mixer calculation.pdf

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2003 Residuals and Biosolids Baltimore, Maryland USA February 19-22, 2002 THICK SLUDGE DIGESTION – RESEARCH, DESIGN AND VALIDATION OF KEY PROCESS UNIT OPERATIONS Aidan Cumiskey Mick Dawson , Martin Tillotson* Monsal, BHR Group Limited, * Yorkshire Water Tel: ++ 44 (0)1623 429500; Fax: ++ 44 (0)1623 429505; email: [email protected]; Web: www.monsal.com ABSTRACT Advances in the design of anaerobic digesters have seen a trend towards digestion of thicker sludges in the US, UK and elsewhere. This includes a number of new facilities using bolt on technologies to mesophilic anaerobic digestion such as Cambi Thermal Hydrolysis, Pre- pasteurisation and Biological Hydrolysis. Cambi Thermal hydrolysis utilises thickened sludge and the digestion process is fed at high DS%, typically 12-14%DS thereby intensifying the digestion process and reducing reactor volumes. There is limited experience in the design of digestion systems for such applications including heating, mixing and pumping. As the trend continues towards advanced digestion technology, a greater emphasis will be placed on these basic unit process operations. Monsal have conducted fundamental research into digester heating and mixing technology in collaboration with BHR and Yorkshire water. A number of plants have been designed using design tools generated from the research. This includes the digester mixing systems for a major sludge processing facility in Scotland, which is an advanced digestion facility employing Cambi Thermal Hydrolysis followed by mesophilic anaerobic digestion. This advanced facility treats a design EP of 562,000, processing 16,000 tds/year of hydrolysed sludge utilising two 4000 m 3 digesters. This paper looks at the design of the mixing technology for thick feed sludges for the full scale plant. The background research is examined and reviewed. The conceptual design is examined and reported. The full scale design and testing is also reported and examined being compared with the design stage. The paper looks at the use of fundamental sludge rheology to design effective systems for thick sludge digestion and reports on the effects of thermal hydrolysis pre-treatment. KEY WORDS Thick sludges, Rheology, Biosolids, Monsal, Digestion technology, Mixing, Hydrolysis, Advanced Digestion

Transcript of mixer calculation.pdf

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2003 Residuals and BiosolidsBaltimore, Maryland USA

February 19-22, 2002

THICK SLUDGE DIGESTION – RESEARCH, DESIGN AND VALIDATION OFKEY PROCESS UNIT OPERATIONS

Aidan Cumiskey† Mick Dawson‡, Martin Tillotson*† Monsal, ‡ BHR Group Limited, * Yorkshire Water

Tel: ++ 44 (0)1623 429500; Fax: ++ 44 (0)1623 429505; email:[email protected]; Web: www.monsal.com

ABSTRACTAdvances in the design of anaerobic digesters have seen a trend towards digestion of thickersludges in the US, UK and elsewhere. This includes a number of new facilities using bolt ontechnologies to mesophilic anaerobic digestion such as Cambi Thermal Hydrolysis, Pre-pasteurisation and Biological Hydrolysis. Cambi Thermal hydrolysis utilises thickenedsludge and the digestion process is fed at high DS%, typically 12-14%DS therebyintensifying the digestion process and reducing reactor volumes.

There is limited experience in the design of digestion systems for such applications includingheating, mixing and pumping. As the trend continues towards advanced digestiontechnology, a greater emphasis will be placed on these basic unit process operations. Monsalhave conducted fundamental research into digester heating and mixing technology incollaboration with BHR and Yorkshire water. A number of plants have been designed usingdesign tools generated from the research. This includes the digester mixing systems for amajor sludge processing facility in Scotland, which is an advanced digestion facilityemploying Cambi Thermal Hydrolysis followed by mesophilic anaerobic digestion. Thisadvanced facility treats a design EP of 562,000, processing 16,000 tds/year of hydrolysedsludge utilising two 4000 m3 digesters.

This paper looks at the design of the mixing technology for thick feed sludges for the fullscale plant. The background research is examined and reviewed. The conceptual design isexamined and reported. The full scale design and testing is also reported and examined beingcompared with the design stage. The paper looks at the use of fundamental sludge rheologyto design effective systems for thick sludge digestion and reports on the effects of thermalhydrolysis pre-treatment.

