a strategic intervention to close Ottery St Mary STW and
transfer all sewage treatment to Fluxton STW to satisfy
historical and future growth
by John Riley IEng MIET & Leo Wardell CEng MICE
of new works - Courtesy of South West Water Delivery
and Ottery St Mary STWs are located in the Otter Valley, East
Devon. The town of Ottery St Mary has a population of 4898 and
is situated on the River Otter, approximately 10 miles east of
Exeter, and the nearby hamlet of Fluxton has a population of
approximately 99. The Fluxton site itself is located within a
very small plot within the floodplain adjacent to the River
Otter and an upgrade was required to provide sufficient capacity
for population growth in the Ottery area and achieve a future
Total P permit.
Under this project, 100%
of pass forward flow will be transferred to Fluxton by the
existing Ottery SPS (located within Ottery St Mary STW site
boundary), leaving just the storm tanks operational as a
combined sewer overflow. A whole new process stream will be
constructed at Fluxton to ensure there is sufficient capacity to
a 2031 design horizon.
The upgraded treatment
facilities at Fluxton will provide additional process capacity
to meet the 2031 design horizon, the increase in catchment
growth and the anticipated EA discharge permit of 39mg/l
biochemical oxygen demand (BOD), 60mg/l total suspended solids (TSS)
and 13mg/l ammonia (NH4-N). The Fluxton works upgrade will
ensure that the works not only meets the required treatment
level but also overall operability will be improved by reducing
the operational requirements at Ottery St. Mary.
Location - Courtesy of South West Water Delivery
STW - Courtesy of South West Water Delivery Alliance H5O
Impact of total
During the Asset
Management Programme review it was envisaged that the
Environmental Agency would apply a total phosphorus permit at
the Fluxton STW at levels between 1-2mg/l, based on permitting
standards for Fluxton and Ottery St Mary STWs. However, having
secured the budget approval for the capital maintenance (CM) and
supply and demand (S&D) projects the EA’s National Environmental
Programme was revised and in doing so identified a further
requirement for a total phosphorus (Total P) discharge permit of
<0.63mg/l to be applied from the 31 March 2020.
This level was based on
the EA’s final modelling analysis of the impacts of the
discharge to the River Otter, taking in to account all other
sources within the catchment area.
The project would also
contribute towards the ‘River Water Quality Improved’ outcome
delivery incentives by improving the water quality of the 49km
stretch of water body that Fluxton STW discharges to, which in
turn influences the tidal limit of the watercourse.
The initial secondary
process solution was to upgrade the works with plastic media
filters, this being the optimal process solution to make
provision for the Total P requirement of between 1-2mg/l.
However, due to the newly revised tighter Total P permit
requirements a greater compliance risk was associated with the
use of the plastic filter media solution.
The increased risk was
due to the lower level of effluent solids required to comply
with the tighter Total P standards. A process review was
undertaken to establish the optimum holistic solution in meeting
both CM and S&D and the new Total P requirements.
Existing sludge beds - Courtesy of South West Water
Delivery Alliance H5O
Summary of options
The process review
considered two potential solutions:
Approved plastic media
Activated sludge plant
holistically the ASP process was more cost effective as solids
are removed more efficiently reducing the number of tertiary
filters and associated equipment required to achieve the tight
Total P requirement.
The process review
concluded that the ASP rather than the plastic media filters for
the secondary process for the CM and S&D was the optimal overall
process, and the most cost effective. Due to a number of
complicated interfaces it was deemed acceptable in meeting both
CM, S&D and Total P projects would benefit by concurrent design
A number of options were
reviewed which considered the differences associated with the
introduction of the tighter Total P requirement. These were:
plant and Total P removal: This would require the
addition of two point Claral dosing, a return liquor balancing
tank, increased sludge treatment process capacity and a single
tertiary solids filter.
filters and Total P removal: This would also require the
same scope as for the ASP. However due to the higher levels of
suspended solids in the effluent produced from the plastic media
filters, a two stage tertiary solids removal process would be
required. In addition, as the tertiary treatment process
generates more return liquors, a larger primary settlement tank
would be required.
Preferred option solution
From a process point and
due to the increased permitting requirements in regards to Total
P the ASP solution for the CM and S&D would provide not only a
more robust and reliable process solution but also reduce the
Due to the numerous and
complicated interfaces and the inclusion of the tighter Total P
permitting restrictions due to come into effect, it was decided
that the design and construction would be achieved concurrently
with that of the CM and S&D, combining all three outputs for
The holistic review of
the overall solution had identified that the most cost effective
and least risk delivery solution would be to design and
construct the Total P project concurrently with the CM and S&D
There were no changes to
the capital efficiencies previously calculated (plastic filter
media option) due to the proposed project change.
Rebar construction - Courtesy of South West Water
Delivery Alliance H5O
The scope of the works:
Works at Ottery included
the Installation of a new overflow chamber adjacent to storm
tank. The existing (new) storm screen was to be re-located
upstream of storm tank, with a continuation flow being limited
to 28l/s (existing FFT permit). In addition:
penstock plus magflow meter, or similar.
Divert low level sewer
SPS via new overflow chamber.
Decommission and make
safe existing STW process, including electrical isolation and
removal of chemicals.
The following fall into
the decommissioning scope of works at Ottery STW:
Inlet channel: drain down
and backfill channel with suitable material. Electrically
isolate macerators and remove.
Place equipment to one
side for removal/reuse by SWW.
