9 th European Waste Water Management Conference 12-13 October 2015, Manchester, UK USE OF BIOFILM SOLIDS TO SEED THE NEW ACTIVATED SLUDGE PROCESS AT LIVERPOOL WWTW Black, J., Clarke P. and Molloy R. United Utilities plc, UK Corresponding Author Email [email protected]Abstract This paper describes the seeding technique and results for the process commissioning of the new Sequencing Batch Reactor at Liverpool Waste-water Treatment Works (WwTW). This process replaces the existing secondary treatment: a Biological Aerated Flooded Filter (BAFF), and is the main component of the £200m project to upgrade United Utilities’ second largest waste-water treatment works. Typically activated sludge is imported from other WwTWs to seed a new process. In the case of Liverpool WwTW there was thought to be a high risk that an imported microbial population would fail to adapt to the influent load, which is known to contain a high proportion of trade effluent as well as fluctuating salinity concentrations. Instead, biosolids from the existing BAFF process were used to seed the new plant. Process modelling (BioWIn) was used to predict the theoretical microbial growth rates based on seeding with conventional activated sludge. These results were compared with the actual growth rates achieved. The seeding process using BAFF biosolids compared favourably with predicted growth from an imported sludge. In addition the elimination of the need for tankers to import activated sludge resulted in significant financial savings to the project. Keywords Process Commissioning, BioWin, Bioflm, Activated Sludge, Sequencing Batch Reactor, Biological Aerated Flooded Filter (BAFF) Introduction This paper describes the seeding technique and results for the process commissioning of the new Sequencing Batch Reactor at Liverpool Waste-water Treatment Works (WwTW). It focusses on the innovative technique of using seed sludge from the existing BAFF (biological aerated flooded filter) rather than importing seed sludge from another waste-water treatment works. www.ewwmconference.com Organised by Aqua Enviro Limited
Transcript
9th European Waste Water Management Conference 12-13 October 2015, Manchester, UK
USE OF BIOFILM SOLIDS TO SEED THE NEW ACTIVATED SLUDGE PROCESS AT LIVERPOOL WWTW
Black, J., Clarke P. and Molloy R.United Utilities plc, UK
This paper describes the seeding technique and results for the process commissioning of the new Sequencing Batch Reactor at Liverpool Waste-water Treatment Works (WwTW). This process replaces the existing secondary treatment: a Biological Aerated Flooded Filter (BAFF), and is the main component of the £200m project to upgrade United Utilities’ second largest waste-water treatment works.
Typically activated sludge is imported from other WwTWs to seed a new process. In the case of Liverpool WwTW there was thought to be a high risk that an imported microbial population would fail to adapt to the influent load, which is known to contain a high proportion of trade effluent as well as fluctuating salinity concentrations. Instead, biosolids from the existing BAFF process were used to seed the new plant.
Process modelling (BioWIn) was used to predict the theoretical microbial growth rates based on seeding with conventional activated sludge. These results were compared with the actual growth rates achieved. The seeding process using BAFF biosolids compared favourably with predicted growth from an imported sludge. In addition the elimination of the need for tankers to import activated sludge resulted in significant financial savings to the project.
This paper describes the seeding technique and results for the process commissioning of the new Sequencing Batch Reactor at Liverpool Waste-water Treatment Works (WwTW). It focusses on the innovative technique of using seed sludge from the existing BAFF (biological aerated flooded filter) rather than importing seed sludge from another waste-water treatment works.
Project Background
Project need
Liverpool WwTW was originally built in the 1990s as part of the Mersey Estuary Pollution Alleviation Scheme (MEPAS), to treat waste water from most of Liverpool. The works treats domestic waste water, plus a high proportion of trade effluent. This includes the waste stream from the United Utilities sludge processing plant at Shell Green.
The existing works was built on the water front, within the boundaries of Sandon Dock, and has been upgraded a number of times since initial construction. In 2010 it was decided that a major works upgrade was required including replacement of the secondary treatment BAFF process.
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The project brief for the new secondary treatment process was for an “operator friendly” process to achieve robust 95%ile compliance with the Urban Waste Water Treatment Directive consent of 25mg/l BOD.
Table 1: Final effluent consent Liverpool WwTW
Parameter 95%(24 hr composite)mg/l
Upper Tier Limit(24 hr composite)mg/l
BOD 25 50
COD 125 250
Suspended Solids n/a 250
Ammonia N n/a n/a
Project constraints
There were a number of additional constraints for the project as follows:
Footprint: the new process had to fit within the docks walls of the adjacent Wellington Dock. Planning permission: the site is on the boundary of a UNESCO World Heritage site and
therefore had to preserve the essential character of the dock, and have an acceptable visual appearance.
