Control of Struvite Deposition in Control of Struvite Deposition in Wastewater Treatment PlantsWastewater Treatment Plants

Paul L. BishopAssociate Vice President for Research

University of Cincinnati

11th Annual Central States Water Environment Association Education Conference

April 4, 2006

Incoming Wastewater

Bar Screen

Grit Chamber

Aeration Tanks

PrimaryClarifier

SecondaryClarifier

Chlorine Contact Tank

Plant Effluent

Dissolved AirFlotation Tank

Anaerobic SludgeDigester

Sludge DewateringFacility

Screenings

Grits

Further Dewatering

GravityThickener

Sludge to Incinerator,Farmland or Landfill

Re

turn

Act

iva

ted

Slu

dg

e

Ce

ntr

ate

/Filt

rate

Typical Municipal WWTP Flow DiagramTypical Municipal WWTP Flow Diagram

ProblemsProblems Anaerobic sludge digestion releases

ammonium, magnesium and phosphate, which can form struvite in digesters and downstream dewatering facilities Can result in scaling in pipelines and on walls

of process equipment

Centrate or filtrate from sludge dewatering is usually returned to the plant headworks where it can add to the wastewater burden

StruviteStruvite

Magnesium ammonium phosphate

MgNH4PO4 · 6H2O

Named after Russian diplomat, H.G. von Struve (1772-1851)

White, yellowish white, or brownish white in color FW = 245.41 Specific density = 1.7

Very insoluble in water, pKso = 12.6 – 13.15 at 25oC

NH4+ NH3 (aq) + H+ pKa=9.3

H3PO4 H2PO4- + H+ pKa1= 2.1

H2PO4- HPO4

2- + H+ pKa2= 7.2

HPO42- PO4

3- + H+ pKa3= 12.3

MgOH+ Mg2+ + OH- pK=2.56

MgNH4PO4.6H2O Mg2+ + NH4+ +PO4

3- + 6H2O pK=12.6

Struvite ChemistryStruvite Chemistry

Struvite formation occurs when the conditions are such that the concentration product exceeds the struvite conditional solubility product

-12

-10

-8

-6

-4

-2

0

2

4

6

8

1 3 5 7 9 11 13

pH

Log (

Ps)

-15

-13

-11

-9

-7

-5

-3

-1

1

Log (

ioniz

ati

on f

racti

on)

Mg2+

NH4+

PO43-

MgNH4PO4.6H2O

Ps = conditional solubility product

Kso = solubility product

CT,Mg = total concentration of all soluble magnesium species

CT,NH3 = total concentration of all soluble ammonia species

CT,PO4 = total concentration of all soluble phosphate species

i = ionization fraction for component i

i = activity coefficient for component i

P s C C CK so

T M g T N H T P O

M g N H P O M g N H P O

, , ,3 42

4 43 2

4 43

Conditional Solubility of Struvite vs pHConditional Solubility of Struvite vs pH

Anaerobically digested sludge, anaerobic supernatant (centrate/filtrate)

Mixing & perturbations

Carbon dioxide stripping

pH elevation

Phosphate equilibrium shifts towards PO43-

[Mg2+] [NH4+][PO4

3-] exceeds struvite solubility product (super-saturation)

Nucleation and crystal growth

Struvite precipitates

Struvite Formation in Sludge Struvite Formation in Sludge Dewatering ProcessDewatering Process

Filtrate return line Ball check

Struvite encrusted roller Productivity lost!!Productivity lost!!

(Courtesy Schaner’s Waste Water Products, Inc.)

MgNHMgNH44POPO4 4 . . 6H6H22OO

Chloride concentration increases

Ferric ion acts as an acid, lowering pH

Large volume inorganic sludge generation

Phosphate recovery from ferric phosphate

salt(s) is nearly impossible

Problems with Current Struvite Problems with Current Struvite Control TechniquesControl Techniques

Addition of iron chloride to form

vivianite (Fe3(PO4)2 . 8H2O)

Similar problems with ferric sulfate or alum

ObjectiveObjective

Investigate the use of magnesium hydroxide to remove nutrients in a controlled fashion from digested sludge

