MODELLING THE DYNAMIC RESPONSE OF A SECONDARY …

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MODELLING THE DYNAMIC RESPONSE OF A SECONDARY CLARIFIER TO A

SEA WATER INCURSION AT THE RADFORD STW

Burt, D.1, Ganeshalingam, J.1, Hammond, C.2 and Egarr, D.1 1MMI Engineering Ltd, UK, 2Hyder Consulting, UK

Corresponding Author Tel. 0117 960 2212 Email. [email protected]

Abstract

As part of a recent study of a secondary clarifier optimisation for the Radford coastal

Sewage Treatment Works (STW) in Plymouth, a Computational Fluid Dynamic (CFD)

model has been used to consider the dynamic performance of the secondary

clarifiers during storm flows with sea water incursions. The addition of seawater into

the system increases the density of the liquid phase at the influent producing an

enhanced gravity current. There is significant site evidence to suggest that a sea

water incursion degrades clarifier performance but the mechanisms for this are not

clearly understood. This behavior has not previously been investigated with a

modelling approach

The paper describes the development of a CFD model of the Radford STW final

clarifier No 4 to determine the performance under dynamic changes in influent flow

including storm influx and a 10% seawater incursion for a duration of up to 1 HRT

(Hydraulic Retention Time). Two inlet modifications are investigated; one with an

upturned bellmouth and the other with an Energy Dissipating Inlet to test whether

dispersing and mixing the load at the influent could exert some control over the

enhanced gravity current.

Keywords

Activated Sludge, Mass Flux Theory (MFT), EDI (Energy Dissipating Inlet),

Computational Fluid Dynamics (CFD)

Introduction

Radford Sewage Treatment Works (STW) is managed and operated by South West

Water Ltd and serves a population of approximately 22,000 from the Plymstock area

to the east of Plymouth. Over the next 20 years, it is anticipated that the catchment

population will grow by around 8,000 0. This will result in higher average flow and

loads for treatment. South West Water Ltd and Hyder Consulting Ltd have carried out

a review to identify the asset upgrades required to deal with future growth and

historical performance issues, particularly sea water incursion.

The source of the sea water is a result of seawater ingress in to the sewer system. This

sea water is consequently conveyed to the STW.

mailto:[email protected]




Figure 1: Aerial View of the Radford STW and low-lying coastal areas (Google

Maps, 2013)

A number of options for extending Radford STW were considered including increasing

the volume of the activated sludge aeration tanks, increasing the area of final

settlement tanks and improving the characteristics of the activated sludge. All

options involve increasing the capacity of the oxygen transfer system, remedial works

to reduce saline infiltration and measures to reduce the degree of filamentous

foaming.

There is significant site evidence to suggest that a sea water incursion degrades

clarifier performance but the mechanisms for this are not clearly understood. This

behaviour has not previously been investigated with a modelling approach.

In the initial phase of Computational Fluids Dynamics modelling, the performance of

the existing secondary clarifiers is assessed under steady state conditions to identify

potential improvements for current and future flow conditions.

The secondary clarifier design was then tested for influent flow profiles approximating

a diurnal flow cycle for a storm flow event as well as for a seawater incursion event

where an influent flow was contaminated with 10% by volume of seawater. The

performance of the secondary clarifier, measured by the effluent concentration and

bed depth, were monitored over the period of the storm and seawater incursion

event and the following hours up to 6 hydraulic detention time (HRT).

This paper mainly discusses the secondary clarifier response to a storm and sea water

incursion.

Methodology

Mass Flux Theory (MFT) is the standard and basic design approach to determine

whether a tank has sufficient surface area to settle the solids load at a given process

condition. However, Computational Fluid Dynamics modelling has significant




advantages over 1-dimensional Mass Flux Theory (MFT) by using the actual geometry

of the secondary clarifier to resolve the flow patterns within the tank which will include

non-ideal flow behaviour such as flow recirculation.

The CFD model developed for this work has been extended from the IAWQ drift flux

model for activated sludge and water mixtures in secondary clarifiers 0. The solids

settling behaviour is included using a drift flux model; hindered settling is incorporated

with the Takaćs 0 double exponential function which is an extension of the Vesilind

settling curve. Sludge mixture density follows Larsen 0, and the sludge rheology

model of Bokil & Bewtra 0 is used to describe the non-Newtonian behaviour of the

liquid-solid mixture in the settled sludge layer. The development of these models were

tested and validated against three independent sets of site data 0.

