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Two plants on a small site - the Millbrook challenge

Dean Stiles

Southern Water's Millbrook wastewater treatment works upgrade, like so many in the UK, is an upgrade of an old existing works. What were the complex challenges to overcome the site's constraints?

The irregularly shaped footprint available to use, overhead high-voltage cables, poor ground strength, and existing buried services all had to be consideredThe irregularly shaped footprint available to use, overhead high-voltage cables, poor ground strength, and existing buried services all had to be considered

Project specs
  • £25M capital works project
  • The upgrade of Millbrook WwTW is one of the largest capital non-infrastructure schemes delivered on behalf of Southern Water for its AMP5 investment programme
  • Built by 4Delivery, a joint venture between Veolia Water, Costain and MWH
  • Delivery of the scheme was a challenge given the scale of the new infrastructure required and the myriad of physical constraints
  • Meticulous planning of the two-staged schedule with the utmost heed to health and safety, existing plant operation, and the environment
  • Completed September 2014

The original Millbrook wastewater treatment works was built in the 1930s when it comprised a series of rectangular storm tanks. Upgrades in the 1960s and 1990s saw the addition of a carbonaceous activated sludge plant to provide secondary treatment.

The site receives wastewater from the Southampton area with a current population equivalent of some 135,000. Located within the Western Docks, the site is sized for a flow to full treatment of 850l/s before discharging into the River Test estuary.

The upgrade was triggered by the Environment Agency’s (EA) National Environmental Programme that introduced a new discharge permit condition of 10mg/l of total nitrogen to satisfy the Habitats Directive. The existing activated sludge plant was not designed to produce effluent anywhere near this standard.

The £25M upgrade was carried out as part of Southern Water’s AMP5 plan, and delivered by 4Delivery, the joint venture between Veolia Water, Costain and MWH. It was completed in September 2014.

“The main challenge for Southern Water was how the nitrogen limits could be achieved using the existing assets at the works while maintaining capacity on a site that had space issues,” says Stewart Garrett, senior project manager, Southern Water.

“With two existing plants in place there was not a great deal of room and space was going to be an issue. The irregularly shaped footprint available to use, overhead high-voltage cables, shallow groundwater table, poor ground strength, and existing buried services all had to be considered,” Garrett says.

The process
The treatment technology selected to provide the total nitrogen removal function was a four-stage Bardenpho activated sludge plant (ASP) process, which differs from conventional carbonaceous ASP in its ability to remove nitrogen through denitrification to produce nitrogen gas, says Paul White, civil engineer at 4Delivery.

“This is achieved through two alternating stages of anoxic and aerated zones. Nitrified liquors are returned to the upstream of the process where anoxic conditions and influent carbon enables denitrification to occur.

Further reduction of residual nitrogen occurs in the secondary anoxic zone, but due to depletion of carbon in the mixed liquor (BOD/COD reduction), an external source is provided in the form of methanol dosing.”
An additional 80% of settlement tank capacity (surface area) was provided for removal of biomass from the secondary effluent to ensure clarity of the final effluent.

“Sizing of the Bardenpho ASP process had to consider the projected PE figure of 155,000 (2020/21 horizon). Further consideration was given to the high-strength centrate and filtrate liquors generated from the on-site sludge treatment centre (STC) that are returned into the wastewater process,” he says.

Analysis showed that in terms of ammonia concentration, these combined liquors contributed approximately 125,000 PE in addition to the crude influent load. A detailed optioneering exercise was carried out to select the most efficient way of treating this non-crude influent.

White says: “A dedicated high-rate treatment process was considered to treat the liquors at-source. However, 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 in the Bardenpho process.

“This meant that the site would comprise of a simple, robust process with an overall lower number of assets, thus representing best-value for the client. This would also allow replacement of the existing, shallow ASP tanks. However, accommodating 33,500m3 of process volume into this constrained site presented a significant design challenge,” White says.

The design
A detailed 3D model that included the existing site and below ground services was particularly useful for planning new pipeline routes and avoiding clashes, says Martin Tresidder, construction manager at 4Delivery.

“Modelling of new structural assets was instrumental in evaluating the construction sequence and phasing activities. By including external constraints such as the overhead HV cables, the model was also used to help plan safe methods of working.”

