rehabilitating and replacing an existing asset to improve
operation, reduce risk and increase resilience
by Chris Mellett MEng (Hons) CEng MIET & Mark Thomas BSc
escalator screens installed within screens chamber -
Courtesy of Arup
Welsh Water (DCWW) is carrying out a strategic package of works
in the Llanelli and Gowerton catchments with the aim of reducing
spills into the Loughor Estuary. Bynea Sewage Pumping Station (SPS)
is a key asset, receiving sewage flows from the local catchment
and transferring them to Llanelli WwTW. The site originally
operated as a sewage treatment works but in 1995 was converted
to a sewage pumping station. The pumping station was upgraded in
2003 by adding storm pumping and storm storage capacity.
Bynea Sewage Pumping Station serves an upstream catchment with a
population equivalent of 19,698. Combined flows from the
catchment are transferred via gravity sewers and are received at
the pumping station wet well. Flows up to the pass forward flow
(PFF) of 501l/s are pumped to Llanelli WwTW for treatment. The
foul pumps discharge into a 700mm diameter rising main that is
1,700m in length and discharges to the inlet works at Llanelli
Flows in excess of the
PFF are pumped to the circular storm storage tanks on site.
Excess storm flows spill over a high level weir around the edge
of each tank. There are two overflow pipes, the use of which
depends upon tide conditions.
The primary discharge is
down the sea outfall to the Loughor Estuary; however, when the
outfall is tide-locked, then the overflow is to an unnamed tidal
pill. Return storm flows are pumped back to the wet well.
During AMP5 Welsh Water
carried out a project to resolve sewer flooding in the upstream
catchment by increasing the capacity of the sewer network. A
performance assessment of Bynea SPS was carried out to identify
any risks at the pumping station that could impact on the
The assessment determined
a number of issues:
Foul pumps not achieving
the required PFF, unreliable and failing repeatedly; primarily
due to blockages.
Large amounts of solids
settling out within the rising main, reducing its capacity and
increasing the pumping head.
Rising main air valves
inaccessible for maintenance.
No storm pump standby
capacity and the storm pump rising main inadequately sized.
Manual intervention is
required to empty the storm tanks.
Inadequate screening of
overflow discharges and screen prone to blinding.
Capacity of the mains
power supply reached and exceeded on occasions.
inadequately sized to run all duty pumps.
It was determined that
the pumping station and rising main would be dealt with under
separate projects. The scope of this paper covers only the
pumping station project.
View of construction works
Courtesy of Arup
Courtesy of Morgan Sindall
The key drivers for the
design were meeting consent requirements and reducing the risk
of flooding in the upstream catchment.
Hydraulic modelling of
the sewer network was used to determine the flows received at
the pumping station. The peak inlet flow is 850l/s, therefore
given a PFF of 501l/s, excess storm flows of up to 350l/s are
The existing pumping
station included a grit trap that was not maintained and had
become disused. The design reinstated this grit trap, preventing
solids being carried forward and reducing deposition in the
rising main. The design also caters for screening of all
incoming flows at the pumping station. This provides benefit by
reducing the risk of pump blockages and removal of rags from
flows passed forward to Llanelli WwTW.
The hydraulic design
considered the need to avoid surcharging of the upstream
incoming gravity sewers and hence influenced the depths of the
new screens chamber and pumping station shaft (approximately 6m
and 8m deep). The new inlet screens and screen channels were
designed to operate over a wide flow range, from 50l/s to
The water retaining
screens chamber was designed as an in-situ reinforced concrete
structure. The design uses thicker walls with less
reinforcement, which saved construction time and cost. The
chamber is designed so that in dry weather it can be temporarily
isolated from incoming flows and then ‘free-drained’ into the
new pumping station structure, to avoid the requirement for
pumping out prior to access for maintenance.
The pumping station wet
well was designed to incorporate both foul and storm pumpsets.
The wet well was physically scale-modelled to optimise the
hydraulic design, which enabled a smaller diameter shaft. The
pumping station shaft structure design to account for flotation
takes benefit from the jacking collar, skin friction and
significant volumes of benching to reduce the overall volume of
concrete used. Welsh Water and the pump supplier were closely
involved to develop a design with good access to all
maintainable components. Access arrangements over the pumps and
wet well avoid the need for operators to remove floor covers or
work over unprotected openings. Fixed walkways and landings with
handrailing were incorporated along with stairway access to
assist with pump removals, while valves were located above
ground to remove the need for confined space entry.
The position of both the
new screens chamber and pumping station shaft allowed them to be
constructed and then partially-commissioned off-line, prior to
connection to the existing incoming gravity sewer, avoiding the
need for significant temporary overpumping.
Installation of sheet piles for screens chamber
excavation - Courtesy of Morgan Sindall
Construction ongoing within pumping station shaft
caisson - Courtesy of Morgan Sindall
A 600mm diameter ductile
iron pipe connects the new foul pumps to the existing rising
main. The connection point is to a ductile iron section of the
rising main upstream of the transition to GRP pipework, to avoid
the difficulties associated with connecting dissimilar
The existing storm
storage tanks were modified to allow them to be drained by
gravity, cutting the costs associated with the old pumped return
system. The new rising main pipework feeding the storm storage
tanks was designed for off-line installation, enabling the storm
tanks to be kept in service throughout the construction phase.
