major improvement to Lancashire treatment works as part of
United Utilities £150m investment programme
by Steven Smythe MSc MICE CEng
Bank - new treatment stream in operation - Courtesy of C2V+
Bank Wastewater Treatment Works (WwTW) serves a population of
approximately 5,800. The catchment of this Lancashire village is
mainly residential and much of the surrounding land is dedicated
to agriculture. The treatment works is on the edge of the
village and is situated close to the River Douglas, near its
confluence with the Ribble Estuary. The treatment works has
recently seen a £4.5m investment by United Utilities (UU). This
investment was driven by the requirements of the Bathing Waters
Directive to enable the operation of the storm tanks overflow,
to meet the discharge frequency limits. Additionally, minor
operational improvements were identified to be addressed as part
of the solution.
The project was awarded
to C2V+, a joint venture between CH2M and VolkerStevin, in
November 2015 following a competitive tender as a design and
build contract. The proposed solution comprised:
Replacing the existing
process with a new treatment stream.
The new treatment stream
to operate with a flow-to-full treatment (FFT) of 64 litres per
second (l/s), (the existing treatment stream in contrast served
a FFT of 36l/s). This provided the treatment (process and
hydraulic) capacity, which meant the storm water could be
managed with a small increase in installed storm tank volume.
The revised FFT reflected
the existing consented 6DWF and allowed the solution to work
within the existing consent regime, which includes BOD of
25mg/l, SS of 60mg/l (both as 95-percentiles) and UV
disinfection applied to all flow that passes through the main
(No.) PCC piles being driven to depths of up to 19.5m to
support the PST, ASP, FSTs and storm return pumping
station - Courtesy of C2V+
Installation of ASP base reinforcement
Courtesy of C2V+
Details of the treatment
plant forming the new works is as follows:
Replacement inlet works
screens (one of the improvement drivers): Huber screens.
Updated control for
control of FFT: Eric Wright Water/Adsyst
A new 13.5m diameter
precast concrete primary settlement tank: Shay Murtagh.
A new precast concrete
aeration tank 18m x 9m x 5m deep (split into two lanes) and 2
(No.) new 12m diameter precast concrete final tanks: Shay
Scrapers for the
settlement tanks: TOT Technical.
Fine bubble diffused
aeration system: Suprafilt.
Return activated sludge
(RAS)/surplus activated sludge (SAS) and other pumping
ancillaries: Eric Wright Water.
New ultraviolet plant
(existing discharge consent requirement) to treat the revised
FFT: Trojan Technologies.
Conversion of old process
tanks to storm storage duty (reusing assets with residual life):
C2V+/Eric Wright Water.
Installation of mixing
equipment to storm tanks, motor control centre (MCC) and control
systems: Lloyd Morris Electrical.
Erection of precast wall units ASP walls
Courtesy of C2V+
PST, ASP and FST.
Placement of ASP PCC walkways prior to post-tensioning -
Courtesy of C2V+
Activated sludge is at
the heart of the process solution but the aeration basin design
has been tailored to suit site needs, by having a two lane basin
with three process cells in each lane:
3. Aerobic cell
with reduced diffuser array.
The purpose of the
proposals is to achieve a settleable sludge and low OPEX/TOTEX
costs for a plant that will experience a wide range of hydraulic
loading. At times of high hydraulic loading, the sewage will be
relatively weak due to dilution from the storm flow element.
Therefore the intended
impact is to address known operational issues to reduce time and
effort maintaining and operating the site to deal with the
settleability issues and its associated impacts on the process
C2V+ selected an
anaerobic selector (as compared to an aerobic selector),
primarily based on its experience with them providing reduced
SSVIs as compared to aerobic selectors. The significantly
reduced energy usage associated with eliminating the high DO
aerobic selector leads to an operational cost saving in
comparison. The aerobic portion of the aeration basin is still
designed to provide satisfactory oxygenation in order to meet
the site consent.
particularly the adoption of an anaerobic selector zone, was
innovative for UU and not covered by their original design
standards. UU worked with C2V+ to understand the proposal prior
to its acceptance.
Activated sludge plant aerobic zones and completed final
settlement tanks - Courtesy of C2V+
The activated sludge
system was sized with sufficient volume and blower capacity to
allow the plant to function well should the primary settlement
tank not be used due to planned maintenance works (e.g. scraper
inspection). In addition, two final tanks were provided to allow
similar operational flexibility.
Due to logistical
constraints on site, proximity to customers and issues with
congestion, there was a need to minimise construction time on
site. This resulted in the team having to begin construction of
the final settlement tanks first and work their way out from the
back. An off-site solution was proposed using the Dutchland
system, of which the project at Hesketh Bank is one of the first
uses in the UK.
Dutchland tanks consist
of precast concrete wall units manufactured off-site and
post-tensioned in situ to achieve high residual compression. The
precast wall units were placed on a reinforced concrete base
slab, which is in turn supported by piled foundations due to
uneven ground conditions.
These, together with the
post-tensioned precast concrete settlement tanks (which
incorporated a precast concrete internal launder channel),
enabled the project to beat the programme and budget allocations
for in situ construction. All of the tanks passed their water
tests first time and in the minimum period.
Many Design for
Manufacture and Assembly (DfMA) benefits were recognised on the
Hesketh Bank project. Using DfMA meant the wall units for the
12m diameter FSTs could be erected in a single day. In contrast,
using traditional formwork for the walls would have proven
extremely difficult in such a tight working area, and a larger
working platform would have been needed, causing further access
reinforcement also reduced the depth of the walls for the PST
and FSTs, reducing material costs and wastage. Using PCC panels
reduced delivery traffic on the site entrance road, which is a
quiet residential country lane. The factory-produced units are
of a higher quality finish than that which would be achieved by
in situ construction and eliminate site wastage, concrete
sampling and curing times before striking formwork.
sludge plant walls progression
plant under construction
Using DfMA saved a total
of 12 cumulative weeks project time and 25% of the cost of
traditional methods. The system also reduced customer impact and
waste, as well as improved health and safety, reducing the
amount of time spent on site. In addition to the high quality
finish, the precast wall units have proven to be more effective
in achieving water tightness; active reinforcement results in
less cracking and greater longevity. This process also allowed
construction schedules to overlap and reduced weather-related
C2V+ís customer team took
an active role in assisting the delivery (site) team to build
early links with the neighbours. This was particularly important
as the site is served by an access track shared with a number of
residents and landowners. The site team made efforts to ensure
that a 10 mph speed limit was respected at all times, that the
neighbours were kept aware of when the construction traffic
might be heavy and that the surface of the track was kept in a
wall units lifted directly from delivery wagons and
placed into the tank - Courtesy of C2V+
Working with Shay Murtagh,
one of the precast concrete tank suppliers, affirmed the benefit
of using a supplier with their own labour force who understood
the product and know where to focus their effort for best
effect. The walls have good line and level, due to the attention
to detail in setting up the packing onto which the units were
placed, with the water test passing first time.
UU is a strong advocate
for the application of DfMA and has been delighted with the
outcome at Hesketh Bank. C2V+ tier two partner Eric Wright were
key to the successful installation of the mechanical and
electrical equipment along with the control automation, and
worked alongside the site team in a collaborative approach
enabling the completion targets to be achieved.
Works have progressed on
time and completion is anticipated at the end of July 2017. Flow
through the works is being treated efficiently and the quality
of the effluent being produced is meeting or is of a higher
quality than the standards required meeting the regulatory
and publishers would like to thank Steven Smythe, Technical
Manager, and Mike Ryding, Senior Project Manager, both with
C2V+, for providing the above article for publication.