innovation in the use of combined 3D BIM model across suppliers
supports design and build for off-site manufacture on
constrained redevelopment site
by Thomas Stell
site of only 1.26 hectares, this constrained site is
surrounded by redevelopment and exisitng housing - Courtesy
1966, Southern Water’s Woolston WwTW uses conventional activated
sludge plant (ASP) to treat sewage flows from part of the
Southampton conurbation, serving a population of approximately
62,000 people. The site occupies an area of 1.26 hectares. The
existing sewerage system which feeds into the WwTW collects both
wastewater and stormwater from the Woolston catchment. At
present treated wastewater is discharged into the River Itchen
Estuary through the final effluent outfall. Existing storm and
final effluent discharge points will remain unchanged during the
works. Construction works are now in the third year on the £63m
environmental improvement scheme to redevelop and completely
modernise the works.
4Delivery, a joint
venture between Veolia Water, Costain and MWH, is undertaking
the design and construction of the project on behalf of Southern
Water. The permanent works treatment process will treat flows up
to 36.9MLD, in compliance with the new discharge permit
requirements. All future permanent processes will remain within
the boundary of the existing site.
The wastewater treatment
Inlet works comprising:
Coarse and fine screens
and screenings processing.
Fat, Oil, Grease and
Grit, (FOGG) removal and processing.
High rate primary
sedimentation Lamella process.
Membrane Bio Reactor (MBR)
Modified Ludzack Ettinger
(MLE) Activated Sludge Plant (ASP) for the biological removal of
Membrane filtration tank,
utilizing microfiltration hollow fiber membranes.
Existing ASP converted
into a storm tank.
Odour control comprising
of chemical scrubbing and carbon units.
The sludge recycling
Sludge storage and
dewatering and processing.
image from inside of the primary building - Courtesy of
The Woolston WwTW
discharges into the River Itchen Estuary within the Solent and
Southampton water Special Protection Area (SPA) which has been
designated as a Nitrate sensitive zone by the Environment
Agency. Southern Water is required to provide Nitrogen removal
for the Woolston catchment to comply with the European Urban
Wastewater Treatment Directive (91/271/EEC) and associated Urban
Waste Water Treatment (England and Wales) Regulations 1994 as
well as Nitrogen removal requirements under the habitats
directive. A new effluent discharge permit requirement of 15mg/l
total nitrogen (TN) has been imposed as part of a coordinated
strategy involving changes at other WwTWs.
Dispersion modelling predicts that the impact due to odour from
the existing WwTW at Woolston is substantial and extensive. Once
the installation is completed, with a stack on the primary
building for the central odour control facility, all ground
level impact remains below the target value of 1.5 ouE/m3 at all
Woolston WwTW visualisation - Courtesy of Southern Water
The Woolston WwTW site is
small in size and has a congested layout creating a challenging
area in which to construct three large building structures. In
order to provide significant programme savings 4Delivery and
Southern Water have leased an area of land from a neighbouring
development owned by the Homes & Community Agency (HCA). The
enabling works have been constructed and were commissioned in
These process units
provide temporary primary treatment of wastewater, stormwater
storage and sludge storage to allow Southern Water to continue
to meet their environmental permits whilst construction of the
new works, within the extents of the Woolston WwTW site, is
completed. Secondary treatment (ASP) within the existing
Woolston WwTW site is used to complete the treatment of
wastewater whilst the new primary and secondary treatment
processes are constructed. Demolition of the existing process
units took place in early 2016. At the time of writing (May
2017) the permanent civil works construction is 65% complete and
the M&E installation has commenced.
Once the permanent
process streams are commissioned, the enabling works primary
settlement tanks will be utilised as part of the secondary
treatment to allow the existing final settlement tanks to be
demolished. This creates the opportunity of constructing the new
sludge facilities ahead of the programme.
Enabling works construction - Courtesy of Southern Water
Precast construction is
being utilised where possible on the scheme to deliver value,
reduce duration and improve health and safety. The consented
storm overflow (CSO), lamella structure and ASP/membranes tanks
are being fabricated, supplied and installed by Carlow Precast.
The design of these
structures has been developed specifically to make use of the
benefits of precast techniques. These include the ability to
have pipe couplings precast into wall units and for complicated
hydraulic features to be formed within a factory environment.
This enables improved quality to be achieved and avoids some of
the risks associated with in situ works.
