A Guardian investigation has spoken to whistleblowers who say UK water companies are knowingly failing to treat legally required amounts of sewage, and that some treatment works are manipulating wastewater systems to divert raw sewage away from the works and into rivers and seas.

It is well known that water companies are dumping large volumes of raw sewage into rivers and seas from storm overflows but an investigation by the Guardian and Watershed Investigations reveals that the industry’s “dirty secret” is bigger, broader and deeply systemic.

By law, every wastewater treatment works must treat a minimum amount of sewage as stipulated in their environmental permits. Four whistleblowers have told Watershed that a large proportion regularly fail to do so and are not reporting it to the environmental regulator.

The insiders say the amount of sewage reaching a works is being “manipulated at the front end” by “flow trimming”, which can be done a number of ways including by “manually setting penstocks to limit the flow”, by “dropping weir levels” and by “tuning down pumps at pumping stations”. The diverted raw sewage makes its way into ditches, rivers and seas.

One industry insider says they “have personally surveyed works and found valves operated and diversion pipes installed so that part of the flow arriving is deliberately diverted to an environmentally sensitive stream, rather than into the works, so that the works passes compliance of sanitary parameters.

The Environment Agency and Ofwat are currently undertaking an investigation into potential widespread non-compliance by water and sewerage companies at sewage treatment works.