LIVERPOOL WWTW SBR CARBONACEOUS TRIAL Abstract...5 SAS during ‘aerate’ phase 6 SAS during...

11th European Waste Water Management Conference

3rd – 4th October 2017, Leeds, UK

www.ewwmconference.com

Organised by Aqua Enviro

LIVERPOOL WWTW SBR CARBONACEOUS TRIAL

Akinola, O.1, Black, J.1, Sherwood, A1. and Hornsby J1. 1United Utilities, UK

Corresponding Author Email [email protected]

Abstract

In January 2017, a Trial commenced on the Sequencing Batch Reactor (SBR) Plant at Liverpool

WwTW. The 16-basin SBR Plant is normally operated in ‘nitrification mode’. Annual aeration savings of

approximately £300k were previously estimated if carbonaceous operation is adopted. The Trial was

carried out on 2 No. basins with a target aerobic sludge age of 4 days. The key objectives of the Trial

were to assess process risk, OPEX benefits, and the likely impact on the sludge stream. Process

modelling using BioWin was carried out in advance to provide guidance on key operating parameters.

Liaison with site operators and managers was necessary throughout the Trial to implement changes

and monitor performance. Key Performance Indicators (KPIs) included: effluent quality (particularly

‘hard’ COD), sludge age, settleability, microscopy and energy consumption. The Trial results

demonstrated effluent quality consistently below consented limits and 37% aeration energy reduction.

Keywords

Basin, Carbonaceous, Configuration, Energy, MLSS, Operation, SAS, SBR

Introduction

The 16-basin SBR Plant at Liverpool WwTW was designed to operate in ‘nitrification mode’, due to

historical issues with ‘hard COD’ received at the Works and the need to ensure adequate treatability.

There is currently no ammonia consent at Liverpool WwTW. It has been previously estimated that by

adopting carbonaceous operation (i.e. not removing ammonia), annual aeration savings of

approximately £300k could be achieved.

The process of transitioning from ‘nitrification mode’ to ‘carbonaceous mode’ requires some

reconfiguration of the process and could entail some process compliance risk. Therefore, it was

necessary to undertake a Trial to further understand the risks, limitations and reconfiguration needed.

Background

Trial Objectives

The objectives of the Trial were as follows:

• To quantify the process risk of carbonaceous operation (in particular, BOD & COD compliance,

sludge settleability, microbiology, odour)

• To understand how best to manage the transition from ‘nitrification mode’ to ‘carbonaceous

mode’

• To understand any reconfiguration required on the SBR and sludge treatment processes to

enable full carbonaceous operation of the SBR

• To understand the optimum operating conditions for carbonaceous operation

• To quantify the OPEX savings and costs of operating a carbonaceous SBR

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Site Details and Permit

Treatment Process

Treatment at the Works consists of: preliminary treatment followed by primary sedimentation, and

secondary treatment in the SBR Plant. Furthermore, there are storm tanks present which provide

storage during periods of high flows. Primary and secondary sludge thickening are carried out

separately, followed by combined digestion on site.

Permit

The Permit for Liverpool WwTW includes consents for the following:

• Flow to Full Treatment (FTFT) ≈ 346 Ml/d

• BOD: 25mg/l (95%ile) / 50mg/l (UTL) / 70% Removal [Urban Wastewater Treatment Directive

(UWWTD)]

• COD:125mg/l (95%ile) / 250mg/l (UTL) / 75% Removal (UWWTD)

• Suspended Solids: 250mg/l UTL

• Several metals including iron and aluminium; and organic compounds such as chloroform and

trichloroethene

SBR Operation

The 16No. SBR basins are continuously filled and are operated according to a 4 hour cycle as shown

in Table 1. This cycle is repeated throughout the day for each pair of basins.

Table 1: Typical SBR Cycle

Time (Hours) 0 – 1 1 – 2 2 – 3 3 – 4

Phase Fill-Aerate Fill-Aerate Fill-Settle Fill-Decant

During each cycle, Surplus Activated Sludge (SAS) can be removed from the basins during the ‘aerate’

(hour 1) or ‘decant’ (hour 4) phase.

Methodology

Prior to start of the Trial, process calculations and modelling1 using BioWin 5.0 software, were carried

out. The Carbonaceous Trial was then carried out on 2No. basins (namely Basins 5 and 6), operating

as a hydraulically-linked pair, with basin 1 as the Control. The target aerobic sludge age was 4 days,

and this equated to a Mixed Liquor Suspended Solids (MLSS) target range of 1,500 to 2,000mg/l.

