montage of proposed Heigham WTW Upgrade and Raw Water
Water Treatment Works (WTW) is the main supply of water to
Norwich and supplies an average of 42Ml/d of water sourced from
the River Wensum at several locations. Potable water is pumped
into the Norwich Public Water Supply Zone feeding a multitude of
storage reservoirs, booster pumps and properties. The main raw
water abstraction for the treatment works has historically been
taken from Costessey Pits, a bankside storage area next to the
River Wensum approximately 7km away from the water treatment
works and away from the urban area of Norwich providing a more
favourable water quality and buffering capacity to manage sudden
water quality changes in the river.
As part of the EU
Habitats Directive, the stretch of the River Wensum between
Costessey Pits and Hellesdon Meadow has been designated as a
Special Area of Conservation (SAC). The Environment Agency (EA)
has reviewed the abstraction licences provided to Anglian Water
for the River Wensum and determined that abstraction of water at
Costessey Pits shall be reduced by April 2019.
Business plan solution
To ensure the continued
supply of water to the public, a number of options were
considered including; desalination of seawater on the Norfolk
coast, indirect reuse of wastewater from Norwich and the
construction of a large new raw water storage reservoir just
outside Norwich. The preferred AMP6 business plan option
however, was to use the existing abstraction point at Heigham
WTW, which falls outside of the SAC, to compensate for the
reduction in abstraction from the river at Costessey Pits. This
solution included a pumping station and an 8km long, 800mm
diameter pumping main to transfer the raw water to Costessey to
enable re-utilisation of the existing 5 days storage capacity.
The stored water would then be conveyed from the pits to the
water treatment works via the existing pipeline.
Schematic of proposed business plan solution - Courtesy
of Anglian Water
Current treatment process
The current works process
stream comprises of a river intake system where the water is
screened prior to being pumped to the large deposit reservoir
which provides around a day’s storage. However, due to the
flashy nature of this river source and limited on site raw water
settlement capacity, feed water is normally supplied from
Costessey Pits which provide a better quality and more reliable
A set of low lift
transfer pumps feed the works where the raw water is pre-ozonated
and then coagulated with ferric chloride prior to flocculation
and dissolved air flotation (DAF) clarification. This is
followed by dual media (anthracite and sand) rapid gravity
filtration (RGF) and then main ozonation prior to granular
activated carbon (GAC) adsorption.
Finally the water
undergoes disinfection by superchlorination, caustic dosing for
pH correction and orthophosphoric acid dosing for plumbosolvency
control. Sulphur dioxide reduces the chlorine residual prior to
ammonia dosing to produce chloramine for network protection.
Treated water is stored on site and delivered in to supply by a
set of high lift pumps. Dirty washwater from the RGFs and GAC
adsorbers is sent to settlement tanks where supernatant
discharges to the environment and sludge is pumped to sewer.
Sludge from the DAF process gravitates to sewer.
Operational risk review
Sustainability Scheme is part of the programme of works the @One
Alliance was asked to deliver as part of their AMP6 Integrated
Main Works framework agreement with Anglian Water. An initial
review of the business plan solution, along with some
particularly challenging raw water quality conditions in
December 2016, highlighted that the pipeline transfer solution
failed to mitigate a number of treatment risks associated with
the existing treatment process.
The abstraction point at
Heigham falls within the Urban Area of Norwich, is downstream of
a storm water overflow, chemical plant and other water recycling
works discharges. Currently, it feeds almost directly into the
works with only limited buffering capacity provided by the Large
Deposit Reservoir (LDR). Therefore, relying on this alternative
raw water source presents a greater treatment challenge to the
The existing DAF
clarification process is well suited to removing algae but not
high levels of suspended solids. Under poor raw water conditions
in the river, turbidity levels can reach 150 NTU; the DAF
process struggles when levels approach double figures. Costessey
Pits do not perform well as settlement ponds as originally
envisaged and the LDR at Heigham gives limited settlement
capacity. Consequently, it has been estimated that there is a 1
in 6 year risk that Heigham WTW may be unable supply water for
up to 3 days, even with the new pipeline transfer solution in
Furthermore, during high
river flow and turbidity events, water abstracted at Heigham can
contain a significant level of cryptosporidium oocysts. The
works currently provides no absolute treatment barrier to these
oocysts and although it can be argued that a well operating
surface water plant with clarification and filtration can
provide effective removal, in the case of Heigham, the
vulnerability of the DAF process to higher turbidities means
this cannot be relied upon. The poorer quality of clarified
water under these conditions affects filter performance and
therefore the risk of cryptosporidium breakthrough into the
final water increases significantly.
