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

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 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 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 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

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

Struvite encrusted roller MgNH4PO4 . 6H2O Ball check Filtrate return line Productivity lost!! Struvite encrusted roller (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

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

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.

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

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

Nutrient Removal from Digested Sludge

Pilot Scale Experimental Results on Phosphate Removal from Centrate

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

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

Effect of Mg(OH)2 on Activated Sludge Settleability

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

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

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

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