There’s rather more to designing a sustainable building than specifying low-embodied energy materials and making sure it will require little energy and maintenance in use. Low water demand and the ability to get the nutrients discharged in the sewage back to the land are important too.

One thing householders don't want to fail is their wastewater treatment system – the pollution, the health hazard, the cost and not least the embarrassment factor are all potentially serious. And yet, one wastewater treatment system provider says that such failures are very likely. As Jason Walsh asks, is he right?