The precast units arrive
on flatbed lorries and are lifted directly into their permanent
position using a series of mobile cranes. This avoids the need for
storage (space availability is low on site) and double handling.
Once placed, the units are joined together by short sections of
in situ concrete which then creates a monolithic structure. This
method means that large structures can be constructed in far
shorter durations compared with entirely in situ approaches.
There is also a significantly lower demand for on-site
operatives which dramatically improves systems of safe working.
constructed lamella structure - Courtesy of Southern
The precast design also
considered a number of complex mechanical interfaces and the
activities required for their installation. For example, the
structure included a number of temporary access ways to
facilitate installation of the fine bubble diffused aeration
plant. In addition the precast structure also provided plinths
with anchor bolts for interfacing with the building
superstructure steelwork. These interfaces were managed at
design phase by federating supplier models and developing these
Currently (May 2017), the
CSO and lamella structures are fully constructed and the
ASP/membranes are approximately 80% complete. For completed
sections mechanical install is underway and will be
part-commissioned in the summer of 2017.
Due to the works
hydraulic profile, much of the mechanical equipment is
significantly elevated above building floor levels. To meet the
maintenance requirements of this equipment together with safe
access and egress, a complex arrangement of structural
steelwork, access platforms, staircases, pipework and cable
routing is required.
A detailed and considered
approach was required to facilitate the design, incorporating
more than 30 (No.) independent 3D mechanical models, numerous
steelwork interfaces and a heavily congested floor-level plant
Due to the specific site
requirements, the design approach demanded a greater level of
definition when compared with less constrained designs that are
often passed to the supply chain for development as soon as the
access requirements are defined.
Fabricated stainless steel pipework - Courtesy of
Fabricated steelwork for the Veolia FOGG plant during
off-site assembly - Courtesy of 4Delivery
Management of interfaces
throughout the development of the structural steelwork design
was key to integrating the mechanical, electrical and civil
solutions whilst maintaining sufficient control of the overall
design to achieve a harmonised and cost-effective solution.
The design set out to
maximise the application of off-site fabrication to reduce
on-site personnel, total man hours and high risk construction
activities such as welding. This approach took a number of
forms, including: incorporating plant onto skid-based designs
transportable by lorry; design of steelwork process chambers
both in single unit and bolted construction formats;
pre-assembled flooring assemblies; welded pipework to flanged
termination points. Design of component parts considered
interface terminations, installation sequence, size,
weight/lifting and component repetition to facilitate the
under construction - Courtesy of Southern Water
Off-site inspection and
integrity-testing where possible further reduced site-based
installation activities. The combination of these methods
allowed for safer working conditions to be realised and a
significant reduction in the on-site installation programme.
Odour control system
Historically the existing
treatment works at Woolston has always suffered with odour
issues. The complete redevelopment of the works includes for
efficient treatment processes which significantly reduce odours
produced. For the odours that are produced these will be treated
by a dedicated odour control system supplied by Air Technology
Odorous air is drawn from
the following process units:
Odorous air is drawn from
both the primary treatment and sludge buildings, this is then
treated within the second stage (carbon filter) of the odour
control plant. Non odorous air drawn from the secondary process
units (ASP and membranes) along with the secondary treatment
building is extracted and discharged directly via the stack.
Fresh air is supplied to
each building by a dedicated ventilation system. Fresh air is
supplied to each MCC kiosk to provide a positive pressure
within. Heat is dissipated through louvres and directly
extracted from the larger VSD drives and the waste heat re-used
by discharging to the building space.
BIM and model interfaces
The use of Building
Information Modelling (BIM) on the Woolston project provides a
collaborative working environment between all the stakeholders
and supply chain employed on the project. The federated model
gives a greater understanding by all parties of the design.
To facilitate design
development, the 4Delivery design team has created an
intelligent 3D model. This provided the ability to ensure that
the spatial requirements of the design were met to facilitate
The model is being used
as a focus of the collaborative solution delivery effort and has
produced construction information as well as being an essential
aid to plan construction and operational activities. The image
below shows how the model has been used to aid
setting out crane radii for different loads.
By working with the
supply chain in 3D and importing different supplier models into
the intelligent 3D model it has proven an effective way of clash
checking as well as managing interfaces between different
suppliers scopes and civil/mechanical/electrical termination
3D model image
showing crane radii setting out - Courtesy of Southern
and publishers would like to thank Thomas Stell, Civil
Engineer with MWH Global, for providing the above article