Control of Struvite Deposition in Wastewater Treatment Plants Paul L. Bishop Associate Vice President for Research University of Cincinnati 11th Annual Central States Water Environment Association Education Conference April 4, 2006

Typical Municipal WWTP Flow Diagram Incoming Wastewater

Bar Screen

Dissolved Air Flotation Tank

Screenings Gravity Thickener

Grit Chamber

Further Dewatering Primary Clarifier

Aeration Tanks

Secondary Clarifier

Chlorine Contact Tank

Plant Effluent

Return Activated Sludge

Anaerobic Sludge Digester

Sludge Dewatering Facility

Sludge to Incinerator, Farmland or Landfill

Centrate/Filtrate

Grits

Problems §  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

Struvite §  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

Struvite Chemistry NH4+ ⇔ NH3 (aq) + H+

pKa=9.3

H3PO4 ⇔ H2PO4- + H+

pKa1= 2.1

H2PO4- ⇔ HPO42- + H+

pKa2= 7.2

HPO42- ⇔ PO43- + H+

pKa3= 12.3

MgOH+ ⇔ Mg2+ + OH-

pK=2.56

MgNH4PO4.6H2O ⇔ Mg2+ + NH4+ +PO43- + 6H2O

pK=12.6

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

Conditional Solubility of Struvite vs pH 2+

Mg

1 -1

4 2

NH4

0 -2 1 -4

3

5

7

9

+

11 13

-3 -5 -7 -9

-6 -8

MgNH4PO4.6H2O

-10 -12

PO4

-11

Log (ionization fraction)

Log (Ps)

8 6

-13

3-

-15

pH

Ps = conditional solubility product Kso = solubility product

Ps = CT , Mg CT , NH3 CT , PO4

Kso = α Mg2+ α NH + α PO3− γ Mg2+ γ NH + γ PO3− 4

4

4

4

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 γi = activity coefficient for component i

Struvite Formation in Sludge Dewatering Process Anaerobically digested sludge, anaerobic supernatant (centrate/filtrate) Mixing & perturbations Carbon dioxide stripping pH elevation Phosphate equilibrium shifts towards PO43[Mg2+] [NH4+][PO43-] exceeds struvite solubility product (super-saturation) Nucleation and crystal growth

Struvite precipitates

MgNH4PO4 . 6H2O

Filtrate return line

Struvite encrusted roller

Ball check

Productivity lost!!

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

Problems with Current Struvite Control Techniques §  Addition of iron chloride to form vivianite (Fe3(PO4)2 . 8H2O) "   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 §  Similar problems with ferric sulfate or alum

Objective §  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)2: Basic Properties that are Important to Wastewater Treatment Applications

Magnesium Hydroxide Dissolution Kinetics 10 9.5 9

pH

8.5 8 7.5 7 6.5 6 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00

Time (min)

Titration Curves of Several Neutralization Chemicals 14 12

pH

10 8 6 C

D

4

A

B

2 0 0

0.002

0.004

0.006

Titrant Added (eq.) A = calcium hydroxide; B = pure magnesium hydroxide; C = sodium carbonate; D = as-received magnesium hydroxide slurry

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Buffering Capacity(eq/pH)

Relative Neutralization Capacity

Relative Neutralization Capacity and Buffering Capacity of Several Neutralization Reagents (at pH = 8.5) 0.0035 0.003 0.0025 0.002 0.0015 0.001 0.0005 0

1

2

3

4

1

2

3

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

4

Summary Mg(OH)2 has unique features compared with other commonly used chemicals: §  slow dissolution process §  high neutralization capacity §  high buffering intensity

Sludge Digestion Enhancement Using Mg(OH)2

NH3-N, PO43--P, Mg2+, Ca2+ and SO42- Changes During Anaerobic Sludge Digestion

Biogas Production Profiles During Anaerobic Sludge Digestion Biogas Volume (L)

300 250 200 150

Mg(OH)2 reactor Control reactor

100 50 0

0.00

100.00

200.00

300.00

400.00

Digestion Time (hours)

500.00

600.00

Summary 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 Digested Sludge and Sludge Supernatant Using Mg(OH)2

3-

9

160 140 120 100 80 60 40 20 0

8.5 8 7.5

0

100

200

300

400

7 500

Time (min) Phosphate---No mix control Phosphate---Mg(OH)2=100mg/L pH---No mix control pH---Mg(OH)2=100 mg/L

Phosphate---Mixed control Phosphate---Mg(OH)2=250mg/L pH---Mixed control pH---Mg(OH)2=250 mg/L

pH

PO4 -P (mg/L)

Nutrient Removal from Digested Sludge

Pilot Scale Experimental Results on Phosphate Removal from Centrate 70 air stripping only

50 40 30

settling period

aeration period

PO43- -P (mg/L)

60

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

20 10 0 0

50

100

Time (min)

150

200

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

310

100

effluent

Primary + secondary treatment systems

10

300 Sludge digester

300 Filtrate/centrate

210

sludge cake

Sludge dewatering

90

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

100 Treated filtrate/ centrate

107 7

effluent

Primary + secondary treatment systems

10

97 Sludge digester

97 Metal phosphate precipitation reactor

Filtrate/centrate

68

Sludge dewatering

P-containing chemical sludge

61

sludge cake + chemical sludge

29

90

Summary §  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 Dewaterability of Activated Sludge: Applications of Mg(OH)2

350

700

300

600

250

500

200

400

150

300

100

200

50

100

0

0 0

20

40

60

80

Mg(OH)2 Dosage (mg/L)

100

120

SV(ml/L)

SVI

Effect of Mg(OH)2 on Activated Sludge Settleability

Surface Charge Density Changes vs Mg(OH)2 Dosage COO-

---Mg2+ ---

-OOC

Relative Surface Charge

NH3

NH3

2 1 0 -1 0

20

40

60

80

100

-2 -3 -4 -5 -6 -7

Magnesium Hydroxide Addition (mg/L)

Mixed Liquor Sedimentation Curves under Different Mg(OH)2 Dosage Conditions Height of water/sludge interface (cm)

180 160

Mg(OH)2: 0 mg/L Mg(OH)2: 100 mg/L

140

Mg(OH)2: 300 mg/L Mg(OH)2: 500 mg/L

120 100 80 60 40 0:00:00

0:14:24

0:28:48

0:43:12

0:57:36

Time (hour:minute:second)

1:12:00

1:26:24

Sludge Dewaterability Changes with the Addition of Mg(OH)2 CST (seconds)

290 285 280 275 270 265 260 255 0

50

100

150

Mg(OH)2 Dosage (mg/L)

200

Summary §  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

Conclusions §  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 the potential application of Mg(OH)2 in EBPR processes

Conclusions §  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

Conclusions §  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)2 Application Locations in Municipal WWTP Incoming Wastewater

Bar Screen

Dissolved Air Flotation Tank

Screenings Gravity Thickener

Grit Chamber

Grits

Primary Clarifier

Ma Hy gne dr siu ox m ide 2

Centrate/Filtrate

Further Dewatering

H

1

Secondary Clarifier

Chlorine Contact Tank

Plant Effluent

3 Magnesium Hydroxide

Sludge Dewatering Facility Magnesium Hydroxide

M ag yd n e ro siu xi m de

Aeration Tanks

Return Activated Sludge

Anaerobic Sludge Digester

Sludge to Incinerator, Farmland or Landfill

4