SEE05: What is the true carbon cost of Blue-Greening?

"Carbon accounting was first introduced into the UK water industry during the 2009 Price Review (PR 09). This saw water companies required to account for embodied and operational greenhouse gas emissions with a shadow price of carbon introduced into the business planning process, so that the sustainability impact of investment decisions could be demonstrably accounted for. As part of PR09 planning UK water companies also produced a 25-year strategic direction statement and many placed sustainability at the core of this roadmap. Water UK has also produced a series of sustainability indicators for the industry most of which relate to water supply performance but also include measures of total energy use, greenhouse gas emissions and renewable energy generation." [IND05, p22]

In the Price Review it states "Each company should audit and quantify the greenhouse gas impact of its proposed strategy, and develop a clear analysis of the balance between local environmental quality improvements and the wider impacts of greenhouse gases." [GAI11, Section 3.3.2]

There is a key Government policy objective on climate change mitigation and adaption stating "in line with the objectives of Defra’s mitigation and adaptation plans to help deliver the UK’s obligation to reduce greenhouse gas emissions by 80% by 2050 and work to carbon budgets stemming from the Climate Change Act 2008". This is set out in the National Policy Statement for Waste Water [GAI12, Section 2.2].

In consideration of the above, a Blue-Green study of London should include a full carbon account of predicted adopted solutions with appropriate ranges applied to each application, e.g. rainwater harvesting and SUDS, allowing for the number of applications to increase over time. In Philadelphia it was shown that costs associated with CO2 emissions were negative for their BG solution, i.e. a benefit, and positive for the tunnel alternative [CAS05].

In order to judge how different schemes compare, e.g. grey only (Tunnel), BG only or grey + BG, the boundary conditions / assumptions for the assessments require careful scrutiny ensuring that appropriate comparisons are valid. For example, for a tunnel scheme, consideration may be required over the life-cycle of the scheme on the carbon associated with:

Blue-Greening applications are essentially local (a decentralized system) and so any cumulative pumping costs are likely to be insignificant in comparison. That said, a similar comprehensive carbon accounting process would be required to cover the life cycle of the BG applications possibly inclusive of some of the key benefits highlighted on the Home Page. Arguments may necessitate a similar period, e.g. 125 years. Combinations of approaches would be needed to compare against grey + BG options.


Major infrastructure projects in the UK are usually required to carry out carbon accounting which clarify the sustainability credentials of the project. It is understood that this information is not currently in the public domain for the Thames Tideway Tunnel scheme but should be made available with all assumptions for assessment and possibly amendment in order for fair comparisons to be made against BG and combined (grey + BG) solutions.