Original subtitles

This time, we reveal the relentless struggle

to keep water flowing to our cities.

One of these pops can fill up

an Olympic sized swimming pool in 20 seconds.

The hidden army working to keep our water safe.

You've got a less than 15 minutes lifespan

once you enter into this water.

And the future of our water waste.

People flush their toilet

and it turns into fuel for cars and buses.

Half the world's population

live in urban areas and the number is rising every second.

Each day is a race against time

to house, feed, water and move more than 3.5 billion people.

It's an impossible daily challenge.

The slightest glitch can bring a city to its knees

and every 24 hours, it has to be done all over again.

This is the story of how cities work.

(upbeat music)

For centuries, cities have been built

near a fresh water supply.

Without it, we'd be lucky to live three days.

But this most basic human need can be deadly too.

Get the supply wrong and it can poison us.

Get too close and it can drown us.

Water,

we need it and must respect it.

With every passing year,

the challenge of providing water to billions of people

becomes harder and harder.

Nowhere is this more apparent than the city of Los Angeles.

(dramatic music)

This is the key to LA survival.

The Hoover Dam on the mighty Colorado River.

It is a whole state away,

but the reservoir it controls, Lake Mead,

is a critical supplier of drinking water to the city.

The route to LA runs 250 kilometers

downstream to the Colorado river aqueduct.

Greg Hare is one of the pilots charged

by the authorities in Southern California

with keeping this precious structure safe.

When I'm doing security patrols,

I'm mainly looking for people

who aren't supposed to be there.

If I see something that I believe is suspicious,

I will call our dispatch office and have them

take the appropriate action.

(tension music)

This manmade aqueduct

provides Los Angeles and Southern California

with approximately 4 billion liters of water a day.

Because of its importance to the state,

surveillance was heightened after the 9/11 attacks.

Our special security patrols

are tied to whatever the terrorist alert level is.

But it's a huge challenge.

The Colorado river aqueduct is a 389 kilometer network

of tunnels, canals and pumping plants.

It's sheer size makes it virtually impossible

to protect efficiently from the ground.

We patrol the aqueduct with the airplane

from the air.

The reason we do that

is because we can cover the entire length of the aqueduct.

But Greg's aerial patrol

allows him to police the remotest points

of the aqueducts route where few ground patrols

are able to reach.

Desolate areas where people aren't around to watch it,

you could have something going on out there

that you wouldn't know about unless you could fly out there

and take a look at it real quick.

People aren't the only threat.

Greg and his team run constant checks

to scan the waterway for signs of damage.

(suspenseful music)

The aqueduct must carry the city's water

across no less than three hostile mountain ranges.

In the 70 years since it was built,

the population of California has risen by more than 500%.

The aqueduct is a backbone of our water supply system.

It's been the most reliable

delivery of water for the last 70 years

for Southern California.

The challenge was to get water from the Colorado river,

some 242 miles, into Southern California

over several mountain ranges in a very dry and hot desert.

Don Nash oversees all five pumping stations

along the aqueduct, including the Julian Hind's Plant.

We're at Hind's Pump Plant.

This is the last lift on the Colorado River aqueduct.

It's taken about 25 hours for the water to get here

from the Colorado River.

This station is the last and most powerful

of the pumping stations.

Each day, it must defy gravity

to lift over 4 million tons of water

over 130 meters to the top of Eagle mountain.

And this is the last 441 feet it takes

to overcome the last mountain range

to get us into Southern California.

The power needed

to lift the water is staggering.

There's one pump of nine that we have here.

One of these pumps can fill up an Olympic sized swimming

pool in 20 seconds.

It's actually discharging right here underneath our feet

and going up the hill some 441 feet to the top of the hill.

Don's 912,500 horsepower pumps

lift water over the mountains,

but there's another natural threat

waiting just on the other side.

(tense music)

This is Lake Mathews,

the final reservoir on the Colorado River aqueduct

where the city's water is stored

before flowing into LA for treatment.

