The need to cut nitrogen levels in effluent being discharged into the River Test. The National Environmental Programme (NEP) specifies a new discharge permit condition of 10 mg/l of total nitrogen (TN) for WwTWs.
Southern spends £25M to meet NEP conditions
Due to the need to cut nitrogen levels in effluent being discharged into the River Test, Southern Water has invested £25M in an upgrade to the Millbrook wastewater treatment works. Nick Myall looks at a project in which the upgrade work had to be carried out in an area where the physical constraints of the site heightened the challenge of keeping it operational.
Keeping the WwTW working at full capacity during the construction of the new Bardenpho activated sludge plant.
The upgrade work had to be carried out in an area where the available footprint for the new buildings was limited and where overhead cables exist.
“From an engineering point of view, this has been an exciting scheme to work on. It will have huge environmental benefits – the end result will be a much greener treatment works that will treat wastewater to even higher standards before releasing it into the Solent.”
Jon Kenrick, project manager, Southern Water
“It was vital to keep the WwTW running during the upgrade work. A significant part of this project’s uniqueness comes from the two-part construction process that was adopted to keep existing capacity up to sufficient levels.”
Stewart Garrett, senior project manager, Southern Water
Originally constructed in the 1930s the wastewater treatment works (WwTW) at Millbrook in Southampton’s Western Docks serves a population of approximately 135,000 and discharges into Hampshire’s environmentally sensitive Test Estuary. It has undergone two upgrades during the 1960s and 1990s and incorporates a carbonaceous activated sludge plant (ASP), however the latest upgrade is arguably the most significant to date.
The project, which involved a £25M capital works scheme, was part of Southern Water’s AMP5 plan and was delivered by a joint venture between Veolia Water, Costain and MWH (4Delivery) and completes this September.
The physical constraints of the site itself heightened the challenge of keeping a working site operational while carrying out the upgrade work and protecting the environment says Southern Water senior project manager Stewart Garrett.
“A key driver for this upgrade was the section of the National Environmental Programme (NEP) which specifies a new discharge permit condition of 10 mg/l of total nitrogen (TN). This was a new requirement that wasn’t being measured previously. The main challenge for Southern Water was how this could be achieved using the existing assets at the WwTW while maintaining treatment capacity on a site that had space issues. It was clear that Millbrook’s existing ASP was not capable of producing effluent that met the new standards.”
The treatment technology that Southern Water selected to extract the TN was a 4-stage Bardenpho ASP process, which differs from a conventional carbonaceous ASP in its ability to remove nitrogen through denitrification to produce nitrogen gas. Two alternating stages of anoxic and aerated zones are employed to achieve the end result.
“Nitrified liquors are returned to the upstream of the process where anoxic conditions and influent carbon lead to denitrification with a further reduction of residual nitrogen occuring in the secondary anoxic zone,” explains Garrett.
Treatment of the high-strength centrate and filtrate liquors generated from the on-site Sludge Treatment Centre (STC), and that are then returned into the wastewater process, also had to be considered. A whole-life cost assessment showed that the most cost-effective and lowest carbon approach was to combine the liquors with the crude load and treat it in the Bardenpho process. This simplified the process on site and reduced the number of assets required, saving Southern Water money in the process. This also led to the replacement of the existing, shallow ASP tanks. However, accommodating 33,500cu m of process volume into the limited space available at Millbrook site posed a significant challenge.
Physical constraints and design
Due to the limited space available and the number of existing buildings on the site constructing the new Bardenpho ASP was always going to be difficult. As a result three dimensional modelling of the site was used to plan pipelines and to avoid clashes with existing below-ground services.
“With two existing plants in place there was not a great deal of room and space was clearly going to be an issue. The irregularly-shaped footprint available to use, overhead HV cables, a shallow groundwater table, poor ground strength, and existing buried services all had to be considered,” says Garrett.
The physical geometry of the large Bardenpho ASP structure had to take all of these constraints into account while remaining in step with the governing process design. In an effort to reduce footprint, the process depth was set at 7m - the upper limit for ASP plants. However this still meant a structure 92m long and 62m wide was required. A total of four lanes were selected to provide operational flexibility and were arranged in a three-pass serpentine arrangement to ensure optimum plug-flow conditions.