Throughout the Trial, monitoring of the Trial and Control basins was undertaken.

_________________________________________________________________________________ 1 Discussed in a separate section

Configuration of Trial and Control Basins

In order to achieve (and subsequently maintain) the required aerated sludge age of 4 days and MLSS

target of 1,500 to 2,000mg/l, it was necessary to reduce the MLSS in the Trial basins by increasing the

amount of SAS solids being wasted during each cycle.

Initially, the Trial and Control basins were set according to the configuration in Table 2 below:






Table 2: Initial Basin Configuration

Basin Configuration

5 SAS during ‘aerate’ phase

6 SAS during ‘decant’ phase

1 SAS during ‘aerate’ phase

However, following a number of mechanical issues with the SAS Drum Thickeners approximately seven

weeks into the Trial, surplussing for all the remaining basins was changed to occur during the ‘decant’

phase. The following basin configuration (Table 3) was thereby adopted for the remainder of the Trial.

It is worthy of note that these mechanical issues were unrelated to the Trial.

Table 3: Final Basin Configuration

Basin Configuration




A key advantage of surplussing solids from the basins during the ‘decant’ phase is significant reduction

in SAS volumes. Therefore, this would result in overall less pressure on the Secondary Drum

Thickeners’ capacity and contribute to more resilient operation of the Sludge Thickening Plant.

Process Set-points

Throughout the Trial, the following process set-points were adjusted and recorded for the Trial basins:

• volume of SAS removed (m3 per cycle)

• duration of SAS event (minutes)

Monitoring

As part of the Trial, qualitative and quantitative monitoring was undertaken.

Qualitative monitoring comprised of daily visual inspections of the Trial and Control basins, weather and

weekly odour readings. Quantitative monitoring comprised of daily/weekly collection and analyses of

effluent samples, MLSS and SAS samples, and weekly downloads of relevant site data. Determinands

of particular interest included Ammonia, Biochemical Oxygen Demand (BOD), Filtered BOD (BODF),

Chemical Oxygen Demand (COD), Filtered COD (CODF), Nitrate (NITR), pH, Total Suspended Solids

(TSS), and % Dry Solids (DS). Furthermore, weekly microscopy was carried out for the MLSS and/or

SAS samples.

Preliminary BioWin modelling and Outputs

In order to predict potential impacts of operating in carbonaceous mode, BioWin 5.0 modelling software

was used. One of the 16No. basins was modelled to assess typical performance of a single SBR basin.

The models were run in nitrifying, transition and carbonaceous modes at 11°C (i.e. worst-case

temperature). Furthermore, several iterations were performed by varying a number of factors including

duration of model run, Dissolved Oxygen (DO) concentration, alpha factor, SAS pump flow rate and

SAS volume removed per cycle.

Figure 1 is a layout of one of the model iterations.






Figure 1: BioWin Model Layout

The input data for the models was SBR feed data from Performance Tests undertaken in June 2016 as

this provided a robust data set. The duration of the model runs was initially 2 weeks, then this was

increased to 4 weeks to observe the MLSS trend over a longer period of time. More detailed analysis

of BioWin showed that it would typically take approximately 40 days for the MLSS in an SBR to stabilize.

The duration of the model runs was therefore increased to 8 weeks in order to assess the MLSS, SAS

concentration and effluent quality trends over this ‘stabilization’ period.

The following sub-sections summarise the outputs of the modelling exercise.

Transition from Nitrification to Carbonation Mode

MLSS and SAS

The transition models suggested that it would take up to two weeks to reach the target MLSS range of

1,500 to 2,000mg/l. The differences in ‘SAS aerate’ and ‘SAS decant’ configurations were also

highlighted. The output SAS concentration from the modelled ‘decant’ configuration was much higher

(about 4 times greater) than the ‘aerate’ configuration; thereby, enabling a significantly reduced SAS

volume to be removed from the basin (approximately one-quarter of that in ‘aerate’).

SBR Effluent Quality

Overall, in transitioning from nitrifying to carbonaceous mode, the following trends were

observed from the 95%-ile effluent quality results. These can also be seen in Figure 2Figure 2:

Effluent Quality – Nitrifying vs. Carbonaceous mode (BioWin)

.

• Ammonia increased

• BOD change was negligible (slight reduction)

• COD reduced

• Solids reduced






Figure 2: Effluent Quality – Nitrifying vs. Carbonaceous mode (BioWin)

Blower Energy Trends

The BioWin models showed that switching the SBR basins to carbonaceous mode (without making any

changes to current DO set-points2) would result in 40 to 50% reduction in blower energy consumption.

Impact of Dissolved Oxygen Control in Carbonaceous Mode

Blower Energy Trends

By halving the DO set-points that the basins currently operate to, the models also suggested potential

additional3 aeration energy savings of approximately 25%. There is an opportunity for this to be explored

on site during the imminent blower optimisation work.

Effect of Reduced Aeration on Microbiology

Results from the BioWin models showed that nitrification was still occurring even at significantly reduced

basin MLSS concentrations in carbonaceous mode.

While modelling in carbonaceous mode, the impact of reducing the DO set-points on the nitrifying

microbiology was observed. The nitrifying microorganisms modelled are classified into three groups,

namely, Ammonia Oxidising Biomass (AOBs), Nitrite Oxidising Biomass (NOBs) and Anaerobic

Ammonia Oxidising Biomass (AAOs). AOBs and NOBs are aerobic organisms which oxidise ammonia

to nitrite, and nitrite to nitrate respectively; while AAOs are anaerobic organisms which oxidise ammonia

using nitrite, to nitrogen gas and nitrate.

Figure 3, Figure 4 and Figure 5 highlight the impacts of halving the DO set-points on these organisms.

From these graphs, it can be observed that in carbonaceous mode, steady populations of AOBs and

AAOs were maintained, while a steady reduction in NOBs is observed. Once the DO set-points were

halved however, the AOBs reduced steadily, the NOBs reduced more rapidly and the AAOs increased

to almost twice their previous concentration. Thus, the AAOs show a competitive advantage over the

aerobic nitrifiers (AOBs and NOBs) when DO levels are reduced. ______________________________________________________________________________________________________________________________________

2 See Appendix for current blower DO profile applied across all basins; 3 Additional savings estimate is a proportion

of carbonaceous energy consumption






Figure 3: Impact of Reduced Aeration on AOBs (Carbonaceous Mode)

Figure 4: Impact of Reduced Aeration on NOBs (Carbonaceous Mode)






Figure 5: Impact of Reduced Aeration on AAOs (Carbonaceous Mode)

Trial Results and Discussion

Flows

Figure 6: Average Flows into SBR Basins






Figure 6 shows the average flows which passed through the entire SBR Plant during the Trial. There

were some periods of heavy rainfall (average flow > 300Ml/d) and drier periods (average flow <

200Ml/d). During the Trial, the 20%-ile and 99%-ile flows were 187Ml/d and 344Ml/d respectively. In

2016, the 20%-ile and 99%-ile flows recorded on site were 167Ml/d and 343Ml/d respectively.

Therefore, the flows experienced during the Trial were representative of the typical flow range on site.

MLSS

Figure 7 shows the trends in Mixed Liquor Suspended Solids (MLSS) throughout the duration of the

Trial. From this graph, it can be observed that there was a significant reduction of MLSS in both Trial

basins as the Trial progressed. Furthermore, within 1 month of the Trial, basin 6 reached the target

MLSS range of 1,500 to 2,000mg/l (average – 1,750mg/l); however, throughout the Trial, basin 5 did

not reach this target. This may have been hindered by wider site issues.

For example, the MLSS concentrations at the beginning of the Trial were elevated (above 4,000mg/l)

for all three basins. These concentrations were above the operational target of 3,800mg/l. In addition,

in early March, an increase in MLSS concentrations was observed for both Trial basins. Both these

instances of solids increase were attributed to increased SBR feed loads, which ultimately exerted a

strain on (and contributed to mechanical issues with) the SAS Drum Thickeners, due to the need for

higher SAS removal rates across the entire SBR Plant. The mechanical issues with the SAS Thickeners

resulted in a reduction in throughput; and consequently, a backlog of sludge to be processed by the

Thickeners. This eventually led to a build-up of solids within the basins.

Furthermore, from 24th to 28th March, basin 6 was out of service due to a mechanical issue. This issue

was considered unrelated to the Trial. During this time, flow in and out of the basin was stopped, and it

was placed in continuous aeration. There was therefore no effluent or MLSS sample collected on those

days.

