The Transport Department confirmed that it was the first time the system had been beaten this year after the overflow cavern beneath Snow Hill filled with water, but was unable to drain during an uncharacteristically wet August.
On two occasions last month, the department said, ‘very diluted’ sewage was discharged to sea.
Duncan Berry, the assistant director for waste water treatment at the Transport Department, said: ‘The system is designed to do this in extreme events such as we had last month.
‘We filled the cavern and discharged during the very heavy rainfall in the early hours of 13 August and then again on 26 August.’
Between those two dates, Mr Berry explained, persistent rain meant the cavern did not get a chance to pump water back into the sewage treatment network.
The cavern is a huge hole 25 metres below Snow Hill car park. It was built to stop raw sewage being discharged at sea via the Island’s old sewerage network.
From about the 1950s foul and surface water began to be separated in the majority of the Island’s network but making similar changes to St Helier’s underground infrastructure was not possible. In the 1990s work began to create an overflow cavern to deal with heavy rain when the sewerage system came under pressure.
‘With the recent heavy rainfall the cavern has completely filled and we discharged very diluted sewage to sea,’ Mr Berry added.
‘This is the first time the cavern has overflowed this year. On average this only happens once or twice per annum.
‘The spills are very dilute and will be rapidly dispersed in the conditions. All waste water networks are designed to spill when the flows are so high. We are very lucky to have the added benefit of the cavern to protect our aquatic environment from what would be many more spills.
‘We have used the cavern so far this year approximately 50 times, but this is the first time it was beaten. The total volume of the cavern is 25,000 cubic metres equivalent to 25,000 tonnes.’
Mr Berry said that the Environment Department and aquaculture industries – fishermen and shelfish farmers – are notified when spills occurred.
