SEQUENCING BATCH REACTORS - PAST, PRESENT AND FUTURE€¦ · particular two solutions led the...
Transcript of SEQUENCING BATCH REACTORS - PAST, PRESENT AND FUTURE€¦ · particular two solutions led the...
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SEQUENCING BATCH REACTORS - PAST, PRESENT AND FUTURE
Smyth, M.1, Horan, N.J.2 1Aqua Enviro, 2The University of Leeds
Email: [email protected]
Abstract
A sequencing batch reactor (SBR) is a variant of the activated sludge process: a suspended
growth, variable-volume, wastewater treatment technology. SBRs were wisely adopted by
the UK Water Industry in the mid to late 1990's and into the early 2000's (AMP 2 & AMP
3). However during this period it became apparent that some SBRs were prone high
suspended solids losses during the decant phase of the cycle and on occasions the sludge
blanket itself. Often this was a result of filamentous bulking, in particular caused by Microthrix
parvicella. The process therefore failed to consistently deliver a compliant final effluent and
as a result SBRs fell out of favour, and in some cases were even replaced in AMPs 3 & 4 with
conventional activated sludge processes.
In the past 5 years there has been an increase in the uptake of SBRs and they look set to be
the choice of technology for organic waste digestion plants treating dewatering liquor. In
addition 2014 will see the completion of the £200million upgrade to Liverpool WwTW with 16
basins on 2 levels.
This papers looks to:
review the causes of the problems witnessed in the 1990's which included:
filamentous bulking and foaming); poor sludge age control, resulting in partial or
unintentional nitrification; inadequate blower capacity and shortened cycle times
in high flow conditions.
evaluate how these factors have been accounted for in present day designs,
consider whether design modifications will result in a trouble free future for SBRs.
Keywords
Microthrix, foaming and bulking, sludge age, nitrification inhibition, liquor treatment
Introduction
The activated sludge process is the preferred technology worldwide for large, domestic
wastewater treatment plants. It can be configured to remove carbonaceous material,
nitrogen and phosphorus. There have been many variants of the process since its discovery
100 years ago by Arden & Lockett (1914) and the Sequencing Batch Reactor (SBR), a
suspended growth, variable volume wastewater treatment technology, is one example.
SBRs became widespread in the UK Water Industry in the mid to late 1990's and into the early
2000's (AMP 2 & AMP 3). During this period however it became apparent that some SBRs
were prone to losing high levels of suspended solids during the decant phase of the cycle
and on occasions the sludge blanket itself. The process appeared to encourage the
development and growth of a particular filamentous bacterium Microthrix parvicella, which
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once established can in a very short period of time cause severe episodes of sludge foaming
and bulking.
The process therefore failed to consistently deliver a compliant final effluent and
consequently SBRs fell out of favour and in some cases were even replaced in AMPs 3 and 4
with conventional activated sludge processes.
In the past five years there has been an increase in the uptake of SBRs and they look set to
be the choice of technology for organic waste digestion plants treating dewatering
liquor. In addition 2014 will see the completion of the £200million upgrade to Liverpool WwTW
with 16 basins on 2 levels.
Process Description
A sequencing batch reactor (SBR) is a variant of the activated sludge process; a suspended
growth, variable volume wastewater treatment technology where treatment takes place in
a single tank (circular or rectangular) and therefore removes the need for an independent
secondary sedimentation tank and recycle system (Gerardi, 2010). SBRs operate in cycles
which, for predominantly domestic wastewaters, are typically 4 to 6 hours in length and can
be configured for carbonaceous treatment, nitrogen and/or phosphorus removal (Wilderer
et al. 2001). Usually a site will operate with at least 2 basins (e.g. Nairn WwTW) and up to as
many as 16 (Liverpool WwTW).
Figure 1: Example cycle times
A basin on a four hour cycle will therefore have six cycles each day and in a four basin
configuration 2 will always be filling and aerating, 1 will be in the settle phase and the 4th in
Decant and Idle.