KEY WORDS

Thick sludges, Rheology, Biosolids, Monsal, Digestion technology, Mixing, Hydrolysis,Advanced Digestion

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INTRODUCTION

Digester mixing is a key unit process operation that has come under scrutiny in recent yearsas the nature of the anaerobic digestion process changes to meet emerging industry trends.Gas mixing is still the most widely accepted form of mixing of digesters in the UK. There arewide variations in the design of gas mixing systems, with both confined and unconfinedsystems in operation. However, it has generally been accepted that traditional forms of gasmixing systems can be energy intensive. One of the key benefits of gas mixing systems hasalways been the absence of moving parts inside the digester.

In recent years there has been a proliferation of large blade impeller based systems installedin new digesters along with other forms of mechanical mixers such as the draft tube type.One of the drivers for mechanical based mixing systems has been improved energy efficiencycompared with traditional gas mixing designs. With the improvements in sludge screeningson the newer sludge plants, it is anticipated that previous operational problems such asragging will not occur, however it is still early days in the UK to get reliable, long termexperience.

Design of many of the gas systems were based on empirical correlations or rules of thumbusing:

Power input per unit volume (Wm-3)Gas flowrate per unit volume of sludge (m3h-1m-3)Gas flowrate per unit area of digester (m3h-1m-2)

This design methodology did not take into account the sludge characteristics (DS%,rheology) or the dimensioning of the digester (aspect ratio, floor slope). In particular therewas little information available on mixing systems for thicker sludges, a trend which hadprimarily started from 1995 onwards as part of the second UK water industry investmentcycle with a move from traditional sludge feed thickness to digesters of 4.0 DS% towards6.0% and beyond.

In addition to thicker sludges becoming the norm, a number of proprietary processes becameavailable on the market. These include Cambi Thermal hydrolysis, pre-pasteurisation andbiological hydrolysis systems. Many of these processes seek to optimise the digestion processand utilise thicker feed sludges, normally above 6.0% DS. The nature of processes alsoresults in significant changes to sludge rheology and these must be taken into account toensure correct design and operation of the digestion process. These greater demands on themixing technology have largely been un-quantified and fundamental work is required toensure that the implementation new processes are not constrained by the prevailing unitprocesses of pumping, heating and mixing.

A collaborative research project was initiated between BHR Group Limited, Yorkshire WaterServices and Monsal aimed to characterise digested sludge rheology and investigate the effectof sludge rheology, digester geometry and mixing system design on mixing performance atboth laboratory and pilot scales. A primary driver of this work was to build knowledge on therheology of thick sludges now becoming more prevalent in the UK and elsewhere. A numberof different mixing systems were investigated which included impeller mixing, jet mixingand continuous unconfined gas mixing and sequential unconfined gas mixing systems. Some

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of the findings from the rheology survey and laboratory scale work have been published1,2,but several of the key findings and developments are reviewed for the purposes of this paper.On the basis of this research work, a novel method of digester gas mixing was developed –the Monsal SGM system which is particularly suited to thick sludge digestion.

The Monsal SGM system differs from conventional gas mixing systems. It injects gasthrough individual nozzles to provide high localised mixing energies. The sequencing orderand duration take into account the digester parameters including the feed positions, feedduration, sludge recirculation position and outlet positions. This integrated approach to gasmixing design now means that superior gas mixing systems can compete in terms of energyefficiency with the mechanical based systems.

REVIEW OF RESEARCH

There were a number of key findings from the research:

Thick Sludge Rheology

The effect of digesting high dry solids sludges was investigated in this work. The objectivesof this work was to develop a better understanding of the rheological properties of sewagesludge and provide good quality data which could be used for further work. The surveyeddigested sludges ranged from 2.5 to 5% DS. To provide thicker sludge samples a 10%DSsludge was formed by evaporation in an oven at 35°C. Three sludges (2.5% DS, 5% DS and10% DS) were chosen as representative of the surveyed range of sludge rheologies and areshown in Figure 1.

It can be seen that the increase in apparent viscosity between 2.5% and 5% DS is an order ofmagnitude with a further order of magnitude increase between 5% and 10%. When designingeffective mixers, the ability to predict the influence of viscosity becomes more important inthe thicker sludge bands (5-10 DS%) because of the more arduous mixing duty.

