Next time you’re at the supermarket, look at the contents of your trolley and consider where most end up:the sewer.

Stripped of packaging (a separate waste problem), most of what’s left - food and drink, washing liquid, chlorine bleach, biological washing powder, drugs and toilet tissue - finds its way down loos and plugholes, into small pipes leading to bigger pipes leading to tubes nearly as big as train tunnels.

Eventually the entire revolt ing torrent flows into a sewage treatment works, where it will emerge as a wide, deep, cloudy, greenish river.

This is raw sewage, stuff most of us hardly ever think about, except to fret in case it is ending up on the beach, or in rivers. To ensure that doesn’t happen (and it is not often, anyway) some people ponder sewage all the time.

One such person is Rob Nason, production manager at Thames Water’s Long Reach Sewage Treatment Works on the south side of the Thames in Kent. Sixty million gallons of raw sewage flow into Rob’s plant every day, and practically all of it is subsequently discharged into the Thames estuary, just a stone’s throw away from two 12-foot inlet pipes. But it takes about 15 hours for the sewage to travel through tanks, conduits and whirlpools at the treatment works, after which it is transformed.

The liquid returned to the river is clear, odourless, slightly discoloured and streaked with soapsuds. The coarsest solids are filtered out - plastic bags, disposable nappies, Christmas trees and motorcycle frames - and taken away for landfill.

But the bulk of the solids are converted into a crumbly, dark brown, not particularly smelly substance that looks like gardener’s potting compost. Rob Nason has an area the size of a tennis court spread two feet thick with it at any one time. Nationally, more than half of this substance - around 500,000 tonnes a year - is used as agricultural fertiliser (a good reason for washing vegetables before you cook them).

Down the tubes

Rob Nason has some funny things come down his inlet pipes, but his biggest challenge is plastic cotton-buds. This is because they float end-on, passing through holes in the 6mm mesh screens designed to filter most inorganic debris out of raw sewage.

Once it’s been filtered, and grit has been settled out of it, the sewage flows into huge settlement tanks where, over the next four hours or so, it separates - some solids sink to the bottom and are sucked out, and others rise to the surface, where they are skimmed off. The “primary” treatment is now complete, and at this stage in some areas the cloudy liquid, from which about 50 per cent of polluting material has been removed, would be discharged.

At Long Reach, however, the sewage also receives “secondary” treatment. This so-called “activated sludge process” was developed in the 1920s by two men with a bucket who discovered that if they blew warm air through sewage they could convert the cloudy liquid into a less offensive clear fluid with a brown sediment. This discovery made them rich, but the process was not applied industrially until the 1960s. It works because the warm, oxygen-rich environment causes innumerable bacteria present in the sewage to go into a feeding frenzy and eat any organic matter they can find. After about seven hours of this, another 45 per cent of polluting material has been removed, and after passing through a final settlement tank, the treated sewage can be released into the river.

Meanwhile, the sludge and scum that have been collected along the way are channeled into huge “digester” tanks, as big as small houses, where, during a couple of weeks of slow cooking, the organic matter degrades, producing methane gas that is piped off to fuel the plant’s electricity generators. (Long Reach supplies almost all of its own power in this way). The “digested sludge” that remains at the end of the process is dried, pressed, and emerges as the composty substance described before. It is full of tomato seeds, which pass unscathed through our intestines and the sewage plant’s digestive system. Apparently farmers who use sludge as fertiliser often find rogue tomatoes growing among their crops.

Flushed with success

We like to keep it out of sight and out of mind, but sewage systems seem to be something Britain can be proud of. Nearly all of us - 96 per cent - are connected to the main system (the comparable figure for France, for example, is 51 per cent), and 83 per cent of us are served by sewage treatment works, a proportion matched by only a handful of countries worldwide. On the other hand, in England and Wales only 19 per cent are connected to treatment plants which provide the most sophisticated form of purification, called “tertiary” treatment (in Scotland the figure falls to 3 per cent). Worst of all, in England and Wales 10 per cent of us produce sewage that receives only the most basic treatment or none at all, and for northern Scotland this figure is 59 per cent.

