COMMISSIOINING OF ENHANCED ENZYMIC HYDROLYSIS AT …

17th

European Biosolids and Organic Resources Conference

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COMMISSIOINING OF ENHANCED ENZYMIC HYDROLYSIS AT LANCASTER WwTW

Leach, K.1 and Edgington, R.M.2 1United Utilities PLC, 2 United Utilities,

Corresponding Author Tel. 01925 237000 Email [email protected]

Abstract

Lancaster sludge treatment centre treats the indigenous and imported sludges from the

surrounding area. In 2011, an enhanced enzymic hydrolysis plant (EEH) was commissioned in

front of the existing mesophilic digesters. The plant is designed to treat 7080tdspa and to meet

an enhanced treated quality standard.

This paper will discuss the design, commissioning and the performance of the EEH plant and the

key lessons learned.

Keywords

Enhanced Enzymic Hydrolysis, Safe Sludge Matrix, CHP, log kill, E.Coli, Digestion

Enhanced Enzymic Hydrolysis

In the UK, the main advanced digestion processes are Enhanced Enzymic Hydrolysis (EEH) and

thermal hydrolysis (TH) to produce an enhanced treated sludge. EEH, consists of Stage 1 (three

reactors in-series at 32-42OC), followed by Stage 2 (one heating reactor, and two holding tanks

all at 55OC), before feeding into the parallel operated mesophilic anaerobic digester (MADs).

This is shown in Figure 1

420C

Figure 1: Process Flow Diagram of conventional EH and EEH (Le et al 2007)

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The first EEH plant was installed at United Utilities Blackburn WwTW in 2005 and showed a

volatile solids destruction (VSd) of around 55% and excellent E.Coli destruction (Le 2006). Later

plants have been installed at Cambridge (2007), Kings Lynn (2008), Great Billing (2009) and Eign

(2009) (Bungay 2008); the same paper gives details on their design. The sites show a VSd of

about 50 to 55% (Asaadi 2008, Riches 2012). With greater VSd than conventional mesophilic

anaerobic digestion, more biogas is produced which leads to greater energy generation

potential; at Kings Lynn 0.75MWh/TDS is generated (Riches 2012).

Heating in stage 1 has always been by hot water heat exchangers. Initially, heating in stage 2

was obtained using hot water heat exchangers. However, more recent plants from Kings Lynn

onwards have used direct steam injection into the sludge to reduce the risk of viviantite scaling

(due to Iron salts being used to remove phosphorous on the treatment works).

Design Parameters of Lancaster EEH Plant

Lancaster WwTW serves a population equivalent of circa 100,000. The works is a conventional

carbonaceous activated sludge plant followed by nitrifying tricking filters. The indigenous co-

settled sludges, along with imported sludges, were processed with strain presses, thickened on

gravity belt thickeners and digested in three MADs followed by secondary digesters and

operational storage. The works is unusual in that the imported sludges are discharged at a

remote tanker terminal (2 miles away, due to poor vehicle access on narrow country lanes) and

then pumped to the works.

In 2008, design commenced on the provision of an EEH plant, prior to the MADs, to meet the

sludge quality driver for the enhanced sludge standard to meet:

• 6 log kill in E.Coli

• Less than 1000 E.Coli No/gDS

• Absence of Salmonella in 2gDS

Initially the plant was design to treat 7080tdspa (equating to 370m3/d), an increase of the then

current sludge make up of 6490tdspa. The increase was due to a growth in population in the

Lancaster catchment and in imported sludges that would be Iron dosed for P removal. This then

gave six EEH reactors each with a sludge working volume of 270m3 each. However, during the

design period, the Price Review 2009 was underway and that had an impact on the scheme, as

the provision of Iron dosing for P removal at Lancaster itself and transfer of Settle WwTW

sludges for treatment at Lancaster had to be taken into consideration. Although the Price

Review sludge figures were very provisional at circa 8300tdspa, it was agreed that the design of

the EEH plant would be flexible to take into account any future approved sludge figures. This

was simply done by increasing the height of each reactor, thus increasing their sludge working

volume to 370m3 each, giving the plant a potential maximum throughput of 492m3/d. Also,

within the layouts, allowances were made for an additional EEH reactor for future expansion.