Wind: the waterfront at Liverpool is subject to regular strong winds, sufficient to compromise settlement processes (as occurred at other similar locations)
Saline intrusion: infiltration of saline water into the sewer network, particularly at high tides, means there are fluctuating salinity levels in the feed to the works.
Trade effluent: approximately one third of the influent load is trade effluent, including significant proportions of slowly bio-degradable BOD, and non bio-degradable soluble COD.
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Existing WwTW(SandonDock)
River Mersey
Wellington Dock(for new 2ndary treatment)
Figure 1: Aerial photo showing Liverpool WwTW
Solution description: scope
The solution chosen was a sequencing batch reactor (SBR) process with 16 basins, on two levels: 8 upper and 8 lower. Alternatives, including continuous flow activated sludge plant, and a membrane bioreactor were found to be higher in whole life cost.
The overall dimensions for each basin are 40m wide (decanter side) and 49.5m long, including all pre-zones. The base case design was for a bottom water level of 4.1m and a top water level of 5.65m (plus allowance for contingencies).
The base case design chosen was a fixed four hour cycle with continuous fill, i.e.
2 hours fill and aerate 1 hour fill and settle 1 hour fill and decant
The process was designed to have 12 day aerobic sludge age (i.e. 24 day total sludge age). A 12 day aerobic sludge age was found to be necessary following pilot scale trials which identified residual treatability concerns with using a 4-6 day (carbonaceous only) sludge age. An intermediate sludge age (6-10days) was considered high risk, as it would result in partial nitrification, and a process that would be difficult to control, with poor settle-ability. Thus a long sludge age, with associated nitrification was chosen to achieve robust compliance with the BOD and COD standards, irrespective of there being no ammonia standard.
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Figure 2: Aerial view showing upper tier of 8 SBR Basins under construction
Process commissioning challenges
One of the main challenges with process commissioning for the new SBRs was how to seed the basins with viable activated sludge. Pilot plant trials at the works had shown that seeding using an imported sludge risked non-compliance with the effluent quality standards. This was because it would take a few days for the microbiology to adjust to the atypical influent received at Liverpool WwTW. In addition the costs and associated logistics for tankering in the required quantities of seed sludge from another waste-water treatment works were best avoided.
Process commissioning approach adopted
Use of Seed Sludge from BAFF plant
As an alternative to seeding with imported activated sludge, an approach using indigenous secondary sludge was investigated. This involved using thickened back-wash returns from the existing BAFF plant, to provide sludge containing the required microorganisms. The benefit of using indigenous sludge was that the micro-organisms were adapted to the nature of the influent load at the works. However, the seed sludge was from a fixed film process, rather than a suspended growth based process. Pilot plant trails on site established that if a sufficient quantity of BAFF plant sludge was used as seed sludge the SBR could be “kick started” in a single cycle to produce an effluent suitable for discharge.
Pilot Plant Trial
The feasibility of using the BAFF plant to seed the SBR process was tested on the SBR pilot plant: a 45m3 tank, 3.05 m diameter, with a 6.0m top water level. This was carried out approximately 2 years ahead of the full scale commissioning. The results comparing measured growth in biomass (MLSS) with that predicted by BOD yield are summarised below.
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Liverpool SBR commissioning: pilot plant trial using BAFF seed sludge - May 2013
predicted mlss measured mlss
start of surplussing
Figure 3: Pilot plant results for seeding trial
The results demonstrated that the proposed seeding method was viable. The results also showed that the actual growth rate could be predicted with some confidence from simple BOD based theory. The trial was repeated, with similar results. This gave the Project Management Team sufficient confidence with the method to adopt it for the full scale commissioning.
SBR Cycle times
Achieving good effluent quality on the very first cycle was critical, as this would enable effluent to be drawn off, and then more organic load added. To achieve a sufficient level of treatment on the first batches meant the SBR would need longer than the operating design 4 hours cycle. It was also recognised that the organic load to be fed to the SBR basins initially would need to be sufficient to produce additional activated sludge (to ramp up the process), without overloading a basin with organic load, such that the effluent quality was non-compliant. i.e. not too much, and not too little. With this in mind the initial cycle time was set at 24 hours, being progressively shortened to the design target cycle time of 4 hours over several days.
