Sustainable Municipal Drinking Water Treatment Coagulation and Rapid Mix Basis for a rational design?

Aggregation Intermolecular bonding Mixing Energy Dissipation Diffusion Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering

Sedimentation of Small Particles?

 How could we increase the sedimentation rate of small particles?

Increase d (stick particles together)

V t

 2

d g

18  

p

 

w



w

Increase g (centrifuge) Increase density difference (dissolved air flotation) Decrease viscosity (increase temperature)

Definitions

 Coagulation: The process of adding a sticky solid phase material (adhesive nanoglobs) that attaches to the colloids so they can attach to each other (the topic of these notes)  Flocculation: The process of producing collisions between particles to create flocs (aggregates) (next set of notes)

extra

Stages of colloid removal

Coagulation Flocculation Floc Blanket Sedimentation Filtration

Nanoglob deposition Collisions More Collisions Gravity!

Last chance!

The Conventional Coagulation Myth*

 “The purpose of addition of coagulant chemicals is to neutralize the negative charges on the colloidal particles to prevent those particles from repelling each other.  Coagulants due to their positive charge attract negatively charged particles in the water.” http://www.thewatertreatments.com/wastewater-sewage treatment/coagulation-flocculation-process/ * This myth has been the dominant “theory” of coagulation for close to 100 years

Sante Mattson

J. Phys. Chem.

,

1928

,

32

(10), pp 1532–1552,

DOI:

10.1021/j150292a011 Publication Date: January 1927 extra

Electrostatic hypothesis inconsistencies

 The coagulant self aggregates – this is inconsistent with the positive charge that should prevent aggregation   Electrostatic repulsion extends only a few nm from the surface of a colloid – and the coagulant adhesive nanoglobs are many times larger than the reach of the repulsive electrostatic force The electrostatic hypothesis doesn’t provide any attachment mechanisms. It only provided a mechanism for colloids to get close together. The hypothesis that London van der Waals forces result in attachment neglects to account for the presence of water in the system. Water molecules will also be attracted to surfaces by London van der Waals forces and thus there will be competition between the coagulant and water. Thus eliminating repulsion is NOT sufficient to produce a bond between the colloids.

extra

The electrostatic hypothesis wrongly predicts…

 a stoichiometric relationship between coagulant and colloid concentrations.  that positively charged coagulant precipitates don’t aggregate.

 resuspension of colloids at a high dosage of coagulant that causes the surface charge to become positive

Sweep Floc: the mystery mechanism

 The majority of water treatment flocculation is not explained by charge neutralization.  Sweep flocculation is used to “explain” the flocculation that occurs where charge neutralization predicts flocculation should not occur.

 Sweep floc overcomes the repulsive surface charge by some other unexplained mechanism (sweeping???)

100 years with the wrong hypothesis!

 Surface charge and charge neutralization does explain some aggregation and sedimentation processes under very low shear conditions  The water treatment application was always perceived to be so complex that a detailed understanding of the mechanism was assumed to be impossible  Results that didn’t fit the hypothesis such as sweep flocculation were explained as the result of the complex nature of the problem extra

Coagulation: The adhesive nanoglob hypothesis

 Aluminum coagulants (alum and polyaluminum chloride) produce adhesive nanoglobs of precipitated aluminum hydroxide that attach to surfaces including other nanoglobs  The attractive attachment force must be stronger than the hydrogen bond between water molecules so the nanoglobs can push water out of the way Predictive performance model for hydraulic flocculator design with polyaluminum chloride and aluminum sulfate coagulants Karen A. Swetland, Monroe L. Weber-Shirk*, Leonard W. Lion

Sticky Nanoglobs?

 Effective flocculation only occurs when the coagulant is in the solid phase  Al(OH) 3 surfaces likes to stick to itself and to other  Why is the solid phase Al(OH) 3 are trying to figure this out…) sticky? (We

The problem of

WET

 Hydrogen bonding between water molecules and also between a water molecule and electronegative atoms (oxygen) in the colloidal surfaces provides competitive bonds that normally prevent bonding between colloids (water gets in the way)  Making wet objects stick together is hard  We need a bond between molecules that is stronger than hydrogen bonds (to push water away)

Strong Intermolecular Bonds: Stronger than hydrogen bonds?*

 We know that aluminum and iron work as coagulants. But why?

 Both coagulants have an oxidation state of +3 and both precipitate as X(OH) 3 d + d + d - Al d +++ Al d +++ d + H d + O H d + *Edge of knowledge alert

Stronger than a Hydrogen Bond?

