Anglian Water reuses tanks for stormwater storage

Anglian Water is repurposing five disused sludge holding tanks at Leighton Linslade Water Recycling Centre to increase stormwater storage without constructing an equivalent set of new tanks. The £2.5 million scheme, delivered through the company’s @one Alliance, will add 2,374 cubic metres of storage, taking total capacity from 1,782m³ to 4,156m³.

The existing structures are being converted from their former role in sludge handling to provide storage for excess wastewater during periods of heavy rainfall. Retaining the tanks allows the delivery team to make use of civil engineering assets already present at the site, reducing the requirement for new structures and the construction materials associated with them. Their new function will be integrated into the water recycling centre’s existing operations.

Incoming wastewater flows can rise rapidly when rainwater enters the sewer network, particularly in areas where surface drainage and wastewater share the same pipes. Treatment facilities have limits on the volume they can process at a particular time, so additional storage provides a means of retaining excess flows until there is sufficient capacity to treat them.

During a period of high incoming flow, storm tanks can hold wastewater that would otherwise place additional pressure on the treatment process. Once flows have fallen, the stored contents can be returned for treatment. The available storage volume therefore influences how the site responds to rainfall, although the extent to which overflows are reduced also depends on the intensity and duration of rainfall, the incoming sewer network and the treatment capacity available afterwards.

The conversion will more than double the storage available at the works, providing additional capacity to retain incoming flows during wet weather. The increase is defined by the engineering design rather than a measured reduction in overflow events. Its operational contribution will depend on the volume entering the site and the space available when heavy rainfall occurs.

The project team has selected an existing group of tanks to provide that increase, avoiding the need to develop equivalent new storage structures from the ground up. The conversion still requires engineering work to make the former sludge tanks suitable for their new duty. The condition of the structures, their connections to the treatment process and the requirements of the new operating arrangement will determine which existing components can be retained.

Tank reuse depends on the ability of a structure to accommodate the loads and operating conditions associated with its proposed function. An assessment may need to consider structural integrity, access, containment, pipework and the equipment used to move flows into and out of storage. Where the main structures remain suitable, adapting them can reduce the quantity of new concrete and reinforcement required compared with constructing additional tanks.

The project team expects the reuse approach to reduce embodied carbon by 88% against a traditional new build, although that comparison remains subject to confirmation. The estimate relates to the construction approach for the additional storage, not an established reduction in the total operating emissions of the wastewater treatment site. The completed assessment will need to reflect the materials and construction activities associated with the conversion as well as the alternative against which it is compared.

Embodied carbon includes emissions associated with producing, transporting and installing construction materials, together with relevant construction processes. New tank construction can require excavation, foundations, concrete, reinforcement and associated works. Reusing existing structures can avoid a portion of that material demand, while refurbishment may still involve additional components, engineering modifications and construction activity.

The extent of the saving consequently depends on how much of the existing infrastructure can remain in service. Retaining a structurally sound tank can avoid replacing a large part of the original asset, whereas extensive reconstruction would increase the material demand associated with reuse. Any carbon comparison must consider equivalent storage provision and the scope of work required under the competing construction options.

These choices also change the procurement requirements of an infrastructure project. Building new tanks creates demand for substantial quantities of construction materials and the equipment needed to install them. Converting existing structures can shift more of the work towards surveys, refurbishment, pipework modification, controls and connections, while reducing some of the bulk materials and earthworks that would otherwise be purchased.

The £2.5 million Leighton Linslade investment therefore combines a defined capacity increase with the reuse of industrial assets that have ceased performing their original function. Its commercial and environmental characteristics depend on whether the retained structures can provide suitable service over the intended operating period. The project does not establish that reusing redundant infrastructure will always be preferable to replacement at other wastewater facilities.

Anglian Water is delivering the work through its @one Alliance, which brings together companies involved in planning and constructing water infrastructure. Alliance arrangements allow project teams to coordinate engineering design, construction and the management of work at operational sites. At Leighton Linslade, the conversion must be undertaken alongside the existing treatment activities and connected to the facilities responsible for managing storm flows.

Once the additional storage is available, the treatment works will have greater capacity to retain incoming wastewater when rainfall causes temporary increases in flow. The storage will still need to be operated in coordination with the treatment process so that retained contents can be processed after the peak has passed. Its effectiveness will depend on the available treatment capacity and the frequency with which storage is needed during wet weather.

The space available when a storm begins will also be affected by how much wastewater remains stored from earlier rainfall. If treatment capacity has not allowed the tanks to be emptied, the additional volume available for a subsequent event will be reduced. The enlarged storage provision must therefore be managed alongside the works’ ability to process retained flows between periods of heavy rainfall.

Adapting the existing tanks will increase the site’s capacity to manage peak inflows while retaining much of its established civil infrastructure. The embodied carbon estimate remains subject to confirmation, while the operational performance of the expanded storage will depend on rainfall conditions and the ability to return retained wastewater for treatment.