Phased approach
Having established that the BAFF plant sludge could seed the SBR process a further consideration was how to phase the commissioning of the SBR basins. This meant predicting the production rate of excess sludge (SAS) from the first pair of basins commissioned, and ensuring that this would be sufficient to seed the next pair of basins.
Based on the pilot plant trial results and predicted growth rates, an overall schedule to commission all 16 basins was prepared, and aligned with the other site construction and commissioning activities.
BOD based sludge growth prediction
An approximate prediction of activated sludge production based on BOD was made. This was used to determine the required loading rate, setting a target f:m ratio below 0.3 kgMLSS / kgBOD. Secondly it was used to predict the growth rate of MLSS assuming a BOD:sludge yield of 1.2kgmlss/kgBOD. An extract of the calculations is shown in Table 2 to illustrate the calculations for a pair of basins.
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Table 2: MLSS growth prediction calculations for seeding and first 4 days of basin commissioning. (basins 13&14)
Day 1 Day 2 Day 3 Day 4
Starting MLSS, mg/l 750 813 876 938
BOD load treated, kg/d 494 494 494 988
Average Aerated F:M, kgMLSS / kgBOD/d
0.09 0.09 0.08 0.18
Net increase in solids, kg/d
505 505 505 1,009
Finishing MLSS Calculated, mg/l
813 876 938 1,064
The initial seed value of 750 mg/l was achieved by using biosolids from the BAFF backwash.
By this method it was predicted that it would take approximately 2 weeks to ramp up a basin from initial seeding to a point at which it was treating the design organic load at the desired 4-hour cycle rate.
The reliability of this method is limited by the assumption of sludge yield based on BOD. Its usefulness in practice was further constrained by uncertainty in advance of accurately knowing what the organic load being fed to a basin on any day would be. (It is possible to control the volume fed, but the concentration, and thus the load would depend on rainfall and other factors).
COD based sludge growth prediction
A second COD based prediction for activated sludge production was made using BioWin as a process modelling tool. The default BioWIn settings were used for reaction rates and sludge growth.
The BioWin model is shown below, along with the BioWin outputs.
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Crude
return liquors
Ferric dosing (off)
Primary sludge SAS
Final Effluent
SBRs 2-16
SBR Basin 1
Figure 4: BioWin model set up to model commissioning
-
500
1,000
1,500
2,000
2,500
3,000
3,500
4,000
0 5 10 15
MLS
S, m
g/l
day
SBR seeding: predicted MLSS growth BOD model BioWin target
Figure 5: Predicted growth rates of MLSS by two methods: BOD yield, and COD model (BioWin)
The BioWin based model predicted similar, though slightly slower growth rates than the equivalent BOD yield based model. As with the BOD yield method, this method was subject to a number of limitations, such as the COD characterisation. The fact that the two methods both predicted 12-15 days to initially “commission” a basin was assumed as sufficient agreement.
Settle-ability
As well as treating the organic load a key requirement to the proper operation of the SBR was that the mixed liquors would settle sufficiently to provide a clear effluent to be decanted. This was particularly a concern for the first few cycles as the “blanket-forming” qualities of the seed sludge were far less certain than for a conventional activated sludge.
Intensive checking of the settle-ability of the mixed liquors was carried out for the first few batches during commissioning.
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Nitrification requirement
It should be noted that the initial commissioning requirement was to meet the effluent quality requirements for BOD and COD. There was no short term requirement for the SBR to be fully nitrifying. It was assumed that nitrification would begin of its own accord over a period of several weeks by controlling the sludge age.
BioWin modelling was used to provide an approximate indication of the timescales by which nitrification could be expected to occur. The prediction is shown below.
0
5
1015
20
25
30
35
40
0 10 20 30 40 50
mg/
l
Days
Predicted time line for nitrificationAmmo mg/l
Figure 6: Predicted change in nitrification following process establishment
The prediction, using BioWIn, assumed using a 12 day aerobic sludge age once the initial process establishment (MLSS of 3,000mg/l) had been achieved. The model indicated it would take approximately 40 days to achieve nitrification down to 1mg/l.
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Results
Microscopy
Microscopic analysis of the seed sludge (from the BAFF plant) and subsequently of the activated sludge from the SBR basins was carried out on a daily basis. Some examples are shown below.
Figure 7: BAFF Sludge photo taken April 2015 (day <0)
It can be seen that there are signs of biological activity within the seed sludge, however it appears to be limited.
Figure 8: Seed sludge photo taken 1st May 2015 (day 1)
Seed sludge photo taken on 2nd May 2015 (seeding of the SBR commenced on 1st May 2015) showing small dispersed flocs with Crawling Ciliates and Filamentous Bacteria present.