 Al is very weakly electronegative and thus it maintains a charge of +3 when combined with 3 oxygen (oxygen keeps all of the electrons)  Hypothesis*: intermolecular bonds between oxygen and aluminum are stronger than intermolecular bonds between oxygen and hydrogen O - H = 3.5 - 2.1 = 1.4

O - Al = 3.5 – 1.5 = 2 Polarity of water Polarity of Al(OH) 3 * Edge of knowledge alert

Aluminum Sulfate Chemistry Alum [Al

2

(SO

4

)

3

*14.3H

2

O]

 A widely used coagulant  Typically 10 mg/L to 100 mg/L alum is used (0.9 to 9 mg/L as Al)  High concentrations (stock solutions) don’t precipitate because the pH is low  The alum precipitates when it blends with the water in the water treatment plant  The primary reaction produces Al(OH) 3 Al 2 (SO 4 ) 3 + 6H 2 O  2Al(OH) 3 + 6H + + 3SO 4 -2 pH = -log[H + ]

Acid Neutralizing Capacity (ANC or Alkalinity) Requirement

Al 2 (SO 4 ) 3 + 6H 2 O  2Al(OH) 3 + 6H + + 3SO 4 -2  ANC is measured as mg/L of CaCO 3  How much ANC is consumed by alum?

Molecular Formula Molecular mass eq/mol Molecular mass/eq Simple guide

Alum

Al 2 (SO 4 ) 3 *14H 2 0 600 g/mol 6 100 g/eq 1 mg/L Alum consumes

Calcium Carbonate

CaCO 3 100 g/mol 2 50 g/eq 0.5 mg/L Calcium Carbonate ANC This sets the maximum alum dose that can be used for low alkalinity waters

Polyaluminum Chloride (PACl)

 Slowly titrated with a base (in the chemical plant) to produce a meta-stable and soluble polymeric aluminum (partially neutralized)  Consumes less alkalinity (ANC)  PACl forms flocs more rapidly than does alum (plant operator observation)  Aluminum mass fraction is higher than in alum (no 14.3 H 2 O) so the mass of PACl required is less than for alum (1-15 mg/L)

10 1 0.1

10 1   2 0.1

Aluminum Solubility

0.01

5 6 Theoretical Al solubility Experimental PACl solubility Minimum EPA MCL Maximum EPA MCL Maximum dose Al pH 7 0.01

5 6 Theoretical Al solubility Experimental PACl solubility Minimum EPA MCL Maximum EPA MCL Min dose Al pH 7 8  4  1 pH control is critical!

Coagulation fails at low pH and high pH because the coagulant becomes too soluble 8

Coagulation Geometry

Clay platelets Coagulant nanoglobs

Surface charge of Particles and Natural Organic Matter (NOM)

 NOM significantly increases the required coagulant dose in some waters  Charge density (conventional explanation)  Clay: 0.05 to 0.5 m eq/mg (1 mg/L clay ≈ 1 NTU)  Fulvic acid 5 to 15 m eq/mg C  Alternative explanation - NOM has a larger surface area* per unit mass than colloids and thus provide many attachment sites for adhesive nanoglobs

Rapid Mixers

 A high-intensity mixing step used before flocculation to disperse the coagulant(s) and to initiate the particle aggregation process*  In the case of hydrolyzing metal salts, the primary purpose is to quickly disperse the salt so that contact between the simpler hydrolysis products and the particles occurs before the metal hydroxide precipitate is formed* (This is probably not true)  “This process is poorly understood…”* *Water Quality and Treatment 5th edition p 6.57

extra

On Rapid Mixing…

“In summary, little is known about rapid mix,

much less any sensitivity to scale

. However, the models and data reviewed suggest the need to be on the lookout for certain effects. From what is presently known, it can be speculated that since coagulant precipitation is sensitive to both micro- and macro mixing, scale-up must consider not only energy dissipation rate, but also the reaction injection point and the contacting method.” Mixing in Coagulation and Flocculation 1991 page 292 extra

Traditional rapid mix units

 Backmix mechanical reactors  In-line blenders  Hydraulic Jump  In-line static mixers

Traditional Design

 Conventional design is based on the use of G (velocity gradient) as a design parameter.  G does not characterize the mixing caused  by turbulence (G is valid for laminar flow) Rapid mix units are fully turbulent

G

  In part because of the error in choice of parameters, conventional design guidelines are not able to characterize the effects of scaling  

This is the traditional approach

Power, height, and G

G



V P



V



Q

   1 2

P

 2  

P

 

P G

2 

g

“velocity gradient” caused by mixing Reactor volume Power required Power required to lift water

