The construction of the project will comprise four main phases:
1. Site set up and enabling works
2. Construction and assembly
3. Wet and dry commissioning of the new WWTP
4. Decommissioning of the existing WWTP
Phase 1 – Site set up and enabling works
To manage the construction phases outlined above a main site compound will be built during site set up in Phase 1, its exact location is yet to be established but it be outside of the earth bank screen which will encircle the facility. Satellite compounds will also be established at existing WWTW, at the shaft locations on the transfer tunnel, and at the discharge chamber adjacent to the River Cam. These satellite compounds will be smaller compounds and will provide offices, welfare and storage areas in order to limit movements between the main compound and the above locations.
The main compound will be established as soon as works commence and will remain for the duration of the construction phase. The satellite compounds will remain only for the period of the construction work they are managing.
Construction access
It is anticipated that construction access will be via Horningsea Rd. Sufficient parking and storage areas will be provided at site compounds.
Phase 2 – Construction and Assembly
Construction of the new Facility
The facility itself will be encircled by an earth bank screen to visually screen all but the tallest elements of the waste water treatment process from view. It envisaged that the earth bank will be constructed solely from material excavated from the works (e.g. tunnels and pipelines) in order to prevent the need to import additional material. Detailed below is a summary of the key elements of the facility and an indication of construction techniques likely to be used.
Bases, Walls and ground and suspended slabs
Reinforced insitu cast concrete will be used to construct the bases, walls and slabs of the tanks and chambers that form the structural elements of each of the above ground process tanks. Where possible, we may seek to replace insitu concrete techniques with either precast concrete or an alternative material, such as recycled plastic. These alternative techniques are often used to form smaller chambers, or for the walls and suspended slabs within the process tanks. For smaller above ground tanks, glass coated steel tanks sitting on a concrete slab will be used. These will be brought to site in segments and assembled in their final location.
Pipework
Between the process tanks there will be below and above ground interconnecting pipework. The below ground pipework will generally be constructed using open cut techniques. The above ground pipework will be supported on galvanised steel frames. Some of the above ground pipework will be clad in insulation to protect it from freezing.
Access, mechanical and electrical equipment Platforms
To support mechanical and electrical equipment and provide access to the tanks, galvanised steel walkways and platforms will be constructed. These will be fabricated off site and imported to site for installation.
Process and control buildings
Some of the mechanical equipment and the electrical control panels will require housing in process buildings or kiosk. These buildings will be in the form of Kiosks or galvanised steel frame structures with profiled steel cladding. The choice of is dependent on the size and type of equipment housed within it. For the kiosk solution these will be fabricated off site and imported to site as a complete unit or as segmental units which will be assembled on site. For the steel framed solution, they will be fabricated offsite before being imported to site where they will be erected and cladding panels installed once the structure is complete and self supporting.
Mechanical equipment
The mechanical equipment required for each process within the facility will be manufactured offsite and delivered for installation into or adjacent to the process tanks and buildings noted above. They will be installed on site by specialised sub-contractors.
Electrical Equipment and cabling
As with the mechanical equipment, the primary electrical equipment and control panels will be assembled and tested off site and then imported to site and installed (e.g. Motor Control Center (MCC) panels). Smaller electrical components which have to be fitted and connected locally to mechanical equipment will require insitu installation. Both high and low voltage electrical cables are required to link mechanical and electrical equipment together. Above ground cables will be supported on steel cable trays. Below ground cables will generally be installed in cable ducts. Some High voltage cables may be laid directly into the ground.
Phase 3 – Wet and dry commissioning
Once a process unit is assembled and the civil, mechanical and electrical works are complete the unit is ready to be tested. Three types of test are usually required, namely water testing, dry testing and wet commissioning.
The water test checks that the tank or pipe will hold water at the design pressure and not leak.
Dry testing checks that the mechanical and electrical equipment has been installed correctly and works when required, producing its anticipated output e.g. air flows for blowers, switch limits for control panels.
