Muck and brass – the dirty work of generating clean power from waste

Any time is a good time to visit the Esholt water treatment works. We all need reminding that a reliable sewage disposal system is one of the foundations of civilisation and we should be grateful every day for the people who make ours work.

Yorkshire Water has used Esholt for some time as a showcase for its work at the dirty end of the business. All over the site are collections of bottles which can be held up to visitors to illustrate the difference between what arrives every day, from 700,000 customers in and around Bradford, and what is later pumped into the River Aire, between Shipley and Guiseley.

Most of the visitors are children, learning how the Victorians and Edwardians changed society and how modern technology has built on their amazing feats of construction. But lately, the story has had a new chapter and the visitors have included a lot more grown-ups wanting to talk mathematics.

Our national effort to do something about our "carbon footprint" can reasonably be said to have started in 2002, when the second Blair government's Renewables Obligation started to take effect.

It meant electricity distributors had to either generate some in renewable ways or buy Renewable Obligation Certificates from others who had done so. The system was designed so that a ROC was worth much more than the electricity it represented. Whatever you think about "global warming", ROCs have done what they were invented for – to drive investment in alternatives to fossil fuels.

For a number of reasons, the water companies were natural contenders for some of this new business. And several strands of green generation technology are being used and improved on daily at Esholt – all powered by you know what.

Esholt is not the biggest of Yorkshire Water's sewage plants but it has become the centre for experimentation with energy recovery, because a major rebuild was required any way, thanks to river protection rules which came into force shortly before the ROCs idea.

The dual-purpose project has been fully up and running for two years and Yorkshire Water is keen to show it off – first, to prove it might actually hit the target of reducing its carbon footprint by 20 per cent by 2020 and, second, to assure customers it is missing no opportunity to cut costs.

Electricity is second to manpower in its outgoings and earnings from ROCs, plus savings on spending, already amount to 5m off a bill of 50m, business-wide. At Esholt, the figure is nearly 30 per cent.

The raw materials for this achievement arrive out of a tunnel starting four kilometres away, under Frizinghall, Bradford. The Mayor of Bradford celebrated its completion, in 1924, by driving a locally-built Jowett car along it. At the time, the purpose was simply to take the city's sewage. It used to all flow into Bradford Beck and the life expectancy around the beck was about 30 in Victorian times, compared with 75 in the "model village" of Saltaire.

The first stage of the process, which has since grown up at the end of the tunnel, is to filter out big junk like bricks and road grit, nappies and false teeth, and the odd television and unwanted Christmas jumper which have been dropped down manholes.

The second stage is a fine filter, which is designed mainly to remove bits of toilet paper and is inclined to get clogged by cotton buds on sticks, which are the curse of the works.

After screening, the torrents are diverted to settling tanks, where the solids sink.

On the way, the slurry now powers two hydro-turbines over a total drop of about 18 metres – generating 180 kilowatts at peak times (like the morning surge known by the staff as The Breakfast Run) and 2,000-3,000 kw hours a day on average.

The sludge from the settling tanks would once have been incinerated – one reason sewage treatment eats power. Now half of it goes into two anaerobic digesters of the kind farms and food businesses are being encouraged to build.

They stew the organics and produce a methane-based gas which drives two 600kw heat-and-power units – basically, adapted diesel engines with exhaust heat-recovery systems. The heat keeps the digesters warm. In other circumstances, it might be piped into premises.

After the settling tanks, the remaining dirty water goes into "activated sludge tanks", which are also big electricity consumers. The process depends on aeration, like a fish tank, and the trick is to put in just enough oxygen for the bacteria which eat the nasties, but no more than necessary, because of the expense of the pumps.

The hands who look after this have been redesignated Energy and Process Technicians and energy efficiency has become part of their performance targets.

Andrew Calvert, the engineer now working under the title of energy generation optimisation manager, says the change, and the education and devolved responsibility which went with it, have done wonders for pride and productivity. He points to a man hosing muck off a grille.

"It looks a simple job but we need him to care about it. Too much water and you change the composition of the slurry and lose hundreds of pounds of generation potential."

The aeration tanks, by the way, are the place to discuss nightmares about drowning in sewage. They are seven metres deep and the bubbles mean "negative buoyancy" – ie, if you fell in, you would not surface. Occasionally, divers do have to go in there.

After aeration, the slurry goes to the final settlement tanks, the water goes into the river and the last of the sludge goes back to the digesters and/or the incinerator.

By this time, the cells of the sludge have become quite tough. But the company engineers have found a way to make them easier for the digesters to break down – meaning up to 30 per cent more gas per ton recovered. It is one of the lessons being learned here which the parent company, Kelda, will eventually take out to the market place.

Andrew Calvert can offer a couple for free, for farmers considering manure power.

"First, it has to be as fresh as possible,whether it is human or animal. Leave it hanging around for a couple of months and it is dead. Second, you want a slurry that gives you maximum fuel efficiency but can still be pumped around without clogging everything up. We find six per cent solids is about right."

An important part of the economics of digesters is the remainder product, which can make good fertiliser.

At Esholt, it is planted with a range of clean-up plants – as part of an ongoing inquiry into which works best – and can then be sold to farmers, for use on a limited range of crops.

Mixed with grit and minced wood, it can be turned into British Standard topsoil. It is not in garden centres yet, and the branding operation will be a tricky one, but finding new markets for it is an essential step towards the next stage in the move away from incineration.

"The digesters are the first big success," says Andrew. "They are better for the environment than incineration and cheaper than incineration, with a lower carbon footprint and an end product you can spread on fields. They work well and the generators work well. It is the pipes and valves in between them which tend to cause problems but we are developing the right maintenance rgime.

"Wind turbines need much less attention. But they are more expensive upfront. Overall, I would think the digesters are going to give us the better return on investment.

"They would not be profitable without the ROCs earnings at the moment. The Government is not giving money away for the sake of it. But the price of electricity is expected to rise another 40 per cent by 2020, and learning to generate our own is essential preparation for that."

Esholt is also home to an experimental "pyroliser" – basically a super-efficient incinerator designed to burn vapourised rubbish and generate more energy than it uses. The company hopes it will show the way to squeezing a few extra calories out of dried toilet paper.