challenging the conventional solution, driving efficiencies
through design, and delivering exceptional outcomes
by Daniel Buxton MEng CEng MICE
(published October 2018)
AAD - Courtesy of MMB/M7
Water set out its vision in its Sewage Sludge Strategy 2040 how
it intended to deal with the number one waste management
challenge all water utility providers face, the sustainable
management and disposal of wastewater sludges. With ever
tightening nutrient and heavy metal limits on agricultural
usage, and more pressure than ever for companies to be more
self-sufficient with their energy usage, many are turning to
more advanced and innovative ways to treat wastewater sludges.
Previously, in 2010,
Welsh Water delivered the first part of its vision by
implementing two advanced anaerobic digestion (AAD) facilities
in South Wales at Cardiff Wastewater Treatment Works (WwTW), and
Afan WwTW. Since late 2014, Mott MacDonald Bentley (MMB), on
behalf of Welsh Water’s Capital Delivery Alliance, has been
delivering the second phase of Welsh Water’s Sludge Strategy
aim, the £56.2m North Wales Sludge Strategy (NWSS). At its heart
the NWSS is a waste management strategy, but also aims to
deliver both Opex savings and enable the business to meet the
Biosolids Assurance standards from June 2018. The other
principle drivers behind the strategy are:
Production of an
‘enhanced’ standard biosolid.
Improve dewaterability of
biosolid product reducing transport costs.
destruction increasing gas yield and reducing transport costs.
Reduced reliance on aging
assets for sludge treatment.
selected option schematic - Courtesy of MMB
The initial concept
solution for NWSS was to replicate the solution for South Wales,
and develop two AAD sites, one in North East and one in North
West Wales, to treat a combined sludge volume of
22,000tDS/annum. However, through initial sludge data analysis,
the team identified the potential to do something different by
utilising existing assets which would ultimately maximise the
benefit to Welsh Water and the customer.
The team embarked on a
detailed feasibility study to determine whether two AAD sites
would provide the best option.
This required an
extensive period of data collection and analysis, looking into
the volumes of sludge to be transported and their origin. A
transport model was built using ‘as tendered’ prices per mile,
utilising knowledge already existing within the Welsh Water
logistics team. Early engagement with the supply chain also
meant the latest technological advances could be utilised when
devising the solutions and conducting the OPEX calculations.
Porthmadog dewatering building - Courtesy of MMB
To further supplement the
feasibility study the team also embarked upon:
forecasts by conducting on-site process assessments utilising
crude sewage data and comparing that to imported and exported
assessments to ensure safe, secure outlets remained wherever the
AAD sites were located.
An in-depth resilience
scenario review to look at potential failure modes across North
Wales, ensuring that a single site option was robust enough to
become the centralised sludge treatment centre.
Asset condition surveys
to understand what plant could be reused within the new
solutions and what plant required replacement.
to ensure whatever solution was picked allowed for mitigation
measures to ensure the development minimised its impact on the
environment and local surroundings.
This information was then
subject to a strategy option identification process, where
feasibility-level designs were analysed on a whole life cost
basis, so selection of the preferred solution could be made via
a risk and value workshop. Multiple options were analysed
including location and number of plants.
The selected option for
the strategy was the construction of a single £37m Advanced
anaerobic digestion (AAD) Plant at Five Fords WwTW, near Wrexham.
A further £19.2m would then be invested in the decommissioning
and conversion of seven satellite wastewater sludge processing
sites to export dewatered biosolids for processing through AAD
at Five Fords WwTW.
The existing site at Five
Fords WwTW was a conventional anaerobic digestion facility, with
liming following digestion. The site had a treatment capacity of
10,000tDS/annum and included; a liquid import centre with two
stage screening, 2 (No.) 3,814m3 digesters complete
with heat exchanges and natural gas hot water boiler, 1 (No.)
1,350m3 gas bag, 4 (No.) gravity belt thickeners,
lime treatment facility and existing dewatering and cake storage
In 2015 a new gas to grid
facility was commissioned at Five Fords WwTW. The facility
cleans, upgrades and pumps the biogas generated through the
digestion process into the national gas grid.
