⏳ Estimated Reading Time: 7 - 9 minutes

Describe best practices in sewage treatment.

Early Sewage Disposal

A sketch of the cross-section of a street and the underground sewer system beneath it. — **Early sewage systems involved waterways under the streets, becoming the sewers of today.**

Before the advent of sewage treatment plants (STPs) and processes, raw sewage from toilets, sinks, bathtubs, showers, and kitchens was dumped untreated directly into the nearest waterway. Raw sewage contains microorganisms that can cause diseases such as cholera, cryptosporidiosis, Escherichia coli (E. coli) diarrhea, encephalitis, gastroenteritis, giardiasis, hepatitis A, leptospirosis, poliomyelitis (polio), salmonellosis, shigellosis, and typhoid fever, among other dangerous diseases.

As we learned about microbes and diseases, one of the first areas we addressed was the deposition of raw sewage. Early systems collected raw sewage in small waterways, canals, and ditches and directed it away from public drinking water supplies. Later, these collection systems became the sewers used today in cities to collect sewage and direct it to sewage treatment plants. As a result of these sewage collection and treatment systems, deaths from diseases caused by microorganisms found in raw sewage dropped dramatically. Today, most developed countries and many developing countries have effective sewage treatment plants.

Explore the interactive map below to see how death rates decline as sanitation increases. Which countries have the highest death rates from unsafe sanitation? Which have the lowest?

Sewage Treatment Plants

Sewage treatment plants serve as an important barrier between human society and natural aquatic ecosystems. Prior to the development of such plants, millions of humans died from coming into contact with diseases found in contaminated wastewater discharged into rivers and streams. In poorer areas of the world, the lack of sewage treatment facilities still contributes to many human deaths annually.

Raw sewage enters the sewage treatment plant as wastewater, which is the total mixture collected from all drains. This is, believe it or not, 99.9% water and 0.1% waste. Despite this, it is still brown and very smelly, containing water from these sources:

🛀🏻 Greywater - wastewater from sinks, bathtubs, showers, dishwashers, and clothes washers
🚽 Blackwater - water from flushed toilets
🧼 Soaps and Detergents
🍔 Food Waste from dishwashers and garbage disposals
🧻 Toilet Paper
🧫 Organic Matter
🦠 Pathogens

Sewage is a threat to public health because it has pathogens in it, such as fecal coliform bacteria. Fecal coliform bacteria in itself is not usually toxic, but high levels of it indicate the presence of more toxic bacteria, so we use it as one of several standards for measuring water quality. Sewage also has high levels of inorganic nutrients, and this can cause algal blooms and eutrophication. Sludge is the solid component of waste that needs to be decomposed.

Sewage treatment is broken into several stages, depending on the design of the STP: preliminary treatment, primary treatment, secondary treatment, and sometimes a third stage called advanced or tertiary treatment.

Sewage Treatment Plant Process
Raw sewage enters the treatment plant, goes through several processes until clean water is discharged from the plant. — ***Sewage moves through several stages as it is cleaned.***

Explore the dropdowns below to learn more about each phase of the sewage treatment process. Have you ever visited a sewage treatment plant? How did it smell?

0️⃣ Preliminary Treatment (click to reveal)

Preliminary treatment may or may not be used in any given Sewage Treatment Plant. In fact, some STPs may only use primary treatment combined with disinfection.

The image depicts three steps of preliminary wastewater treatment: Raw Wastewater Inflow, the Screening Process, and the Grit Chamber. — ***Preliminary treatment removes the large debris from sewage.***

In preliminary treatment, incoming wastewater called influent enters the plant and debris such as limbs, toys, litter, and other non-liquids are screened out and hauled to the local landfill or waste-to-energy plant.

1️⃣ Primary Treatment (click to reveal)

A clarifier tank used in wastewater treatment. — ***Primary treatment allows heavy particles to sink and then they are removed.***

Primary treatment focuses on removing large particles from the wastewater.

In wastewater treatment plants that do not have preliminary treatment, debris flows through a bar screen, is trapped, and is manually raked from the screen. This removes the largest particles from the wastewater.

Then, the water moves to the grit chamber where large particles are allowed to settle out.

Next, primary clarifier tanks allow 30-50% of the suspended particles to settle and then they are removed. Secondary treatment works by adding oxygen to the wastewater that leaves primary treatment. The wastewater flows over a bed of rocks and percolates through them in a trickling-filter system.

