When we learned we could tour South Tahoe Public Utility District’s Wastewater Treatment Plant I got a little excited. The only other Sewer Tour we've taken was the très merveilleux one in Paris.

"STPUD is excited to offer free public tours at its wastewater treatment plant. The 45-minute tours will provide residents with a behind the scenes look at how the facility operates and the critical role it plays in cleaning wastewater. Participants will have the opportunity to tour the treatment plant and learn about the process of how the wastewater is cleaned and safely reused."Our tour took us through the very complicated process of water treatment with our first stop being the operations control room and lab. The operations department is staffed 24/7/365, including holidays. There are always 2 to 4 operators on duty. They are constantly performing process control tests to ensure the treatment process at each stage is functioning properly, making sure there’s the right mix of bugs, enough oxygen, when to pump the water out of basin, and responding to any alarms going off in the field. One of the ways that they monitor all that is through SCADA, Supervisory Control and Data Acquisition. Operations uses this computer software to monitor everything that happens at the plant, as well as the 80 water and sewer field facilities. There was something very comforting about this control room!The plant processes about 3.7 million gallons of sewage a day and can treat up to 7.9 million gallons a day. There are three treatment processes for treating wastewater: physical, biological, and chemical.  The first stage of the sewage treatment begins in this building. This is part of the physical treatment, as they physically remove things from the wastewater. It moves though various bar screens. Any objects that were flushed down the toilet are caught here. Often, they see rags, toys, underwear, anything that is large enough to be caught by the screening prong.
This building also has a grit chamber. The water is slowed down allowing the dense or heavy material to sink and is collected in these bins. Some of the dense material caught here is sand and grit, seeds, and corn kernels. All materials that are caught at this stage from the screens and grit chamber are taken to a landfill.
This contraption is a carbon odor scrubber. It has three stacks of charcoal type material that filter out the odor. The large pipes shoot the scrubbed air up and disperse it so that the smell isn’t concentrated. So interesting.
The two tanks behind Lauren are all primary clarifiers, or settling tanks. Here the flow is slowed down to allow for solids to settle and any fats, oils or grease to float. The top of the basin has skimmers that capture the fats oils and grease. At the bottom there is a rake arm moving slowly, helping the solids collect in the center point.

At this point, the treatment process diverges, with settable solids being transported up to the bio building, and the water with suspended solids flows down to the aeration basin. We began with with the bio building. This is where they produce what is called “cake.” Here they are trying to further remove liquids from the solids. To accomplish this a centrifuge is used which spins the solids at high speeds and adds polymers to thicken the sludge, further separating the solids from the liquids. Only 3% of what enters this building is turned into cake, while the remaining 97%, known as centrate, is sent back to the Headworks to be processed with the rest of the raw wastewater.The District can recycle 100% of all water and solids for beneficial reuse. The solids are placed in boxes and hauled to Bentley Agro Dynamics in Nevada five days a week. The biosolids are used to make compost for land application on livestock grazing fields. They ship out an average of five trucks per week, with 273 truck loads, or 2.3 million pounds, shipped out in 2024.
We then continued our wastewater journey. The water with suspended solids flows down, via gravity, to the Aeration Basin. Using gravity at the plant reduces the need for pumping, lowering power consumption and associated costs. This is the stage where most of the work happens by tiny microorganisms or biological treatment. The wastewater with suspended solids enters the basin and are consumed by microorganisms, known as activated sludge, but commonly referred to as “bugs.” These bugs are aerobic, which means they need oxygen.Oxygen pumped into the basins, which is why it's bubbling. The bugs ingest the food, metabolize it for energy as they move around, expel carbon dioxide, and have a limited life cycle - just like humans. One of the many jobs Operations does is conduct tests on the water to ensure the bugs are happy, healthy and that the balance of microorganisms is right for them to do their job. It is their job to make sure that they are treating the wastewater to the standard of their permit and that the water used by ranchers is of appropriate water quality.

Then the effluent flows to the secondary clarifiers, where they are once again using physical treatment to slow the water down and separate the solids and liquids. This is where they get rid of the bugs and have fairly clean water.