Storing vast quantities of water in the middle of the desert

produces unexpected challenges.

The lake is a breeding ground for tiny intruders.

The advantage of having water in a lake

is that you can store it.

The disadvantage is that now it's a large enough reservoir

that is going to behave as a lake.

Dr. Rick DeLeon and his team of divers

are fighting an ongoing battle

against the mighty army of small,

but persistent quagga mussels.

These mussels can actually grow to very high densities.

100,000 per square meter is not uncommon.

They have been documented to grow up to 700,000

per square meter.

The mussels have no direct effect

on the quality of the water,

but if left unmonitored,

they could block supply pipes to the city

with potentially devastating consequences.

And when the mussels colonize,

they can actually close that space

and not allow much water to go through

and put a strain on the pumps.

So they need to be removed

to allow the water to go through uninterrupted.

(suspenseful music)

Dr. DeLeon and his team

place test plates in the lake to monitor

the mussels growth rates.

Today, divers Steve and Kelly,

are retrieving one that's three months old.

(tense music)

Steve and Kelly, are the quaggas growing just on

just rocks or are they also growing on the silt?

Just on the,

just on the rocks at this point.

I think (indistinct).

After a 30 minute dive,

Steve rises to the surface with the test plate.

It is completely covered in quagga mussels.

That's surprising how

much they're still growing in there.

What depth was this?

This was 53 feet. 53.

So deeper than we thought.

The test plate reveals

a steady increase in the number of quaggas.

One of the reasons why we monitor is to see

is there frequency of which they need to go in there

and clean up.

For Dr. DeLeon,

this means that not long from now

a full-scale unblocking operation will be needed

to keep water flowing to Los Angeles.

For us, it's really a matter of control over time

to reduce the impact that it can have on us

and prevent them from interfering

with our normal operations.

Humans cannot survive without a constant supply

of drinking water.

In the next 20 years,

our cities will grow to such a point

that demand will outstrip supply

from rivers and lakes by 40%.

For cities like Barcelona,

that means a constant battle against drought.

The impact is already being felt by Aigues de Llobregat,

one of the city's main drinking water suppliers.

We have not enough water to supply all these people.

In the last five years,

Barcelona has turned to a radical new source

that could solve supply problems forever,

seawater.

But seawater has a higher salt content

than the human body can process.

Drinking large quantities could result in death.

In 2009, the city built the biggest desalination plant

of its kind in Europe on Catalonia's shoreline.

Overseen by plant director,

Carlos Miguel Centeno.

(Carlos speaks in a foreign language)

This brightly colored super hub of pipes and pressure pumps

is equipped to bring an average of 200 million liters

of sea water into the plant a day.

To make it drinkable,

it's killer salt content needs to be removed.

This desalination happens

through a process called reverse osmosis

and takes place inside these blue tubes.

Seawater is pumped through a coiled membrane

fine enough to filter out the salt molecules in the water.

It allows pure water

to flow through to the next stage of its treatment.

And just minutes into the day shift,

engineers have found a problem with a pressure pump.

Carlos Miguel must act fast.

A faulty pump could seriously impact

Barcelona's water supply.

(Carlos speaks in a foreign language)

Seawater can't keep flowing in

during the membrane change

so the control room shuts down a section of the plant.

This is not good news for Carlos Miguel

as less water is now available to the city.

Changing membranes is no easy task.

These seven heavy cylinders

must be perfectly placed in their tube

for their power and pressure connectors to work.

The team add a coat of glycerin

to ensure they slide in smoothly,

and apply some heavyweight muscle.

Once the tube is tightly sealed,

the pump can startup again,

bringing supply back to full capacity.

Pumping seawater through an army of over 17,000 membranes

is not enough to guarantee a safe drinking supply.

The city's water must go through several purification stages

before it's fit for consumption.

And it's here in the plant's laboratory

where critical testing takes place

before releasing drinking water into the city.

(Carlos speaks in a foreign language)

Biochemist Maria (indistinct)

leads a team of scientists

whose job it is to test samples

from every step of the process.

