| 1. Trunk Sewer:Wastewater discharged from your home or business flow into a lateral sewer, where it is conveyed by trunk sewers to the wastewater treatment plant. |  |
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| Headworks: Wastewater first passes through an aboveground headworks where the volume is measured and a portion can be diverted to the underground treatment facilities, otherwise, the flow goes to a second aboveground headworks. In both headworks, large solids, which would interfere with pumps and other equipment are removed using mechanical bar screens. Screenings removed from the wastewater are hauled to a landfill for disposal. |
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| 3. Primary Sedimention Basins:The wastewater flows through primary settling tanks, known as "primaries". Six primaries are located aboveground and two underground. |  |
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| Primary treatment consists of holding wastewater for about two and a half hours to allow heavy particles to settle to the bottom as "sludge" and light particles to float to the top as "scum".(Photo above: Primary Effluent Pump, Photo at left: Primary Sed Tanks) |
| 4.Equalization Basins:During rain storms and daily peak flow periods, primary treated wastewater is diverted to underground equalization basins. Here, up to 2.8 million gallons of wastewater can be held until the peak period passes. Equalized primary wastewater is pumped to the secondary treatment process. |
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| Primary Effluent Pump Station:Wastewater treated in the aboveground primaries can either be pumped to the secondary process or flow to the equalization basins. |
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| Degritter:Sludge is collected and pumped to a cyclone degritter which removes heavy grit particles such as sand, egg shells, and coffee grounds. Grit is hauled to a landfill for disposal. (Photo Below: The degritter is the blue structure on top of the gravity thickener) |
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| Gravity Thickener:Degritted primary sludge is concentrated in gravity thickeners until it contains about 5% solids and 95% water. |
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| Anaerobic Digester: Thickened primary sludge is anaerobically digested just as people digest their food. Here methane forming bacteria feed on sludge contained in a tank devoid of oxygen. The digesting sludge is heated to an average of 98 oF to speed up the process that takes anywhere from 12 to 25 days. |
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|  | Electricity Generator and Gas Flare:Methane gas, produced during digestion, is burned to heat the digesters or used to generate electricity. Any remaining gas is burned in an excess gas burner. |
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| Centrifuge:Digested sludge, or biosolids, is dewatered using a high speed centrifuge. The centrifuge spins the water from the biosolids until a cake containing 20 to 25% solids remains. Depending on the season, the biosolids are hauled to a landfill where they are used as daily cover or to a site where they are beneficially reused as a soil amendment. |
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Hi Allyssa,
ReplyDeleteThank you for your post. I appreciate you including a schematic of the flow of water processing for the Daly City water treatment facility and showing pictures with step-by-step process descriptions. I think for many of us, we do not think too often about where our water comes from or how it is treated. This system of processes goes to show how far we have come in terms of regulation by the EPA and the guidelines set forth in the SWDA and CWA.
Hi Allyssa,
ReplyDeleteWow, what an informative post! IT really helps to understand the process when you have a step by step list with images. It is so interesting the process that our water goes through to get to us.