upgrade of water quality through installation of new pumping
plant and equipment
by Duncan Tompkins MEng
constructed treatment works at Ladderedge - Courtesy of MMB
Wallgrange Solvents and pH Improvements Scheme required the
replacement of two existing borehole pumps and the design and
construction of a water treatment works sized to treat 20ML/day.
The installation of this treatment works has directly addressed
a number of water quality issues and has delivered a more
resilient supply to thousands of customers. The scheme,
delivered by Mott MacDonald Bentley (MMB), involved the
construction of a new water treatment works on the site of the
existing Ladderedge Distribution Service Reservoir (DSR), and
the replacement of borehole pumps at Wallgrange Borehole Pumping
A number of water quality
issues are present in the raw water abstracted from the
Wallgrange boreholes, including high levels of turbidity,
frequent bacteriological detections, high levels of chlorinated
hydrocarbons, low pH and an amber cryptosporidium Drinking Water
Safety Plan (DWSP) risk. The high level of chlorinated
hydrocarbons from Wallgrange BH2 had seen it taken out of
operation reducing output from Wallgrange by 10ML/day.
All these water quality
issues were to be addressed by this project, with a Drinking
Water Inspectorate commitment to resolve pH and solvents issues
In addition, there were
several site operational issues at Wallgrange which have been
resolved as part of the project. These included:
chlorine gas to Wallgrange in winter as the site access road is
The need for boreholes to
be run to waste on start-up due to high levels of turbidity.
This could take up to four hours and required operational staff
to be on site to manage the process.
Wallgrange BPS is part of
the Ladderedge Control Group which consists of four borehole
sites and Tittesworth Reservoir and surface water treatment
works. All of these sites feed Ladderedge DSR.
There are two boreholes
at Wallgrange. Abstracted water was first treated by a chlorine
gas system at Wallgrange, then by a temporary UV system and
dosed with orthophosphoric acid (PO4) at Ladderedge DSR before
entering Ladderedge DSR.
Scope of works
In order to deliver a
sustainable and efficient solution, in relation to the scheme
drivers, the following works were undertaken:
At Wallgrange Borehole
Installation of new
borehole pumps with variable speed drives and downhole sand
separators to reduce the amount of sand being drawn.
Removal of chlorine gas
► At Ladderedge
Distribution Service Reservoir
Construction of a steel
portal framed building to house the following; Bollfilters, an
antiscalant dosing system prior to aeration tower to prevent
scaling at the aeration tower, hypochlorite dosing rig and a new
motor control centre (MCC).
Installation of an
aeration tower to reduce total chlorinated solvent
concentrations and raise the pH.
Installation of end
suction low lift pumps to pump the partially treated water
through the rest of the treatment processes and an associated
Installation of a new
factory built assembly (FBA) UV treatment plant to replace the
existing rig, to address faecal indicator bacteria and
Installation of a sodium
hypochlorite dosing system for disinfection - this was achieved
via the installation of a static mixer inside an underground
Installation of a new
sewer pumping station and transformation of the salt saturator
bund into a settlement tank.
In addition there were
further associated works including installing process and storm
water drainage, ducting, inlet and outlet pipework, construction
of a chemi-safe area and modifications to the site road.
Antiscalant and hypochlorite dosing rigs - Courtesy of
Efficient design and
build and factory thinking
The project utilised a
number of factory build assembly (FBA) products, removing the
need for on-site construction of these items.
The standard Severn Trent
Water (STW) UV kiosk, used across STW’s AMP6 UV batch, supplied
by Lintott was installed on-site. The repeated use of this
product gave additional efficiencies as MMB design, construction
and commissioning staff could transfer design, construction
techniques and best practice from previous projects into this
The aeration tower was
designed and manufactured off-site by Forbes providing a more
cost-effective solution for the removal of solvents than a more
traditional granular activated carbon (GAC) treatment plant.
Gee and Co also supplied
factory built dosing skids, for hypochlorite and anti-scalant
Overall the use of FBA
products has provided significant cost and programme savings.