After its journey from the sea and a battle against time,

this water is now safe for the city to drink.

Desalination is important for Barcelona

because with this new supply of water,

we have 20% of the water that Barcelona needs.

Barcelona's use of the sea

could be at the forefront of a water revolution,

a way to guarantee future supply

for the world's growing city populations.

(suspenseful music)

Cities don't just depend on water for drinking.

Waterways provide a vital transport link for moving goods

in and out of our cities.

London Gateway.

Today engineers are taking delivery of a fleet of cranes

among the biggest in the world.

They're approaching British shores

after a six week journey from Shanghai.

This marks a turning point for London.

These key cranes are capable of unloading

some of the largest cargo ships in the world

at unprecedented speed.

Engineer, Russ Broughton is supervising the unloading.

It's a really big day

in the life of London's nearest port.

Yeah, we're all very excited about it here.

The UK is an island nation

and 90% of our goods come in by sea

and it's nothing new to have our ports in London.

It's what the Romans first did 2000 years ago.

It may be the same waterway

but London is now a capital city

with millions of inhabitants.

The key point is that this is the biggest consumer market

in the UK.

This is Europe's largest economic area.

So we're once again,

gonna bring the biggest ships in the world

to the biggest point of consumption in the UK.

To do that, we need some big pieces of kit.

Each crane is 138 meters tall,

taller than the London Eye.

It's the culmination of years of planning

and millions of pounds spent,

but Russ and his team are worried.

There's no crane in the world

big enough to lift each 2000 ton steel structure

onto dry land.

The only way to unload them

is by dragging them off the ship.

This can only happen at high tide

when the deck is level with the quayside.

The tide is flowing pretty quick at the moment.

Then we've got one hour

to get this crane back onto the land.

It's 2000 ton, we're on a moving vessel

and if we don't do it in that hour

then we ended up with 2000 ton of metal

sticking out at the Thames

and I'm not sure how we're going to get that out.

The ebb and flow of the Thames tide

can change water levels by a massive seven meters.

Russ and the team only have one shot to get it right.

To roll the crane off the ship,

engineers must lay down

four industrial sized bridging tracks.

The vessel and the dock must be perfectly aligned

for the crane to slide off.

The tracks must be fitted in a matter of minutes,

but the team can't begin to put them into position

until the ship and the quayside are almost level.

This is it now.

This is the last couple of minutes

to just put the final bridge in to position.

Any second now, this crane's gonna start rolling off.

The rails are aligned,

but the worst is not over.

Yeah, this is it.

the ropes are all gone tight now

and you can see the crane starting to move.

This is the bit that we've all been waiting for

and it's do or die.

They must make this work or risk costly delay.

We're now at high tide.

We've got about 45 minutes to an hour

to get this off safely onto the land.

There's another complication Russ can't control.

So he relies on engineers aboard the vessel.

As the crane is unloaded,

its weight threatens to destabilize the ship.

To keep it level,

engineers must pump thousands of liters of water

into the ballast tanks on the other side of the ship.

If they get it wrong,

the crane could slide off the ship and into the Thames.

On the quayside,

engineers have wired in a network of winches, pulleys

and wire ropes.

This choreography of slow but meticulous pushing and pulling

begins working in combination to control

every move of this giant structure.

A sudden movement or level change in the water

could spell disaster.

(suspenseful music)

It's taken over 40 minutes of coaxing,

but finally the crane makes it onto dry land.

The final bit is just to get an inch perfect

so when we lower it down,

then I want to see it lands centrally on the rails.

British and Chinese engineering teams

have completed this mission.

And the construction of London's newest port

is a step closer to realization.

Thankfully, there were no problems on this one

and it all went very smoothly.

Very happy here.

Sweet, isn't it?

You never know but it all went very well.

Waterways may serve to expand our cities,

but rivers can be treacherous pathways for city dwellers.

Hundreds of boats carrying thousands of passengers

use the Thames every day.

