Wastewater Treatment Plant

WASTEWATER TREATMENT PLANT PROCESS DESCRIPTION

Influent Pumping and Pretreatment
Raw sewage is conveyed to the plant by two deep trunk sewers which converge at the Influent Pump Station.  A mechanical bar screen removes coarse and stringy debris from the wastewater and then it is pumped to the pretreatment structure.  It is sampled and metered and it flows through the Aerated Grit Tank where denser, easily settable material is removed.  The pretreated wastewater then proceeds to the Primary Settling Tanks.  Flows in excess of the primary tank capacity are automatically diverted around them.
Primary Treatment
The first step in the sewage treatment process is the removal of the suspended solids carried by the wastewater.  After screening and grit removal, the pretreated sewage flows through the four circular primary settling tanks where the suspended material drops out by gravity.  The purpose of this process is to reduce the solids load to the secondary treatment process.  The material collected on the bottom of the primary tanks is withdrawn as liquid primary sludge and pumped to the plant’s sludge treatment facilities.  Floating material is skimmed off and collected in the scum wells.  Settled wastewater flows over the discharge weirs and continues on to the Primary Splitter Boxes and then to the Aeration Tanks.

Secondary Treatment
Secondary treatment is accomplished in the four rectangular Aeration Tanks and two circular Final Settling Tanks.  Primary settled wastewater flows into the aeration tanks where it mixes with a suspension of live micro-organisms known as activated sludge.  The mixture is aerated and mixed in order to satisfy the respiration requirements of the organisms as they oxidize the organic matter in the wastewater.  Alum solution is added for phosphorus removal and the mixture of activated sludge and wastewater, known as mixed liquor, the flows to the final settling tanks where the sludge settles out.  The clarified, treated wastewater, now stripped of approximately 90% of its suspended and dissolved organic pollutants, continues on to the filtration step.  The activated sludge is concentrated and collected on the settling tanks bottoms and continuously returned to the aeration tanks by means of the Return Activated Sludge Pumps.
Because the activated sludge mass grows and multiplies as it utilizes the wastewater as its food course, excess sludge is periodically wasted to the plant’s sludge treatment facilities along with the primary sludge.

Tertiary Treatment 
Following secondary treatment, the wastewater is filtered as a final polishing step.  The tertiary treatment facilities include the Filter Influent Pumps, the four Tertiary Filters, Backwash Tank and Pumps, and Backwash Waste Pumps.  The material trapped in the filter is removed by backwashing and then recycled through the Final Settling Tanks.
The filtered effluent is disinfected by ultraviolet radiation and discharged to the Sandusky River.

Aerobic Digester Facilities
Primary sludge and waste activated sludge are transferred to the three circular Aerobic Digesters by the Primary Sludge Pumps.  Sludge reduction and stabilization is achieved under prolonged aeration as the micro-organisms consume any remaining nutrients and then starve and waste away.  Air for the digesters is supplied by three centrifugal blowers located in the Digester Control Building.  There are also transfer pumps and a grinder for moving the sludge among the tanks and to the Sludge Processing Building.

Sludge Processing
Aerobically digested sludge is pumped to the Sludge Processing Building and dewatered to a solid cake on the Belt Filter Press.  The sludge cake is then fed to the Biosolids Pasteurization Unit where it is blended with lime and heated to kill pathogens and produce Class “A” or “B” product for agricultural application.  The processed material may be immediately loaded onto trucks for disposal, or accumulated on-site in a covered storage area. 

DESIGN DATA

Basis of Design

Permit Limitations

The Bucyrus Wastewater Treatment Plant was designed to serve a future population of 15, 500 and currently (2016 provides service to 12,500 persons.  The plant has an average daily design capacity of 3.4 million gallons per day (mgd) and a peak capacity of 6.0 mgd.  It is permitted under the National Pollution Discharge Elimination System (NPDES) Permit No. 2PD00021*MD.  The plant effluent is discharged to the Sandusky River.  Final effluent limitations, as defined by the permit, are as follows: