use of biofilm solids to seed the new activated sludge … · 2019. 3. 22. · sequencing batch...

9th European Waste Water Management Conference

12-13 October 2015, Manchester, UK

www.ewwmconference.com

Organised by Aqua Enviro Limited

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.

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.






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.

Existing WwTW(Sandon Dock)

River Mersey

Wellington Dock(for new 2ndary treatment)






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.






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.

0

500

1000

1500

2000

2500

3000

3500

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

mg/

l

Days from initial seeding

Liverpool SBR commissioning: pilot plant trial using BAFF seed sludge - May 2013

predicted mlss measured mlss

start of surplussing






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.






Figure 4: BioWin model set up to model commissioning

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.

Crude

return liquors

Ferric dosing (off)

Primary sludge SAS

Final Effluent

SBRs 2-16

SBR Basin 1

-

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






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.

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.

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50

mg/

l

Days

Predicted time line for nitrificationAmmo mg/l






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.






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.

Figure 11: Predicted growth rates of MLSS, and actual growth rate for basins 13 and 14

It is clear that the initial actual growth rate was lower than the predicted. This is thought to be due to a

number of factors:

-

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

4,500

5,000

0 5 10 15 20 25

MLS

S, m

g/l

day

SBR seeding: predicted MLSS growth BOD model BioWin target Actual






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.

Figure 12: Effluent quality for SBR basin 13 during initial ramp up phase

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






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.

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.

0

10

20

30

40

50

60

70

80

26/04/2015 16/05/2015 05/06/2015 25/06/2015 15/07/2015 04/08/2015 24/08/2015 13/09/2015 03/10/2015

mg/

l

date

Combined Effluent Ammonia - all basinsAMMO Ammonia Out






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


use of biofilm solids to seed the new activated sludge … · 2019. 3. 22. · sequencing batch...

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