A leading US water expert has suggested that Sustainable Drainage Systems (SuDS) in tackling combined sewer overflows (CSOs) for London is a realistic alternative to Thames Water’s proposed £4.1 billion Tideway Tunnel for London and that more studies are needed before any final decisions are made. The utility has acknowledged that SUDS has an important role to play in addressing the issue but argues that there is no choice and that London needs both a Tunnel and SUDS to address its problems.

Dr Mark Maimone, Senior Vice President CDMSmith, a US engineering consultancy, has been in London this week to undertake a series of high-level meetings explaining how the city of Philadelphia Green City, Clean Waters strategy could successfully be used in London. The city is aiming to tackle 85 per cent of CSO discharges through SuDS measures over a 25 year period.

On Monday Dr. Maimone was among a number of experts who met to discuss how London can learn from the Philadelphia experience at a high-level meeting in the House of Lords. Deputy Liberal Democrat leader Simon Hughes M.P, who has already expressed concern about funding arrangements for the supersewer and Lord Selbourne, who chaired the independent commission into the Thames Tunnel, were both present at the meeting.

Following the meeting Dr. Maimone has responded to Thames Water’s latest publication on its Tunnel consultation website setting out the case for the Tunnel.

The utility says that it has looked at the role of Sustainable Drainage Systems (SuDS) in tackling combined sewer overflows (CSOs) using case studies from Philadelphia, Portland and London.

The statement says that Philadelphia aims to tackle 85 per cent of CSO discharges through SuDS measures over a 25 year period and that while the work has started, it cannot yet be deemed successful. Portland has removed approximately eight million m3 of storm water a year from the system using SuDS (35 per cent of total CSO volume), but tunnels still proved to be a necessity to meet the required levels of control.

Tideway Tunnel essential to tackle scale of discharges in timescale required

Thames Water commented:

"A tunnel solution akin to the Thames Tideway Tunnel may still prove necessary to meet the required environmental standards.“

"London’s specific circumstances dictate that the proposed Thames Tideway Tunnel is essential to tackle the scale of the discharges in the timescale required. SuDS could and should play an important role in extending the life of the Thames Tideway Tunnel, but can in no way replace the need for it.“

In an article by Richard Aylard, Thames Water’s Director of External Affairs and Sustainability, in Thames Estuary Partnership’s magazine ‘Talk of the Thames’ at the end of November, he explained why the proposed Thames Tideway Tunnel and Sustainable Urban Drainage Systems (SuDS) each have a vital role to play in tackling sewage discharges to the River Thames.

Mr. Aylard commented:

"At Thames Water we are enthusiastic about the potential of local or targeted SuDS, which is why they feature in our proposals to tackling sewer flooding to customers’ properties in the Counter’s Creek catchment in west London. We expect that SuDS will increasingly be a part of other retrofit schemes and will play a key role in ensuring that new developments do not add to existing problems. However, the idea that SuDS might, on their own, somehow provide a solution to the modern day scandal of sewage discharges to the River Thames is a very different matter, and one that literally doesn’t hold water in modern London, any more than it did in the 1850s.

"The problem of combined sewage discharges to the River Thames needs practical solutions that reflect London’s conditions, will achieve the required standards, and can be implemented quickly. The river and London as a whole urgently need the Thames Tideway Tunnel and also, over time, to implement SuDS to prevent the existing problems growing worse. The two approaches are complementary and we need to utilise both of them."

Dr Maimone has now questioned a number of the arguments set out by Thames Water explaining why the Philadelphia approach is a not an appropriate solution to the problems of London’s ageing sewerage system.

In response to the utility’s comment that "Philadelphia has indicated that SuDS will, in an average year, control 19 to 25 million cubic meters of CSO discharges out of the total estimated at 38 to 49 million cubic meters. That’s a level of control of 50 per cent, far less than the required minimum of 85 per cent.“ Dr. Maimone said that the numbers are not close to being correct, describing them as "absurd misinformation."

According to Dr. Maimone, Philadelphia discharges over 60 million cubic meters of CSO in an average year. The program is designed to capture 85% of pollutants, and the total volume controlled will also be at around 85%, once the SUDS program is complete, meeting the 85% control target set by the US Environmental Protection Agency. He added:

"To say that this is less than the required minimum means that the US EPA and Pennsylvania Dept. of Environmental Conservation just signed agreements in error.“

In response to Thames Water’s comparison between a projected increase of 74,000 people by 2031 in Philadelphia, and a projected increase of additional one million people in the area served by CSOs in the same time frame in London, Dr. Maimone says that in Philadelphia, population growth is seen as an advantage to the SUDS program. Under the stormwater regulations, all new development results in the automatic addition of SUDS, significantly reducing stormwater contribution to the sewers and more than offsetting any increase in foul water by the population increase.

