Preliminary treatment consists of raw sewage pumping, and removal of inorganic material from the sewage.

The preliminary treatment building, seen in the photo above, contains three Archimedes screw pumps to lift the sewage from the sewers, 12 feet below the ground, into the preliminary treatment building. The screw pumps are in the sloping concrete structure on the left of the building. Each screw pump is 72 inches in diameter, 40 feet long, driven by a 100 horsepower motor and has a capacity of 15 million gallons per day. Normally, only one screw runs at a time. During wet weather periods, two screw pumps are required, and during heavy rainstorms, all three pumps are required.


This photo provides an idea of the size of these screws. This photo was taken in 2003 during replacement of an old worn out screw pump. The screw shown here was being replaced because of wear and tear from grit in the sewage, the vanes were worn, and the gap between the vanes and the concrete channel was excessive and decreased the capacity of the screw pump.






This photos shows the 100 hp motors that turn the screw pumps from the top. These 100 hp motors drive the screw pumps through a gear reducer which reduces the speed of the motor from 1440 rpm to drive the screws at 29 rpm.







Immediately after the wastewater is pumped up into the plant, the wastewater must first pass through bar screens that have iron bars spaced 3/8 inch apart. The bar screens catch large pieces of debris such as plastic bags, plastic water bottles, sticks, etc. that could damage equipment in the wastewater treatment plant.

A mechanical rake on the front of the bars scrapes the material caught by the bar screens and lifts that material up into a screw conveyor which moves the material to a dumpster for disposal.






After the sewage passes through the bar screens, it enters aerated grit chambers. These are deep tanks that have air introduced from the bottom on just one side of the tank. The air rising along just one side of the tank causes the water to move around in a swirling motion causing heavy material, sand, grit, etc. to be collect at the bottom of the tank.





Two 15 horsepower centrifugal blowers are used to supply air to the aerated grit tanks. A screw conveyor at the bottom of the tank moves the grit out of the tank and back into a hopper under the preliminary treatment building.







Grit bucket elevators are used to lift the grit from the hopper. The grit buckets in the back of this photo are driven by a pair of chains that drive buckets to scoop out grit from the hopper and the lift the grit high enough to dump the grit into a chute where it is moved by an auger to a grit washer.









The grit enters the grit washer. Inside the grit washer is an inclined screw that raises the grit while it is cleaned and drained. Heavier grit settles while the organic material tends to remain in suspension and flow out of the washer in an overflow pipe where its returned to the beginning of the treatment process. From there, the grit is moved by an auger to a hopper for pickup by a waste hauler for disposal in a landfill.




This photo shows two conveyors. On the right is a silver colored conveyor that moves the grit to the far side of the building. The grit conveyor is inclined to promote drainage as the grit travels the length of the building to the hopper. The grit is disposed by a waste hauler for disposal in a landfill. On the left and much harder to see is the conveyor for screenings collected by the bar screens talked about early in the tour. The conveyor moves the screenings to what's called a screenings washer.



This screenings washer (with yet another inclined screw conveyor to promote drainage) is located at the end of the screenings conveyor. The screenings fall into the screenings washer where water cleans away any organic material from the screenings. The wash water is returned to the plant for treatment. The screenings are carried up the inclined screw, and as they enter the cylindrical portion of the screenings washer, the screenings are compressed to squeeze out even more water. The dry screenings then fall into a hopper which is eventually taken by a waste hauler to a landfill for disposal.


The preliminary treatment building can sometimes have undesirable odors. An odor control system has been provided to remove 99% of the hydrogen sulfide gases which cause the odor. The white piping draws air out of the building from many different locations and it is the inlet to the odor control equipment. Inside the black box are canisters of carbon coated with a chemical that will attract and hold the hydrogen sulfide molecules. Clean air is exhausted out the vertical stack on the right. As the carbon becomes saturated with the hydrogen sulfide molecules, the removal efficiency decreases and the carbon is backwashed to restore its adsorptive capacity.

At this point in the process, wastewater has completed the preliminary treatment phase, and it is ready to receive primary treatment.

1. Preliminary Treatment
2. Primary Treatment
3. Roughing Filters
4. Activated Sludge
5. Effluent Filtration
6. Effluent Disinfection
7. Sludge Digestion
8. Sludge Dewatering
9. Sludge Storage
10. Other Unique Features