No additional treatment processes were required at Lancaster and the final process flow

diagram for sludge treatment is shown in Figure 2.

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Figure 2: Process Flow Diagram of Lancaster Sludge Treatment Centre

Iron dosing for P removal is a future requirement for Lancaster WwTW and is already practiced

at the feeder works. Based on this, it was decided that direct steam injection would be used for

the second stage heating to reduce the risk of viviantite formation.

Details of Lancaster EEH Plant

The contract was delivered by United Utilities Process Alliance Partners South. Due to the cost of

the project, the scope book was divided into four major sub contract areas; Siloxane removal

system, EEH plant, CHP kit Framework and Steam & Hot Water Plant.

Key project deliverables:

• The EEH plant comprising of 6 No. clad & thermally insulated reactor vessels, access

platform and stairway. 1st& 2nd stage sludge heating / circulation systems and a 3rd

stage air blast cooling system.

• New 525kW CHP packaged system.

• New boiler building, two steam generating boilers and ancillary equipment

• New boiler fuel oil storage tank and oil transfer pumps

• Gas booster system complete with duty / standby gas compressors

• Modification to existing digester / EEH feed pumping system

• New pumps & modifications to existing Digester hot water pumping system pipe-

work

• New Siloxane removal plant and coalescing chimney stack.

• New instrument air compressors in kiosk.

GBT's

2 off

Primary

Digesters

3 off

Poly

Secondary

Digesters 6 off

Operating

Storage

Tanks 4 off

Sludge

holding

Wash water

Enhanced

Enzymatic

Hydrolysis

Post

Screening

MBT's

4 off

Co-settled sludge

from PST

Sludge to land

Imports

S

C R E

E

N

Pre Screens

sludge

holding Tank

2 Import

Tanks

FiltrateGas Accumulator

Returns to

head of works CHPFlare Sludge

Storage

Tank

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• New process water storage tank and booster pumps in kiosk.

• Modifications to the existing and the introduction of new HV and LV electrical

services

Figure 3: Lancaster EEH

Commissioning

The original design of the EEH plant was based upon the maximum processing capability of 492

m³/d. During early commissioning meetings, it became apparent that those quantities of sludge

would not be available, as the daily site sludge production approximately 300m3/d. it was

therefore decided that the system would be modified to accommodate a maximum of 400m³/d

to enable commissioning. Reducing the available sludge capacity would increase the retention

time and increase the risk of foaming within the reactors and the digesters. The EEH plant

throughput was reduced, by isolating reactor No.1, allowing the remaining reactors to operate

at design levels for the commissioning period. Site operations began to store sludge in the

upstream process to enable the required sludge flow rates during commissioning.

Following wet testing the reactors were drained and dried before indigenous sludge was

introduced. Sludge feed was alternated every 30 minutes between the MADS and EEH to sustain

the output of the digestors during this period.

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Imported enhanced treated sludge was not used to seed the EEH process as the project output

was not reliant on producing an enhanced sludge product immediately, this would be claimed

following the completion of the future dewatering project.

Transfer of sludge commenced on the 10th Jan 2011, and filling took approximately 5 days. The

EEH plant was then re-commissioned without heating stage 1 or stage 2. Heating was applied to

both the first stage in the form of hot water and the second stage in the form of steam on the

31st January 2011. The sludge temperatures at the two main stages of the process were now

raised to the required set points of 42oC and 55oC.

Six weeks after operating on indigenous sludge a number of the sludge feed and recirculation

pumps failed due to damage to the internal pump lobes. These pumps failed before the first

scheduled inspection date. The Ethylene Propylene Diene Monomer (EPDM) coated lobes were

replaced with a Natural Buna Rubber (NBR) lobes more suited to the fine grit particles

associated with the coastal location of the site. See Figure 4

Figure 4: Damaged EPDM Sludge Pump Lobes

EEH outlet pathogen monitoring commenced shortly after the commencement of heating to

stage 1 and stage 2. The destruction of E.Coli and Salmonella was gradual as the temperature

increased and first fill of sludge was displaced from the EEH as can be seen in Figure 5.