BIM model snapshot of submerged membrane plant -
Courtesy of Anglian Water
In light of the risks
detailed above, the @One Alliance was asked to develop an
alternative solution to upgrade the existing clarification and
filtration processes at Heigham WTW to cater for a more
challenging raw water quality envelope. The key challenges for
the upgraded treatment works were defined as:
A source water has a
highly variable water quality with significant peaks of algae up
to 31,500 counts/ml and turbidity up to 200 NTU.
The source water shows a
risk of microbiological contamination.
Water quality temperature
ranges from 0.9° to 22°C.
The final product water
is the main drinking water supply for the City of Norwich and
thus a resilient solution is required.
Feed water supply is
restricted and high water recovery is required such that reject
flows are limited to a maximum of 2800m3/day.
Through an extensive
procurement exercise, the design team considered a range of
options including roughing filters, pre-clarification and sand
ballasted lamella clarification. However, they concluded that a
submerged ultrafiltration membrane plant was the solution which
best suited the limited footprint available whilst leaving the
lowest level of residual risk. Additionally, Anglian Water has
significant operational experience in submerged membrane
filtration at their Clapham (Bedford) and Hall (Lincoln) WTWs.
Following this process,
GE Water and Process Technologies were selected as the submerged
membrane technology suppliers. They proposed their ZeeWeed‐500
membrane as the primary filtration step. This reinforced, hollow
fibre ultrafiltration membrane has a strong track record in
similar applications and offers a number of key benefits:
The reinforced membrane is strong and capable of dealing with
high solids loads.
tolerance: The low fibre packing density allows for high
solids tolerance to up to 5,000mg/l process solids in drinking
water application. A more conservative limit of 2,500mg/l TSS is
proposed for Heigham to take into account solids from the feed
water, solids generated during coagulation, stickiness of the
algae present and solids returned from any waste treatment.
chemistry: The membrane uses PVDF chemistry which is low
fouling whilst also highly resistant to oxidants, which can be
used for chemical cleans. At Heigham the plant is designed to
treat raw water coagulated with ferric sulphate, clarified
wastewater returns and potentially sticky algae at certain times
of the year. The membrane selection enables the widest range of
cleaning options to remove any foreseen foulants.
ensuring particulate/microbial quality downstream: The
membrane is a barrier to particulate matter and micro-organisms
which ensures reliability of performance downstream,
irrespective of feed water quality. At Heigham achieving 4 log
removal for cryptosporidium and coliforms is important and has
been proven at other drinking water sites.
adapting to differing water quality: The membrane is
assembled into a modular system. The high solids tolerance, low
fouling chemistry and the modular nature of the system as well
as the particulate, bacteria and virus barrier provided by the
membrane ensure a robust system design able to adapt to
differing water quality.
of proposed submerged UF membrane solution - Courtesy of
Working closely with GE
and other supply chain partners, the project team are currently
developing the detailed design for the new submerged membrane
plant solution. This will be integrated within the existing
treatment works process and will replace the existing DAF and
RGF treatment stages. The existing structures for these two
processes will be re-purposed to provide buffering of flows,
both to and from the membrane stage, to deliver a consistent
output to the downstream processes.
Raw water from the LDR,
Costessey Pits or direct from the River Wensum will be pre
ozonated, coagulated and strained prior to the submerged
membrane process. Reject flows from the membrane process will be
clarified and returned to the LDR with the waste sludge
discharged to sewer along with waste cleaning chemicals from the
clean in place process.
Construction is due to
commence in autumn 2017 with handover scheduled for December
and publishers would like to thank Dr Stewart Bell, Head of
Water Process with Skanska Utilities, for providing the
above article for publication. The author thanks Anglian
Water for permission to publish this case study.