Wet Commissioning is when the plant starts to treat the effluent as it is designed. This is a planned sequence of activities that seeds the process tanks with the biological enzymes and the sludge centre with sludge that each process can treat. This operation will start to transfer flows from the existing works to the new works. To speed up the process biological enzymes can be taken from the existing Milton works.
The sewage transfer main from the existing Cambridge facility to the new site
To transfer the sewage and storm water flows from the existing Cambridge facility to the new facility we will construct a 2.5km long tunnel, 15-20m below the ground using a pipe jacking techniques.[FS2]
To use the pipe jacking method we will need to construct six shafts, three which will be temporary, with 2 installed as permanent structure for inspection and maintenance purposes. The temporary shafts will be used to install the 2.4m diameter pipes. Once the pipes are installed these temporary shafts will be removed. The 2 other shafts will be part of the permanent works design.
Treated effluent pipeline to the River Cam
The treated effluent main will be installed using traditional pipelaying techniques and, generally, the three 1.5m diameter pipes will be installed within an open excavation. The excavations will vary in depth as the ground level varies. Prior to laying the pipes a working easement will be established up to 40m wide and fenced on both sides. The easement width will allow for a sufficient area to stockpile topsoil, sub soil, allow room to string out the pipes and provide a working area to lay the pipes whilst also allowing access to the rest of the pipeline and the outlet chamber. The easement will be accessed from the compound on the main site.
Adjacent to the River Cam there will be a discharge structure built into the existing flood embankment. It is envisaged that it will be constructed using insitu cast or precast concrete and is likely to be built within a sheet pile cofferdam (an enclosure built within a body of water). The cofferdam will be designed to maintain the flood protection levels currently provided by the flood embankment. Only when the new structure is complete and connected to the flood embankment will the temporary protection be removed. The discharge structure will be accessed along the pipeline easement from the main construction compound
Earth bund, landscaping and habitat creation
The proposals include enclosing the WWTP facility within a high circular earthwork bank, formed from material excavated as part of the construction activities. This new landform aims to screen all but the very tallest elements of the WWTP from most directions. Tree stands of native woodland are proposed around the northern, western and southern sides of the site, which would introduce new habitat and help to screen the works from viewpoints such as Horningsea and Fen Ditton villages.
The landscaping for the proposed WWTP is being developed to screen the facility in a way that integrates it into the landscape and supports biodiversity gains. The timing of planting, which may include grassland, woodland, tree lines and hedgerows, could be planned in a way that advances the screening effects. Planting activities could extend beyond the facility on nearby land, but this would be subject to agreements with the landowners.
At this stage the proposals include:
- Provision of a new area of woodland of up to 25 hectares;
- Provision of new extents of species rich grassland up to 35 hectares;
- New lengths of hedgerows that may provide up to 9500 linear metres; and
The creation of a new landscape feature formed from excavated material.
Need images+ consultation video of landscape proposals
Figure 2‑21: Proposed new pedestrian access and bridleways as present at Phase Two Consultation
| Figure 2‑20: Indicative drawing of the Proposed Development |
Phase 4 – Decommissioning
Once the new WWTW is operational, the existing site at Milton will be decommissioned. The demolition of the site will be undertaken by future developers and therefore will not fall within the scope of the Development Consent Order (DCO). Anglian Water will need to surrender all permits including the Industrial Emissions Directive (IED) permit and undertake remedial works as required.
[FS1]Show on GIS map, if we have info
[FS2]Possibly insert video -similar to this:
https://www.bing.com/videos/search?view=detail&mid=3D2318976CB28146AD623D2318976CB28146AD62&q=pipe jacking video&shtp=GetUrl&shid=a98bed07-805a-4a08-adfe-4a6970aface8&shtk=UGlwZWphY2tpbmcgTWljcm90dW5lbGxpbmcgQW5pbWF0aW9u&shdk=Uk9BRCBcdTAwMjYgUklWRVIgQ1JPU1NJTkcgY291cnRlc3kgSXNla2k%3D&shhk=dntBIXpJS2mJwEN4fDwbKfCri2%2FwO51PFAYAkk1ngM0%3D&form=VDSHOT&shth=OVP.-CVjy7xsWOk4X4NyjfBOWQEsDh