Predicting Sludge Viscosity

Understanding the rheology of sludge to be mixed is essential and a key goal of the researchwas to develop better predictive methods for estimating sludge viscosity.

The accepted UK industry standard prediction method is described in the Water ResearchCentre (WRc) TR185 report4. Figure 2 shows an example of a comparison between themeasured rheology of a digested sludge and that predicted from TR185, in this case theagreement between measured and predicted viscosity is poor. Generally, the TR185predictive method is not very accurate for thicker digested sludges and a different model wasadopted in this work. Rheological properties were fitted using the Herschel-Bulkley model1

and a modified predictive correlation arrived at from digested sludges.

As can be seen, the actual data for the sludge is considerably different to that predicted byTR185 whereas the Herschel-Bulkley model closely matches the experimental data.

Effect of Gas Mixing System Configuration

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The performance of two unconfined gas mixing methods (sequential and simultaneous), twoliquid jet orientations, and an impeller system were investigated and compared in terms ofblend time and active volume. The overall results are presented elsewhere1,2; only the gasmixing systems will be discussed here. Two gas mixing configurations were investigated asshown in Figure 3 below.

Sparger A provides a central core of diffusers which operate continuously. This configurationis similar to the standard adopted by a number of the UK water companies. The continuousgas flows simultaneously through all the nozzles generating an axial flow mixing pattern.

Sparger B provides two cores, an outer and inner. However the gas flow is injectedindividually through each nozzle for a period of time then sequenced in a pre-determinedway.

The results from research were as follows:

• At equal net power inputs, with well spaced diffusers, sequential gas mixing achievesgreater active volume and shorter blend times than a continuous system.

• The superiority of sequential gas mixing increases as the sludge DS% and henceviscosity increases.

• Nozzle sequence, gassing duration per nozzle and sequence integration with feedcycle are important.

Case Study : Thick Sludge Digestion

The plant is an advanced digestion facility employing Cambi Thermal Hydrolysis followedby mesophilic anaerobic digestion. This advanced facility treats a design PE of 562,000,processing 16,000 tds/year of hydrolysed sludge utilising two 4000 m3 digesters. The projectis a Private Finance Initiative (PFI) and will be operated by a consortium for 25 years. Energycosts therefore become more significant.

The sludge centre is a regional plant processing a number of sludges from satellite works,imported sludges and the indigenous sludge produced at the new works on site (table 1).

The contractual mixing system performance criteria were:

1. Feed sludge dispersal within 120 minutes2. Active volume > 90%3. Less than 5% feed volume short circuiting

The design feed sludge is thermally hydrolysed and fed to the digesters at 37-40 oC with 10-12DS%.

Thermal hydrolysis is a pre-treatment process heating raw sludge to approximately 165 oCand 6 bar pressure prior to digestion. The high temperatures and pressure result in cell lysis.The resultant hydrolysed sludge is fed to the digester at much higher DS%, typically 12%resultant in high organic loading (see table 2). The digesters on this plant are of a moderndesign with a good aspect ratio (1: 1) for mixing (see figure 8). This combination of high feed

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solids coupled with a need to minimise power use led to the client selecting the Monsalsequential gas mixing system which could be designed using the available rheologicalinformation from similar plants.

Hydrolysis changes the properties of sewage sludge. Rheology data provided by the maincontractor is shown in Figure 5. These parameters were input into the mixing model derivedin the stage one research. In general the hydrolysis process changes the rheological propertiesof sludge, the resultant effect of hydrolysis is to effectively make the sludge behave as if itwere thinner than its equivalent non-hydrolysed counterpart. This is shown in Figure 5 wherea 12% hydrolysed sludge is compared with two digested sludge samples at 7-8%DS. It can beseen that the rheological properties are similar. This approach has allowed an optimisedsolution to be designed to meet the client’s needs on this site, in particular with achieving thedesired active volume (>90%) at low mixing energies.

During the design, as in the other case studies, an integrated energy input approach wasadopted with particular attention paid to dispersing this thick feed sludge. Hydrolysed sludgeenters the digester at 40°C and there is considerable danger of an 'inactive zone' developingas the hot feed accumulates on the surface. This problem also occurs in conventionaldigesters and has been shown to result in a high acid concentration zone in the digester andserious foaming and other operability problems including untreated sludge short-circuiting tothe digester outlet.

