delivering improved performance and raising environmental
discharge standards, as part of United Utilities AMP6 investment
by Jonathan Starling CEng MEng MCIWEM
Steven Smythe CEng MSc MICE & Peter Isherwood BEng
works on site - Courtesy of C2V+
Wastewater Treatment Works (WwTW) is located to the north of the
coastal town of Allonby, Cumbria, serving a population
equivalent of less than 1000 within a dominantly rural catchment
area. The works provides treatment via a three stage process,
including rotating biological contactors (RBCs) and ultraviolet
dosing. Allonby WwTW is situated on the coast and discharges
into the Irish Sea via an existing dedicated long sea outfall.
Expansion and improvement works to the site were identified by
United Utilities to improve bathing water quality, to align with
the Environment Agency targets for coastal waters and to
accommodate future population growth. It was one of the first
projects to be delivered under United Utilities’ AMP6
Construction Delivery Partner (CDP) framework and was completed
by C2V+ in 2016.
The primary aim of the
project was to improve the effluent quality from Allonby WwTW
and to reduce the frequency of unplanned discharges from the
outfall, to comply with the Environment Agency quality drivers.
For compliance with EC Bathing Waters this meant no more than
one spill per five bathing seasons (May to September inclusive)
and for compliance with Shellfish Waters, no more than 10 spills
per annum. To achieve this, it was proposed that the existing
WwTW was to be upgraded so that the amount of flow going through
the full treatment process was doubled from 5 litres per second
(l/s) to 10l/s. In addition, a significant expansion of the
storm water storage capacity was proposed to reduce the number
of storm spills to below the target levels.
The project was awarded
to C2V+, a joint venture between CH2M and VolkerStevin, in March
2015, following a competitive tender as a design and build
contract. The key elements of works comprised:
Expansion of the existing
works capacity through installation of additional process units,
to provide full treatment to incoming flows of 10l/s.
treatment to reduce bacteriological levels within the effluent.
existing works storage capacity to significantly increase the
stormwater storage capacity of the site and reduce the frequency
of unplanned spills from the outfall.
The existing works at
Allonby WwTW provides fairly traditional treatment processes,
comprising inlet screens and lift pump station, twin septic tank
primary settlement units, twin rotating biological contactors (RBCs),
‘Dortmund’ type humus tanks, an ultraviolet dosing channel and a
discharge tidaI pumping station, with a tidal storage tank.
There is no storm overflow at the site and all flows entering
the works need to be discharged via the sea outfall.
Cofferdam installed for FtFT construction - Courtesy of
Due to the location of
the outfall, the works were only permitted to discharge during a
fixed duration tidal window, either side of high tide. As such,
the project needed to take into account the interaction of the
tidal cycle with the storm events and ensure that sufficient
on-site storage was provided, to reduce the number of spills
from the outfall to less than one per bathing season and 10 per
annum. Hence, whilst incoming flows did not generally exceed
35l/s, the outfall needed to be sized to discharge 80l/s to
ensure both incoming and any stored flows could be discharged
within the tidal window.
It was identified at an
early stage that the catchment was heavily impacted by surface
water runoff. In addition, the main sewer pipe into the works
was aged and identified as having significant defects. Due to
its location near a water channel on the foreshore, it was
believed to be allowing significant volumes of brackish water to
enter the pipe system.
Not only was this adding
additional unnecessary flow to the works, but it was also
introducing sediment and the high salt content was of concern
due to its potential negative impact on the works
microbiological process units. Replacement of this section of
pipeline was therefore considered critical to the successful
operation of the works and prioritised as an early action.
Due to the nature of the
catchment, dry weather flows were very low (<3l/s) and below the
range of most pumps. To accommodate the flow to full treatment
flow range, variable speed pumps were determined to be the most
suitable equipment, operating within a range of approximately 4
to 10l/s. The wet well enabled incoming flows to be stored and
passed forward in batches to provide practical delivery of dry
weather flows whilst meeting the peak flow requirements.