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Figure 10: Activated sludge from the SBR basins (days 7)
It can be seen that within 7 days there were signs of healthy floc growth, including larger colonies of Stalked Ciliates, Crawling Ciliates and a few Filamentous bacteria typically associated with activated sludge.
Mixed liquor suspended solids (MLSS) growth
The growth in MLSS for the first 15 days is shown below.
9th European Waste Water Management Conference 12-13 October 2015, Manchester, UK
Withdrawal of activate sludge to seed further basins
Because the first two basins seeded had quickly achieved final effluent quality, it was decided to accelerate the commissioning and seed the next 2 basins with activated sludge from the first basins over days 8-10. Thus growth for the first two basins was significantly slowed over this period.
Influent strength
The influent strength during the initial commissioning period was far lower than had been assumed: the average influent COD strength for the period was 285mg/l, compared with an anticipated influent COD of 375mg/l.
Slow Growth
The modelling predictions assumed that the seed biomass would have a similar level of biological activity to that of conventional activated sludge. This is unlikely to have been the case, partly due to a higher proportion of inert solids, and also a higher proportion of inactive biomass.
Settle-ability
Initial settle-ability was extremely good, enabling the first batches to be decanted with little solids content in the final effluent.
Subsequent settle-ability has continued to be good, with SSVI3.5 typically in the range 90-100ml/g during the commissioning period. However, at the time of writing the basins had only recently begun to de-nitrify, so it is too soon to assess the longer term settle-ability.
Effluent quality
The primary objective of the initial seeding was to achieve a final effluent quality that could be discharged. The results for initial effluent quality are shown below.
0
20
40
60
80
100
120
140
160
0 5 10 15 20 25 30
mg/
l
days from initial seeding
SBR Basin 13, effluent quality
SOLI
BOD
COD
AMMO
Figure 12: Effluent quality for SBR basin 13 during initial ramp up phase
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The results show that it was possible to maintain effluent quality whilst ramping up the basin to design load. It can also be seen that nitrification had begun to occur around day 18.
The high COD value on day 18 is probably a spurious result as the suspended solids were low.
Subsequent nitrification
As stated earlier, full nitrification was not required to meet the discharge consent standard, but rather to ensure process stability. The build-up of nitrifying bacteria was allowed to take place without any additional seeding.
In addition to the slow growth rate of nitrifying bacteria, the sludge age was occasionally shortened by withdrawing additional mixed liquors to seed other SBR basins. Thus, it took several weeks for full nitrification to be achieved.
The combined effluent quality ammonia is shown below.
Combined Effluent Ammonia - all basinsAMMO Ammonia Out
Figure 13: Effluent quality for all SBR basins during commissioning
At the time of writing two basins had yet to be commissioned. However it can be seen that full nitrification across most of the basins has taken approximately two months (60 days). This compares favourably with the initial prediction of 40 days for the first basin.
Conclusions
Use of bio-film solids as seed sludge
The use of the biofilm solids as a seed sludge was successful in getting the SBR biological treatment process started: this was both in terms of proving sufficient break-down of the organic load, and having the settle-ability characteristics.
The biofilm sludge probably did not provide as high a level of biological activity as an equivalent mass of imported activated sludge, however it was sufficient to “kick-start” the process for carbonaceous removal.
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This approach has delivered the benefits of reduced costs, reduced tanker movements, and reduced process risk.
Use of process modelling as a commissioning tool
Predictive calculations using BOD as a basis, coupled with the use of BioWin as a (COD based) process tool has provided confidence in the commissioning approach adopted.
Given the number of assumptions required in the modelling, and the variability of influent loads, any modelling outputs are best regarded as indicative, rather than definitive predictions.
References
Artan N and Orhon,D (2005) Mechanism and Design of Sequencing Batch Reactors for Nutrient Removal. Scientific and Technical Report no19, IWA Publishing London.
Black J (2014) BioWin Modelling of the new SBR for Liverpool. Activated Sludge Past Present and Future Conference.
Boon A (2001) Sequencing Batch Reactors. Proceedings of Aqua Enviro Technology Transfer Conference Sequencing Batch Reactor III Client and Contractor Experiences pp 35-41
Halladey L and Coleman P (2001) Success or Failure: Three Key SBR Design Criteria. Proceedings of Aqua Enviro Technology Transfer Conference Sequencing Batch Reactor III Client and Contractor Experiences pp 35-41
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