Most journeys go off without incident

but the river can be a dangerous place.

There's many hazards on the Thames,

mainly the tide and the traffic.

So other the vessels going up and down

and the weight and tide,

more barges and boats and bridge piers.

This is a difficult environment

for the rescue workers who police it.

Last year, the emergency services

were called out over 850 times,

making this the busiest stretch of water

in the country for rescuers.

It's the job of the Royal National Lifeboat Institution,

a charity operating along the coast of the British Isles

to act as the river's first responders.

Helmsman Stantod has served almost a thousand hours

for the team.

The Thames can be quite a ruthless beast.

The currents and the coldness of the water

coming down from the mountains.

It makes it a very hostile place.

This gives rescue services

little or no time to save a casualty once they fall

into the Thames.

You've got less than 15 minutes lifespan

once you enter into this water.

(tension music)

RNLI Tower Station base

is one of four on the Thames

set up in the wake of the 1989 Marchioness disaster

which claimed 51 lives.

Since then, the RNLI has rescued almost 3000 people

from the Thames.

For volunteer John Mullen,

there's no other service completely committed to saving

lives on the river.

The river is a problem

for the standard emergency services.

So the RNLI presence here gives a committed rescue service.

No other job, just to go out and help people

that need the help here and there.

The single biggest challenge probably

is trying to prepare for an event

that you don't quite know how it's gonna pan out.

We've been to people that's been having babies,

we've been to fires,

we've been to people are stranded in the mud.

It could be anything.

That's part of the appeal of this job.

(bell ringing)

Right now, the London coast guard

has issued an emergency call.

The RNLI crew has 90 seconds to get on the water.

On duty is helmsmen Michael Neild,

and crew members Colin McCarthy and Giles Harrison.

(suspenseful music)

A suspected incident at Westminster Bridge

has put all the capital's rescue services on high alert.

Police, fire and ambulances are on their way.

(indistinct) on the sea.

But even more testing

is the absence of information they're given.

The information that comes through to the coast guard

is, you know, usually done in a panic.

It's not by any means the exact picture

of what you're gonna get when you get there.

(indistinct)

We got a report of a male who threatened to jump

the Westminster Bridge.

(indistinct) at this moment are able,

with surrounding cameras, to look on the bridge.

If a person falls into the Thames,

he or she only has minutes to live

in water that's 10 to 15 degrees Celsius.

After that, a fearsome undercurrent,

and the chances of drowning are high.

It's not clear whether anyone actually went into the water,

but the emergency services

have to take every report seriously.

Crew members Michael, Colin and Giles

search every meter of the river.

The London Tower, we've searched both sides of the bridge.

We've got nothing obvious.

Fits your description if you don't have him.

Five foot seven and medium build. Over.

London Tower. Yeah, that was all received.

Now an obvious-

Helmsman Michael Neild needs to stay alert.

Powerful currents surround the bridges foundations

could easily suck their vessel into its walls.

Nice. He's on his phone I think.

Leaning on the side.

There's no further information

to suggest anyone did go into the water.

It's starting to get dark.

If there was anyone in the river,

fast moving currents would have carried them far downstream.

Eventually, the search is called off.

Just no trace basically.

Let's hope police have tried all avenues

and got to a point where they decided that's it.

There's no point in issuing anymore.

(dramatic music)

The team head back to their base.

It's not a question of if,

but when the next call will come in.

And for these young unsung heroes,

the battle on the Thames continues.

I'm not sure I'd really wanna work anywhere else.

There's always something going on here

which is a nice thing.

That's what I enjoy the most.

Water can be our worst enemy,

but it can also save our lives.

It protects us from the most devastating

of everyday disasters.

Fire.

In 2011, the US fire department received reports

of structural fires every 65 seconds.

We save lives

and there is nothing more that you can do

that's more gratifying than to save a life.

In New York, there are almost 20,000 people

per square kilometer that depend on the fire department's

protection at all times.

The entire force is helpless without fast access

to massive amounts of high pressured water.