A close working relationship between Southern Water, 4Delivery and civil, structural and geotechnical engineers was instrumental in the smooth execution of the project.
Garrett notes: “Collaboration was key in the design and construction phases and Southern Water developed a very close relationship with 4Delivery during the project which contributed significantly towards its overall success.”
This close working relationship led to a series of design workshops which culminated in agreement on the following design approach:
1) Treatment of the made ground layers using Vibro-stone columns which create less noise and are more cost-effective than pre-cast concrete piles. The structural design had to allow for up to 50mm of potential settlement.
2) A propped cantilever system with tie-beams between opposing walls at coping level was utilised to avoid unnecessarily thick walls. This allowed the 7.5m high walls to taper in thickness from 500 to 350mm.
3) The base slab design was also more efficient as a result of adopting this solution, being only 300mm thick and 500 mm thick below the main walls.
This approach brought about a number of benefits. The propped cantilever design cut the amount of in-situ concrete that was required and the tie beams made installation quick and simple, with no requirement for connection into wall reinforcement. The beams were designed in reinforced concrete and included overhangs at each end for a simple hooked connection which will help to ensure their longevity. In total, 68 16m long beams were made to hold the treatment tank, each weighing eight tonnes. The tank itself is the length of ten buses and home to millions of bacteria that will break down particles in the wastewater. The site also incorporates two final settlement tanks, measuring 33 metres in diameter.
The construction phase
Commenting on the scale of the construction phase Garrett says: “During the construction of the upgrade over 10,000 cubic metres of concrete were used, enough to fill four Olympic swimming pools. In addition 11,000 tonnes of reinforced steel, 1,500 metres of new pipeline and 3,000 metres of new ducting were used.”
To create space for the Bardenpho ASP, two existing primary settlement tanks and an ASP structure had to be demolished.
However, as Garrett observes, the day-to day operations had to continue: “Millbrook serves the Southampton region which has a population of approximately 135,000 and the site treats a Flow to Full Treatment (FFT) of 850 l/ per second. Serving such a large region it was clear that the current permanent condition of the plant had to be maintained during the upgrade work.”
To ensure this treatment capacity was maintained at Millbrook a phased approach to construction of the new tank was adopted. Initially the greenfield area was cleared, the ground was treated and two of the four ASP lanes and two 33m diameter conical final settlement tanks were constructed.
Next all the process pipework, mechanical and electrical plant to enable commissioning for carbonaceous treatment of a proportion of FFT was installed. This additional process capacity then allowed the redundant assets to be decommissioned and demolished.
Finally, the remaining two ASP lanes were constructed in the cleared space with the completed structure commissioned for full biological nutrient removal (BNR), enabling decommissioning of the other ASP.
The limited space and overhead HV cables meant that 4Delivery used crawler cranes as the main method employed to construct the principal structures, with further mobile cranes in support. Installation of the propped cantilever tie beams for the new ASP structure was particularly challenging due to the site logistics and involved the use of a 350 tonne mobile crane to enable lifting of the eight-tonne beams into position.
The environmental credentials of this project were also high with 90% of the waste material created during the upgrade work being recycled. Soil was used as infill on site and any concrete that had to go off site was sent to recycling centres to be used by the building trade.
A successful outcome
Commenting on the success of the upgrade Stewart Garrett says: “The project ran from November 2011 to September 2014. Although the capacity at the WwTW has not increased, influent is now treated to a much higher standard. As a result effluent discharges into the Test Estuary contain less nutrients and the spread of environmentally damaging algae growth has been cut.”
The principal contractor 4Delivery brought knowledge and expertise from its joint venture partners Veolia Water, Costain and MWH. A collaborative approach to design helped to overcome physical constraints at Millbrook and led to an innovative approach to the construction of the new infrastructure required.
With a large proportion of the UK’s population in the region and a lengthy coastline, water companies in the south east are pioneering this cutting edge upgrade process while paying close attention to health and safety, existing plant operation and the environment.
Indeed, the same process is being mirrored across Southampton Water at Marchwood where Slowhill Copse WwTW is undergoing a similar upgrade helping to ensure that the waters of the Test Estuary are protected for years to come.
Nick Myall is a freelance water and environment writer
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