A significant proportion of the overall design effort was focused on the optimum civil and structural arrangement for the large Bardenpho ASP structure.

“To minimise footprint, the process depth was set at 7m, which is at the upper limit for typical ASP plants. This still entailed a structure 92m long and 62m wide. Four lanes were selected to provide operational flexibility and arranged in a three-pass serpentine arrangement to ensure optimum plug-flow conditions.”

Tresidder continues: “As the structure was being designed for construction in reinforced concrete, great effort was made to ensure an efficient design. The draft outline design involved a simple 700mm thick base on a precast concrete piled foundation and 500-700mm thick walls.

“A co-ordinated approach was applied for development of the design, involving group discussion between the teams, civil, structural and geotechnical engineers. Early contractor involvement was key to these discussions.”
Following these design workshops it was agreed to use a layer of Vibro-stone columns as it is more cost effective than piling and far less noisy. However, Vibro-stone columns required the structural design to allow for up to 50mm of potential settlement.

To avoid excessively thick walls, the structural design adopted a propped cantilever system with tie-beams between opposing walls at coping level. This meant that the 7.5m high walls could taper in thickness from 500 to 350mm, as the beams help resist the hydrostatic load imposed on the walls.

The base slab design was also more efficient as a result of adopting this solution: 300mm thick and 500mm thick below the main walls.

“Use of the innovative propped cantilever design helped save a significant volume of in-situ concrete. Settlement was anticipated but the structure was designed to withstand the effects, Tresidder says.

“The tie beams were designed to make installation as quick and simple as possible, without need for connection into the wall reinforcement. For longevity, given the coastal environment of the site, the beams were designed in reinforced concrete and included overhangs at each end for a simple hooked connection,” he says.

Banagher Precast Concrete in Ireland made the specialist items: 64 beams each 16m long weighing 8tonnes.

Construction
Two existing primary settlement tanks and an activated sludge plant structure had to be demolished to generate sufficient space for the new Bardenpho activated sludge plant. But the site had to maintain consent compliance throughout construction.

A phased approach to construction of the new tank was adopted: first clearing the greenfield area and treating the ground, then constructing two of the four new ASP lanes and two, 33m diameter conical final settlement tanks.

This was followed by installation of all associated process pipework, mechanical and electrical plant to enable commissioning for carbonaceous treatment of a proportion of flow. This additional process capacity allowed the redundant assets to be decommissioned and demolished. The remaining two ASP lanes were constructed in the newly available space and the full structure commissioned for full biological nutrient removal enabling decommissioning of the other ASP.

“By phasing the construction in this way, the existing site was given a significant boost in terms of available treatment capacity during construction,” Tresidder says.

During construction more than 10,000m3 of concrete and 1,500 tonnes of reinforcement was placed. More than 1.5km of new pipelines up to 1.2m in diameter were laid together with 3km-plus of cable ducts. In addition, more than 100 tonnes of new above-ground steelwork was installed.

“All of this work has been undertaken in a working site with limited space and overhead high-voltage cables,“ Tresidder says.

Crawler cranes were used to aid construction of the major structures, with further mobile cranes of various sizes 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 lift the 8-tonne beams into position.”

Revisions
The upgrade of Millbrook WwTW is one of the largest capital non-infrastructure schemes delivered on behalf of Southern Water for its AMP5 investment programme.

The principal contractor, 4Delivery, brought knowledge and expertise from its joint venture partners Veolia Water, Costain and MWH.

Delivery of the scheme was a challenge given the scale of the new infrastructure required and the myriad of physical constraints.

Innovation in the design was required to overcome these, helped by a collaborative approach with early input from the construction team. This avoided the need for revisions on the ground.

The delivery team meticulously planned the two-staged schedule to ensure construction could proceed with the utmost heed to health and safety, existing plant operation and the environment.

The plant became fully operational in September when effluent discharge had significantly reduced nutrient input into the Solent. This forms part of a regional environmental programme that safeguards the quality of local waters for the benefit and enjoyment of local communities.

“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 wastewater to even higher standards before releasing it into the Solent,” Garrett says. 

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