G

2   Equivalent potential energy measured as a height used for mixing Equivalent potential energy as a function of G

Traditional Design Guidelines: Mixing with a Propeller

Residence Time (s)

0.5

10 – 20 20 – 30 30 – 40 40 – 130

“velocity gradient” (G) (1/s)

4000 1500 950 850 750

Energy dissipation rate (W/kg)

16 2.25

0.9

0.72

0.56

Equivalent height (m)*

0.8

2.3 – 4.6

1.8 – 2.8

2.2 – 2.9

2.3 – 7.5

No mention of scale effects

G

2 

g

from Environmental Engineering: A Design Approach by Sincero and Sincero. 1996. page 267 * A measure of mechanical energy converted to heat

Hydraulic Energy Constraint

 If we use the same amount of mechanical energy in hydraulic water treatment plants as is used in mechanical water treatment plants we will need between 0.8 and 7.5 m of water height change just to power the rapid mix unit!!!!!

 Rapid mix is one of the largest energy consumers in mechanical plants  We need to be more efficient (and hence smarter)

Caveat: It is a short walk to the edge of knowledge

extra  The analysis presented next was designed to achieve adequate mixing so that nanoglobs have a chance to attach to all colloids  Coagulant self aggregation occurs at the same time as mixing  Self aggregation (especially in high alkalinity waters) can have a significant detrimental effect* because it reduces the surface area of the adhesive nanoglobs  There is a significant opportunity for further research that will lead to an improved understanding of the rapid mix process

Why might RAPID mixing be necessary?

 Vague answer is that we need to mix the adhesive nanoglobs with the water  But why RAPID?

 IF RAPID mixing matters then there must be something bad that happens if the mix is SLOW  Self aggregation of nanoglobs into microglobs 

Nonuniform distribution of nanoglobs between colloids

Why might RAPID mixing be necessary?

 Self aggregation of nanoglobs will begin when diffusion blends enough raw water with the coagulant to raise the pH so that the nanoglobs become sticky – this diffusion may occur in a few seconds  If the nanoglobs are only dispersed in a fraction of the raw water then some raw water colloids won’t receive any nanoglobs  We will find that we need an energy dissipation rate of >1 W/kg to distribute nanoglobs between colloids

Rapid Mix: From macro to nano scale (in a few seconds)

Length scale m mm Transport Process Rapid Mix flow dimension Large scale eddies Small scale eddies Kolmogorov scale

L K

    3   1 4 Result m m Molecular diffusion nm Molecular scale

Three steps for mixing

 Large scale eddies to mix at the dimension of the reactor (Macro mixing)  Generate large eddies  Flow expansion with dimensions similar to the dimension of the reactor  Small scale eddies to mix down to the Kolmogorov length scale (Micro mixing)  Generate energetic tiny eddies so that turbulence can mix to the length scale of colloid separation  Molecular diffusion to finish the job

Turbulence – Mixing – Energy Dissipation

 The turbulent eddies cause stretching and thinning of concentration gradients and “shuffle” packets of fluid  The intensity of the turbulence can be characterized by the rate at which mechanical energy is being lost to thermal energy  

Kg

 

J

     

s

2    

m

3 2

s

 

How Far Can Turbulence Mix? (Kolmogorov length scale)

 Dissipates

energy

from the mean flow through chaotic eddies and through

viscosity

where the kinetic energy is converted to heat  Turbulence is a great mixer down to the Kolmogorov scale!

Let  = 1 W/kg Kolmogorov length scale

L K

Kolmogorov time scale 200 150 100 50 

K

Viscosity kills inertia (and eddies)!

    3   1 4   1 2 30 m 1 ms m 0 1 1000 10 100 Energy dissipation rate (mW/kg) , viscosity, for water is 10 -6 m 2 /s

Average distance between colloids

 What is the average volume of water “occupied” by a colloid?

 Need to know colloid diameter (

D Colloid

) and colloid volume fraction ( 

Colloid

)

L Separation



D Colloid

   6 

Colloid

  1 3 

Colloid

  3

D Colloid

6

V Occupied



Colloid



V Colloid V Suspension



C Colloid



Colloid

Floc volume Suspension volume

V Occupied

 

D

3

Colloid

6 

Colloid

Hypothesis: Energy Dissipation Rate for Micromixing

 If the Kolmogorov length scale is large compared with the distance between colloids, then distribution of nanoglobs between colloids will be non-uniform

L K

    3   1 4  Therefore… set energy dissipation rate to make Kolmogorov length scale less than separation distance between colloids

L Separation



D Colloid

   6 

Colloid C Colloid

  1 3 

RM Min

 4

D Colloid

   6  3 

Colloid C Colloid

  4 3

Energy dissipation rate for uniform nanoglob application

200 200 150 150

L Separation



D Colloid

   6 

Colloid C Colloid

  1 3 100 50 100 50

L K

    3   1 4   0 1 10 100 Turbidity due to clay (NTU) 10000 We need an energy dissipation rate of approximately

3 W/kg

to ensure uniform application of nanoglobs to colloids!

