How is sewage transformed into drinkable water?

NARRATOR: The average person flushes the toilet 2,500 times a year, sending approximately one to three and a half gallons of water and waste down the drain with every flush. We'd rather not think about it, but the process of how sewage is transformed back into drinking water is one of the great underappreciated marvels of modern life. But how does all this happen? Gather around-- in this ChemMatters video we're going to tell you the story of sewage-- a substance that we'd rather ignore but one that resurfaces in unexpected ways.

If it comes from homes, sewage is a mix of water from sinks, showers, toilets, washing machines, and dishwashers. Also called wastewater, this unpleasant mix of substances contains human waste, food, grease, soap, and other organic matter. Unsurprisingly, sewage is full of bacteria and viruses. But amazingly, all of these substances are removed from sewage in wastewater treatment plants. To get more acquainted with this process, we visited Blue Plains Advanced Wastewater Treatment Plant in Washington DC, which treats a whopping 370 million gallons of sewage every day.

Let's look at the four main stages of water treatment to see how wastewater goes from polluted to pure. In order, the stages are physical treatment, biological treatment, filtration, and disinfection. In the physical treatment stage, various materials are physically removed from wastewater. First, wastewater passes through special bar screens which catch large debris, like branches or rocks. Next, smaller pieces of materials-- such as sand and silt, stones, and facial tissues-- are trapped in grit chambers.

The wastewater then flows to settling tanks where particles denser than water sink to the bottom, while particles that are less dense than water form a layer at the top. This top layer, called liquid grease, is a mixture of oils, waxes, and soaps. The liquid grease is skimmed off and sent to a landfill. The heavier, sunken material is collected and treated to form a sludge called biosolids that is later used as fertilizer. Every day 65 trucks haul the biosolids away from Blue Plains to fertilize the soil in Maryland.

The next step is biological treatment. While we often think about bacteria as organisms that might cause harm, bacteria are actually used in the wastewater treatment process to remove contaminants from water. The darker reddish color you see here in Blue Plains' biological treatment tanks come from the bacteria used in this process. One of their most crucial roles is to convert nitrogen-containing compounds into harmless nitrogen gas, which can then bubble out of the water into the atmosphere. This process occurs in two stages-- nitrification and denitrification.

One of the most common nitrogen-containing compounds in wastewater is ammonia. See the bubbles in these nitrification tanks? That's the oxygen that is added to help the nitrifying bacteria convert ammonia into nitrate ions. Then the bacteria use some more oxygen to convert the nitrite ions into nitrate ions. Next denitrifying bacteria convert the nitrate ions into nitrogen. This is a reaction that requires another substance to be oxidized as the nitrate is reduced.

Methanol is one of the best choices for such a substance because it's usually present in wastewater. Nitrate is reduced to nitrogen according to the following reaction. Methanol is oxidized to carbon dioxide as follows. These reactions occur together in what chemists call an oxidation reduction reaction, or redox reaction. At the end of this process, the bacteria sink to the bottom, leaving clear water at the top.

The third step in wastewater treatment is filtration. After biological treatment, inorganic salts, like sodium chloride and sodium sulfate, remain in the water. These salts can be removed using filtration techniques such as reverse osmosis. Osmosis is a process in which water moves through a semi-permeable membrane that allows water, but not the salts that it contains, to flow from an area of low-solute concentration to an area of high-solute concentration. This process doesn't require any energy. Reverse osmosis, on the other hand, uses pressure to force water to move in the opposite way.

Here we have a cross-section of a typical purifying tube used in the filtration process. The working part of the tube is a very thin semi-permeable membrane that only allows a small amount of water to pass through. Because only water can go through the membrane, the liquid on the other side is pure water. On the opposite side, the solution becomes even more contaminated as the salt content rises.

The final step is disinfection, where the filtered water is treated with chlorine or other compounds that kill disease-carrying bacteria. The final product is probably cleaner than most tap water and as safe as the safest bottled product. But does this sanitized sewer water feed directly into our homes? Actually, no. Treatment plants throughout the United States often empty fully treated water into lakes, rivers, and streams so that it enters the water cycle. It evaporates in the atmosphere and ultimately condenses and falls as rain.

Here is the site where Blue Plains treated water is released into the Potomac River in Washington DC. This water percolates into the ground to enhance the water supply. Months later, it works through the soil and becomes actual drinking water again. Although we often take it for granted, wastewater treatment plays a crucial role in providing us clean drinking water. According to the US Environmental Protection Agency, every major body of water in the United States contains some purified wastewater. For more, here's Akile Testaye, Director of Wastewater Treatment at Blue Plains.

AKILE TESTAYE: It give us the opportunity to constantly be protecting the environment, which is exactly what we're doing. It is very dynamic work in here, process in here. It's challenging in a way. It's not as simple as what sort of comes in and is discharged-- there's a lot to it. There's a lot of challenge. And so that part is what is very interesting in the job.