How Do We Pump Sewage?

This is the Crossness Pumping Station in London.  Its ornate architecture and elaborate ironwork   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.   In today’s episode, we’re talking  about sewage lift stations. This video is sponsored by HelloFresh,  America's #1 meal kit. More on that later. I have a video 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   video 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. Let me show you what I mean with  this demonstration I set up in my garage. This is a small submersible pump designed  for moving water out of basements or pools.   It’s nothing fancy, just an electric motor with  a small impeller inside a molded plastic case.   I took off the screen so we can  really see how much it can handle.   The submersible pump moves clear water just fine. Toilet paper is no issue even for this small pump.  It can break apart the paper with no problem.   It can even manage small suspended  solids like these plastic balls.   The particles stay in solution forming  a slurry similar to raw sewage.   However, larger solids in the stream can easily  clog the pump. Anything bigger than the spaces   in the impeller just won’t make it through. And,  clogging really becomes a problem with stringy   debris. These are quote-unquote “flushable”  wipes. They may make it past a toilet,   but you can easily see why these types of  wipes are a problem in a wastewater stream.   I really don’t feel like pouring a bunch of grease  into my demonstration here, but I’m sure you can   imagine how congealed fats and oils would only  exacerbate the problem of clogging this pump. It’s clear that 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. My   garbage disposal can handle all of the solids  I tested with the submersible pump with no   issues with clogging. The wipes gave it a little  trouble, but it was still able to chew them up. 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 video breaks that convention  to help you understand a little about   the challenges and solutions of managing  wastewater to keep your city clean and safe. It’s time for everyone’s favorite segment of   me trying to cook while my wife  tries to capture that on video. We are not expert chefs or foodies, but cooking is  one of our favorite ways to spend time together.   That’s why we’re thankful for HelloFresh, the  sponsor of this video, for converting cooking   from a chore into our favorite thing to do on  date night, or date afternoon in this case. Our little helper likes to open the box, and  he tried to quality-test the ingredients early   this time, but I promise they’re usually  washed before going into anyone’s mouth.   Then he’s off for nap time, so we  can goof around and cook lunch.   HelloFresh even has options for  20-minute meals, easy cleanup,   and low prep so you can take advantage  of life’s little windows for eating well. They also have options for family-friendly meals,  pescatarian, vegetarian and more, so it’s a really   nice way to jump start a change in your diet  if that’s one of your goals for the new year. The pre-portioned ingredients mean  there’s less prep and less food waste,   and the packaging is already recycled  content and most of it is recyclable as well. Go try it yourself at HelloFresh.com. If you use  code PRACTICAL16, you’ll get up to 16 free meals   plus 3 surprise gifts. Let us know in the  comments who rolled the better burrito.   That’s HelloFresh.com and use my code PRACTICAL16.   Thanks, HelloFresh, and thank YOU for  watching. Let me know what you think.

Channel: Practical Engineering

Views: 1,371,566

Keywords: Crossness Pumping Station, raw sewage, lift station, sewers, wastewater, pumping station, wet well, force main, Sewage pump, basket screen, grinder, air release valve, engineer, practical engineering, Grady

Id: eHAsuPVBwYM

Length: 12min 14sec (734 seconds)

Published: Tue Feb 01 2022

Reddit Comments

[u/ZestyData]

Ima level with you, while Practical Engineering is a good candidate for /r/mealtimevideos, not when it comes to sewage related videos.

[u/dholmestar]

This is the last topic I want to watch a video on while I'm eating

[u/Mad_Maps]

I didn’t see anything gross about this video. Very informative!

[u/drumstikka]

This ain't made for eatin'

[u/Proof-Injury-8668]

i used to do maintenance work on condos. The engineers built the their sewer below the cities. Lots of old people live there. Old people like to flush stuff that shouldn’t be flushed. Flushable wipes destroy the really expensive shredder pumps one needs to fix the problem. It would go out all the time. When it did it would entail getting roto router out to the property, at a nice low price of $3000 because they a have to do whats called a confined space entry to fix the pumps or switches that fry when they get plugged.

[u/Redditusername00001]

Practical engineering is probably my favorite YouTube channel.

[u/StopSendingSteamKeys]

/r/pooptimevideos