1000 100 10 1 Below 10 NTU the mixing in the flocculator should be adequate!

0.1

1 1000 0 1 10 100 1000 Energy dissipation rate (mW/kg) 10000 10 100 Turbidity due to clay (NTU) 1000 

RM Min

  4

D Colloid

   6 3 

Colloid C Colloid

  4 3

After turbulence: Let diffusion begin!

500 NTU clay suspension 2 m m diameter clay (4 m m cylinder) P DH = 8 Separation distance for clay is 32 m m Kolmogorov length scale of 100 m m Adhesive nanoglobs Adhesive nanoglobs

After turbulence: Let diffusion begin!

500 NTU clay suspension 2 m m diameter clay (4 m m cylinder) P DH = 8 Separation distance for clay is 32 m m Kolmogorov length scale of 32 m m Adhesive nanoglobs Adhesive nanoglobs

Turbulent Micromixing followed

extra

by molecular diffusion

 Large scale eddies move packets of fluid around at the scale of the flow (or the scale of the separation distance of the injection  points …Smaller eddies move packets of fluid over smaller length scales…  Smallest scale eddies create high concentration gradients at Kolmogorov scale (viscosity kills turbulence at this scale)  Molecular diffusion finishes the job

Velocity gradients increase interfacial area

extra  Turbulence causes chaotic movement of the fluid with spatially and temporally varying velocity gradients (not like this image!)   Net effect is to increase the interfacial area between the fluids being mixed by stretching and deforming the fluids What will I find if I take 1 m m 3 samples and measure the concentration after short exposure to turbulent mixing?

Turbulent Mixing

extra 10 cm Cowen - "An Experimental Investigation of Scale-Dependent Plume Physics"

How Fast can Diffusion Mix?

 What are the units of diffusion?

 Diffusion is the product of a velocity and a path length – [L 2 /T]

D Molecular



V Diffusion L Diffusion

 How long will it take for diffusion to blur the concentration gradient left by turbulent mixing?

D Molecular



L Diffusion t Diffusion L Diffusion t Diffusion



L

2

Diffusion D Molecular

extra

Molecular Diffusion Quantified

extra

D M



k T B

3  m

d P MW

 

P V N P A

Einstein relation (by Albert Einstein in 1905) kb    23  joule K N.A

 6.0221415 10 23  mole  1 Boltzmann’s constant Avogadro’s constant  .AlOH3

 2420 kg m 3

MW

 

P N A



d P

3 6

d P

 6

MW



P N A

  1 3

D M



k T B

3  m Fluid dynamic viscosity 

P N A

1 3 6

MW

Molecular weight For Al(OH) 3 the molecular diffusion coefficient is 10 -9 m 2 /s

Turbulent Mixing so that Molecular Diffusion can finish the job

extra

L K

    3   1 4

L D



D t M Diffusion

Turbulence will mix to the Kolmogorov length scale Diffusion will mix this far in this time

L K



L D

Set the length scales equal 10     3   1 4 

D t M Diffusion

If we use a 1 W/kg micromix event then diffusion will take 1 s

t Diffusion

   3 2

D M

1 10 100 Energy dissipation rate (mW/kg) 1000 This assumes that diffusion rather than shear is the dominant transport mechanism. Need to model collision time between clay particle and nanoglob based on shear transport to determine whether shear or diffusion dominate.

How do we generate intense turbulence?

 We need to be converting mechanical energy (kinetic energy) to thermal energy  We want “concentrated” head loss! (this shouldn’t be too hard to achieve!)  Therefore use minor loss (related to a change in flow geometry) rather than major loss (from shear at the solid boundaries)  Almost all minor losses are caused by

expansions (We need a flow E X P A N S I O N)

Flow Expansion

h e



V in

2 2

g

  1 

A in A out

  2  The control volume analysis gave us the

total

energy loss, but it doesn’t give us the energy dissipation rate.

