UK introduction of the revolutionary Volute disc press at Nigg
WwTW leads to
significant savings through high efficiency sludge treatment
by Robert Mannion & Geoff Cooper-Smith
installation of 2 (No.) ES354 Volute units (a total of eight
drums) in operation shortly after commissioning - Courtesy
of KWS Grampian.
Water Services Grampian (KWS Grampian) has a 28 year design,
build, finance and operate contract with Scottish Water to
operate and maintain four wastewater treatment plants serving
the north east of Scotland; two sites in Aberdeen (Nigg and
Persley) and two sites to the north (Peterhead and Fraserburgh).
In addition to treating wastewater, KWS Grampian operate sludge
treatment facilities able to receive imported sludge and liquid
wastes. Nigg WwTW located to the south east of the City of
Aberdeen on the shore of Nigg Bay serves a PE of 250,000. The
works was built to provide full treatment to a flow of 1600l/s
and achieve the requirements of the Urban Waste Water Treatment
Directive (BOD: 25mg/l and COD: 125mg/l).
Nigg WwTW treatment
A pumping station lifts
the flow up from the preliminary treatment stages whence it
gravitates through the primary and secondary treatment stages
prior to discharge into the North Sea. Due to the compact nature
of the site primary treatment is provided by lamella settlement.
Lamella settlement uses inclined plates or tubes to provide high
settling area in a small plan area; flow travels up these plates
or tubes with the solids ‘falling’ onto the inclined surface and
falling into the hopper below. These solids are subsequently
removed from the hopper and at Nigg WwTW pumped to the sludge
treatment centre (STC).
Secondary treatment is
provided by a two-stage biological aerated flooded filter (BAFF).
The BAFF utilises a down-flow plastic open-structured media,
followed by an up-flow through a bed of granular clay media of 3
to 6mm in diameter. This media is regularly backwashed with the
resulting dirty water pumped to the lamella stage for treatment.
In addition to the sludge
generated on-site, the Nigg STC receives sludge (in either cake
or liquid form) from the surrounding area of an additional
200,000 people. After reception and blending, sludges are
dewatered and thickened, with the assistance of polymer
addition, and pre-treated by thermal hydrolysis before entering
2 (No.) anaerobic digesters with a combined heat and power
system. Digested sludge is then dewatered, assisted by further
polymer addition, with the resulting product applied to land.
Nigg WwTW and the STC - Courtesy of Evergreen Water
As built, the dewatering
and thickening stages deployed 7 (No.) belt presses, but by 2014
these were requiring intensive maintenance and considered to be
at the end of their asset life and thus in need of replacement.
Consequently, KWS Grampian commenced an evaluation of options
which included the Amcon Volute disc press.
The evaluation concluded
through a highly structured business case which considered odour
generation potential, health and safety, the ability to cope
with rags and the total and whole-life cost, that the belt
presses used to dewater and thicken the sludges fed to the
thermal hydrolysis process should be replaced by Volute
technology; calculations, based on pilot-scale trials in
Aberdeen, clearly demonstrated that the whole life cost of the
Volute technology was around two-thirds that of any competition.
Based on this appraisal
KWS Grampian placed an order for the provision of a 2 (No.)
Amcon Europe ES354 Volute disc press units to replace five belt
presses. These were to represent the first ever full-scale
installation in the UK.
The Volute process
The Volute ES Series
thickening and dewatering press was introduced by Amcon, a
Japanese company in 1992. To date, there are over 3,000
dewatering machines sold in around 50 countries. It is a cost
effective, energy efficient, versatile, robust and revolutionary
dewatering and thickening technology. Evergreen Water Solutions
introduced the Volute in to the UK municipal market in 2011.
Image of the planned installation at Nigg - Courtesy of
Evergreen Water Solutions
Volute is a relatively
simple process utilising an integrated flocculation tank, a
special spiral tapered screw sitting inside a disc drum of
alternate fixed and moveable stainless steel rings.
Performance is enhanced
by the addition of polymer to the liquid sludge and the action
of the screw transports sludge up the inside of the disc drum
rings which induce a squeezing and shearing action on the sludge
resulting in the release of liquid. This filtrate is then
allowed to gravitate out between the fixed and moving rings and
collected in a filtrate pan for discharge to the head of the
works. Once the thickened sludge reaches the top of the disc
screw press it is further dewatered by a permanent end plate
where more pressure is applied to the sludge to improve
The dry sludge cake is
allowed to fall down a chute where it can be collected and
forwarded for further treatment.
Volute achieves three
process steps in a single process stage; flocculation,
thickening and dewatering. The low power required to operate the
Volute disc screw makes it 90% more energy efficient than a
typical centrifuge and as its washwater requirement is minimal
(just a spray-bar system for odour control) consumption of water
is significantly less than that of a belt press.
Volute also operates with
very little noise (63dB), little vibration, is self-cleaning and
resistant to clogging, which means very little maintenance is
required (typically 12,000 hours operation between services) all
of which results in a very low whole life operating cost.