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The EEH plant first achieved a reduction in E.Coli of <1000 No/gDS and destruction of

Salmonella, monitored as Presence/Absence(PAB) approximately 10oC lower than the second

stage temperature setting of 55oC .

With reactor No.1 isolated and low sludge volume, foaming within the EEH reactors became an

issue due to increased retention time. Prior to the project anti-foam was dosed on the MAD

sludge feed line as it exited the GBT building, however this had been bypassed to allow for

installation of the EEH plant. Foaming within the sealed EEH reactors was not a problem;

however this caused foaming in downstream digesters, which required the reintroduction of

anti-foam dosing. Anti-foam was initially slug dosed via the EEH s, but this was later improved by

adding an automated system that dosed into the MAD sludge feed pipe work when transferring

sludge from the EEH, based on the detection of foam levels within the MAD’s.

0

500000

1000000

1500000

2000000

2500000

01/02/2011 00:00 06/02/2011 00:00 11/02/2011 00:00 16/02/2011 00:00 21/02/2011 00:00 26/02/2011 00:00

E.C

oli N

o./g

DS

0

0.2

0.4

0.6

0.8

1

1.2

Salm

on

ell

a P

AB

's

E.Coli Salmonella PABs

Figure 5: EEH E.Coli start up performance

Pathogen Performance

Performance Test 1

The EEH performance test was undertaken between the 14th February and 14th March 2011

returned good E.Coli destruction, all counts were below the scope book requirements of <1000

No/gDS, with significant numbers of zero detection. However the plant failed to regularly

achieve the required log kill stated in the project requirements of 6 log reduction see figure 6.

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Steam plant failures on a number of days caused delayed batches, but temperature

performance was maintained. Detailed investigation of the plant did not identify any further

operational issues or defective equipment, the sampling procedure was considered to be the

most likely cause of higher than expected E.Coli detections.

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

11/02/2011 16/02/2011 21/02/2011 26/02/2011 03/03/2011 08/03/2011 13/03/2011 18/03/2011 23/03/2011

E.C

oli L

og

Kill

Figure 6: Performance Test 1 E.Coli and Salmonella Data 14th

February and 14th

March

2011

Performance Test 2.

The performance test was repeated and sampling procedures were reviewed to elimate any

possible sources of cross contamination. The log kill performance proved difficult to achieve due

to a low Log of E.Coli in the EEH feed sludge. Average feed count was 6.37 Log, with a 2.38 Log

on average at the outlet of the EEH plant for the duration of performance test. With such a low

level of E.Coli in the feed sludge, near zero detection of E.Coli would be needed to achieve the

enhanced treated sludge standard of 6 log reduction. Although the E.Coli performance remained

below 1000 E.Coli No/gDS for the majority of the test, E.Coli numbers monitored at the EEH

outlet increased and were now typically around 220-250 No/g/DS compared to large numbers of

zero detection observed in the first test. There were also a number of detections above the

1000 No/gDS E.Coli target that also couldn’t be explained. Once again no evidence of defective

equipment or control issues were identified that may have lead to the decline in performance

since the early test.

Salmonella performance monitored as Presence /Absence (PABS) at the outlet of the EEH has

remained consistent with zero detection on all but 4 samples since the plant reached the

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required temperature set points. As the required testing method used is not quantitative it is

not possible to ascertain a conclusive correlation between E.Coli and Salmonella destruction.

Compliance sample point

Following the failure of the 1st and 2nd performance tests, the EEH sample point was questioned

as a possible contamination source. It was thought that the detection of E.Coli may be

attributable to contamination from stagnant sludge that remained in and around the sample

pipe work and sample point. Due to limited drainage within the vicinity of the EHH plant it was

difficult to flush the sample tap sufficiently to ensure a representative sample. A new sample tap

was installed, with a sludge drainage point that allowed flushing of the sample pipework prior to

sampling. Figure 8.

