Southern Water has reported a fifteen-fold improvement in its performance to prevent sewer flooding incidents in the past year.

The achievement is the result of a concerted effort to improve the performance of the sewer network and a £multi-million pound investment programme by the water company.

During the winter of 2013/14, 60 towns and villages suffered from ground, river or surface water forcing itself into the sewer system. This resulted in Southern Water taking emergency action to prevent internal sewer flooding and restrictive toilet use.

The company sent a fleet of nearly 150 tankers to ferry wastewater from the inundated sewers to local treatment works. When the tankers couldn’t take the wastewater away quickly enough, the company installed overpumps to pump the wastewater, at a rate typically 25 times faster than a tanker, directly from the sewers into a tank where it is passed through an elementary filtration system before being discharged into nearby rivers, streams or ditches.

It also set about surveying and sealing its sewers – investing £12 million alone in towns and villages prone to groundwater infiltration. Although further work will be carried out this year, there has already been a huge improvement in performance.

During last winter (2014/15) Southern Water needed to tanker in only two of the 60 vulnerable locations and overpump in only one, compared to performance a year earlier when tankers were employed at 30 locations and overpumps installed at 24.

Moreover, in some locations groundwater levels rose 15 metres above the trigger levels of the previous year when tankers and overpumps had to be commissioned – but the sewer network continued to operate soundly without external assistance.

Southern Water’s Director of Operations Phil Barker commented:

“Our overall network is vast, measuring some 40,000km (approx 25,000 miles) in length – about the same length as the circumference of the earth. Not surprisingly, parts of it are ageing which means there will always be work to do, particularly in locations such as those regularly subjected to groundwater sewer flooding.”

“With the spring weather being drier than usual this year, people tend to forget just how wet the past three winters have been but we have been working hard and innovatively throughout all the seasons to improve the performance of our sewer network in locations where it has been vulnerable.”

The project, which is ongoing, has resulted in a much improved performance. Much of the work uses modern engineering techniques which means it can be conducted remotely from manholes instead of excavating the highways..

Southern Water is also addressing the issue of groundwater flooding which can cause work to be interrupted – the firm is presently trialling an engineering technique which stabilises the saturated ground around a sewer, effectively building a groundwater dam so that repairs can continue to be made from inside the sewer pipe. This is the first time that Southern Water has adopted the ground stabilisation technique – more commonly used to repair canal walls – to aid its sewer repair work.

At the height of the weather emergency, the company’s engineers invented a new type of mobile treatment tank which was quickly rolled into position at key sites to provide a much higher level of wastewater treatment when pumping wastewater to watercourses. The tanks (which have caught the eye of judges at national awards events) afford a similar level of treatment to that at a standard wastewater treatment works, even when the wastewater is already heavily diluted by groundwater,.

Another ‘first’ is the introduction of Infiltration Reduction Plans (IRPs) –included by the Environment Agency (EA), in its Regulatory Position Statement which required water companies across the UK to publish plans which effectively set out their strategies for dealing with groundwater flooding of the sewer network. Southern Water has responded by publishing the first IRPs to be agreed by the EA in the UK. They now form the template for other water companies to follow.

The company has also tightened its maintenance regimes and made improvements to its pumping stations to ensure that, should groundwater enter the network, it can be removed as quickly as possible.

Phil Barker concluded:

“Naturally, we are hoping for a drier than usual winter during 2015/16, but if the elements conspire against us for the fourth consecutive year we’re confident that our ongoing investment in the sewer system will again stand us – and our customers - in good stead.”