Southern Water is being investigated by the Environment Agency, which is carrying out pollution checks on the River Ouse at Lewes.
It has come about after the mystery collapse of a huge storage tank during a gale just before Christmas.
It spilled up to 900,000 litres of storm sewage onto the site in Ham Lane.
Now the Environment Agency is trying to establish why the tank broke open and is monitoring the river for pollution.
The agency is also checking how many times Southern Water has discharged sewer water into the Ouse over the past two years.
The investigation has been revealed by ‘The Pennant’ on the Lewes Rowing Club website, www.lewesrowingclub.co.uk
Jamie Benton, Environment Management Team Leader, said: “We are still investigating the incident which resulted in the storm tank collapsing. The results of our investigation will determine any enforcement approach that we decide to take.” Enforcement action could include prosecution if the agency feels it is justified.
Nine temporary plastic tanks have been hired by Southern Water at Ham Lane at an estimated cost of £25,000 per week.
The site acts as an emergency store when heavy rain floods the town’s drainage system, which handles rain and sewage from homes and offices.
Mr Benton continued: “In times of high rainfall foul sewer pipes and pumping stations in Lewes and elsewhere can fill up with rainwater and, in extreme cases, become flooded. When this happens there is a risk that people will not be able to flush toilets or empty sinks, baths, showers etc and, in extreme cases, could flood people’s homes or businesses.
“The flooding in the sewer is therefore alleviated by allowing a water company to discharge the sewage into rivers, but only in accordance with a permit from the Environment Agency, which will have strict conditions attached to it to minimise the impact on the receiving watercourse.
“The main condition requires that the ‘storm sewage’, as it is known, is settled and screened to remove the solids, so that only effluent is discharged to the watercourse. The storm sewage will also be diluted in strength by the rainfall, and the increased amount of flow in the receiving river during high rainfall will also dilute the sewage even further, leading to a considerably diluted sewage when it enters a watercourse.”
A spokesman for Southern Water said: “The system of temporary tanks we are using is performing at least as well as the tank that collapsed, and the flows are settled and screened.
“The current discharge is the same as it would have been before the collapse, which is consented by the Environment Agency. The cause of the collapse is being investigated.
“It’s important to stress many things can affect the quality of river water – Public Health England’s Swim Healthy leaflet explains this.”