You would struggle to find a lovelier view anywhere than that from Pencefn, a hilltop farm near Tregaron in mid-Wales.

Lush meadows with sheep grazing peacefully roll down towards the valley of the Teifi, renowned for its salmon and sea trout. Close by are the Cambrian Mountains, where the river begins its journey at the limpid Teifi Pools.

But dwarfing the main farm are the towers and tanks of an anaerobic digester. The Government-subsidised ‘green guzzler’ turns animal excrement, human food waste and specially grown rye into methane gas, which is burnt in a generator to make supposedly environmentally friendly electricity for the National Grid.

Last December, just a few months after it was built, the digester triggered an ecological catastrophe.

Unnoticed by the farm’s owners, brothers Jim and William Lloyd, a pipe from a storage vat sprang a leak.

Thousands of gallons of black, toxic slime began sliding slowly downhill across those verdant meadows to a nearby stream – a tributary of the Teifi. The result was a poisonous ‘tsunami’, a flood of putrid sludge that flowed down the stream and into the river for hours. The consequences were devastating, and are likely to last many years.

This week, an investigation by this newspaper has revealed:

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