But the biggest problem facing the sewage treatment industry (apart from plastic cotton-buds) is the fact that our ability to produce noxious waste outstrips our ability to process it into harmless substances.

In 1994, the Environment Agency published evidence of the “feminisation” of British fish. In some rivers, it said, nearly all the male fish examined were found to have egg tissue in their testes. This was partly linked to the presence of “gender bending” chemicals derived from industrial waste, and partly to oestrogens. These hormones are naturally excreted by women in an inactive form, but are thought to be activated by the sludge treatment process. So the very effort to make sewage cleaner may have created a new type of pollution.

Keep it clean

As sewage treatment works go, the Long Reach plant is large, processing the effluents of 800,000 people in north Kent. Most of the UK’s 8,300 treatment works are smaller. A few are bigger - such as the two huge plants on the banks of the Thames which, for more than a century, have treated the copious outpourings of London.

In their day, the Victorian plants were communities unto themselves, with hundreds of workers, and their own schools and churches.They were built on a grand scale, but isolated from the people they served for reasons of hygiene. Today, Long Reach is maintained by a staff of about 11 and a few hundred metres away a new housing estate is being built .

Minimising the impact the plant has on new residents will be another challenge for Rob Nason. But you can be sure prospective buyers won’t be thinking about what impact their effluents will have on him when they flush the loo. Our sewage system may be one of the great public conveniences of modern life, but it is the one we take most for granted.

YOU CAN’T PLEASE EVERYBODY

The equations balancing commercial, consumer and environmental interests are often complicated. One of the first effects of the EC Water Directive, which regulates sewage disposal, was to ban the dumping of sewage sludge at sea, from 1998 onwards. This was generally welcomed, but it has created a huge problem for the treatment plants - in the form of growing stockpiles of loam-like dried sludge.

The water companies are keen to sell this stuff to farmers, because it solves their disposal problem. Most people agree this seems like an ecologically satisfactory solution. However, some sewage contains industrial waste unsuitable for fertiliser. Moreover, it is not currently necessary to pasteurise sludge before selling it to farmers, so there is a risk that it might contain pathogens (disease-causing agents), in particular the dangerous E coli 0157. In theory, this could contaminate crops and cause disease. A lobby led by the supermarkets (worried about legal liability for such illness) is trying to persuade the Government to make pasteurisation compulsory. If that happened, additional expense would be incurred through the need for further machinery and (dirty) electricity. The pasteurisation process could also prove smelly for people living near the plant. In the meantime, treatment plants are building incinerators to burn sludge and produce electricity. Even Friends of the Earth agree this is probably the least damaging option for now.

VISIT THE SEWAGE WORKS

Visiting a sewage works may not sound very appealing, but forward-thinking companies welcome school-age visitors, and many have education officers to teach children about water and sewage systems. Visits provide a chance for children to see an important industrial process with direct environmental impact. And treatment plants aren’t as nearly as smelly as you imagine.

Make your own sewage One of the expectations of the national curriculum for science is that older primary children learn how to separate mixtures of materials by sieving, filtering and evaporating. You can give them experience of this by mixing and then separating classroom “sewage”.

Follow this simple recipe:

* To one plastic aquarium of clean water, add plastic bags, LEGO bricks, a few stones and cotton buds. Have these dissolved?

* Pour in a cup of salt. Stir. Has the salt dissolved? How do we know? l Add gravel, a handful of soil and some sand. Have these dissolved? There will be soluble bits in the soil.

* Add twigs, pebbles and sheets of loo paper. Leave to marinate.

* Separate and clean. Garden sieves remove large debris; kitchen sieves remove small items.

* Pour the rest through a filter. Is the water clear? Pour into a saucer and leave it on a window sill. You may get back some salt crystals.

For educational visits to North West Water, call the booking line on 01925 233 233, and for Severn Trent Water, call Judith Wynn on 0121 722 4000. For a copy of Thames Water’s, Educational Resource for Schools and Colleges, call 0845 9200800.