How Sewage Wastewater Is Treated

Most of us take modern conveniences like electricity and indoor plumbing for granted. We flip a switch or turn the tap and assume that light will come on and water will run. When we flush the toilet or drain water from the sink, we never consider where that wastewater is going.

Whereas people used to throw their refuse out the window and wait for the rain and wind to carry it away, we eventually discovered that living in squalor contributes to the mass spread of diseases like dysentery, especially in crowded, urban settings. Over time, sewer systems emerged as a means of carrying human waste away from habitations.

Where does wastewater go, and what happens to it once you flush it away? If you’re interested in understanding this vital part of modern infrastructure, here are a few basics concerning wastewater treatment.

The Treatment Process

When wastewater goes down the drain or gets flushed away, it is carried away from your house through pipes to treatment facilities designed to remove a variety of contaminants through physical, biological, and chemical processing. The intended outcome is separating the sewage, or waste, from still-usable water, which is released back to the environment or used for secondary purposes like landscape irrigation for example.

Wastewater goes through several stages of treatment to separate out contaminants before it is deemed safe for use or ultimate disposal (i.e. released back to the environment). Here are the basic stages of treatment.

Screening and Pre-treatment

The initial treatment of wastewater includes filtering out the largest physical waste, such as non-human-waste items commonly flushed down drains. This could include sanitary items, diapers, toiletries that aren’t toilet paper (cotton balls, Q-tips, etc.), cleaning rags, toys, and whatever other odd items people purposely or accidentally flush.

Because these bulky items can damage finer filtration equipment, they must be removed before the real treatment process can begin. This stage also removes any heavier gravel and debris that is introduced to the sewage system from outside, such as through rainwater runoff.

Primary Treatment (Physical/Chemical Treatment)

During the primary stage of treatment, water can be treated with chemicals such as coagulants and/or polymers and placed in settlement tanks where solid human waste and other organic solid matter can sink to the bottom of the tank, forming a heavy sludge that is regularly swept away for further treatment. This stage also allows lighter waste products like fats and oils to rise to the surface, where they can also be skimmed away.

Secondary Treatment (Biological Treatment)

With solid waste removed, water is next moved to aeration tanks where air is pumped into the water. This process allows bacteria in the water the best opportunity to break down any remaining biological waste that was dissolved into the water, or that managed to escape primary treatment.

Tertiary and Final Treatment

The final treatment of wastewater is a two-step process. Once the water has been aerated, it goes again to a settlement tank so that the byproducts of bacterial treatment can settle to the bottom and be removed. The water then passes through a physical filter, such as a sand-filter designed to remove any remaining organic and inorganic contaminant particles. A sterilization step, such as chlorination or UV light, are commonly used at this stage for the purposes of disinfecting the water.


At this point water is considered treated. It can now be safely reintegrated back into a river, lake, lagoon, marsh, or another body of water. It can also be routed for land applications like watering public or private landscaping such as parks and golf courses.

Wastewater Facilities

Wastewater treatment is a complex process that requires dedicated space and specialized equipment, as well as proximity to both urban areas and drainage points (such as a river where clean water can be released following treatment). However, with the advent of new treatment technologies, these once large and potentially odorous facilities are now being built in and amongst densely populated urban areas. In fact, with treatment technologies where they are today, treated sewage water can be made cleaner than the water we drink.

Why Treat Wastewater?

In addition to keeping human habitation clean and free of hazardous contaminants, treating wastewater is essential to protecting the environment from the side effects of dumping human waste. In most cases, wastewater treatment merely speeds up the natural process of filtering and breaking down waste products, without the environmental consequences that a lack of treatment would cause.

More importantly, however, human additives like chemicals, hormones, and synthetic materials (like phthalates) can be removed before water is reintegrated into clean water supplies. This helps to ensure safer, cleaner water for people, animals, plants, and the environment at large.

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