Figure 7: MLSS Trends






Sludge Settlement

Throughout the Trial, both Trial basins showed very good settlement. For example, the Stirred Sludge

Volume Index (SSVI) was significantly less than 100ml/g, compared to the UU Asset Standard of

120ml/g. In addition, the Control basin generally showed good settlement; however, there were some

occasions during which the SSVI was greater than 100ml/g. Figure 8: SSVI Test

shows the SSVI samples at the end of a test.

Figure 8: SSVI Test

As settleability was not an issue during the Trial, a detailed standardised SSVI (SSVI 3.5) test was not

considered necessary for each sample. From the entire Trial dataset, approximate SSVI 3.5 figures for

all three basins were as follows: Basin 5 – 79ml/g, Basin 6 – 81ml/g and Basin 1 – 79ml/g. Therefore,

SSVI 3.5 was consistent across the Trial and Control basins.

SAS

For the Liverpool WwTW SBR basins, SAS can be removed during the ‘aerate’ or ‘decant’ phases of

the cycle. Due to increased SBR feed loads and consequent mechanical issues with the SAS Drum

Thickeners in March, there was need for a reduction in the sludge volume to be processed by the

Secondary Thickening Plant. Therefore, Operations adjusted all the remaining SBR basins to surplus

sludge during the ‘decant’ phase (including Trial basin 5).

Following the repair and recovery of the SAS Drum Thickeners, Operations decided to maintain the

sludge surplussing regime of the basins in ‘decant’ phase for the foreseeable future, as this puts less

pressure on the Drum Thickeners’ capacity; thereby contributing to more resilient operation of the

Thickening Plant.

Table 4 highlights advantages and disadvantages of either method of SAS removal.






Table 4: ‘SAS Aerate’ vs. ‘SAS Decant’ Configuration – Advantages & Disadvantages

SAS Removal Phase Advantages Disadvantages

Aerate

Relatively automatic sludge age

control Greater sludge volumes (3 times ‘decant’ phase volume)

Less intensive operator intervention Secondary sludge processing

capacity exceeded – major additional

capital expenditure (CAPEX) likely

Higher operational expenditure

(OPEX) – i.e. SAS pumping and

sludge processing

Decant

Much lower sludge volumes

(up to one-third of ‘aerate’ phase

volume)

More intensive operator intervention

(MLSS control)

Within volumetric capacity of

secondary sludge assets –

no/minimal additional CAPEX

Potential adverse impacts on process

due to more unknowns e.g. mass of

basin solids

(no evidence of this during Trial)

Potential OPEX savings (i.e. reduced

SAS pumping and sludge

processing)

Settleability risk

(no evidence of this during Trial)

The SAS volumes removed from the basin(s) with the ‘SAS decant’ configuration varied throughout the

Trial. This was mainly due to a number of factors; for example, difficulties in estimating mass of sludge

being withdrawn from the system at any given time (as this depends on sludge settleability), and

changing sludge concentrations at the base of the SBR during ‘settle’ and ‘decant’ phases. Therefore,

close monitoring of basin MLSS was essential for this configuration. The ‘optimum’ range for basin 6

SAS volume in order to maintain the MLSS target, was found to be between 65 and 85m3 per cycle (in

‘SAS decant’ configuration). As a comparison, the maximum SAS volume withdrawn from basin 5 whilst

it was in ‘SAS aerate’ configuration, was 185m3 per cycle.

The SAS pump flow rate typically used on site is 70l/s. Triplicate sampling of SAS was carried out for

both Trial basins in order to observe the change in SAS concentration over each withdrawal event. At

this rate of 70l/s during the ‘decant’ phase, it was observed that the SAS concentration reduced

significantly over the event – on average from approximately 0.8% Dry Solids (DS) at the start through

to 0.4% DS at the end.






Therefore, in the last two weeks of the Trial, the SAS flow rate was reduced from 70 to 50l/s for both

Trial basins and the resulting impact on SAS concentrations noted. It is worthy of note that both basins

5 and 6 were surplussing sludge during the ‘decant’ phase at this point. Following this reduction in SAS

flow rate, the SAS concentrations were approximately 1% DS at the start, and 0.5 to 0.6% DS by the

end of the event for both basins. This therefore suggested that a reduction in SAS pump flow rate

resulted in more consistent SAS concentration throughout the SAS event, as there was a lower

possibility of ‘rat-holing’ (i.e. drawing off more dilute sludge).

Effluent Quality

Daily spot and 24-hour composite samples were generally collected for the Trial and Control basins.

