[Note that this article is a transcript of the video embedded above.]

This is the Stickney Water Reclamation Plant in Chicago, the largest wastewater treatment plant in the world. It serves more than two million people in the heart of the Windy City, converting all their showers, flushes, and dirty dishwater, plus the waste from countless commercial and industrial processes into water safe enough to discharge into the adjacent canal which flows eventually into the Mississippi River. It all adds up to around 700 million gallons or two-and-a-half billion liters of sewage each day, and the plant can handle nearly double that volume on peak days. That’s a lot of olympic sized swimming pools, and in fact, the aeration tanks used to biologically treat all that sewage almost look like something you might do a lap or two in (even though there are quite a few reasons you shouldn’t). However, flanking those big rectangular basins are rows of circular ponds and smaller rectangular basins that have a simple but crucial responsibility in the process of treating wastewater. We often use chemicals, filters, and even gigantic colonies of bacteria to clean sewage on such a massive scale, but the first line of defense in the fight against dirty water is usually just gravity. I’m Grady, and this is Practical Engineering. In today’s episode, we’re talking about settlement for water and wastewater treatment.

This video is part of a series on municipal wastewater handling and treatment. Rather than put out a single video overview of treatment plants (which many other channels have already masterfully done), we’re taking a deep dive into a few of the most interesting parts of converting sewage into clean water. Check out the wastewater playlist linked in the card above if you want to learn more.

The job of cleaning water contaminated by grit, grime, and other pollutants is really a job of separation. Water gets along with nearly every substance on earth. That’s why it’s so useful for cleaning and a major part of why it does such a good job carrying our wastes away from homes and businesses in sewers. But once it reaches a wastewater treatment plant, we need to find a way to separate the water from its inhabitant wastes so it can be reused or discharged back into the environment. Some contaminants chemically dissolve into the water and are difficult to remove at municipal scales. Others are merely suspended in the swift and turbulent flow and will readily settle out if given a moment of tranquility. That’s the trick that wastewater treatment engineers use as the first step in cleaning wastewater.

Once it passes through a screen to filter out sticks and rags, sewage entering a wastewater treatment plant’s first step, or primary treatment, is the simple process of slowing the wastewater down to allow time for suspended solids to settle out. How do you create such placid conditions from a constant stream of wastewater? You can’t tell people to stop flushing or showering to slow down the flow. Velocity and volumetric flow are related by a single parameter: the cross-sectional area. If you increase this area without changing the flow, the velocity goes down as a result. Basins used for sedimentation are essentially just enormous expansion fittings on the end of the pipe, dramatically increasing the area of flow so the velocity falls to nearly zero. But just because the sewage stream is now still and serene doesn’t mean impurities and contaminants instantly fall to the bottom. You’ve got to give them time.

How much time is a pretty important question if you’re an engineer because it affects the overall size of the basin, and thus it affects the cost. Particles falling out of suspension quickly reach a terminal velocity, just like a skydiver falling from a plane. That maximum speed is largely a function of each particle’s size, and I have a demonstration here in my garage to show you how that works. I think it’s intuitive that larger particles fall through a liquid faster than smaller ones. Compare me dropping a pebble to a handful of sand. The pebble reaches the bottom in an instant, while the smaller particles of sand settle out more slowly. Wastewater contains a distribution of particles from very small to quite large, and ideally we want to get rid of them all.

As an example, I have two colors of sand here. I sifted the white sand through a fine mesh, discarding the smaller particles and keeping the large ones. I sifted the black sand through the same mesh, this time keeping the fine particles and discarding the ones retained by the sieve. After that, I combined both sands to create a gray mixture, and we’ll see what happens when we put it into a column of water. This length of pipe is full of clean water, and I’m turning it over so the mixture is at the top. Watch what happens as the sand settles to the bottom of the pipe. You can see that, on the whole, the white sand reaches the bottom faster, while the black sand takes longer to settle. The two fractions that were previously blended together separate themselves again just from falling through a column of water.

