On Sunday, August 30, signage was put up at Littlehampton beach warning people of untreated sewage in the sea.
It came after Southern Water alerted Arun District Council to the incident, which had taken place 5km off the coast and at the mouth of the River Arun.
It was later discovered to have been caused by faulty valve and testing found the water has since returned to an “excellent” rating.
In a post on social media, Arun District Council warned swimmers of the incident.
It wrote: “This morning we were notified by Southern Water of a discharge of untreated sewage into the sea 5km off the coast of Littlehampton and Climping as well as at the mouth of the River Arun.
“We are adopting a precautionary approach and are putting up signs at the beach advising against bathing in the sea and suggest that people and dogs do not go into the sea to swim or paddle.”
Southern Water said the discharge sewage followed heavy rainfall and occurred due to its storm overflow system.
A spokesman for the company said: “On Saturday night, August 29, Littlehampton received 5.4 times the average daily rainfall for August and to protect homes, schools and businesses from flooding the storm release system operated as it is designed to.
“Separately, during repairs at one of our pumping stations mechanical problems were encountered, and a valve could not by fully opened causing an additional release to prevent flooding. Because of the risk of harm to the environment, we immediately raised this an incident and put in place a plan to mitigate the risk.
“In addition to extra expert engineers mobilised to the pumping station we moved a tanker fleet of 50 vehicles the area to carry flows if needed, prepared overland pipes to bypass the valve, stood up additional teams and carried out environmental monitoring.
“Overnight, the valve was successfully opened and the tanker fleet and pipeline were stood down.”
Storm overflows operate to protect areas from flooding, releasing excess water to prevent sewage from backing up into homes and businesses.
However, the system has faced heavy criticism as it often can pollute rivers, seas and sensitive habitats.
Earlier this year, a Channel 4 docu-drama – Dirty Business – explored water companies role in sewage pollution, telling the stories of people who believe their lives have been affected by polluted waters.
The Southern Water spokesman added: “We’re committed to ensuring cleaner rivers and seas for everyone in our region, and reducing storm overflows is a major part of this.
“Storm overflows act as a safety valve to protect communities from flooding when sewers become overwhelmed during heavy rainfall. However, we recognise the current level of their use and the impact on rivers and seas is not acceptable, and we are working hard to reduce it
“Our £1.5bn Clean Rivers and Seas Plan is already starting to make a difference, preventing more than 700 releases in the first year.
“This is a clear sign that our approach is working – and we’re now scaling up resources, accelerating projects and expanding our partnerships to go even further, faster.”
The incident comes amid growing criticism of Southern Water and its efforts to protect the environment.
In March this year, the Environment Agency found “poor maintenance” during its 730 inspections of Southern Water wastewater assets and told the company to make 400 improvements.
Southern Water said its turnaround plan in April 2023 “delivered real change by spring 2025” and the “vast majority of the 400 required actions relate to relatively minor breaches”.
It cited “a range of improvements”, such as reduction in overall pollution by 30 per cent, internal sewer flooding by 40 per cent and leakage by 18.5 per cent.