

Sludge produced at the WWTP is land applied as fertilizer in neighboring farm fields. In warmer months, the sludge is applied at about 2% solids concentration directly from the secondary digesters. Sludge cannot be applied when the ground is frozen, therefore the facility stores sludge for a period of up to six months. To save space, the sludge is dewatered in a belt filter press to about 12% solids. Polymer is added at the belt filter press to aid in the separation of the sludge from the water. The sludge cake is loaded into trucks and sent to the on-site sludge storage building until warmer months, when it is land applied.