Figure 2: 4 basin mode operating on 4 hour cycle times
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There are a number of features which distinguish the different types of SBR systems available
(often referred to by their acronyms: IDEAL, ICEAS, CASS, CAST, JetTech) in the market place,
including the:
Cycle times, can be fixed times with fixed phases (figure 2 and 3), fixed times with
fixed phases or both variable. Successful operation requires an automated
control system (Bungay et al. 2007) or knowledge-based Intelligent Environmental
Decision Support System (Sottara et al. 2014).
Feeding regime which can be true batch process or can be permit during the
decant phase at high flows (figure x).
Configuration of a selector which can be external to the tank (captive
contactor), internally configured and baffled Internal selector) or can exploit the
whole basin by manipulating the fill regime (whole basin selector).
Decant mechanism which can be fixed or floating (figure x), which should include
a scum/foam guard to ensure that floating material is not entrained (Wisaam et
al, 2007).
Type of aeration which can range from fine bubble to jet aeration (figure x).
A feature of an SBR is that it is variable volume, the level in the SBR will at any point in time be
between the bottom water level (BWL, typically in the region of ~4 metres) which is reached
at the end of the decant period and a maximum top water level (TWL, typically ~6
metres). Whether or not the TWL is reached is dependent upon the incoming flow, but it
cannot be exceeded otherwise untreated/partially treated wastewater and/or mixed liquor
would be discharged.
For some plants if the flow treated and the level measured in the SBR looks set to exceed TWL
the cycle time may be shortened (e.g. from 4 to 2-2.5 hours) and the option to introduce
filling during the decant period introduced, thus permitting continuous fill. This change in
cycle times generally occurs when either a basin is out of service (meaning that
proportionally other basins must treat more flow) or during wet weather events when full flow
to treatment conditions prevail. In the latter case the influent is very dilute and thus a
shortened aeration period is not detrimental.
Figure 3: Storm Cycle
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Figure 4: Floating decanter system and jet aeration
Problems Encountered with SBR Technologies
SBRs became widespread in the UK Water Industry in the mid to late 1990's and into the early
2000's (AMP 2 and AMP 3) and were successfully marketed on their claimed simplicity, small
footprint leading to reduced capital costs and especially their operational flexibility. In
particular two solutions led the market, the CASS (Cyclic Activated Sludge System offered by
Earth Tech Engineering, now AECOM) and the Jet Tech Omniflow SBR (US Filter). Although
both technologies are fundamentally very similar, there are enough differences to make their
design, construction and operation very different (Kirkwood, 2001).
However, a number of problems associated with the operation of sequencing batch
reactors were seen in this period (table 1) but by far the most serious (in terms of
environmental impact and frequency) was the loss of the sludge blanket during the decant
phase leading to catastrophic consent failures on total suspended solids, BOD and COD,
reviewed here. The major cause of this was filamentous sludge bulking and foaming.
Table 1: Problems encountered with SBRs
Problem Contribute to foaming and bulking?
Aeration systems prone to blocking due to
settlement in the air-off phases. Biofilm formation
risk very high.
Yes, ability to flush out toxic intermediary
products from nitrification reduced
Air in the recirculation pipework due to air
passing the isolation valve and being retained by
the head of water above the pipe.
No
Faulty or leaking recirculation valves and leaking
valves on the decanter system No
Lack of IDSC (inlet distribution and sludge
consolidation manifold) leading to short circuiting
and disruption of the sludge blanket.
Yes, selector effect difficult to achieve
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Shortened cycle times during high flow
conditions
Yes, leads to inadequate dissolved
oxygen provision
Aeration unable to ramp up to DO set points
quickly enough
Yes, ability to flush out toxic intermediary
products from denitrification (nitrous and
nitric oxides) reduced
Aerated sludge age measurement and control Yes, nitrification enhances the likelihood
of a bulking & foaming event
Load balancing between SBRs Yes, unintentional or partial nitrification
Poor sludge dewaterability Yes, influences sludge age control
From the perspective of plant performance the major problems has been one of sludge
foaming and bulking. This is a phenomenon in which the aeration basin is covered with a
thick and stable foam and the sludge settles poorly during the settle phase. As a result it is
very difficult to achieve the necessary solids consent.