The seven satellite sites
are all existing sludge treatment centres, with five being
conventional digestion facilities and two liming facilities. All
treat biosolids for disposal to agriculture, and the digestion
facilities generate biogas for on-site usage and power
generation through existing CHPs.
Fords as beginning of construction - Courtesy of MMB/M7
Following selection of
the strategy single option, the proposed solution at each of the
sites was developed in detail. Commencing in early 2016, design
was conducted collaboratively with the supply chain and the
client, culminating in multiple risk and value reviews, HAZOPs,
and Access, Lifting & Maintenance (ALM) reviews. The following
is a breakdown of the final solutions.
The design capacity of
the proposed Five Fords AAD facility is 47.9tDS/d and all biogas
generated through the new process will be diverted to the
existing on site gas to grid facility. The process stages of the
2 (No.) belt reception
units for imported cake with 30m3 storage.
First stage cake dilution
and 90m3/hr cake pumping station.
2 (No.) 300m3
cake storage silos where imported cake and dewatered indigenous
sludges are blended.
building where indigenous primary sludges, SAS and screened
imported liquid sludge (existing liquid import centre) are
Final effluent filtration
and UV disinfection centre.
Dual stream thermal
hydrolysis plant and three-stage sludge coolers.
Automated digester grit
removal and classification system.
Existing cake building
upgrade and new extension to increase storage.
2 (No.) odour control
units for cake imports and post-THP.
Liquor treatment plant
utilising Anammox technology (MBBR) to treat post-THP liquors
before discharge to the existing WwTW.
Bay sludge storage tanks - Courtesy of MMB
The solution at each of
the satellite sites was similar in its intention, but due to the
size and condition of each of the sites the investment required
ranged from £0.3m to £6m. The notional solution at each of the
Liquid sludge import
screen and well.
Enhanced screening via
Polymer dosing and
Cake export system
delivering into 29t trailers.
Sludge buffer storage
coupled with pump mixing.
Odour control unit.
Decommissioning of the
belt reception units - Courtesy of MMB
Efficiencies & lessons
learned through detailed design & delivery
Throughout the design and
delivery process the team strived to ensure past lessons were
learned from, and future efficiencies and innovations were
learned: Through delivering several sludge treatment facilities
across the UK, MMB and the Capital Delivery Alliance were able
to ensure mitigation was put in place to avoid previously
encountered issues. Lesson learned workshops were held with the
Cardiff and Afan AAD delivery teams before detailed design
commenced to capture pertinent points from their journey, but
also key site visits were made across the UK to get operations
Direct outcomes of these
lessons learned included:
A two-stage dilution
system with hot FE to ensure a more robust control of the %DS
feed to the THP, but also reduce the steam requirement due to
Use of warm FE for
polymer make-up to improve solids capture in dewatering.
Use of belt reception
units rather than hoppers to reduce excavations during
construction and mitigate conducting maintenance activities in a
pit during operation.
Use of direct coupled
cake transfer pumps to the bottom of the cake silos removing any
bridging risk and subsequent starvation of flow to the THP.
treatment plant model - Courtesy of MMB
Innovation: As part of the delivery the team strived to
embrace the latest advances in technology when it came to the
selection of plant. Some highlights are:
Due to the production of
high strength liquors via the post-digestion (and therefore,
thermal hydrolysis) centrifuges a liquor treatment plant is
required to reduce the ammonia load returned back to the WwTW,
providing resilience and protection to the current discharge
consent. This liquor treatment process would conventionally be
an activated sludge process, reducing ammonia via traditional
This traditional option
was challenged due to its high chemical and power usage, and a
detailed option selection process was undertaken where Veolia’s
moving bed biofilm reactor (MBBR) HYBAS technology was selected.
This is a purpose-designed high strength liquor treatment
technology, utilising anaerobic ammonium oxidation (Anammox)
Compared to conventional
treatment there is a large OPEX saving as the Anammox bacteria
convert ammonium and nitrite directly to N2 anaerobically,
minimising the need for aeration and alkalinity dosing.
