Activated sludge from the plant that is no longer needed to treat the incoming wastewater is discharged or “wasted” as WAS or waste activated sludge. Wasting is necessary to keep a good balance of mass (treated solids) and food (incoming wastewater) for the biological process.  A poor balance of mass to food could lead to either untreated wastewater or excess solids leaving the plant through the effluent discharge.

Sludge Holding

During treatment of the wastewater at NOWWTP, Waste Activated Sludge (WAS), is produced as a result of the biological treatment in the activated sludge process. A large portion of the WAS is biodegradable and at the WWTP it is further treated in the aerobic sludge digestion process. The main purposes of the digestion process are to make the treated sludge suitable for ultimate disposal and to reduce the quantity of sludge needing disposal.

There are two (2) Sludge Storage Tanks, both measuring 50 feet in diameter and a depth of 28 feet. The WAS is removed from the Secondary Settling Process by two (2) WAS pumps located in the RAS/WAS Pumping Station and transferred to the Sludge Storage Tanks via a 4 inch line. At the Sludge Storage Pump Room (located between the two tanks) the line splits into two (2) lines, one for each tank, which goes over the top of the side wall and expands to a six (6) line that is discharged at the bottom of the tank.

The Sludge Storage Tank contents are aerated by a coarse bubble aeration system and a floating mixer is used to keep the contents mixed while in storage.

The Pump Room contains the Decant Pumps (one for each tank), Sludge Grinders and Sludge Transfer Pumps.  Two (2) Blowers used for aeration of the tanks are located adjacent to Sludge Storage Tank 2.

biosolids1 biosolids2

Sludge Dewatering

The Waste Activated Sludge (WAS) stored in the Sludge Storage Tanks is dewatered prior to final disposal by two (2) Dewatering Centrifuges manufactured by GEA Westfalia Separator.


The sludge is pumped out of storage, blended with polymer and sent to the Centrifuges for dewatering. Polymer enhances the dewatering capability of the solids. One Centrifuge is located in the Solids Handling Building near the Sludge Storage Tanks, and the other centrifuge is located in the sludge load out area.

The centrifuge spins at a rate of 4,300 rpm to utilize centrifugal force to separate water from the feed sludge. The water, known as centrate, is discharged by gravity to the Plant Drain Pump Station where it enters the wet stream treatment process. The dewatered cake is discharged to a screw conveyor and then onto the Belt Conveyor, in the sludge load out area, for disposal by truck.

The NOWWTP currently hauls all biosolids with the use of in-house staff. The operational goal is to haul material offsite for beneficial reuse. The major site for beneficial reuse is Quasar’s Three Creeks Facility, were biosolids are converted first to methane and then to electricity.