Gallons of raw sewage was discharged into the sea at Burton Bradstock, West Bay, Seatown and Charmouth on March 16 when storm overflows released wastewater at the Wessex Water sites to prevent properties from sewer flooding after heavy rainfall.
Raw sewage overwhelmed underground networks and was dumped into the sea.
Charity Surfers Against Sewage said there had been 400,000 sewage discharges into UK waterways in 2020, and it had collated more than 150 health reports of people becoming unwell after entering the water.
A spokesperson from Surfers Against Sewage said: “Sewage pollution is being poured into our blue spaces at an alarming rate and water companies are getting away with it.
“In 2020 alone, sewage was released into the environment over 400,000 times equating to 3.1 million hours of discharge.
“We’ve called on the government to address this sewage scandal.
“We’ve reported the health risks this pollution puts on all water users.
“We’ve exposed the regulators and testing regime for ignoring the truth. Now we need to take our message to #EndSewagePollution straight to the doors of water company HQs.”
Wessex Water said the waste that was emptied off the Dorset coast on Wednesday was ‘highly diluted’ although provides information to local councils, Surfers Against Sewage and others when storm overflows operate that could ‘potentially affect bathing waters and inland swimming sites’.
A Wessex Water spokesperson said: “We’re committed to completely eliminating the discharge of untreated sewage, starting with storm overflows that discharge most frequently and those that have any environmental impact.
“Every month we’re investing more than £2.5million to reduce storm overflows, with £150million being spent between 2020 and 2025.
“In an ideal world we wouldn’t have storm overflows at all.
“They are a legacy from the past to protect properties from flooding which have no place in the 21st century.”
A National Day of Action on Water Quality is being organised by charity Surfers Against Sewage on April 23 to protest against the use of CSOs by water companies.
The protest against Wessex Water is being held in Bath.