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City of Paso Robles
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Wastewater Division Treatment Plant
3200 Sulphur Springs Road
Paso Robles, CA 93446
(805) 237-3865
(805) 237-3886 FAX
Mon-Fri 7am to 3:30pm


The Treatment Plant Process (continued):

Secondary Clarifiers  |  Bio Solids  | Disinfection
Polishing Ponds  |  Laboratory

Previous Page:
Physical Treatment  |  Primary Treatment  | Trickling Filters

Secondary Clarifiers

Image numbers are in parenthesis.

This is the final process before disinfection. The three secondary clarifiers (13) slow the stream again and remove any sloughings or larva that has come from the secondary trickling filters. The secondary sludge is then pumped (14) to the digester.

13. 14.



The sludge which was removed during the primary and secondary processes is pumped to the anaerobic digesters (15). The digesters are heated and mixed using boilers (16) and external mixing pumps. The methane gas produced by the decomposing of the sludge is used for fuel for the boilers and the excess methane is burned off at the flare (17).

15. 16. 17.

With a detention time of 20 to 30 days the digested sludge is then pumped to a holding tank (18) where the heavier sludge is allowed to settle to the bottom. The heavier sludge is then pumped to a belt filter press (19), where cationic polymer is mixed prior to distribution onto the press.

18. 19.

The sludge (20) is squeezed between porous belts for the final product called “cake” (21) or bio-solids. These bio-solids (22) are then hauled to the landfill for disposal. Bio-solids can also be used for soil enhancement and mixed with green waste for composting.

20. 21.



The stream then flows to the chlorine contact chambers (23) where gaseous chlorine (23A) is injected into the stream to kill the coliform bacteria. Total Coliform or MPN (most probable number) is used to determine the effectiveness of disinfection.

23. 23A.

The Plant is in the process of converting to Sodium Hypo Chlorite (bleach) for disinfection. Other forms of disinfection may also be used other than chlorine to prevent the formation of tri-halo-methane, a by product of the oxidation of organic matter by the chlorine. The methods being considered are Chlorine Dioxide and UV (ultra violet).

More about The Treatment Plant Process:
Polishing Ponds  |  Laboratory

Previous Page:
Physical Treatment  |  Primary Treatment  | Trickling Filters

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