Design and construction
The order placed with
Evergreen Water Solutions was for the design, manufacture,
testing, inspection, delivery, commissioning, training and
handover of 2 (No.) Amcon Europe ES354 Volute dewatering
presses. This included the provision of mixing and flocculation
tanks and any necessary modifications to both the feed and
reject pipework and the cake discharge arrangement to suit the
existing cake pumps. Evergreen Water Solutions was also
responsible for ensuring that their PLC system was compatible
with that of KWS Grampian and providing a control philosophy for
maintaining the required dry solids concentration of the sludge
cake product as part of the PLC control.
As it was absolutely
essential that such a major asset should have minimal downtime
during the install, Evergreen Water Solutions collaborated, and
worked very closely, with the client throughout the project,
with just two weeks allowed for the mechanical installation by
Evergreen, two weeks for the electrical installation by KWS
Grampian and one week for commissioning.
During the ten week lead
time for delivery of the units, 3D plans were produced, the
units manufactured and FAT tested in the Czech Republic and
subsequently delivered to the STC at Nigg.
pristine and Nigg bound Volute unit undergoing FAT
testing in the
Czech Republic - Courtesy of Evergreen Water Solutions
This identified that the
Volute units needed to be raised up 700mm on stainless steel
stands to ensure that the existing positive displacement cake
pumps feeding the sludge to the THP would not need modification.
During this lead in
period KWS Grampian was responsible for removal and disposal of
the existing belt presses from the STC building, and this was
completed in August 2015. KWS Grampian also assisted with the
off-loading of the Volute equipment when it arrived on site and
hired in a centrifuge to provide sludge thickening and
dewatering, to keep the thermal hydrolysis plant (THP)
operational during the project.
Installation of the
Volute equipment, into the area previously occupied by the belt
presses commenced in September 2015. This included modifications
to existing pipework e.g. sludge feed pipework, filtrate drain,
washwater, and the improvements to the polymer dosing system
(provision of Solitax probes) and was complete by mid-October
requirement, to supply the integrated spray bars, was for the
provision of 15l/s every 15 minutes per drum (the two Volute
units present a total of eight (No.) disc drums), equivalent to
0.5m3/hour. Access platforms and walkways were also provided and
installed during this period for inspection of the disc drums
and flocculation tanks.
KWS Grampian subsequently
undertook the electrical cabling, made all the electrical
connections and installed some additional sensors to improve
control and set-up interfaces between the PLCs and SCADA to
allow control and the exchange of data between the LCP and the
main motor control centre (MCC).
This allowed Evergreen
Water Solutions to undertake electrical testing and subsequent
commissioning, which was completed in December 2015. However,
although the units were in use from this time and feeding the
THP some subsequent on-site electrical issues had to be resolved
and the polymer dosing system set up to suit the requirements of
the Volute. These were soon resolved and the process considered
to be fully functional during March 2016, allowing full training
of KWS Grampian staff to ensue.
Approximately 95% of the
Volute plant and equipment was constructed off-site.
What a comparison! Looking across the area where the
belt presses stood towards the Volute machines -
Courtesy of KWS Grampian
Progress to date
Since introduction to
service at the Nigg STC, the twin Volute machine has exceeded
the Kelda Water Services Management and operations team
expectations for being reliable and robust process, achieving
high utilisation and delivering a consistently dry sludge cake.
Although feed quality can
be quite variable (in the range 2.1 to 3.3% dry solids) the
Volute units have demonstrated an ability to produce a sludge
cake with a dry solids content in excess of 30%. However, this
is not ideal and so the target is to actually achieve a cake of
around 25% dry solids which can be fed directly to the thermal
This is being achieved
consistently with very little variation (+/- 2%) and is one of
the attractions of Volute; its operation can easily be ‘tuned’
to deliver the desired quality of product.
The 2 (No.) Volute units
operate at a range of 35-70m3/hour (duty/standby or
duty/assist). The Nigg Sludge Treatment Centre treats the
equivalent of 300,000m3 and 400,000m3 of sludge per annum,
equivalent to the processing of between 8,000 and 10,000T of dry
solids. As expected solids capture is excellent at around 98%,
which usually results in a filtrate containing around 600mg/l of
Over the first year or so
of operation KWS Grampian embarked on a programme of
optimisation and this has resulted in a further reduction in
operating cost as their good work has allowed a polymer dose
around 3kg per tonne of dry solids to be utilised.
With 95% of the build
off-site, the Volute installation for KWS Grampian at Nigg WwTW
has proved very successful and is consistently delivering the
target dry solids sludge cake they require on site, 24 hours
operation a day 7 days a week, suitable for feeding directly
into the a thermal hydrolysis process at 16.5%.
By utilising a polymer
dose of just 3kg per tonne of dry solids, using much less energy
than a centrifuge and much less water than a belt press,
operating costs are low.
The disc press
in action, showing the rings of a Volute drum - Courtesy
of KWS Grampian
Having become the first
user of Volute technology in the UK KWS Grampian is now treating
their sludge for a significantly lower cost than previously. As
a result of this success Evergreen Water Solutions has
subsequently been invited to provide Volute technology at the
KWS Grampian Persley site and this is expected to occur in 2017.
This paper was
prepared by Robert Mannion,
Managing Director, and Geoff Cooper-Smith, Lead Process
Engineer, both with Evergreen Water Solutions