1. 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

2. Typical Municipal WWTP Flow Diagram

3. 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

4. Struvite Magnesium ammonium phosphate MgNH4PO4 · 6H2O
Named after Russian diplomat, H.G. von Struve ( )
White, yellowish white, or brownish white in color
FW =
Specific density = 1.7
Very insoluble in water, pKso = 12.6 – at 25oC

5. Struvite Chemistry Struvite formation occurs when the conditions are
NH4+ Û NH3 (aq) + H+ pKa=9.3
H3PO4 Û H2PO4- + H+ pKa1= 2.1
H2PO4- Û HPO42- + H+ pKa2= 7.2
HPO42- Û PO43- + H+ pKa3= 12.3
MgOH+ Û Mg2+ + OH- pK=2.56
MgNH4PO4.6H2O Û Mg NH4+ +PO H2O pK=12.6
Struvite formation occurs when the conditions are
such that the concentration product exceeds the
struvite conditional solubility product

6. Conditional Solubility of Struvite vs pH
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
gi = activity coefficient for component i

7. 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

8. Struvite encrusted roller
MgNH4PO4 . 6H2O
Ball check
Filtrate return line
Productivity lost!!
Struvite encrusted roller
(Courtesy Schaner’s Waste Water Products, Inc.)

9. 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

10. 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

11. Characterization of Mg(OH)2: Basic Properties that are Important to Wastewater Treatment Applications

12. Magnesium Hydroxide Dissolution Kinetics

13. Titration Curves of Several Neutralization Chemicals
A = calcium hydroxide; B = pure magnesium hydroxide; C = sodium carbonate; D = as-received magnesium hydroxide slurry

14. Relative Neutralization Capacity and Buffering Capacity of Several Neutralization Reagents (at pH = 8.5)
1 = pure magnesium hydroxide; 2 = sodium carbonate; 3 = calcium hydroxide; 4 = as-received magnesium hydroxide slurry.

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

16. Sludge Digestion Enhancement
Using Mg(OH)2

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

18. Biogas Production Profiles During Anaerobic Sludge Digestion

19. 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

20. Nutrient Removal from Anaerobically Digested Sludge and Sludge Supernatant Using Mg(OH)2

21. Nutrient Removal from Digested Sludge

22. Pilot Scale Experimental Results on Phosphate Removal from Centrate

23. Total phosphorus mass balance without metal phosphate precipitation from centrate/filtrate
effluent
Influent
Primary + secondary
treatment systems
100
310 10
300
Sludge digester
300
sludge cake
Filtrate/centrate
Sludge dewatering
210 90
Total phosphorus mass balance with metal phosphate precipitation from centrate/filtrate
effluent
Influent
Primary + secondary
treatment systems
100
107 10 97
Treated filtrate/
centrate 7
Sludge digester 97
sludge cake +
chemical sludge
Metal phosphate
precipitation reactor
Filtrate/centrate
Sludge dewatering 68 29 90
P-containing chemical sludge 61

24. 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

25. Improving the Settleability and Dewaterability of Activated Sludge: Applications of Mg(OH)2

26. Effect of Mg(OH)2 on Activated Sludge Settleability

27. Surface Charge Density Changes vs Mg(OH)2 Dosage
COO-
---Mg2+ ---
-OOC
NH3
NH3

28. Mixed Liquor Sedimentation Curves under Different Mg(OH)2 Dosage Conditions

29. Sludge Dewaterability Changes with the Addition of Mg(OH)2

30. 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

31. 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

32. 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

33. 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.

34. Potential Mg(OH)2 Application Locations in Municipal WWTP