Southern Water discharged enormous volumes of raw sewage into protected coastal waters for nearly six years causing “very considerable environmental damage” because it was cheaper than treating it, a court has heard.

This was “the worst case brought by the Environment Agency in its history”, the court was told. Southern Water had acted “deliberately” and had reaped “considerable financial advantage” by allowing the discharges.

The Environment Agency’s biggest ever investigation uncovered how the privately owned company dumped untreated sewage into seas that were home to shellfish beds and were supposed to be some of the most protected marine environments in the country.

Andrew Marshall, appearing at the sentencing hearing for the regulator, told Canterbury crown court that Southern Water, which is ultimately under the control of Greensands Holdings, opened storm tanks to release raw sewage into coastal waters in north Kent and the Solent to increase its own financial benefits. The company, also allowed storm tanks to be kept full and to turn septic, instead of putting millions of litres of raw sewage through the treatment process as required by law.

In one wastewater treatment plant, Millbrook, outside Southampton, the equivalent of 371 Olympic-sized swimming pools of sewage or 746m litres, were released into Southampton water in four years and eight months.

“The company charges its customers for treating wastewater and they are required by legislation to properly treat that wastewater,” said Andrew Marshall.

“But at 17 wastewater treatment plants the company chose to discharge raw sewage to the environment, mainly coastal waters, and it was doing that because it was a far cheaper alternative than to properly treat it.”

The company’s illegal dumping of raw sewage into waters that were home to shellfish beds was discovered after the quality of shellfish on the north Kent and south coast plummeted because of high levels of faecal contamination and failed to meet quality standards.

What was happening, Marshall said, took place between 2010 and 2015, was deliberate and was known about at corporate level.

“It was known about and permitted at at a high level in the company, was brought about deliberately, by a deliberate lack of control and investment and … has caused very considerable environmental damage by the release of raw sewage into coastal waters,” he said.

Southern Water, whose operating profit in the year 2019/20, the court heard, was GBP213m, has pleaded guilty to 51 counts of knowingly permitting entry to coastal waters of poisonous, noxious or polluting matter and/or waste matter and/or sewage effluent, namely untreated sewage otherwise than as authorised by an environmental permit.

The investigation was the largest in the 25-year history of the agency. It was the result of long-running inquiries into environmental damage in the seas off north Kent and Southampton, particularly the contamination of shellfish with faecal bacteria.

The dumping of sewage took place along the north Kent coast, which includes Whitstable and Herne Bay, where multiple environmental protections are in place covering the marine environment. It stretched along the Solent, between Southampton and Chichester, waters which are also covered by protected water status. Both areas were home to shellfish beds, which as a result of the contamination were slapped with bans or made unviable because of the faecal bacteria levels, the court heard.

As well as dumping sewage into the sea, Southern Water failed to report its illegal discharges to the agency, leaving investigators searching for other sources before their criminal inquiry led them to 17 wastewater treatment plants. “There was a degree of puzzlement because there was nothing reported from Southern Water,” Marshall said.

Marshall said: “Clear themes of corporate offending were revealed … known faults not being addressed, lack of funding to maintain and improve infrastructure at sites …

“The facts reveal long-term corporate knowledge of the situation. It was not some site worker who was making a decision by themselves … this was company held knowledge that has revealed … the worst case brought by the Environment Agency in its history.”

The result of the long period of pollution of coastal waters was very considerable environmental damage, and financial harm to the community and an industry attempting to provide sustainable food for the future, the court heard.

For Southern Water the criminal activity reaped them “considerable financial benefit,” the court heard.

Marshall said: “It (Southern Water) is paid for treating sewage … every consumer has to pay. What it was doing was avoiding the cost of these operations that it was being paid to carry out …

“It did not pay for the maintenance and repair of its equipment or the improvements necessary. It under-reported its performance. There has been considerable financial advantage to the company … It was being paid for something it was not doing.”

The investigation found that at the 17 sites there were 10,741 discharges of raw sewage, 78% of which (8,400) were illegal.

The prosecution is focusing on 6,971 unpermitted discharges of raw sewage to the environment, which amounted to 61,704 hours of releases, or a duration of seven years.

At the Eastchurch wastewater treatment works investigators found a sheet of board had been put into the system to stop wastewater flowing into the treatment section of the plant. Instead raw sewage was collected in storm tanks and dumped into the environment.

Storm tanks that were supposed to be emptied after high flow through the system were kept full, which meant they released more toxins into the sea when they were discharged. The treatment works were being deliberately run at less than half their proper capacity of treating waste water, the court heard.

Southern Water was in court for the sentencing hearing, which is expected to last several days.