In a CFD model, the fluid dynamics within the secondary clarifier are resolved in 2D or

3D. CFD models can therefore provide contour maps of flow profiles and solids

concentration in the secondary clarifiers. The CFD model therefore allows the

calculation of the height of the sludge blanket and the effluent suspended solids (ESS)

concentration, which are both indicators of how close the tank is to failure i.e. the

point at which the blanket is spilt. Neither of these parameters can be determined

from mass flux theory.

CFD models can be used to test different internal geometry configurations such as

the stilling well design which may incorporate a McKinney baffle or Energy Diffusing

Inlets (EDIs).

Geometry

Radford STW has four secondary clarifiers 0. Of the four secondary clarifiers, the tank

with a diameter of 18 m and a side wall depth of 2.3 m was considered and

discussed in this paper. This tank is fitted with a diffuser drum of 3.5m diameter and

1.05m depth below top water (TWL). The existing tank (as built) and proposed EDI

modification, marked in dark blue, for the CFD model are presented in Figure 2. The

EDI has two rows of four ports, each spaced at uniform pitch centres (eight ports in

total).

As built Secondary Clarifier Secondary Clarifier with an EDI device

Figure 2: Radford STW Secondary clarifier geometry details with and without

proposed EDI modification.

172 mm

9000 mm

1750 mm

1050 mm

100 m

m

11.4

°

2300 m

m

250 mm 300 mm

200 mm

3800 m

m

6100 m

m

60°

175 mm

9000 mm

1800 mm

1650 mm

11.4

°

2300 m

m

6100 m

m

4 Ports 280x280mm1210 mm

4 Ports 280x280mm

900 mm




Sludge Characteristics

A series of settlement tests were carried out at Radford STW to characterize the

hindered settling behavior of the activated sludge 0. The data was analysed to

determine the SSVI3.5 value for the subsequent MFT and CFD analysis.

Figure 3(a) shows the Vesilind coefficients from the measured data. Based on these

coefficients, the sludge equates to SSVI 105 ml/g for the Pitman0 and White0

correlation.

(a) Vesilind coefficients from the measured data (b)Sludge settling correlations compared with the

experimental data.

Figure 3: Radford STW Activated Sludge characteristic

Figure 3 (b) shows a comparison of the measured hindered settling velocity with the

“standard UK” Pitman0 & White0 correlation for a SSVI3.5 value of 105 ml/g. Hence, all

the CFD and MFT calculations were undertaken using “Pitman & White” settling

characteristics to ensure that the results would be representative of the site sludge

samples.

A limitation of this work is that the effect of the local concentration of sea water is not

considered. The main influence on the settling performance of the tank is assumed to

be the change in the mixture density at the inlet due to the presence of sea water.

Results and Discussion

Influent Optimisation - Steady State Simulations

Initially, the performance of the existing secondary clarifier was assessed under steady

state flow conditions (a) with the existing internal geometry arrangement and (b) with

an EDI.

The EDI design developed by MMI comprised two rows of 4 counter current discharge

ports. This bespoke design of EDI device is discussed in detail in Reference 0. The EDI

induces a swirling (circumferential) component to the flow.

The process conditions that were assessed in this phase of work are presented in Table

1. Figure 4 presents CFD results at State Point ‘C’. Both the as built tank and the EDI

design show very similar performance. The built tank shows that there is a density

waterfall and flow re-entrainment which creates stirring. However the depth of the

y = 5.7051e-0.456x

0.01

0.1

1

10

0.0 2.0 4.0 6.0

V0, S

ett

ling

ve

locity [m

/h]

Sludge Concentration [kg/m3]

Vzs = Vo exp(-nX)

n = 0.456 m3/kgVo = 5.705 m/h

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0 1 2 3 4 5 6 7 8Se

ttlin

g V

elo

city

[m/h

]

Sludge Concentration[kg/m3]

Measured

Pitman and White




tank prevents transport of high concentrations of solids to the effluent weir. The EDI

shows improved sludge consolidation and therefore has a slightly lower sludge bed

than the as built tank.

Table 1: Influent Optimisation – Secondary clarifier performance with and

without an EDI design

Run Process

flow

condition

QEff /

tank

QRAS/

tank

Inlet

MLSS

SSVI

3.5

As built EDI design

[L/s] [L/s] [mg/l] [ml/g

]

ESS

[mg/L

]

Bed

Depth

[m]

ESS

[mg/L

]

Bed

Depth

[m]

A FFT1 40.4 24.2 3500 105 13 2.27 8 2.22

B FFT1 40.4 24.3 4000 105 11 1.98 9 1.95

C FFT1 40.4 24.3 3500 120 10 1.83 10 1.95

D FFT2 52 31.2 3500 105 10 1.85 10 2.03

E FFT2 52 31.2 3000 120 12 1.72 12 2.03

Figure 4: Contour plots showing the distribution of solids concentration through a

2D slice of the tank on a log scale (1 mg/L ≤ Conc. ≤ 10000 mg/L) for

the state point C.