The design information for the mixing system is presented in table 3

DESIGN

Application of the research has been used in the following way:

• Selection as sequential mixing as preferred option for thicker sludges• Use of BHR Group’s Herschel-Buckley model to more accurately predict sludge

rheology• Use of predictive models for active volume and blend time• Use of flow visualisation techniques to improve nozzle positioning

The approach that the partners have taken is to produce a mixer-sizing model thatincorporates rheological data, either taken from measurement or predicted using the yieldstress prediction model produced within this project. Both approaches have been used in thecase studies that follow.

The design of the mixing systems used an integrated approach taking into account the feedpositions, outlet positions and sludge re-circulation lines.

Sequential Gas Mixing is carried out using high efficiency sliding vane compressors and gasis injected into the digester via the use of suitable gas solenoids. Control of sequencing is viaa PLC and both cycle times and duration of opening can be altered. This allows moreflexibility to fine tune the system during plant commissioning.

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TESTING

Tracer studies have been used in the water industry to determine the actively mixed volumesand flow patterns within operational sewage sludge digesters. In addition, tracer tests areused to demonstrate effective performance of newly commissioned digester mixing systems.

The mixing performance of the digester was tested during November 2001.

The digester tested is cylindrical in shape and constructed with a sloping base. The installedworking volume of the digester is reported to be 4000m3. The digester feeding system is“pump-in, overflow-out”, with a single raw sludge entry point near the base and a singledigested sludge over flow pipe located within a concrete chamber close to the digester roof.The digester feed pump is reported to run in an automatic sequence of 15 minutes ON – 15minutes OFF.

The main objectives of the work were as follows:

! To conduct a tracer test on one of the two live digesters at Nigg WWTP, under normaloperating conditions.

! To determine the blend time within the digester! To determine the Residence Time Distribution (RTD) within the digester so that the

active volume, minimum retention time and short-circuiting can be derived.

RTD and Flow Pattern Characterisation

Mixing vessels usually exhibit a flow pattern that lies somewhere between a completelyback-mixed flow pattern and a plug flow pattern. A completely back-mixed system wouldexhibit an RTD curve as shown in Figure 6 featuring exponential decay of the tracer pulseconcentration at the outlet. An ideal plug flow system is one where each element of thevessel contents would have the same residence time. The RTD curve for a plug flow systemwith some dispersion is also shown in Figure 6. Sewage sludge digesters typically exhibit afully back-mixed RTD, ensuring contact between feed sludge and the active biomass.

Results

The results are summarised in table 4.

Based on the initial 12-hour mixing test period the results showed that the actively mixedvolume of the digester was completely mixed within 75 minutes. The tracer signal remainedrelatively flat after the initial mixing and did not indicate any pockets of lithium solution thatwere subsequently reintroduced back into the active zone, while the mixing test was running.

Based on the data gained from the remaining 36-day washout curve, the actively mixed zonefor the NIGG Digester consisted of approximately 93% of the total digester capacity. Hence,the percentage dead volume in the digester was approximately 7%.

The calculated degree of short-circuiting was 5.8% of the average feed volume.

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The nominal retention time of the digester, at an average feed volume of 168 m3/day was 23.8days. The calculated mean retention time from the linear regression of the washout curvewas 23.5 days.

Mixing Test

The results of the initial 12-hour mixing test period can be seen in Figure 7. The test showedthe following:

Fully mixed conditions achieved at: 75 minutesMean concentration of lithium between 75 and 720 minutes: 3.02 mg/l

The fully mixed blend time of 75 minutes was based on the time after which all the correcteddata was within the limits of +/- 5% of the mean concentration.

Sludge Feed Rate Data

The sludge feed profile can be seen in Figure 7 over the 36-day test period (15th November –21st December 2001).

In order to calculate the hydraulic performance of the digester in terms of active volume andshort-circuiting, Levenspiel’s model (Reference 7) was applied to the experimental data.Levenspiel’s model assumes a perfectly mixed zone, with a dead zone attached and a bypass.One of the requirements of the model is that a constant or mean feed rate is used to achievemeaningful results.