This city solution is a network of 110,000 fire hydrants

visible on every city street.

They connect the city's water mains directly

with the fireman's hose.

For firefighter Michael Gallo,

hydrants are critical in the fight against fire.

Fire hydrants in the city of New York

are of paramount importance.

We rely on them.

It doesn't matter how good my rig is

or how fast they get there,

without water we don't put fires out.

(dramatic music)

6:00 a.m.

One of the city's vital hydrants

has been knocked down by a car.

The Department of Environmental Protection

is dispatching a replacement.

A broken hydrant could mean the difference

between life and death.

John Castiglioni and his team have just two hours

to fit the replacement.

But some hydrants in the city date back over 50 years.

This is the lead from back in the day.

As you see, it's soft.

This was around the joint,

and here's today's technology.

See the comparisons.

This rubber gasket stays in the retaining plan.

Back in the day they didn't have that technology.

The hydrant went in around the pipe

and they used to make up the difference of space

with the leg.

Now today's technology, you're using a rubber gasket,

the retaining gland,

and the bolt.

They go in.

It holds everything in place.

(dramatic music)

As the work starts,

there's an unusual amount of water coming out

of the mains pipe,

making the attachment of the hydrants

retaining gland an impossible task.

The team bring in a high powered suction hose

to rid the area of excess water.

For now we're gonna install the hydrant.

Right on the hydrate branch,

we're gonna fold it together to block it off,

find the bolts up,

lock up the back

and then we're gonna all come to the hydrant gate.

It makes you to hide your work properly.

It's a tried and tested plan.

It takes five men and a crane

to heave the 300 kilogram hydrant into place.

They're trying to connect the hydrant

to an older section of branch pipe.

And however many ways they try, it doesn't fit.

Being that you're trying to repair something

that was installed between, say, 1900 and 1960s,

it makes the job very challenging.

May not know if the job's going to take one hour

or two hours.

(indistinct)

Right now there is no space.

We're gonna have to cut the existing hydrant branch back

a little more towards the street

to give us a little more room.

The hydrant needs to fit perfectly.

A centimeter out

and it will leak valuable water and pressure

needed in the event of a fire.

The team need to shave off some of the existing iron pipe

to get it to the right size.

At times it can be frustrating,

especially if you're working in any kind of weather.

But today, right now, it's a nice day.

So it's just part of the game.

You want everything to run smoothly

but you expect little problems to go little by little.

Try to get past all the bumps in the road.

Not knowing what they're going to find

every time they dig up the road

is part of the job for John and his team.

Each job is much different

which makes it very interesting,

each day to day job.

At the end of the day, the jobs all get completed.

All the guys come in,

come into work safe and then go home safe.

The branch pipe sits right next to a network

of vital city services.

The team must be careful not to puncture gas, water,

telephone and internet lines.

There's a lot of utilities in the way.

Steam means and electric lines,

oil and static lines.

So it's much harder to repair the hydrants.

Finally the pipe is down to size

and the workers try the installation again,

this time it works.

But the job isn't quite over.

The team must check all valves are tightened securely

to avoid any valuable water seeping out

and that the hydrant drains its own water correctly.

The last step in installing a hydrant

is making sure that the gasket and the retaining gland

and the bolts are tight

and you have the backing in the back of the hydrant

so when you open it up,

any pressure against the hydrant, it's fine.

And you open it up to make sure it's running fine as well.

Once the final checks are made,

the surface area is restored with a layer of concrete.

But they take for granted

the work that goes behind the hydrant to keep it working.

This hydrant is now ready for action,

along with the other 110,000 in the city.

It's been a challenging day for the team

as they continue to keep New York

armed against the flames.

Moving water in and out of our cities

is vital to our survival.

Protecting and maintaining the structures

that make this possible is a massive challenge.

In San Francisco, the threat to its water infrastructure

is much greater.

In the next 20 years,

the chance of an earthquake of 6.0 magnitude or higher

hitting the area is as high as 60%.