Manufacture in a controlled environment also allowed greater
levels of quality assurance and control of construction health
and safety risks. Being able to reduce the number of on-site
construction activities was highly valuable on such a large
The ability to factory
test these items of plant also allowed potential commissioning
issues to be minimised before delivery to site, and gave
assurance over the products’ functionality.
As part of the scheme the
redundant salt saturator bund was reused to create a settlement
tank to reduce the suspended solids content of the Bollfilter
backwash water, which is discharged into the sewage system. The
associated kiosk, previously used for control, was also reused
to provide an on-site welfare facility. This yielded significant
cost and embodied carbon savings and reduced the health and
safety risk associated with the construction of these two items.
Bollfilters and lifting equipment - Courtesy of MMB
Due to some uncertainty
in borehole yields which had the potential to affect the
performance of the aeration tower, there was a potential that
recirculation pumps may be required. In addition, the treatment
of water within the aeration tower had the potential to cause a
rise in turbidity which would have required the installation of
A phased approach was
taken to allow for the additional features that may have been
required and an allowance made within the programme to retrofit
these once commissioning had commenced and there was a greater
degree of certainty in the treatment process.
This approach provided a
cost-effective approach meaning that unnecessary items of plant
were not installed. It will also allow greater flexibility for
any alterations that are required to the treatment process in
Health and safety and
MMB and STW realise that
health and safety performance is imperative and that health and
safety leadership comes from all levels of the company. The
Wallgrange UV scheme has a proactive health and safety culture
which was demonstrated by the high levels of positive
intervention reporting. The leadership shown by Rob Boniface
(Site Delivery Manager) and Paul Smith (Works Manager) resulted
in them receiving an internal JN Bentley ‘Be Recognised’
During the collaborative
planning process, which aims to engage and bring together the
wider project team, the ability to gain permission to shutdown
the Wallgrange borehole to replace the borehole pump was
identified as a potential risk to the project programme.
This early identification
allowed MMB to challenge and work collaboratively with the
borehole pump supplier on their lead and delivery time and
highlighted the need for early engagement with STW’s network
control team. The shutdown, required to allow the installation
of new borehole pumps, was carefully planned to minimise the
time needed to cut off supply to the reservoir. The works were
also planned to fall outside of the higher risk summer months
when demand is often high and available supplies are lower. This
minimised the risk of a cancelled shutdown or reservoir levels
becoming low during the works. The ability to move the programme
forward away from the summer months also reduced this risk
A number of residential
properties are located close to and within view of the new
treatment works. In order to engage with local residents, they
were invited to an open day where an overview of the scheme was
provided and stakeholders had the opportunity to ask any
questions about the scheme.
Severn Trent’s ‘DOLLY’
bus was utilised to provide refreshments and further information
to customers. This positive engagement with the local community
lead to an agreement to undertake some additional tree planting
to provide screening to the treatment works.
The completed Wallgrange
Solvents and pH Improvements Scheme has directly addressed the
water quality and operational issues which were present prior to
the schemes construction. This has allowed borehole two to be
brought back into service increasing the maximum yield from the
Wallgrange Boreholes. This will provide greater resilience to
the Ladderedge Control Group during drought periods protecting
the water supply to thousands of customers.
The reuse of existing
assets and use of FBA, standard products and package plants
allowed for an efficient, cost-effective and sustainable
delivery of the project.
Provision in the design
for additional treatment allowed for a phased approach to
construction and now gives flexibility for future on-site
treatment. Early collaborative planning allowed programme risks
to be identified and mitigated allowing efficient programme
and publishers would like to thank Duncan Tompkins, Project
Leader, Rob Boniface, Project Delivery Manager, and Karen
Chiu, Project Leader, all with Mott MacDonald Bentley for
providing the above article for publication.
The author acknowledges the following: Mark Smith, Programme
Manager, David Ralphs, Operations Manager, Tim Rose, Process
Engineer and Sarah-Jayne O’Kane, Communications, all with
Severn Trent Water.