Can use waste flue gas desulfurization sludge as a source of Mg(OH)2

Characterization of Mg(OH)Characterization of Mg(OH)22: Basic : Basic

Properties that are Important to Properties that are Important to

Wastewater Treatment ApplicationsWastewater Treatment Applications

6

6.5

7

7.5

8

8.5

9

9.5

10

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00

Time (min)

pH

Magnesium HydroxideMagnesium Hydroxide Dissolution KineticsDissolution Kinetics

0

2

4

6

8

10

12

14

0 0.002 0.004 0.006

Titrant Added (eq.)

pH

A BCD

A = calcium hydroxide; B = pure magnesium hydroxide; C = sodium carbonate; D = as-received magnesium hydroxide slurry

Titration Curves of Several Titration Curves of Several Neutralization ChemicalsNeutralization Chemicals

00.10.20.30.40.50.60.70.80.9

1

1 2 3 4

Rel

ativ

e N

eutr

aliz

atio

n C

apac

ity

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

1 2 3 4

Buf

feri

ng C

apac

ity(

eq/p

H)

1 = pure magnesium hydroxide; 2 = sodium carbonate; 3 = calcium hydroxide; 4 = as-received magnesium hydroxide slurry.

Relative Neutralization Capacity and Relative Neutralization Capacity and Buffering Capacity of Several Buffering Capacity of Several

Neutralization Reagents (at pH = 8.5)Neutralization Reagents (at pH = 8.5)

Mg(OH)2 has unique features compared with other

commonly used chemicals:

slow dissolution process

high neutralization capacity

high buffering intensity

SummarySummary

Sludge Digestion EnhancementSludge Digestion Enhancement

Using Mg(OH)Using Mg(OH)22

NHNH33-N, PO-N, PO443-3--P, Mg-P, Mg2+2+, Ca, Ca2+2+ and SO and SO44

2-2- Changes Changes

During Anaerobic Sludge DigestionDuring Anaerobic Sludge Digestion

0

50

100

150

200

250

300

0.00 100.00 200.00 300.00 400.00 500.00 600.00Digestion Time (hours)

Bio

gas

Vol

um

e (L

)

Mg(OH)2 reactor

Control reactor

Biogas Production Profiles During Biogas Production Profiles During Anaerobic Sludge DigestionAnaerobic Sludge Digestion

SummarySummary

Applying magnesium hydroxide into an anaerobic sludge digester can:

Result in greater destruction of COD and SS Enhance the production rate of biogas Increase overall treatment efficiency Reduce level of nutrients in the supernatant that must

be

returned to the plant’s headworks Increase the nutrient content in the generated biosolids

for agricultural use Improved sludge dewaterability, which will ease the

operation of the down stream sludge dewatering facilities

Nutrient Removal from Anaerobically Nutrient Removal from Anaerobically

Digested Sludge and Sludge Supernatant Digested Sludge and Sludge Supernatant

Using Mg(OH)Using Mg(OH)22

020406080

100120140160

0 100 200 300 400 500

Time (min)

PO

43- -

P (

mg/

L)

7

7.5

8

8.5

9

pH

Phosphate---No mix control Phosphate---Mixed controlPhosphate---Mg(OH)2=100mg/L Phosphate---Mg(OH)2=250mg/LpH---No mix control pH---Mixed controlpH---Mg(OH)2=100 mg/L pH---Mg(OH)2=250 mg/L

Nutrient Removal from Digested SludgeNutrient Removal from Digested Sludge

0

10

20

30

40

50

60

70

0 50 100 150 200

Time (min)

PO

43- -

P (

mg/

L)

air stripping only

air stripping + 200 mg/L MgCl2

air stripping + 400 mg/L MgCl2

air stripping + 100 mg/L Mg(OH)2

air stripping + 200 mg/L Mg(OH)2

air stripping + 400 mg/L Mg(OH)2

aera

tion per

iod

sett

ling p

erio

d

Pilot Scale Experimental Results on Pilot Scale Experimental Results on Phosphate Removal from CentratePhosphate Removal from Centrate

Primary + secondarytreatment systems

Sludge digester

Sludge dewatering

effluent

sludge cake

Influent

Primary + secondarytreatment systems

Sludge digester

Sludge dewatering

effluent

sludge cake +chemical sludge

Influent

Filtrate/centrate

Filtrate/centrateMetal phosphateprecipitation reactor

100

100

310

210

300

300

10

90

107 10

29 90

61

7Treated filtrate/centrate

P-containing chemical sludge68

97

97

Total phosphorus mass balance without metal phosphate precipitation from centrate/filtrate

Total phosphorus mass balance with metal phosphate precipitation from centrate/filtrate

SummarySummary Use of Mg(OH)2 to remove nutrients from

anaerobically digested sludge is effective

only if the sludge is well digested.

