a complex project with tight timescales to meet regulatory
by James Henderson CEng MIET
3D model of
the new GAC filter bed - Courtesy of AECOM
Water’s largest water treatment works (WTW) is being upgraded as
part of their AMP6 business plan to ensure they continue to
provide a robust treatment solution for the removal of
pesticides from water abstracted from the River Thames. The
scope of the works is to increase the granular activated carbon
empty bed contact time (GAC EBCT) to a minimum of 15 minutes to
enhance pesticide removal. Trant Engineering has been appointed
as principal contractor and has teamed up with AECOM for the
provision of 8 (No.) new GAC filters, clean backwash tank and
pumping station, high voltage works, increased sludge storage
and associated equipment. The (circa) £12m project started
construction in January 2017 and is due for completion in July
Built in 1974, the water
treatment works was designed to output 160Ml/d of flow with
water abstracted from the River Thames.
Water quality in this
abstraction varies depending on weather patterns and land
activities in the Upper Thames. This site requires a multistage
treatment process to manage the risk from pollutants and produce
water that complies with the Water Supply (Water Quality)
Regulations 2000. These include the following processes:
Following periods of
heavy rainfall and seasonal applications of pesticides to
agricultural land, elevated levels of pesticides in the raw
water abstractions are detected. Chemicals are used on oil seed
rape and wheat predominantly grown in the Thames catchment and
are mainly detected in high levels in the abstraction from
September to March.
The site currently has 18
(No.) GAC filters split into three streams of six filters each
(nominated as streams A, B and C). Streams A and B were built in
1972-1974; Stream C was added in 1992.
that the existing EBCT should be increased to provide a more
robust treatment solution for the times of elevated pesticides
levels abstracted from the River Thames.
sludge tank under construction - Courtesy of Trant
commissioning of sufficient additional GAC filters to increase
the EBCT to 15 minutes, to enhance pesticide removal, which
requires the installation of additional GAC filters, while one
filter is out of service for regeneration and one filter out of
service for backwashing. In addition, the GAC filters must lower
turbidity, aluminium and iron levels.
Removal Project - Key Participants
Main D&B Contractor
Morrison Utility Services
Trant - Control,
Automation & Technology
QST Construction Services
Saint Gobain PAM
Epoxy Coated Steel pipe
Third Light Media
Invicta Valves Ltd
GAC filters - proposed
GAC filters (Stream D)
The installation of 8
(No.) combined filter adsorbers in reinforced concrete, constant
level - outlet controlled with a monolithic plenum floor
manufactured by Cadar.
Normal flow rate:
Normal peak flow rate:
Emergency peak flow rate:
GAC media specification:
AquaSorb CS 10 x 20 mesh in all filters. The media size was
selected because the GACs are not only acting as adsorbers but
Media EBCT: 15 minutes
based on maximum design flow and with one filter out of service
for regeneration and one filter out of service for backwashing.
Sweetening flow of 5% of
maximum filter flow to be provided to filter out of service to
prevent anaerobic conditions developing within the GAC.
Each filter is 9.4m wide,
12.8m long, giving a total filter area of 120m2 and a
volume of 138m3. The 8 (No.) have a total filter area
of 960m2 and the total filter volume is 1,104m3.
Construction of 8 (No.) GAC filters - 4 (No.) filters
either side of a central gallery
Courtesy of Trant Engineering
New backwash system
The backwash system used
for the existing GAC filters did not have sufficient capacity
for the new GAC filters. Therefore, as part of this project, it
was necessary to construct a new backwash system including clean
backwash water storage tank, backwash pumps, air blowers and
Each filter operates for a minimum of 24 hours between
backwashes, under all inlet turbidity conditions, and 36 hours
between backwashes in normal operation (when inlet turbidity
post ozone is <1 NTU and algae <4350 cells/mL) without any
deterioration on the turbidity, aluminium and iron removal
performance. The design is such that the capacity exists for one
new filter and one existing filter to convey waste backwash
water up to and including the thickened sludge tanks
The new stream backwash
system operates independently of the existing backwash system
and has sufficient capacity in the new backwash water tank to
allow for two successive backwashes without recharge.