Aerial view of works in progress - Courtesy of C2V+
It was determined that
the additional treatment capacity should comprise new, similar
process units installed in parallel with the existing. Process
calculations confirmed that the existing RBCs were adequately
sized to cater for the new design flows. Other process areas
were expanded through supplementary units, which were
identically sized to the existing where possible to ensure the
level of process treatment and hydraulics were balanced across
the units. The solution comprised the following key elements:
Replacement of an
approximately 300m section of pipeline on the main incoming
sewer to ensure structural integrity and reduce infiltration
into the pipe.
Manholes were designed to
accommodate variations in beach level through elevated top level
and extended depth of concrete surround.
New 6mm mechanical screen
within the inlet works, installed within an existing building
without top roof access, through component installation via the
main access door (Haigh).
New flow to full
treatment (FtFT) pumping station fitted with variable speed
pumps to cater for flow range of 4 to 10l/s. The wet well was
provided with an enlarged storage capacity.
New buried GRP septic
tank unit and desludging line.
New Dortmund type
rectangular humus tank with new humus sludge return pumping
New ultraviolet treatment
plant comprising in-channel light units to treat the revised
FtFT to reduce bacteriological levels to below consent standards
New tidal pumping station
equipped with two duty/standby variable speed pumps rated at
80l/s each and associated pipework.
Diversion of site
drainage into the FtFT wet well to operation of the sludge
return pumping station.
New flow distribution
Installation of a new
motor control centre (MCC) and automated control systems.
Works in progress - Courtesy ofCourtesy of C2V+
The existing hydraulics
and planning conditions required that the above ground
structures were generally limited to small chambers and control
buildings only. Hence, the new works, including the new septic
tank, the humus tank and the large 16m x 9m x 6m deep dual
pumping station and effluent storage tank were all designed and
installed below ground with very little above ground impact. The
need to enter below structures was minimised through the
provision of access to mechanical components, controls and
sensors from ground level. Safe access to the chambers was
provided through planned working areas and access covers.
Construction of the works
was successfully undertaken between 2015 and 2016, with the new
works being commissioned for operation in March 2016. All
completion targets were successfully achieved.
As the works needed to
remain operational throughout the construction programme, the
amount of offline construction was maximised with connections
and tie-ins designed to be undertaken during short shutdown
periods. Where possible, existing assets were re-used to
optimise the efficiency of investment and avoid abandoned assets
The site for the new
tidal storage area was constrained by the site boundary and
required construction within a relatively small working area
between existing tanks and services. This was achieved by the
installation of a temporary sheet pile cofferdam. Significant
dewatering of the excavation was required to control groundwater
levels during the excavation.
Completed works - low above ground impact of new septic
tank and humus tank - Courtesy of C2V+
Reductions to carbon
Consideration of the
project in its entirety included reduction of infiltration in
the incoming flow, hydraulic planning of the outfall tidal
system, to establish the size of the works as well as the
storage capacity which was optimised to ensure environmental
quality targets were met, whist avoiding over-sized and
potentially under-utilised facilities being installed.
Space on site was
constrained by the existing structures and the provision of
storage capacity required the provision of deep tanks.
Therefore, optimisation of the design was required to reduce the
depth of tanks and the lift requirements of the pumping
stations. In addition, waste was minimised through effective
reuse and incorporation of the existing assets.
Working in cooperation
with United Utilities, C2V+ has successfully delivered another
important stage in the continued improvement of Cumbria’s
beaches through the expansion and upgrade of the treatment works
completion aerial shot - Courtesy of C2V+
in 2016, the new works at Allonby is now operational, and is
producing a significantly improved quality of effluent from the
site. Relatively little is actually visible of the significant
changes at Allonby WwTW and the site may appear relatively
unchanged to the casual observer. However, the structures and
pipework installed below ground have made a substantial
improvement to the operation and capacity of the works such that
it now meets the required quality standards and number of spills
events and is delivering real benefits to both the community and
and publishers would like to thank Jonathan Starling, Design
Manager, Steven Smythe, Technical Manager, and Peter
Isherwood, all with C2V+, for providing the above article