To avoid complete collapse,

the city is defending one of its oldest structures,

its sewer system, against the next big one.

Today, veterans sewer inspectors, Lorenzo Hale

and Fred Gonzalez are assessing a section of the sewer

for structural damage.

But before their descent, they must check

for a dangerous occupational hazard, methane gas.

We're about to put the gas meter

hose inside the pit hole here just to test the atmosphere

for any toxic gas or any methane,

cause any little spark, if it's methane gas,

could cause an explosion.

The guys have the go-ahead

but for their own safety,

they have just one hour underground

before they must resurface.

San Francisco has one of the oldest sewers in the US.

Originally built for a population

of approximately 50,000 people,

it's now a wastewater network for a city 16 times that size.

It spans a staggering 1500 kilometers

and can carry over one billion liters

of city wastewater a day.

The stress this puts on older sections of the network

weakens its structure considerably.

And it's Fred Gonzalez's job to protect it from collapse.

99% Of the people

probably don't even realize what's underneath the city.

Until they have a problem,

nobody really cares or understands

what goes on under the ground.

Their workspace is cramped, hot and dark.

What we look for in this joint

is any sand coming through

or any kind of infiltration of water.

Sand coming through a joint in the tunnel

indicates a weakness that in the event of an earthquake

could cause a total collapse.

What happens is if sand starts coming through,

there's no sand between this and the street.

If enough sand washes away and a bus drives over it,

the bus will actually fall into a hole.

This joint is looking good for now,

but a section of the wall further along the tunnel

is cause for concern.

The wall has been damaged during the winter rainy season

when these tunnels run at full capacity.

If you look right here, all these here is signs.

At one time, the water level was this high,

up to here.

All these are all remnants of toilet paper

and stuff that sticks to the wall.

So when it rains, this runs full capacity almost.

This sewer collects more than 300 million

liters of wastewater a day,

enough water to fill 120 Olympic size swimming pools.

But when it rains heavily,

it can be six times that amount of water.

After every rainy season,

we've got to really inspect these big sewers

because that's when a lot of damage is being done

which is gonna just cause big problems later.

(tension music)

Structural weaknesses in any part of the sewer

must be dealt with straight away.

Just marking the wall of this mine to be plastered.

Lorenzo and Fred have reached their time limit

and climb up to fresher air,

but the job isn't quite finished.

A team of plasterers is on its way

to carry out the emergency repair.

Strengthening this small section of sewer

looks simple enough.

But like Lorenzo and Fred,

this team has a short window of time

in which to carry out their work.

Potentially, toxic gases and dark and damp conditions

mean workers can't stay underground for more than an hour.

This constant cycle of inspection and repair

is the only way to keep San Francisco sewage structure safe.

These aren't the sewer's only hidden heroes.

Across the city, another rapid response team

has received a call out.

(upbeat music)

People might wash a lotta grease down the drain

and over a period of time it blocks the drain.

Or they might

flush too much toilet paper.

There's a blockage

in one of the smaller sewer pipes

that connects the main drain,

causing raw sewage to leak onto the streets.

Veteran service engineer, Jose Ledesma,

is proud of his part in keeping sewage flowing

under the city.

I've been working for this sewer word department

for 16 years

and I've been doing this kind of work for 30, 35 years now.

And I'm impressed with what San Francisco has got.

It's got good equipment,

good tools for the sewer problems that we encounter.

And today is no different.

We're responding to an emergency.

It's actually called a side service emergency backup.

What it's doing, it's overflowing.

It's not going out to the main.

But the size of the pipe

means that Jose can't go down to fix it.

Instead, it's a job for Betsy.

This is it.

This is the one that gets all the work done, right here.

Betsy is a 60 centimeter long

stainless steel explosion-proof video camera

that can get to places humans can't.

On this job, Jose and Warren Best

are hoping she can show them what the mystery blockage is.

It's a tool.

You gotta treat it like part,

part of your job.

Without this here, you can't do nothing.

We ain't gonna have no results on what the problem is.

