As we celebrate one year since the new sewers’ adoption code came into force, we look at the new code’s requirements and identify five main reasons why a precast concrete attenuation tank should be the number one option for most SuDS applications.

The new sewers’ adoption Design & Construction Guide (DCG) came into force in April 2020. What is unique about this new ‘Sewers for Adoption’ guide is that it allows Sustainable Drainage Systems (SuDS), such as ponds, swales and underground stormwater attenuation tanks, to be adopted by water authorities in England.

There is a very wide range of underground attenuation tank systems available. It is important to understand that not all these systems meet the strict requirements of adoption under the DCG. In this article, we explore a number of these requirements. We also look at other factors usually considered by Water Companies and other asset owners. We identify five main reasons why a precast concrete attenuation tank is likely to be the safe, appropriate option for most SuDS applications.

1. Access for inspection & cleaning

Clause 3, of C7.8 in the new DCG code, requires tanks to have provisions for access for inspection and cleaning. More crucially, the DCG states that this should also include means of removing any sediment and preventing it from being washed downstream.

The UK has an average rainfall of 1,254mm per annum, and has an increased threat of flooding and persistent rain in the next few decades due to climate change. Despite silt traps, significant amount of sediment and debris enter attenuation tanks every year. The process of removing and retrieving that sediment from inside tanks could be a major challenge for some attenuation tank systems. Despite the fact that some geocellular systems are accessible via CCTV and remote-controlled maintenance equipment, it is not clear how sediment trapped inside such systems can be effectively collected and retrieved without being washed downstream. Only piped systems and modular attenuation tanks can offer sufficient access for maintenance and removal of sediment.

Some plastic pipe systems can be used for attenuation and offer man-entry for maintenance. But the ability of such systems to take sufficient water jetting pressure during cleaning remains uncertain. Within piped attenuation tank systems, only concrete pipeline systems can offer effective water jetting resistance (≥ 4,000 psi) sufficient to remove hardened sediment, grease and FOG.

2. Structural integrity and reliability

attenuation tanks are expected to be installed under fields, car parks and minor roads where a wide range of loads would be presumed. The strength of a tank and its ability to maintain structural integrity over its lifetime is key for its use. Concrete attenuation tanks made of modular precast units, box culvert units and concrete pipes have a long track record and proven performance over their expected design life. The units are designed to Eurocodes and BS 9295, and would account for lateral forces, such as acceleration and braking forces, where applicable.

Lightweight alternatives, such as geocellular tank, may not have such proven track record. Unless strictly designed to C737, the long-term performance of such units against a wide range of loads is yet to be understood fully. Decades after the installation of an underground concrete attenuation tank, articulated trucks can brake over that tank multiple times without any fear of collapse. It is not clear if a decades’ old lightweight tank can offer the same guarantees.

3. Life Expectancy

The expectation within the industry is for any infrastructure to last and function for 100-120 years. This is the life expectancy used in Susdrain’s B£ST tool to assess the sustainability and functionality of SuDS projects. It is a life expectancy that most lightweight attenuation systems, including geocellular tanks, do not meet. The main European standard for geocellular tanks, EN 17152-1, suggests a design life of 50 years. Some plastic pipeline standards, such as EN 13598, offer testing to fulfil a service life of 50 years or more. For concrete attenuation tanks made of modular units, box culverts or concrete pipes, Series 1700 of the Specification for Highway Works in England (NG 1704) suggests that such products are deemed as highway structures capable of fulfilling a design life of 120 years.

4. Carbon footprint

Despite concrete attenuation tanks being significantly heavier than plastic alternatives, there is a significant difference between both systems on embodied and operational carbon emissions. Most geocellular tank systems are made of mould injected polypropylene, which has a cradle-to-grave carbon footprint 30-40 times higher than an equivalent mass of precast drainage products. This massive difference is likely to double if a 100+ years SuDS development service life is considered. The end-of-life impacts of a precast concrete attenuation tank are also likely to be lower. A precast concrete tank can be dismantled and reused after the end of life. Even if demolished after the end of its service, the crushed concrete will absorb significant amounts of carbon dioxide and will be used as aggregates. On the other hand, a geocellular tank retrieved after 50 or 60 years will need to be recycled or incinerated, leading to further emissions which may exceed 900 kg CO2/t and 1340 kg CO2/t respectively. Leaving the plastic tanks underground after use will cause the plastic to decompose over hundreds of years, leading to further Greenhouse gas emissions.

5. Space constraints

Not all sites in need of SuDS can afford sufficient space for ponds or wetlands. Underground tanks are likely to be used in most existing urban sites as tanks can be built under estate roads and car parks. Precast concrete tanks offer a superior option in terms of space constraints as units can be installed in the tightest of places adjacent to existing structures without any concerns over structural integrity. Concrete attenuation tanks can also be installed with lower cover depths and can be installed much deeper compared to geocellular tank alternatives.

SuDS is still a relatively new concept. There have been lots of high-profile applications for almost all attenuation tank systems, but only time can tell whether these applications are successful and offer the asset owners value for money. As the use of attenuation tanks grows over the next few years, it will be possible for the industry to learn more about the strengths and flaws of each attenuation tank solution.