by Jon Rogerson, Gabriella Giuffre, Tom Pawson & Mark Calcutt
delivery area and storage tanks - Courtesy of Tom Pawson,
Netheridge Sewage Treatment Works (STW) is one of Severn Trent
Water’s key sites in the south west of the region serving the
city of Gloucester, discharging final effluent to the River
Severn. The works currently serves a population equivalent of
197,940 which is predicted to increase to 218,815 by the 2034-35
design horizon. The site is also a strategic sludge treatment
centre for Severn Trent and processes inter-works sludge from
over 100 regional STWs. It also processes trade waste from
regional and national traders which are tankered to the site and
treated by the digestion stream. Severn Trent invested in
combined heat and power (CHP) engines during the AMP5 programme
to maximise the energy generated from the biogas produced by the
site digestion plant. Therefore, the facility provides a revenue
stream to Severn Trent in addition to significantly reducing the
site power imported from the grid.
Over the years the site
has suffered from poor control of the digestion stream feed
arrangement resulting in large fluctuations in the digesters’
gas production. This issue is exacerbated by the range of trade
wastes which are accepted into the site, which can exceed COD
values of 800,000mg/l.
The existing arrangement
has been to blend the indigenous sludges, imported sludges and
trade wastes in an attempt to control the digestion process.
CiM6, a partnership between Costain and MWH for AMP6, were
awarded a contract to assess the existing digestion process and
develop a solution to provide an improved sludge feed control
system and maximise continuous gas production.
The project value is
approximately £8.1m and was due for handover back to Severn
Trent in the Spring of 2017.
Primary sludge gravity belt thickeners - Courtesy of Tom
with Severn Trent, CiM6 developed a solution that provides a new
sludge and trade waste imports facility with additional sludge
buffering and storage. A new control system has been implemented
which ensures that flow and load into the digestion stream is
controlled to maximise continuous gas production. In addition,
improvements were delivered to the indigenous sludge route to
ensure that both primary sludge and SAS are thickened to an
optimum point of 6% dry solids. The solution overall comprises
the following scope items:
6mm 2D sludge and trade
waste screens for imports tanker discharges.
Sludge and trade waste
above ground tanks for buffer storage with pumped mixing.
transfer pumping stations.
Primary sludge gravity
Powder polymer makeup
Odour control plant.
Using the concept of a
‘petrol forecourt’ design with multiple discharge points and a
one way traffic management system, CiM6 developed the solution
to be offline from the existing works processes during
construction and commissioning to minimise interfaces and
disruption to the day to day running of the works. Imported
sludges into the facility are screened before being transferred
into the pre-digestion blending tank.
Trade wastes are
controlled differently; due to the high variability in COD
values of the incoming trade wastes, imports are separated into
medium strength and high strength wastes, with a COD value of
300,000mg/l being the cut off criteria between the two.
Using the quality
monitors in the discharge loggers and the intelligent control
system linked to the transfer pumps, the trade wastes are
blended to provide a feed stock with an optimum COD value for
the digestion process. Low strength trade waste continue to be
discharged into the main treatment process.
The indigenous primary
sludge is mechanically thickened to 6% dry solids using gravity
belt thickeners with a polymer coagulant. Thickened indigenous
sludge, imported sludge and trade waste are mixed in a
pre-digestion blending tank before being fed to the digesters.
By optimising the
digester feed to provide a consistently high calorific value
sludge feed, gas production is maximised and managed within the
site operations to generate electricity for both the site and
the National Grid.
Sludge imports delivery area and storage tank - Courtesy
of Tom Pawson, Costain
and production control
In order to meet the
efficiency demands of AMP6, CiM6 has embarked on a change
programme, working with Severn Trent, to make collaborative
planning and daily production control business as usual. Weekly
production control sessions using lean techniques and software
to bring programmes to the forefront of the design process
helped to provide a focussed engineering stage prior to
Daily production control
on site with the fully integrated supply chain teams helped meet
a challenging programme and provide a consistent health and
safety message to all construction workers.
percentage complete metrics and utilising an active plan to
protect and issue management workflow has allowed the project
team to deliver the scheme under budget and achieving all the
Off site manufacture and
One of Severn Trent’s
capital efficiency streams is to promote Design for Manufacture
and Assembly (DfMA). The strategy has been actively pursued on
the Netheridge Imports project to great effect. Assets such as
the polymer mixing and dosing kiosk, imports screens, washwater
filters and pump mechanical assemblies have all been
manufactured and tested off site to reduce the installation
time. In reducing time subcontractors spend on site, this
approach has also reduced the risk of accidents and near misses,
ensuring that this project remains a safe site for all
The design for the sludge
processing units has been an evolution from previous CiM6
projects for Severn Trent. The arrangement for the gravity belt
thickeners and imports screens for example has previously been
seen at Wanlip STW and the design for Netheridge has built on
those successfully installed assets, taking feedback from
operators and maintainers.
trade waste and sludge imports area and tank storage -
Courtesy of Tom Pawson, Costain
Netheridge STW has long
been a strategic site for Severn Trent in the south of the
region with many outlying sites and traders relying on the
successful operation of the site. The Netheridge Sludge Imports
project will provide the site with additional trade waste
imports capacity and assurance within the digestion process to
allow Severn Trent to accept further trade waste in the future.
The improvements to the
digester feed stock assets and the control functionality will
give the site maximum opportunity to be a profitable and
sustainable site for Severn Trent to operate.
and publishers would like to thank Jon Rogerson, Civil
Regional Discipline Lead, Gabriella Giuffre, Regional
Project Delivery Lead, Tom Pawson, Project Manager, all with
CiM6, and Mark Calcutt Programme Manager with Severn Trent
Water, for providing the above article for publication.