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We know there are better things to read about right? Sewage, waste removal and delivery of potable water are the cornerstones of modern plumbing so why should we neglect such an important aspect of the plumbing trade?

So in the following few pages we are going to give you the basics on sewage, waste, how it’s removed from the workplace and living space and how we keep the potable water supply safe from contamination. Before we get into the different types of sewage treatment systems, it’s probably a good time to get familiar with the different terminology used in waste water treatment.


Plumbing SewageSewage – Is any liquid waste containing animal, vegetable or chemical by-products in suspension or solution. It may include subsoil or subsurface waters, human waste including fecal matter. It is also the liquid and solid wastes conveyed away from homes, business or institutions combined with subsoil and storm drainage.

Solids in Solution – Solids in solution are dissolved in the liquid.

Solids in Suspension – Solids that are in suspension are solids not dissolved by the solution.

Organic Waste – Any waste material coming from something living. These organic wastes are composed of proteins, carbohydrates and fats.

Inorganic Waste – Is waste made of mineral matter like silica or dissolved minerals like iron, hydrogen sulfides, manganese, magnesium, etc.

Aerobic Bacteria – Bacteria living, active and occurring in the presence of oxygen.

Anaerobic Bacteria – Bacteria that lives and grows in the absence of free oxygen. These bacteria get their oxygen by decomposing substances that contain oxygen.

OK so we know that safe disposal of sewage is a huge concern for us here in the US and abroad so how is it done? Most buildings in the US carry sanitary wastes and water through drainage piping to a centralized or municipal treatment facility.

There are two different types of sewage treatment systems but they both have the same basic functions, to take sewage and make it harmless and non offensive.

  • Private Sewage Treatment System – A private sewage treatment system is any system not owned or operated by a municipality. Private sewage disposal systems can be as simple as a single tank septic system or as complex as a multi-stage.
  • Community or Municipal Sewage Treatment System – A municipal sewage treatment system is owned and operated by city, county, state or a private company responsible for safely treating waste water, removing bulk solids and the effluent water is placed back in the environment.

It’s All About the Sludge

Big or small, simple or complex, sewage treatment systems have processes common to all. Here are the six fairly common processes used for sewage treatment.

Screening or Bulk Filtering – This is  the process that removes large solids both organic and inorganic i.e. tree branches, heavy paper products, rags, etc. These are removed using metal screens. Sand, gravel, grit, ash etc. fall into a grit or detritus chamber.

Primary Sedimentation – After the screening takes place the sewage enters a sedimentation tank. During this process lighter solids like grease rise to the top of the tank and the heavier solids sink to the bottom creating two layers. The top is called the scum layer and the bottom is the sludge layer. Around 50% of all solids sink to the  bottom by gravity. Anaerobic bacteria begins to decompose the sludge layer breaking down the organic solids to stable, soluble substances and ammonia. The sludge must be removed periodically to maintain the tanks capacity for sedimentation.
Aeration –  OK so things aren’t done yet, there is still some bad stuff in whats left, and that stuff is organic and very susceptible to to aerobic bacteria. Aeration can occur naturally on land or in bacterial aeration beds. The process puts oxygen into the sewage and the process oxidizes the organic solids reducing them to water, CO2 gas and nitrogen. At the end of this stage the solids are reduced greatly, solids are about 90% broken down.

Final Settlement – This process again settles out the remaining solids in the effluent from the aeration tank. The stuff that is left behind is called the activated sludge, and it called that because it is literally active with a biologically rich culture of aerobic bacteria.

Filtration – This is a process to further reduce any pathogens from the sewage

Chlorination – This would be the final step in a very complex treatment system. These last two steps are almost never included in a private septic system. However if these two steps are included the effluent can be made potable which is kinda weird but possible.

Please stay tuned for further details on septic and sewage treatment systems.

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