The reasons for foaming and bulking in SBR systems operated in the UK are well but not fully
understood. However one organism in particular, Microthrix parvicella, was routinely
identified at sites experiencing loss of the sludge blanket. Microthrix is most frequently
found where the (Eikelboom, 2002):
F/M <0.2/d
Fats and grease present in wastewater.
Low temperatures <15°C, so generally more of a problem in the winter and
spring.
Large anoxic zones and elevated nitrate levels both of which are associated with
nutrient removal plants.
Low DO concentrations can also be contributory.
Therefore a nitrifying plant operating in winter with inadequate or absent scum handling
systems on the primary tanks and that struggles to reach >1.0 mg/l dissolved oxygen quickly
after the anoxic zone, is a prime candidate for Microthrix proliferation.
Figure 5: Microthrix parvicella Gram stain, wet mount and Neisser stain (x100)
Microthrix is not unique to SBRs, but when compared to conventional activated sludge
processes, it is not so flexible at dealing with the problems of bulking and foaming once they
have occurred, because the SBR combines both biological treatment and solids separation
in the same basin. In a conventional system the operator has a choice of where to manage
the a foaming incident, it can be retained and managed in the aeration basin where it
poses little threat to the consent, this choice is not open to the SBR operator.
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Figure 6: Foam contained in an oxidation ditch
When present Microthrix is effective at disrupting the floc structure of the sludge, it reduces
the rate of separation of treated water from the mother liquid which leads to reduced
settling velocities and SSVI3.5 (Stirred Sludge Volume Index) typically in the range 120-180 ml/g
(Trumper et al., 2005). The presence of the organism in the mixed liquor also leads to a highly
unstable sludge blanket-treated effluent interface that is easily disturbed by the motion and
energy generated by the decanter. Furthermore the organism is hydrophobic and when a
plant reaches 'tipping point' Microthrix has the ability to migrate to the surface of the basin
and produce a thick, mousse like foam. Operators have reported that over the course of a
weekend a plant can progress from having a 'small corner of foam' to in extreme cases
exiting the basin itself.
Figure 7: Extreme foaming
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Figure 8: Foaming SBR
Microthrix is opportunistic and given the right conditions it can easily outcompete well settling
floc-forming bacteria that designers and operators alike, aim to promote.
Plants that nitrify and denitrify encourage Microthrix and this is due to the way that the
organism deals with the toxic intermediate products of denitrification (Gerardi, 2002),
effectively performing a shunt reaction from nitrate to nitrogen gas.
Figure 9: The Microthrix shunt
Toxic intermediaries are flushed out in the aeration zone, if however there is a time lag in the
provision of oxygen and achieving for example a minimum of 1 mg/l dissolved oxygen in
these condition Microthrix has the advantage. This scenario is more likely to occur with SBRs
where the cycle begins with an unaerated fill period or includes a fill option in the decant
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phase. In conventional activated sludge plant design the part of the aeration lane following
the anoxic zone is preferentially loaded with diffusers in anticipation of the high oxygen
demand resulting from the combination of oxygen starved sludge and sewage.
A plant that is nitrifying will operate with a longer sludge age (at the same temperature) than
a plant designed for carbonaceous treatment alone.
Figure 10: Sludge age and nitrification
In theory, for a carbonaceous plant, maintaining the appropriate sludge age and avoiding
nitrification should be fairly straightforward, however sludge age control for SBRs is more
complex. In order to accurately calculate it and assess the likelihood of nitrification the
following are measured the:
In-basin temperature. This is straightforward provided that a temperature monitor
has been provided.
Mass of MLSS in the SBR. As the level in the SBR varies throughout the cycle, the
operator must either adjust the value recorded to BWL and use the BWL volume
to calculate the mass or measure the level in the SBR at the time of sampling and
calculate the active volume at that time. Where foam is present on the SBR non-
routine sampling techniques must be employed to get a representative sample.