0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12

ESS

[m

g/L

]

Time [Hours]

As Built

EDI Design

ESS = 10 mg/l Bed depth = 1.83m

ESS = 10 mg/l Bed depth = 1.95m

Basecase : As built Case2 : EDI design

Better consolidationof the bed

Stable ESS

Density water fall and re-entrainment

Effluent Solids Conc. [mg/L] vs. Time [Hours]




When analysed for a range of state points the EDI device was found to be marginally

more effective at high flow and solids loading. The EDI design improves the settling

performance resulting in a consolidated sludge bed.

The EDI enhances mixing in the stilling well by inducing a swirling (circumferential)

component to the flow. This usually increases mixing within the stilling well and

improves settling performance. Hence the addition of the EDI gives improvements in

effluent solids stability.

Storm Flow Event - Dynamic Simulation

The secondary clarifier with and without an EDI was assessed to determine the

performance of the secondary clarifiers for a storm flow event over a 24 hour period.

This event is considered with MLSS (3500 mg/L), SSVI (120 ml/g) and a constant RAS

flow.

Figure 5: Selected storm flow event for 24 hour period and fitted curve for CFD

analysis.

Figure 5 shows the influent flow profile representing the storm flow event. During this

storm, there is an increase in flow of factor 3 when compared to the flow rate at the

beginning of the storm flow event. This increase in flow occurs in a 14 hour period.

Figure 6 shows the effect of the dynamic flow on the effluent suspended solids

concentration where the ESS spikes up to 27mg/l and 13mg/L for the as built tank and

EDI design respectively. Therefore, in this analysis, the effect of sea water is not

considered, and the tank performance is assessed for the storm profile only. The peak

ESS concentration is approximately double for the existing design than the peak value

for the EDI design therefore confirming that for an increase in flow, the tank with an

EDI fitted operates better than the existing inlet arrangement. The results also indicate

that there is typically a delay of approximately 1 hour between the peak storm flow

entering the secondary clarifier and the response of the ESS concentration.

Figure 7 shows that the sludge blanket position rises to a minimum of 1.7m below TWL

for the as built tank and to 1.9m below TWL for the clarifier with an EDI fitted which

0

10

20

30

40

50

60

0 4 8 12 16 20 24

Flo

w [

L/s

]

Time [Hours]

Site Flow per FST, L/s Fitted CFD -Flow , L/s




confirms that the sludge bed continues to be more consolidated under storm

conditions with the EDI.

Overall, the secondary clarifier performs well under storm conditions with and without

the EDI modification. The EDI acts as a momentum dissipater, providing a degree of

protection from a typical storm event, halving the maximum ESS (13 vs 27 mg/l) and

keeping the blanket lower in the tank (1.9 vs 1.7 m below TWL).

Figure 6: Effect of storm flow on the ESS concentration for the secondary clarifier

with and without an EDI influent modification.

Figure 7: Effect of storm flow on the settle sludge bed for the secondary clarifier

with and without an EDI influent modification

Saline Seawater incursion - Dynamic Simulation

A seawater incursion event was assumed such that the influent flow is constant at

52L/s and contaminated with 10% by volume of seawater during the first 2.6 hours (1

0

10

20

30

40

50

60

0

5

10

15

20

25

30

0 4 8 12 16 20 24

Flo

w ,

[L/s

]

Effl

ue

nt

soli

ds

[mg

/L]

Time [Hours]

As Built - ESS EDI- ESS Flow [L/s]

0

10

20

30

40

50

60

0

0.5

1

1.5

2

2.5

3

0 4 8 12 16 20 24

Flo

w ,

[L/s

]

Slu

dg

e b

ed

de

pth

be

low

TW

L [m

]

Time [Hours]

As Built- SBD EDI- SBD Flow [L/s]




hydraulic residence time) of the storm flow. This event is considered with values of

MLSS of 3500 mg/L, SSVI of 120 ml/g and recycle ratio of 0.6. The secondary clarifier

with and without an EDI was tested to assess the performance of the tank during the

seawater incursion event and the following 15.6 hours (6 HRT).