Manufacture and Assembly (DfMA) and standardisation: Due
to the repeatability of the satellite sites, and the numerous
package plants utilised at Five Fords, the team was able to
exploit the benefits that DfMA and standardisation brings. Some
The development of 4
standard products for the satellite sites; dewatering, polymer
dosing and batching, sludge tanks and sludge screen. The design
of these products has been procurement-led and developed
alongside the supply chain following an extensive tendering
process. This work ensured they are as easy to adapt as
possible. This development has saved time in design and key
meetings such as HAZOPs as well as led to procurement
The new boiler house was
designed collaboratively with the supplier Dunphy Combustion.
The boiler house was fully HAZOP’d and ALM’d before being
manufactured, assembled and fully tested in Dunphy’s factory.
The building was then broken down into sections before delivery
design: Procurement has been managed holistically rather
than on a project-by-project basis, saving time in tender bid
analysis. We have always championed procurement-led design,
ensuring we design once, reducing wastage. Innovative delivery
models have been trialled with key supply chain partners,
including pilling and mechanical install, where their design
expertise has been exploited for commercial gain and reduction
THP Stream - Courtesy of MMB
We are now producing and managing more information in
construction than ever before. With the correct workflows,
processes and tools in place the way we deliver projects can be
enhanced, paving the way for truly intelligent delivery and
assets. Through this project we were able to develop and utilise
different tools, including intelligent P&IDs and databases, to
ensure data was managed from inception through to commissioning
in the most efficient way, making the whole process automated.
The team particularly
exploited advancements in modelling, immersive environments and
virtual reality to help the whole project team make efficiencies
through design reviews, maintenance assessments and construction
‘dress rehearsals’. These tools help reduce health and safety
incidents during construction by highlighting clashes before
they happen and reduce time during handover as the operator
feels more confident inheriting an asset they have already
Following completion of
the new access road and bridge over the River Clywedog,
construction commenced at Five Fords in April 2017, with the
first activity being the installation of piling. The scheme was
broken up into four distinct phases to ensure construction
activities could take place in a safe manner, and supplier
lifting procedures did not clash.
Working with the supply
chain, the team strived to remove any large below ground
structures, meaning the majority of plant was installed on slabs
at ground level. This not only made construction safer but also
reduced time and cost. Also, by striving for as much DfMA as
possible mechanical install was reduced on site with the obvious
The satellite schemes
began on site in March 2017 and were phased in the construction
commencement to ensure the team could benefit as much as
possible from lessons learned as delivery teams moved from site
Five Fords AAD
ProjectProcess and package plant suppliers
THP & sludge coolers
Cake belt reception units
& storage silos
CTM Systems Ltd
Liquor treatment plant
Polymer dosing plant
Northern Pump Suppliers
Rotamat and Strainpress
screens, grit removal system & classifier
Air Water Treatment Ltd
Caustic dosing plant
Gee & Co
Dunphy Combustion Ltd
Package pumping plant
Tanks and mixers
Cake loading system
With construction well
developed at Five Fords, the commissioning phase is to begin in
July 2018, with an enhanced product being produced by first
The full scheme will be
completed by end of 2019. The satellite sites will be completed
in line with the import requirements to commission the AAD plant
at Five Fords so all will be completed by summer 2019.
This paper aims to
summarise the work conducted to date on the North Wales Sludge
Strategy, with a real focus on the feasibility phase of the
project and the efficiencies gained through design and delivery.
It also demonstrates how,
through innovative thinking, MMB and the Welsh Water Alliance
challenged initial proposals for two AAD plants in North Wales.
This led the team to conclude on a single AAD Plant option that
has reduced capital investment by £15m, reduced operational
costs by £2.1m/annum, and whole life costs by £40m.
and publishers would like to thank Daniel Buxton, Design
Manager with Mott MacDonald Bentley, for providing the above
article for publication.