What’s the WRRF?

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Recycled water is used in San Luis Obispo to keep our parks and fields green.

What happens to the water that I use?

Do you ever wonder what happens to the water that flushes down your toilet, drains in the shower, or goes down your kitchen sink? The drain is not the end of the line for the water — it gets conveyed to the City’s Water Resource Recovery Facility (WRRF) where it is recycled! Ultimately, the used water is treated, and then either goes to support habitat and beneficial uses in San Luis Creek or is distributed in the City for landscape irrigation. How do we clean up the water to reuse it? Well, that is an amazing story.

“[Students on school tours] observe the chemical, mechanical and biological processes that turn their wastewater into a useable water resource, sustaining aquatic ecosystems in our area,” – Mike Di Milo, Science Discovery Program

What happens at the Water Resource Recovery Facility?

This is where science and art combine. Your used water flows through a network of pipes to arrive at the Water Resource Recovery Facility, and is then sent through a bar screen that separates out any trash or large solids in the water. It then flows slowly through large basins called clarifiers that allow the heavier stuff, the solids, to sink to the bottom. Gravity does the work in this step, like when rocks sink to the bottom of a lake.

WRRF Operator Tom demonstrates the clarity of treated water.

These solids are then collected and treated separately. The water then enters a large tank where air is added and microbes are allowed to thrive.  The microbes devour the remaining nutrients and organic material in the water. Nothing like a hearty breakfast! Next, the microbes themselves are allowed to settle out of the water in a second clarification step. The clarified water goes on, while the microbes are recycled back through the process to do their important work. The clarified water is then cooled, filtered, and chlorinated –  and it’s ready to be reused. That’s nifty, but what about all the stuff that was removed from the water?

Where do the solids go?

Once removed from the water, the solids begin their own journey through the Water Resource Recovery Facility. This rich resource, called biosolids, is thickened to create a concentrated nutrient source and is transferred to anaerobic digesters where more helpful microbes break down the organic material and produce biogas as a byproduct of their digestion – talk about energy rich! The biogas is scrubbed and then fed to a large engine and generator to produce electricity.  Water is then squeezed out of the digested biosolids before they are hauled to an offsite compost facility to produce a valuable soil amendment for agriculture. As you can see, not only is the treated water a precious resource, but so are the solids that become compost and the biogas that can be used to produce energy.

The stages of treatment incrementally make the treated water clearer and clearer. Click on the image to see the clarity up close!

What is the biogas used for?

Biogas, known as such because it is produced biologically and is not a fossil fuel, is used to produce sustainable electricity to help run the plant. Biogas contains mostly methane and carbon dioxide. It’s a fuel for an engine and generator that produce electricity, reducing the amount of electricity the City purchases from PG&E and the amount of fossil fuels used to run the plant. Now that’s the fourth mention of resource recovery!  Did you catch the other three? The heat emitted by the engine is also recovered to help warm the anaerobic digesters, which speeds up digestion and improves biogas production.

How are we making it better?

The upgrades will transform the Water Resource Recovery Facility into a community asset that supports health, well-being and quality of life.  Now that you know a bit more about what happens every day, learn more about the future of the new Facility.

Learn more about resource recovery at the WRRF:

  1. Energy
  2. Compost
  3. Water Supply
  4. Environment
  5. Education
Process diagram and photos of the current treatment processes at the Facility.