However, in the last month of the Trial, the frequency of sampling was reduced for a number of

determinands, as sufficient information had already been gathered.

The following sub-sections discuss the composite effluent results only, as these are representative

over a 24-hour period. Furthermore, the spot effluent results generally showed similar trends across

all determinands.

Ammonia

From the second week of the Trial, the ammonia concentrations in effluent from the Trial basins

increased significantly more than the Control. This was expected as increased amount of solids were

being surplussed from the Trial basins in order to reduce the MLSS concentrations. Consequently, the

sludge age and populations of nitrifying organisms in these basins were being reduced accordingly.

Furthermore, in March, there were two periods of increased effluent ammonia concentrations in the

Trial and Control basins. These corresponded with increased SBR feed loads.

As the Trial progressed, a reduction in ammonia load removed by basin 5 was observed, and by

February, an average load removal of approximately 300kg/d was estimated. In early March (i.e. during

the period of the sludge backlog on site and consequently, increased basin solids), a slight increase in

ammonia load removed (to approx. 420kg/d) was observed for both Trial basins. However, this reduced

towards the end of the Trial. There is therefore potential for further optimisation i.e. achieving further

reductions in OPEX, if ammonia load removal is reduced further.

Through the Trial, nitrification (i.e. conversion of ammonia to nitrate) reduced in basins 5 and 6. A

corresponding reduction in denitrification (i.e. conversion of nitrate to nitrogen gas) was also estimated

for these basins. This was expected because if lower levels of nitrate were being produced through

nitrification, there would be less nitrate available for conversion to nitrogen gas, through the process of

denitrification. On the converse, denitrification for basin 1 remained relatively stable, despite occasional

fluctuations.

BOD

The effluent BOD concentrations for basin 5 were relatively steady for the entire duration of the Trial.

However, on 29th March, there was an atypical result of 25.8mg/l. This sample showed a 77% BOD

removal (from corresponding crude BOD concentration of 113mg/l); therefore, was still within the Permit

conditions. On this day, it rained and visible solids were noted in the effluent; hence, the exceedance

may have been linked to rain which would have contributed to increased flow through the basin,

resulting in solids carryover. The corresponding soluble BOD (BODF) concentration was significantly

lower (8.6mg/l); and this suggests that this total BOD exceedance may have been linked to solids

carryover. However, given that the corresponding COD concentration for that sample was 72mg/l (within

typical range experienced on site), it is possible that this high BOD concentration may have been a






spurious result. Also, by the next day, the effluent BOD concentration for basin 5 had reduced

significantly to 15.8mg/l.

Furthermore, several BOD spikes in effluent from basin 1 (Control) were observed. These spikes also

corresponded with spikes in COD and Total Suspended Solids (TSS) concentrations. Overall in March,

a steady rise in effluent BOD concentration was observed for both basins 5 and 1. This rise

corresponded with increasing SBR feed loads.

Typically, for Trial basin 5, effluent soluble BOD (BODF) concentrations were significantly less than

total BOD concentrations (average of 3mg/l BODF vs. 10mg/l for total BOD). Similarly for basin 1

(Control), typically, BODF concentrations are significantly less than total BOD concentrations (average

of 3mg/l BODF vs. 17mg/l for total BOD). Both basins 5 and 1 generally showed a similar BODF

concentration range of 1 to 10mg/l; thereby, confirming BOD treatability at lower basin MLSS

concentrations.

No distinct difference in BOD load removal was observed between basins 5 and 1; however, in some

instances and on average, results from basin 5 showed slightly higher removal.

COD

Initially, a slight reduction in COD concentrations was observed for Trial basin 5, followed by a period

of relatively steady results. However in March, there were two periods of increased effluent COD

concentrations for the Trial and Control basins. These corresponded with increased SBR feed loads

(potentially linked to several factors including increased Primary and Secondary Thickener filtrate loads

and crude loads coming into the Works). In addition, for majority of the Trial, one of the Primary

Settlement tanks was out of service, and this could have contributed to greater loads in the SBR feed.

However, the effluent COD concentrations for basin 5 were still within the UWWTD Consent of 125mg/l.

Overall, effluent COD results for the Trial basins were typically lower than the Control; and the Trial

basins did not show spikes in COD concentrations compared to the Control. This has contributed to

alleviating major concerns over treatability of (‘hard’) COD in carbonaceous mode.