Betsy's always gonna get the job done.

From the back of the van,

Warren controls Betsy as she travels down into the depths

of the sewer.

We need to pan and tilt, basically,

to see 360 degrees of the pipe.

Any defects that we might have, you know, in the pipe.

All right, right here I'mma increase the speed.

15 meters into her search

and Betsy comes across the problem area.

A large amount of grease from a nearby restaurant

has built up in the pipe.

It needs to be removed and it's Don's job to dislodge it

with a high pressure hose and an industrial solvent capable

of breaking down the grease.

But the cause of the blockage is the usual offending items.

Paper towels are bad.

Diapers too. They don't break down.

Paper towels and diapers

get absorbed, like they could splash,

blow up like 10 times of the volume.

The jets works the blockage free.

It's another battle won for the emergency team.

But maintenance alone is not enough

to keep a city's sewage system working.

In downtown San Francisco,

critical construction work is being carried out

to protect a main wastewater pipe from collapse.

The north shore force main,

a pressurized pipe over two kilometers long

moves untreated wastewater through the city

but it has failed twice in the last five years.

A bypass pipe is being built

while vital repairs are carried out on the main pipe.

This requires digging up the road

right in the heart of the city.

Construction in a city like San Francisco

is very challenging.

We're really webbing through a jungle of infrastructure

when we're trying to replace a soar line.

The team make every effort

to reduce the amount of road surface they need to dig up,

but there's no escaping the dangers

of working in this densely populated area.

Where we are right now,

it's the heart of the financial district.

We have apartments, condominiums, tourists,

visitors, workers, commuters.

During any given day,

the population of San Francisco doubles

when the workforce enters the city.

So we have to maintain that support.

Despite these challenges,

round the clock maintenance and new construction

play a vital part in keeping the city's wastewater

flowing out safely.

But it's only half the battle.

If left untreated, wastewater can spread disease.

The average city dweller uses approximately 200 liters

of water a day,

flushing the toilet,

taking a shower or doing the washing up.

In Stockholm, over 350 million liters go down the plug hole

on a daily basis.

And it ends up here.

This is Henriksdal,

one of the largest underground wastewater treatment plants

in the world.

It's so vast, operating technician Dennis Goffin

depends on his scooter to get around the plant.

Its about 20 kilometers of tunnels,

300,000 square meters of area.

Seven digesters,

about 30 tanks.

It's a big place.

One second you're sitting here in front of a computer screen

and the other second you're down in like

one of the filthiest places in Stockholm.

Dennis is one of a team of 28

whose job it is to process millions of liters

of wastewater a day

disposed of by over 700,000 people.

But after so long in the job,

Dennis no longer thinks about where the waste has come from.

I don't even think that it's people's waste anymore.

I just think that it's just some dirty mess

that has to be cleaned.

You don't even consider that it comes from someone.

Most impressively, 98% of the waste water

that comes through Henriksdal

goes back into the Baltic Sea, clean enough to swim in.

Stockholm is islands and water.

That's all it is.

So we have to protect that environment.

That's why it's important to keep this process going

and keep it getting better each day.

It's a mammoth task.

In the plants control room,

computers monitor the intake

of thousands of liters per second,

which varies based on the time of day.

So here we can see that it fluctuates

in the middle of the day.

It gets us

about 3000 and then at nighttime all the way down

to only 1000, even below that sometimes.

Waste rush hour is right now at 7:00 a.m.

As Stockholm's residents all head to the bathroom,

the plant can often run over full capacity.

We can see that about 12,000 liters a second came in

and that causes trouble.

In just one hour,

the plant is capable of ingesting over

36 million liters of wastewater.

And down in the depths of the mountain,

Dennis' colleague process engineer Hannah Astfelt,

is heading off to monitor the first stage

of the cleaning process.

Henriksdal has a surface area

the size of over 30 football pitches.

Like Dennis, the only way for Hannah to get around

to complete her morning checks

is to take the latest model of sewage plant transport.