On Thames Water’s comment that Philadelphia has a much higher proportion of open space available for implementing SuDS measures, Dr. Maimone says this distorts the program and that SUDS are not in fact being implemented on "open space" in Philadelphia. The city are actually "re-purposing existing space“ - essentially by turning existing impervious areas to SUDS without compromising their existing use. SUDS are being built on roofs, streets, pavements, playgrounds, school properties, parking lots, etc with only some using open space.

With regard to differences in geology, Thames Water says that while London is built on top of a layer of thick clay and saturated gravels that do not easily soak up water, Philadelphia sits on much more permeable soils. According to Dr. Maimone, Philadelphia has a geology that consists of river clay, sands, silts and gravels, covered in many areas by artificial fill material – with highly variable permeability. He points out that permability is also highly variable in London, which sits atop clays, sands and gravels associated with river deposits. Dr. Maimone says that SUDs in Philadelphia are being designed to either infiltrate or slow release, depending on the infiltration rates at the site, which is equally possible in London.

He also refutes the utility’s comment tha Philadelphia’s existing system has 164 discharge points by design, compared with London’s discharges which are concentrated to far fewer points (57). Dr. Maimone says this has "little to do with implementing SUDS or building a tunnel“ and that London will soon have one fourth the amount of CSO volume discharging from one third as many outfalls which will simply be handled in the design of the program.

Dr. Maimone describes Thames Water’s argument that while each of the London CSOs has on average an area connected to it 10 times larger than is the case in Philadelphia whichmeans that per CSO, the average volume discharged in London is twice as much compared to Philadelphia as a "meaningless calculation."

According to Dr. Maimone, while there are 165 outfalls in Philadelphia, the largest 10 outfalls account for about 60% of the total CSO volume which discharge much higher volumes than any of the London outfalls.

He also comments that Thames Water's suggestion that unlike London, Philadelphia has received a disposition (exemption) from wastewater quality standards with the city projected to meet a 50 per cent level of control (with full implementation of their SuDS plan), rather than the 85 per cent minimum required elsewhere in the United States as "completely incorrect“, pointing out that the Consent Order on Agreement with the US Environmental Protection Agency requires Philadelphia to meet the 85% minimum.

EU and USA have moved to a more sustainable approach – clearly not too late for London

Dr. Maimone has also commented on references by Thames Water on the feasibility of a SUDS-based approach in the context of work undertaken by the city of Portland in Oregon in the 1990s, which said that Portland had embarked on an ambitious programme aiming to tackle CSOs via SuDS and set out with the specific aim of removing the need for a tunnel.

Dr. Maimone says this is not true and that the SUDs program has been aimed primarily at separate sewered areas. Portland had to build a tunnel because this was what the USEPA was requiring at the time and this approach has changed at the Federal level in the US to advance broader environmental goals.

In response to Thames Water’s comment that unlike Philadelphia, Portland is legally required to achieve a 96 per cent or higher level of CSO control, a target comparable to the one London would meet with the Thames Tideway Tunnel, Dr. Maimone says that this is true, commenting:

"(This) is exactly the point of shifting the paradigm from the older approach. In the approach of the 1990s, regulations aimed only at reducing CSOs, regardless of their actual impact on water quality and aquatic habitat."

"The EU and US EPA have moved to a more sustainable approach, one that considers the whole environment (air, water, stream baseflow, urban amenities, flooding), and evaluates solutions based on environmental, social, and economic benefits. This change in philosophy was too late for Portland, not for Philadelphia, it is clearly not too late for London."

More thorough analysis would be expected before deciding on a £5bn investment

Dr. Maimone also comments that geologic and infiltration studies such as those being carried out in Philadelphia are required in response to Thames Water’s reference that like Philadelphia, and unlike London, the geology of Portland is more suitable for SuDS; the soils underlying the SuDS areas of the city are more porous and more able to soak up excess rainwater. He says:

"Even a cursory look at existing geologic maps of London show areas of gravel and sand in CSO areas. One would expect a more thorough analysis before deciding on a 5 billion pound investment.“

He also takes issue with Thames Water’s view that SuDS measures alone would be ineffective in London and fail to control CSO discharges to the required level in the required timescale and that unlike Portland and Philadelphia, London is a heavily urbanised city, with relatively little open land available for SuDS measures. In his view, this could only be concluded after a thorough study of the SUDS alternative which has yet to be done.

Dr. Maimone points out that Philadelphia and London have very similar degrees of impervious cover and that describing the CSO areas of Philadelphia as less urbanized than London is absurd. He also makes the observation that New York City is implementing SUDS in Manhattan and that the Thames Water statement has avoided making a comparison between Manhattan and London.

Commenting on Thames Water’s view that the maximum practical level of retrofit SuDS would take over 30 years to implement, cost a projected £13 billion and still not provide the level of CSO control needed, Dr. Maimone says:

"The 13 billion pound number cannot have been adequately researched, based on lack of a serious study and alternatives analysis for such an important investment. In most US cities, SUDS solutions have been roughly equivalent in cost to tunnels, and one would expect something similar in London.“