The discharge was caught on video by open water swimmer Barry Johnston as he was about to take a dip by The Groves with a fellow swimmer.
He smelled the stench and heard the sound of churning pumps as excrement, toilet paper and sanitary products floated out on to the water surface near the Boathouse pub last Saturday morning.
Mr Johnston, who lives in King Street, described the practice as ‘horrendous’ although Welsh Water stresses the discharges were 'heavily diluted' and the action is allowed by the Environment Agency.
He said: “I drop my son off at the City Baths about 9am on a Saturday and then go for a swim on the river for 40 or 50 minutes. I had arranged to meet a friend who wanted to join me, despite the rain, but when we got there it was just vile.”
News of the sewage release comes just ahead of Saturday's annual charity duck race on the Dee which sees hundreds gathered by The Groves with a handful of organisers in the water. More concerning is that drinking water is extracted from the Dee but is treated before being piped into people’s homes.
Mr Johnston, a qualified environmental scientist, suspects water intakes are shut off during a discharge. He is unsure about the impact on the ecology of the river.
He says there needs to be a replacement programme for Chester’s Victorian sewers which handle both rain water and sewage but can’t cope after a deluge.
“They say they can’t afford it but if it was spread over the coming century it might be affordable,” added Mr Johnston, a PhD student at Manchester University, who believes regulators need to get tougher to force change.
In the short run he demands information about sewage discharges is made available to river users.
Mr Johnston, who knows fellow River Dee swimmer and BBC Breakfast presenter Louise Minchin, said: “I want to see what they can see – whether there is more pollution than normal and whether it’s heading my way.”
In the past he even offered to help create an app that people could access online.
A Welsh Water spokesperson said: “During the heavy rain that occurred in the Chester area on Saturday, the emergency relief valve on our wastewater system released diluted wastewater into part of the River Dee. During heavy rain like that, we do have consent from Environment Agency to release diluted wastewater into the river.
“This is essential to prevent internal sewage flooding to homes and businesses in the area. The release over the weekend was compliant with our consent and we also did notify Environment Agency of the release.
“The use of the release valves, known as combined sewer overflows, is common practice across the whole of the water industry, particularly where sewers carry both waste and surface water as is the case in Chester. They help protect properties from the risk of flooding while at the same time minimising impact on the local environment as the releases are heavily diluted with rain water.”
Welsh Water says it is currently liaising with Mr Johnston to explain more about the system and the ‘essential role it plays in protecting public health and the environment’.