zeta potential monitoring for optimised doc removal and...

European Regional Development FundWATER. VANDAAG EN MORGEN

Zeta Potential Monitoring for Optimised DOC removal and Process Control

David Metcalfe – South West Water,

U.K.

DOC2C’s Workshop – Belgium, Oct ‘17


Overview

• DOC

• Coagulation control

• Zeta potential for water treatment

process optimisation

• Manual zeta potential trial / results

• Automated zeta potential

measurement trial

• What now?


DOC at South West Water

• Sources = 95% surface waters,

typically soft water

• Flashy rivers to eutrophic algal

dominated reservoirs

• Increasing DOC concentration trend +

variability

• More stringent requirements for

coagulation to remain optimised at all

times

• Effects on WT processes, GAC,

Disinfection by-products, regrowth

etc.

0

1

2

3

4

5

6

7

2001 2003 2006 2009 2011

TO

C (

mg

/l)

Year

Yearly Average TOC (mg/l)

TOC (mg/l)


Coagulation

• Particles / colliods – negatively charged in water

• Addition of coagulant reduces negative particle charge

• Reduced electrostatic repulsion between particles = coagulation

• Formation of hydroxide “flocs” which can be settled / floated, removal

of dissolved organic carbon etc.

• Optimised dosing critical to downstream process performance and

treated water quality


Coagulation Control at SWW

• Coagulant is expensive both to

purchase and dispose of

• Coagulant dose is currently set by:

• Operator experience

• Jar testing

• Raw colour/turbidity algorithms –

automated coagulant control

(ACC)

• Typically employ a safety factor!


ACC - Other Options

• Feed back trim e.g. UV254nm post filter

• Fluorescence

• UV absorbance scanning + machine learning

• Streaming Current

• Zeta potential


ACC Wish List

• Indicative of current conditions

• Reflective of the actual WTWs process chemistry / hydrodynamics

• Rapid response (particularly for river sources)

• Water quality and process to remain optimised at all times

• Efficient chemical use – reduce overdosing of chemical, < sludge

formation etc.

• Not adversely affected by pH changes

• Affordable, reliable, manageable maintenance, understandable output


Zeta Potential Measurement

• Complex technique however, basic

output

• Laser beam frequency shift caused by

electrophoretic mobility of particles /

colliods

• In a nutshell - particles move to the

electrode of opposite charge at varying

speed determined by magnitude of

charge


Zeta Potential in Water Treatment

• Rapid measurement – 5 mins

• ZP - analogous to surface charge

• Coagulated water ZP near 0mV = good

chemistry. Goldilocks zone -10mV to +5mV

• Research shows this range provides good:

• DOC removal

• Turbidity removal

• WTWs process

Malvern PANalytical - Zetasizer


Initial Zeta Potential Testing

• Manual bench top instrument

• Test whether previous research could be applied our situation

• Establish benefits / drawbacks and opportunities

• Determine whether ZP could help achieve optimal DOC removal and

process performance

• Cost / chemical savings?


Zeta vs. DOC

0

1

2

3

4

5

6

7

-30,00 -25,00 -20,00 -15,00 -10,00 -5,00 0,00 5,00 10,00

DO

C (

mg

/l)

Zeta Potential (mV)

prewley DOC

Northcombe DOC

Drift Al DOC

Drift Fe DOC

Drift PACl Jar tests 22Nov16

Northcombe 211216

Prewley 211216

Allers 090217

Typical optimal zeta potential range


Coagulation Dose / pH Effects

• ZP typically linear to coag. dose

• pH has a significant effect on ZP

• lower coag. pH = increased ZP

• = At lower coag, pH a lower dose

can be applied

• Significant potential savings for soft

waters

• This pH effect is not factored into

other feed forward coagulant

control systems


Companywide Assessment

-20

-15

-10

-5

0

5

10

15

20

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Alle

rs

Me

an

Ze

ta P

ote

nti

al

(mV

)

Sample Point

Optimum range for good destabilisation

Overdose

Underdose

Potential operational setpoint


Zeta Potential Stability

• Coagulated samples ZP is not

stable for extended periods

• Become more negative

• Must measuredonsite for accuracy

• Sample containers could potentially

affect ZP

-4,26

-4,79

-5,6

-5,2-5,41

-6,91

-6,29

-6,77

-8

-7,5

-7

-6,5

-6

-5,5

-5

-4,5

-4

0 1 2 3 4 5

Time (Hrs)

Ze

taP

Ote

nti

al (m

V)


Long Term WTWs Trial

• ZP used to tune the coagulant dose at a reservoir site

• Improved process control

• Reduction of coagulation pH – lime system turned off

• Saving of ~30% coagulant

• Instrument payback < 1 year

• Operators engagement


Troubleshooting


Online Zeta Potential

• Malvern Zeta WT – automated ZP

• SWW commissioning – 1 month ago

• Sample presentation issues - improving

• Some hurdles still to overcome...

• Closed loop control or other options?


What Now?

• Reliability and validation of measurement

• Application to different waters / WTWs processes

• Determine optimal control philosophy and ZP setpoints

• Full automation, trim or inform manual dose changes


Thanks..... Questions?

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zeta potential monitoring for optimised doc removal and...

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