Of course, physicists have used sophisticated fluid dynamics with partial differential equations to work out the ideal settling velocity of any size of spherical particle in a perfectly still column of water based on streamlines, viscosity, gravity, and drag forces. But, we civil engineers usually just drop them in the water and time how quickly they fall. After all, there’s hardly anything ideal about a wastewater treatment plant. As water moves through a sedimentation basin and individual particles fall downward out of suspension, they take paths like the ones shown here. Based on this diagram, you would assume that depth of the basin would be a key factor in whether or not a particle reaches the bottom or passes through to the other side. Let me show you why settling basins defy your intuitions with just a tiny bit of algebra.

You’ve got a particle coming in on the left side of the basin. It has a vertical velocity - that’s how fast it settles - and a horizontal velocity - that’s how fast the water’s moving through the basin. If the time it takes to fall the distance D to the bottom is shorter than the time it takes to travel the length L of the basin, the particle will be removed from the flow. Otherwise it will stay in suspension past the settling basin. That’s what we don’t want. As I mentioned, the speed of the water is the flow rate divided by the cross sectional flow area - that’s the basin’s width times its depth. Since both the time it takes for a particle to travel the length of the basin and the time it takes to settle to its bottom are a function of the basin’s depth, that term cancels out, and you’re left with only the basin's length times width (in other words, its surface area). That’s how we measure the efficiency of a sedimentation basin. Divide the flow rate coming in by the surface area, and you get a speed that we call the overflow or surface loading rate. All the particles that settle faster than the overflow rate will be retained by the sedimentation basin, regardless of its depth.

Settlement is a cheap and efficient way to remove a large percentage of contaminants from wastewater, but it can’t remove them all. There are a lot more steps that follow in a typical wastewater treatment plant, but in addition to being the first step of the process, settlement is also usually the last one as well. Those circular ponds at the Stickney plant in Chicago are clarifiers used to settle and collect the colonies of bacteria used in the secondary treatment process. Clarifiers are just settlement basins with mechanisms to automatically collect the solids as they fall to the bottom. The water from the secondary treatment process, called mixed liquor, flows up through the center of the clarifier and slowly makes its way to the outer perimeter, dropping particles that form a layer of sludge at the bottom. The clarified water passes over a weir so that only a thin layer farthest from the sludge can exit the basin. A scraper pushes the sludge down the sloped bottom of the clarifier into a hopper where it can be collected for disposal.

Settlement isn’t only used for wastewater treatment. Many cities use rivers and lakes as sources of fresh drinking water, and these surface sources are more vulnerable to contamination than groundwater. So, they go through a water purification plant before being distributed to customers. Raw surface water contains suspended particles of various materials that give water a murky appearance (called turbidity) and can harbor dangerous microorganisms. The first step in most drinking water treatment plants is to remove these suspended particles from the water. But unlike the larger solids in wastewater, suspended particles creating turbidity in surface water don’t readily settle out. Because of this, most treatment plants use chemistry to speed up the process, and I have a little demo of that set up here in the studio.

I have two bottles full of water that I’ve vigorously mixed with dirt from my backyard. One will serve as the control, and the other as a demonstration. The reason these tiny soil particles remain suspended without settling is that they carry an electrical charge. Therefore, each particle repels its neighbors, fighting the force of gravity, and preventing them from getting too close to one another. Chemical coagulants neutralize the electric charges so fine particles no longer repel one another. Additional chemicals called flocculants bond the particles together into clumps called flocs. As the flocs of suspended particles grow, they eventually become heavy enough to settle out, leaving clarified water at the top of the bottle. Treatment plants usually do this in two steps, but the pool cleaner I’m using in the demo does both at once. It’s a pretty dramatic difference if you ask me. In a clarifier, this sludge at the bottom would be pumped to a digester or some other solids handling process, and the clear water would move on to filtration and disinfection before being pumped into the distribution system of the city.

Our ability to clean both drinking water and wastewater at the scale of an entire city is one of the most important developments in public health. Sedimentation is used not only in water treatment plants but also ahead of pumping stations to protect the pumps and pipes against damage, with canals to keep them from silting, in fish hatcheries, mining, farming, and a whole host of other processes that create or rely on dirty water. The science of settlement and sedimentation is something that impacts our lives in a significant way and hopefully learning a little bit about it helps you recognize the brilliant engineering keeping our water safe.