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.
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Thanks, HelloFresh, and thank YOU for watching. Let me know what you think.