offline whilst remaining tanks still in operation - Courtesy
of Trant Engineering
steel top rings
stainless steel tank erection in process
Water’s Bristol STW is the largest sewage treatment works within
south west region, serving approximately a catchment of 1.1
million people. The works also accounts for more than 50% of
region’s sludge treatment. It treats around 100 tonnes dry solid
(TDS) per day of indigenous sludge as well as imported sludge to
a compliant standard through digestion and lime treatment.
Previously a major scheme was implemented at the works to
increase electrical self-generation and to reduce land
application rates of compliant treated sewage sludge. An acid
phase digestion (APD) process was commissioned upstream of the
existing mesophilic anaerobic digesters (MAD). The biogas output
from the combined processes was used for generating electricity
through the combined heat and power (CHP) engines.
Thickened sludge is
heated to 36-37°C before being processed through the APD plant
comprising 6 (No.) reactor vessels made in glass coated steel.
Each reactor provides approximately 600m3 storage capacity with
a combined 2 days retention. Biogas is compressed and recirculated through the tanks to mix the sludge. Each of the
reactor is 8.5m in diameter and 13m in height.
In the autumn of 2012,
the following issues were identified during the routine
APD tank roofs were
showing significant signs of glass debonding and corrosion. The
debonding was extending radially from penetrations (gas lifting
and mixing pipework in particular) into the roof and the bolted
stiffeners between roof segments.
Cold storage tanks (i.e.
APD buffer tank and thickened sludge feed tank) showed visible
corrosion on the side panels at the sludge/air interface level.
The top two rings of the sludge buffer tank and the thickened
sludge storage tank were corroding from the inside. The sludge
operating level was reduced below the top two rings in both
Surveys and inspections
Wessex Engineering and
Construction Services (WECS) arranged a number of surveys and
inspections to determine the condition of reactors and cold
tanks. Glass coated steel tank panel thickness is nominally
2.5mm when it is new. Deterioration in the individual panels was
measured and showed a 10-20% reduction in thickness, with the
worst result being 28%, confirmed by ultrasonic thickness and
coating thickness testing.
show a view of defects on tank panels. On continued swab
testing, the Elcometer alarmed constantly with no
visible evidence of damage or defects, this would
indicate that the coating is either porous or defective
to some degree. The third photo clearly shows pitting or
pinholes throughout the surface of the coating
Courtesy of Hayes GFS Ltd
Internal panel of corroded roof where the patch repairs
carried out externally
Courtesy of Hayes GFS Ltd
The conclusion was that
short-term repairs should be made to stop the corrosion
spreading and potentially extend the time until the full
repair/replacement works in AMP6. However concerns remained that
the reduction in roof thickness could lead to the tanks failing
To provide full buffering
capacity during the APD remedial works, replacement to the top
rings of the sludge buffer tank and the sludge storage tank were
carried out first prior to any APD outage. Temporary pipelines
were required to bypass the cold tanks.
Existing APD process
The Avonmouth APD process
is a 6-reactor process by Monsal. In the original design, the
reactors cannot be bypassed individually. In assessing the
methodology for the long-term remedial works to the tanks, it
was identified that all 6 (No.) tanks needed to be taken off
line in order to fit an individual bypass system so that one
tank could be taken offline at a time for the repair. This
required a contingency plan in place to ensure sludge compliance
and treatment capacity were not compromised during the work.
To maintain the sludge
treatment capacity on site, the existing bypass to the APD
process was tested and operated, whilst ensuring no cross
contamination with hydrolysed raw sludge in the MADs.
The following issues were
identified and formed part of the overall solution:
The requirement for, and
time scales associated with, the MADS to be
conditioned/reconditioned to accepting raw/ hydrolysed sludge.
The requirement for other
treatment methods to ensure sludge compliance i.e. liming.
A new approach
It was considered that
conditioning the MADs from hydrolysed sludge to raw sludge and
back could take up to 6 months.
Operations and Workstream
partners including Trant Engineering Ltd and the University of
Bath developed a new approach with a much shorter shutdown
In this approach, the
acid phase digestion plant would be shut down for a short period
with the mesophilic anaerobic digesters essentially isolated.
The APD/MAD digestion plant would be gradually brought back into
operation by introducing sludge back into the APD/MADS in the
following controlled manner:
Feed to APD stopped, MADS
isolated, sludge processed by dewatering followed by liming to
ensure sludge compliance.
APD reactors drained and
purged with nitrogen.
Individual tank isolation
and bypasses installed. Shift work was utilised to ensure
completion on schedule.
pressure and vacuum relief valves installed.
APD filled with raw
sludge and arranged to operate in full 6-reactor mode.