Mass of sludge wasted from the process (Surplus/Waste Activated Sludge). SAS
usually takes place at the end of the decant phase, there may be as little as a 10
minute window to sample every 4-6 hours.
Mass of sludge lost in the final effluent. Impossible to determine if a plant is
suffering from blanket loss and/or foam entrainment into the final effluent.
Amount of time that positive dissolved oxygen is recorded in the cycle. Biomass
actively convert carbonaceous material into new cell matter during those periods
where a source of oxygen and electrons are available. On a four hour cycle with
a two hour fill/aerate it is normal to assume that 50% of the time. However this
estimate may be inaccurate, the actual period is difficult to determine when the
DO set point is not reached quickly at the beginning of the cycle and during high
flows when the cycle time shortens.
The batch nature of treatment also means that basins operating on fixed cycle times will
receive over the course of 24 hours varying loads.
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Figure 11: Diurnal load profile (1)
For example a basin being filled in periods where the morning and evening peak loads are
being experienced will receive a greater load than one that sees part or none of that
period. In the example below for a 3-basin SBR operating on a 6-hour cycle, Basin 3 over the
course of the day receives 85% of the load of basin 1 and 90% of basin 2. These differences
result in different Food: Microorganism ratios and sludge ages across the plant as a
whole. Those basins that receive proportionally less load are more susceptible to
unintentional nitrification/denitrification and filamentous bulking and foaming.
Figure 12: Diurnal load profile (2)
Design and Operating Considerations
The main issue associated with a nitrifying-denitrifying SBR treating domestic sewage is
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Microthrix parvicella and the resultant foam and poor settling characteristics to the activated
sludge. It is not a question of if the organism will be present rather when will its presence
become a threat to consent, a number of design and operation considerations should be
allowed for:
The start of the cycle is crucial, enough blower provision must be in place to flush out
the toxic intermediaries. The first 10 minutes of the aerate period (referred to as the
'Flash Mix') should see the DO achieved in excess of 1 mg/l as quickly as
possible. Blowers should be oversized to achieve this, the rate of oxygen provision not
solely based upon traditional calculation rules for peak loads. Naturally this will result
in increased capital (and operational) costs in the form of additional diffusers, larger
blowers and the need for variable speed blowers to meet the oxygen demand after
the flash mix period.
Incorporate a cycle time that is not fixed for 4 or 6 hours, rather selecting e.g. 4h15
minutes to ensure that individual basins see different load conditions over time.
Waste activated sludge in the aerate period to permit accurate sludge age
control. Increased buffering and thickening capacity downstream will be
required. This strategy also enables operators to accurately control in-basin MLSS
levels and therefore optimise mass flux values on a basin to basin basis.
Utilise the entire decant period. The decanter should be at TWL at the start of the
decant period and the software capable of determining the slowest rate of decant
throughout the full period (rather than reaching a fixed BWL and an idle period
following). This will minimise turbulence with in the basin and disturbance of the
sludge blanket.
If the SBR includes separated zones (e.g. CASS, Jet Tech design does not) ensure that
the option for variable speed return activated sludge (RAS) is included and sufficient
control mechanisms (e.g. aeration, redox, on-line TOC analysis) incorporated to
encourage fully aerobic operation to promote floc forming bacteria (Trumper et al,
2005).
Build in additional settlement capacity. Traditional activated sludge clarifiers are
usually designed for a maximum SSVI3.5 of 120 ml/g/. Domestic wastewater SBR sites
have seen values of 120 or 140 ml/g, the latter provides a greater margin of safety but
increases the overall footprint and capital costs.
Domestic Wastewater Treatment by SBRs in 2014 and Beyond
Liverpool WwTW marks the first large capital investment in SBR technology for domestic
wastewater treatment in the British Isles for over a decade. The £200 million project see the
infilling of Wellington Dock and build of the SBRs (£145m), upgrade to the existing outfall
(£11m) and improvements on the existing site (£45m) (waterprojectsonline.com, 2013).
