[Note that this article is a transcript of the video embedded above.]
The Crossness Pumping Station in London has ornate architecture and elaborate ironwork which belie its original, somewhat disgusting purpose: to lift raw sewage from London’s southern outfall, the lowest point in one of London’s biggest sewers, up to the ground surface where it could be discharged directly into the Thames River. Of course, we don’t typically release raw sewage into waterways anymore, and Crossness has long been decommissioned after newer treatment works were built in the 1950s. It’s now in the process of being restored as a museum you can visit to learn more about the fascinating combined history of human waste and Victorian engineering. But even though we have more sophisticated ways to treat wastewater before discharging it into streams and rivers, there’s one thing that hasn’t changed. We still use gravity as the primary way of getting waste to flow away from homes and businesses within the sewers belowground. And eventually, we need a way to bring that sewage back up to the surface of the earth. But that’s not as easy as it sounds. I’m Grady, and this is Practical Engineering. Today, we’re talking about sewage lift stations.
I have a post all about the engineering of sewers, and today we’re following that wastewater one more step on its smelly journey through a typical city. You can go check that out after this if you want to learn more, but I’ll summarize it quickly here. Most sewers flow by gravity from each home or business toward a wastewater treatment plant. They’re installed as pipes, but sewers usually flow only partly full like smelly water slides or underground creeks and rivers. This is convenient because we don’t have to pay a monthly gravity bill, and it almost never gets knocked out during a thunderstorm. It’s a free and consistent force that compels sewage downward. But, because Earth’s gravity only pulls in one direction, sewers must always slope, meaning they often end up well below the ground surface, especially toward their downstream ends. And that can be problematic. Here’s why.
Sewers are almost always installed in open excavations also known as trenches. This might seem obvious, but the deeper a trench must be dug, the more difficult, dangerous, disruptive, and ultimately expensive construction becomes. In some cases, it just stops being feasible to chase the slope of a sewer farther and farther below the ground surface. A good alternative is to install a pumping station that can lift raw sewage from its depths back closer to the surface. Lift stations can be small installations designed to handle a few apartment complexes or massive capital projects that pump significant portions of a city’s total wastewater flow. A typical lift station consists of a concrete chamber called a wet well. Sewage flows into the wet well by gravity, filling it over time. Once the sewage reaches a prescribed depth, a pump turns on, pushing the wastewater into a specialized sewer pipe called a force main. You always want to keep the liquid moving swiftly in pipes to avoid the solids settling out, so this intermittent operation makes sure that there are no slow trickles during off-peak hours. The sewage travels under pressure within the force main to an uphill manhole where it can continue its journey downward via gravity once again.
Another important location for lift stations is at the end of the line. Once wastewater reaches its final destination, there are no magical underground sewage outlets. Septic systems get rid of wastewater through leach fields that infiltrate the subsurface, but they’re designed for individual buildings and aren’t feasible on a city scale. That would require enormous areas of land to get so much liquid to soak into the soil, not to mention the potential for contamination of the groundwater. Ignoring, for now, the fact that we need to clean it up first, we still need somewhere for our sewage to go. In most cases, that’s a creek, river, or the ocean, meaning we need to lift that sewage up to the surface of the earth one last time. Rather than build wastewater treatment plants in underground lairs like stinky superheroes so we only pump clean water, it’s much easier just to lift the raw sewage up to the surface to be treated and eventually discharged. That means we have to send some pretty gross stuff (sewage) through some pretty expensive and sophisticated pieces of machinery (the pumps), and that comes with some challenges.
We often think of sewage as its grossest constituents: human excrement, you know, poop. But, sewage is a slurry of liquids and solids from a wide variety of sources. Lots of stuff ends up in our wastewater stream, including soil, soap, hair, food, wipes, grease, and trash. These things may make it down the toilet or sink drain and through the plumbing in your house, but in the sewer system, they can conglomerate into large balls of grease, rags, and other debris (sometimes called “pig tails” or “fatbergs” by wastewater professionals). In addition, with many cities putting efforts into conserving water, the concentration of solids in wastewater is trending upward. Conventional pumps handle liquids just fine but adding solids in the stream increases the challenge of lifting raw sewage.
Appropriately sized centrifugal pumps can handle certain types and sizes of suspended solids just fine. Sewage pumps are designed for the extra wear and tear. The impellers have fewer vanes to avoid snags and the openings are larger so that solids can freely move through them. Different manufacturers have proprietary designs to minimize obstructions to the extent possible, but no sewage pump is clog-proof. Especially with today’s concentrated wastewater full of wipes that have been marketed as flushable, clogs in lift stations can be a daily occurrence. Removing a pump, clearing it of debris, and replacing it is a dirty and expensive job (especially if you have to do it frequently). Most lift stations have an alarm when the level gets too high, but if a clog doesn’t get cleared fast enough, raw sewage can back up into houses and businesses or overflow the wet well, potentially exposing humans and wildlife to dangerous biohazards.
A seemingly obvious solution to the problem of clogging is to use a screen in the lift station wet well to prevent trash from reaching the pumps. But, screens have a limitation: they can clog up too. By adding a screen, you’ve traded pump maintenance for another kind of maintenance: removing and hauling away debris. Smaller lift stations with bar or basket screens can get away with maybe a once-a-week visit from a crew to clean them. Larger pump stations often feature automatic systems that can remove solids from the screen into a dumpster that can be hauled to a landfill every so often.
Sometimes using a screen is an effective way to protect against clogging, but it’s not always convenient, especially because it creates a separate waste stream to manage. For example, if a lift station is remote where it’s inconvenient to send crews for service and maintenance, you might prefer that all the solids remain in the wastewater stream. After all, treatment plants are specifically designed to clean wastewater. They have better equipment and consistent staffing, so it often just makes sense to focus the investments of time and effort at the plant rather than individual lift stations along the way. In these cases, there’s another option for minimizing pump clogs: grinding the solids into smaller pieces.
There’s a nice equivalent to a lift station grinder that can be found under the sinks of many North American homes: the garbage disposal. This common household appliance saves you the trouble and smell of putting food scraps into the wastebasket. It works like a centrifugal pump with a spinning impeller, but it also features a grinding ring consisting of sharp blades and small openings. As the impeller spins the solids, they scrape against the grinding ring, shearing into smaller pieces that can travel through the waste plumbing.
Some lift stations feature grinding pumps that are remarkably similar to under-sink garbage disposals. Others use standalone grinders that simply chew up the solids before they reach the pumps. Grinders are often required at medical facilities and prisons where fibrous solids are more likely to find their way into the wastewater stream. Large grinders are also used where storm drains and sewers are combined because those systems see heavier debris loads from rainwater runoff. A grinder is another expensive piece of equipment to purchase and maintain at a lift station, but it can offer better reliability, fewer clogs, and thus decreased maintenance costs.
Of course, clogging is not the only practical challenge of operating a sewage lift station. When you depend on electromechanical equipment to provide an essential service, you always have to plan for things to go wrong. Lift stations usually feature multiple pumps so that they can continue operating if one fails. They often have backup generators so that sewage can continue to flow even if grid power is lost. Another issue with lift stations is air bubbles getting into force mains and constricting the flow. Automatic air release valves can purge force mains of these bubbles, but venting sewer gas into populated areas isn’t usually a popular prospect. Although our urban lives depend on sewers to carry waste away before it can endanger public health, reminders that they exist are usually unwelcome. Hopefully this breaks that convention to help you understand a little about the challenges and solutions of managing wastewater to keep your city clean and safe.
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Source: The Paradise News