The controls for the new
equipment are contained within a motor control centre (MCC)
located in the existing control building. The design enabled
off-line installation so that testing and commissioning of the
new equipment could be carried out while the existing pumping
station remained operational. A touchscreen human machine
interface (HMI) provides a local interface for the operators,
while the programmable logic controller (PLC) is linked to Welsh
Water’s telemetry system for remote monitoring and reporting of
alarms. The control philosophy for the variable speed foul pumps
incorporates features to minimise blockages whilst maintaining
smooth pass forward flows. The existing storm MCC was retained
for existing equipment that forms part of the new storm system,
including storm tank cleaning pumps and instrumentation. The
PLCs in the two MCCs communicate over an ethernet network.
An upgraded mains power
supply was provided, with a new ground-mounted substation
established outside of the site boundary. The existing standby
generator was replaced with a new 500kVA containerised standby
generator including an integral bulk fuel tank. The container
design facilitated rapid installation and provides increased
The site is located down
an access track of approximately 3m width, which is also part of
a frequently used public right of way. Due to the scale of the
structures and the need for large plant to access the works, it
was decided that a separate access track would be constructed
for construction vehicle movements and to safeguard pedestrians
and cyclists in the area. Arrangements were made to provide site
access through an adjacent car park via a purpose made 100m long
track complete with segregation.
900mm concrete pipe being installed
Courtesy of Morgan Sindall
station wet well during commissioning
Courtesy of Arup
The site’s water table is
at approximately 1.8m below ground level. Given the excavations
required for the 8.9m deep pumping station and the 6.1m deep
screens chamber it was decided to install a dewatering system to
enable the construction of the large structures. Seven deep well
dewatering points and one observation point were installed to a
depth of 10m around the main working area to encompass the zone
of influence. The dewatering system was operational for 24 hours
a day over a period of 16 weeks.
Civil works construction
The programme was
developed to enable construction of the two large underground
structures simultaneously in order to achieve the optimum output
and limit the duration of the overpumping on site.
The 12m long, 5.9m wide
screens chamber was constructed with in-situ reinforced
concrete, while the excavation for the chamber was undertaken by
installing a cofferdam allowing for working room.
The 9m deep, 7.5m
diameter pumping station shaft was constructed via a top down
caisson method. Six jacks bolted onto a jacking ring pushed the
bolted concrete segments down as the earth within the caisson
was excavated. A 1.4m mass concrete plug was poured at the
formation level, which enabled the reinforced base and benching
to be constructed.
Whilst working on the
large concrete structures, the separate deep pipelines were
installed to improve efficiency of the programme and completed
whilst the dewatering system was in place.
The cable duct runs used
precast drawpits, which were delivered to site and installed on
the same day. The use of precast concrete enabled a cost saving
of over two thirds compared to traditional plastic
sections/brick work. Excavated material was utilised to create a
screening bund. The screening bund provides effective visual and
noise screening of the site from the public rights of way that
run parallel. This 1,800m3 bund resulted in a saving of 95% of
the waste sent to landfill.
Physical scale model of pumping station
Courtesy of Arup
Courtesy of Arup
Mechanical and electrical
The pumping station shaft
houses four foul pumps and three storm pumps as well as the
riser pipework, handrailing, access bridge and pipe manifolds.
As well as benching within the wet well, the more intricate flow
vanes and baffle plates were fabricated from steel and fixed
within the base of the wet well.
The existing MCC was
fully operational whilst the new one was being installed and a
phasing and commissioning plan was developed to ensure a smooth
transition from the old system to the new operating system. As
the new MCC and associated variable speed drives generate more
heat, a new ventilation system was installed to mitigate the
View of new
inlet screens - Courtesy of Arup
Following completion of
all construction works and once all the necessary equipment was
dry commissioned, the final change over of flows and systems
could begin. The first operation was to make a connection to the
existing foul rising main, with minimal shutdown time due to the
incoming flows. This involved draining down the existing rising
main and overpumping to the storm tanks on site, which was
undertaken during a sustained period of dry weather to minimise
the incoming flows.
A shutdown of the
electrical systems was undertaken with the backup generator
running so the new mains power upgrade could be connected and
the electrical apparatus transferred over to the new system.
Dŵr Cymru Welsh Water
Construction & Infrastructure Ltd
WJ Groundwater Ltd
HOS Civils Ltd
Pumping station shaft
HB Tunnelling Ltd
Whitland Engineering Ltd
Zone Electrical Ltd
Inlet screens &
Motor control centre
General Panel Systems Ltd
Xylem Water Solutions UK
pumping station - Courtesy of Arup
The project has achieved
its aims by delivering improvements in pumping efficiency and
reliability. The reinstated grit removal and new inlet screens
will reduce future maintenance requirements at both Bynea SPS
and Llanelli WwTW. The new pumps achieve the required PFF and in
combination with improved control is resulting in less flow into
the storm storage tanks, which will contribute to a reduction of
spills into the Loughor Estuary.
and publishers would like to thank Chris Mellett, Associate
with Arup, and Mark Thomas, Project Manager with Morgan
Sindall Construction & Infrastructure Ltd, for providing the
above article for publication. The authors thank Dŵr Cymru
Welsh Water for their assistance with this paper.