SBR technology was selected for the site due to financial and space constraints. Awareness
and mitigation of the issues with SBRs is evident in the design approach and final solution with
the final design being informed by onsite pilot plant data and Biowin Modelling and CFD,
neither of which were available to designers in AMP2:
Nitrification - whilst the site is not required to nitrify it has been designed for this
purpose (and to denitrify) with a 12 day aerated sludge age due to concerns
over treatability of the wastewater (high industrial fraction) at sludge ages of 4-6
days (carbonaceous) (Black, 2014). Microthrix parvicella would therefore be
expected to be present in the biomass.
Measures to control the organism include those previously listed:
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o increased blower capacity to flush out toxic intermediaries by
increasing the air flow provision rate from 11,800 to 16,000 m3/hr and
the diffuser density from 5.6 to 9.6%.
Figure 13: Biowin modelling of dissolved oxygen (Black, 2014)
wasting activated sludge in the aerate cycle thus permitting better control
over in basin MLSS and sludge age.
the option to recycle activated sludge.
optimising decanter control philosophy.
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The CFD modelling has been employed to simulate a range of worse case scenarios taking
into account poor performance of primary settlement tanks, a maximum SSVI of 120 ml/g
and piles within the basin and it is perhaps these two factors which (in the authors’ view)
have the greatest unknown. Microthrix disrupts the floc structure of the biomass, the
rheology of the mixed liquor and the stability of the sludge blanket-treated effluent interface
whilst more conservative design select a design SSVI of 140 ml/g.
The settlement characteristics are further complicated as the cycle incorporates a
continuous fill-decant, which will disturb the sludge blanket. The reason for the fill-decant is
that Liverpool WwTW is a 1.5 DWF works as opposed to the conventional 3 DWF. The SBRs will
therefore operate more frequently at FTFT than a 3 DWF works. This factor does however
mean that a shortened or altered 'storm' cycle is not required, which helps with ease of
operation. Whilst a continuous fill means that each basin will receive equal flow and load
over the day the same the oxygen requirement at the beginning of the fill/aerate cycle will
vary, being greater where peak load conditions have been experienced in the fill/settle and
fill/decant cycles. The propensity for a slow ramp up in dissolved oxygen profiles is
exacerbated in this situation and hence the risk of bulking.
Other areas of CFD modelled uncertainty in the authors’ view include the potential for saline
intrusion, which can result in density currents plus the presence of high levels of dissolved
solids and sulphate; insufficient alkalinity for full nitrification; and the high proportion of
industrial COD in the influent. Combined these factors increase the likelihood of bio-fouling
of the aeration system and diffusers in the non-aerate phases which in turn result in reduced
rates of oxygen provision and transfer, increasing the bulking and foaming risk.
Figure 14: DO ramp up profiles
Liquor Treatment by SBRs in 2014 and beyond
In recent years there has been an increase in the uptake of SBRs for liquor treatment in the
organic waste digestion sector. This move is being driven by the costs associated with the
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transport of whole digestate, limited availability of landbank and to help achieve the
requirements of PAS110 in terms of the residual biogas potential limit.
Whilst the basic principles of sequencing batch reactors apply in that treatment takes place
in a single tank or set of tanks the nature of the influent is vastly different and as a result cycle
times are much longer and the problems encountered different. SBRs treating dewatering
liquor do not suffer from the Microthrix problem since this organism is almost always
associated with low strength domestic wastewaters in nitrifying plants (Eikelboom, 2000).
In fact these SBRs rarely suffer from filamentous bulking although their performance is
frequently affected by high levels of suspended solids passed forward to the basins from the
dewatering process. Whilst a centrifuge or belt press operating in the wastewater industry
would be expected to remove >98% of the solids fed to the unit in this industry 90% is more
common. In practice therefore a system fed with 3% dry solids (30,000 mg/l) would pass
forward 3,000 mg/l to the SBR.
