Step 1: Screening and Pumping The incoming wastewater passes through screening equipment where objects such as rags, wood fragments, plastics, and grease are removed. The material removed is washed and pressed and disposed of in a landfill. The screened wastewater is then pumped to the next step: grit removal.
Step 2: Grit Removal In this step,
heavy but fine material such as sand and gravel is removed from the
wastewater. This material is also disposed of in a landfill.
Step 3: Primary Settling The
material, which will settle, but at a slower rate than step two, is
taken out using large circular tanks called clarifiers. The settled
material, called primary sludge, is pumped off the bottom and the
wastewater exits the tank from the top. Floating debris such as grease
is skimmed off the top and sent with the settled material to digesters.
In this step, chemicals are also added to remove phosphorus.
Step 4: Aeration / Activated Sludge
In this step, the wastewater receives most of its treatment. Through
biological degradation, the pollutants are consumed by microorganisms
and transformed into cell tissue, water, and nitrogen. The biological
activity occurring in this step is very similar to what occurs at the
bottom of lakes and rivers, but in these areas the degradation takes
years to accomplish.
Step 5: Secondary Settling Large
circular tanks called secondary clarifiers allow the treated wastewater
to separate from the biology from the aeration tanks at this step,
yielding an effluent, which is now over 90% treated. The biology
(activated sludge) is continuously pumped from the bottom of the
clarifiers and returned to the aeration tanks in step four.
Step 6: Filtration The clarified
effluent is polished in this step by filtering through 10 micron
polyester media. The material captured on the surface of the disc
filters is periodically backwashed and returned to the head of the plant
Step 7: Disinfection To assure the treated wastewater is virtually free of bacteria, ultraviolet disinfection is used after the filtration step. The ultraviolet treatment process kills remaining bacteria to levels within our discharge permit.
Step 8: Oxygen Uptake The treated
water, now in a very stabilized high quality state, is aerated if
necessary to bring the dissolved oxygen up to permit level. After this
step, the treated water passes through the effluent outfall where it
joins the Oconomowoc River. The water discharged to the river must meet
stringent requirements set by the DNR. Pollutant removal is maintained
at 98% or greater.
Sludge Treatment The primary sludge
pumped from the bottom of the primary clarifiers in step three, along
with the continuous flow of waste activated sludge from the aeration /
activated sludge process in step four, must be treated to reduce volume
and produce a usable end product. The sludge treatment process involves
four steps as described here.