How California is Turning Wastewater into Drinking Water

By: Monique Dimitrova

Each year, California produces around 1.46 trillion gallons of wastewater — roughly 4 billion gallons every single day. Sewage is the wastewater from inside people's homes, businesses, and corporations, including waste from sinks, toilets, showers, and washing machines. As California faces chronic drought conditions, water is becoming an increasingly limited resource. It is crucial to implement sustainable practices that recycle our water so that we have enough for agriculture and drinking.

So how do we prevent raw sewage from polluting the environment and instead turn it into usable water again? This question was one for environmental engineers to answer, and their solution was a four-stage centralized sewage treatment process.

The first stage, known as primary treatment, is the physical removal of solid objects from sewage. This is done through screens and grates in pipes that lead to large tanks, where solid waste settles at the bottom. Solid waste can include human waste, food particles, plastics, and other debris.

The second stage, secondary treatment, uses a biological process rather than physical separation. The sewage goes into an aeration tank where bacteria remove organic material through decomposition, producing carbon dioxide gas and inorganic sludge that settles at the bottom. These tanks are oxygenated to increase the rate at which bacteria decompose the organic matter. The leftover sludge, composed of biosolids, is then sent to methane digesters, which produce electricity from methane — adding another layer of sustainability to the whole process.

The third stage, tertiary treatment, uses ecological or chemical processes to remove nutrients still remaining in the water. Ammonia, nitrates, and phosphates are all removed during this stage.

In the fourth and final stage, disinfection, the treated wastewater is exposed to disinfectants such as chlorine, ozone, or UV light to kill pathogens before it is discharged. Although this water is not yet suitable for drinking, it can be used for cooling or irrigation, or returned to rivers and the ocean under strict regulatory standards.

More advanced processes can take this further, turning treated wastewater into actual drinking water through reverse osmosis. This process uses pressure to force wastewater through a set of plastic membranes with holes just slightly larger than an individual water molecule, allowing only water to pass through while salts, pharmaceuticals, pesticides, heavy metals, and pathogens are filtered out. Facilities like the Orange County Water District's Groundwater Replenishment System are already doing this at scale, making it a real and growing part of California's water future.

These filtration processes allow us to be more sustainable with our water usage and prevent sewage from polluting our land and oceans — and increasingly, they may be the key to keeping California's taps running.

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