The reason for the reduced performance is because food waste is a predominantly an
organic material and after homogenisation and dilution contains almost no settleable solids
(unlike a primary or secondary sludge). It is largely a colloidal suspension of organic material
with the fibres associated with the feedstock. During the digestion process the particle size is
further reduced due to hydrolysis, and the fibrous material swells and largely resists
biodegradation. The anaerobic biomass does not produce exopolysaccharide and
therefore whole digestate is uncharged and thus requires larger poly doses and potentially
an additional source of cations (e.g. iron) to aid flocculation. Thus food waste digestate
comprises a mixture of anaerobic biomass and swollen fibrous material. The fibrous material is
loath to shed water and due to its large size will block conventional filter media, making this a
material that is difficult to filter. (Baddelely et al., 2014)
Figure 15: The sludge particle
As a result the SBR contains a large but difficult to quantify amount of digested solids, which
makes sludge age control challenging and on occasions impossible. Furthermore digestate
produced from food waste digesters (where the measured hydraulic retention time is <40
days and organic loading rate >3kg.VS/m3/d) usually contain much higher levels of volatile
fatty acids and ammonia-N (relative to sewage sludge digesters), modelling of the levels to
be anticipated and/or upfront bench scale trials are essential to determine the
carbonaceous and nitrogen loads as well as the dewaterability potential.
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Irrespective of the readily biodegradable (RBCOD) fraction the concentration of ammonia-N
nitrogen to be treated by the SBR will be in the range 1,000-5,000 mg/l. At these levels
additional challenges are posed when aiming to achieve nitrification and denitrification, if
required.
Figure 16: Nitrification-Denitrification
In order to achieve nitrification 7.14g of alkalinity per gram of ammonia-N oxidised to nitrate
is required. Supplementary dosing, usually in the form of sodium hydroxide, may well be
required, where this is the case pH control becomes critical as with increasing pH
the proportion of free ammonia is increased.
Figure 17: Free ammonia
Free ammonia is toxic both to Nitrosomonas (ammonia to nitrite) and also Nitrobacter (nitrite
to nitrate) with inhibition for the former from 10-30 mg/l and as low as 0.1-1 mg/l for
Nitrobacter. At pH 9.3 free ammonia and ammonium are in equilibrium.
Figure 18: Equilibrium curve for the effect of pH on free ammonia concentration (Gerardi,
2002)
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Accumulation of nitrite within the activated sludge process can also lead to inhibition of
Nitrobacter, typically in the range 150-200 mg/l. Temperature can also be an issue. For plants
treating liquors over 30oC both Nitrobacter and Nitrosomonas will become partially inhibited,
quantifying the inhibition is challenging and required laboratory/ bench scale investigation.
In order to account for these factors a purpose designed balancing tank where pH,
temperature and load can be balanced are required. Incorporating denitrification into the
SBR cycle time will also aid with pH control and reduce the alkalinity requirement, however
an additional source of carbon will be required in the form of for instance, . molasses.
Conclusions
SBRs treating domestic wastewater are prone to filamentous sludge bulking and foaming, the
organism responsible is usually Microthrix parvicella. Where present the design SSVI can be
exceeded resulting loss of the sludge blanket, the organism also disrupts floc structure which
leads to an unstable sludge blanket-treated effluent interface.
SBR cycle times encourage the proliferation of Microthrix especially where the process is
configured to nitrify and denitrify. Unlike floc forming bacteria the organism is able to cope
with the toxic intermediate products of denitrification by performing a shunt from nitrate to
nitrogen gas.
Accurate sludge age control is a necessity in attempting to control Microthrix and this is not
easily achieved in SBRs where different basins receive varying flows and loads over the
course of the day and/or during storm/shortened cycle times.
Further design considerations to minimise filament levels include oversizing blowers to rapidly
flush out toxic intermediate products during the fill/aerate period, adopting a cycle time that
manages catchment specific load conditions, wasting mixed liquor during the aerate phase
to control sludge age accurately and adopting a conservative design SSVI3.5 at the design
stage.
SBRs are becoming more popular for the treatment of liquors from dewatered food waste
digestate. Whilst sites do not appear to suffer from filamentous bulking on the whole, food
waste digestate dewaters poorly, leading to large influxes of digestate solids into the SBR
which complicates sludge age control, reduces potential throughput and compromises final
effluent quality.
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