Furthermore, both basins 5 and 1 showed a similar effluent soluble COD (CODF) concentration range

of approximately 20 to 80mg/l; thereby, confirming (‘hard’) COD treatability at lower basin MLSS

concentrations.

Basins 5 and 1 (Control) showed similar COD removal throughout the Trial. COD load removal

increased from early- till about mid-March, and this corresponded with increased basin MLSS following

the sludge backlog on site, as well as increased feed loads. This increased removal was expected as

an increase in MLSS would result in more microbiological organisms being available to provide more

treatment, and increased feed COD load meant more COD was available for removal. COD load

removal then reduced, before increasing again in the last week of the Trial. This latter increase was

also attributed to increased basin MLSS and feed loads around the same time.

Total Suspended Solids

Effluent TSS concentration from basin 5 was generally steadier than the Control (basin 1). This was

attributed to there being less solids in basin 5, as well as good settlement of solids during the ‘settle’

and ‘decant’ phases.

For the Control basin, spikes in BOD, COD and TSS concentrations sometimes occurred when the

basin MLSS was greater than 4,000mg/l, during high SBR feed loads, rainfall and when SSVI was






slightly above 100ml/g. However, the Trial basins did not show such spikes. This has contributed to

alleviating concerns over ‘carbonaceous mode’ operation.

Summary: Basin 5 vs. 1

Table 5: Effluent Quality Summary – Basin 5 vs. 1

Table 5 summarises the effluent quality for basins 5 and 1, from the point at which basin 5 MLSS had

reduced to a relatively stable level (20th February) up until the end of the Trial.

Table 5: Effluent Quality Summary – Basin 5 vs. 1

Determinand

(mg/l)

95%-ile

Consent

(mg/l)

Basin 5 Basin 1

Average 95%-

ile Risk ratio 4 Average

95%-

ile Risk ratio 4

Ammonia - 32.4 48.8 N/A 16.4 32.8 N/A

BOD 25 11 23 2.2 17 59 1.4

COD 125 65 91 1.9 70 159 1.8

TSS 250 (UTL 5) 21 27 12.0 44 145 5.7

The risk ratios for basin 5 were significantly higher than those for basin 1 (Control), particularly for BOD

and TSS. Furthermore, the 95%-ile effluent results for basin 5 were all within the Permit limits; whereas,

the Control showed 95%-ile exceedance for the determinands listed above. Therefore, these results

confirm that process risk is significantly reduced at lower basin MLSS (i.e. in/approaching

‘carbonaceous mode’).

4 This is the Consent/Average ratio. Typical operational target for BOD and COD > 2 5 Upper Tier Limit (UTL) value i.e. absolute maximum

Blower Energy

After about three weeks into the Trial (from 3rd February 2017), a steady decline in blower energy for

the Trial basins was observed (Figure 9). From that point to 24th March, the reduction in total estimated

blower energy consumption was approximately 37%. Furthermore, from 3rd February, the difference

between the total energy6 for the Trial basins and that for basins 1 and 2 was approximately 1,000kWh

on average. Applying this across all 16No. basins would correspond to a potential annual aeration

energy saving of approximately £292,000.

From the first week of March, the blower energy consumption for the Trial and Control basins was seen

to increase. This was attributed to several factors including increased basin MLSS concentrations

following issues with the SAS Drum Thickeners (and resulting sludge backlog) and increased SBR feed

loads. An increase in temperature also occurred, and this resulted in increased nitrification (and






therefore, increased oxygen demand) as well as a reduction in oxygen transfer efficiency. The combined

effect of all these factors was increased blower power output in March.

Furthermore, during the last few weeks of the Trial, there were a few instances where discrepancies

between the DO concentrations of basins 5 and 6 were observed. Following temporary removal and

cleaning of the DO probes, these discrepancies seemed to be removed and the DO concentrations for

both basins returned to similar levels. These occurrences may likely have contributed to increased air

demand from the blowers during these times; and consequently, increased blower energy consumption.

Lastly, there is currently no air flow measurement in the pipes supplying air to the basins. This

information would have been useful to better understand the factors contributing to blower energy

consumption. Going forward as time progresses, by monitoring the air flow to the basins, it would enable

site to attribute the blower energy consumption to either blower or diffuser efficiency.

6 Between 28th and 30th March, a data gap in blower power data was observed. This was identified as a site-wide issue.

Due to several issues on site from 24th March, the blower energy consumption after this date has been omitted from

calculations and Figure 9.

