What is wastewater?

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If you just bought your first property with a septic tank, you are probably wondering about two things: what is wastewater and why do we bother treating it?

These basic questions are profoundly important, not only for your property, your drinking water, and your health, but also for the planet we must protect for future generations.

To answer them, let’s start with a definition. In simple terms, wastewater is any water that has been contaminated by humans, most often as a result of:

  • flushing toilets and urinals (this waste is known as blackwater)
  • bathing, showering, and washing clothes and dishes (greywater)
  • commercial and industrial activities

As you might expect, wastewater is composed almost entirely of water. The remaining portion – approximately 0.1% - includes organic matter, inorganic compounds, nutrients, and microorganisms.

Organic matter
fish underwater

Organic matter in wastewater includes proteins, carbohydrates, fats, oils, greases, and synthetic compounds found in certain detergents and emulsifiers.

Without proper treatment, these pollutants enter lakes and rivers and become a food source for various microorganisms. To consume this food, the microorganisms require dissolved oxygen. In watercourses with high concentrations of organic matter, this process of decay can reduce oxygen levels so dramatically that larger aquatic species suffocate and die.

Inorganic compounds
inorganic compounds

Inorganics in wastewater include compounds with sodium, potassium, magnesium, copper, lead, nickel, and zinc. In many cases, these pollutants are the results of commercial and industrial activities.

Unlike organic matter, inorganics are difficult or impossible for microorganisms to break down. If they enter lakes and rivers, they remain there, making the watercourse increasingly hazardous for humans and animals alike.


Nutrients in wastewater include phosphorus and nitrogen compounds that often originate in human waste and cleaning products like laundry detergent and dishwasher soap.

If nitrogen and phosphorus enter lakes and rivers, they contribute to a process called eutrophication. It begins with excessive algae growth that prevents sunlight from reaching plants below the water’s surface. As native plant species die, bacteria that feed on decaying plant matter multiply and consume more and more oxygen until the watercourse can no longer support life.


Certain microorganisms in wastewater are hugely beneficial because they break down organic matter that would otherwise pollute the environment.

Pathogens in untreated wastewater are a different story. If these bacteria, parasites, and viruses contaminate water resources, they can undermine public health by causing serious and sometimes deadly illnesses in humans.

Why do we treat wastewater?

A basic understanding of wastewater pollutants makes it easy to see why effective treatment is so important. Without it, we risk the things we care about most.

  • public health
  • groundwater and drinking water
  • lakes and rivers
  • fish and other aquatic species
  • livestock
  • property values


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