Everybody needs a bike or a moped to work here.

This motorized trait is a definite upside

to a job that sees Hannah exposed to thousands of liters

of her fellow residents' waste.

As we can see, there's a lot of debris from the humans.

There's

sanitary articles, condoms,

food.

Step screens extract any debris

bigger than three millimeters from the water.

That includes 10 tons of earbuds,

tampons and condoms a week.

These items could cause devastating damage

to machinery and bring the plant to a halt.

I think people sort of think

that they can throw anything into toilet,

but for us it's a big problem

because these material

creates a great damage

if they continue for the process.

But the items aren't limited

to what goes down the toilet.

The city's drains also empty into this facility

with some surprising results.

A bicycle, a canoe,

and we have seen driving license

and even teeth from people.

So I don't think we can be any more shocked.

Here at the sediment tanks,

gravity does the work.

Solid matter or sludge sinks to the bottom

whilst cleaner water flows from the top.

But it doesn't always go to plan.

Today, Dennis and Hannah have noticed something strange

during their daily inspection,

an abnormal buildup of sludge.

We're seeing sludge that should be sinking,

but for one reason or another

some bad bacteria is formed

and not been taken care of

in the biological cleaning process

which means it comes down to this process

and creates these carpets which causes it to float up.

With epic volumes of wastewater

to process every day, Hannah and Dennis can't afford

to let one defect in the chain affect the next.

However small a problem, it could escalate

in a matter of minutes

and have a detrimental effect on the quality of clean water

going out of the plant.

The team leave nothing to chance

and draft in another young member

who gives the sludge a helping hand to sink to the bottom.

This is the regular everyday business,

to just go through the entire process.

Does it smell bad?

That means it's probably a problem somewhere.

Does it look bad?

My girlfriend doesn't know what my job entails.

No one knows what I work with.

People don't even know how big this place is.

One of the final processes

known as the biological stage

is critical to making this water clean enough to swim in.

And it's completely invisible.

Good bacteria devours the organic matter

and nitrogen in the waste.

And 24 hours later, the result is this,

water in this jacuzzi style outflow

is on its way back to the Baltic Sea

at an average speed of 2000 liters per second.

Our job is to clean the water

from this dirty mess that we saw

into water that we can actually put back into the Baltic Sea

with a clear conscience.

You don't have to go back that far

to see a time where you could not swim in Stockholm.

It was actually illegal to swim

anywhere in the city of Stockholm

and now you can swim anywhere.

You can ask anyone in Stockholm.

In most water treatment plants,

this is where the story ends,

but Henriksdal is one of the few plants around the world

reducing the amount of water waste even further.

While some plants discard their sludge,

Dennis and his team consider it

one of their most prized products.

Well here we mix the primary sludge

with the secondary sludge,

with the organic matter that comes into our organic station.

This all gets mixed together.

And with these pumps,

pumps into the digesters to begin creating biogas.

Our advantage is that we are so big,

that we get so much in,

that we can actually do this stuff,

that we can actually make this much gas.

After leaving Henriksdal,

waste matter goes through a further refining stage

before arriving at Keolis bus depot in the form of biogas.

Here, a fleet of about 120 buses

take minutes to fuel after a 22 hour shift

on the city's roads.

But the difference is that this biogas

is entirely renewable and much less polluting

than ethanol or diesel.

It's the way forward for technical advisor, Eric Norden.

I'm proud to say that our fleet here runs on biogas.

I would like more companies to go with it because it's good.

It's really good.

It's actually amazing that you can make fuel other ways.

You know, people flush their toilets

and it turns into fuel for cars and buses.

Henriksdal turns a record

one and three quarter trillion liters of wastewater a week

back into clean water and biofuel,

making this plant a model for the future.

(lighthearted music)

Cities around the world depend on the skill of engineers,

scientists, inspectors and technicians

to keep our water flowing.

Without this hidden army working tirelessly

around the clock,

our survival, well-being, safety and prosperity

would be seriously at risk.

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