Recondition MADs to
hydrolysed sludge by gradually ramping up sludge feed through
the APD into the MADS (sludge was initially essentially raw but
quickly became hydrolysed as the APD sludge reached its required
Full sludge compliance
Liming and additional
The enabling works,
comprising the installation of additional valves and pipework,
allow each APD reactor to be isolated and taken out of service
individually for inspection/repair without affecting sludge
Tank 4 out of service on human-machine interface -
Courtesy of WECS
Bypassing an isolated
Bypassing an isolated APD
reactor whilst continuing to operate the APD Plant on the
remaining 5 (No.) reactors requires additional sludge transfer
pumps and bypass pipework for the tanks. The bypass pumps
operate in such a way as to mimic the existing gas lifting
transfer system between the tanks. The gas lift system will
continue to operate for the non-isolated tanks as per normal
When bypassing Tanks 1 or
6, no sludge transfer pumps are required. When Tank 1 is taken
out of service, temporary pipework will enable Tank 2 to be fed
using the existing APD feed pumps and allow the contents of Tank
2 to be recirculated through the existing heat exchanger. For
Tank 6, the existing digester feed pumps can be utilised to move
sludge from Tank 5 via the temporary bypass pipework for Tank 6
during its replacement.
Avonmouth site works
within its existing hazard analysis and critical control points
(HACCP) plan (i.e. critical control points of 33.5°C minimum and
sludge feed of 1,863m3/d maximum) while operating on the 6 (No.)
Historic data of sampling
directly from Reactor 5 of the APD plant suggest that the
operation of five reactors is likely to be successful, albeit
there may be a marginal increased risk of HACCP failure.
If there is a failed
HACCP bacteria sample (or failure of the critical control plan),
then the existing exceedance strategy would have to be enacted
including re-sampling from farm stockpile (more than once if
necessary) and liming to avoid non-compliant sludge being
actually spread to land.
Additional funding was
agreed for a one month proving period operating the APD on five
rather than six reactors. This trial was carried out prior to
commencement of any tank repairs to confirm continued sludge
Preparation of concrete tank base rebate - Courtesy of
The proving trial
required liming of all digested sludge cake and started in April
2016. Tank 6 was selected as the tank out-of-service because no
bypass pumps were required and it was the simplest way to
convert the plant to five-tank operation.
The outcome of the trial
has validated five-tank operation and confirmed liming was not
necessary during APD repairs. Gas production during the trial
with five tank operation varied but has not shown any
significant reduction compared to that produced by six tanks,
albeit that this is based on a limited dataset.
During the proving trial,
APD Tank 6 was inspected from low level. Apart from large areas
of corrosion observed on the roof, tank panels have developed
glass coating defects at random locations.
The coating defects
development was unknown although previously it was thought to be
stress fracturing of the coating caused by over-torquing of the
fixings. No effective methods for repairing damaged panels would
guarantee a long-term solution.
An enhanced enzymic
hydrolysis (EEH) feasibility study was also undertaken with the
objective to determine whether an EEH plant provides more
certainty to meet Biosolids Assurance Scheme (BAS)standards. The
conclusion was conversion to an EEH plant would not guarantee a
BAS compliant product.
Duty/duty sludge transfer pumps for Tank 4 bypass in
operation - Courtesy of WECS
Due to high temperatures
and varying levels of chloride a conversion to EEH would
necessitate the use of stainless steel for Tanks 4, 5 and 6.
High capital and operation cost without any other significant
benefits to the process established there is not a strong
business case to upgrade the existing APD to EEH plant now.
However, future regulatory drivers to have EEH process means
that provision for EEH would be necessary providing an ‘advance
treated’ standard in accordance with the safe sludge matrix.
The cost of full
replacement in stainless steel has previously been observed to
be only marginally higher than full replacement in glass coated
steel. Since it is not possible to be certain that if replacing
in glass coated steel the issues seen would not re-occur, the
recommended solution was to replace all 6 (No.) APD reactors
roof and panels with stainless steel 316L with provision of
additional flanges for future EEH plant upgrade.
In 2016, operational
issues were discovered with Tank 4. Local repairs were
subsequently carried out to the roofs of all tanks.
In order to accelerate
the overall repair programme WECS awarded Trant Engineering Ltd
the NEC Engineering and Construction Contract Option C contract
for Tank 4 and 5. Meanwhile a delivery and procurement strategy
for the remaining tank repairs was also developed. It was
envisaged that the APD remedial scheme would be a rolling
programme with no gap in between tank repairs to minimise
disruptions to site operation.
Trant started the major
APD remedial work in August 2016. The repair work to date has
seen successful whole tank replacement of Tanks 4 and 5 in
stainless steel 316L and their integration into the existing APD
plant on budget and on time. Sludge sampling data on the 5 (No.)
APD tanks during the construction period have been positive and
fully compliant with HACCP.
The new tanks have
included the provisions of additional flanges for future EEH
plant upgrade, which are isolated and blanked off. With proper
planning it is possible to keep the plant live during any future
upgrade. Improvements on gas lifting and mixing connections to
the roofs have also been made to reduce the risk of future gas
gas draw-off pipe off the roof - Courtesy of WECS
Future planned work
Over the next 12 months,
WECS will continue working closely with the contractor and
Operations. Using knowledge gained from the replacement of the
first two tanks, far more accurate costs and risks will be
assessed for the whole-tank replacement of the remaining four
tanks. The anticipated completion for the scheme is summer 2018.
and publishers would like to thank Ningyi Shu, Project
Manager with Wessex Water, for providing the above article
for publication. The author thanks Simon Osborne, Programme
Manager with Wessex Water for his support for the paper.