As I step out of the car and shake the hand of Bob Porter, manager of the Bristol Sewage Treatment Works at Avonmouth, he asks if I'd found the place OK.

"I normally tell people to come down to King's Weston Lane, then just follow your nose," he laughs.

In fact, the smell of the works is not nearly as overpowering as I'd imagined it to be.

Despite the fact that the bodily waste of up to a million people from Bristol and the South West is circling around in great sludge vats behind me, there is only the faintest hint of methane in the air.

Don't take my word for it, you'll be able to smell it for yourself when the plant is opened to visitors as part of the Bristol Doors Open Day on Saturday, September 12.

The annual event, which allows members of the public free access to dozens of otherwise inaccessible Bristol buildings, is always popular – with thousands each year taking the opportunity to look around historic venues such as the Corn Street Exchange building, Colston's Almshouse in St Michael's Hill and the Clifton Rocks Railway.

Although taking a tour of the region's biggest sewage works may not be everybody's cup of tea, Bob says the open days always prove popular with anyone with a penchant for engineering.

"We've been opening our doors for the annual event for eight years, and we get more and more visitors each time – last year we had more than 400 people. People are genuinely fascinated to know how we deal with the vast amounts of waste that come to us each day."

Make no mistake about it – we are talking about vast amounts of waste. The works, which is fed by Bristol's four major sewer systems, treats on average 210 megalitres per day – that is 210 million litres of sewage.

"The maximum that we can handle in storm conditions is 4,400 litres a second," Bob says. "After that the incoming waste is diverted into the two large storm tanks, which can store a combined 25,000 cubic metres of liquid.

"We can hold it there until the flow from the sewers calms down enough after the storm, so that our systems are able to start treating it." The Wessex Water plant, which employs just 15 men (much of the process is now operated by computers), is undergoing £100m worth of infrastructure investment. It is state-of-the-art in the world of sewage treatment works.

"It's been here since the early 1960s," says Bob, who started as a fitter 30 years ago.

"Before this place was built, the raw sewage from Bristol would simply be pumped into the River Avon, with tonnes of liquid chlorine being pumped into the river to break it down.

"These days, with the sewage treated to such a high standard, it's hard to believe that was ever allowed. Now the water coming out at the end of the process is incredibly clean when we pump it into the Severn."

The process begins with a single gaping hole into the sewer system, with the waste spewing out of it somewhat grotesquely.

From this flow, a pair of enormous metal screws rise out of the water, constantly turning and lifting the sewage up into the air.

"These Archimedean screws, as they're known, lift the sewage up 7.8 metres to enable it to flow by gravity through the next stages of the works," Bob explains.

He leads the way through a series of mechanical contraptions that continue to filter the great sludgy river of waste.

The next stage is the "coarse screen". These are great metal jaws, that bite together as the waste is washed through. The jaws spit out the largest pieces of non-biological waste on to an unsavoury-looking conveyor belt.

This has to be the most unsightly part of the process, with everything from feminine hygiene products to contraceptive waste merrily making its way out to a waiting skip.

There are certainly no cuddly toys on this particular conveyor belt.

But the great river of waste doesn't stop. It continues to flow straight on to the next stage in the process. This is the "grit channel", where fine screens with holes just 6mm wide remove the grit that has amassed in the waste as it has made its way through miles of sewers.

With all the unwanted rubbish removed from the flow, it's now time to start the treatment of the sewage.

Bob leads the way to the "primary settlement tanks".

"The sewage flows first into a chamber, and is then evenly distributed between six tanks, each of which holds up to four million litres," he says.

The sewage spends between two and six hours flowing through these funnel-shaped tanks, during which time the suspended solids in the sewage settle to the bottom, forming sludge.

A bridge travels around each tank, connected to a series of scrapers, which help to push the sludge to a central point.

The sewage then heads on into the "sequencing batch reactors". There are 11 of these enormous vats – each one equivalent to the size of an Olympic swimming pool.

"Air is bubbled up from the bottom, which feeds the bacteria that is alive in the tanks. This bacteria helps to break down the waste," Bob explains.

The sludge then gets separated into final settlement tanks.

Meanwhile, the water is channelled off, by which point it is clean enough to be channelled into the Severn Estuary. However 16 million litres of it a day is reused – being delivered to the nearby Seabank gas-fired power station for cooling purposes. Then the sludge moves on to what is perhaps the cleverest part of the entire process.

"The two large tanks in front of us are known as digesters," Bob explains. "They work exactly like a human body. They are fed with 1,212 cubic metres of sludge each day, and they are kept at a constant heat – somewhere around human body temperature.

"The sealed units are left for around 14 days to go through an anaerobic process which allows bacteria to break down the organic matter and purify the sludge.

"In other words, they digest the sludge, just like the human body digests food.

"Working with them is like working with a human body," Bob adds. "If there is an impurity allowed in, the digesters can get an upset stomach, and you soon know about it, because it lets off a terrible smell.

"You have to treat the digesters with a little tenderness. It really is almost like they're alive."

During this process, the methane that is omitted by the waste is captured, and used as an energy source, somewhat ingeniously, to power the plant.

The treatment works has its own generating station, where the waste gases are used to generate £800,000-worth of green energy each year via five 1.1mw engines.

When the sludge leaves the digesters, it is dried out to make "cakes" that can be used to fertilise fields of non-foodstuff crops.

"It's amazing how much reusable material we're able to get out of all this waste," Bob says. "Whether it's the electricity that's generated, the water for cooling the local power station, or the sludge that's transformed into a fertiliser. In fact, these days, there's very little waste at all."