What Happens When I Flush? A Story About Your Poop…

It isn’t magic, it is your water bill working for you…

When you wash, brush, or flush, the water leaves your home and flows mostly by gravity down pipes to one of our wastewater treatment plants.

There are over 4,600 miles of pipe used to collect the wastewater in the collection system. Utility crews clean pipelines to prevent blockages and spills. They also clear blocked pipes, maintain pumping stations, repair damaged pipes, and connect new customers. Our plant operators work 24/7 to protect public health and the environment.

What is wastewater, and where does it go?

Used water (wastewater) from inside homes and businesses flow through thousands of miles of pipes to one of five wastewater treatment plants. The used water includes detergents, food, paper, and industrial & human waste. The collection system includes pipes, manholes, and pumping stations.

But how does it all work? Below is a quick explanation:

Pumping Influent

Wastewater flows by gravity from homes and businesses to the wastewater treatment plant. It enters the plant at a low elevation, usually near a creek. The wastewater is pumped uphill to begin treatment. Gravity moves water through the treatment process.

Preliminary Treatment

Screens remove large objects such as bottles,  branches, wipes, and trash. Grit chambers remove grit, sand, sediment, and gravel.

A big part of the wastewater treatment process is to separate the solids from the liquids and the liquids from the solids…


Primary Treatment

Wastewater flows slowly through large tanks called primary clarifiers. This allows the heavier organic solids to settle down to the bottom of the tank. The settled material or primary sludge is pumped from the tank to a digester for further treatment.

Secondary (Biological) Treatment

Air bubbles are forced through the wastewater to encourage certain types of useful bacteria and microorganisms (microscopic single-cell organisms, think amoeba) to grow. These ‘bugs’ consume organic pollutants in the wastewater. They break pollutants (i.e. ammonia) into simpler forms (nitrates).

Final Clarification

The bacteria and microorganisms from the biological treatment phase are settled out of the wastewater in large tanks called final clarifiers. The bacteria/bugs are reused again in the treatment process…

Very similar to the primary clarifiers, the water moves slowly to allow the settling process to occur. Settled material or sludge that is pumped from the bottom of the clarifiers goes to digesters for further processing.


Even though the wastewater flowing from the final clarifiers appears to be clear, there may be very tiny particles remaining. The flow is passed through a bed of sand or fine mesh screens called an effluent filter. This filter removes these particles.


The processed wastewater (effluent) is disinfected with ultraviolet (UV) light to neutralize any remaining harmful microorganisms.

Cascade Returns Water to Creek

The final stage of treatment is disinfection and then the water flows over a cascade of steps to the creek. The cascade steps looks and acts like a large waterfall putting oxygen from the atmosphere back to the water. The water may have a foam look as it travels down the creek due to the oxygen.



Digesters are large tanks where the solids removed from the clarifiers are heated, mixed, and treated with biological processes to remove harmful bacteria, break down fats and oils, and overall reduce the volume of solids. 


The solids removed from the digesters still contain a large amount of water. This water is removed for treatment. The treated biosolids are collected and returned to the environment. The biosolids are nutrient-rich byproducts of wastewater treatment. Biosolids are land applied on agricultural or farmers’ fields to as soil amendments and a source of fertilizer. 

Wastewater treatment operators and our laboratory professionals test both the water and solids throughout the process.

Did you know…

  1. Our oldest plants were built in the 1920s.
  2. Part of designing for treatment plants includes ensuring space for expansion for generations.
  3. Staff can take parts of the wastewater plant offline for cleaning, repairs, and upgrades all while treating the constant flow.

Additional resources:

What does a growing city mean to wastewater treatment plants?

Why does wastewater cost more than water on my bill?

How does CLTWater respond to a wastewater overflow?

Is the odor from a wastewater plant or sewer manhole?