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 later 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):
Details of where the TTP will be placed within Mogden.
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?
Specifics of changes in Traffic volumes, air, dust and vibration impacts and long term impacts.
How will TDRA works impact on the operation of Mogden’s core business of treating sewage?
Constructing TDRA will men that Mogden can never be used as a 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 in repeating that claim?
The main tunnelling works for the treated sewage tunnel will still have to happen and still need a starting point. This will involve 24/7 works probably on the the western side of Mogden.
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.
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 lessen any of those impacts.
If the TTP is moved to a different location in site then, some people will 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 also 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 seems to have started decided that 75db is an appropriate daytime level of significant effect. This is unrelated to any baseline noise levels. This means that what would otherwise be counted as significant noise levels in the daytime are not.
Traffic: The traffic assessment was a desk based assessment. There was a potential for movements of up to 100 HGV per day – with other construction traffic on top of that. Use of the northern site entrance / exit to be used sometimes was suggested with no specifics.
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 one for works of varying types in Mogden from 2027 for 54 months. There is some mention of the need to construct “containment” in the site which seems to be needed imminently – that may or may not be deferred. These will mean concurrent works at Mogden impacting the traffic volumes etc and also increase 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.