Figure 9: Total Daily Blower Energy






Microscopy

Figure 10: Trial Basin - Example Image from Microscope (x40 Magnification)

Figure 10 is an example microscope image for basin 6. This shows good floc structure and

representative life forms in the basin.

As carbonaceous mode was achieved in both Trial basins, overall observations from microscopy were

as follows:

• Good floc formation and generally low filament density in both Trial and Control basins

• Trial basins: a greater proportion of lower life forms i.e. free-swimming flagellates, crawling

ciliates and some higher life forms i.e. stalked ciliates; thereby, indicative of a lower sludge age

• Control basin: Much lower proportions of lower life forms i.e. very few flagellates and much

greater proportions of higher life forms; thereby, indicating a higher sludge age

Odour

Throughout the Trial, no impacts on odour were observed.

Comparison of BioWin and Trial Results

Both the preliminary BioWin modelling and the Trial showed the following results for carbonaceous

mode operation:

• Effluent quality well within Permit limits (BOD, COD and TSS)

• Blower Energy reduction – 37% vs. 50% prediction by BioWin (Black, 2016)

• Potential additional aeration savings if DO set-points are halved (BioWin results)

• Continued nitrification even in ‘carbonaceous mode’

The BioWin modelling exercise suggested that it would take up to two weeks to reach the target MLSS

range. However, it took four weeks to achieve this target during the Trial, and only basin 6 reached the

target. This delay may have been contributed to by elevated starting basin MLSS concentrations (i.e.

>4,000mg/l versus the preferred site operating range of 3,000 to 3,600mg/l).

Furthermore, BioWin generally suggested that more treatment would be provided in carbonaceous

mode i.e. lower average effluent concentrations for Ammonia and BOD. However, this may be linked to

differences in sewage characteristics between North America and UK e.g. COD/BOD ratios; as well as

under-/over-exaggerated processes or assumptions within the BioWin software e.g. settling, mixing and

nitrification.






Conclusions

From the Trial, the following conclusions about carbonaceous mode operation were drawn:

1. Operation of the Liverpool WwTW SBR basins in carbonaceous mode is possible and is

capable of delivering consistently compliant effluent quality. Operational risk ratios (i.e.

Consent/Average) from the point of relatively stable MLSS in the Trial basins, were 2.2 and 12

for BOD and TSS respectively for basin 5; whereas those for basin 1 (Control) were 1.4 and

5.7. This confirmed that process risk is significantly reduced at lower basin MLSS (i.e.

in/approaching ‘carbonaceous mode’)

2. Removing surplus solids from the basins during the ‘decant’ phase is preferable due to several

benefits such as: significantly reduced sludge volumes (up to one-third volume of ‘aerate’

phase) and consequent reduced operation of secondary sludge processing assets, and OPEX

savings

3. For the ‘SAS decant’ basin configuration, the required SAS volume per cycle varied throughout

the Trial, in order to achieve (and subsequently maintain) the MLSS target of 1,500 to

2,000mg/l. The ‘optimum’ range of SAS volume for the Trial basin that achieved this target

(basin 6) was found to be 65 to 85m3 per cycle

4. Close monitoring of basin MLSS is required, particularly if the ‘SAS decant’ configuration is

adopted

5. A lower SAS pump flow rate of 50l/s resulted in more consistent sludge quality over a typical

SAS event, as this prevented “rat-holing”

6. Significant savings in blower energy consumption was achieved – average of 1,000kWh/d

savings estimated. This may have been higher if a steady basin MLSS concentration was

maintained (although this was affected by wider issues on site e.g. existing sludge treatment

capacity and resulting sludge backlog issues)

7. The Trial objectives were achieved; thereby, rendering the Trial successful

8. Following the success of this Trial, the roll-out of carbonaceous mode operation across all the

16No. SBR basins at Liverpool WwTW, has begun.

Acknowledgements

I would like to thank Jeremy B., Andrew S., and Jon H. from United Utilities, for their review and

constructive criticism while writing this paper.

References

• EnviroSim. (2016) BioWin 5.0 Software

Appendix

Current Blower Control DO profile

Time

(cycle min) 0 15 30 45 60 75 90 105 120

DO Target

(mg/l) 0 0.37 0.75 1.12 1.50 1.67 1.85 2.05 2.4


LIVERPOOL WWTW SBR CARBONACEOUS TRIAL Abstract...5 SAS during ‘aerate’ phase 6 SAS during...

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