South Elmsall Sewage Treatment Works and other Constructed Wetlands
Philosophy
The primary objectives for using PowerCem products in wetland construction are:
1. To significantly reduce the carbon footprint of the construction
2. To preserve virgin aggregate resources
3. To avoid the use of a manufactured membrane
4. To create a facility that can be maintained simply and safely throughout its intended
design life
5. To use materials that can be fully recycled at end of life of the facility, with zero waste
Project Details
The goal of the project was the creation of 5no. wetland cells at Yorkshire Water Sewage
Treatment Works (STW) in South Elmsall, Yorkshire, covering a total area of approximately
25,300m2 in order to provide sewage water treatment and retention capacity and help reduce
storm overflow discharges into Frickley Beck, a tributary of the River Don. Pollutants and
nutrients from the storm water will be broken down and removed by a planting regime within the
cells. The wetlands will accommodate a flow of 440 l/s of diluted waste water.
Methodology
The design of the wetlands was engineered to ensure a balanced cut/fill; no soil to landfill and
no imported soil or granular material.
The wetland was constructed in a field adjacent to the existing sewage treatments works,
between the railway line and the beck.
The superficial vegetation was stripped off and the topsoil removed and stockpiled.
The bulk earthworks were carried out to create the plateaus for each of the 5 cells.
The bases of the cells were stabilised insitu, to a depth of 250mm, utilising RoadCem and
cement in the ratio 1.2kg/m³ and 120kg/m³ respectively.
The topsoil was stabilised ex-situ, placed in 250mm layers and compacted, to form the
perimeter and internal embankments to the cells.
The embankments were formed oversized and trimmed back to the final profile after 1 week.
Prior to commencement laboratory tests had been carried out to prove the impermeability and
strength of the stabilized soils. The target permeability was to match the characteristics of
puddle clay..
The first cell to be completed was filled with water, estimated to be in the region of 6 million
litres, which successfully proved the low permeability and robustness of the construction.
The stabilised embankments have been cut through to enable the installation of the in-situ
concrete inlet and outlet structures and the precast concrete inter cell cascades. Examining the
embankments in section at the cuts gave further confidence on the strength and consistency of
the stabilized material.
Carbon Footprint
The original design intent was to export all of the topsoil to landfill and to import a similar
quantity of clay to form the cells.
It is estimated that this would have involved around 3,000 heavy goods vehicle movements.
Access to the STW is along a single-track road, through South Elmsall train station car park.
The original design included construction of a 650m long temporary access road across fields to
the main road to the south of the site, requiring 2 bridges over the beck and a formal junction at
the main road.
Stabilising the site won materials reduced the HGV movements to simply delivery of the
RoadCem and cement and the construction plant.
The temporary road was not required with the RoadCem design, creating a very significant
saving in quarried stone and the carbon footprint associated with the construction of the haul
road. At the end of the project the field would have been reinstated and the construction
materials would have been waste due to contamination with soil from the vehicles.
Based on detailed studies of other projects in the UK and globally it is estimated that the
reduction in carbon footprint would be in the region of 80%. The data to carry out the detailed
study at South Elmsall is not available.
The omission of the liner is a further saving in carbon footprint in terms of the manufacture and
transport of the membrane.
Technology
Cement and lime simply ‘glue’ the particles together, resulting in a brittle material with little
tensile strength.
RoadCem significantly enhances the stabilisation process by wrapping the particles in a needle
like matrix, which can be observed at a microscopic level.
The resultant material has good tensile strength, a high modulus and is relatively impermeable.
This means that large areas can be stabilised with excellent crack control, such that movement
joints are not required.
A greater proportion of the cement is included in the hydration process which improves the
durability of the stabilised material.
Contaminants are locked into the microstructure, preventing leaching of contaminants.
RoadCem can be used with soils which are not suitable for lime, or cement stabilisation such as
soils with high organic content and clays with high sulphate content and high-volume change
potential.
Please have a look at the following construction video below.
Further details of wetland construction can be found on our website: www.constructed-wetlands.co.uk