£2.25m Poolend and Highate pH Correction and Plumbosolvency
Scheme required the design and construction of a 20Ml/d
treatment plant. Poolend BPS is located near Leek in
Staffordshire. Water from Poolend and Highgate is blended with
water from other sources and supplies thousands of properties
across Staffordshire. The successful completion of the scheme by
design and build contractor Mott MacDonald Bentley (MMB) on
behalf of Severn Trent Water (STW), ensured Poolend remains a
robust water treatment site and leaves a lasting legacy of high
quality water to service the needs of people in Staffordshire.
The drinking water safety
plans for Poolend and Highgate identified a deteriorating trend
for pH and lead, and an elevated risk of bacteriological issues,
making the project a high priority for STW. In light of this, in
the summer of 2014, the Drinking Water Inspectorate (DWI) and
STW brought forward completion of the project from June 2018 to
June 2016. This reduced the anticipated duration of the
programme by 50% and gave 18 months to deliver the project.
To add to the challenging
programme, Poolend is located in the ‘green belt’ and also
designated as a Special Landscape Area, meaning it required
early environmental consultations and planning permission to
extend the water treatment site area and to construct the new
In order to achieve a
successful outcome an innovative delivery model was deployed by
STW. MMB was involved from the offset to value engineer the
project and avoid a circular or protracted design phase. A
collaborative team, empowered to make decisions was brought
together to ensure fast paced delivery. Three main efficiency
levers were applied namely:
The proposed solution was
to rationalise the sites at Poolend and Highgate into one, by
provision of common:
Sodium hydroxide dosing.
Sodium hypochlorite dosing.
Water softener plant for carrier water system.
Orthophosphoric acid dosing.
Utilisation of existing UV disinfection at Poolend.
Proposed location of the new treatment building
Courtesy of MMB
Courtesy of MMB
The works at Poolend
involved significant network reconfiguration, converting
Highgate WTW into a raw water supply which is pumped to Poolend
site for pH correction, disinfection, plumbosolvency and UV
Through careful design
and rigorous challenge, the team was able to reuse the existing
main between Poolend and Highgate to allow flow going back into
area, previously supplied by Highgate. Carbon footprint
calculations indicated a 40% saving on embodied carbon, £300,000
on a 1.8km 180mm pipe, and four months on the programme by
reusing the main and by avoiding the installation of the new
main. The delivery team performed a pressure monitoring on the
existing main to ensure that customers continue to receive ample
pressure in all demand scenarios.
Combination of Poolend
and Highgate to a common treatment
Highgate is located
approximately 1.3km to the north of Poolend. In order to provide
a common treatment for both Highgate and Poolend borehole
sources, the existing main from Highgate to Ladderedge had to be
diverted into the Poolend treatment system, effectively
rendering it a raw water source. Part of the treated water is
returned to the booster station at Highgate, which feeds Heaton
distribution service reservoir (DSR).
The Highate flow was
connected into Poolend upstream of the existing UV plant, which
had been installed at Poolend as part of a separate project.
Although the UV plant was sized for the combined flow, the
pipework between where the flows merge, and the UV inlet and
also the outlet pipework had to be upsized to accommodate the
The new treatment
processes provided by MMB (see process diagram below)
includes caustic dosing for pH correction, sodium hypochlorite
dosing for marginal chlorination, and phosphoric acid dosing for plumbosolvency control. All chemicals are injected into a new
static mixer installed dowsntream of the UV plant, meaning that
the old UV plant, chlorine dosing plant and phosphoric dosing
plant at Highgate are no longer required.
Poolend - Highgate process flow diagram - Courtesy of
MMB - Click to enlarge
All new chemical dosing
systems were designed for the combined flow from Poolend and
The method selected for
pH correction was dosing of sodium hydroxide at 50%
concentration. The dosing control is flow proportional with trim
to a target pH. The new sodium hydroxide plant includes two bulk
storage tanks designed for 25 tonne chemical delivery, dosing
cabinet and control panel.
