Thames Water puts the main contributing factors to sewage flooding down to the loss of green space and soft landscaping, combined with the increase in paved-over gardens and more intense development across London.

Water can no longer soak away into the ground. Instead it runs off roofs, driveways, paved gardens and parking areas straight into the drains, down to the combined sewer system and from there into the river at an accelerated pace.

One of the solutions put forward is the interceptor tunnel behind the Thames - a huge engineering proposition and an enormous investment, but this was not included in the recent Price Review report for the water industry for the next five years

"Plans for an interceptor tunnel were not included at DEFRA's request," said a Thames Water spokesman. "We have been part of the Thames Tideway Steering Group with the Environment Agency and the Port of London Authority, and we submitted a report on the possible solutions to DEFRA in June. We await a decision on that."

The current sewage system dates from the Victorian era and was designed to combine storm water with the sewage at times of high rainfall. The increased speed of rainwater run-off with urbanisation fills the tunnels more quickly, leading to combined sewage and rainwater overflows into the Thames.

The spokesman continued: "We are ready, willing and able to put whatever solution in place that ministers feel most appropriate. We accept that the current situation is not sustainable in the long term, but it is because of the scale of investiment that ministers asked us to submit the report. It will have an impact on customers' bills."

He continued that since privatisation in 1989, Thames Water had invested a billion pounds on improving the quality of sewage treatment, which has had a significant impact on river water quality throughout the region, including the Thames.

"Despite the overflows into the Thames, it does still support a diverse eco-system. We have made substantial improvements to the quality of sewage treatment," said the spokesman.

He put Thames Water's side of the argument about pollution incidents from Mogden, and stated that the event on the River Thames on August 3 was caused by an exceptional storm with rainfall across the catchment, not Mogden Sewage Treatment Works, which discharges into the river just downstream of Richmond Lock.

The spokesman confirmed that the total sewage overflow from Mogden on 3 August was 216 million litres and that at the time one of 24 primary tanks was out of commission for planned refurbishment but that this not reduce the hydraulic capacity of the works. There was also an unusual sludge arriving in the sewage but this represented no reduction in the site's effective capacity.

The spokesman continued that the storms tanks were not in use prior to the storm on 3rd August but were empty when it began, and insisted that the impact of the sewage on the river Thames and the subsequent death of fish was caused by an exceptional storm.

"In the price review, although the tunnel is not included, we have made a provision for an increase in the capacity at sewage treatment works and this will help to lessen storm discharges into the Thames in frequency and volume, Mogden included," he continued. "We are increasing the capacity of sewage treatement works on the tidal Thames so that during storm conditions they would be able to hold back more storm sewage so that less goes into the river."

The company also says that closure of other local sewage treatment works at Kew increased Mogden's flow by only 4%, and a diversion of additional flow from Iver South amounted to less than 0.2% of the flow to Mogden. There has also been a 25% increase in Mogden's capacity. On Perry Oaks evaporation ponds and sewage treatment, now an expanse mostly beneath T5, Thames Water say that it has has not been closed yet.

"Further, it does not handle raw sewage and its eventual closure will not divert any additional flow to Mogden," theThames Water went on.

In reply to an assertion that the company sold off land which could have been used for expansion of the storm tanks at Mogden, the spokesman said: "Mogden's peak treatment capacity was increased in the late 1990s to reduce the need to use or have additional storm tanks. Land sales were made in 1983, 1986, 1993 and 2000, and all of this land was outside the operational perimeter of the site. Storm tank capacity has been increased on two occasions, in the 1960s and in the 1990s by something like 25%.

"Thames Water has proposed increasing treatment capacity of the works, using land we already own within the operational perimeter, and this project (totalling almost £100million) has been included in Ofwat's draft determination."

According to the spokesman, the company has invested in excess of £80million at Mogden over the last five years, exceeding the value of its land sales.

See Thames Water's website on water efficiency

Friday, October 15, 2004

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