Austin Water Utility has two major wastewater treatment plants: Walnut Creek and South Austin Regional. The two major plants have total permitted capacity of 150 million gallons per day. 

The plants receive wastewater flow from Austin Water Utility's sanitary sewer collection system and treat it before returning it to the Colorado River. Austin Water Utility's biosolids facility, Hornsby Bend, receives sludge generated by the treatment processes at the wastewater plants and uses it to create compost for land application and public sales.

Austin's original wastewater treatment, Govalle Wastewater Treatment Plant was decommissioned October 2006.

Wastewater enters the headworks of a plant and flows through screens that remove large objects such as sticks and trash. From there it flows into grit basins in which heavier inorganic materials such as sand and small stones settle out. This material is transferred to a dump container along with the materials removed by screening. The wastewater then flows by gravity into the primary clarifiers, where a large portion of the waste settles to the bottom of the clarifiers and is pumped to sludge processing atHornsby Bend. The Hornsby Bend Biosolids Management Plant plays a key role in that effort by recycling Austin's biosolids and yard trimmings. Hornsby Bend receives the nutrient rich sewage solids reclaimed from the millions of gallons of wastewater that Austin's wastewater plants treat every day. All of Austin's sewage solids are pumped toHornsby Bend where they are treated to kill pathogens in the solids and the resulting "biosolids" are recycled.

Next, the wastewater flows to equalization basins from which measured amounts of the wastewater are pumped into aeration basins. There, activated sludge--essentially a large mass of beneficial microorganisms--consumes most of the remaining organic matter and produces more microorganisms. In secondary clarifiers, microorganisms grown in the aeration basins settle out and are returned to aeration basins. As the mass of microorganisms grows, a portion is removed and pumped to Hornsby Bend to be processed with the wastes from the primary clarifiers.

With more than 95 percent of wastes removed, the wastewater then passes to the chlorine basins for disinfection. To further reduce suspended solids, the wastewater flows through filters or another clarifier. Then a small amount of sulfur dioxide or sodium bisulfite is applied to remove the chlorine before the fully treated water is discharged to the Colorado River or reused through the City's Water Reclamation Program.