Figure 8 shows that both designs discharge substantial effluent solids following the

event up to 5 x HRT) as a result of the enhanced density current. The peak effluent

discharge is about 20% (304 vs. 381 mg/l) lower for the EDI. The mechanism for the ESS

rise is due to the influent density current that lifts the sludge blanket as shown in Figure

9. Both designs took approximately 13 hours (5 x HRT) to recover to consent levels for

ESS concentration. The EDI influent modification had only a minor effect reducing

peak effluent discharge by 20% relative to the original installed equipment. Hence,

the influence of the EDI for the saline intrusion event is less significant.

Figure 8: Effect of seawater incursion on the effluent suspended solids

concentration for the secondary clarifier with and without an EDI

influent modification.

0

2.5

5

7.5

10

0

50

100

150

200

250

300

350

400

0.0 2.6 5.2 7.8 10.4 13.0 15.6Time [Hours]

Sea

wat

er

[%]

Effl

ue

nt

solid

s [m

g/L]

As built - ESS EDI - ESS Sea water [%] @ influent




Figure 9: Effect of salt incursion on the influent density current and sludge bed

within the secondary clarifier.

Figure 10: Effect of seawater incursion on the settle sludge bed for the secondary

clarifier with and without an EDI influent modification

Figure 10 shows that the sludge bed rises to a minimum of 0.65 m below TWL for the

secondary clarifier with an EDI and to 0.75m below TWL for the existing tank.

Conclusions

CFD analysis has been undertaken for the secondary clarifiers at Radford STW, for the

existing tank design and retrofitted with an EDI. The CFD model was used to calculate

Basecase : As built

T = 2.6 Hours (1HRT)No seawater present at the influent

T = 2.6 Hours (1HRT)10% (v/v) seawater present at the influent

Basecase : As built

0

2.5

5

7.5

10

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

0.0 2.6 5.2 7.8 10.4 13.0 15.6

Time [Hours]

Sea

wat

er

[%]

Slu

dge

be

d d

ep

th b

elo

w T

WL

[m]

As built - Bed depth EDI - Bed depth Sea water [%] @ influent




and compare the clarifier performance with respect to effluent solids concentrations

and sludge bed depths.

The results suggest that the EDI works well as a momentum dissipater but works less

well for a seawater incursion. The EDI therefore provides a degree of protection from

a typical storm event without sea water ingress, halving the maximum ESS and

keeping the blanket lower in the tank.

The trend information suggests that the EDI gives improved performance for both

steady state behaviour and dynamic loads for the secondary clarifier. However, the

absolute differences in effluent solids carry over are not significant for this secondary

clarifier except during the peak storm flow.

This modelling study confirms that a sea water ingress pulse can significantly degrade

final clarifier performance at a coastal STW and that it is difficult to control such

incursions with tank inlet modifications alone.




References

South West Water – Radford STW, “Supply and demand evaluation”, 12 September

2012.

Ekama, G.A., Barnard, J.L., Gunthert,F.W., Krebs,P., McCorquadale, J.A., Parker, D.S.

and Wahlberg, E.J., Secondary Settling Tanks, Theory, Modelling, Design and

Operation, International Association of Water Quality, Scientific and Technical Report

No 6, (IAWQ), 1997.

Takács, I., Patry, G.G., and Nolasco, D., “A Dynamic Model of the Clarification

Thickening Process”, Water Res, 25(10), 1991.

Larsen, P. “On the hydraulics of rectangular settling basins, experimental and

theoretical studies”, Dept of Water Resources Engineering: Lund Institute of

Technology, Lund University, 1977.

Bokil, S.D. and Bewtra, J.K. “Influence of Mechanical Blending on Aerobic Digestion of

Waste Activated Sludge”, Proc., 6th Int. IAWPRC Conf. on Water Pollution Res., Int.

Assoc. on Water Pollution and Control, London, 421-438, 1972.

Burt, D.J. ‘Improved Design of Settling Tanks using an Extended Drift Flux model”, PhD

Thesis, University of Bristol, UK, January 2010.

Pitman, A.R. “Settling Properties of Extended Aeration Sludge”, J. Wat. Pollut. Control

Fed. 52(3), 524-536,1980.

White, M.J.D., Settling of Activated Sludge, Technical Report TR11, Water Research

Centre, Stevenage, UK, 1975.

Wahlberg, E.J. and Keinath, T.M. Development of settling flux curves using SVI. J. Wat.

Pollut. Control Fed. 60 (12), 2095-2100, 1988.

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