Septic System 101

Watch the video to see how a septic systems work.  Keep an eye out for the baby!


How Septic Systems Work

When you flush your toilet, take a shower, or rinse food down the sink, you are creating wastewater. Wastewater is made up of two very different components: liquids and solids.

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Septic holding tank.

The solids are captured in your septic system’s “holding tank.”  The holding tank is a big concrete box where your wastewater sits quietly for a while so that solids can sink to the bottom or float to the top. The solids are then stored or “held” in the holding tank until they can be removed by a septic system contractor.

While the solids are being held, bacteria break-down the waste and reduce its volume. The solids in your holding tank shrink as they break down, but they never disappear. Eventually, the solids have to be cleaned-out.

Once the solids have separated from the liquids in the holding tank, the liquids flow out of the tank to the “leaching field.” The leaching field spreads the liquids over a large underground area. Beneficial bacteria in the soil digest the pollutants in the liquid, and then the purified water percolates through the ground to join your community’s groundwater.

Septic vs. Sewer. What’s the difference?

  • Septic systems treat wastewater, then recycle it locally, into the ground, where it helps to sustain ponds, streams, wetlands and wildlife.
  • Sewers send wastewater to a facility to be processed, which might be many miles away from the homeowner’s property.  In the Neponset River Watershed, sewer systems lead to the Deer Island Treatment Plant, where it’s processed and then released into Boston Harbor.

Contrary to what many people think, septic systems are better for the environment than sewers.  Water-testing data shows that most water pollution problems in the Neponset Valley are caused by sewer systems. 


Septic systems, like all mechanical things, require maintenance.

Leaking and poorly maintained septic systems release nutrients and pathogens (bacteria and viruses) that can be picked up by stormwater and discharged into nearby waterways. Pathogens can cause public health problems and environmental concerns. Learn how your septic system works to eliminate stormwater pollution, and to avoid costly clean outs and repairs.

For septic system owners, follow a few key steps to keep your system working properly. A healthy septic system saves you money and helps to keep our groundwater healthy!


Click here to learn how to maintain your septic system.