Advanced Secondary Settling Return to Process Page

What is Advanced Secondary settling and why is it done?    After the wastewater has been through Advanced Secondary Treatment, the wastestream is once again full of organic material from the bacteria that has sloughed from the two biotowers.  At this stage, the remaining solid material is in very small particles, which do not settle as easily as large clumps of material would.  For this reason, the capacity of the Advanced Secondary settling tanks is more than the combined volume of the Primary and Secondary settling tanks combined.  This allows the wastewater the maximum amount of time to slow down and allow the solids to settle out.  On an average day, it takes the wastewater about sixteen hours to be completely processed through the Treatment Facility.  For the wastewater, nine of those hours is spent in the Advanced Secondary Settling tanks.    The Advanced Secondary settling tanks use a different design than the Primary and Secondary tanks.  The Advanced Secondary tanks are round in design with a sloped conical bottom and a central sweep mechanism.  This allows all the solids to be pulled toward the center of the tank, instead of to one end.  Solids that are collected during this stage are pumped automatically three times a day into the beginning of the secondary settling system, where the smaller tertiary particles can combine with the larger secondary solids.  After this final settling stage, the wastewater is ready for its last treatment step - disinfection.