Communities across Wales are being put at risk as the sewerage system becomes increasingly over-burdened, a TV documentary will claim tonight.

According to the Marine Conservation Society, the system is now too reliant on more than 3,000 sewage overflow pipes that spill directly into rivers and seas if there is added pressure from heavy rain.

On tonight’s BBC Wales Week In, Week Out programme, Thomas Bell, from the Marine Conservation Society, says: “The system of overflow pipes is so large and so poorly regulated that in some cases it amounts to a licence to pollute.”

Earlier this year, the scale of the problem was illustrated when organisers of the Towy River Festival at Carmarthen called the event off after untreated sewage appeared in the water.

One of the organisers, Ian McCue, said: “No-one expects in 2008, raw sewage to be entering our river every time it rains. We’ve asked Welsh Water to clean it up, but they just let it go. As long as it gets taken away by the tide, that’s fine. But there are beaches further down this river that people use, and they’re complaining.”

Welsh Water operations director Peter Perry tells the programme that, despite investing billions of pounds improving sewage treatment, “In addition to sewage from homes and industry, we are dealing with surface water, and those volumes of surface water can be considerable. If we’re not able to allow the rainfall to discharge away to our overflow, then the very worst scenario our customers fear can happen, and that means the sewage flooding of somebody’s home, which I think all of us would agree is an absolutely terrible experience.

“If we were to replace the sewerage system, you are talking of a huge investment. That £13bn would levy a charge of £10,000 to every household in Wales. In addition to that, can you imagine the disruption it would cause and the time scale?”

Mr Bell said: “No-one wants to see raw sewage ending up in their houses, but it’s pretty clear people want to see clean beaches and clean seas, if for no other reason than if there’s a clean sea there is less health risk if you go swimming in it. People with families don’t want to run the risk of going to the beach and coming home with sick kids.”

Even if the wholesale replacement of the sewerage system is unrealistic, there are a number of things we are doing that are making the situation worse.

At present, huge quantities of varied rubbish are flushed down the toilet, adding to the burden on the system.

And, as environmental consultant Chris Baines tells the programme, new building is reducing the ability of rain water to drain into the ground naturally.

“In a typical suburban area with large gardens, most of the rain is going to find somewhere to seep into the ground. But if you look at new developments taking place, they are so much more tightly packed together. There are little areas of green, but most of it is hard tarmac with concrete blocks – there is nowhere for the rain water to go. Ten years ago many areas were absorbing the rain water, while now they can’t.

The European Commission has warned the UK Government it is in breach of EU rules and it is likely that a case will be taken to the European Court of Justice.

Week In, Week Out – Down the Drain goes out on BBC1 Wales at 10.35pm tonight.