construction and commissioning of a £43m sludge treatment centre
at Thames Water’s Basingstoke STW by the eight2O Alliance
by Kerry Gutteridge BEng CEng MICE
STC - Construction Progress June 2017 - Courtesy of eight2O
Basingstoke Sludge Treatment Centre (STC) incorporates liquid
and caked, raw sludge import facilities together with thermal
hydrolysis and anaerobic digestion plant which will process
53tDS/day of sludge from Basingstoke, Farnham, Fleet, Guilford
and Newbury. This will fulfil Thames Water’s strategy to cease
unsustainable liming activities at the four satellite sites up
to the 2036 design horizon. Combined heat and power (CHP)
engines, installed as part of the project will generate
sufficient green electricity to satisfy the STC and the existing
STW requirements and surplus power will be exported to the Grid.
The sludge treatment
centre is being constructed and commissioned by the eight2O
alliance which is the largest alliance in the water sector. The
partners involved are Thames Water and two design and build
joint ventures made up of Costain, Atkins, Black & Veatch (CABV)
and Skanska, MWH (now part of Stantec) and Balfour Beatty (SMB).
MWH also leads programme control and IBM is the technology
At Basingstoke STC, MWH
UK began the detailed design in March 2015. Costain started
construction in September with targets of achieving ROCs
accreditation on the new CHP engines before the scheme closed in
March 2017 and will achieve full operational benefits by March
At an early stage in the
project both the thermal hydrolysis process (THP) and a new
variant, the intermediate thermal hydrolysis process (iTHP),
were considered. The latter has been shown to produce more
biogas than THP which in turn results in more power generation.
However for sites with a high proportion of imported cake, such
as Basingstoke, the cost of diluting the cake prior to primary
digestion and then dewatering the primary digestate prior to
thermal hydrolysis exceeds the revenue generated from exporting
electricity from the site, so iTHP was not economically viable
in this installation.
The PR14 business plan
solution was based on expanding the existing sludge treatment
facility within a very small site area which would have caused
operational, demolition and construction phasing difficulties.
In view of this MWH recommended that the STC be built off–line
on some of the old stormwater treatment plots immediately to the
east of the existing STW.
This provided numerous
benefits in terms of minimising interfaces with the existing
operational works during construction and the optimisation of
plant and services’ layout was not compromised by lack of space.
Stormwater treatment plot before construction
Courtesy of Costain
Excavation for cake import bunker
Courtesy of Costain
The existing sludge
treatment plant at Basingstoke STW treats 12.9tDS/d and consists
of gravity belt and picket fence thickeners, primary and
secondary digesters, digested sludge dewatering belts and a
covered cake storage area.
Electricity from 2 (No.)
existing CHP engines is used on site. The existing digesters and
CHPs will be decommissioned once the new STC is commissioned.
The STC is designed for a
throughput of 53tDS/d; under normal operation however it can
accommodate up to 70tDS/s in order to catch up with a backlog of
sludge imports when the THP and steam boiler are shut down for
annual servicing and during infrequent unplanned shut downs.
At the normal throughput
of 53tDS/d, the CHPs will generate 2.23MW and satisfy the 1.32MW
total demand of the STC and the existing STW and the remainder
will be exported to the Grid.
The STC comprises the
Imported sludge cake
Imported liquid sludge
collection and transfer to STC.
Liquid sludge blending,
buffering and screening.
polyelectrolyte storage, preparation and dosing for pre-THP
Sludge cake back-mixing
THP feed silo.
Thermal hydrolysis plant.
Sludge cooling and
transfer to digesters.
collection and transfer.
dewatering and storage (existing).
Biogas storage and
Dual fuel boiler (biogas
and diesel oil).
Odour control plant.
Liquor collection and
return; partial flow to the existing aeration plant, the
remainder to a liquor treatment facility.
Final effluent supply.
Potable water supply
(reverse osmosis plant).
Standby generator for
Auxiliary fuel storage.
Building on previous
Whilst THP technology has
been used in the wastewater industry for some time now and the
actual THP optimised, it is the design and operation of
ancillary systems that determine the success of the plant.
Indeed there are currently projects being undertaken across
numerous water companies to ‘optimise’ the operation of existing
installed THP plants.
THP being lifted into position - Courtesy of Costain
The project team worked
closely to build on previous experiences and good knowledge of
existing THP plants to make Basingstoke THP the best it can be,
some points of particular note:
collection tower: A digested sludge collection tower has
been constructed to counter issues and problems associated with
digester surging (volume changes). THP sludge is very
digestible, producing a lot of gas compared to non-THP
digestion. As soon as THP sludge is fed into the digester,
within minutes biogas is produced. Thus, the digester feed rate
has a significant effect on the operation of the digesters; an
increase in feed rate results in an increase in gas entrainment,
resulting in a density fall and rapid volume increase. This
effect causing a ‘surge’ and sudden discharge from the digester.