The new sodium
hypochlorite plant also contains two fully bunded bulk storage
tanks, dosing cabinet and new control panel delivered to site as
a pre-assembled package plant. The dosing control is flow
proportional with residual trim.
A new orthophosphoric
acid dosing plant was installed at Poolend to deliver a stable
dose in the range 0.5-1.5 mg-P•L-1. The plant includes a bulk
storage tank in a bund, dosing cabinet and control panel and it
was delivered as a pre-assembled package plant in a security
Carrier water is required
for sodium hydroxide and sodium hypochlorite dosing. The carrier
water needs to be softened to eliminate any risk of scaling in
the dosing lines and injection lances. A salt saturator and
duty/standby duplex softeners based on ion exchange were
installed at Poolend.
The softened water pressure was boosted by new carrier water
pumps before dosing back into the process main. As the carrier
water is required for both sodium hydroxide and sodium
hypochlorite dosing, the same system was used for the both
chemicals. Following detailed chemical modelling, an additional
saving was made by using common dosing lines and common
withdrawable dosing lances.
The softening system
produces a waste stream therefore a new wet well and sewage
pumping station were installed on site to remove this waste, and
a new 1.2km foul rising main was laid and was connected in to a
local sewer. This main is now taking the softener, sampling
waste together with domestic foul sewage.
New sodium hydroxide
dosing, sodium hypochlorite rig, softener carrier water system
and the new control panels were installed in a new
security-rated steel framed building at Poolend. Construction of
a new building involved expanding beyond the existing developed
site boundaries (although still on STW owned land). This ground
was previously falling away steeply from the developed site so
major earthworks were undertaken to provide a level platform for
the new building, tanker delivery area, tanker access road and
turning area. Earth retaining structures were constructed where
there was insufficient space to batter back the slope to a safe
angle within the site boundaries.
The new phophoric dosing
system was installed in a secure GRP kiosk in the location of
the previous kiosk.
steel clad building during construction - Courtesy of
New fill point cabinets
for sodium hypochlorite, sodium hydroxide and phosphoric acid
were installed in a new common chemical delivery area, which
includes two 3-way valves with interlock system and two
interceptor tanks for separate chemicals, as well as new safety
shower with head tank and eye bath.
The Poolend Delivery team
(MMB, Asset Creation, Operations and NCAM) excelled in
delivering the DWI obligation on time, through hard work, strong
collaboration and great commitment, embracing a strong ‘One
Severn Trent’ ethos.
planning from feasibility to commissioning, the team delivered
the programme of works in half the time of similar sized schemes
in AMP5, taking just 14 months from design to completion. Work
started in October 2015 with a clear milestone of completing
earthworks (4,500 tonnes of soil shifted) and the raft slab by
The steel-clad treatment
building was started in January and ready for fit out in six
weeks. This left just four and a half months to install and
commission the disinfection, sodium hydroxide and softener
plants and to reconfigure the network and the existing UV plant.
Factory-build assembly significantly reduced the amount of time
spent on installation of the dosing plants. In parallel the
waste main using directional drilling was built in three weeks.
The interconnecting pipework required six separate network
interventions which were planned and executed flawlessly.
From start to finish, the
Poolend delivery team has shown massive commitment to the
collective goal of achieving the DWI obligation, which was met
with time to spare.
The excellent health and
safety culture of the site delivery team saw the completion of
construction without lost time incidents or serious near misses.
The team has reported 970 positive interventions per 100,000
worked hours, which demonstrates their great engagement and a
lead measure of performance.
delivered project ensures that customers in Staffordshire will
continue to be provided with a high quality supply of water.
collaborative delivery team - Courtesy of MMB
and publishers would like to thank Anna Czerwiec, Project
Leader with MMB, for providing the above article for
publication. The author thanks the following for their
Guillaume Appert, Programme Manager with Severn Trent Water,
Jamie Colwell, Design Manager with Mott MacDonald Bentley,
and Sarah-Jayne O’Kane, Communications with Severn Trent