The system is capable of
achieving filter bed expansion of 25% for granular activated
carbon media densities within the range of 220-550kg/m2.
Air scour rate: 40m/hr.
30 minutes or 414m3 based on 3 (No.) bed volumes.
Housed in a new RC structure 3 (No.) new 150kW variable speed,
end-suction centrifugal variable speed driven pumps have been
installed. Working on a duty/assist/standby basis, the total
flow rate is between 730l/s to 1330l/s for water temperature
range 2 to 25°C. Each backwash duration is in the range of 6-9
minutes, but is typically 8 minutes or 414m3 based on 3 (No.)
Air scour blowers:
Housed in the same area as the backwash pumps are 2 (No.) 75kW
air blowers. Working on a duty/standby basis, the blowers
provide 4800Nm3/h of flow during the air scour process of the
backwash cycle. Each cycle duration can range from 3-6 minutes,
but is typically 5 minutes.
water storage tank: Construction of a new covered
concrete single compartment, clean backwash water tank, with a
working capacity of 1328m3. The design is based on
two wash volumes on a 9 minutes wash at 25°C without
replenishment. The tank has internal dimensions of 22.3m x 12.4m
with a tank top water level at 5.3m.
New sludge storage
Prior to the project
commencement the existing sludge reception tanks were reported
to overflow on a regular basis, indicating that existing
hydraulic balancing capacity was insufficient at times. Due to
the increase in backwash water flow rates from the new filters,
an increase to both hydraulic and compositional balancing was
required to minimise overflow events in the future and to
provide better control of thickener solids, chemical dosing and
solids removal efficiency in downstream processes.
Piling for the new clean backwash water tank and pumping
Courtesy of Trant Engineering
To control the predicted
sludge concentrations within the sludge reception tanks to
within a maximum-to-minimum ratio of 2:1, a minimum volume of
between 1000-1200m3 is required to be maintained in
the tanks at all times.
It was calculated that a
minimum operating volume of 2000m3 will be required
to minimise the potential for overflow during times peak
hydraulic loading. The current combined storage volume is only
1,550m3; therefore to increase capacity a fourth
tank, similar in construction to the existing three tanks, was
Using Trant Engineering’s own specialist in-house panel
manufacturing division (Trant - Control, Automation &
Technology), Trant constructed a new MCC to power the new 2
(No.) 90kW air scour blowers (soft starts) and 3 (No.) 132kW
backwash pumps (VSD). The MCC also included power supply and
control of the package plant for air compressors, 42 (No.)
Rotork actuators for penstocks and valves, 8 (No.) pneumatic
flow control valve with closure on power failure and the
building services for new plant room and GAC filter gallery.
Trant - Control,
Automation & Technology conducted a full harmonic analysis of
the site, resulting in the inclusion of a 150A active harmonic
filter that was incorporated into the new MCC.
10 (No.) electromagnetic
14 (No.) ultrasonic level
8 (No.) differential
9 (No.) turbidity
1 (No.) iron analyser.
1 (No.) aluminium
2 (No.) pH analyser.
Full PROFIBUS control
system for actuators and instrumentation (PROFIBUS DP and PA).
PROFIBUS network, PROFIBUS interface panels, and PROFIBUS
pneumatic control for local override systems.
Local operator console
and integration with existing SCADA fibre optic network with
modification to the SCADA server for the new plant and
Modification to existing
For the addition of new
sludge tank jet mixers and control penstocks.
The (approximately) £12m project started construction in January
2017 and is due for completion in July 2018. Only close
collaboration forged with the operations and project teams from
Affinity Water, Trant and AECOM has made the achievement to date
possible and the entire team are keen to ensure this
collaborative approach is maintained in order to also achieve
the committed end date of the project.
and publishers would like to thank James Henderson, MEICA
Manager with Trant Engineering Ltd, for providing the above
article for publication.