Municipal Sewage-Primary Treatment Facility

June 21, 1995
October 2013 Update

The title of this paper should be, "How Is A 6" Screw Press Used To Treat 3,500,000 Gallons Per Day Of Sewage Water?"

The answer is found at the City of San Diego (California) Sewer Pump Station Number 77.  This facility performs primary treatment of 3.5 mgpd of combined residential and industrial sewage wastewater.  A small Vincent Press is used in the process.

The primary treatment at Pump Station 77 removes all of the coarse material from the stream.  99% of the settleable solids are removed along with 40% of the suspended solids.  (Settleable solids are those that settle out in two hours, whereas suspended solids are those that take up to five years to settle out.)  All of the solids removed from this sewage are hauled to landfill in dumpsters.

The first step of treatment is a coarse climber screen.  This uses a belt screen with 5/8" perforations.  As the screen becomes blinded, the water level rises to the point where the screen is actuated.  Once actuated, the screen lifts the solids from the sewer flow and drops the material into a dumpster.  The climber screen captures large items, mostly rags and plastic film.

A bar grate is kept available for emergency service.  Manually cleanable, it is used in the event of failure of the climber screen.  The bars on this grate are about 1" apart.

The second treatment is a vortex separator.  It removes heavy material, such as grit, sand, egg shells, and coffee grounds, through cyclonic action.  Large pumps are used to create a whirlpool effect.  The slurry from the large vortex separator is pumped to a smaller cyclone separator.  The solids removed in this device go to a dumpster, while the water goes back into the wastewater stream.

The third treatment uses a Hycor rotating drum filter.  This 4’ diameter drum has discs with ¼" separations.  The drum filter typically removes cigarette filters, small pieces of paper, and small pieces of vegetable food waste (leaves and stems).

From here all the waste flow goes to the fourth treatment, which uses a set of Hycor Discostrainers.  These large 6’ diameter discs have 250 micron (.010") mesh screens.  They remove the one last suspended solid from the waste stream: paper fiber.  This is mostly pulped toiler paper.  (A hot water spray is used to keep these screens from blinding.)

The fifth, final, and most important (to Vincent) treatment is the dewatering of this paper pulp.  The discharge pulp from the Discostrainer is very wet, with about 96% moisture.  The landfill authorities will accept only material of 50% or more solids. Therefore the paper pulp is run into a 6" Vincent press prior to being dropped into the dumpster.

Now you know how a 6" press can treat 3,500,000 gpd of sewage wastewater.

(Incidentally, almost none of the fecal material is captured in any of the primary screens.  The particles are so small that they pass through all of the screening devices.  It is all removed in the secondary treatment operation.)

Following the five stages of primary treatment, the sewage water is pumped to secondary treatment facilities three miles away.  The pump station also provides 1.0 mgpd of sewage to an experimental secondary treatment facility.  That plant, described in Pressing News #7, uses water hyacinths to remove biological waste from the wastewater.

October 2013 Update: 

Vincent has never received a single order for any spare parts for the VP-6 screw press used on this job.  That tells us that the plant was likely shut down.  We did hear that the hyacinth portion proved un-economical because of the large amount of land required for the ponds. 

The small amount of paper fiber treated at the San Diego installation could easily be handled by other equipment in the system.  There was no strong need for the screw press.

Vincent Corporation does not serve the municipal biological sewage market.  Our experience has been that screw presses very rarely work satisfactorily with biological sludge.

Issue 28