The Basics of Wastewater Treatment

How does a septic system work?

A septic system, also called an onsite wastewater treatment system, collects waste, treats the water, and then returns the treated water back into the ground. Although the process and science behind the treatment of wastewater can be rather complex, the process can be broken down into three stages: wastewater source, pre-treatment and final treatment.

What and how much is in the system determines how hard the system is going to work to process the wastewater. Residential systems are designed to process human waste. The introduction of other items like F.O.G. (fats, oils and grease), food, soaps, and detergents make the treatment system work harder or could cause the system to fail. If, for instance, there is too much bleach, it will kill the bacteria in the pretreatment, sending solid waste into the distribution field causing it to become clogged.


Never flush:

  • Cooking grease or oil

  • Non-flushable wipes, such as baby wipes or other wet wipes

  • Photographic solutions

  • Feminine hygiene products

  • Condoms

  • Dental floss

  • Diapers

  • Cigarette butts

  • Coffee grounds

  • Cat litter

  • Paper towels

  • Pharmaceuticals

  • Household chemicals like gasoline, oil, pesticides, antifreeze, and paint or paint thinners

The next step is the pre-treatment. This will most likely take place in a processing tank, trash tank or septic tank. This is where the solids will be stored allowing them to decompose. The solids will settle onto the button creating the sludge layer and fats, oils, and greases will rise to the top creating the scum layer. The clear liquid (effluent) in the middle will flow to the final treatment as water is introduced to the tank. The volume of the tank will always stay the same but the percent of solids and scum will gradually increase with time. Tanks should be pumped when the combination of sludge and scum reach 25%-33% combined.

The last step in the treatment process is the dispersal of the effluent into the environment. There are several ways to do this but the most common are gravity flow into a leach field or trenches. Here the effluent breaks down and slowly filters though the earth until it eventually is reintroduced into the ground water. This is why it is very important not to put pharmaceuticals and chemicals into a wastewater treatment system. For more information please visit: https://www.epa.gov/septic


You can also give us a call at (303) 720-0401. At Black Diamond Pumping, we are always more than happy to answer in any questions you have! We are the local experts in wastewater treatment in Colorado.