HERE'S one of those daft questions no-one is really likely to know the answer to. What have Prince Charles' Highgrove House, the M40 motorway services station at Oxford and a barn conversion at Mickleton got in common?

No, they haven't all been the scene of a midnight flit by Lady Di. Rather less glamorous than that.

They all have reed-bed sewage systems producing re-cycleable water.

In this day and age, when we are urged to be as "green" as we reasonably can, it makes sense to be able to re-use all that household water that would otherwise go straight down the drain.

Of course, we're not really talking drinking water, but there's no reason at all why water that's been through the reeds shouldn't wash your car, flush your toilet or water your garden. Indeed, with only minor additions to the system, it can even be used in your washing machine.

Which leads me to Rick Hudson, who lives in Woodstock Road, Worcester, but spends most of his working week out in the countryside, where environmentally friendly homeowners are looking for something rather more inspiring than the routine septic tank.

Rick has been one of the pioneer installers of reed-bed systems in this country, having gone through a quantum change in lifestyle from a university academic to a decidedly more spiritual being.

In his teens in the 1950s, he was in on the embryo scuba diving scene and wanted to be a marine zoologist. He studied marine science at university in Australia and spent six-and-a-half years as a lecturer.

But he'd always had a "green" edge - when living near Melbourne he built his own adobe hut home out of clay bricks - and after moving to North America changed to a more spiritual and holistic approach.

Lecturing went out of the window and, always handy with his hands, he began carpentry, cabinet making and joinery.

Moving to England in the early 90s, he took charge of several rural building projects, one of which was a barn conversion at Mickleton, near Evesham.

"There was a problem fitting a suitable sewage system," Rick explained.

"The normal method would have been a septic tank to a soakaway network. But here there was a high water table and the ground was flat clay, which made dispersal of the liquids difficult.

"You could get a package treatment plant, but it was costly and wouldn't have worked particularly well on that site.

"It was while I was searching for a solution that I came across reed-beds. They were very new in this country at that time and the early ones had received some bad press.

"This was because they had originated in Germany in the 80s and the first ones were based on a horizontal system, which doesn't work on everything. Then a vertical system was developed, which operated better, but the best solution is often a combination of the two."

Rick travelled down to Newnham-on-Severn, south of Gloucester, to see one of the advanced reed-beds in action and returned convinced this was the answer for the Mickleton project, particularly as the owner had expressed an interest in being able to recycle the water.

Almost along the lines of the famous ad "He liked the product so much he bought the company", Rick decided to found his own company, Cress Water, to instal reed-bed systems and carry out water management and landscape projects, which he has been doing successfully all over the country for the last 10 years.

He's in good company too, because Prince Charles has long had a system at Highgrove and Rick himself was called in to design one for the Sandringham Estate to cater for 450 people in the estate houses.

But if you think that's a high number, then hats off to the bosses of the M40 motorway services station just outside Oxford. When it was being built and they were unable to wait for it to be connected to a new mains sewage network, they opted for a self-contained reed-bed system that caters for around 20,000 people a day. So you know it works.

No doubt that system, with its huge blocks of reeds, cost more than the "from £6,000 plus VAT" quote of installing one in an individual country property.

That, incidentally, includes a septic tank.

Because most reed-bed filtrations deal only with the liquids, you still need a septic tank for the solids, which is emptied, on average, once a year.

The way the reed-bed system works is that the liquid from the septic tank is pumped through a series of beds of common river or Norfolk reed - the same as that used for thatching - that filter the water, which is eventually returned to a holding pool from where it can be taken for a variety of uses.

"It's an ecologically sensible and cost-effective method of waste water treatment," said Rick.

Although the reeds grow from seedlings set in gravel and eventually reach 6-8ft tall, the system will begin to work immediately and is really effective after only two weeks. So it's not a case of installing the beds and then waiting for the reeds to rise tall.

Finally, one more question. How much water does each person in Britain use every day?

The answer is 250 litres (65 gallons), or 125 of those two-litre plastic bottles you buy in supermarkets.

Makes you think.