Removing phosphate from the side waste

stream will: reduce the nutrient load to the headworks of the

treatment plant (this is a current practice that

adversely affects the overall treatment efficiency)

lower the potential for struvite formation, which is

a frequently occurring O&M problem in many

municipal wastewater treatment plants

generate a slow release fertilizer

Improving the Settleability and Improving the Settleability and Dewaterability of Activated Sludge: Dewaterability of Activated Sludge: Applications of Mg(OH)Applications of Mg(OH)22

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120

Mg(OH)2 Dosage (mg/L)

SV

I

0

100

200

300

400

500

600

700

SV

(ml/L

)

Effect of Mg(OH)2 on Activated Effect of Mg(OH)2 on Activated Sludge SettleabilitySludge Settleability

-7

-6

-5

-4

-3

-2

-1

0

1

2

0 20 40 60 80 100

Magnesium Hydroxide Addition (mg/L)

Re

lativ

e S

urf

ace

Ch

arg

e

COO- -OOC---Mg2+ ---

NH3 NH3

Surface Charge Density Changes vs Surface Charge Density Changes vs Mg(OH)2 DosageMg(OH)2 Dosage

40

60

80

100

120

140

160

180

0:00:00 0:14:24 0:28:48 0:43:12 0:57:36 1:12:00 1:26:24 Time (hour:minute:second)

He

igh

t o

f w

ate

r/s

lud

ge

inte

rfa

ce

(c

m)

Mg(OH)2: 0 mg/L

Mg(OH)2: 100 mg/L

Mg(OH)2: 300 mg/L

Mg(OH)2: 500 mg/L

Mixed Liquor Sedimentation Curves under Mixed Liquor Sedimentation Curves under Different Mg(OH)Different Mg(OH)22 Dosage Conditions Dosage Conditions

255

260

265

270

275

280

285

290

0 50 100 150 200

Mg(OH)2 Dosage (mg/L)

CS

T (

seco

nd

s)

Sludge Dewaterability Changes with the Sludge Dewaterability Changes with the Addition of Mg(OH)Addition of Mg(OH)22

SummarySummary By charge neutralization, sweep

flocculation and Mg2+ bridging between the EPS matrices of the microorganisms, Mg(OH)2 is effective in improving the settleability of activated sludge

Besides enhancing the overall sludge digestion process efficiency, Mg(OH)2 application to anaerobic sludge digester can also generate a digested sludge that is easier to dewater

Mg(OH)2 improved the biological phosphate uptake and release behavior of activated sludge

Mg2+ was found to stimulate the phosphate uptake during aeration periods

The pH increase caused by Mg(OH)2 addition enhanced phosphate release during the anaerobic sedimentation period

Research results provide supporting evidence for thepotential application of Mg(OH)2 in EBPR processes

ConclusionsConclusions

ConclusionsConclusions

Magnesium hydroxide can effectively improve the settleability of mixed liquor during sedimentation in secondary clarifier and the dewaterability of anaerobically digested sludge in sludge dewatering

Magnesium hydroxide can enhance the overall process efficiency of anaerobic sludge digestion due to improved pH/alkalinity and the supplementation of Mg2

ConclusionsConclusions Magnesium hydroxide is effective in removing nutrients

from anaerobic supernatant, thus reducing the nutrient load

returned to the headworks of the plant

It minimizes the risk of struvite formation and generates a

good plant fertilizer

Magnesium hydroxide is superior to other commonly used

chemicals in this regard FeCl3, alum and lime.

Aeration (for mixing) plus magnesium chloride (Mg2+

source) plus struvite seeding proves to be a good process

for controlled struvite crystallization.

Potential Mg(OH)Potential Mg(OH)22 Application Locations Application Locations

in Municipal WWTPin Municipal WWTP