Due to the nature of commissioning there was some variability in the feed rate over the 36-day test period.

The mean sludge feed rate over the whole of the test period was calculated to be 168m3/dwhich, is superimposed onto the chart in Figure 8.

The results for the 36-day washout curve are presented in Figure 9. Linear regressionconducted on the results can be seen in Figure 10. It can be seen that the best-fit line to theanalytical data is very good with and R2 value (correlation coefficient) of 0.9383 (the closerthis value is to 1 the better the fit is). Figure 10 gives the values of the y intercept and thegradient of the curve. This data can then be applied to the Levenspiel model.

The performance criteria for the mixing test states that the mean sludge retention exceedstwelve days, that the active volume is a minimum of 90% and that short-circuiting is less than5%. The results showed that 92.8% of the digester volume was active and the mean sludgeretention time was 23.5 days. The short-circuit volume was calculated to be 5.8% of theaverage daily feed volume. This exceeded the limit by 0.8 %.

Discussion of results

The full scale system as designed meets the performance tests required under the contract.

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The use of fundamental rheology has allowed for a more accurate design of a the mixingsystem for thick sludges.This has been validated by full scale performance testing.

A review of the mixing system design is presented in table 3. This demonstrates very lowspecific mixing energies of 1.49 W/ m3 with a daily power consumption of 90.2 Wh/ m3/day.This is very favourable compared with the stated power consumption of other mechanicalsystems5 ; draft tube 2 – 3.5 W/ m3 and impellers about 2 W/ m3.

The system is currently run 24 hours a day. Now that the dispersion information is availabletaking the more conservative dispersion time of 90 minutes, it is possible to optimise the runtimes further to reduce mixing energies.

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CONCLUSION

With increasing pressures on the sludge digestion Energy efficient gas mixing systems cannow be designed which can compete with the best mechanical systems in terms of energyconsumption. The work to date by the research partners has shown this and they have beenemployed at a number of sites across the UK.

• Fundamental research has improved our understanding of sewage sludge rheology

• Sequential gas mixing is more effective at mixing thick sludges that a continuoussystems at the same net energy inputs

• Design guidelines have been developed for mixing thick sludges in digesters

• Effective gas mixing systems can be designed with specific mixing energies of 1- 2W/ m3.

• The Monsal SGM system compares favourably with mechanical systems with specificmixing energies in the 40 –90 Wh/ m3/day.

REFERENCES

1. DAWSON M., CHRISTODOULIDES, J., FAWCETT, N. AND BRADE, C. “AComparison of Mixing Systems in a Model Anaerobic Digester” 5th EuropeanBiosolids and Organic Residuals Conference, Aqua-Enviro, Wakefield, UK. 19-22Nov. 2000

2. BARKER, J., DAWSON, M. “Digester Mixing: Theory and Practice” 3rd EuropeanBiosolids and Organic Residuals Conference, Aqua-Enviro, Wakefield, UK. 16-18Nov. 1998

3. WHORLOW, R.W., Rheological Techniques, Ellis Horwood Ltd., 1980

4. MAY, P.” Renewal of Mogden STW digester heating and mixing systems” SludgeDigester Mixing and Heating systems, Cranfield, UK. 22 May 2001

5. LAHNER E, WERNER U. “ Draft tube sludge mixer – a unique solution for theprocess of digestion” Sludge Digester Mixing and Heating systems, Cranfield, UK.22 May 2001

6. FROST, R. C. “How to Design Sewage Sludge Pumping Systems” Technical ReportTR185, Water Research Centre, 1983

7. Levenspiel, O., “Chemical Reaction Engineering”, Wiley International Ed., 1972.

ACKNOWLEDGEMENTS

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The authors would like to thank EarthTech Engineering, in particular Martin Jolly and SteveWright for their help in preparing this paper.