As the digesters work on
hydrostatic displacement there may be a surge out of 3 main
tanks, the digested sludge collection tower acts as a balance
tank which reduces the risk of rapid volume loss and allows
sludge levels to be managed by the forwarding pumps.
Kirk GFS tanks installed. Digested sludge collection
tower on right in front of digester
Courtesy of Costain
digesters: This has been an issue on many of the earlier
sites; which is due to what the ‘digestive’ bacteria excrete (a
washing up liquid like substance) that creates a stable layer on
top of the sludge. In theory, if there is a continuous feed,
with a continuous discharge then all should be well.
However, small variations
in gas production happen as soon as sludge stops
overflowing/discharging from a digester; this results in
degassing in the digester and a thicker (higher density) of
sludge in the discharge pipe which cannot readily be pushed
through. Pressure keeps building up until the ‘slug’ of higher
density sludge can be pushed through.
On the outlet pipes of
the digesters at Basingstoke, an air lift system has been
installed to encourage the density in the riser pipe to remain
relatively low by introducing air into the system.
Basingstoke STW has a 1mg/l ammonia consent. As part of the
project, Basingstoke will import three times its indigenous
sludge production thus generating more ammonia load which will
put strain on the STW to maintain consent. To overcome this
issue a Demon LTP is being installed for ammonia removal; a low
energy and low sludge production solution.
availability: There is no natural support fuel to rely
on as providing a new gas supply to site was considered too
expensive so the THP will in effect run-itself, using biogas to
run the plant. A small diesel storage has been provided to
enable ‘start-up’ during the commissioning phase and following
Use of low grade
heat from CHP engines: To heat final effluent for
dilution of sludge pre-THP and also heating the hot well. Both
reduce steam consumption which means more biogas available for
facility: THP needs to be fed at a relatively consistent
feed dry solids to achieve optimal performance. Incoming cake is
too thick to feed directly. The use of screened liquid sludge to
dilute pre-dewatered sludge down to 20% dry solids before going
to the THP feed silo has been provided. This saves on polymer
usage in the pre-dewatering process.
Use of low grade
heat: to keep LTP bacteria warm.
To realise the potential
fast delivery benefit of a design and construct approach, civil
designs were called for in advance of receiving ‘official’
supplier design information, thus the handling of interfaces
with key plant suppliers was crucial to the success of the
project. At the time of writing (August 2017) circa 8000m of
underground pipework, 6000m of above-ground pipework and 5000m
of cabling has been installed.
Over 3500m of underground pipework has been installed -
Courtesy of Costain
one of the pipe corridors filling up quickly
Courtesy of Costain
In the absence of
supplier information, experience based design decisions were
important in facilitating the installation of such services, in
particular designating two main above ground service corridors
and locating plant/setting the site falls to facilitate the
gravity pipe runs.
were minimised to reduce potential for delays due to potential
groundwater and increased construction costs associated with
excavations. There were only two pumping station shafts (each
approximately 7m deep) and one large cake import bunker (20 x 15
x 5m deep) that required substantial excavation.
Importantly, much of the
mechanical plant (e.g. FE transfer pumping station), was skid
mounted to reduce installation time on site and some of the
larger plant items, including the THP plant, were assembled
off-site prior to being dismantled and reassembled on site to
minimise installation issues.
construction underway. Dewatering belts
polymer dosing kiosk in the foreground
STC - Drone aerial view
eight2O Drone Service
Over 40 suppliers were
engaged, mainly under NEC Option C Target Cost, from local
contractors to as far afield as the Netherlands. The largest
value contracts were FSD (pipeline supply and design) £4m and
CAMBI (THP plant) circa £3m. The smallest value contract was
with NBK (Nick Building Contractors) circa £20,000.
The longest lead in time
from inception to delivery was approximately 10 months, and
nearly 160 CEs have been raised and closed to date. Spending
peaked between November 2016 and March 2017 at circa £1.3m per
Richard Alan Engineering
Imported Cake reception
CTM Systems Ltd
Kirk Environmental Ltd
Dunphy Combustion Ltd
Seepex and Xylem
Liquor Treatment Plant
Plasticon UK Ltd
Mixing System Mix and
Waste Gas Burner
Flare Products Ltd
HRS Heat Exchangers Ltd
The programme was driven
by the need to achieve ROCs accreditation of the engines by
March 2017. Ideally this meant that all plant would be up and
running; the site would be receiving and processing sludge
imports, the THP would be in operation and digesters producing
the gas required to run the engines.
To ‘de-risk’ the
programme MWH suggested and designed a temporary gas main which
was installed across the site from the existing gas holder to
allow engine start up in advance of March 2017. Currently the
CHP engines are running, the digesters have been seeded and one
is being brought up to temperature.
First imports are
expected in June, ramping up to a fully commissioned works by
2 (No.) CHP
engines in foreground. Engine start-up achieved to
programme using temporary gas supply - Courtesy of
and publishers would like to thank Kerry Gutteridge,
Principal Civil Engineer with MWH (now part of Stantec), for
providing the above article for publication.