Following installation of the new sample tap and incorporation of sampling technique based on

the EA sludge sampling best practice (EA 2003) an intensive sampling programme was

undertaken. However there was no improvement in the number of E.Coli detected.

Microbiological swabs taken from the old and new sample points prior to sampling and in all

cases returned zero detection of E.Coli. Thus the initial thought of contamination with old sludge

was disproved.

Figure 8: New EEH Outlet Sample Point

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Steam Injector

Whilst undertaken a training demonstration on the removal and inspection of the steam injector

in June 2012, significant damage to the steam injector lance and lance assembly pipe work was

discovered. A rupture had perforated the end of the lance within the location of the steam

injection ports. (Figure 11). The lance is precision machined from high grade stainless steel and

inserted inline with the flow of sludge on the recycle pipework of reactor 4 .

Steam is dosed at constant pressure of 6.4bar and pasteurises the sludge while also providing

heating to stage 2 sludge as it is re-circulated through reactor 4. The steam exists radially

through a number of steam ports to ensure uniform exposure to the sludge. It is thought that

following the rupture of the lance, the reduction in back pressure would have resulted in the

steam taking a preferential route from the damaged area, preventing adequate pasteurisation,

and possibly accounting for the drop in E.Coli performance across the EEH.

Due to the absence of specific data monitoring for the volume and pressure of steam delivered,

it is impossible to ascertain when the lance failed. On installation of the replacement injection

lance, a rigorous inspection schedule will be implemented. Performance testing will also be

undertaken on the EEH plant to ascertain if the sub-optimal steam injection was accountable for

the reduced E.Coli performance during the initial testing phase.

Figure 11: Steam Injector Lance Damage

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Process Performance

Volatile Solids Destruction

Sludge feed to the EEH is on average 5.42 %DS with a range of between 2.7 – 11.7%DS based on

data collected from site. The plant achieves a Volatile solids destruction (VSd) of around 55%

which is in consistent with the hydrolytic model proposed by Le et al (2007). Figure 9 shows the

Lancaster VSd plotted against Le’s model with comparable results particularly against the other

UU EEH site at Blackburn. Lancaster has seen an increase in VSd from 41% to 55% on

commissioning of the EEH plant upstream of the existing mesophilic digesters, with associated

improved gas yield and post digestion settle-ability.

0%

10%

20%

30%

40%

50%

60%

70%

0 5 10 15 20 25

Retention time (d)

Vo

latile

so

lid r

ed

uctio

n

EH +MAD Blackburn MAD Lancaster Bromborough EH1 Asaadi et al 2011 EH1 Asaadi et al 2011 EEH Asaadi et al 100% Primary. WEF 1998 50% Primary. 50% WAS. WEF 1998 100% WAS. WEF 1998

Figure 9: Hydrologic model Le et al (2007) inclusive of Lancaster Plot

Gas Production and Mixing

Reduction in sludge throughput since completion of commissioning has lead to increased

foaming, difficulties in maintaining digester temperature and erratic gas production. Average

gas production is 276 m3/hr. Operational savings were calculated upon a throughput of 370m3/d

but average throughput since project completion is 270-290m3/d. Sludge throughput has not

increased as expected, the plant has yet to export electricity, although increasing sludge imports

from other sites is being developed through the future development of a cake re-liquidization

project.

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Methane gas composition monitored at the feed to the two CHP engines is typically 63-65%.

During commissioning the newer 525kW CHP engine was prone to Lean Oxygen (Lenox) failures

caused by poor gas mixing. Gases from the EEH plant contain predominately CO2, gas should

have mixed sufficiently with the bio gas from the digesters prior to the CHP, however due to the

Siloxane removal plant not being commissioned and thus bypassed due to safety issues at other

site, the initial gas feed to engines contained high composition of CO2 resulting in a number of

Lenox fails. The EEH gas was rerouted to improve blending with the biogas, Lenox operation has

been further improved by maintaining a more consistent mixing regime in the EEH Reactors and

widening the tolerance band of the new CHP engine inline with the existing plant 323 kW CHP

engine. The presence of a gas bag would have insured homogenous gas mixing but due to

project constraints was excluded from the scope of the project.