 The energy dissipation rate varies with location in the expanding jet

Jet Mixing

1 0.8

0.6

0.4

0.2

0 0 2 4 6 14 16 8 10 x/D.Jet

12 

Centerline

 

x

50  3

D V Jet

3

Jet

2

D Jet

 4 Minimum x for which this relationship is valid is 7

D Jet



Centerline

 50 3

D V Jet

3

Jet

 7

D Jet

 2

D Jet

 4 x is distance from the jet origin VJet      4 6 8 10     m s  JetMax      896 3084 7401 13530     W kg (Baldyga et al., 1994 Baldyga, J., Bourne, J. R. and Zimmermann, B., 1994, Investigation of mixing in jet reactors using fast, competitiveconsecutive reactions. Chem. Engn 0 Sci. 49, 1937.



Max

   50 4   3

V Jet D Jet



Max

  P

RoundJet V Jet

 3

D Jet

1   50 5 4   3  0.431

P Jet   DJetArticle   JetMax  3 1 VJet      0.482

0.485

0.487

0.477

    P

RoundJet

 0.4

The value we will use in our analysis. Further work is require to determine the best value of this parameter for different jet geometries.

18 20

A

Energy Dissipation Rate



Max

  P

RoundJet V Jet

 3

D Jet

Three orifices, same velocity

Which jet has the highest energy dissipation rate?

B C Which jet has the highest shear (or velocity gradient)?

Big eddies create smaller eddies

 Which jet has the largest eddies?

 Which jet will make the smallest eddies first?

 Which jet will dissipate energy the fastest?

Re 

VD

 Higher energy dissipation rates give a smaller Kolmogorov scale!

Rapid Mix in a jet?

Combined macro and micro mix

Pin to keep plate in place Access pipe for coagulant delivery tube Orifice plate Coagulant delivery tube

Orifice Diameter to Obtain Target Mixing

(Energy Dissipation Rate, Kolmogorov Length Scale) 

Max

  P

RoundJet V Jet

 3

D Jet A Orifice

P 

vc A Jet D Orifice

P 

vc D Jet



Max

   P

RoundJet D Jet

 4

Q

2

D Jet

  3 Substitute for D Jet and solve for D Orifice

D Orifice

     4

Q

P

RoundJet

1  

Max

  7 3  1 P

vc

The orifice must be smaller than this to achieve the target energy dissipation rate

Rapid Mix Head Loss



Max V Jet

  P

RoundJet V Jet

 

D Jet D Jet



Max

P

RoundJet

 1 3  3

D Jet

     4 P

RoundJet Q

1  3

Max

    7 3 50 This is one of the scale effects for rapid mix Why?

40

h e

 2

V Jet

2

g h e

 

D Jet



Max

 2

g

P 2

RoundJet

2 3

h e

   4 P

RoundJet



Q

 2

Max

2

g

P 2

RoundJet

  7 2 30 20 10 0 1 10 Flow rate (L/s) 100 1000

What can you do to reduce the required head loss through the rapid mix for large plant flows?

 Rapid Mix head loss increases with Q

h e

   4 P 2

RoundJet



g

 How could we maintain a high energy P 2

Q

 2

Max RoundJet

  2 7 dissipation rate while reducing the velocity and the head loss?

  P

RoundJet V Jet

 3  What else is needed?



Max D Jet

 Use a plate with multiple orifices in the rapid mix pipe and add macro mixing upstream

Hypothesis: Macomixing minor loss coefficient and length scale

 The eddy velocities are comparable to the mean velocities for a minor loss coefficient of 1  The minimum distance in the direction of flow (L) for mixing over the dimension perpendicular to the direction of flow (D) would then be equal to D. (L>D)  This length is the pipe minimum length before the micromixing event (unless the two are combined in a single orifice)

Mechanized Rapid Mix vs. Hydraulic Rapid Mix

 Vampire loads  A 100 L/s AguaClara plant costs approximately $600,000  After 25 years the electricity cost for mechanized rapid mix would be $230,000  “Another way to give is to not take…”  The total energy cost for a package plant that uses a total of 400 J/L is 1.5 million USD!

400 J L  50¤ 10 9 J  100 L  s 25 yr  1577846 ¤

Reflections

 At lower than design flow rates the energy dissipation rate will be lower than the target 

Max

  P

RoundJet V RoundJet

 3

D Jet

 The hydraulic rapid mix head loss is tiny compared with traditional designs  For flow rates less than 1000 L/s it probably isn’t necessary to mix in two stages (micromix and macromix are combined)

Edge of Knowledge Reminder

 We need to test the hypothesis that colloid spacing and Kolmogorov length scale influence performance  The energy dissipation rate in a jet is not uniform and we are using the max energy dissipation rate in this analysis  We haven’t addressed loss of coagulant precipitate to reactor walls