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TABLES AND FIGURES

Table 1 – Raw sludge make up to sludge processing plant

Catchment Load (TDS per day) Sludge TypeNigg STW 18.9 50% high rate PSTs (lamella)

50% BAFF (non- nitrifying)Persley STW 2.75 Non nitrifying ASP without

primary settlementFraserburgh STW 2.75 Primary + ASP from fish

processing (non- nitrifying)Peterhead STW 6.55 Primary + ASP from fish

processing (non- nitrifying)Imported Cake 1.37 Municipal

Imported Liquid 6.03 Municipal

3rd party imports 5.5 Industrial from fishprocessing

TOTALS 43.85

Table 2 – Design conditions for digesters at Aberdeen

Sludge Type Primary + SASDigester temperature 35 oCRaw sludge feed 365 m3/dayDigester Design HRT 18 days @10% feedTS raw sludge 10-12 %VS raw sludge 75 %Digester organic loading 4.1 kgVS/m3.dayTS digested 7-8%

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Table 3 - Operational data of Monsal SGM System

AberdeenDigester volume 4000 m3

Immersion depth 17 m

Compressor rating 216 m3/h

Adiabatic Compressionpower

6.97 kW

Isothermal expansion power 5.96 kWPower consumption duringgas mixing

1.49 W/ m3

Absorbed power onCompressor Motor

15.1 kW

Energy efficiency of mixingsystem

39%

Operational time of gasmixing

24 h

Daily power consumption 362.4 kWh/dDaily power consumption perdigester volume

90.6 Wh/ m3/day

Table 4 - Summary of mixing test results

Days (1-36)value of C0 used c0 = 3.09

average V, m3 4000

average Q, m3/day 168M (from line fitted to wsht data) -0.042606

k (from line fitted to wsht data) -0.045813

R2 0.9383VA m3 3712.8

Q1 m3/day 158.2

% flow short-circuited 5.8

% available volume used 92.8MRT1 = VA/q1= -1/m 23.5

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MRT (ideal) = V/Q 23.8

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Figure 1: Range of apparent sludge viscosities found from site survey

Figure 2: Log-log plot showing comparison of apparent viscosity - TR185 predicted andmeasured for a 4% DS digested sludge

0.01

0.1

1

10

100

0.01 0.1 1 10 100

Shear Rate (s-1)

App

aren

t Vis

cosi

ty (P

a.s)

2.5% DS

5% DS

10% DS

Increasing DS

0.01

0.1

1

Vis

cosi

ty (N

/m2)

10 100 Shear rate (s-1)

Measured

TR185 median

TR185 upper

TR185 lower

Herschel-Bulkley

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Figure 3: Different sparger arrangements used at pilot scale

Figure 4: Section of the digesters at Aberdeen

xx

x

1

23

4

5

6 7

8

(b) Sparger B

9

10

11

12

x

(a) Sparger A

Inlet position

Inlet position

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Figure 5: Rheology data for a 12%DS hydrolysed sludge compared with non hydrolysedsludge

Figure 6: RTD curve for a fully back mixed system

Rheogram to compare Hydrolysed sludge with digested sludges of 7 to 8 %DS

0.1

1

10

100

0.01 0.1 1 10 100Shear rate (s-1)

App

aren

t vis

cosi

ty (P

as)

Hydrolysed

8% DS digested sludge

7.6% DS digested sludge

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Figure 7: Nigg Digester: corrected data for the initial 12 hour period

NIGG WWTP - 15/16 Nov 01Digester Lithium tracer test

0

1

2

3

4

5

6

0 100 200 300 400 500 600 700

Time (minutes)

Lith

ium

Con

cent

ratio

n (m

g/l)

Test Samples Mean Concentration

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Figure 8: Sludge feed rate over 36-day test period with mean sludge feed rate

Figure 9: Lithium tracer washout curve for the Nigg Digester over 36-day test

0

50

100

150

200

250

0 5 10 15 20 25 30 35

TIME (DAYS)

SLU

DG

E FE

ED R

ATE

(m3/

d)

ACTUAL

MEAN

168 m3/d

y = 2.9538e-0.0426x

R2 = 0.9383

0.0

0.5

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1.5

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2.5

3.0

3.5

0 5 10 15 20 25 30 35 40Time (days)

Con

cent

ratio

n (m

g/l)

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Figure 10: Washout curve – Linear regression on Nigg Digester

y = -0.0426x - 0.0412R2 = 0.9383

-1.80

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.000 5 10 15 20 25 30 35 40

Time (days)

ln (C

/Co)

ln C/CoLinear (ln C/Co)