Drain down and clean PST
tanks, core holes in tank walls to allow drainage of rainwater.
Backfill with suitable material.
Remove metalwork to humus
tanks and dispose off site.
Sludge storage covers to
Electrically isolate the
lower control building from main control building and ensure
covers are secured.
MCERTS flowmeter at OSM
outfall to be electrically isolated at control building.
Impeller change to
The scope of the works:
capacity at Fluxton STW to meet expected population growth to
2031 design horizon and to comply with the new 0.63mg/l Total P
permit required by 31 March 2020. Works required the inclusion
New Huber standby screen
to be capable of passing 109l/s required for compliance with
Extension of existing
inlet screen channel required to accommodate new bypass channel
with hand raked screen to suit.
M&E refurbishment of
existing Jeta grit trap including new grit classifier.
Upgrade of inlet PS to 60
Primary and Secondary
1 (No.) 12.5m diameter
primary settlement tank.
3 (No.) compartment
selector and 1 (No.) ASP provided in two 50% streams.
2 (No.) 14m diameter FSTs,
including RAS/SAS pumping station.
Fluxton STW Sludge
New unthickened (raw)
sludge tank to provide two days raw sludge holding.
New sludge thickener
package 25m3/hr raw sludge capacity.
Conversion of existing
sludge tanks to thickened sludge holding tanks to provide five
days thickened sludge storage capacity.
In addition, the
decommissioning scope of works of existing assets at Fluxton
included the following:
Screw pump inlet channel
Electrically isolate the
screw pump inlet channel and backfill sump.
Drain down and clean.
Leave desludge valves open to allow drainage of surplus
diffusers etc and mechanical equipment. Backfill tank to coping
with suitable granular material.
remove and dispose of scraper bridges. Core drainage holes in
tank walls (assume 2 (No.) 150mm diameter holes per tank) and
backfill with suitable material.
Backfill exposed sludge
sump with suitable granular material.
Flows arriving at Fluxton
STW will be screened to 6 mm 2D at the works inlet, prior to
flow control. A new standby screen is being installed to improve
The flow to full
treatment of the new works had been based on the calculated 3DWF
of 60l/s. As per the existing permit, flows above 3DWF (FFT)
will be diverted to the storm tanks. The revised 3DWF has been
calculated as 60l/s and the overflows will be adjusted to suit
this revised flow rate.
The required storm
storage volume had been calculated as 194.4m3 based on providing
2 hour retention at peak flow to storm tanks. The storm storage
tanks at Fluxton provide 418m3 of storage and was therefore
With the existing site
continuing to operate during the new construction phase it was
important to maintain a small footprint in line with the
planning application process. The existing sludge beds were to
be broken up and material removed from site.
Base and wall construction - Courtesy of South West
Water Delivery Alliance H5O
The removal of material
from site would have been extremely difficult due to restricted
routes and site manoeuvrability, increasing time and cost on
site. During the construction development phase the Balfour
Beatty site team proposed the reuse of this material, mainly
concrete, and they were subsequently able to reuse up to 90% as
base material for the formation of the new PST, FSTs and ASP
The scheme not only
included the added phosphorus removal but included the
decommissioning of Ottery St Mary STW and so the flows generated
at Ottery would directly impact Fluxton STW.
Careful planning on how
both sites interconnected was vital to the programme due to a
lengthy commissioning period. A number of sub-contractor
interface programmes hinged on the civil works through direct
labour, and failure to meet construction deadlines would have a
detrimental effect on the ability of the sub-contractor to meet
deadlines. Procurement, construction, M&E interface, and
commissioning would all be affected if one deadline was delayed.
This was a very
definitive constraint that allowed for no flexibility in the
program. This included the need to incorporate a new grit plant
on to the program that would allow for up to 90% of the arisings
from the existing sludge beds to be reconstituted and reused for
the bases for the concrete structures. With this change the site
team was able to reduce not only the frequency and size of
traffic around site, but also the local transport routes to and
from the works. This had a major effect on the local population,
as the site is located deep in agricultural territory.
fabrication - Courtesy of South West Water Delivery
Due to the complex nature
of the scheme and with subcontractor involvement key, it was
vital that a detailed programme was developed which clearly set
milestones within the project. For the norm this program was
essentially managed by the site team. However, due to the nature
of this work and a very constrained period in which to deliver,
it was agreed that a number of meetings and approval processes
would have to be met prior to the next stage of the program
commencing. This would include approval from all parties and
included milestones such as outline and detailed design stages,
pre-construction phase plan, health and safety checks,
procurement schedules, weekly site meetings, strategic monthly
progress meetings, monthly commercial meetings, and design
It was also decided that
for ‘critical works’ a full desk top review and site meeting
would take place to ensure all aspects of the works and risk
would be captured, and included mitigation and contingency
planning. Finally, post project evaluation and lessons learnt
are programmed in on completion of the project to ensure good
and bad points from the project are passed on.
In order to reduce the
risk from flooding and to aid future programmed operating and
maintenance the site team led by Robin Goodman worked tirelessly
with the design consultants Pell Frischmann through design
manager Leo Wardell to ensure that were practical, pipework,
pump stations, and storage containment, were calculated,
designed, and positioned away from the threat to flooding.
and publishers would like to thank John Riley, Engineering
Manager with Balfour Beatty, and Leo Wardell, Associate
Director with Pell Frischmann, for providing the above
article for publication.