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Virtual Tour

Influent Pumping
Raw sewage from all the sewage collection systems combines into one influent pipeline that divides between two influent pump stations each with a trash rake at the inlet of the pump stations’ wet wells.  The trash rakes allow water to pass through, but remove large debris such as paper towels, plastic wrappers, rags, washcloths, baby wipes, and diapers.  This enables the raw sewage to be pumped, without clogging, to the fine screen building via three flow sensors.

Influent Pump Station 
Influent Pump Station

 Trash and debris collected from trash rake
Trash and debris collected from trash rake

Influent Screening
Fine screens remove solids and debris to prevent damage to downstream pumps and other mechanical equipment.  They also increase the efficiency of the facility by reducing solids buildup in downstream basins.  The captured screenings are dewatered before being discharged to a dumpster to be hauled off site for disposal. 

The screened wastewater flows by gravity to two pairs of grit removal units where dense particles, particularly grit, is settled out of the wastewater.  The captured grit is pumped through grit cyclones and classifiers to wash organics from the grit and then hauled off site for disposal. 

Dumpster where trash and debris is collected

Primary Clarification
Two primary clarifier junction boxes distribute flow to four primary clarifiers.  Sludge that settles to the bottom of the clarifier is removed as primary sludge and scum that floats on the surface of the clarifier is removed as primary scum.  Primary sludge and scum are pumped to the fermenters.  Ferric chloride can be added to the primary clarifier junction box for chemical precipitation of phosphorus.  The precipitated phosphorus settles and is removed with the primary sludge.  The primary clarifiers overflow treated wastewater to the biological reactor inlet box.  The primary clarifiers can be bypassed at the primary junction box allowing flow from the grit removal units to be sent directly to the biological reactors.

In the fementers, primary sludge settles and thickens and scum rises to the fermenter surface.  Both scum and the thickened sludge are pumped to the digesters.  Solids retention in the fermenters encourages microorganisms to release volatile fatty acids (VFA), which is a nutrient that is pumped to the biological reactors to aid in the biological removal of phosphorus.
Primary Clarifier 

Secondary Biological Treatment
The biological reactor inlet box distributes flow to an Enhanced Nutrient Removal (ENR) system that includes twelve biological reactors, six secondary anoxic tanks, and six final clarifiers for biologically and chemically removing nutrients from the wastewater.

Biological Reactors and Secondary Anoxic Tanks
The biological reactors receive incoming wastewater from the primary clarifiers and discharge to the secondary
anoxic tanks where treatment continues.  A large population of microorganisms consumes organic carbon compounds, nitrogen, and phosphorus.  The resulting mixture, called activated sludge, flows from clarified effluent and discharges to the tertiary filters.  Most of the settled activated sludge is recycled back to the biological reactors.  A small percentage of the settled activated sludge is wasted from the system.

ENR uses three chemicals to enhance the removal of phosphorus and nitrogen.  Two coagulants, ferric chloride and polymer, are added to the final clarifier influent to precipitate phosphorus which then is removed from the system with the waste activated sludge (WAS).  Volatile fatty acids from the fermenters are added to the biological reactors and acetic acid is added to the secondary anoxic tanks to provide food for microorganisms that are part of the biological phosphorus and nitrogen removal system.


Biological Reactors

Final Clarifier
Mixed liquor from the biological reactors flows to six clarifiers where the solids, mostly microorganisms, settle by gravity and the scum rises to the clarifier surface leaving treated wastewater to overflow to the clarifier weirs and discharge to the tertiary filter.  Most of the settled solids are returned to the biological reactors, as return activated sludge (RAS).

Final clarifiers collect secondary scum from the liquid surface and settled RAS from the bottom of the clarifiers.  RAS pump stations draw the secondary scum and RAS from clarifiers and discharge to the biological reactors.  After this process is performed throughout the clarifiers, sludge is discharged into storage tanks.

A small controlled portion of the RAS and scum is removed as WAS by a variable position motorized valve and directed to a gravity belt thickener.  The thickened WAS from the gravity belt thickener is pumped to the digesters.  The volume of WAS is controlled to maintain a target concentration of solids in the biological treatment system.

Ferric chloride and polymer can be added to the secondary clarifier flow distribution boxes for chemical precipitation of phosphorus.  The precipitated phosphorus settles and is removed with the WAS.

RAS Junction


Final Clarifier

Effluent Filtration
Treated wastewater from the secondary clarifier flows to the tertiary filter pump station that raises the elevation of the wastewater to it to flow by gravity through the tertiary filters.  The tertiary filter pump station and the tertiary filters can be bypassed and flow from the final clarifier sent directly to the chlorine contact tanks.

Tertiary Filter Pumps

Chlorine Contact Tanks
Filtered wastewater from the tertiary filter flows to four chlorine contact tanks.  Sodium hypocholorite is added upstream of the chlorine contact tanks where pathogenic microorganisms are killed.  The chlorinated water is dechlorinated by sodium bisulfite before discharging to the cascade aerator.

Chlorine Contact Tanks 


Post Aeration and Cascade Aeration
The cascade aeration channel accepts flow from the chlorine contact tanks and is designed to raise the dissolved oxygen concentration of the treated wastewater before it is discharged to the North River.

Treated wastewater being discharged into the North River
using cascade aeration

Solids Handling
The solids handling facility include three sludge storage tanks, two gravity belt thickeners, a belt filter press, and two anaerobic digesters.  WAS and some secondary sludge are thickened prior to being pumped to the digesters.  Secondary scum from final clarifiers 5 and 6 and the secondary anoxic tanks is pumped to the sludge storage tanks before being pumped to the digester.  Primary sludge and primary scum are thickened in the fermenters before being pumped to the digesters.  Digested sludge is dewatered on the belt filter press and delivered to the sludge storage facilities.

The sludge storage tanks and a complex of yard piping and valves provide numerous discharge options which allow the operator to direct just about any kind of sludge to one of the following destination points:

  • Anaerobic digesters
  • Gravity belt thickeners
  • RAS pip as it returns to the biological reactors
  • Biological reactor 8
  • Belt filter press
  • Fermenters
  • Sludge Lagoon
Sludge Storage

Dewatered Biosolids

Laboratory Testing
The HRRSA laboratory is accredited by the Division of Consolidated Laboratory Services (DCLS) under the Virginia Environmental Laboratory Accreditation Program (VELAP).  HRRSA received accreditation on June 15, 2010.  The laboratory provides analysis of samples as required by the Virginia Pollutant Discharge Elimination System (VPDES) permit issued to HRRSA and analysis of process samples to ensure proper operation of the North River Wastewater Treatment Facility (NRWTF).  Laboratory tests are run on the wastewater and sludge at various stages throughout the process.  Our on-site laboratory runs CBODTSSpH, TRC, TP, ammonia, and alkalinity tests.  A commercial laboratory performs additional tests including nutrients and E. coli.