2️⃣ Secondary Treatment (click to reveal)

Secondary treatment works by adding oxygen to the wastewater that leaves primary treatment. The wastewater flows over a bed of rocks and percolates through them in a trickling-filter system.

The stages of secondary wastewater treatment: aeration, secondary settling, activated sludge, and pasteurization. — ***Secondary treatment uses aerobic bacteria, settling tanks, and anaerobic bacteria to further clean organic matter from sewage.***

Aerobic microorganisms feed on the organic matter present in the sewage, removing it. As the microorganisms feed, they become heavier and clumped together, making them easier to remove. This aerated water flows to final settling tanks where the heavier microbes settle to the bottom to be removed. Some of them are reserved for use in the next round of secondary treatment.

Then, activated sludge, a mixture of detritus-feeding organisms, is added to the water. The activated sludge anaerobically works to decompose the organic waste still in the wastewater. After this, treated sludge (biosolids) are separated from the water, and the water is aerated. Mulch is added to the sludge and decomposers break down the sludge and mulch to a humus-like material. The water is pasteurized, or heated, to kill any pathogens left in the water.

In secondary treatment, the thickened sludge is sent to anaerobic digesters where anaerobic microorganisms digest the organic waste. After digestion, the sludge is dewatered and can be composted, sent out as fertilizer to land applications, or simply sent to a landfill unless another beneficial use is available.

A product of this digestion is methane gas, also called biogas. Biogas can be used to produce heat or electricity either for the STP or the local community. Not all STPs have this step as part of their process but using biogas can help defray the costs of operating the STP or benefit local citizens or businesses.

After this, treated sludge (biosolids) is separated from the water, thickened, and sent to a sludge processing facility.

3️⃣ Tertiary Treatment (click to reveal)

Because advanced or tertiary treatment is somewhat expensive, it’s not used at all STPs.

This stage of the sewage treatment process uses ecological or chemical processes to remove pollutants that are still in the water after the earlier stages of treatment.

Two common pollutants that are difficult to remove are nitrogen and phosphorus, both of which contribute to the eutrophication of waterways. Because of the presence of these nutrients, this stage is sometimes called biological nutrient removal.

The remaining water is then disinfected to kill any pathogens left in the water and sent to the nearest waterway. Disinfection processes include the addition of chlorine, ozone, or ultraviolet radiation, all of which kill pathogens.

In some cases, the disinfecting chemical is neutralized prior to the effluent being released into waterways to protect aquatic organisms.

Essential Knowledge

Primary treatment of sewage is the physical removal of large objects, often through the use of screens and grates, followed by the settling of solid waste in the bottom of a tank.

Secondary treatment is a biological process in which bacteria break down organic matter into carbon dioxide and inorganic sludge, which settles in the bottom of a tank. The tank is aerated to increase the rate at which the bacteria break down the organic matter.

Tertiary treatment is the use of ecological or chemical processes to remove any pollutants left in the water after primary and secondary treatment.

Prior to discharge, the treated water is exposed to one or more disinfectants (usually, chlorine, ozone, or UV light) to kill bacteria.

Discharge

A serene landscape with the text: “We all live downstream…”—a reminder of our interconnectedness within ecosystems. — **Almost everyone's waste treatment plant is downstream from someone else's.**

When we clean our sewage, we release it back into the environment. An effective sewage treatment plant (STP) will clean the water to a level that is often cleaner than the “fresh” water in our rivers and streams. Water that is this clean could be used as drinking water, or at the very least, used to water our lawns or wash our cars, but people, understandably, have an issue with drinking former wastewater.

Still, unless you live at the head of a river, it is likely that your community is downstream from someone’s wastewater treatment plant. Environmental scientists like to say,

“We all live downstream.”

Our sewage treatment plants have pretty strict regulations regarding the levels to which our water must be cleaned before it is released back into the environment. Other countries, however, do not have the same regulations regarding the release of sewage. In developing countries, sewage is often released straight back into the environment without being cleaned. These waterways can eventually run into the oceans, releasing sewage into the ocean.

Downstream control tries to rid water of the pollutant after it has entered the water. Sewage and drinking water treatment plant are both downstream methods of controlling water pollution. Drinking water treatment plants purify water before it is sent to humans to drink and sewage treatment plants clean water after it has been dirtied by humans and before it is emptied back into surface waters.

AP Exam Tip

You need to know what happens in each step of a sewage treatment plant.

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