Sludge Dewaterability

The selected option for the forthcoming Sludge Cake Project is the installation of 3 No. Belt

Presses. A belt press trial undertaken on Lancaster EEH sludge produced cake dry solids reliably

around 25%DS, this is comparable with dry solids of 23-26% obtained at sites with EH and EEH

pre-treatment and centrifuge equipment reported by Asaadi et al 2011. Following the

completion of the EEH project operations observed improved settleability within the operational

storage tanks. Sludge is stored in open tanks prior to export, on settlement the supernatant is

decanted and returned to the head of the works. Improved settlability has seen a reduction of

£110K in transport sludge to land transport costs, a reduction which will be further increased

with the completion of the sludge cake project.

Current Performance

The plant has not been able to match the earlier E.Coli destruction witnessed during the initial

performance test. The identification of the steam injector issue discussed in the paper is one

explanation for the deterioration in performance. While the reduction in sludge throughput

shouldn’t had resulted in reduced E.Coli performance it was significantly different to the flow

rates undertaken during the initial performance trial. Following the commissioning period,

sludge throughput reduced from 400m3/day to 270-290m3/day thus increasing retention time

within the reactors. The plant was unable to sustain the required throughput of sludge, which

resulted in the EEH plant holding batches and failing to feed the digestors for a number of days.

This issue is further compounded by the plant only taking sludge imports on week days.

Consistent running of the EEH plant although at lower throughput provides a steady feed rate

to the digesters resulting in a more consistent gas supply and mixing of bio-gas for the CHP

engines. Operational changes to sludge throughput are now limited to ±10% flow change.

Conclusion

• The plant achieves the enhanced sludge standard by meeting the MAC <1000 E.Coli

No/gDS , due to the low E.Coli in the feed sludge being below the guidance values the

log kill is not considered for compliance. Following completion of the sludge cake

project, the end product will be re-validated as an enhanced product.

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• Volatile solids reduction of 55% is achieved, resulting in improved gas production,

improved post digestion settleability and is consisted with previously reported values.

• Cake solids of 25% from belt presses are comparable with those attained from other EH

and EEH sites using centrifuge technology. Assadi et al 2011

• Investigations and monitoring are ongoing to identify the cause of the damage to the

steam injector and the potential impact this had on E.Coli destruction across the EEH.

• Increased sludge imports and inclusion of Ferric dosing on site will increase processed

sludge volume to maximise the efficiency of the EEH plant and realise the full electricity

export potential of the new 525kw CHP engine.

Acknowledgements

The author would like to thank the site team at Lancaster WwTW and the project team for there

assistance, time and knowledge that was invaluable in the production of this paper.

References

Asaadi, M. (2008). Review of the Performance of an Advanced Digestion. Presented at 13th

European Biosolids and Organic Resources Conference. Aqua Enviro, Leeds.

Bungay, S. and Abdelwahab, M. (2008). Monsal Enzymic Hydrolysis – New Developments and

Lesson Learnt. Presented at 13th

European Biosolids and Organic Resources Conference. Aqua

Enviro, Leeds.

Le,S. et al (2006). Enzymic Hydrolysis Technology Demonstration – Production of Enhanced

Treated Biosolids for Agricultural Recycling. . Presented at 11th

European Biosolids and Organic

Resources Conference. Aqua Enviro, Leeds.

Riches, S. et al (2011) Advanced Digestion in Anglian Water – Summary of AMP4 Delivery

Experience and AMP5 Delivery Plan. Presented at 16th

European Biosolids and Organic Resources

Conference. Aqua Enviro, Leeds.

Environment Agency (2003). The Microbiology of Sewage Sludge (2003) - Part 2 - Practices and

procedures for sampling and sample preparation. Bristol

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