Sewage treatment | Aquatic and Terrestrial Pollution | AP Environmental Science | Khan Academy

This is my cat, Rubiks. One of the many amazing things about Rubiks is that he naturally works to keep himself clean. His barbed tongue is really good at getting rid of the dust and dirt that he gets in his fur every day, but sometimes he needs a little help getting clean.

Healthy lakes and rivers are kind of like cats. They naturally clean themselves. They have lots of spaces where contaminants from water can settle and to be filtered out by the soil and organisms like plants. The clean water produced by these systems is important for irrigation, drinking water, and the ecosystem.

But when humans add sewage and storm water pollution into rivers too quickly, natural ponds and streams can't keep up. The pollution can then build up and become a danger to wildlife and people. To prevent this from happening, many urban areas in developed countries use water management systems to treat their water waste.

Let's say I'm giving Rubiks a bath one day. When I unplug the drain, where will the water go? First, it will travel through a long network of sewer pipes, where it will be joined by water waste from homes and businesses and maybe some storm water from catch basins along roads.

Eventually, it will reach the water treatment plant, where it'll go under primary sewage treatment. Primary sewage treatment, which is also called physical sewage treatment, is where the treatment plant physically takes out large objects from the water. This prevents the objects from clogging up the pipes and pumps in future treatment steps.

During primary treatment, the water goes through a series of screens with progressively smaller and smaller openings. The screens sift out objects like sticks, rocks, rags, plastic bottles, and Rubik's hair. Even though the screens take out many of the larger objects, there's still a lot of grit in the water afterwards. Grit is made up of small loose particles of sand, gravel, coffee grounds, food waste, and other pieces of material.

To get rid of the grit, the treatment plant pumps the water through long narrow tanks that slow down the flow enough that the grit has time to settle to the bottom. All that thick soft mud that gathers at the bottom of the grit tank is called sludge, and it's collected and treated. The water then moves on to secondary or biological treatment.

During this stage, the wastewater is aerated or mixed with air to create perfect conditions to help increase the population of aerobic bacteria. Aerobic bacteria are important because they quickly feed on and break down the organic waste in the water. Sometimes bacteria-loaded sludge called activated sludge is also added to help grow even more bacteria.

When the bacteria have plenty of oxygen, they metabolize the organic material into carbon dioxide and yes, more sludge. The sludge, again, settles to the bottom of the tank and is collected for treatment. I like to think of secondary treatment as kind of like being a good caretaker to millions of microbial pets. When we provide our microscopic pets with oxygen and food, then they help us out by doing the cleaning work.

That's why secondary treatment is also called biological treatment because it uses living organisms. Like primary treatment, secondary treatment generates lots of sludge. Some of the sludge may be recycled and added to water entering secondary treatment. This increases the population of aerobic bacteria in the wastewater so the bacteria remove the organic waste faster.

The sludge collected from primary and secondary treatment can be dried and buried as a waste product, or dehydrated and heated to make a soil-like material called biosolids that can be used in agriculture. By the time the primary and secondary treatment steps are completed, about 85% of the organic pollution has been removed from the wastewater, but there are still some disease-causing bacteria and viruses that are present in the water.

So, most treatment plants add another step to secondary treatment: disinfection. Some treatment plants may use ultraviolet light treatment to prevent disease-carrying bacteria from being able to multiply, or treatment may use chlorine or ozone treatment processes to kill bacteria and viruses. Chlorination will kill more than 99% of the harmful bacteria.

In some places, the wastewater treatment plant directly releases cleaned water into drinking water sources, agricultural water sources, or recreational water bodies. In these cases, the water may go through yet another treatment process called tertiary treatment or advanced treatment. Tertiary treatment looks different at different water treatment plants.

Sometimes it uses a more targeted approach to the removal of specific wastewater components. Some examples include additional biological treatments capable of removing nitrogen and phosphorus or physical chemical separation techniques such as filtration, carbon absorption, distillation, and reverse osmosis for removing a variety of chemicals. Despite being effective, these techniques are expensive and not widely used.

After treatment, the newly cleaned water will go out into the world, perhaps to an ocean or to a river or maybe even into Rubik's next bath.