Preliminary Treatment

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Headworks HMI Cover Page

Grit Removal Process  To Primary Clarifiers barscreen.jpg grit_truck.jpg grit_basin.jpg grit_pumps.jpg

Headworks

The existing Headworks, placed into service in 1977, provides influent flow measurement, screening, influent pumping, and grit removal.  This building was renovated in 1991 during the Phase 1A project to upgrade the existing treatment systems, improve ventilation, and provide odor control facilities.  The influent flow metering equipment has been modified under Phase 2 Expansion project in 2009.

The ventilation system provides approximately 30 air changes per hour through the Headworks building. This exchange rate scours odorous gases out of the building and maintains a good working environment for operations staff.  The exhaust air from the Headworks building is used to supply oxygen to the biological treatment process in the nearby Trickling Filters.  The combined exhaust from the Headworks and Trickling Filters is chemically treated to remove odorous compounds prior to release to the atmosphere.

Influent Flumes. An average of 23 million gallons per day (mgd) of wastewater from the cities of Littleton and Englewood enters the plant separately through two interceptor pipes.  The Englewood interceptor is 60-inches in diameter.  The Littleton interceptor is 66-inches in diameter.  Two Parshall flumes located at the entrance to the Headworks building measure flows.  One flume is dedicated to measuring City of Englewood flows and the other flume measures City of Littleton flows.  Each flume has a rated free-flow capacity of 80 mgd.  Influent flows are measured and recorded on a continuous basis.  Several manual gates upstream of the flumes allow the two separate influent flows to be combined and routed through either flume.

Screening. Following the influent flumes, flow passes through four mechanically cleaned bar screens. The screens have a 1/2-inch clear opening between bars and effectively remove most rags, large debris, and stringy materials from the liquid flow stream. Screenings are automatically collected and discharged into a grinder where the screenings are macerated before falling into a trough. A high velocity flow of recycled effluent carries the screenings through the trough to a pump. The ground screenings are pumped to one of two screening compactors which washes and compacts the material prior to discharge into an open hopper. An open-bed dump truck receives the ground, dewatered screenings which are transported to final disposal at a sanitary landfill. It is important to remove these materials to prevent damage or clogging of the plant's equipment.

Influent Pumps. After screening, raw sewage pumps lift the flow approximately 30 feet into either two grit basins or two grit concentrators. Raw sewage pumping is provided by six dry well, vertical shaft centrifugal pumps. Each pump has a rated capacity of 20.1 mgd. Each pump discharges individually just above the water level of the grit basin distribution channel. An air gap of several inches precludes reverse flow when the pumps are shut down, eliminating any need for check valves on the pump discharge piping.

The raw sewage pumps are each equipped with electronic pulse-width modulation (PWM) type variable speed drives. Pump speed is varied to maintain a constant level in the Headworks wetwell. The pumps start and stop automatically in an operator selectable sequence. Operation with a constant wetwell level achieves several purposes including:

1.  Reduces the drop downstream of the influent flumes minimizing turbulence and air entrainment.  Minimal turbulence reduces stripping of H2S and other odorous compounds to the air.

2.  Eliminates frequent pump cycling associated with a variable level control strategy.

3.  Minimizes static lift on the existing pumps to achieve the rated capacity of the existing units.

4.  Increases submergence of existing pumps to minimize potential for vortexing and air entrainment.

The wetwell water surface level is set as high as possible while maintaining scouring velocities at low flow. The barscreen rake mechanism operates frequently enough to avoid high headloss or breakthrough of rags and debris at high flow. The diurnal flow peaking factor of 1.5 does not cause excessive velocities through the barscreens.

Grit Removal. Screened raw sewage is pumped to the grit influent channel by the raw sewage pumps. Flow from the Filter Influent Pump Station is combined with the raw sewage flow in the grit influent channel. From the influent channel, the sewage/nitrified effluent can flow to either Grit Tank 1, Grit Tank 2, Grit Concentrator 1 and/or Grit Concentrator 2. Normally, the grit tanks are isolated and the grit concentrators are used for grit removal.

The grit concentrators are hydraulically-driven separation devices that utilize boundary layer effects to create a concentrated grit slurry and a degritted sewage stream. The degritted sewage flows out of the concentrator, over a weir, to the grit effluent channel. The grit slurry is pumped from the bottom of the concentrator to grit classifiers for further processing. A water (3W) supply is delivered to the concentrator as an underflow stream during operation to assist in the separation process.

The grit pumps draw the grit slurry from the bottom of the concentrators and pump the slurry to the grit classifiers. The grit pumps are recessed impeller pumps with variable speed drive.

Grit classifiers are hydraulically-driven separation devices that utilize centrifugal force to separate grit from the sewage stream. A 3W supply is delivered to the classifier during operation to assist in the separation process. After separation, the sewage and supply water are returned to the raw sewage wet well. The grit falls from the classifier directly into an associated grit dewaterer.

In the grit dewaterers, the grit from the classifiers is dewatered prior to being deposited in a truck for off-site removal. Rinse water is added to the dewaterers to prevent against dewaterer run-dry conditions. Dewatering is accomplished by transporting the grit up an inclined cleated belt. The grit then falls out of the discharge chute at the end of the belt into the screenings/grit hopper or disposal vehicle. The excess rinse water drains from the dewaterer and is returned to the raw sewage wet well.

Grit and screening, that are discharged to the screenings/grit discharge hopper, normally fall through the hopper and are deposited in a truck for off-site disposal. When a truck is not available, a slide gate at the bottom of the hopper is closed and the grit/screenings are temporarily collected in the hopper. 

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