As the world continues to be gripped by the unprecedented Covid-19 pandemic, there has been very little coverage of the historic change to the codes governing the adoption of sewers by water authorities. One year ago, the new Design & Construction Guide (DCG) code, previously known as Sewers for Adoption, came into force. The new code introduced significant changes to the principles around sewers’ adoption, as for the first time a number of Sustainable Drainage Systems (SuDS) became adoptable by water authorities.

Adoption has been a major obstacle to the wide introduction of SuDS in England for several years. The new code comes after water companies decided to take the initiative on SuDS responsibility in the absence of any plans by Government. Water UK noted that laws dating back to the Victorian era were used to reinterpret the meaning of ‘sewers’ and include a range of SuDS solutions into the new adoption code. For the first time, a wide range of natural SuDS, such as swales, retention basins and wetlands, became adoptable by water authorities. Engineered SuDS, such as soakaways and underground attenuation tanks, can also be adopted by water authorities.

However, it is important to note that not all SuDS solutions are adoptable under the new code. The Water UK document suggests that adoptable SuDS would only include assets that “convey and return stormwater flows to sewers, surface water bodies or groundwater”. Adoptable SuDS need to have an effective point of discharge. The Code also sets a range of requirements on different SuDS solutions such as attenuation tanks. For example, a tank will need to have provisions for access for inspection and cleaning with means to remove and retrieve trapped sediment in the tank. It is likely that such requirement will significantly limit the field of attenuation tank solutions available for adoption as a significant proportion of geocellular tanks in the UK market may not have provisions to enable sediment trapped inside tanks to be collected and removed without being washed downstream.

The code also refers to standards such as C737, on structural design of geocellular tanks, which requires such systems to be designed with its long-term compressive strength (usually significantly lower than short-term) taken into account. Other structural issues associated with lateral forces on underground tanks, such as vehicles’ braking and acceleration forces, may further limit the options for attenuation systems to be adopted by water companies.

Other aspects, such as tanks’ life expectancy, embodied carbon, and cost of maintenance may all affect the choice of what attenuation tanks to construct and adopt.

More information on sewers adoption can be found at Water UK’s website (here). A good source of information on SuDS include Susdrain (www.susdrain.org) and our new precast concrete SuDS solutions website (www.precastsuds.co.uk).