TDRA, Mogden STW and surrounding areas.
In May Thames Water announced significant changes in the design of the TDRA project.
One of the announced changes was that the location of the Tertiary Treatment Plant in within the Mogden STW site would be changed. No details were provided. As of Mid August no further updates have been made.
Thames Water are LIKELY - but not confirmed yet - to run a consultation on this change sometime this year. Be Prepared.
Please get your MP and local councillors to really push Thames Water on the following points (and these are just some of the concerns with TDRA both the impact around Mogden and with the wider scheme):
Where will the TTP now be constructed within Mogden site?
How will TDRA works impact on the operation of Mogden’s core business of treating sewage?
What are the proposed treatment processes for the TTP?
What are the construction and operational costs changes?
Will there be a change in how long the TDRA works will take?
What about the specifics of changes in Traffic volumes, air, dust and vibration impacts and long term impacts?
Constructing TDRA will mean that Mogden can never be used as an Indirect Potable Reuse source – thus limiting the possible water resources available to London. How does this make sense?
The “sweetening flow” will not materially improve the output of Mogden into the river at Isleworth – why do Thames Water insist on repeating that claim?
Specific construction issues:
The issues of 24/7 works, HGV traffic and other construction issues will still affect everyone that lives around Mogden - But now Thames Water has just left everybody hanging whilst they decide when they will confirm the changes.
The main tunnelling works for the treated sewage tunnel will still have to happen; This means 24/7 works on the western side of Mogden.
In the 2025 consultation there were already some issues with the information provided by Thames Water on how TDRA construction would impact people living and working around the Mogden STW site (see below). It seems unlikely that any changes will genuinely reduce any of those impacts.
If the TTP is moved to a different location in Mogden then some people may be saved from noise impacts but other may find they face a longer period of significant levels of noise. Similarly with traffic (and associated dust and vibration impacts) and general construction and excavation storage and removal; all may be increased by the change in plans.
Examples of some of the 2025 issues were:
Noise: Significant noise effects at Trevor close, Hillary drive and Bankside area on the Eastern side / and Wainwright and Harvesters on the west side. Thames Water have decided that 75db is an appropriate daytime level of significant effect (in other words anything below that is deemed not significant). This is unrelated to baseline noise levels which were measured at between 50 and 60db. This means that the relative change in noise level is not being taken into account.
Traffic: In 2025 one of the potential issues was for movements of up to 40 HGV movements a day in 2029 and 100 HGV per day for the period mid 2030 to late 2031– with other construction traffic on top of that. Will the changes to be announced mean a reduction or increase in any of these impacts?
Space: Mogden is constantly described as “space constrained”. In 2025 there was no reference to the Mogden Master plan in terms of other works at Mogden over the next several years. There has been no detail on potential impacts of other works in conjunction with TDRA. Thames Water has a capital investment forecast which includes mention of works of varying types in Mogden from 2027 for 54 months. This would then be ongoing at the same time as the start of construction of TDRA! Concurrent works at Mogden means even more impact on traffic volumes, air quality etc and also increasing risk around the working of the site.
In 2025 RAPID highlighted that there is no “… detailed plan for [an] alternative option if the space at Mogden Wastewater Treatment Works is no longer available to use. The space is limited and in high demand and finding an alternative space may lead to changes in the TTP layout, design, and location within Mogden. This could lead to an impact on the scheme costs and programme.”.
So the potential of changing the location of the TTP has been on the cards for a long time! And it is incredible that Thames Water has still not described how construction of the TDRA infrastructure will not interfere with the operation of Mogden STW nor with the need to increase the capacity of Mogden over time.